Mineral-deposit model for lithium-cesium-tantalum pegmatites
<p>Lithium-cesium-tantalum (LCT) pegmatites comprise a compositionally defined subset of granitic pegmatites. The major minerals are quartz, potassium…
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Mineral-Deposit Model for Lithium-CesiumTantalum Pegmatites Chapter O of Mineral Deposit Models for Resource Assessment Scientific Investigations Report 2010-5070-O U.S. Department of the Interior U.S. Geological Survey
Cover. Giant crystals of beryl from the Bumpus pegmatite mine in Maine. The vintage 1927 photograph is from Perham (1986), used with permission.
Mineral-Deposit Model for Lithium-CesiumTantalum Pegmatites By Dwight C. Bradley, Andrew D. McCauley, and Lisa M. Stillings Chapter O of Mineral Deposit Models for Resource Assessment Scientific Investigations Report 2010-5070-O U.S. Department of the Interior U.S. Geological Survey
U.S. Department of the Interior SALLY JEWELL, Secretary U.S. Geological Survey Suzette M. Kimball, Director U.S. Geological Survey, Reston, Virginia: 2017 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit ://www.usgs.gov or call 1-888-ASK-USGS. For an overview of USGS information products, including maps, imagery, and publications, visit ://store.usgs.gov/. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Bradley, D.C., McCauley, A.D., and Stillings, L.M., 2017, Mineral-deposit model for lithium-cesium-tantalum pegmatites: U.S. Geological Survey Scientific Investigations Report 2010-5070-O, 48 p., ://doi.org/10.3133/sir20105070O. ISSN 2328-0328 (online)
Contents Abstract 1 1.0. Introduction 2 2.0. Deposit Type and Associated Commodities 2
2.1. Name 2
2.2. Nomenclature and Synonyms 2
2.3. Brief Description 10
2.4. Associated Deposit Types 11
2.5. Primary and By-Product Commodities 12
2.6. Trace Constituents 12
2.7. Example Deposits 12 3.0. History of Pegmatite Research 13 4.0. Regional Environment 14
4.1. Tectonic Environment 14
4.2. Temporal (Secular) Relations 15
4.3. Duration of Magmatic System and (or) Mineralizing Process 15
4.4. Relation to Structures 15
4.5. Relations to Igneous Rocks 18
4.6. Relations to Sedimentary Rocks 18
4.7. Relations to Metamorphic Rocks 18 5.0. Physical Description of Deposits 18
5.1. Dimensions, Form, and Shape 18
5.2. Host Rocks 18 6.0. Geophysical Characteristics 18 7.0. Hypogene Ore Characteristics 19
7.1. Mineralogy and Mineral Assemblages 19
7.2. Zoning Patterns 21
7.3. Paragenesis 23
7.4. Textures, Structure, and Grain Size 23 8.0. Hypogene Gangue Characteristics 24
8.1. Mineralogy and Mineral Assemblages 25 Hydrothermal Alteration 26 10.0. Supergene Ore and Gangue Characteristics 26 11.0. Geochemical Characteristics 26 11.1. Trace Elements and Element Associations 26 11.2. Fluid Inclusion and Melt Inclusion Thermometry and Chemistry 26 11.3. Stable Isotope Geochemistry 27 11.4. Petrology of Associated Igneous Rocks 27 11.5. LCT Pegmatite Geochronology 27 11.6. Environment of Mineralization 28 12.0. Theory of Pegmatite Origin 28 12.1. Ore Deposit System Affiliation 29 12.2. Sources of Metals 29 12.3. Sources of Melt Fluxes 29 12.4. Chemical Transport and Differentiation 30 12.5. Nature of Geological Traps that Trigger Ore Precipitation 30
13.0. Geological Exploration and Assessment Guide 30 13.1. Regional-Scale Favorability 30 13.2. District-Scale Vectors 30 13.3. Alteration Haloes 31 13.4. Geophysical Guidelines 32 14.0. Geoenvironmental Features and Anthropogenic Mining Effects 32 14.1. Soil and Sediment Signatures Prior to Mining 32 14.2. Drainage Signatures from Mining of LCT Pegmatites 32 14.3. Climate Effects on Geoenvironmental Signatures 33 14.4. Mining Methods 33 14.5. Ore-Processing Methods 33 14.6. Metal Mobility from Solid Mine Waste 33 15.0. Knowledge Gaps and Future Research Directions 34 16.0. Acknowledgments 36 References Cited 36 Appendix. Grade-Tonnage Data and Plots 47 Figures
1. Photographs of pegmatites 2
2. World map showing the locations of noteworthy pegmatites and related granites categorized by type 10
3. Graph showing aluminum silicate phase diagram showing pegmatite classes 11
4. Pegmatite classes from Cˇerný and Ercit (2005) as clarified by London 12
5. Schematic cross section showing schematic cross section through Appenines and Tyrrhenian Sea in the Late Miocene, at about 7 Ma (million years before present) showing simultaneous foreland thrusting and hinterland extension, and location of the Elba pegmatites when they were emplaced 14
6. Age distributions covering most of Earth history, back to 4000 million years before present (Ma). A, Probability plot of crystallization ages of igneous rocks. B, Histogram of detrital zircon ages from Pleistocene and modern sediments, using unfiltered data from the global dataset of Voice and others (2011). C, Histogram of ages of lithium-cesium-tantalum (LCT) pegmatites, updated from the global compilation of McCauley and Bradley (2014) 16
7. Map of the Wodgina district, Western Australia, showing fault control, from Sweetapple and Collins (2002) 17
8. Cross section through the Tanco pegmatite, Manitoba, Canada, showing zoning, from Stilling and others (2006) 19
9. Cross section through the Altai No. 3 pegmatite, Inner Mongolia, China, from Zhu and others (2006) 20
10. Cross section through the giant Greenbushes pegmatite, Australia, from Partington and others (1995) 21
11. Phase diagram for lithium silicates under conditions of quartz saturation, from London (1992) 22
12. Map showing idealized concentric, regional zoning pattern in a pegmatite field, adapted from Galeschuk and Vanstone (2005) after Trueman and Cˇerný (1982) 23
13. Map of the Ghost Lake pegmatite field, Ontario, Canada, showing regional zoning, from Breaks and Moore (1992) 24
14. Deposit-scale zoning patterns in an idealized pegmatite, modified from Fetherston (2004) after Cˇerný (1991a) 25
15. Graph showing garnet compositions in pegmatites as a function of degree of fractionation, from Selway and others (2005) 25
16. Graph showing trace element abundances relative to upper continental crust, Little Nahanni pegmatite group, Canada, based on data from Barnes and others (2012) 27
17. Maps showing lithium and rubidium geochemical anomalies over the buried Tanco pegmatite, Manitoba, Canada, adapted from Trueman and Cˇerný (1982) 31
18. Graph showing paleolatitude plotted against time for selected northern Appalachian pegmatites 35 Appendix Figures
A1. Graph showing grade-tonnage plot for lithium in lithium-cesium-tantalum (LCT) pegmatites, based on data in table A1 48
A2. Graph showing grade-tonnage plot for tantalum in lithium-cesium-tantalum (LCT) pegmatites, based on data in table A2 48 Tables
1. Noteworthy lithium-cesium-tantalum (LCT) pegmatites of the world 4
2. Common lithium-, cesium-, and tantalum-bearing minerals in lithium-cesiumtantalum (LCT) pegmatites 21 Appendix Tables
A1. Grade-tonnage data for lithium in lithium-cesium-tantalum (LCT) pegmatites 47
A2. Grade-tonnage data for tantalum in lithium-cesium-tantalum (LCT) pegmatites 47
Mineral-Deposit Model for Lithium-CesiumTantalum Pegmatites By Dwight C. Bradley, Andrew D. McCauley, and Lisa M. Stillings Abstract Lithium-cesium-tantalum (LCT) pegmatites comprise a compositionally defined subset of granitic pegmatites. The major minerals are quartz, potassium feldspar, albite, and muscovite; typical accessory minerals include biotite, garnet, tourmaline, and apatite. The principal lithium ore minerals are spodumene, petalite, and lepidolite; cesium mostly comes from pollucite; and tantalum mostly comes from columbitetantalite. Tin ore as cassiterite and beryllium ore as beryl also occur in LCT pegmatites, as do a number of gemstones and high-value museum specimens of rare minerals. Individual crystals in LCT pegmatites can be enormous: the largest spod umene was 14 meters long, the largest beryl was 18 meters long, and the largest potassium feldspar was 49 meters long. Lithium-cesium-tantalum pegmatites account for about one-fourth of the world's lithium production, most of the tantalum production, and all of the cesium production. Giant deposits include Tanco in Canada, Greenbushes in Australia, and Bikita in Zimbabwe. The largest lithium pegmatite in the United States, at King's Mountain, North Carolina, is no longer being mined although large reserves of lithium remain. Depending on size and attitude of the pegmatite, a variety of mining techniques are used, including artisanal surface mining, open-pit surface mining, small underground work ings, and large underground operations using room-and-pillar design. In favorable circumstances, what would otherwise be gangue minerals (quartz, potassium feldspar, albite, and muscovite) can be mined along with lithium and (or) tantalum as coproducts. Most LCT pegmatites are hosted in metamorphosed supracrustal rocks in the upper greenschist to lower amphibo lite facies. Lithium-cesium-tantalum pegmatite intrusions generally are emplaced late during orogeny, with emplacement being controlled by pre-existing structures. Typically, they crop out near evolved, peraluminous granites and leucogran ites from which they are inferred to be derived by fractional crystallization. In cases where a parental granite pluton is not exposed, one is inferred to lie at depth. Lithium-cesiumtantalum LCT pegmatite melts are enriched in fluxing com ponents including H2O, F, P, and B, which depress the solidus temperature, lower the density, and increase rates of ionic diffusion. This, in turn, enables pegmatites to form thin dikes and massive crystals despite having a felsic composition and temperatures that are significantly lower than ordinary granitic melts. Lithium-cesium-tantalum pegmatites crystallized at remarkably low temperatures (about 350-550 °C) in a remark ably short time (days to years). Lithium-cesium-tantalum pegmatites form in orogenic hinterlands as products of plate convergence. Most formed during collisional orogeny (for example, Kings Mountain district, North Carolina). Specific causes of LCT pegmatiterelated magmatism could include: ordinary arc processes; over thickening of continental crust during collision or subduc tion; slab breakoff during or after collision; slab delamina tion before, during, or after collision; and late collisional extensional collapse and consequent decompression melting. Lithium-cesium-tantalum pegmatite deposits are present in all continents including Antarctica and in rocks spanning 3 billion years of Earth history. The global age distribution of LCT pegmatites is similar to those of common pegmatites, orogenic granites, and detrital zircons. Peak times of LCT pegmatite genesis at about 2640, 1800, 960, 485, and 310 Ma (million years before present) correspond to times of collisional orog eny and supercontinent assembly. Between these pulses were long intervals when few or no LCT pegmatites formed. These minima overlap with supercontinent tenures at ca. 2450-2225, 1625-1000, 875-725, and 250-200 Ma. Exploration and assessment for LCT pegmatites are guided by a number of observations. In frontier areas where explora tion has been minimal at best, the key first-order criteria are an orogenic hinterland setting, appropriate regional metamorphic grades, and the presence of evolved granites and common granitic pegmatites. New LCT pegmatites are most likely to be found near known deposits. Pegmatites tend to show a regional mineralogical and geochemical zoning pattern with respect to the inferred parental granite, with the greatest enrichment in the more distal pegmatites. Mineral-chemical trends in common pegmatites that can point toward an evolved LCT pegmatite include: increasing rubidium in potassium feldspar, increas ing lithium in white mica, increasing manganese in garnet, and increasing tantalum and manganese in columbite-tantalite. Most LCT pegmatite bodies show a distinctive internal zonation featuring four zones: border, wall, intermediate (where lithium,
2 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites cesium, and tantalum are generally concentrated), and core. This zonation is expressed both in cross section and map view; thus, what may appear to be a common pegmatite may instead be the edge of a mineralized body. Neither lithium-cesium-tantalum pegmatites nor their parental granites are likely to cause serious environmental concerns. Soils and country rock surrounding a LCT pegma tite, as well as waste from mining operations, may be enriched in characteristic elements relative to global average soil and bedrock values. These elements may include lithium, cesium, tantalum, beryllium, boron, fluorine, phosphorus, manganese, gallium, rubidium, niobium, tin, and hafnium. Among this suite of elements, however, the only ones that might present a concern for environmental health are beryllium and fluo rine, which are included in the U.S. Environmental Protection Agency drinking-water regulations with maximum contami nant levels of 4 micrograms per liter and 4 milligrams per liter, respectively. 1.0. Introduction Lithium-cesium-tantalum (LCT) pegmatites are granitic rocks that form relatively small igneous bodies and are char acterized by large crystals and a variety of distinctive textures (fig. 1). The LCT family of pegmatites takes its name from its characteristic enrichment in lithium, cesium, and tantalum. These rocks account for about one-fourth of the world's lith ium production (Naumov and Naumova, 2010), one-tenth of the beryllium (Foley and others, 2016), most of the tantalum, and all of the cesium (U.S. Geological Survey, 2011). Lithiumcesium-tantalum pegmatites are also mined for quartz, potas sium feldspar, albite, muscovite, cassiterite, industrial and gem beryl, gem tourmaline, and high-value, museum-quality specimens of many rare minerals. Pegmatites of this type have been found on all continents, including Antarctica, in orogenic belts ranging in age from Mesoarchean to Cenozoic. Table 1 lists the main pegmatite districts, and figure 2 shows their global distribution. This report is part of an effort by the U.S. Geological Survey to update existing mineral deposit models, and to develop new ones, in preparation for a national mineral resource assessment. The deposit models of this series are intended to cover the same kinds of information for all deposits, but the outline has been tailored to suit the characteristics of LCT pegmatites, and where possible, to minimize repetition. In keep ing with the overall purpose, aspects of pegmatite geology that might directly or indirectly help in the search for new depos its are emphasized. The recent, comprehensive synthesis by London (2008) provides more information on many of the top ics covered herein, and is the single best resource for research on pegmatites. An abridged version of the present report was published by Bradley and McCauley (2013). 2.0. Deposit Type and
Associated Commodities 2.1. Name Lithium-Cesium-Tantalum (LCT) Pegmatite. 2.2. Nomenclature and Synonyms London (2008) defined pegmatite as, "an essentially igneous rock, commonly of granitic composition, that is distinguished from other igneous rocks by its extremely coarse but variable grain-size, or by an abundance of crystals with skeletal, graphic, or other strongly directional growth-habits. Pegmatites occur as sharply bounded homogeneous to zoned bodies within igneous or metamorphic host-rocks." A number of schemes for pegmatite nomenclature have been proposed; London (2008) provided an annotated sum mary. For granitic pegmatites, the simplest subdivision is two fold and informal. The common pegmatites are igneous rocks having pegmatitic textures made up of the standard rockforming minerals of granite, which include quartz, potassium feldspar, plagioclase, muscovite, biotite, and local accessory minerals, the most conspicuous being tourmaline, garnet, and apatite. The rare-element pegmatites also contain anomalous abundances of elements relative to what are trace amounts in ordinary granites. These include Be (commonly as beryl), Li (commonly as spodumene or lepidolite), Ta (commonly as tantalite-[Mn] or tantalite-[Fe]), and (or) Cs (as pollucite). The "rare elements" are not to be confused with the "rare-earth elements," as discussed further in Section 2.4. The rare-element pegmatites are subdivided into two end member compositionally-defined families. The lithiumcesium-tantalum (LCT) pegmatites are the products of extreme fractional crystallization of orogenic granites. Most such granites were derived from metasedimentary rocks (S-type granites) rich in muscovite (London, 1995), although certain LCT pegmatites are related to granites derived from igneous Figure 1 (following page). Photographs of pegmatites. (Photographs by Dwight Bradley unless otherwise noted.) A, Molds of giant spodumene crystals in the Etta lithium-cesium-tantalum pegmatite, Black Hills, South Dakota; note person for scale near right edge of picture. Photograph from Schaller (1916). B, Unidirectional solidification texture (UST) defined by alignment of spodumene laths in the San Luis #1 pegmatite, Argentina. C, Graphic granite from the Berry-Havey pegmatite, Maine. D, Skeletal crystals of quartz in potassium feldspar, Las Cuevas pegmatite, Argentina. E, Miarolitic pegmatite pocket in Paleocene granite, western Alaska Range. F, Layered aplite, or "line rock" in a common pegmatite, Popham Beach, Maine. G, Tourmaline halo in metasedimentary host rocks adjacent to Las Cuevas pegmatite in Argentina.
