A preliminary report on the geology of the volcanic-rock-hosted Barite Hill gold deposit, Carolina slate belt
Text and copies of slides for a paper presented at the meeting of the Southeastern Section of the Geological Society of America in Winston-Salem, North…
Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.
U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY A Preliminary Report on the Geology of the Volcanic-Rock-Hosted Barite Hill Gold Deposit, Carolina Slate Belt Sandra H.B. Clark , David D. Greig , and Norman L. Bryan Open File Report 92-305 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code). Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1 USGSRestonVA Nevada Goldfields, Inc., 1801, Suite, 1620, Denver, Colorado 80202 Consultant, 2 Partridge Court, Aiken, South Carolina 29802
A PRELIMINARY REPORT ON THE GEOLOGY OF THE VOLCANIC-ROCK- HOSTED BARITE HILL GOLD DEPOSIT, CAROLINA SLATE BELT Sandra H.B. Clark, U.S. Geological Survey, MS 954, Reston, Virginia, 22092 David D. Greig." Nevada Goldfields, Inc., 1801, Suite, 1620, Denver, Colorado, 80202 and Norman L. Bryan, Consultant, 2 Partridge Court, Aiken, South Carolina 29802 Text and copies of slides for a paper presented at the meeting of the Southeastern Section of the Geological Society of America in Winston-Salem, North Carolina, March 19, 1992. SLIDE 1 Location in Slate Belt. The Barite Hill mine is located in the southern part of the Carolina slate belt of South Carolina just north of the Georgia border. SLIDE 2 Mine scene. Barite Hill went into production early in 1991, with open-pit mining and heap-leach extraction. SLIDE 3 Background. Exploration has been active in the area since the mid-1970's, first for base metals, later for barite, and then for precious metals. Detailed geologic maps of the area and descriptions of other deposits were done at the University of Georgia mostly between 1977-82. Exploration work for Amselco from 1985-88 resulted in excellent maps and reports of Barite Hill. A small-scale study of Barite Hill was initiated by the U.S. Geological Survey in 1989, shortly after the property and data were purchased by Gwalia. The paper we are presenting today incorporates and summarizes some of the ealier work and gives preliminary results of the new work. SLIDE 4 Summary. This is a summary of the main features of the deposit. Reserves are estimated to be 1.658 million tons of 1.3 g/t Au and 6.2 g/t Ag in the oxidized zone. Plans are for mining only the oxizided material because the amount of pyrite in unoxidized material. Host rocks are mainly sheared Lower Cambrian felsic volcanics with greenschist facies metamorphism. Schistosity and bedding dip steeply to the northwest; bedding is slightly overturned. Mineralized zones are lenticular with a pyritic low gold stage followed by a cross cutting high gold, low base-metal stage. There is widespread sericitic alteration. Gold occurs preferentially in fragmental rocks and is and associated with quartz and pyrite+barite. Gold and silver occur in native form and electrum and as tellurides with silver also as selenides. The mineralized zones are cut by a shear zone and NW-trending faults. SLIDE 5 Regional geologic map. The Barite Hill mine is about 21/2 miles south of the Dorn mine at McCormick and is in the Lincolnton-McCormick district, which includes several gold and base metal mines and prospects on the flanks of the Present address, Gwalia Group, 16 Parliament Place, West Perth, Western Australia 6005
Lincolnton metadacite. The district also includes aluminum-silicate rich zones, such as the Graves Mountain kyantite mine and andalusite that outcrops near the Searl's pig farm mine. The Barite Hill deposit is in the unit locally referred to as the felsic pyroclastic sequence, which lies between the Lincolnton metadacite and an upper sedimentary sequence composed mainly of argillites. The district is in greenschist facies metamorphic rocks of the slate belt northwest of the Modoc zone. The rocks we'll describe today have greenschist facies mineral assemblages, but the prefix "meta-" has been omitted on most of the names to make the slides more legible. SLIDE 6 Correlation Chart. The Lincolnton metadacite has been dated as early Cambrian and is thought to be comagmatic with the felsic pyroclastic sequence, and may be equivalent to the Persimmon Fork and Uwharrie Formations, although there are questions about the ages, especially for the Albemarle Group and Uwharrie Formation. SLIDE 7 Mine location. The Barite Hill mine consists of two areas, the main pit area and Rainsford South. Detailed mapping of the felsic pyroclastic sequence by Bill Gunter allowed differentiation of units of predominantly volcanic origin from units that are predominantly clastic and defined map patterns that suggest folds. Schistosity is commonly developed both parallel to bedding and about 5 to 10° from the bedding, with fabrics and orientations that suggest a relationship of schistosity to folding and shearing. SLIDE 8 Barite Hill, main pit area. This is the detailed map of the lithologies in main pit area, from mapping by Joel Padgett and Dave Watkins from both drill core and trenches. The units dip steeply to the northwest, and where tops are found, they indicate overturned bedding, which is consistent with the regional interpretation of the section becoming younger to the southeast. The felsic volcanic unit is typically composed of fine grained sericitic metatuffs commonly with quartz eyes, but also includes coarse-grained tuffs and breccias. The sedimentary unit includes medium- to coarse-grained epiclastic rocks that are typically chloritic and contain a variety of clasts including feldspar, quartz, and lithic material. There are also very fine grained argillites and siliceous layers that commonly occur at the tops of volcaniclastic units and above mineralized lenses and could include exhalites and vitric tuffs. Mineralization is bounded on the south by a sill or stock of feldspar porphyry with plagiogclase and minor quartz phenocrysts in a fine-grained light green matrix. The unit is probably dacite, but is here called a feldspar porphyry, until more detailed petrograhic and chemical work is done. Numerous fine-grained mafic rocks are also present in section. Most are probably dikes or sills, but some have amygdaloidal tops and may be flows. Lenses of sulfide-rich rock, mostly pyrite, some places with basemetals, show up as gossan-like material at the surface. Quartz-barite rocks in which the protolith is obliterated, shown here in pink, are a conspicuous feature of the deposit, SLIDE 9 Barite outcrop. ...and form prominent outcrops on the southeast side of the hill. SLIDE 10 Map of main pit area with fragmental rocks and shear zone.
