Results of automated scanning electron microscope (SEM) analyses of rock and stream sediment samples from the Taurus porphyry copper deposit area, Tanacross quadrangle, eastern Alaska
<p>Numerous porphyry copper-molybdenum-gold and epithermal deposits define a belt that extends from Eastern Alaska to western Yukon, Canada. An orientation…
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U.S. Department of the Interior U.S. Geological Survey Open-File Report 2022–1046 Results of Automated Scanning Electron Microscope (SEM) Analyses of Rock and Stream Sediment Samples from the Taurus Porphyry Copper Deposit Area, Tanacross Quadrangle, Eastern Alaska
Results of Automated Scanning Electron Microscope (SEM) Analyses of Rock and Stream Sediment Samples from the Taurus Porphyry Copper Deposit Area, Tanacross Quadrangle, Eastern Alaska By Karen D. Kelley, Katharina Pfaff, and Garth E. Graham Open-File Report 2022–1046 U.S. Department of the Interior U.S. Geological Survey
U.S. Geological Survey, Reston, Virginia: 2022 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: Kelley, K.D., Pfaff, K., and Graham, G.E., 2022, Results of automated scanning electron microscope (SEM) analyses of rock and stream sediment samples from the Taurus porphyry copper deposit area, Tanacross quadrangle, eastern Alaska: U.S. Geological Survey Open-File Report 2022–1046, 12 p., ://doi.org/10.3133/ofr20221046. Associated data for this publication: Kelley, K.D., Graham, G.E., and Peterson, M.L., 2020, Geochemical data for stream water and stream sediment samples from the northeast part of the Tanacross quadrangle, Alaska: U.S. Geological Survey data release, ://doi.org/10.5066/P94KBWD3. ISSN 2331-1258 (online)
Acknowledgments We thank Kristian Price and Sarah Bala (U.S. Geological Survey) for doing all the sieving and Wilfley table separates. Kelsey Livingston (Colorado School of Mines) helped prepare many of the pucks used for automated SEM analyses. We thank Bronwen Wang (U.S. Geological Survey) and Kelsey Livingston (Colorado School of Mines) for their helpful reviews.
Contents Figures
1. Map of Alaska showing location of the Yukon-Tanana Upland region in the Tanacross quadrangle in eastern Alaska, along with the locations of Copper and lode gold deposits, the distribution of major faults, and location of Jurassic
2. Topographic map showing locations of stream sediment samples along with
4. A, overview backscatter electron images of one mineralized and B, one background stream sediment sample highlighting the distribution and
5. Phase map of particles containing svanbergite in sediment sample 17TCIM001,
Conversion Factors International System of Units to U.S. customary units Multiply By To obtain Length centimeter (cm) inch (in.) millimeter (mm) inch (in.) meter (m) foot (ft) kilometer (km) mile (mi) kilometer (km) mile, nautical (nmi) meter (m) yard (yd) Area square kilometer (km2) acre square kilometer (km2) square mile (mi2) Mass gram (g) ounce, avoirdupois (oz) kilogram (kg) pound avoirdupois (lb) metric ton (t) ton, short (2,000 lb) metric ton (t) ton, long (2,240 lb) Grams per metric ton (g/t) Ounces per metric ton Density kilogram per cubic meter (kg/m3) pound per cubic foot (lb/ft3) gram per cubic centimeter (g/cm3) pound per cubic foot (lb/ft3) Energy electron volt (eV) kiloelectron volt (keV) Abbreviations BSE backscattered electron EDX energy dispersive X-ray PCIM porphyry copper indicator minerals SEM scanning electron microscope Cu copper Au gold Mo Molybdenum Ag Silver Pb Lead Zn Zinc
Results of Automated Scanning Electron Microscope (SEM) Analyses of Rock and Stream Sediment Samples from the Taurus Porphyry Copper Deposit Area, Tanacross Quadrangle, Eastern Alaska Karen D. Kelley,1 Katharina Pfaff,2 and Garth E. Graham1 Abstract Numerous porphyry copper-molybdenum-gold and epithermal deposits define a belt that extends from Eastern Alaska to western Yukon, Canada. An orientation study conducted near the Taurus porphyry deposit was designed to test methods that require minimal sample collection, preparation, and analytical time to determine the viability of indicator mineral studies as a reconnaissance exploration method. Bulk stream sediments and altered and mineralized rocks were sieved to the 0.105−0.25 millimeter fraction (+140, −60 mesh) and passed over a shaking table to create a moderate to heavy mineral separate that was mounted