2.0 Deposit Type and Associated Commodities 3 A E F G D B
Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites Table 1. Noteworthy lithium-cesium-tantalum (LCT) pegmatites of the world. For comparable information on other types of pegmatites and lithium granites, see McCauley and Bradley (2014). [Numbers for pegmatites in the Pegmatite number column are only given for pegmatites whose coordinates are known. Numbers are assigned by latitude, from north to south. Numbers and short names for pegmatites in the Pegmatite number column and the Short name column are shown in layers in the original Illustrator version of figure 1, which is available on request from the authors. Numbers for pegmatites in the Pegmatite number column are are assigned by latitude, from north to south. Ages and 2-sigma errors in the Age and Error columns are rounded to the closest integer. Use in Lithium-Cesium-Tantalum age plot? column is used to select the ages plotted in the Lithium-Cesium-Tantalum age distribution. "Y" means yes. "Wrong kind" means not plotted because it is the wrong kind of pegmatite for that plot. "Duplicate" means not plotted because it another pegmatite from the same group is plotted instead. "Filtered" means not plotted because the age has dubious accuracy and (or) precision] Peg matite number Clas sification Pegmatite body Short name Region and country Age in millions of years Error in millions of years Geon Era Dating method
dated Latitude (decimal degrees Longitude (decimal degrees) References and notes Use in LCT age plot? (Y or other) Lithium or Tantalum resource S small L large LCT Pilgangoora Pilgangoora Australia Mesoarchean U/Pb Columbitetantalite -21.042 Kinny, 2000 Y LCT Wodgina Wodgina Western Australia Mesoarchean Pb/Pb Columbitetantalite -21.210 Kinny, 2000 Y LCT Pakeagama Lake Favourable Lake Ontario, Canada Neoarchean U/Pb Columbitetantalite -93.379 Breaks and others, 1999; Smith and others, 2004 Y S LCT Fairservice Mavis Ontario, Canada Neoarchean U/Pb Columbitetantalite -92.656 Breaks and Moore, 1992; Smith and others, 2004 Y S LCT Gullwing Lake Gullwing Canada Neoarchean U/Pb Monazite -92.531 Larbi and others, 1999 Y S LCT Silver Leaf Greer Manitoba, Canada Neoarchean U/Pb Columbitetantalite -96.360 Camacho and others, 2012 Y S LCT Big Whopper Big Whopper Ontario, Canada Neoarchean U/Pb Columbitetantalite -94.060 Breaks and Tindle, 2002; age from Smith and others, 2004; coordinates are for nearby Marko's pegmatite. Tonnage from Breaks and Tindle, 2002 Y — LCT Tanco and Silverleaf Tanco Manitoba, Canada Neoarchean U/Pb Columbitetantalite — — Camacho and others, 2012 Y LCT Quebec Lithium Corporation Lacorne Quebec, Canada Neoarchean U/Pb Monazite -77.808 Location from ://www.mindat.org; Ducharme and others, 1997 Y S LCT Bikita Bikita Zimbabwe Neoarchean — — -20.092 Symons, 1961; age from Melcher and others, 2013 Y LCT Benson Benson Zimbabwe Neoarchean U/Pb Columbitetantalite -17.016 Age from Melcher and others, 2013; location from ://www.mindat.org Y S LCT Greenbushes Greenbushes Western Australia Neoarchean U/Pb Zircon -33.867 Partington and others, 1995 Y LCT Vasin-Mylk Vasin-Mylk Russia Neoarchean U/Pb Columbitetantalite Kudryashov and others, 2004 Y S LCT Kokobin Kokobin Ghana Paleoproterozoic U/Pb Columbitetantalite -0.986 Age from Melcher and others 2008; location approx. Date from placer material Y S LCT Goltsovoya Goltsovoya Russia — Paleoproterozoic — — Vladimirov and others, 2012; location from Mindat; Filtered S LCT Vishnyakovskoe Vishnya-kovskoe Russia Paleoproterozoic U/Pb Columbitetantalite Sal'nikova and others, 2011 Y S LCT Nykopingsgruvan Uto Sweden Paleoproterozoic U/Pb Columbitetantalite Romer and Smeds, 1994 Y S LCT Rosendal Rosendal Finland Paleoproterozoic U/Pb Tapionite Lindroos and others, 1996 Duplicate S LCT Kaatiala Kaatiala Finland Paleoproterozoic U/Pb Columbitetantalite Alviola and others, 2001; location from Mindat
Duplicate S LCT Skogsbole Skogsbole Finland Paleoproterozoic U/Pb Tapionite Lindroos and others, 1996 Duplicate S
2.0 Deposit Type and Associated Commodities 5 Table 1. Noteworthy lithium-cesium-tantalum (LCT) pegmatites of the world. For comparable information on other types of pegmatites and lithium granites, see McCauley and Bradley (2014).—Continued [Numbers for pegmatites in the Pegmatite number column are only given for pegmatites whose coordinates are known. Numbers are assigned by latitude, from north to south. Numbers and short names for pegmatites in the Pegmatite number column and the Short name column are shown in layers in the original Illustrator version of figure 1, which is available on request from the authors. Numbers for pegmatites in the Pegmatite number column are are assigned by latitude, from north to south. Ages and 2-sigma errors in the Age and Error columns are rounded to the closest integer. Use in Lithium-Cesium-Tantalum age plot? column is used to select the ages plotted in the Lithium-Cesium-Tantalum age distribution. "Y" means yes. "Wrong kind" means not plotted because it is the wrong kind of pegmatite for that plot. "Duplicate" means not plotted because it another pegmatite from the same group is plotted instead. "Filtered" means not plotted because the age has dubious accuracy and (or) precision] Peg matite number Clas sification Pegmatite body Short name Region and country Age in millions of years Error in millions of years Geon Era Dating method
dated Latitude (decimal degrees Longitude (decimal degrees) References and notes Use in LCT age plot? (Y or other) Lithium or Tantalum resource S small L large LCT Seinäjoki Seinäjoki Finland Paleoproterozoic U/Pb Tapionite Alviola and others, 2001 Y S LCT Haapaluoma Haapaluoma Finland Paleoproterozoic U/Pb Columbitetantalite Alviola and others, 2001 Y S LCT Orrvik Orrvik Sweden Paleoproterozoic U/Pb Columbitetantalite Romer and Smeds, 1994 Y S LCT Ullava Ullava Finland Paleoproterozoic U/Pb Columbitetantalite Alviola and others, 2001 Y S LCT Varuträsk Varuträsk Sweden Paleoproterozoic U/Pb Columbitetantalite Romer and Wright, 1992; location from Mindat Y S LCT Tin Mountain Black Hills South Dakota, United States Paleoproterozoic U/Pb Apatite -103.714 Krogstad and Walker, 1994 Y S LCT Brown Derby Quartz Cr. Colorado, United States Mesoproterozoic Rb/Sr — -106.626 Hanley and others, 1950; Rb-Sr age from Aldrich and others, 1957, who quoted the error at "<5%" Filtered S — LCT Lower Jumbo Lower Jumbo Arizona, United States Mesoproterozoic U/Pb Zircon — — Unpublished age by Bradley and McCauley Duplicate S LCT Midnight Owl White Picacho Arizona, United States Mesoproterozoic U/Pb Zircon -112.546 Jahns, 1952; unpublished age by Bradley and McCauley Y S LCT Harding Harding New Mexico, United States Mesoproterozoic Ar/Ar Muscovite -105.634 Karlstrom and others, 1997 Y S LCT Skantorp BorkenasOrust Norway Mesoproterozoic U/Pb Columbitetantalite Romer and Smeds, 1996 Y S LCT Homestead Homestead Namibia Neoproterozoic U/Pb Columbitetantalite -28.769 Melcher and others, 2013 Y S LCT Ruhembe Ruhembe Burundi Neoproterozoic U/Pb Columbitetantalite -2.761 Romer and Lehmann 1995; intercept age Y S LCT Kivuvu Kivuvu Burundi Neoproterozoic U/Pb Columbitetantalite -2.834 Romer and Lehmann 1995; intercept age Y S LCT Manono Manono Zaire Archean — — -6.251 Von Knorring and Condliffe, 1987; age from Melcher and others, 2013 Y LCT Wamba Nigeria Neoproterozoic Rb/Sr Muscovite Küster 1995; location approximate Filtered S LCT Kenticha Kenticha Ethiopia Paleozoic U/Pb Columbitetantalite Küster and others, 2009 Y LCT Bupo Bupo Ethiopia Paleozoic U/Pb Columbitetantalite Küster and others, 2009 Duplicate S LCT Parelhas Parelhas Brazil Paleozoic Ar/Ar Biotite -6.689 -36.463 Araujo and others, 2005 Y S LCT Mamoes Mamoes Minas Gerais, Brazil Paleozoic U/Pb Columbitetantalite -6.918 -36.724 Baumgartner and others, 2006 Y S LCT Capoeira Capoeira Brazil Paleozoic U/Pb Columbitetantalite -6.685 -36.637 Baumgartner and others, 2006 Y S LCT Rubicon Rubicon Namibia Paleozoic U/Pb Columbitetantalite -22.103 Broccardo and others, 2011; Diehl and Schneider, 1990 Y LCT Sutlug Sutlug Russia Paleozoic U/Pb Zircon Kuznetsova and others, 2011 Y S
6 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites Table 1. Noteworthy lithium-cesium-tantalum (LCT) pegmatites of the world. For comparable information on other types of pegmatites and lithium granites, see McCauley and Bradley (2014).—Continued [Numbers for pegmatites in the Pegmatite number column are only given for pegmatites whose coordinates are known. Numbers are assigned by latitude, from north to south. Numbers and short names for pegmatites in the Pegmatite number column and the Short name column are shown in layers in the original Illustrator version of figure 1, which is available on request from the authors. Numbers for pegmatites in the Pegmatite number column are are assigned by latitude, from north to south. Ages and 2-sigma errors in the Age and Error columns are rounded to the closest integer. Use in Lithium-Cesium-Tantalum age plot? column is used to select the ages plotted in the Lithium-Cesium-Tantalum age distribution. "Y" means yes. "Wrong kind" means not plotted because it is the wrong kind of pegmatite for that plot. "Duplicate" means not plotted because it another pegmatite from the same group is plotted instead. "Filtered" means not plotted because the age has dubious accuracy and (or) precision] Peg matite number Clas sification Pegmatite body Short name Region and country Age in millions of years Error in millions of years Geon Era Dating method
dated Latitude (decimal degrees Longitude (decimal degrees) References and notes Use in LCT age plot? (Y or other) Lithium or Tantalum resource S small L large LCT Felder Ridge Felder Ridge Antarctica Paleozoic Rb/Sr — -80.430 Faure and Felder, 1984 Filtered S LCT Tastyg Tastyg Russia Paleozoic U/Pb Zircon Kuznetsova and others, 2011 Y S LCT Naipa Alto Ligonha Mozambique Paleozoic U/Pb Zircon -15.737 Neiva and Leal Gomes, 2010 Y S LCT "metapegmatite" — Czech Republic Paleozoic U/Pb Columbitetantalite — — Glodny and others, 1998 Y S LCT Wendersreuth Quarry Wendersreuth Germany Paleozoic U/Pb Monazite, zircon Glodny and others, 1998 Y S LCT La Totora La Totora Argentina Paleozoic U/Pb Columbitetantalite -32.500 -65.500 von Quadt and Galliski, 2011 Y S LCT Marropino — Mozambique Paleozoic U/Pb Columbitetantalite -15.500 Melcher and others, 2013; location approximate Y S LCT — Majahayan Somalia Paleozoic Rb/Sr Muscovite Kuster 1995; 2 point isochron; location approximate Filtered S LCT Moneia — Mozambique Paleozoic U/Pb Columbitetantalite -15.938 Melcher and others, 2013; location approximate Y S LCT San Luis II San Luis II Argentina Paleozoic U/Pb Columbitetantalite -32.983 -65.983 von Quadt and Galliski, 2011 Y S LCT Stranakelley Leinster Ireland Paleozoic Rb/Sr — -6.537 O'Connor and others, 1991 Filtered S LCT Brazil Lake Brazil Lake Nova Scotia, Canada Paleozoic U/Pb Columbitetantalite -65.997 Kontak and others, 2005 Y S LCT Las Cuevas Las Cuevas Argentina Paleozoic Ar/Ar Muscovite -32.386 -65.707 Galliski and Marquez-Zavalia, 2011; unpublished age by Benowitz and Bradley Y S LCT Clark Ledge Clark Massachusetts, United States Paleozoic U/Pb Zircon -72.873 Location from ://www.mindat.org; age from Bradley and others, 2013 Y S LCT McHone Spruce Pine North Carolina, United States Paleozoic U/Pb Zircon -82.083 Wise and Brown, 2009; unpublished age by Buchwaldt, Bowring, and Bradley Y S LCT Foote Kings Mtn. North Carolina, United States Paleozoic U/Pb Columbitetantalite -81.354 Kesler, 1942; unpublished age by Buchwaldt, Bowring, and Bradley Y LCT Beryl Mountain Beryl Mtn. New Hampshire, United States Paleozoic U/Pb Zircon -72.294 Page and Larrabee, 1962; provisional age from Bradley and others, 2013 Filtered S — LCT Oldrich pegmatite dike Oldrich Czech Republic Paleozoic U/Pb Monazite — — Novak and others, 2008 Duplicate S LCT Oldrich Oldrich Czech Republic Paleozoic U/Pb Monazite Novak and others, 2008 Y S LCT Eibenstein Pegm Eibenstein Austria Paleozoic Sm/Nd Garnet Ertl and others, 2004 Filtered S — LCT Puklice — Czech Republic Paleozoic U/Pb Columbitetantalite — — Melleton and others, 2012 Duplicate S
2.0 Deposit Type and Associated Commodities 7 Table 1. Noteworthy lithium-cesium-tantalum (LCT) pegmatites of the world. For comparable information on other types of pegmatites and lithium granites, see McCauley and Bradley (2014).—Continued [Numbers for pegmatites in the Pegmatite number column are only given for pegmatites whose coordinates are known. Numbers are assigned by latitude, from north to south. Numbers and short names for pegmatites in the Pegmatite number column and the Short name column are shown in layers in the original Illustrator version of figure 1, which is available on request from the authors. Numbers for pegmatites in the Pegmatite number column are are assigned by latitude, from north to south. Ages and 2-sigma errors in the Age and Error columns are rounded to the closest integer. Use in Lithium-Cesium-Tantalum age plot? column is used to select the ages plotted in the Lithium-Cesium-Tantalum age distribution. "Y" means yes. "Wrong kind" means not plotted because it is the wrong kind of pegmatite for that plot. "Duplicate" means not plotted because it another pegmatite from the same group is plotted instead. "Filtered" means not plotted because the age has dubious accuracy and (or) precision] Peg matite number Clas sification Pegmatite body Short name Region and country Age in millions of years Error in millions of years Geon Era Dating method
dated Latitude (decimal degrees Longitude (decimal degrees) References and notes Use in LCT age plot? (Y or other) Lithium or Tantalum resource S small L large — LCT Dolni Bory — Czech Republic Paleozoic U/Pb Monazite — — Novak and others, 1998 Duplicate S — LCT Sedlatice — Czech Republic Paleozoic U/Pb Columbitetantalite — — Melleton and others, 2012 Duplicate S — LCT Jeclov — Czech Republic Paleozoic U/Pb Tnt — — Melleton and others, 2012 Duplicate S LCT Rozná Rozná Czech Republic Paleozoic U/Pb Columbitetantalite Melleton and others, 2012 Y S LCT Dobra Voda Dobra Voda Czech Republic Paleozoic U/Pb Columbitetantalite Melleton and others, 2012 Duplicate S — LCT Rozna — Czech Republic Paleozoic U/Pb columbitetantalite — — Melleton and others, 2012 Duplicate S — LCT Chvalovice — Czech Republic Paleozoic U/Pb columbitetantalite — — Melleton and others, 2012 Duplicate S LCT Palermo Palermo New Hampshire, United States Paleozoic U/Pb Zircon -71.890 Whitmore and Lawrence, 2004; age from Bradley and others, 2013 Y S LCT Ctidruzice Ctidruzice Czech Republic Paleozoic U/Pb Columbitetantalite Melleton and others, 2012 Y S LCT McAllister McAllister Alabama, United States Paleozoic Ar/Ar Muscovite -86.248 Foord and Cook, 1989; age from Snee with uncertainty estimated Y S LCT Chedeville Chedeville France Paleozoic Ar/Ar Lepidolite Raimbault, 1998 Y S LCT Kara-Adyr Solbelder Russia Paleozoic U/Pb — Kuznetsova and others, 2011 Y S LCT Anderson Anderson Connecticut, United States Paleozoic U/Pb Zircon -72.592 Age from Bradley and others, 2013 Y S LCT Shuk-Byul' Shuk-Byul' Russia — Paleozoic — — Kuznetsova and others, 2011 Duplicate S LCT Mt. Mica (Irish Pit) Mt. Mica Maine, United States Paleozoic U/Pb Zircon -70.472 Wise and Brown, 2010; age from Bradley and others, 2013 Y S LCT Lipovy Log Lipovy Log Central Urals, Russia Paleozoic Re/Os Molybdenite Mao and others, 2003 Y S LCT Weinebene Koralpe Austria Mesozoic Sm-Nd Garnet Göd, 1989; Habler and others, 2007 who bracketed age between 238 and 256 Ma Filtered S LCT — Brissago Switzerland Mesozoic U/Pb Zircon Vignola and others, 2008 Y S LCT Altai #3 Altai China Mesozoic U/Pb Zircon Wang and others, 2007 Y LCT Jiajika Jiajika China Mesozoic U/Pb Zircon ://www.mindat.org/loc-146947.; Li Jiankang and others, 2013 for SHRIMP age Y LCT — Orlovka Transbaikalia, Russia Mesozoic Rb-Sr — Reyf and others, 2000 Filtered S LCT Zavatinskoe (=Zavitino) Zavatinskoe Russia — Mesozoic — — Vladimorov and others, 2012; location from ://www.mindat.org; Filtered LCT Oktyabrskaya Malkhan Transbaikalia, Russia Mesozoic Ar/Ar Muscovite Zagorsky and Peretyazhko, 2010 Y S
8 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites Table 1. Noteworthy lithium-cesium-tantalum (LCT) pegmatites of the world. For comparable information on other types of pegmatites and lithium granites, see McCauley and Bradley (2014).—Continued [Numbers for pegmatites in the Pegmatite number column are only given for pegmatites whose coordinates are known. Numbers are assigned by latitude, from north to south. Numbers and short names for pegmatites in the Pegmatite number column and the Short name column are shown in layers in the original Illustrator version of figure 1, which is available on request from the authors. Numbers for pegmatites in the Pegmatite number column are are assigned by latitude, from north to south. Ages and 2-sigma errors in the Age and Error columns are rounded to the closest integer. Use in Lithium-Cesium-Tantalum age plot? column is used to select the ages plotted in the Lithium-Cesium-Tantalum age distribution. "Y" means yes. "Wrong kind" means not plotted because it is the wrong kind of pegmatite for that plot. "Duplicate" means not plotted because it another pegmatite from the same group is plotted instead. "Filtered" means not plotted because the age has dubious accuracy and (or) precision] Peg matite number Clas sification Pegmatite body Short name Region and country Age in millions of years Error in millions of years Geon Era Dating method
dated Latitude (decimal degrees Longitude (decimal degrees) References and notes Use in LCT age plot? (Y or other) Lithium or Tantalum resource S small L large LCT Little Three Mine Little Three California, United States Mesozoic Ar/Ar Muscovite -116.794 Symons and others, 2009; OrtegaRivera, 2003 Duplicate S LCT Himalaya Mine Himalaya California, United States Mesozoic Ar/Ar Muscovite -116.798 Fisher, 2002 Y S LCT — Little Nahanni Northwest Territories, Canada Mesozoic U/Pb Apatite -128.833 Barnes, 2010 Y S LCT Khaltaro Haramosh Pakistan Cenozoic Ar/Ar Muscovite Laurs and others, 1996 Y S LCT Fonte del Prete Elba Italy Cenozoic Rb/Sr — Aurisicchio and others, 2002 Y S LCT Whabouchi Whabouchi Quebec, Canada — — Neoarchean — — -75.846 Laferrière and others, 2011 Undated LCT McCombe Root Ontario, Canada — — Neoarchean — — -91.700 Selway and others, 2005; location approximate Undated S LCT Vodorazhdel noye Vodorazhdelnoye Russia — — — — — — Location from ://www.mindat.org Undated S LCT Animikie Red Ace Animikie Wisconsin, United States — — — Paleoproterozoic — — -88.353 Sirbescu and others, 2008 Undated S LCT Cer Mountain Cer Mtn. Serbia — — — — — — Lazic and others, 2009 Undated S LCT Black Mountain Black Mtn. Wyoming, United States — — — — — — -107.442 Hanley and others, 1950 Undated S LCT Forcarai Forcarai Spain — — Paleozoic — — -8.350 Fuertes-Fuentes and others, 2000 Undated S LCT Alijó Covas de Barroso Portugal — — Paleozoic — — -7.784 Charoy and others, 2001; location is generalized Undated S LCT Feli Sn deposit Feli Spain — — Paleozoic — — -6.868 Roda-Robles and Pesquera, 2007 Undated S LCT Buckhorn Crystal Mtn. Colorado, United States — — — — — — -105.373 Hanley and others, 1950 Undated S LCT Talbuzanak Talbuzanak Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Myouke nyama Myoukenyama Japan — — — — — — ://www.mindat.org/loc-37307. Undated S LCT Eshkashim Eshkashim Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Pachighram Pachighram Afghanistan — — — Mesozoic? — — Orris and Bliss, 2002 Undated S LCT Kantiway Kantiway Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Panjsher Panjsher Afghanistan — — — — — — Orris and Bliss, 2002 Undated S LCT Marid Marid Afghanistan — — — Mesozoic? — — Orris and Bliss, 2002 Undated S LCT Nilaw-Kolum Nilaw-Kolum Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Kurghal Kurghal Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Parun Parun Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Alinghar Alinghar Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S
2.0 Deposit Type and Associated Commodities 9 Table 1. Noteworthy lithium-cesium-tantalum (LCT) pegmatites of the world. For comparable information on other types of pegmatites and lithium granites, see McCauley and Bradley (2014).—Continued [Numbers for pegmatites in the Pegmatite number column are only given for pegmatites whose coordinates are known. Numbers are assigned by latitude, from north to south. Numbers and short names for pegmatites in the Pegmatite number column and the Short name column are shown in layers in the original Illustrator version of figure 1, which is available on request from the authors. Numbers for pegmatites in the Pegmatite number column are are assigned by latitude, from north to south. Ages and 2-sigma errors in the Age and Error columns are rounded to the closest integer. Use in Lithium-Cesium-Tantalum age plot? column is used to select the ages plotted in the Lithium-Cesium-Tantalum age distribution. "Y" means yes. "Wrong kind" means not plotted because it is the wrong kind of pegmatite for that plot. "Duplicate" means not plotted because it another pegmatite from the same group is plotted instead. "Filtered" means not plotted because the age has dubious accuracy and (or) precision] Peg matite number Clas sification Pegmatite body Short name Region and country Age in millions of years Error in millions of years Geon Era Dating method
dated Latitude (decimal degrees Longitude (decimal degrees) References and notes Use in LCT age plot? (Y or other) Lithium or Tantalum resource S small L large LCT Darra-i-Pech Darra-i-Pech Afghanistan — — — Cenozoic — — Orris and Bliss, 2002; coordinates for southeast part of field Undated S LCT Darrahe-Nur Darrahe-Nur Afghanistan — — — Cenozoic — — Orris and Bliss, 2002; coordinates for northeast part of field Undated S LCT Shahidan Shahidan Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Surkh-Rod Surkh-Rod Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated S LCT Taghawlor Taghawlor Afghanistan — — — Cenozoic — — Orris and Bliss, 2002 Undated LCT Guano Guano China — — — Mesozoic — — ://www.mindat.org/loc-216639. Undated S LCT Nagatar eyama Nagatareyama Japan — — — — — — ://www.mindat.org/loc-53666. Undated S LCT Nanping Nanping China — — — — — — Nanping #31. ://www.mindat.org/ loc-216998. Undated S LCT Maoantan Maoantan China — — — Mesozoic — — ://www.mindat.org/loc-224897. Undated S LCT Sakangyi Sakangyi Myanmar — — — — — — Zaw, 1998 Undated S LCT Khnefissat Khnefissat Mauritania — — — Mesoarchean — — -15.571 Gunn and others, 2004 Undated S LCT BastarMalkangiri BastarMalkangiri India — — — — — — Pal, 2007 Undated S LCT Santa Ana Santa Ana Oaxaca, Mexico — — — — — — -96.900 ://www.mindat.org/loc-21304. Undated S LCT Komu Igbeti Nigeria — — — Neoproterozoic — — Adetunji and Ocan 2010 Undated S LCT Phuket Phuket Thailand — — — — — — Suwimonprecha and others, 1995 Undated S LCT Gatumba Gatumba Rwanda — — Neoproterozoic — — -2.000 Hulsbosch and others, 2013; Graupner and others, 2010 Undated S LCT Urucum Urucum Brazil — — — — — — -19.023 -41.460 Viana and others, 2003 Undated S LCT Manjaka Manjaka Madagascar — — — — — — -20.000 ://www.mindat.org/loc-2271.; location approximate Undated S LCT Tabba Tabba Australia — — Mesoarchean — — -20.667 Fetherston, 2004 Undated S LCT — Volta Grande Minas Gerais, Brazil — — Paleoproterozoic — — -21.021 -44.691 Lagache and Quéméneur, 1997 Undated S LCT Karibib Karibib Namibia — — Paleozoic — — -21.938 Jacob and others, 2000 Undated S LCT Niobe Niobe Australia — — — — — -27.707 Fetherston, 2004 Undated S LCT Edon Edon Australia — — — Archean — — -29.307 Fetherston, 2004 Undated S LCT Marion Marion Australia — — — Archean — — -31.078 Fetherston, 2004 Undated S LCT Tantalite Tantalite Australia — — — Archean — — -31.097 Fetherston, 2004 Undated S LCT Bald Bald Australia — — — Archean — — -31.516 Fetherston, 2004 Undated LCT Deans Deans Australia — — Mesoarchean — — -32.307 Fetherston, 2004 Undated LCT Cattlin Cattlin Australia — — — — — -33.564 Fetherston, 2004 Undated S
10 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites rocks (I-type granites). In contrast, niobium-yttrium-fluorine (NYF) pegmatites are the products of extreme differentiation of the so-called anorogenic, A-type granites. Some pegmatites share characteristics of both families (Černý and Ercit, 2005). Another approach to categorizing pegmatites is based on the pressure-temperature metamorphic conditions of the pegmatite host rocks (fig. 3). Expanding on an earlier depthrelated classification scheme of Ginsburg and others (1979) and (Ginsburg (1984), Černý and Ercit (2005) recognized five pegmatite classes: the abyssal, muscovite, muscovite-rare element, rare element, and miarolitic classes. The newer scheme includes a number of mineralogically based "types" and subtypes." London (2008, p. 23) and Martin and De Vito (2005) pointed out a number of problems with this scheme, and questioned its overall utility. One of the problems is that the depth in the crust at which a pegmatite was emplaced need not correspond to that at which the host rocks were previously metamorphosed. The Černý and Ercit (2005) scheme is reproduced in figure 4. 2.3. Brief Description Lithium-cesium-tantalum pegmatites are extremely coarse-grained granitic rocks that form small but mineralogically spectacular igneous bodies. These rocks account for about one-fourth of the world's lithium production (Naumov and Naumova, 2010), one-tenth of the beryllium, most of the tantalum, and all of the cesium (U.S. Geological Survey, 2011). In addition, LCT pegmatites are mined for tin, high purity quartz, potassium feldspar, albite, kaolinite, white mica, gem beryl, gem tourmaline, and museum-quality specimens of many rare minerals (Glover and others, 2012; Simmons and others, 2012). The main minerals are silicates. Most LCT pegmatites are the differentiated end members of peraluminous, S-type granitic melts. Some are related to metaluminous granites and some to I-type granites (Martin and De Vito, 2005). They are highly enriched in the incompatible elements Li, Cs, and Ta, and are distinguished from other rareelement pegmatites by this diagnostic suite of elements. The Figure 2. Locations of noteworthy pegmatites and related granites categorized by type. Note the widespread distribution of lithiumcesium-tantalum (LCT) pegmatites, which are listed in table 1. Economically important LCT pegmatites are shown by larger red squares. Locations for the other types of pegmatites are from McCauley and Bradley (2014).
2.0 Deposit Type and Associated Commodities 11 melts from which LCT pegmatites crystallize are enriched in fluxing components, including H2O, F, P, and B, which depress the solidus temperature, lower the density, and increase rates of ionic diffusion (London, 2008). Pegmatites can thus form thin dikes and massive crystals despite their felsic composition and significant subliquidus undercooling. Lithium-cesium-tantalum pegmatite bodies have various forms including tabular dikes, tabular sills, lenticular bodies, and irregular masses (Cameron and others, 1949). In some cases, an LCT pegmatites can be spatially and genetically linked to an exposed parental granite; in other cases, no such parent can be observed at present levels of exposure. Most LCT pegmatites are hosted in metasedimentary or metavolcanic (supracrustal) country rocks, which are typi cally metamorphosed to low-pressure upper greenschist to amphibolite facies (Černý, 1992). Less commonly, LCT bod ies intrude granites or gabbros. In some districts, pegmatites show a regional mineralogical and geochemical zoning pattern surrounding an exposed or inferred granitic pluton, with the greatest enrichment in incompatible elements in the more distal pegmatites (Trueman and Černý, 1982). On the scale of a single pegmatite body, three types of min eralogical and textural zonation are recognized. First, most LCT pegmatites have a distinctive, concentric zonation featuring four zones: border, wall, one or more intermediate zones (where Li, Cs, and Ta are generally concentrated), and core. Second, another textural pattern, which is seen in narrow LCT pegmatite dikes, is layering, with layering more common in the footwall of the pegmatite. Third, a few LCT pegmatites are unzoned. Pegmatites are mined using a variety of techniques. These include artisanal surface mining (Kibaran Belt, Burundi: Romer and Lehmann, 1995; Brinkmann and others, 2001; Mutima and Li, 2010), open-pit surface mining (Greenbushes, Western Australia: Fetherston, 2004), small underground workings (San Diego County, California, United States: Symons and others, 2009), and large underground operations using room-and-pillar design (Tanco, Manitoba, Canada: Burt and others, 1982). 2.4. Associated Deposit Types Common Granitic Pegmatites: The vast majority of gra nitic pegmatites lack rare-element enrichments and on this basis can be lumped together. The common pegmatites are igneous rocks having pegmatitic textures made up of the standard rockforming minerals of granite, which are mainly quartz, potassium feldspar, plagioclase, muscovite, and local megascopic acces sory minerals that may include garnet, apatite, and tourmaline. This informal category includes abyssal and muscovite classes of pegmatites as described in Černý and Ercit (2005), which are believed to have originated through anatexis. The mineral ogically simplest, least fractionated pegmatites within an LCT pegmatite field can be considered common pegmatites, although in this case they are linked genetically with the LCT pegma tites. Common pegmatites are important sources of economic, ceramic-grade feldspar, ultrapure quartz, and muscovite (for example, Spruce Pine district, North Carolina, United States: Lesure, 1968; Glover, 2006; Glover and others, 2012). NYF Pegmatites: As noted in Section 2.2, the other end member of rare-element pegmatites is the niobium-yttriumfluorine (NYF) type. Niobium-yttrium-fluorine pegmatites, which also have been referred to as rare-earth element pegmatites, are characteristically enriched in Nb Ta, Ti, Y, REE, Zr, Th, U, Sc, and variably F, but are impoverished in the rare alkali elements Li, Rb, and Cs (Ercit, 2005). An example is the Cretaceous Malosa pegmatite field in Malawi (Martin and De Vito, 2005). The term rare-earth element pegmatites is a potential source of confusion, because the name is similar to that of rare-element pegmatites; see figure 4 for clarification. Some pegmatites have a mixed NYF-LCT signature, and have been interpreted as the consequence of contamination of an NYF melt with local sources (Martin and De Vito, 2005). Examples include the Anjanabonoina pegmatite in Madagascar (Martin and De Vito, 2005) and the McHone pegmatite, Spruce Pine district, North Carolina, United States (Wise and Brown, 2009). Rare-Element Enriched Granites: Certain granitic plutons show enrichments in rare elements such as lithium, tantalum, tin, and fluorine. Rare-element-enriched parts of these gran ites are typically zones within a larger granite body. Examples include the Yichun Li-F granite of China (Schwartz, 1992) and the Beauvoir granite of the Massif Central, France (Raimbault, 1998). Such granites are only known from the Phanerozoic (Tkachev, 2011). Hydrothermal Veins: Certain pegmatite districts include hydrothermal quartz veins that contain tin-, tantalum-, niobium-, and (or) tungsten-bearing minerals. These are known from the Pilbara Craton in Western Australia, where they appear to be offshoots of mineralized pegmatites (Sweetapple, 2000). kyanite Pressure, in kilobar (kb) Temperature, in Celsius (C°) andalusite sillimanite sillimanite 1,000 Rare Element Miarolitic Abyssal kyanite andalusite spodumene petalite Muscovite Figure 3. Aluminum silicate phase diagram showing pegmatite classes. Adapted from figure 3.3 of London (2008), which was based on Cˇerný (1991b) and Ginsburg (1984). The fields for rare-element and miarolitic pegmatites have been extended schematically into lower temperatures in accord with recent estimates of crystallization temperatures (see Section 11.3 of this report).
12 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites A relation between pegmatites and gold-bearing quartz veins has been suspected since at least the 1930s (Landes, 1937). The two types of deposit are found in the same regions and are broadly coeval, for example, in the Damaride oro gen of Namibia (Nex and others, 2011). The possibility of a genetic connection (as opposed to merely a spatial one) has not been studied in detail and will require state-of-heart geochronology. 2.5. Primary and By-Product Commodities Globally, various pegmatites of the LCT family have yielded economic quantities of the rare-metal ores of lithium, tantalum, niobium, tin, beryllium, cesium, and rubidium (Černý, 1991a). Abundances of these elements vary greatly from pegmatite to pegmatite, but in general the less fraction ated bodies are enriched only in beryllium and phosphate. Spodumene and petalite from LCT pegmatites were histori cally the main source of lithium metal and various lithium compounds, but with most lithium production now from brines, the majority of lithium silicate production from the LCT deposits instead goes to the ceramic industry (Kesler and others, 2012; Gruber and others, 1968). Bradley and others (2016) provided a recent synthesis of the geology, environ mental geochemistry, and global resource outlook for lithium; Schulz and others (2016) similarly covered tantalum. Potas sium feldspar, plagioclase, quartz, and muscovite are produced as industrial minerals from some pegmatites belonging to the LCT family (Glover and others, 2012). Pegmatites and their contact zones are also important sources of semiprecious gemstones and high-value, museumquality specimens of rare minerals (Simmons and others, 2012). Among the gemstones are the beryl varieties aquama rine, heliodor, and emerald; the spodumene varieties kunzite and hiddenite; topaz; watermelon tourmaline; and chrysoberyl. The diversity of mineral species in the most evolved LCT pegmatites is impressive; at Tanco, for example, 104 minerals have been identified (Černý, 2005). 2.6. Trace Constituents The LCT pegmatites take their name from their enrich ments in Li, Cs, and Ta. They also tend to be enriched in Be, B, F, P, Mn, Ga, Rb, Nb, Sn, and Hf (London, 2008). 2.7. Example Deposits United States: Tin Mountain pegmatite, Black Hills district, South Dakota (fig. 2). This district includes an estimated 22,000 pegmatites associated with the Paleoproterozoic, peraluminous Harney Peak granite (Redden and others, 1982). Only about two percent of the pegmatites are zoned, and these have been mined for muscovite, potassium feldspar, and rare metals such as Li, Be, Nb, Ta, Sn, and Cs. The most highly differentiated PEGMATITE FAMILY based on geochemistry: LCT or NYF PEGMATITE CLASS based on mineralogy plus emplacement depth Subclass Type Subtype 1. ABYSSAL 1a. HREE 1b. LREE 1c. U 1d. B-Be 2. MUSCOVITE 3. MUSCOVITE-RARE ELEMENT 3a. REE 3b. Li 4. RARE ELEMENT 4a. REE allanite-monazite euxenite gadolinite 4b. Li beryl beryl-columbite beryl-phosphate complex spodumene petalite lepidolite elbaite amblygonite albite-spodumene albite 5. MIAROLITIC 5a. REE topaz-beryl gadolinite fergusonite 5b. Li beryl-topaz spodumene petalite lepidolite Figure 4. Pegmatite classes from Černý and Ercit (2005) as clarified by London (2008, p. 22). Pegmatite classes in red type are in the lithium-cesium-tantalum (LCT) family; those in blue type are in the niobium-yttrium-fluorine (NYF) family. Abbreviations are as follows: B, boron; Be, beryllium; HREE, heavy rare-earth element; Li, lithium; LREE, light rare-earth element; REE, rare-earth element, U, uranium.