in epoxy and subsequently analyzed using automated scanning electron microscope (SEM) techniques. Seven polished thin sections of core were also analyzed. Among the advantages of automated SEM techniques compared to visual mineral identification are that thousands of grains can be rapidly identified in each sample (about 1 hour per sample) and small quantities of indicator minerals that may be missed during traditional visual analyses can be detected. Automated SEM analyses of stream sediment and rock samples show that specific minerals (chalcopyrite, bornite, and jarosite) are indicators of potential mineralized areas. Svanbergite, an aluminum sulfate phosphate mineral, was identified in mineralized rocks and in nearly all stream sediment samples (up to 9 kilometers) downstream from the Taurus and other porphyry occurrences but not epithermal occurrences. It was not identified in areas with no known mineralization and thus it is possibly one of the best indicator minerals for porphyry copper (+/- molybdenum, gold) occurrences. 1U.S. Geological Survey. 2Colorado School of Mines. Introduction The presence of specific minerals in a sample of surficial material (stream sediment, soil, glacial till) may indicate proximity to a mineralized area, and therefore, can provide a useful tool for exploration or mineral resource assessments. For example, specific minerals have been identified in surficial materials that are indicative of diamonds (McClenaghan and Kjarsgaard, 2001; 2007), gold (Au) and platinum group element deposits (McClenaghan and Cabri, 2011), and numerous other deposit types. Minerals such as chalcopyrite, molybdenite, bornite, gold, and other copper sulfide minerals like chalcocite or covellite may specifically indicate the presence of porphyry copper (Cu) deposits. These are called porphyry copper indicator minerals (PCIM). Methods for the recovery and identification of PCIMs from glacial sediments for porphyry Cu exploration have been tested in the glaciated terrain of Canada (for example, Averill, 2001, 2011; Chapman and others, 2015, 2018; Hashmi and others, 2015; Plouffe and others, 2016; Pisiak and others, 2017; Mao and others, 2016; Plouffe and Ferbey, 2017, 2019) and Alaska (Kelley and others, 2011; Eppinger and others, 2013), and one recent study in stream sediment samples has been completed near the Casino deposit in Yukon, Canada (McClenaghan and others, 2020; 2022). Most of the published PCIM studies have used heavy mineral concentrate samples (derived from till or sediment) that require labor intensive magnetic and heavy liquid separa tions, followed by mineralogical determinations made visually using a binocular microscope. In this paper, we present a method we tested in eastern Alaska near the Taurus and Bluff porphyry copper deposits within the Tanacross quadrangle in the Yukon-Tanana Upland region (fig. 1). The method includes collection and preparation of bulk sediments from active streams with minimal sample collection and reduced preparation time. We also used recently developed automated techniques for determining mineralogy. Automated techniques rapidly yield results for an enormous number of minerals. The advantages of automated SEM techniques compared to visual mineral identification of indicator minerals (physical evidence
2 Results of Automated Scanning Electron Microscope Analyses of Samples from Eastern Alaska of the presence or absence of mineralization or alteration) are numerous and include: (1) small abundances of indicator minerals are detectable that may be missed during traditional visual analyses. The mere presence of a few indicator mineral grains in a sample may indicate a region that warrants more detailed examination and sampling; (2) indicator mineral grains can be subsequently analyzed to determine chemistry, which provides information about the nature of the mineraliz ing system; (3) minerals relatively lighter than heavy mineral concentrates produced by heavy liquid separations typically occur in alteration halos surrounding ore zones, and these minerals are detected by automated SEM techniques. Such alteration halos in the case of porphyry deposits may extend for many kilometers away from the core of the deposit; and (4) automated SEM techniques allow for the identification of mineral intergrowth textures, such that a matrix or assemblage of indicator minerals can be defined that might be specific to a deposit type. This study represents one of the first indicator mineral studies to utilize stream sediment samples and pro cessing without heavy liquid or magnetic separations. Some mineral exploration companies are utilizing similar methods (Agnew, 2015) but details of the results are typically not publicly available. In addition to the indicator mineral work presented here, stream sediment geochemistry (Kelley and others, 2020) and hydrogeochemistry (Kelley and Graham, 2021) results are also available. 