2.0 Deposit Type and Associated Commodities 13 pegmatites, such as the Tin Mountain body, are dikes with lepidolite, spodumene, and pollucite. Various aspects of the Tin Mountain pegmatites have been described by Krogstad and others (1993), Krogstad and Walker (1994), Walker and others (1996), Sirbescu and Nabelek (2003), and Teng and others (2006). Canada: Tanco pegmatite, Bird River Greenstone Belt, Manitoba (fig. 2). The Neaorchean Tanco pegmatite is the most fractionated igneous body on Earth (London, 2008), and the most thoroughly studied pegmatite (Černý, 1982c, 2005; Stilling and others, 2006). It is a subhorizontal, lenticular, zoned, LCT rare-element pet alite pegmatite measuring about 1,520 m by 1,060 m by 100 m. It has economic concentrations of Li, Cs, and Ta. Underground mining uses the room-and-pillar method. Tanco is the world's primary source of Cs (Butterman and others, 2004). China: Altai Number 3 pegmatite, Altai Orogenic Belt, Inner Mongolia (fig. 2). This Triassic deposit is one of several pegmatites in the Altai Metallogenic Province. The Altai Number 3 pegmatite is a vertical stock of zoned pegmatite. It was mined for 50 years, beginning in 1950, by open-pit methods. It is mineralized in Li, Be, Nb, and Ta. Aspects of the deposit and its genesis have been discussed by Huanzhang and others (1997), Wang and others (2000), and Zhu and others (2006). Australia: Greenbushes pegmatite, Yilgarn Craton, Western Australia (fig. 2). This Neoarchean LCT rare element pegmatite is enriched in Li, Ta, and Sn, and has been extensively mined by open-pit methods for many years (Partington, 1990; Partington and others, 1995). It consists of a series of zoned dikes, each tens to hundreds of meters in thickness and hundreds of meters to kilome ters in length, intruded into greenstones. The pegmatite crystallized as it was emplaced into a shear zone, and it has been significantly deformed. Among LCT pegma tites, the Greenbushes deposit contains the largest Ta resource (Fetherston, 2004) and the largest Li resource (Kesler and others, 2012). Zimbabwe: Bikita pegmatite, Zimbabwean Craton (fig. 2). This large, zoned LCT pegmatite carries resources of Li, Cs, and Be (Symons, 1961; Cooper, 1964). It is 1,700 m long, varies in width from 30 to 70 m, and dips 15 to 45 degrees. Minerals hosted in the pegmatite include pet alite, lepidolite, spodumene, pollucite, beryl, eucryptite, amblygonite, and bikitaite. The Bikita pegmatite is poorly dated, but is probably Neoarchean. Ethiopia: Kenticha pegmatite district, Adola Belt (fig. 2). These Cambrian pegmatites have been mined since the early 1990s, with an emphasis on the signifi cantly weathered regolith (Küster and others, 2009). The pegmatites intrude greenschist to lower amphibo lite facies schists and serpentinites. The pegmatites range from tens of meters to one kilometer in length. 3.0. History of Pegmatite Research Summaries of the history of pegmatite research can be found in Jahns (1955) and London (2008). Pegmatites played an important early role in modern science. Before chemistry and geology had diverged along their now-separate academic paths, the elements niobium (in 1801), tantalum (in 1802), lithium (in 1817), and rubidium (in 1861) were all discovered and (or) isolated from unusual minerals found in pegmatites. The problem of pegmatite genesis has long attracted researchers. During the 1800s and early 1900s, various early workers put forth a range of models for genesis of pegma tites. De Beaumont (1847) and Hitchcock (1883) advocated an igneous origin; Hunt (1871) proposed what essentially is a hydrothermal origin; and Lindgren (1913) placed peg matites somewhere between these two regimes. In the latter half of the 1900s, one of the most influential researchers was Richard Jahns, who proposed that complex pegmatites form an aqueous vapor phase that separates from a silicate melt phase (Jahns and Burnham, 1969). Recent treatments of pegmatite petrogenesis include those by London (2005a, 2008, 2014), Simmons and Webber (2008), and Thomas and others (2012). Based in part on high-temperature petrologic experiments, London (1992, 2005a) developed the idea of constitutional zone refining. During crystallization of a pegmatite melt, such a process involves flux-rich incompatible elements building up in the residual melt immediately in advance of the crystalliza tion front. The fluxing components reduce the solidus tem perature to far below the normal minimum range for granites, they increase rates of diffusion in the melt, and they supress crystal nucleation rates leading to enhanced crystal growth on the few nuclei that do form. Classification schemes have evolved in parallel with other lines of pegmatite research. Fersman (1931) related pegmatites to one of seven magma types, beginning a trend of correlating pegmatites with genetically similar granites. Landes (1933) offered a simplified classification based on whether the pegmatite had mineralogy of acid, intermediate, or basic compositions. Ginsburg and Rodionov (1960) and Ginsburg and others (1979) adapted the depth-zone granite classification of Buddington (1959) to pegmatites. Černý (1991a) further refined the depth-zone classification and used trace-element abundances to make a distinction between what he called LCT and NYF pegmatites. Černý and Ercit updated this classification scheme in 2005. At about the same time, Martin and De Vito (2005) advocated a petrogenetically based classification, with LCT pegmatites being related to S-type, or more rarely, I-type, orogenic granites and NYF pegmatites being related to anorogenic, A-type granites. London (2008) and Simmons and Webber (2008) presented overviews of previous classifications. During World War II and the early Cold War years, the U.S. Geological Survey completed detailed, deposit-scale mapping of hundreds of domestic pegmatites. The resulting regional syntheses remain crucial to mineral resource assess ment in the United States, and to the study of individual
14 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites pegmatite deposits that in most cases are no longer well exposed. Key reports include Kesler and Olson (1942), L. Page and others (1953), J. Page and Larrabee (1962), Stoll (1950), Hanley and others (1950), and the widely cited treatise by Cameron and others (1949) on pegmatite structure. 4.0. Regional Environment 4.1. Tectonic Environment All LCT pegmatites are emplaced into orogenic hinterlands. That is, they are found in cores of mountain belts where metasedimentary and granitic rocks predominate. Many of the world's largest LCTs are found in Archean and (or) Paleoproterozoic orogens; these pegmatites originated in the hinterlands of orogenic belts that have long since lost all topo graphic expression. The fundamental connection between LCT pegmatites and tectonics is that they are indirect products of plate convergence. To date, however, few LCT pegmatites have been closely tied to specific, well-documented plate-tectonic events or settings at the orogen scale. In part, this is because of problems of pegmatite geochronology and, in part, to a dearth of investigation on this topic. At present, only a few anecdotal findings bear mention. One generalization is based on a global survey by Černý (1991c), that most LCT pegmatites are late syntectonic to early post-tectonic with respect to host rocks. Lithium-cesium-tantalum pegmatites that can be most confidently related to an orogen's tectonic evolution are in Elba, an island off the west coast of Italy. The pegmatites are of LCT type, as indicated by pollucite, lepidolite, and the eponymous elbaite. Elba is located in the extended hinterland of the Apennine orogen, an Oligocene to late Miocene eastdirected arc-continent collision zone (Malinverno and Ryan, 1986). The Apennine orogen and its hinterland are a remark able example of paired tectonic belts that migrated in tandem across strike (fig. 5). The two belts migrated to the east, much like a "tectonic wave," such that a given locale first experi enced thrust shortening, then extension. The country rocks at Elba are part of a thrust stack of continental and oceanic rocks. The pegmatites are spatially and genetically associated with the Monte Capanne pluton, which has a Rb-Sr isochron age of 6.9 Ma (million years) (Dini and others, 2002). The pluton and its associated pegmatites have been exhumed from an emplacement depth of approximately 4.5 kilometers (km) along a down-to-east extensional detachment. Extension was syn-magmatic (Smith and others, 2010) and was a conse quence of slab rollback of the lower plate (Malinverno and Ryan, 1986). Magmatism, mainly crustal melts with a mantle influence, was the consequence of decompression melting because of extension (Malinverno and Ryan, 1986) or lowerplate delamination (Serra and others, 1993). All of this hap pened in a relatively arid setting as recorded by the widespread latest Miocene evaporites throughout the Mediterranean region (Rouchy and Caruso, 2006). A number of older pegmatite districts are clearly related to collisional orogenesis, but they cannot yet be tied to par ticular plate-tectonic events. The Appalachian-CaledonianHercynian orogen includes about two dozen LCT pegmatite districts that range in age from 395 to 264 Ma and are broadly related to collisions that led to formation of the supercontinent Pangea (Bradley and others, 2012). In the Paleoproterozoic Sveconorwegian collisional orogen of Sweden, lepidolitebearing pegmatites were emplaced after deformation and metamorphism, and just before late-orogenic extensional col lapse (Romer and Smeds, 1996). In a new synthesis of Siberian pegmatites, Zagorsky and others (2014) related the Ordovician Tastyg and Sutlug pegmatites of the South Sangilen province to a transition from collision to post-collisional extension and strike-slip. Gem-bearing LCT pegmatites of San Diego County, California (Fisher, 2002) are widely held to have formed above a continental-margin subduction zone (Ortega-Rivera, 2003). Most of the pegmatites intruded into plutonic rocks of the Peninsular Ranges Batholith (Symons and others, 2009), which contains multiple intrusive phases dated from circa (ca.) 140 to 80 Ma (Ortega-Rivera, 2003). A 40Ar/39Ar age of 95.4±0.3 was reported from the Himalaya pegmatite (Snee and Foord, 1991). Tectonic interpretations of events at ca. 95 Ma are problematic (Todd and others, 2003); possibilities include crustal thickening because of collision of an outboard arc (Johnson and others, 1999), crustal thickening driven by westerly-directed intra-arc shortening (George and Dokka, 1994), or lithospheric delamination after collision between the Peninsular Ranges arc and North America (Hildebrand, 2009). Although the pegmatites are relatively young, the platetectonic record is debatable because of the loss by subduction of vast amounts of seafloor. EXPLANATION Fault—Arrow show relative motion Sinking of subducting slab leading to rollback Direction of advance of orogenic belt with respect to Apulian plate extension crustal melts mantle-influenced remnant arc shortening flexural foredeep Apennines Po Basin Tyrrhenian Sea asthenosphere convection asthenosphere Late Miocene (about 7 Ma) Apulian continent crust Apulian subcontinent mantle lithosphere Corsica slab rollback Elba pegmatites NOT TO SCALE Figure 5. Schematic cross section through Appenines and Tyrrhenian Sea in the Late Miocene, at about 7 Ma (million years before present) showing simultaneous foreland thrusting and hinterland extension, and location of the Elba pegmatites when they were emplaced (adapted from Serri and others, 1993).
4.0. Regional Environment 15 4.2. Temporal (Secular) Relations Recent compilations by Tkachev (2011) and McCauley and Bradley (2014) have revealed that the global age distribu tion of LCT pegmatites is similar to the age distributions of common pegmatites, orogenic granites (Condie and others, 2009), and detrital zircons (Voice and others, 2011) (fig. 6). The global LCT pegmatite maxima at ca. 2640, 1800, 960, 485, and 310 Ma are times of collisional orogeny and super continent assembly (Bradley, 2011). Between these pulses were long intervals of little or no LCT pegmatite formation. The global LCT minima overlap with supercontinent tenures at ca. 2450-2225, 1625-1000, 875-725, and 250-200 Ma, as established from global minima in the abundances of pas sive margins and detrital zircons (Bradley, 2011). Figure 6A provides a first-order filter in the search for LCT pegmatites in frontier regions. The main characteristics of LCT pegmatites have not changed much over time. The oldest and largest LCTs, how ever, are Archean. Among these giants are the Greenbushes pegmatite in Western Australia (2527 Ma: Partington and others, 1995), the Bikita pegmatite in Zimbabwe (ca. 2650 Ma: Černý and others, 2003), and the Tanco pegma tite in Manitoba, Canada (2640 Ma: Herzog and others, 1960). Martin and De Vito (2005) noted that Archean LCT pegmatites were derived from metaluminous parent melts, in contrast with younger pegmatites that are related to peraluminous melts. This change may relate to the secular evolution of the world's sedimentary mass (Veizer and Mackenzie, 2003), which is evi dent from the increase through time of δ18O values in zircons (Valley and others, 2005). 4.3. Duration of Magmatic System
and (or) Mineralizing Process A widespread misconception is that individual pegmatite bodies, with their giant crystals (fig. 1A), must have taken millions of years to crystallize. A number of modeling results suggest otherwise. Thermal modeling by Webber and others (1999) suggested that the San Diego County LCT pegmatite dikes crystallized in a matter of days to years, depending on dike thickness. Based on modeling of heat diffusion, London (2008) likewise suggested that rare-element pegmatites, particularly dike swarms and independent tabular intrusions, cooled during a period of days to weeks. Other modeling suggests that granite segregation and emplacement occurs over the order of thousands of years (Petford and others 2000, Harris and others, 2000). Modeling by Baker (1998) suggested that rare-element pegmatite dikes cannot propagate outward from a parental 10 cubic kilometers (km3) batholith within 10,000 years of batholith emplacement, because the surround ing country rocks need time to heat up first. However, the model suggests that 100,000 years of heat diffusion would allow for dikes to reach as far as 10 km from this host granite. Well-dated LCT pegmatites in at least two orogenic belts, the northern Appalachians (see Section 4.1) and the Sveconorwegian orogen (Romer and Smeds, 1996), were emplaced in multiple episodes spanning more than 100 million years. Thus, the conditions needed to form LCT pegmatites can repeatedly exist in an orogenic belt, although not neces sarily in the same location. This can be seen on an even longer time scale in the Minas Gerais region of Brazil, where LCT pegmatites were emplaced during both the PaleoproterozoicTransamazonian orogeny and Neoproterozoic to Cambrian Brasiliano-Pan-African event (Viana and others, 2003). Zagorsky (2009) observed that age gaps of tens of mil lions of years between pegmatites and spatially associated plutons are common, with the granite plutons being older than the pegmatites. This has generally been interpreted to indicate that an unexposed pluton of the same age as the pegmatite must exist at depth. Another possibility (Zagorsky, 2009) is that there was a late, independent stage of pegmatite magma tism. High-resolution pegmatite geochronology (Section 11.5) will be needed to address this problem. 4.4. Relation to Structures Most LCT pegmatite bodies show some structural control, with the specifics being a function of depth of emplacement and varying from district to district. At shallow crustal depths, pegmatites tend to be intruded along anisotropies, such as faults, fractures, foliation, and bedding (Brisbin, 1986). At relatively deeper crustal levels, commonly in high-grade metamorphic host rocks, pegmatites are typically concordant with the regional foliation, and form lenticular, ellipsoidal, or "turnip-shaped" bodies (Fetherston, 2004). In certain districts (for example, Wodgina, Western Australia), pegmatite bodies are concentrated along or near major deep-crustal faults (Sweetapple and Collins, 2002) (fig. 7). The Tschupa-Loukhi pegmatites of northwestern Russia were emplaced into antiforms located at the junctions of multiple fold systems (Černý, 1982b). In the Pilbara Craton, pegmatites in metasedimentary rock sequences assume the form of sheeted swarms or stockworks, whereas pegmatites in mafic and ultramafic rocks form large, discrete bodies (Nisbet, 1984). Sweetapple (2000) and Sweetapple and Collins (2002) noted that pegmatites within a district tend to occupy "structures of convenience," and readily crosscut and link different structures. Anecdotal observations like these provide useful exploration guidelines within established pegmatite districts, as well as features to look for in frontier regions. Another aspect of pegmatite structure involves correla tions between structure and degree of fractionation. Černý (1991c) noted that among zoned pegmatites in a district, those with gentler dips tend to be more enriched in lithium, cesium, and tantalum than steeply dipping bodies. A classic but inaccessible example is in an area of two kilometers of relief in the Hindu Kush of Afghanistan (Rossovskyi and Shmakin, 1978). This is not a hard-and-fast rule, however, because some large pegmatites (for example, Greenbushes: Partington and others, 1995), are steeply dipping bodies.
16 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites 1,800 2,638 2,700 1,880 1,055 Probability Number of zircon ages Number of pegmatites Pangea Rodinia Nuna Sclavia and Superia Assembly time of supercontinent EXPLANATION B. Detrital zircons from modern sands C. Lithium-cesium-tantalum pegmatites A. Igneous crystallization ages Phanerozoic Proterozoic Archean n 31,574 n 66 n 21,655 1,000 1,000 2,000 3,000 4,000 Age in millions of years (Ma) present (Ma). A, Probability plot of crystallization ages of igneous rocks. These are mainly U-Pb zircon ages before about 250 Ma and a mix of 40Ar/39Ar and U-Pb zircon ages between 0 and 250 Ma. Data are from the Dateview geochronology database, maintained by Bruce Eglington, University of Saskatchewan (://sil.usask.ca/Databases.htm). B, Histogram of detrital zircon ages from Pleistocene and modern sediments, using unfiltered data from the global dataset of Voice and others (2011). C, Histogram of ages of lithium-cesium-tantalum (LCT) pegmatites, updated from the global compilation of McCauley and Bradley (2014). The three age distributions are broadly similar. Maxima correspond to times of assembly of the supercontinents. Figure 6. Age distributions covering most of Earth history, back to 4000 million years before
4.0. Regional Environment 17 Wodgina pegmatite Cs, Ta Sn, Ta Sn, Li Sn, Ta Sn Sn Sn Sn Be, Ta Be, Sn Be Be, Nb EXPLANATION Archean rocks Fault—Arrows show relative motion Pegmatite location with chief commodities Chief commodities identifier Supracrustal rocks Intrusive rocks Metamorphosed chert, iron formation, and pelite Metamorphosed sandstone and conglomerate Metamorphosed volcanic rocks Metamorphosed ultramafic rocks Older rocks Migmatite and granodiorite Beryllium Cesium Lithium Niobium Tin Tantalum Yule granitoid complex Marginal leucocratic phase Porphyritic granitoid Medium granitoid Sn Sn Ta Be Cs Nb 3 KILOMETERS 3 MILES Figure 7. Geologic map of the Wodgina district, Western Australia, showing fault control, from Sweetapple and Collins (2002).
18 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites 4.5. Relations to Igneous Rocks Lithium-cesium-tantalum pegmatites are the most highly differentiated products and last magmatic components to crystallize from calc-alkaline granitic melts. Parental granites are typically peraluminous, S-type granites, although some Archean examples are metaluminous, I-type granites (Martin and De Vito, 2005). In some districts (for example, Ghost Lake, Alberta, Canada: Breaks and Moore, 1992), pegmatites show a regional zoning pattern with respect to the parental granite, with the greatest enrichment in incompatible elements in the more distal pegmatites. Presumably because of this effect, some peg matites (for example, Brazil Lake, Nova Scotia: Kontak, 2006) have only an inferred spatial and genetic relation to a buried plutonic parent. Another implication is that the structurally high est pegmatites in an LCT field are particularly promising. Genetic links between a pegmatite and its parental granite have been postulated on various grounds. In the clearest cases, the two can be linked by physical continuity (Greer Lake, Canada: Černý and others, 2005). In other cases, some com bination of textural, mineralogical, geochemical, isotopic, and geochronological information provide evidence for a genetic link (Pilbara Craton, Australia: Sweetapple and Collins, 2002). Simmons and Webber (2008) noted that LCT pegmatites sur rounding an inferred parental granite cannot be traced to source body via a feeder dike. Zagorsky (2009) suggested that, in certain cases, geochemical and geochronologic data contradict the perceived relation between pegmatites and spatially related granites, suggesting that these pegmatites and granites may be likened to brothers and sisters rather than parents and children. 4.6. Relations to Sedimentary Rocks Pegmatites are never found intruding unmetamorphosed sedimentary rocks, so any relations between LCT pegma tites and sedimentary rocks are indirect. The lithium in many LCT pegmatites was likely derived, in part, from clays in the metasedimentary source rocks of the parent S-type granites. 4.7. Relations to Metamorphic Rocks Lithium-cesium-tantalum pegmatites typically occur in metasedimentary and metaigneous rocks of low-pressure upper greenschist to amphibolite facies (ca. 500-650 °C and ca. 200-400 MPa: Černý, 1992). Contacts between pegmatites and metamorphic host rocks are typically sharp. Alteration haloes are discussed in Section 13.3. 5.0. Physical Description of Deposits 5.1. Dimensions, Form, and Shape Pegmatites do not form in isolation, but as members of larger populations. Černý (1991b) suggested that the term pegmatite group be used to identify the basic genetic unit. Pegmatites within a group are cogenetic bodies numbering tens to hundreds, and occupying an area of a few tens of square kilometers. Černý (1991b) defined the terms pegmatite field, pegmatite belt, and pegmatite province, for successively larger entities; in our view, these are cumbersome subdivisions. A use ful general term, which we employ here, is pegmatite district. Even the largest LCT pegmatite bodies are much smaller than typical granitic plutons. The Manono-Kitotolo pegmatite in the Democratic Republic of the Congo, one of the largest in the world, has a surface exposure 12 km long and as much as 400 m wide (Ngulube, 1994). The Greenbushes pegmatite in Australia is 3 km long, 40 to 250 m wide, and 400 m deep (Partington and others, 1995). The Kings Mountain pegmatite in North Carolina consists of a swarm of dikes, each as much as 75 m wide and hundreds of meters long (Kunasz, 1982). At the other extreme, the subeconomic Little Nahanni pegmatite group in the Northwest Territories, Canada consists of a series of planar dikes no wider than 2 m that can be traced along strike for only a few hundred meters (Barnes, 2010). Lithium-cesium-tantalum pegmatite bodies have various forms, including tabular dikes, tabular sills, lenticular bodies, and oddly shaped masses (Cameron and others, 1949). The giant Tanco pegmatite in Manitoba, Canada is a subhorizon tal lenticular body (Černý, 1982b; Stilling and others, 2006) (fig. 8). The Altai No. 3 pegmatite in Inner Mongolia, China, is a subhorizontal sheet that feeds upward into a vertical stock (Zhu and others 2006) (fig. 9). The Greenbushes pegmatite dike swarm dips 40-50° and was emplaced into a shear zone (Partington and others, 1995) (fig. 10). 5.2. Host Rocks Lithium-cesium-tantalum pegmatites are hosted in a vari ety of metamorphic and igneous rocks. Most economic deposits are emplaced into metasedimentary and metaigneous belts of amphibolite or upper greenschist facies rocks (Černý, 1992). Some LCT pegmatites remain as segregations within their par ent granite (for example, Greer Lake leucogranites: Černý and others, 2005), but most are emplaced into surrounding country rock. In more competent rocks such as granites, pegmatites commonly follow fractures. Pegmatites intruded into schists tend to conform to foliation. Many pegmatites are partly concor dant and partly discordant (Cameron and others, 1949). 6.0. Geophysical Characteristics Lithium-cesium-tantalum pegmatites do not have a strong geophysical signature. The granitic composition of pegmatites means that their density is often only marginally different from metasedimentary host rocks, and, in any case, their small size would make anomalies difficult to resolve. However, in an established pegmatite field, gravity anomalies can be identi fied, which are even capable of detecting zonation within the pegmatite. A survey of the Tanco pegmatite found the wall zones to be gravity lows and the spodumene-rich core to be a
7.0. Hypogene Ore Characteristics 19 gravity high (Trueman and Černý, 1982). Being extremely low in iron, pegmatites do not stand out in aeromagnetic surveys. Some rare-element pegmatites have elevated levels of uranium, which can be readily identified by radiometric surveys. Groundpenetrating radar has been tried in exploration for gem pockets at the deposit scale (Patterson and Cook, 2002), but the method has not been successful. 7.0. Hypogene Ore Characteristics The distinction between "ore" and "gangue" is blurred in LCT pegmatites because quartz, muscovite, plagioclase, and potassium feldspar, which make up the bulk of all pegmatites, may themselves be viable mineral commodities. Those components are discussed in Section 8. In this section, minerals of the rare elements are discussed. 7.1. Mineralogy and Mineral Assemblages Lithium minerals. Lithium reserves in LCT pegmatites are mostly in the silicates spodumene (LiAlSi2O6), petalite (LiAlSi4O10), and lepidolite (Li-mica, (table 2). Spodumene is a lithium pyroxene that typically forms euhedral tabular or lath-shaped crystals, which generally are white, beige, or light green (London, 2008). Gem spodumene varieties include pink kunzite, yellow triphane, and green hiddenite (Simmons and others, 2012). Spodumene can form in α-spodumene and β-spodumene varieties depending on the pressure-temperature conditions of crystallization; the α form is most common in nature. Petalite is a phyllosilicate that forms in large, subhedral, club-shaped, white or light gray crystals. In general, petalite is associated with the most highly differentiated LCT pegmatites. Spodumene and petalite represent different phases in the Li2O-Al2O3- SiO2 system (London, 1984) and can provide a rough paleobarometer (fig. 11). Whereas petalite is normally compositionally pure, spodumene commonly takes up ferric iron, an impurity that can complicate refining and diminish the value of the mineral for uses in ceramics. The feldspathoid eucryptite (LiAlSiO4) is also formed in lithium-rich pegmatites that crystallized under the lowest pressure and temperature conditions shown in figure 12; it looks like quartz, but can be identified by its red fluorescence in short-wave ultraviolet light (London, 2008). Lepidolite is a pink to lavender mica solid solution of the muscovite-trilithionite-polylithionite series (London, 2008). In the Tanco pegmatite, lepidolite can contain 6.02 weight percent (wt%) Li2O, 4.91 wt% Rb2O, and 2.75 wt% Cs2O (Černý, 2005). In addition, lithium phosphate minerals, mainly montebrasiteamblygonite, lithiophilite, and triphylite, are present in some LCT pegmatites. Figure 8. Cross section through the Tanco pegmatite, Manitoba, Canada, showing zoning, from Stilling and others (2006).
20 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites Tantalum-niobium-tin minerals. Tantalum and tin oxides make up another important group of ore minerals found in LCT pegmatites (London, 2008). Tantalum mineralization predominantly occurs as columbite-tantalite Columbite-tantalite compositions evolve toward Ta over Nb and Mn over Fe with increasing fractionation of the melt. Tantalum may also occur as microlite in more highly differentiated pegmatites. Tin is found as cassiterite (SnO2). Oxides in pegmatites are reddish or yellowish brown to black in color, with a reflective, submetallic luster and high density, which distinguishes them from tourmalines. Although they are common in other granitic rocks, irontitanium oxides are uncommon in rare-element pegmatites. Tantalite-Mn has a particularly high specific gravity, as high as 8.1, which distinguishes it from oxides such as cassiterite, which has a specific gravity of 6.5-7.0 (Fetherston, 2004). Pollucite. Cesium is mined exclusively from pollucite ((Cs,Na)(AlSi2O6).nH2O as per table 2, or more simply, CsAlSi2O6). It occurs only in highly fractionated LCT pegmatites, notably Tanco and Bikita. Pollucite forms large, white to light gray anhedral crystals, which are brittle and shatter into splintery fragments. Micas may contain significant Cs2O, but not as an essential structural component (London, 2008, p. 53). Beryl. Beryl (Be3Al2Si6O18) is by far the most common of the beryllium minerals in LCT pegmatites. Pegmatitic beryl was the main source of beryllium until rhyolite-hosted bertrandite deposits at Spor Mountain, Utah, United States (Foley and others, 2012) came to dominate the world market. Ordinary green beryl is commonly found in the wall zone as inwardflaring crystals, and just outside the quartz core (London, 2008, p. 78). Gem beryl varieties from miarolitic cavities ("pockets") include the pale blue aquamarine, pink morganite, and yellow heliodor. Emerald gets its color from Cr; it forms in the exomorphic haloes of pegmatites that intruded ultramafic rocks. Tourmaline. Tourmaline typically forms black crystals immediately inside the margins of pegmatites, oriented perpendicular to the wall of the intrusion and flaring inward. This black tourmaline is commonly referred to as schorl, but is actually a solid solution between schorl and elbaite, also including components of olenite and foitite (London, 2008, p. 64). Tourmaline indicates an abundance of boron, which is an important flux in the melt and forms tourmaline after interacting with iron and magnesium from host rocks. Pink to green elbaite is a semiprecious gemstone found in the more highly fractionated inner zones of LCT pegmatites (core and core margin); high-end specimens can sell for $25,000 per carat (Simmons and others, 2012). Figure 9. Cross section through the Altai No. 3 pegmatite, Inner Mongolia, China, from Zhu and others (2006). Inset shows pegmatites, including the Altai No. 3, in black.
7.0. Hypogene Ore Characteristics 21 Other gemstones and rare minerals. Although the focus of this report is in the resources of the rare metals lithium and tantalum in LCT pegmatites, many deposits that would otherwise be subeconomic are mined for gemstones and (or) for high-value museum specimens of rare minerals (Simmons and others, 2012). An example is the Palermo #1 pegmatite in New Hampshire, which has yielded 151 valid minerals including 13 new species (://www.mindat.org/loc-3942.; Whitmore and Lawrence, 2004). 7.2. Zoning Patterns District scale. As noted in Section 2.3, at the district scale, LCT pegmatites typically show mineralogical and geochemical zonation that is broadly concentric surrounding an exposed or inferred granitic pluton—both in map view and in cross section (fig. 12). The most proximal and least evolved pegmatite bodies in an LCT field contain only the standard rock-forming minerals of granite, such as quartz, potassium feldspar, sodic plagioclase, muscovite, and biotite, with lesser garnet, apatite, tourmaline, and (or) zircon. Further outward are pegmatites containing beryl. Beryllium is incompatible in common minerals or silicic melts; it crystallizes as beryl at relatively low abundances (London, 2005b). In the next zone outward, columbite forms along with beryl. Tantalite and lithium aluminosilicates precipitate in the next zone outward. Finally, the most evolved and, ideally, most distal pegmatites contain pollucite. Regional pegmatite zonation is recognized adjacent Figure 10. Cross section through the giant Greenbushes pegmatite, Australia, from Partington and others (1995). Table 2. Common lithium-, cesium-, and tantalum-bearing minerals in lithium-cesium-tantalum (LCT) pegmatites. Compositions are from London (2008). Lithium minerals Chemical formula Notes Amblygonite-montebrasite group (solid solution): Amblygonite Montebrasite (solid solution) Cookeite Elbaite Eucryptite Holmquistite Lepidolite Lithiophilite-triphylite group (solid solution): Lithiophilite Triphylite Petalite Spodumene LiAlPO LiAlPO LiAl (AlSi O )(OH) NaLi Al Al (Si O )(BO ) (OH) ) Li Mg Al (Si O )(OH) (K,Rb)(Li,Al) (Al,Si) O (OH,F)
LiAlSi O LiAlSi O pegmatites alteration product of spodumene; also in veins in slate belts pegmatites pegmatites LCT pegmatite aureoles pegmatites pegmatites pegmatites pegmatites Cesium minerals Pollucite (Cs,Na)(AlSi O ).nH O pegmatites Tantalum minerals Tantalite-columbite group (solid solution): Tantalite-Mn Columbite-Mn Wodginite Microlite O O O (Na,Ca) Ta O (O,OH,F) pegmatites pegmatites pegmatites
22 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites to the Separations Rapid pluton in Ontario (Breaks and Tindle, 1997), the Ghost Lake Batholith in Ontario (Breaks and Janes, 1991) (fig. 13), the Osis Lake granite complex in Manitoba (Černý and Brisbin, 1982), and the Black Wonder granite in Colorado (Černý, 1982b). The most highly fractionated rareelement-enriched pegmatites only constitute about two percent of regional pegmatite populations (Ginsburg and others, 1979; Stewart, 1978). Deposit scale structure. Most individual LCT pegmatite bodies are concentrically, albeit irregularly, zoned. Zoning is both mineralogical and textural. Generalizing from detailed, deposit-scale mapping of hundreds of U.S. pegmatites, Cameron and others (1949) identified four main zones: border, wall, intermediate, and core zones (fig. 14). The following is summarized from Cameron and others (1949, 1954). See also London (2014) for a modern treatment of zoning in pegmatites. The outermost, or border zone, which may or may not be present, is immediately inside the sharp intrusive contact between the pegmatite and country rock. Typically the border zone is 1 cm thick, fine-grained, and composed of quartz + muscovite + albite. Essentially it is a chilled margin of aplite. The wall zone is typically less than 3 m thick, although in some cases it is as thick as 10 m. The largest crystals seldom exceed about 30 cm. In general, the grain size is somewhere between that of the fine-grained border and that of the intermediate where the notoriously giant crystals are to be found. The essential minerals are albite + perthite + quartz + muscovite. Graphic intergrowths of perthite and quartz are common. Wall zones are mined for muscovite. Tourmaline, beryl, and columbite may be present. The intermediate zone or zones comprise everything between the wall and the core; there may be one or several of these zones. They may be discontinuous rather than complete shells, and in some pegmatites they are absent altogether (Cameron and others, 1949). The essential minerals are plagioclase and potassium feldspars, micas, and quartz. In more evolved LCT pegmatites, various rare-element phases, such as beryl, spodumene, elbaite, columbite-tantalite, pollucite, and Li-bearing phosphates, are present. Overall grain size is coarser than in the wall zone. In pegmatites that have them, gem-lined pockets are located in the intermediate zones (Simmons and others, 2003). The core of many zoned pegmatites is monomineralic quartz. In the core zone of some LCT pegmatites, quartz is associated with perthite, albite, spodumene or other Li-bearing aluminosilicates, and (or) montebrasite (London, 2008, p. 28). Whereas the four-fold zonal scheme is classic and widely observed, important LCT pegmatites such as Tanco, Altai Number 3, and Greenbushes are differently zoned (Stilling and others, 2006) (figs. 8-10). London (2008) suggested that the zonation features are controlled extrinsically by emplacement setting, rather than intrinsically by composition. Some LCT pegmatites are unzoned. For example, the largest lithiumpegmatite resource in the United States, at Kings Mountain, North Carolina, consists of closely spaced dikes of spodumenealbite-quartz pegmatite that show only a minor gradation, with lithium concentrated toward the center (Kunasz, 1982). In their classic synthesis, Cameron and others (1949) recognized two types of secondary units: replacement bodies and even younger fracture fillings. Both of these, if present, are superimposed on any pre-existing zones in pegmatites. Deposit-scale mineralogy. Norton (1983) presented a scheme describing the mineralogical zonation of lithiumbearing pegmatites, modified from a similar one by Cameron and others (1949). Norton (1983) stressed that these are mineral assemblages, and that they do not correspond one-for-one with the structural zones shown in figure 14. Working inward, the Norton (1983) mineral assemblages are: Sodic plagioclase + quartz + microcline Plagioclase + quartz Quartz + sodic plagioclase + perthite ± muscovite ± biotite Perthite + quartz Sodic plagioclase + quartz + spodumene (or petalite or montebrasite or both) Quartz + spodumene Quartz + microcline Quartz Lepidolite or lothian mica + sodic plagioclase + quartz + microcline The above list covers only the major mineral phases, those comprising at least 5 percent of the rock by volume. The plagioclase-rich outer units may include accessory beryl, garnet, tourmaline, apatite, and (or) columbite. The inner, Li-mica-rich units typically include tourmaline, beryl, topaz, apatite, other phosphates, and (or) Sn-Ta-Nb oxides (London, 2008, p. 32). Figure 11. Lithium silicate phase diagram under conditions of quartz saturation, from London (1992).
7.0. Hypogene Ore Characteristics 23 7.3. Paragenesis Lithium-cesium-tantalum pegmatites crystallize from the outside inward. In an idealized zoned pegmatite, first the bor der zone crystallizes, then the wall zone, then the intermediate and lastly, the core and core margin, the latter con taining the highest concentration of fluxing components. As discussed in Sections 4.3 and 11.3, all this happens in days to years and at temperatures that are remarkably low for igneous rocks. As crystallization proceeds, the remaining pegmatite melt becomes progressively enriched in incompatible metals including Li, Be, Rb, Cs, Nb, Ta, and Sn, and also in the flux ing components H2O, B, F, and P. Crystals in pegmatites get as large as they do through the process of constitutional zone refining, whereby fluxing components are concentrated in the melt immediately next to the faces of rapidly growing crystals (London and others, 1989; London, 2008). The fluxes modify the melt structure, and thus lower magma viscosity, increase diffusion rates, and suppress crystal nucleation (Webber and others, 1997, 1999). Fluxing effects are additive (Linnen and Cuney, 2005; London, 2008; Nabelek and others, 2010). Mineral assemblages in LCT pegmatites are mainly of primary origin. Among the important secondary assemblages is SQI, the acronym for a fine-grained intergrowth of spodu mene and quartz that formed through the replacement of petalite. Secondary alteration of pollucite and albite can create a solid solution with analcime, the sodium analogue of pol lucite (Lagache and others, 1995). 7.4. Textures, Structure, and Grain Size Pegmatites are known for their massive crystals that are meters or even tens of meters long (fig. 1A). A giant muscovite from the Inikurti pegmatite in India measured about 4.5 m by 3 m by 3 m (Rickwood, 1982). The largest spodumene crystal on record was 14.3 m long, from the Etta Mine in the Beryllium Lithium, beryllium, tantalum, tin, (cesium, rubidium) Limit of pegmatite halo around granite Parental granite Lithium, beryllium, niobium greater than tantalum EXPLANATION Metasedimentary or metaigneous country rock Granite Pegmatitic granite Pegmatite Boundary between zones of rare-element enrichment Fault NOT TO SCALE Figure 12. Idealized concentric, regional zoning pattern in a pegmatite field, adapted from Galeschuk and Vanstone (2005) after Trueman and Cˇerný (1982). Characteristic rare-element suites of the most enriched pegmatites in each zone are indicated. The most enriched pegmatites tend to occur distally with respect to the parental granite.