175°E 175°W 180° 175°E 175°W 180° 51° 53° 51° A I S L A N D S L E U T I A N 70°N 65°N 60°N 55°N 50°N 175°E 170 E 175°W 180° KENAI PENINSULA A L A S K A P E N I N S U L
Y u k o n
R e r CHUKCHI SEA KOTZEBUE SOUND BERING STRAIT NORTON SOUND BERING SEA BRISTOL BAY GULF OF ALASKA PACIFIC OCEAN PRINCE WILLIAM SOUND SEWARD PENINSULA RUSSIA KODIAK ISLAND B R O O K S Valdez Nome
Anchorage Fairbanks R A N G E PEBBLE PEBBLE WHISTLER WHISTLER FORT KNOX FORT KNOX POGO DONLIN DONLIN BOND CR/ORANGE HILL BOND CR/ORANGE HILL Juneau Ketchikan Mt. Churchill Denali fault Tintina fault COFFEE CASINO TAURUS CANADA UNITED STATES Tok TANACROSS QUADRANGLE TANACROSS QUADRANGLE 65°N 180°W 60°N 175°W 55°N 120°W 135°W 150°W 165°W 180°W 500 KILOMETERS 300 MILES Porphyry deposit Lode gold deposit Paleogene igneous rocks Cretaceous igneous rocks Yukon-Tanana Upland Jurassic igneous rocks EXPLANATION Base from US Geological Survey 1:16,000,000-Scale digital data NAD 1983 Alaska Albers Figure 1. Map of Alaska showing location of the Yukon-Tanana Upland region in the Tanacross quadrangle in eastern Alaska, along with the locations of Copper (+/- molybdenum, gold) and lode gold deposits, the distribution of major faults, and location of Jurassic Period, Cretaceous Period and Paleogene Period igneous rocks. The Taurus deposit and others in the immediate region of Taurus are similar to the Casino deposit in Yukon, Canada.
Methods 3 Characteristics of Mineralized and Altered Rocks in the Taurus Region There are at least three mappable porphyry systems in the Taurus area (East and West Taurus, Bluff, and Dennison) representative of multiple pulses of mineralization span ning approximately 6 million years (Kreiner and others, 2020). All are associated with Late Cretaceous Period igne ous rocks (fig. 1). Other poorly described porphyry systems include Oreo, Pushbush, and Baggage (fig. 2). Several silver (Ag)–Au–Cu (+/- lead [Pb], zinc [Zn]) occurrences north of Taurus are described as possible porphyry (Gill, 1977) or epithermal deposits of assumed younger (Late Cretaceous to early Tertiary Period) age (Gill, 1977; U.S. Geological Survey, 2008). These include the Pika Canyon, NE Pika Canyon, South Pika, and Fishhook occurrences (fig. 2). All porphyry deposits in the Taurus region are associated with plutons characterized by small stocks, plugs, and dike swarms of predominantly quartz monzonite, granodiorite, and granite, and local syenite bodies (Kreiner and others, 2020). The Taurus deposit has two main mineralized centers: West Taurus and East Taurus, with a combined inferred resource of 68.3 million metric tons (Mt) at 0.275 percent Cu, 0.032 percent molybdenum (Mo), and 0.166 grams per metric ton (g/t) Au (Harrington, 2010; Lasley, 2018). At East Taurus, the supergene and hypogene Cu–Mo–Au mineralized zones are overlain by an approximately 50 meter (m) thick leach cap characterized by oxidation and argillic alteration (Harrington, 2010). Hypogene mineralized rocks include chalcopyrite and molybdenite with weak gold enrichment, and distal galena ± sphalerite zones. The Bluff occurrence has been drilled, but detailed information is not available. Tourmaline-rich, sericitepyrite alteration and tourmaline breccia pipes are abundant across the Taurus, Bluff, and Dennison localities (Kreiner and others, 2020). Arsenic-rich pyrite and arsenopyrite have been noted in select samples. Methods The purpose of this study was to test methods that require minimal sample collection, preparation, and analytical time to determine the viability of indicator mineral studies as a recon naissance exploration method. The sampling method, sample preparation, and analytical method are described below. Stream Sediment Collection and Preparation We collected 8–10 kilograms (kg) of bulk sediment from 47 stream sites within an area of 1,150 square kilometers (km2), including mineralized drainages and those distal to any known deposits (fig. 2). At each site, we collected specifi cally from stream locations most likely to contain moderate or heavy minerals, such as point bars, gravel bars, behind and under large boulders or between cobbles; however, streams in the area vary from slow moving with mostly mud to silt sized material to high flow velocity streams with abundant coarse material. Samples were screened to less than 10 mesh (<2 mm) and air dried and sieved in the laboratory to the 0.105–0.25 mm fraction (+140, –60 mesh sieve sizes). This size fraction was chosen based on previous studies that showed this approximate fraction provided optimal results for automated mineralogy (Wilton and Winter 2012; Wilton and others, 2017). Smaller grain size (approximately <0.100 mm) samples required longer analytical times (30–50 percent longer) and the minerals were more difficult to definitively identify. Larger grain sizes (approximately greater than 0.250 mm) resulted in a significant decrease in the number of analyzed particles in a grain mount and the range of observ able intergrowth textures decreased. The 0.105–0.25 mm size fraction was further processed using a shaking table (also called a Wilfley table) to separate lighter from denser minerals; our specific gravity threshold between heavy and light minerals using the shaking table is about 2.6–2.8. This process is designed to minimize the occur rence of the lighter minerals (for example, quartz and feldspar), and it optimizes collection of minerals of intermediate to high density or dense-light intergrowths. Heavy liquid or magnetic separation techniques were not used in the processing. The resultant concentrate for each sample was then partitioned using a splitter to produce an approximate 0.5 grams (g) separate that was mounted in a 2.5 centimeter (cm) diameter epoxy mount and the hardened puck was polished (fig. 3). Rock Sample Collection and Preparation Seven samples were collected from approximately 15–20 cm interval of drill core from the Taurus deposit; there are two samples representative of the leach cap, two of the supergene, and three hypogene mineralized zones. General descriptions of the leach cap, supergene, and hypogene zones are provided in Harrington (2010). One unmineralized igneous outcrop sample was collected and processed in the same manner as the sediment samples. Each of these samples were crushed and sieved to 0.25–0.105 mm fraction and processed using the Wilfley table, like the method described for stream sediment samples. In addition, seven polished thin sections of hand samples from core were included in the automated mineral ogy to obtain in situ textural relations and mineral associations. Identifying the prominent indicator minerals in each of the rock samples is a critical component for comparison to the stream sediment analyses. Sample Analysis Automated scanning electron microscopy (auto mated mineralogy) was done in the Mineral and Materials Characterization Facility at the Colorado School of Mines, Golden, Colo. Samples were loaded into the TESCANVEGA-3 Model LMU VP-SEM platform and analysis was
4 Results of Automated Scanning Electron Microscope Analyses of Samples from Eastern Alaska CANADA UNITED STATES
! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !! ! ! ! ! ! ! ! !
18TCIM047 18TCIM018 18TCIM023 18TCIM020 18TCIM021 18TCIM054 18TCIM053 18TCIM039 18TCIM048 18TCIM049 18TCIM038 18TCIM037 18TCIM052 18TCIM050 18TCIM051 18TCIM036 EAST TAURUS PIKA CANYON SOUTH PIKA; FISHHOOK NE PIKA CANYON BAGGAGE 17TCIM001 17TCIM003 17TCIM016 17TCIM015 17TCIM017 18TCIM044 18TCIM043 18TCIM042 18TCIM046 18TCIM041 18TCIM040 18TCIM029 18TCIM045 17TCIM011 17TCIM012 18TCIM032 17TCIM010 18TCIM030 17TCIM002 WEST TAURUS 17TCIM014 17TCIM013 17TCIM004 17TCIM005 17TCIM008 17TCIM009 18TCIM057 18TCIM058 18TCIM060 17TCIM007 DENNISON BLUFF PUSHBUSH 18TCIM056 18TCIM059 OREO 63°40’0”N 63°50’0”N 141°40’0”W 141°20’0”W 141°00’0”W 63°30’0”N 63°20’0’N Base from U.S. Geological Survey digital data, 1:400,000 Geographic projection, North American Datum of 1983
! Stream sediment location
Porphyry occurrence or deposit Epithermal occurrence or deposit EXPLANATION 5 KILOMETERS 3 MILES ALASKA Map area Figure 2. Topographic map showing locations of stream sediment samples along with porphyry and epithermal deposits in the study area.