24 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites Black Hills, South Dakota (Page and others, 1953, p. 55). Rickwood (1982) recorded an 18-m-long by 3.5-m-wide beryl crystal from the Malakialina pegmatite in Madagascar. The largest known microcline, from the Devil's Hole pegmatite in Colorado, measured 49 m by 36 m by 14 m, and weighed almost 16 million kg (Rickwood, 1982). Other distinctive pegmatite textures include unidirectional solidification textures (fig. 1B), graphic intergrowths (fig. 1C), skeletal crystals (fig. 1D), miarolitic cavities (fig. 1E), and "line rock" (banded aplite) (fig. 1F). Replicating and understanding these textures has been a primary concern in experimental studies of pegmatites. The layered or graphic growth of crystals near the border zones of pegmatites is explained by the high viscosity of low-temperature melts, which limit diffusion (London, 2009). Some rare-element pegmatites are characterized by miaro litic cavities, or pockets, that are filled with well-formed crystals. These structures are commonly associated with oriented growth textures, including "line rock" (banded aplite), graphic granite, and euhedral crystals that grew into the miaroles. 8.0. Hypogene Gangue Characteristics The distinction between "ore" and "gangue" is blurred in LCT pegmatites because quartz, muscovite, plagioclase, and potassium feldspar may themselves be viable industrial minerals. Nonetheless, in the Section 8.1, these phases are separated from the ore minerals of the rare elements. Spodumene-beryl-tantalite zone Ghost Lake batholith Beryl-columbite zone Interior beryl zone Albite zone N EXPLANATION Holmquistite alteration zone Granite Pegmatites Granite and pegmatite Boundary between zones of rare-element enrichment in pegmatites Metasedimentary rock— Sillimanite grade 1.5 KILOMETERS 1.5 MILES Figure 13. Ghost Lake pegmatite field, Ontario, Canada, showing regional zoning, from Breaks and Moore (1992).
8.0. Hypogene Gangue Characteristics 25 8.1. Mineralogy and Mineral Assemblages Volumetrically, even the most evolved granitic peg matites are mainly quartz, sodic plagioclase, and potas sium feldspar. Establishing an accurate bulk composition for a pegmatite body is difficult because of large crystal size and monomineralic zoning patterns. For the few peg matites where this has been done, the bulk composition is close to that of a minimum-melt granite. We quote modal proportions from the Tanco pegmatite based on Stilling and others (2006) and from the Tin Mountain pegmatite from Norton (1994). Quartz. Quartz is the most abundant phase in most LCT pegmatites. At Tanco, it comprises about 33 percent of the rock and at Tin Mountain about 34 percent. Norton (1983) noted that not all zones of pure quartz in pegmatites are at the physical core of the intrusive body. Silica occurs as the lower-temperature form, α-quartz; only in exceptional cases of high temperature and low pressure crystallization does β-quartz form (Martin, 1982). In certain LCT pegma tite districts (for example, Spruce Pine, North Carolina), quartz has low trace-element abundances and is mined as a source of high-purity silica for the semiconductor industry (Glover, 2006). Plagioclase. Plagioclase is nearly as abundant as quartz in most LCT pegmatites. At Tanco it comprises about 29 per cent of the rock; at Tin Mountain, 27 percent. The anorthite content of plagioclase is low in LCT pegmatites, rarely exceeding An6 at the borders and decreasing inward (London, 2008). Thus, all the plagioclase is albite, commonly in the platy form cleavelandite. The Sr, Ba, Ga, and P contents of plagioclase are elevated (London, 2008, p. 46). Plagioclase is mined for ceramics. Potassium feldspar. Potassium feldspar, typically microcline, is next in abundance in LCT pegmatites. At Tanco it comprises about 20 percent of the rock; at Tin Mountain, 12 percent. The Rb, Ba, Pb, and P contents of potassium feldspar are elevated (London, 2008, p. 48). In many pegmatite border zones, quartz and microcline form a graphic intergrowth that grows generally more massive towards the interior of the pegmatite. Potassium feldspar is mined for ceramics. Micas. The principal mica in LCT pegmatites is mus covite. At Tanco it comprises less than 3 percent of the rock; at Tin Mountain, 9 percent. Muscovite is most abundant in the border and intermediate zones. Muscovite is mined for a variety of industrial uses, including paint and wallboard. Muscovites from highly fractionated LCT pegmatites have elevated Li, Rb, Cs, Mn, and F concentrations. "Lepidolite" refers to a family of lavender to pink micas that occur in the intermediate and (or) core zones of more fractionated LCT pegmatites; it is a solid solution of muscovite, trili thionite, and polylithionite end members. Lepidolite com monly occurs as a secondary mineral in fine-grained masses (London, 2008, p. 54). Lepidolite has been mined as a source of lithium (for example, at Bikita). Garnet. Garnet is a common accessory mineral in peraluminous granites and LCT pegmatites (London, 2008). Pegmatite garnets are part of the almandine-spessartine series. Selway and others (2005) reported that on a regional scale, the orange, manganiferous end-member spessartine is associated with the more fractionated LCT pegmatites (fig. 15). EXPLANATION Core zone Intermediate Core margin—Large crystals of tourmaline, beryl, or spodumene Albite zone Wall zone Border zone Figure 14. Deposit-scale zoning patterns in an idealized pegmatite, modified from Fetherston (2004) after erný (1991a). The thickness of the border zone has been exaggerated. Cˇ Primitive Almandine Mn / (Mn + Fe2) Garnet Evolved Magnesium (Mg) in atoms per formula unit (apfu) Spessartine Figure 15. Garnet compositions in pegmatites as a function of degree of fractionation, from Selway and others (2005).
26 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites 9.0. Hydrothermal Alteration During the crystallization and cooling of LCT pegmatites, the transition from magmatic to hydrothermal conditions is gradational (London, 1986b). As described in more depth in Section 11.3, the fluxing components, H2O, B, P, and F, depress solidus temperatures to ca. 350-550 °C, far below the minimum-melting temperature range of 800 to 600 °C for "wet" granite at various pressures (Miller and others, 2003). Evidence for hydrothermal effects on pegmatites is preserved in trapped fluid inclusions, in miarolitic cavities, and in pseudomorphic replacements of coarse minerals (London, 2005a). In the Covas de Barroso pegmatite district of Portugal, Charoy and others (2001) documented the replacement of primary spodumene by albite + muscovite + petalite by K-feldspar + eucryptite. In the same area, Bobos and others (2007) documented two stages in the hydrothermal alteration of spodumene, first spodumene to cookeite + quartz (at ca. 240 megapascals [MPa] and 240 ºC), then cookeite + kaolinite ± mica (at ca. 220 MPa and 220 ºC). Alteration haloes surrounding LCT pegmatites are described in Section 13.3. 10.0. Supergene Ore and
Gangue Characteristics Under conditions of intense tropical weathering, the durable, heavy, resistant phases in LCT pegmatites may survive dissolution and accumulate in residual soils or placers. Pegmatite-derived placer deposits in Pilbara Craton, Western Australia, mainly yield cassiterite and the columbite-tantalite group (Fetherston, 2004). Tantalite-Mn from the Kenticha pegmatite, Ethiopia, has mainly been produced from weathered regolith (Küster and others, 2009). "Coltan," which has gained notoriety as a conflict mineral in Africa (Melcher and others, 2008) is columbite-tantalite that was weathered from pegmatites. It is amenable to mining by artisanal methods. 11.0. Geochemical Characteristics 11.1. Trace Elements and Element Associations Lithium-cesium-tantalum pegmatites take their name from their enrichments in Li, Cs, and Ta; they also tend to be enriched in Be, B, F, P, Mn, Ga, Rb, Nb, Sn, and Hf, and locally, in U and As. Whole-rock trace-element abundances from the Little Nahanni LCT pegmatite in the Canadian Rockies are shown in figure 16. Most incompatible elements in this pegmatite are depleted relative to average upper continental crust. Phosphate enrichment has been linked to a pelitic protolith (London, 2008). Rare earth element (REE) abundances are low in LCT pegmatites; chondrite-normalized spidergrams patterns show minor light REE enrichment and are broadly similar to those in highly evolved P-rich granitic rocks (Barnes and others, 2012). Relative to granites, pegmatites are depleted in mafic components. Most incompatible elements in this pegmatite are depleted relative to average upper continental crust. 11.2. Fluid Inclusion and Melt Inclusion
Thermometry and Chemistry Fluid inclusion studies of pegmatites suggest formation at ca. 350-550 °C and relatively low to moderate pressures. For example, microthermometry suggests that the Tin Mountain pegmatite (Black Hills, South Dakota, United States) crystal-
lized at 400-350 °C at 270±30 MPa (Sirbescu and Nabelek, 2003). The spodumene-bearing Animikie Red Ace pegmatite in Wisconsin, United States, formed at 550-400 °C and ca. 300 MPa (Sirbescu and others, 2008). The lepidoliteand cassiterite-bearing pegmatites of the tungsten-tin belt in Myanmar formed at ~410-230 °C (Zaw, 1998). In the Bohemian Massif, Czech Republic, LCT pegmatites crystallized at 560-500 °C and 310-430 MPa, which is about 100 °C cooler and 100 MPa shallower than common granite pegmatites in the same district (Ackerman and others, 2007). Fluid inclusions are critical to understanding the low-temperature crystallization of pegmatites because H2O concentrations greater than a few weight percent cannot be preserved in crystallized silicic melts; any concentrations above 5-6 weight percent can only be preserved in fluid and melt inclusions (Thomas and Davidson, 2008). Inclusions sampled from a single pegmatite often display evidence for the coexistence of immiscible melts and (or) fluids. Thomas and others (2000) demonstrated that pegmatite melts above 712 °C and 100 MPa are completely miscible, whereas below these conditions they separate into two different melts contrasting sharply in H2O content, density, and viscosity. Fluid inclusions in the Tin Mountain pegmatite in South Dakota suggest the coexistence of two immiscible fluids, one CO2-rich and low density and the other H2O-rich and high density (Sirbescu and Nabelek, 2003). Evidence
from the miarolitic Königshain pegmatites in Germany suggests that fluids unmixed during pegmatite emplacement, when an initially H2O-rich peralkaline melt separated into immiscible peraluminous and peralkaline melt phases (Thomas and others, 2009a). Pegmatites from the Orlovka granite in Siberia reveal the coexistence of at least three immiscible phases, an aqueous CO2-rich supercritical fluid containing elemental sulfur and two different melts (Thomas and others 2009b). The compositions of melt inclusions and fluid inclusions provide valuable insights into the sequence of pegmatite crystallization and nature of low-temperature silica melts. Fluid inclusions in lithium-bearing aluminosilicates in the Tanco pegmatite yielded high concentrations of alkali borate. The boron would have increased the solubility of H2O and could have contributed to complete miscibility between silicate liquid and H2O (London, 1986b). Some melt inclusions have significant enrichments in volatiles. Melt inclusions from pegmatites at the Erzgebirge Sn-W deposit in Germany are peraluminous, with aluminum saturation indices, or ASI (Al2O3/[K2O+Na2O+CaO]) in the range 1.15 to 2.0). These inclusions contain more than 20 weight percent H2O, F, Cl, and P2O5, and have anomalous Sn and Li (Webster and others, 1997; Thomas and Webster, 2000).
11.0. Geochemical Characteristics 27 11.3. Stable Isotope Geochemistry Oxygen isotopes provide a reliable indicator for the involvement of crustal rocks in pegmatite genesis. Whole rock values of δ18O of about 11 parts per thousand (‰) or higher suggest a primarily metasedimentary melt source, except in the Archean when the Earth's sedimentary rocks had not yet matured sufficiently for significantly differentiated oxygen isotopes (Valley and others, 2005). The two primary granite sources for LCT pegmatites have distinctive δ18O values: S-type granites have values greater than 10‰, whereas I-type granites have values less than 10‰. Černý (1991a) recognized a bimodal distribution of δ18O values for LCT pegmatites, with one peak at about 11‰ and the other at 8-9‰. They attributed this distribution to differences in source material for the granites, which has higher δ18O if derived from supracrustal sources and lower δ18O if from juvenile basement. The fractionation of the two lithium isotopes, 6Li and 7Li provides a useful method to analyzing solid and fluid phase interaction, including magmatic differentiation. As 6Li is more easily accommodated into the crystal lattice, 7Li should become progressively more enriched as a melt differentiates. Teng and others (2006) measured δ7Li of +7.9 to +11.4 for pla gioclase, muscovite, and spodumene, and contrasting values of +14.7 to +21.3 in quartz. The distinction is probably related to the presence of 7Li for less coordinated sites, with higher bond energies, such as those in quartz. In the Little Nahanni pegmatites, Northwest Territories, Canada, δ7Li increases with increasing fractionation, as measured by decreasing K/Rb, Nb/Ta, Zr/Hf, and Li/Cs ratios. The δ7Li in these pegmatites also tends to increase with flux content of the melt, notably with increased F (Barnes, 2010). 11.4. Petrology of Associated Igneous Rocks Lithium-cesium-tantalum pegmatites are associated with
and derived from fertile peraluminous granites and leucogranites. These granites are S-type and highly evolved I-type granites (Martin and De Vito, 2005), enriched in volatiles and rare elements. Fertile granites that generate LCT pegmatites are generally late- to post-tectonic calc-alkaline intrusions and post-date regional metamorphism (Černý, 1991c). These granites are leucocratic and contain biotite or muscovite or both, as well as accessory garnet, tourmaline, cordierite, and
(or) andalusite (Černý, 1982a).
Selway and others (2005, their table 1) reported mean values for representative fertile granites from the Archean Superior Craton, Canada. The Separation Rapids pluton contains a mean of 48 parts per million (ppm) Cs (compared to 3.7 ppm for average upper continental crust), 235 ppm Li , (compared to 20 ppm for average upper continental crust), 890 ppm Rb (compared to 111 ppm for average upper continental crust), and 19 ppm Ta (compared to 2.2 ppm for average upper continental crust). Beryllium, Ga, and Nb are similarly elevated. The ratios K/Cs, K/Rb, and Nb/Ta are low. Trueman and Černý (1982) noted that evolved granites have low Ti, Fe, Mg, and Ca contents.
11.5. LCT Pegmatite Geochronology
Lithium-cesium-tantalum pegmatites are amenable to dating using a range of isotopic systems. For most pegmatites, a precise, accurate date has proven elusive. The Rb-Sr, K-Ar, Re-Os, and U-Pb isotopic systems have each been used with mixed results. Figure 16. Trace element abundances relative to upper continental crust, Little Nahanni pegmatite group, Canada, based on data from Barnes and others (2012). Elements above the heavy line are enriched with respect to average upper continental crust, those below are depleted
28 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites A number of LCT pegmatites were dated by the Rb-Sr or conventional K-Ar methods when those were state-of-the-art, many decades ago. The Rb/Sr isochron method is seldom used anymore for dating igneous rocks because of its inherently large uncertainties and open-system behavior, although one advantage of LCT pegmatites over most igneous rocks is the presence of high Rb concentrations in micas and potassium feldspars. For pegmatite geochronology, the conventional K-Ar method has been superceded by the 40Ar/39Ar method, but the latter still has a shortcoming. The closure temperature for retention of radiogenic argon is about 425 °C for mus covite and about 300 °C for lepidolite (Harrison and others, 2009; Smith and others, 2005). These are in the same range as inferred crystallization temperatures for LCT pegmatites (Section 11.2). Presumably owing to slow cooling, many pegmatites show a significant lag between mica ages and the crystallization age as inferred from other geochronometers. The relatively new Re-Os system shows some promise. Many LCT pegmatites contain small amounts of molybdenite, which is the best target for this isotopic system (Stein and others, 2001; Marques, 2013). Minerals in LCT pegmatites that are amenable to U-Pb dating include zircon, the columbite-tantalite group, apatite, monazite, and uraninite. Indeed, LCT pegmatites were the first rocks to be dated by the method. In a seminal paper on isotope geochronology, Boltwood (1907) dated uraninites from two pegmatite districts in Connecticut. During the past 40 years, analytical advances have elevated U-Pb zircon geochronol ogy to a preeminent role in modern geology. Zircons from LCT pegmatites commonly have high U contents and are susceptible to metamictization. In addition, many are full of inclusions that contain common lead. Using the CA-TIMS method (chemical abrasion-thermal ionization mass spectrom etry; Mattinson, 2005, 2011), high resolution concordant ages on zircons from a suite of LCT pegmatites can be obtained (for example, Appalachian LCT pegmatites in Bradley and others, 2013 and in press). The CA-TIMS involves annealing a zircon at high temperatures to repair radiation-induced lattice damage, and then high-temperature, partial dissolution of the zircon in hydrofluoric acid to selectively remove weaker zones that are likely to cause discordance. The remainder is ana lyzed by the TIMS method and concordant results with small uncertainties are generally obtained. An overarching problem, however, is that many concordant zircons in LCT pegmatites turn out to be xenocrysts that are significantly older than the independently constrained crystallization age of the pegmatite. One solution is to select macroscopic (greater than 2 mm) zircons, for which a xenocrystic origin is extremely unlikely. Minerals of the columbite-tantalite group have been dated from several pegmatites and the results are promising (Romer and Wright, 1992; Romer, 2003). Inclusions of bismuthinite, uraninite, rutile, cassiterite, quartz, feldspar, and various niobium-tantalum minerals can be selectively removed by leaching in dilute hydrofluoric acid (Romer, 2003). An advan tage of columbite-tantalite is that, as a crystallization product of highly evolved melts, it is unlikely to occur as xenocrysts. Apatite is another promising target for U-Pb geochronology in LCT pegmatites (Krogstad and Walker, 1994; Chamberlain and Bowring, 2001). A few LCT pegmatites have been dated using mul tiple methods, making it possible to compare results. The Little Nahanni pegmatite group in the Northwest Territories, Canada has been dated in two studies. Mauthner and others (1995) reported U-Pb TIMS age of 81.6±0.5 Ma on columbite (Mauthner and others, 1995). More recently, apatite yielded a 206Pb/238U isochron TIMS age of 90.3±1.9 Ma (Barnes, 2010). Barnes (2010) also reported an Rb-Sr isochron age of 79±11 Ma on muscovite, albite, apatite, and whole-rock. The muscovite results are likely compromised by opensystem behavior in Rb-Sr and are the main source of the large uncertainty. The 40Ar/39Ar results from muscovite also reveal open-system behavior (Barnes, 2010), with 40Ar/39Ar isochron ages from three parts of the same muscovite grain giving dates of 83.7±1.9, 78.8±0.9, and 80.3±2.1 Ma. Lepidolite yielded a 40Ar/39Ar isochron age of 65.8±0.8 Ma (Barnes, 2010). Thus, the age of the Little Nahanni pegmatites remains uncertain, with an age that could be ca. 90 Ma or ca. 80 Ma. Kontak and others (2005) reported 40Ar/39Ar and U-Pb ages from the Brazil Lake LCT pegmatites in Nova Scotia, Canada. Five U-Pb TIMS analyses from pieces of a single columbite-tantalite group crystal define a concordant array from 393±1 to 366±1 Ma (206Pb/238U ages). Koktak and others (2005) interpreted the oldest of these as being close to the crystallization age, with the younger ages recording variable lead loss. In addition, 40Ar/39Ar spot-laser analysis of a large muscovite crystal yielded a disturbed age spectrum spanning 381 to 348 Ma, with generally older results from the core and younger results from the rim. 11.6. Environment of Mineralization Pegmatites may intrude a wide variety of rock types, but they are most commonly found in upper greenschist to lower amphibolite facies metasedimentary and metaigneous rocks (Černý, 1992). Pegmatites are generally emplaced within 10 km of fertile, peraluminous granites or leucogranites. Pegmatites are emplaced at slightly shallower crustal levels than the associated parental granite, which may not be exposed. Pegmatite bod ies commonly show structural control, such as being emplaced along faults, lithologic boundaries, and plutonic contacts. 12.0. Theory of Pegmatite Origin Most pegmatite researchers consider that LCT pegma tites are the products of extreme fractionation of pluton-sized batches of granitic magma. An alternative hypothesis is that LCT pegmatites are the direct products of partial melting or anatexis. London (2005a) summarized the evidence bearing on these two possibilities. There is no question that count less common pegmatites in high-grade metamorphic terranes
12.0. Theory of Pegmatite Origin 29 originated by anatexis and remain essentially in situ; these belong to the abyssal class of Černý and Ercit (2005) (fig. 4). The issue of controversy is whether pegmatites that are strongly enriched in the incompatible suite Li, Cs, and Ta have a comparable origin. Evidence for the fractionation model is provided by LCT pegmatites at Greer Lake in Canada (Černý and others, 2005), which can be traced to a fertile granite. In addition, the White Picacho LCT pegmatites in Arizona (Jahns, 1952) are observed to cross-cut rocks that are too low grade to have undergone partial melting—staurolite grade in the case of White Picacho. Experimentally based fractionation modeling has shown that neither beryl-bearing, nor pollucitebearing pegmatites can be derived by direct anatexis of any plausible source (London, 2005a). 12.1. Ore Deposit System Affiliation Lithium-cesium-tantalum pegmatites are a type of igne ous rare-metal deposits. Ore deposition is more likely from a magma than from an exsolved hydrothermal fluid, and endogenic rather than exogenic. Associated deposit types are discussed in Section 2.4. 12.2. Sources of Metals The relation between peraluminous granites (S-type or evolved I-type) and the LCT signature in pegmatites is well established (London, 2005a, 2008). S-type magmas are derived primarily from pelitic sediments that first were meta morphosed to muscovite- and biotite-rich schists. Partial melts from schists are silicic, peraluminous, and potassic. Micas are the major reservoir of Li, Rb, Cs, Be, and Ba in metaclastic rocks. Micas appreciably melt during the beginning of ana texts, and tend to melt incongruently, so that melts formed by melting micas do not produce much mica when they cool and crystallize (London, 2005b). This is an important factor for the concentration of lithium in the melt, which would otherwise be depleted from the melt during mica recrystallization. Cesium is derived from micas, cordierite, and feldspars in metasedi mentary rocks, but is incompatible in these phases in a crystal lizing granitic melt, so that there are no phases to moderate cesium concentration in fractionated melts (London, 2005a). Tantalum and niobium in metapelites are mainly derived from ilmenite (London, 2005b). I-type granites are associated primarily with subduction and sourced mainly from oceanic slab and mantle sources, and are orders of magnitude less enriched in rare elements than S-type magmas. More extensive fractionation of I-type mag mas is needed to reach an equivalent degree of enrichment. The rare elements and melt fluxes in LCT pegmatites are mainly derived from crustal rocks that have been subjected to some degree of reworking. In Manitoba, two pegmatite districts, the Proterozoic Wekusko Lake field and the Archean Cat Lake-Winnipeg River field, provide a valuable case study for pegmatite enrichment (Černý, 1989). The Wekusko Lake field is relatively depleted in Li, Rb, Cs, Be, P, F, Sn, Nb, and Ta, relative to the Cat Lake-Winnipeg River pegmatites. After accounting for plutonic evolution, level of erosion and volatile activity, Černý (1989) concluded that the fundamental cause for this difference in geochemistry was the evolution of the crust. The crustal sources for the Wekusko Lake field were juvenile volcanic-sedimentary rock sequences derived from depleted mantle, whereas the Cat Lake-Winnipeg River peg matites formed from protoliths that had been enriched during repeated orogenic cycles. 12.3. Sources of Melt Fluxes Fluxing components in pegmatite melts are contributed from the melting of source rocks and subsequently con centrated in the melt by differentiation. Important fluxing components of pegmatite melts include H2O, B, P, and F. Boron enters melts mostly through the melting of tourmaline, which is often formed by seafloor hydrothermal systems, and contributes to I-type magmas. Boron may also be concen trated in marine sediments by adsorption onto clay or mica surfaces, particularly in the vicinity of submarine exhalative vents. Tourmaline melts incongruently so that B may enter partial melts, whereas Fe, Mg, and Al are retained in other aluminous mafic minerals. The melting of apatite in crustal rocks is the major source of P in partial melts. Whereas S-type melts are enriched in P and can crystallize other phosphatebearing minerals in addition to apatite, I-type melts are gener ally depleted in P and do not precipitate phosphate-bearing minerals. Fluorine is mainly derived from micas, particularly biotite, which becomes enriched in F over OH with increas ing temperature and increased Mg content; as the temperature of anatexis increases, the F contributed to the melt by biotite will also increase (London, 2008). Pegmatite melts are able to retain H2O during most of their crystallization histories, which is critical to the formation of pegmatitic texture and structures (Nabelek and others, 2010). The volatiles and fluxes that are typical in enriched pegmatite melts preferentially partition into melts during anatexis, so a small degree of partial melting (0.1-5 percent) of typical sedimentary source rocks will generate rare-element granite melts. However, it is not clear how these small degrees of partial melting would be compositionally distinct from the leucosomes of migmatites, nor is there a clear mechanism for extraction of the melt from the zone of melting. Given equiva lent conditions, a partial melt of 30 percent has an extraction rate 6,500 times greater than a partial melt of only 2 percent (Spera, 1980). However, this difference can be mitigated if the partial melt viscosity is lowered through an increase in the H2O content (from 2 to 6 percent, for example). The mecha nisms of enhancing the H2O contents of pegmatitic melts are still not well understood, but a partial melt could become enriched by the low degree of partial melting, an influx of free fluid during near solidus melting conditions, or flux enhanced silicate liquid—H2O miscibility (Shearer and others, 1992).
30 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites 12.4. Chemical Transport and Differentiation A large amount of fractional crystallization is needed to form rare-element minerals in LCT pegmatites. London (2005b) calculated that relative to subalkaline obsidians, Be must concentrate by 23 times, Li by 163 times, and Cs by 7,833 times to form granite liquidus saturation. The Tanco pegmatite, with its massive pollucite zones, has a bulk compo sition of about 2,800 ppm CsO (Stilling and others, 2006). To attain this bulk composition from a reasonable starting point (6 ppm Cs in average obsidian), 99.9 percent of the melt must have crystallized. The mechanisms for this degree of differen tiation must involve fractional crystallization, filter pressing, and rapid diffusion. 12.5. Nature of Geological Traps
that Trigger Ore Precipitation Lithium-cesium-tantalum pegmatites are intrusive rocks that can be emplaced as far as 10 km from the parental granite (Breaks and Tindle, 1997). As discussed in Section 4.3, ther mal modeling suggests that 10,000 years must elapse, during which time the country rock is heated and weakens, before pegmatites can extend outward to such distances (Baker, 1998). Most pegmatite bodies show some degree of struc tural control but, as noted by Sweetapple (2000), they occupy "structures of convenience." The Greenbushes and related pegmatites in Western Australia were emplaced along the Donnybrook-Bridgetown regional shear zone, which is com parable in scale to the San Andreas fault system (Partington, 1990). But most pegmatites are controlled by relatively minor structures. 13.0. Geological Exploration
and Assessment Guide Guidelines for exploration for LCT pegmatites, or assessment of their potential presence in a region, have been discussed in overviews by Trueman and Černý (1982), Selway and others (2005), and Galeschuk and Vanstone (2005, 2007). 13.1. Regional-Scale Favorability As noted in Section 4.1, LCT pegmatites form in oro genic hinterlands, that is, in the cores of mountain belts. Many of the world's largest LCTs are now exposed in the Archean cratons, but these also originated in orogenic hinterlands. Generally favorable regions contain both fertile peralumi nous granites or leucogranites, and metasedimentary or mafic metavolcanic rocks metamorphosed to upper greenschist to lower amphibolite facies. The identification of possible granitic parents is a key ini tial step in evaluating a region for LCT pegmatite potential. Fer tile, peraluminous granites typically contain coarse muscovite that is green rather than silvery; potassium feldspar that is white rather than pink; and accessory garnet, tourmaline, fluorite, and (or) cordierite (Selway and others, 2005). Fertile granites have elevated rare-element abundances compared to average upper continental crust; low Ca, Fe, and Mg; and atypical elemental ratios (for example, Mg/Li less than 10 and Nb/ is less than 8) (Selway and others, 2005). As noted in Section 2.3, however, not all LCT pegmatites can be linked to a known parental granite; the parent is presumed to lie at depth, but may not be evident. Therefore, the presence of one or more fertile granites is only a general guideline. Similarly, metamorphic grade is a guideline rather than a hard-and-fast rule. LCT pegmatites are unlikely to be found in extremely high-grade granulite facies metamorphic terranes, although it is possible for a younger pegmatite to be emplaced into such rocks. Likewise, LCT pegmatites are unlikely be found in low-grade (medium greenschist facies or lower) metamorphic belts. Unmetamorphosed sedimentary or volcanic successions are absolutely not prospective. Whereas most LCT pegmatite districts are in metamorphic belts, individual pegma tite bodies may be emplaced into granite (Greer Lake, Canada; Černý and others, 2005), gabbro (for example, Pala Chief, California; Symons and others, 2009), or other igneous hosts. 13.2. District-Scale Vectors Surface expression. In areas of good bedrock exposure, LCT pegmatites are not easily missed because they are lightcolored rocks with enormous crystals. The main pegmatites of the White Picacho district in Arizona, for example, are plainly seen in Earth images. Typically, granitic pegmatites are relatively resistant and will stand above their surroundings. This is the case in heavily wooded western Maine, where many of the glacially sculpted hilltops are pegmatites that are con spicuously forested with spruce trees rather than hardwoods. The quartz cores of pegmatites are particularly resistant, and in some climates (for example, Tabba Tabba in Western Australia, Sweetapple and Collins, 2002), the quartz core may be all that is exposed. On the other hand, in some deeply weathered regions, such as the Southern Appalachian Piedmont, pegmatites may have little or no remaining topographic expression, although pegmatite minerals may still be visible as float. Regional mineralogical and geochemical zonation. Selway and others (2005) summarized a number of use ful guidelines related to zoning. Lithium-cesium-tantalum pegmatites typically occur in regionally zoned swarms, with the enriched pegmatites far outnumbered by the common ones. The presence of even a single LCT pegmatite suggests the existence of others nearby. As noted in Section 12.5, LCT pegmatites form as far as 10 km from the parental granite (Breaks and Tindle, 1997), with the distal ones being most fractionated.
13.0. Geological Exploration and Assessment Guide 31 Within a pegmatite district, a number of mineral frac tionation trends may help to reveal regional zonation and thus point toward the most promising ground. Potassium feldspars from the most fractionated pegmatites are characterized by greater than 3,000 ppm Rb, K/Rb less than 30, and greater than 100 ppm Cs (Selway and others, 2005). Similarly, mus covites from the most fractionated pegmatites contain greater than 2,000 ppm Li, greater than 10,000 ppm Rb, and greater than 500 ppm Cs (Selway and others, 2005). Smeds (1992) observed that in Swedish pegmatites, Sn greater than 500 ppm in muscovite is a good indicator of cassiterite, particularly when coupled with low Mg and Fe. As summarized by Selway and others (2005), garnets change both in color and composi tion with increasing fractionation. Fertile granites contain the red, Fe-rich almandine, whereas the most evolved pegmatites may contain orange, Mn-rich spessartine. Tourmaline in fertile granites and the outer zones of LCT pegmatites is black and low in Li and Mn; the inner zones of the most fractionated pegmatites commonly host tourmaline in the pink to green variety elbaite, which is enriched in Li and Mn. Beryl shows color changes with increasing fractionation, from greenish or brownish in less evolved pegmatites, to pale, white, and pink in more evolved bodies (Trueman and Černý, 1982). Whole-rock geochemistry is useful for distinguishing fertile granites from barren ones by means of the majorelement and trace-element abundances and ratios summarized in Section 11.5 (Selway and others, 2005). Among the key indicators are elevated Cs, Li, Rb, Sn, and Ta. Whole-rock geochemistry also is useful in identifying exomorphic enrich ment haloes of Li, Rb, Cs, B, and F (Selway and others, 2005), as described in Section 13.3. Weathering of LCT pegmatites can result in both soil anomalies and indicator minerals. Smith and others (1987) demonstrated that As, Sn, Be, and Sb form a 12- by 20-km halo in lateritic soils surrounding the Greenbushes pegmatite; Nb, Ta, and B form a smaller, 1- by 5-km halo. Cassiterite, tantalite, elbaite, and spessartine are sufficiently dense and durable to be useful heavy indicator minerals; spodumene is dense enough, but degrades rapidly in the near-surface envi ronment. The subsurface, subeconomic Dibs pegmatite near Tanco is marked by soil anomalies in Mo, W, In, Sn, Mn, and Sr (Galeschuk and Vanstone, 2005). 13.3. Alteration Haloes Highly mobile alkali elements such as Li, Rb, and Cs and volatile components such as B and F tend to alter the adjacent country rocks during LCT pegmatite emplacement. For exam ple, a large alteration halo of Li and a smaller one of Rb directly overlie the buried Tanco pegmatite (Trueman and Černý, 1992) (fig. 17). In mafic country rocks, the purple, lithium-bearing amphibole holmquistite may form in haloes as much as 20 m beyond a pegmatite (London, 1986a). In metasedimentary coun try rocks, black tourmaline (fig. 1G) and rare-element-enriched biotite and muscovite form distinctive haloes. EXPLANATION Lithium, in parts per million Greater than 100 20 to 100 Rubidium, in parts per million Greater than 40 20 to 40 Geochemical anomalies over the subsurface Tanco pegmatite Outline of subsurface Tanco pegmatite .5 KILOMETER .5 MILE Lithium Rubidium Figure 17. Lithium and rubidium geochemical anomalies over the buried Tanco pegmatite, Manitoba, Canada, adapted from Trueman and Cˇerný (1982). The two maps cover the same area.