Results 5 initiated using the control program TIMA (Tescan, 2012). Four energy dispersive X-ray (EDX) spectrometers acquired spectra in liberation analysis mode from each point with a user defined beam stepping interval (in other words, spacing between acquisition points) of 5 microns for backscattered electron (BSE) brightness and 15 microns for semiquantitative EDX spectroscopic analyses, an acceleration voltage of 25 kilo electron volts (keV), and a beam intensity of 14. Interactions between the beam and the sample are modelled through Monte Carlo simulation. The EDX spectra are compared with spectra stored in a look-up table allowing a mineral or phase assign ment to be made at each acquisition point. The assignment makes no distinction between mineral species and amorphous grains of similar composition. Results are output by the TIMA software as a spreadsheet giving the area percent, mass per cent, number of mineral grains or percent of mineral grains of each composition in the look-up table. This procedure allows a compositional map to be generated. Composition assignments were grouped appropriately. Results This report is designed to provide basic results (app. 1, table 1.1) and BSE images of pucks (fig. 4) that highlight select minerals. The results of the TIMA analyses in table 1.1 are orga nized by mineral category (oxides, silicates, sulfides, sulfates, phosphates, and tungstates). The reported number of particles or grains of each mineral is given in addition to the total number of grains in the puck. Most studies utilizing heavy mineral concentrate samples from stream or till sediments are designed to significantly minimize quartz and other rock forming miner als, but this is less important with automated SEM techniques because each puck or thin section yields results of tens of thousands of grains (table 1.1). Thus, although some samples contain significant amounts of quartz (greater than 20 percent of total grains in the sample) after sieving and processing with Wilfley, many contain less than 10 percent (table 1.1). The actual number of grains of a specific mineral is not as important in most cases as the presence or absence of a given mineral, especially ore minerals (sulfides) or those associated with them (for example, sulfates or tungstates). For exam ple, 1–2 grains of chalcopyrite in one sample compared to 7–8 grains in another is not necessarily significant. The mere identification of chalcopyrite in a sample is indicative of its presence in bedrock upstream, and confidence in interpretation is strengthened if other copper sulfides, such as bornite, are associated with chalcopyrite in the sample. Indicator Minerals in Bedrock The indicator mineral analytical results of core (samples collected from approximately15–20 cm and processed like sediments, and hand sample polished thin sections) show all samples of mineralized and altered material, with the exception of one leach cap sample, contain pyrrhotite, pyrite, and chalco pyrite, with some samples containing grains of bornite, chal cocite, covellite, galena, molybdenite, and sphalerite (table 1.1). Tourmaline is present primarily in quartz-sericite altered porphyry, leach cap, and the hypogene and supergene zones. 25 millimeter 25 millimeter A B Figure 3. A and B, examples of prepared puck for TIMA analyses.
6 Results of Automated Scanning Electron Microscope Analyses of Samples from Eastern Alaska 5 millimeter 5 millimeter 5 millimeter 5 millimeter 5 millimeter 17TCIM016 Rutile 17TCIM001 Apatite 17TCIM001 Svanbergite 17TCIM001 Rutile A. Mineralized sediment B. Background sediment 17TCIM016 Apatite Figure 4. A, overview backscatter electron images of one mineralized (17TCIM001) and B, one background (17TCIM016) stream sediment sample highlighting the distribution and abundance of select minerals (apatite, top images highlighted in yellow; rutile, middle images in pink; and svanbergite, bottom images in green). Note svanbergite does not occur in the background sediment samples.