32 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites 13.4. Geophysical Guidelines As summarized in Section 6.0, LCT pegmatites lack definitive geophysical characteristics (Galeschuk and Vanstone, 2005). 14.0. Geoenvironmental Features and
Anthropogenic Mining Effects Lithium-cesium-tantalum pegmatites have not been exam ined with the same level of environmental concern as many other metallic ore deposit types. Accordingly, no synthesis covering the environmental geology of LCT pegmatites has been published. These pegmatites may be enriched in many trace elements, including Li, Cs, Ta, Be, B, F, P, Mn, Ga, Nb, Sn, and Hf (London, 2008). Be and F are included in the U.S. Environmental Protection Agency (EPA) primary drinking-water regulations with maximum contaminant levels (MCLs) of 4 micrograms per liter (mg/L) and 4 milligrams per liter (mg/L), respectively. In concen trations greater than the MCL, Be can cause internal lesions and F can cause bone disease (EPA, 2009). Two other elements that may be locally anomalous in the intermediate zones of LCT pegmatites are As and U. Health effects for consuming As at levels above the MCL of 10 mg/L include increased risk of cancer, skin damage, and circulatory problems. Consumption of U at levels above the MCL of 30 mg/L can lead to kidney damage. Manganese is clas sified as a secondary contaminant by the EPA, meaning that high concentrations do not lead to health issues, but can be an aes thetic nuisance. At concentrations higher than the 50 mg/L MCL it imparts a black-brown stain to water and plumbing fixtures and gives water a bitter, metallic taste. Interestingly, F is also a secondary contaminant as well as a primary contaminant, because at concentrations greater than the secondary MCL of 2.0 mg/L it can cause tooth discoloration. Geoenvironmental aspects of several of these elements are covered in USGS Professional Paper 1802,"Critical Mineral Resources of the United States— Economic and Environmental Geology and Prospects for Future Supply": lithium by Bradley and others, 2017; tantalum and nio bium by Schulz and others, 2017; beryllium by Foley and others, 2017; fluorine by Hayes and others, 2017; gallium by Foley and others, 2017; hafnium by Jones and others, 2016; and manganese by Cannon and others, 2017. 14.1. Soil and Sediment Signatures
Prior to Mining The most highly evolved LCT pegmatites are of great est interest for their geoenvironmental signatures because they have the most complex mineralogy and greatest enrichments in rare elements. There appear to be no environmental studies of an unmined LCT pegmatite that is both highly evolved and large enough to be economically viable. Boreal soils above the buried Tanco pegmatite have Li (at least 175 ppm) and Rb (at least 110 ppm) anomalies over the deposit (Trueman and Černý, 1982). The elements Li (20-170 ppm), Cs (0.64-15.7 ppm), Ta (0.2-4.4 ppm), and Sn (2-6 ppm) form a positive soil anomaly over the LCT pegmatite veins of the Little Nahani Pegmatite Group in the eastern Yukon Territory, Canada (Turner and Young, 2008). The subsurface, subeconomic Dibs pegmatite near Tanco is marked by moderate to strong positive soil anomalies in Mo, W, In, Sn, Mn, and Sr (concentrations from less than 10 to ca. 1,000 parts per billion), weak to moderate positive anomalies in Cl, Ga, Ge, Nb, Ta, and Li, and a number of other more complex geochemical signatures (Galeschuk and Vanstone, 2005). Scien tists studying the anomaly at Dibs pegmatite argued that a partial digest (using the Enzyme LeachTM) of upper, B-horizon soils is a better exploration tool than total soil chemistry, because it selec tively targets trace metals, which have migrated from ore bodies at depth to the soil surface, and which were trapped by adsorption to surfaces of soil minerals (for example, manganese oxides). The Enzyme LeachTM partial digest was also used to identify a posi tive Ta (at least 1.2 ppm) anomaly in the B-horizon of soils above the Aubry Pegmatite in northern Ontario (Dimmell and Morgan, 2005). As discussed in Section 13, lateritic soils surrounding the Greenbushes pegmatite define a multielement alteration halo with the following elements (concentrations in ppm): As (1,150), Sb (75), Sn (4,200), Nb (75), Ta (75), W (30), Li (100), B (500), and Be (60) (Smith and others, 1987). It is likely that a geochemical signature in soils and sedi ments surrounding a pegmatite would have been produced from weathering of the deposit, rather than from reactions between the pegmatite and its host rock during its emplacement. This is because interactions of the pegmatite body with its host rock are believed to be limited, because of rapid cooling and low heat content of pegmatite-forming magmas (Černý, 2012). Because the elements listed here are trace elements, it is predicted that adsorption and exchange reactions on clays and Fe-Mn oxides, and complexation by organic matter are the reactions that control elemental distribution and concentration in the solid phase. This statement can be supported by the partial leach studies cited in the previous paragraph, which show anomalous concentrations of indicator elements adsorbed to oxide surface in the soil B horizon. Li will also associate closely with clay minerals. This is illustrated by a large regional study of Li concentration in soils of the southeastern United States, where it was concluded that total soil Li correlated strongly with clay content, ranging from 3.74 to 59.93 mg/kg (Anderson and others, 1988). Cesium, too, is strongly adsorbed to clays (Stevenato and Vos, 1984; Heier and Billings, 1978); however, Ta may lack association with other pegmatite-associated elements (Stevenato and Vos, 1984). Values of pH, Eh, and organic content of soils and sediments surrounding the pegmatites will greatly affect the development of a soil and sediment signature. 14.2. Drainage Signatures from Mining
of LCT Pegmatites Little information is available on this topic. Rahn and others (1996) analyzed stagnant water in LCT pegmatite mine pits in the Black Hills and reported pH between 8.0 and 9.0, which is equivalent to the equilibrium pH of orthoclase (8) and
14.0. Geoenvironmental Features and Anthropogenic Mining Effects 33 plagioclase (8-10) when pulverized in distilled water. Traceelement abundances from waters draining LCT pegmatites, or LCT pegmatite mines, have not been documented. However in a related topic, groundwater in New Hampshire can be found in a fractured silicate aquifer in contact with unmined pegmatite intrusions. Arsenic in arsenopyrite and scorodite in the pegmatites is readily soluble and lends a geochemical signature to the groundwater of 2 to 400 mg/L As (Peters and Blum, 2003). 14.3. Climate Effects on Geoenvironmental
Signatures Lithium-cesium-tantalum pegmatites are located in diverse climatic settings. For example, the Khnefissat pegmatite (Mauritania) is in a hyperarid part of the Sahara Desert; Kings Mountain (North Carolina) has a warm temperate climate; Mt. Mica (Maine) has a cool temperate climate; and the Little Nahanni (Northwest Territories, Canada) is in a region of permafrost. No specific information is available on the geoenvironmental behaviors of LCT pegmatites in these or other climatic regimes. If the pegmatite body is exposed to weathering solutions at or near the surface of the Earth, then it is expected that a weathering "signature," such as a halo of pegmatite-associated trace elements in surrounding soils, would be more extensive in a warm and wet climate compared to one that is cold and dry. Because these trace elements are found associated with oxides and clays in a soil, the soil depth and extent of podzolisation will affect the distribution and retention of a geochemical signature. 14.4. Mining Methods Mining methods for LCT pegmatites depend on the structure and vertical extent of the pegmatite (Garrett, 2004). Historically, most pegmatites were mined by hand, either in shallow open pits or shallow underground workings. When labor was cheap and plentiful, hand cobbing was the preferred method of recovering enormous crystals of K-feldspar, muscovite, beryl, and spodumene. Modern, small-scale LCT pegmatite mines in the United States target gemstones and specimens, rather than rare-element ores, but otherwise the mining methods have not changed much. Artisinal surface mining is still done in areas of deep bedrock weathering of Africa (Melcher and others, 2008). Modern, industrial-scale mines of large LCT pegmatites include both open-pit and underground operations, depending on the size and attitude of the intrusive body or bodies. The open-pit mine at Greenbushes has worked steeply dipping pegmatites to depths of more than 200 m, with an anticipated final mining depth of 270 m (Fetherston, 2004). The Tanco pegmatite was mined by the underground room and pillar method. Rooms were 15-m-wide and pillars were one-half that size, with variable height matching the thickness of the pegmatite. This allowed for 89 percent ore recovery (Garrett, 2004). 14.5. Ore-Processing Methods Lithium-cesium-tantalum pegmatite ore-processing methods depend on the minerals being processed and the desired endproduct grade. Different lithium end uses require different degrees of processing of silicate ores. Large-scale mining operations employ crushing, grinding, and gravity separation techniques to refine the ore and prepare it for further processing (Hatcher and Elliot, 1986). Lithium-ion batteries require pure lithium compounds and refined lithium metal. During the past few decades, brines have mostly, but not entirely, supplanted pegmatites as sources of these commodities (Garrett, 2004). Today, most of the spodumene and petalite that is mined is used as feedstock for specialty ceramics. Various processing techniques have been used for spodumene, which is highly insoluble in its α form (LiAlSi2O6), and must be heated above 1,100 oC to change into β-spodumene (LiAlSiO4) (Garrett, 2004). For ceramics, crushed spodumene, petalite, and eucrypite concentrates are used directly. Some spodumene, however, can contain as much as a few weight percent Fe. This impurity must be removed in order to achieve ceramic grade concentrates, typically by means of early magnetic separation and late stage soda ash additions (Garrett, 2004). Petalite and eucryptite are both exceedingly pure and can be used to provide high quality ceramic concentrates relatively easily. A recent review by Bradley and others (2017) provides additional information about lithium mining. Tantalum ores are separated out by a series of gravity separation stages following crushing to sand size particles. This screening, regrinding, spiral concentration, and final separation using Falcon concentrators and shaking tables can produce a primary concentrate of roughly 17 percent Ta2O5, which is then secondarily processed to create a 40 percent concentrate (Fetherston, 2004). Tantalum processing is complicated by the similarity between the chemical properties of tantalum and niobium, which are separated chemically rather than by smelting (Fetherston, 2004). A recent review by Schulz and others (2017) provides additional information about tantalum mining. Virtually all of the cesium produced in the world is from LCT pegmatites. Cesium is produced from crushed and sorted pollucite by acid digestion (Butterman and others, 2004). 14.6. Metal Mobility from Solid Mine Waste Mine waste from LCT pegmatite operations consists primarily of environmentally benign potassium feldspar, plagioclase, quartz, and muscovite, which exhibit relatively slow rates of weathering. These phases are, however, known to release trace elements in greater-than-stoichiometric proportions (White and Brantley, 1995). The mining history of LCT pegmatites may affect the mineralogy of tailings and waste piles that remain. Tin was the original commodity of interest during early mining at Kings Mountain (Kesler, 1942, 1955), suggesting that lithium-bearing minerals may still remain in some of the early stockpiles. Mineral processing of the pegmatite minerals involves crushing, wet grinding, sieving, gravity concentration, flotation, and collection with
34 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites fatty acid amines. Tailings were discharged to storage ponds and decanted water was reused (Garrett, 2004). It is likely that lithium and other trace elements remained in tailings materials, but these elements may also have leached into the groundwater to be distributed within the local environment. Later, in 1969, mineral processing was changed to improve the recovery of lithium. The new process involved calcining (heating) the spodumene con centrate after collection, then leaching the calcine with hot water. This process allowed additional recovery of lithium, although some lithium was still lost. The efficiency of the overall process has improved over time. For example (Garrett, 2004), when the Lithium Corporation of America began processing spodumene ore from their South Dakota mines in the 1940s, they experi mented with an acid roasting/leaching process that provided an 80 percent yield on a 0.7 percent Li spodumene ore. This was a considerable improvement in efficiency, because prior to adopting the process, their method of flotation and leaching the concen trates provided an overall yield of only 38-43 percent Li. The unrecovered lithium was sent to tailings and waste piles. Another source of lithium to waste and tailings piles could be through dissolution of the lithium minerals in the grinding and sieving steps of processing. For example, spodumene is fairly insoluble in water and dilute acids, but a small amount of dissolu tion has been observed during ore processing and concentration (Aral, 2007, as referenced in Aral and Vecchio-Sadus, 2008). The dissolved lithium will be transported with the processing water, and may concentrate in tailings ponds with concentrations as high as 13 mg/L. One example of groundwater contamination by Li milling occurred at a Foote Mineral processing plant, 40 km northwest of Philadelphia, Pennsylvania. Wastes generated by the process ing of Li minerals (for example, lepidolite), Li metal, and the manufacture of Li halides leaked into the groundwater, which was discovered to contain concentrations of as much as 13 mg/L Li and 20 mg/L B. In 1992, the processing plant was added to the National Priorities List of the EPA, making it eligible for cleanup by the Superfund Program (Agency for Toxic Substances and Disease Registry, 2009; EPA, 1998). Bulk concentrations of uranium and thorium in LCT pegmatites are anomalously low. In Western Australia, however, processing of Ta ores generates enriched waste and concentrates containing from 7.5 to 75 kilobecquerel per kilogram (kBq/kg) of both uranium and thorium (Cooper, 2005). 15.0. Knowledge Gaps and
Future Research Directions Despite major strides in understanding of LCT pegmatites (London, 2005a, 2008, 2011, 2014; Simmons and Webber, 2008; Thomson and others, 2012), a key question remains: why are some orogenic belts well endowed with LCT pegmatites whereas others are poorly endowed? As mentioned in Section 4.1, con nections between LCT pegmatites and tectonics have yet to be unraveled at the level of detail that would bear on deposit genesis or aid in exploration. Specific causes of LCT pegmatite-related magmatism could include ordinary arc processes, overthickening of continental crust during collision or subduction, slab breakoff during or after collision, slab delamination during or after colli sion, late collisional extensional collapse and consequent decom pression melting, and (or) ridge subduction. To relate a particular LCT pegmatite field to events such as these will require precise geochronology for the pegmatite field and for the entire orogenic belt and its flanking foreland basin. Such an effort is now techno logically possible and only requires motivation and funding. Except for the anecdotal observations presented herein, no connections have been documented between LCT pegmatites and paleoclimate. Nonetheless, this is a fertile field for inquiry for all rock assemblages in orogenic belts. Synorogenic climate is hypothesized to have a major, permanent effect on the architec ture of a growing, critically tapered orogenic belt (Hoffman and Grotzinger, 1993; Koons, 1995). Where precipitation rates are low (for example, if the orogenic front faces away from prevail ing winds), erosion rates are slow and the orogen mainly grows laterally, the growth driven by plate convergence. Where precipi tation rates are high (for example, if the orogenic front faces into prevailing winds), erosion rates are high and orogenic growth, which would ordinarily result from plate convergence, is thwarted by erosion off the top. Thus, in leeward-facing orogens, uppercrustal rocks remain at the surface, whereas in windward-facing orogens, the top is eroded away and deeper-level crustal rocks are conveyed syntectonically to the surface, yielding a dramatically different metamorphic field gradient that is not merely an artifact of the erosional level. Many combinations of plate geometry and synorogenic climatic regime can be imagined. The implications for pegmatite genesis and exhumation remain to be investigated. Another possible controlling factor in LCT genesis is the geochemistry of metasedimentary rocks. The mineral chemistry of clays that are precipitating today in the oceans is controlled, to a large extent, by the climatic regimes on the adjacent continents: the deeply weathered tropics yield the aluminum clay, kaolinite, whereas glaciated sources at high latitudes yield the Al-Fe-Mg-Ca clay, chlorite (Griffin and others, 1968). This observation suggests that the endowment of certain trace elements (for example, Li and Cs) in ancient sedimentary successions might be controlled to some extent by paleoclimate. Because LCT pegmatites are mainly related to S-type granites, paleoclimate could help determine whether or not a particular suite of granites will produce LCT pegmatites. Another newly discerned control of protolith clay mineralogy is atmospheric composition (Hazen and others, 2013). During the Phanerozoic, times of high atmospheric O2 correlate with times of high relative abundance of chlorite and low relative abundance of kaolinite. These ideas could be explored through a synthesis of infor mation on metasedimentary country rocks of LCT pegmatites: paleolatitude (a proxy for paleoclimate) and depositional age. As an example of what might be useful on a worldwide basis, fig ure 18 shows inferred depositional paleolatitudes of the metasedi mentary host rocks of the northern Appalachian pegmatites, and emplacement paleolatitudes of the pegmatites themselves.
15.0. Knowledge Gaps and Future Research Directions 35 Cenozoic Mesozoic Paleozoic Cretaceous Jurassic Triassic Permian Devonian Ordovician Pennsylvanian Silurian Cambrian Mississippian Geologic time in millions of years before present (Ma)
Temperate wet belt Temperate wet belt Equatorial wet belt Subtropical dry belt Subtropical dry belt Clark Ledge Acadian orogeny Neo-Acadian orogeny Palermo #1 Mount Mica Taconic orogeny Alleghanian orogeny Late Pennsylvanian orogeny Paleolatitude in degrees -30 -60 -90 Clark Ledge Mount Mica Paleolatitude path of a location along the Laurentian continental margin at the Quebec-Vermont border, interpolated between blue control points Preferred age and permissible age range of lithium-cesium-tantalum pegmatite and identifier Orogeny age range Age of lithium-cesium-tantalum pegmatite and identifier EXPLANATION Figure 18. Paleolatitude (vertical axis) plotted against time (horizontal axis) for selected northern Appalachian pegmatites. The ages of lithium-cesium-tantalum (LCT) pegmatites are shown by red diamonds; for two pegmatites, the diamond shows the preferred age (after Bradley and others, 2016) and the adjacent red line segment shows the permissible age range. The brown, zigzag line tracks the paleolatitude of a point along the Laurentian continental margin at the Quebec-Vermont border, that is, near these pegmatites. Small blue symbols along the paleolatitude line indicate control points. For the Paleozoic, the control points were derived from continent-scale maps by Witzke and Heckel (1988), which are based on climatically sensitive sediments; for the Mesozoic and Cenozoic, the control points were obtained from maps by Scotese (1997), which are based on paleomagnetism and marine magnetic anomalies. The variation in paleolatitude is a consequence of the north-south component of continental drift, first southward, then northward. Relevant latitude-related climate zones are shown in green (equatorial and temperate wet belts) and yellow (subtropical dry belts). This plot reveals that the Palermo #1 and Mt. Mica pegmatites were emplaced in dry belts whereas the Clark Ledge, if the preferred age is correct, was emplaced in a wet belt.
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Appendix. Grade-Tonnage Data and Plots 47 Appendix. Grade-Tonnage Data and Plots Table A1. Grade-tonnage data for lithium in lithium-cesium-tantalum (LCT) pegmatites. Location Tonnage (million metric tons Grade (percent Li2O) Source Bernic Lake (Tanco) (Manitoba, Canada) Jaskula, 2010; Garrett, 2004, Betterman and others, 2005; U.S. Geological Survey, 2011; Selway and others, 2005; Sweetapple, 2000 Separation Rapids, Ontario, Canada Sweetapple, 2000 Georgia Lake, Ontario, Canada Breaks and others, 2008 Lacorne, Quebec, Canada ://finance.yahoo.com/news/acquisition-four-lithium-projects-preissac-120000979., accessed July 28, 2016 Whabouchi Quebec, Canada ://www.marketwatch.com/story/nemaska-files-the-notice-of-project-before-the-comevfor-the-whabouchi-deposit-and-provides-update-on-work-in-progress-2011-09-19-86200, accessed July 28, 2016. (Value for tonnage is measured plus indicated.) Bikita, Zimbabwe Jaskula, 2010; Garrett, 2004 Pilgangoora, Australia Sweetapple, 2000 Greenbushes, Australia ://www.talisonlithium.com/docs/content-documents/15-jun-2011-greenbushes-lithiumoperations-report-ni-43-101.pdf, accessed July 28, 2016 Mount Marion, Australia ://www.neometals.com.au/lithium.php, accessed July 28, 2016 Foote, North Carolina Garrett, 2004 Table A2. Grade-tonnage data for tantalum in lithium-cesium-tantalum (LCT) pegmatites. Location Grade (percent Ta2O5) Tonnage (metric tons) Source Bernic Lake (Tanco) (Canada) Fetherston, 2004 Big Whopper (Canada) Fetherston, 2004 Lac Du Bonnet (Canada) Fetherston, 2004 Thor Lake (Canada) Fetherston, 2004 Morrua (Mozambique) Fetherston, 2004 Muriane (Mozambique) Fetherston, 2004 Marropino (Mozambique) Fetherston, 2004 Tantalite Valley Fetherston, 2004 Uis—Three Aloes (Namibia) Fetherston, 2004 Uis—B1 and C1 Prospects (Namibia) Fetherston, 2004 Eagle (Zimbabwe) Fetherston, 2004 Donsa (Zimbabwe) Fetherston, 2004 Dove 14 (Zimbabwe) Fetherston, 2004 Wanroo (Zimbabwe) Fetherston, 2004 Nanping (China) Fetherston, 2004 Rosendal (Finland) Fetherston, 2004 Forcarey Sur (Spain) Fetherston, 2004 Labelle (Australia) Fetherston, 2004 Alwa South (Australia) Fetherston, 2004 The Bounce (Australia) Fetherston, 2004 Mount Alwa (Australia) Fetherston, 2004 Pilgangoora (Australia) Fetherston, 2004 Tabba Tabba (Australia) Fetherston, 2004 West Wodgina (Australia) Fetherston, 2004 Wodgina (Australia) Fetherston, 2004 Bald Hill (Australia) Fetherston, 2004 Binneringie (Australia) Fetherston, 2004 Cattlin Creek (Australia) Fetherston, 2004 North Ravensthorpe (Australia) Fetherston, 2004 Dalgaranga (Australia) Fetherston, 2004 Niobe (Australia) Fetherston, 2004 Greenbushes (Australia) Fetherston, 2004 Mount Deans (Australia) Fetherston, 2004 Mt. Weld (Australia) Fetherston, 2004 Yalgoo (Australia) Fetherston, 2004
48 Mineral-Deposit Model for Lithium-Cesium-Tantalum Pegmatites Lithium Grade in weight percent of lithium oxide (wt% Li2O) Ore, in million of tons Figure A1. Grade-tonnage plot for lithium in lithium-cesiumtantalum (LCT) pegmatites, based on data in table A1. Tantalum 1,000 Grade in weight percent of tantalum pentoxide (wt% Ta2O5) Ore, in million of tons Figure A2. Grade-tonnage plot for tantalum in lithium-cesiumtantalum (LCT) pegmatites, based on data in table A2.
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