Results 7 Altered monzonite and some mineralized rocks contain topaz. Tungstate minerals (scheelite and wolframite) are present in a few altered or mineralized samples. Jarosite was identified in all altered or mineralized rock samples and svanbergite, an alumi num phosphate sulfate (APS) mineral [SrAl3(PO4)(SO4)(OH)6], was identified in some of the altered or mineralized samples from Taurus, but not relatively unmineralized rocks (table 1.1), and in that sense may represent one of the best indicator miner als of Taurus-like porphyry mineralization. Other minerals such as rutile, apatite, zircon, and sphene (titanite) are present in all rock samples (table 1.1); therefore, their presence alone is not significant as indicator minerals, but chemistry of these has been shown to distinguish mineralized and altered rocks compared to barren intrusive rocks (Scott, 2005; Celis, 2015; Mao and others, 2016). As expected, most sulfide minerals except pyrite are lacking or occur in small quantities in the leach cap samples. Supergene zone samples contain notably more covellite and chalcocite grains than most other rock types, except for one hypogene sample that contains abundant chalcocite and covel lite along with chalcopyrite and bornite (table 1.1). Indicator Minerals in Stream Sediment Samples Based on the presence or absence of known mineralized rocks in each stream sediment drainage basin, the sediment samples are characterized as background (no known occur rences in drainage basin) or mineralized (occurrence is known; Cameron, 1999; U.S. Geological Survey, 2008). For example, sediment samples from McCord Creek that drains the Taurus deposit and other unnamed sites downstream from the Bluff, Dennison, and Oreo porphyry-style occurrences, and those near the epithermal Pika Canyon, NE Pika Canyon, South Pika, and Fishhook occurrences (fig. 2) are distinguished as mineralized sediments (table 1.1). Background sediments dif fer because they are from streams with no known occurrences in the region; however, many sediments contain grains of sul fide minerals that could reflect upstream mineralized bedrock yet to be identified. Minerals observed in rocks but not sediments include molybdenite and covellite and only a few sediment samples contain chalcocite. Bornite is present in small amounts in many samples also contain relatively abundant chalcopyrite (table 1.1). Svanbergite is present in many of the mineralized stream sediment samples but is lacking in those classified as background. Scheelite and wolframite are present in some mineralized sediments, and relatively abundant in one sedi ment classified as background, which also contains a few grains of chalcopyrite, suggesting potential for mineralized rocks upstream. All mineralized sediment samples except for three contain one or more grains of base metal sulfide minerals (galena, sphalerite, or copper sulfide minerals), supporting the idea of using indicator minerals in sediment samples is a valid tool for exploration. For example, all samples from McCord Creek, even those 9 km downstream, contain evidence of the upstream Taurus deposit (table 1.1; fig. 2). Jarosite is also variably abundant in mineralized sediments. Although many background sediment samples are barren of sulfides (except pyrite) as might be expected, many do contain zinc- or leadrich base metal sulfide minerals (for example, 1–2 grains of sphalerite or galena). Puck-Scale Mineral Maps One of the most powerful aspects of automated SEM techniques is BSE images may be generated for entire pucks. This is important for identifying specific minerals of interest because they show the spatial distribution within a given puck. Figure 4 includes puck-scale BSE maps for apatite, rutile and svanbergite for select rock and sediment samples. Apatite and rutile are common indicator minerals in porphyry deposits in general, and svanbergite, as stated above, is observed to be diagnostic of porphyry deposits in our study area. These com positional maps were generated to show the overall quantity and distribution of each mineral in select samples so future mineral chemistry studies may be conducted. The puck-scale maps allow easy navigation to specific grains for subsequent microanalytical analysis. Another powerful tool provided by TIMA is the ability to visualize mineral associations and grain size distribution. Figure 5 is an example of svanbergite grains in sediment sample 17TCIM001, located immediately downstream from the Taurus deposit. Phase maps show grain size distribution of select minerals in any given puck, as well as mineral associa tions. For example, in sample 17TCIM001, svanbergite grains range in size from less than 100 micrometers (µm) to many hundreds of microns, and the dominant associated minerals are quartz and aluminum silicate minerals, most likely pyrophyl lite or dickite (fig. 5). Similar phase maps can be generated for rutile and apatite, with the aim of illustrating some grains are associated with sulfide minerals (possibly indicating hydro thermal origin), and some with minerals such as magnetite or ilmenite (possibly indicating igneous origin). Such informa tion is valuable for distinguishing multiple sources of grains in the sediments. Mineral chemistry studies using microanalyti cal techniques such as electron microprobe and laser ablationinductively coupled mass spectrometry (LA-ICP-MS) are planned for the future.
8 Results of Automated Scanning Electron Microscope Analyses of Samples from Eastern Alaska 100 micrometer EXPLANATION Svanbergite Quartz Pyrophyllite or dickite Unknown Figure 5. Phase map of particles containing svanbergite in sediment sample 17TCIM001, showing grain size distribution and mineral associations. Svanbergite is associated primarily with quartz and aluminum silicate minerals (most likely pyrophyllite or dickite). Each grain of svanbergite can be flagged using the TIMA software and labeled on overview backscatter electron images, making it easy to locate grains for future detailed scanning electron microscopy or other microanalytical work.
References Cited 9 Conclusions Indicator mineral studies of stream sediment samples using automated scanning electron microscope techniques is a relatively new tool that is useful in exploration and min eral resource assessments. Our orientation study conducted near the Taurus porphyry deposit in eastern Alaska shows the 0.105–0.25 millimeter fraction of bulk stream sediments and altered or mineralized rocks contain specific minerals are indicators of potential mineralized areas, specifically sulfide (chalcopyrite, bornite) and sulfate minerals (jarosite). Svanbergite, an aluminum sulfate phosphate mineral, was identified in mineralized rocks and in nearly all downstream stream sediment samples (up to 9 kilometers downstream) from the Taurus and other porphyry occurrences. Other miner als such as apatite and rutile occur in all sediment samples, so their presence alone is not significant, but the chemistry of these minerals may distinguish hydrothermal versus other sources. The automated mineralogy results serve as a founda tion for future mineral chemistry studies using microanalytical techniques such as electron microprobe and laser ablationinductively coupled mass spectrometry (LA-ICP-MS). References Cited Agnew, P.D., 2015, Micro-analytical innovation for indi cator mineral exploration in McClenaghan, B., and Layton-Mathews, D., Application of indicator mineral methods to exploration: Tucson, Ariz., April 18, 2015, 27th International Applied Geochemistry Symposium Short Course No. 2, p. 11, accessed month day, year, at ://www.appliedgeochemists.org/images/Explore/ 27thIAGS-ShortCourse2.pdf. Averill, S.A., 2001, The application of heavy indicator mineralogy in mineral exploration with emphasis on base metal indicators in glaciated metamorphic and plutonic terrains, in McClenaghan, M.B., Bobrowsky, P.T., Hall, G.E.M., and Cook, S.J., eds., Drift exploration in glaciated terrains: Geological Society, London, Special Publication 185, p. 69–81. 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Results of Automated Scanning Electron Microscope Analyses of Samples from Eastern Alaska Appendix 1. Results of TIMA Analyses The results of the TIMA analyses in table 1.1 are organized by mineral category (oxides, silicates, sulfides, sulfates, phosphates, and tungstates). Table 1.1. Number of particles of select indicator minerals in rock and stream sediment samples from the Taurus region. Bulk processed rock and stream sediment size fraction is 0.105 to 0.25 millimeters. [Table is available as a comma separated values (csv) format file for download at ://doi.org/10.3133/ofr20221046. For locations, see figure 2 and Kelley and others, 2020. Ilm, ilmenite; FeOx, iron oxides and hydroxides; Cr, chromite; Cor, corundum; Spn, sphene; Tur, tourmaline; Sp, sphalerite; Aspy, arsenopyrite; , stibnite; Bn, bornite; Ccp, chalcopyrite; Cct, chalcocite; Cv, covellite; Mol, molybdenite; Po, pyrrhotite; Jrs, jarosite; Alu, alunite; Sv, svanbergite; Mnz, monazite; Sch, scheelite; Wlf, wolframite; PTS, polished thin section; SS-Mz, stream sediment, mineralized; Cr, creek; SS-Bkg, stream sediment, background; Bkg, background] Reference Cited Kelley, K.D., Graham, G.E., and Peterson, M.L., 2020, Geochemical data for stream water and stream sediment samples from the northeast part of the Tanacross quadrangle, Alaska: U.S. Geological Survey data release, ://doi.org/10.5066/ P94KBWD3.
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