Two other important features of the geologic setting are a shear zone northwest of the porphyry and zones of fragmental rocks. The fragmental rocks generally have hydrothermal overprinting, but probably include pyroclastic and tectonic breccias. Note also, the late northwest trending faults that cut all of the units. SLIDE 11 Map of main pit area with mineralized zones. The mineralized zones outlined by Amselco work are lenses that plunge gently to the northeast and cut contacts, with the most intense mineralization being roughly within the quartz-barite zone, but not confined to it. Gold values generally drop off in the metasedimentary rock and at the shear zone, and are displaced by the northwest trending faults, suggesting that both the shear zone and faults were active after mineralization. SLIDE 12 Rainsford South. Relationships are similar in the Rainsford South zone, with the major differences being narrower mineralized zones, the absence of the quartzbarite rock, and a greater divergence of mineralized zones from regional lithologic trends. SLIDE 13 Stages of mineralization. Two main stages of gold mineralization have been identified: an earlier low gold and a later high gold stage. There is widespread sericitic alteration, and the high gold mineralization is usually in complex cross-cutting veinlets and is associated with quartz and pyrite with or without barite. There is also base-metal mineralization without gold that is local and specially associated with the feldspar porphyry. Repeat SLIDE 11. Next, we will look at some drill core. I have chosen BHD 6 because it near the edge of a mineralized lens and has more of the protolith preserved and less overprinting than the more intensely mineralized areas. SLIDE 14 Log of BHD 6. BHD 6 goes through about 150 feet of fragmental rocks below the oxidized zone, then through a pyritic layer, and then into fine-grained metasedimentary rocks, with grading suggesting that tops of the beds are toward the bottom of the drill hole. Then there is a coarse clastic zone, the shear zone, seen as intensely sheared metasedimentary rocks, finally, the feldspar porphry. Quartz-barite veins cut both the volcanic and sedimentary rocks. SLIDE 15 BHD 6-52. This is fragmental rock from a depth of 52 feet. It is a polymictic matrix-supported breccia, with clasts of mostly volcanic material. It includes a silicious, pyritic fragment. The white spots are sericite. SLIDE 16 BHD 6-59. At about 59 feet, the fragments are larger and pyrite-quartz stringers start appearing in the matrix, SLIDE 17 BHD 6-75.5. ...but also cutting clasts and earlier veinlets. The rounding of the clasts suggests a hydrothermal origin. SLIDE 18 BHD 6-82. Some of the clasts are very large and include rock fragments, and the matrix here is completely replaced by pyritic veinlets. SLIDE 19 BHD 6-132. Deeper in the hole, coarse bladed barite-quartz veins cut pyritic stringers. SLIDE 20 BHD 6-154. Between 150 and 165 feet the rock is predominantly thinly laminated pyrite with quartz and barite. SLIDE 21 BHD 6-162. Above the pyrite zone, are fine-grained rocks with graded bedding,
SLIDE 22 BHD 6-166. ...that grade into rocks that contain rip-up clasts of the finegrained beds. Further up in the section are intensely sheared rocks, and feldspar porphyry. SLIDE 23 Feldspar porphyry thin section. This is a thin section of the feldpar porphyry in polarized light, showing subhedral plagioclase in a fine-grained matrix that has a schistosity that wraps around the felspar phenocrysts. This field of view is 2.2 mm in the long direction. SLIDE 24 BHD 6 with chemistry. Next we will look at the distribution of metals and barium for BHD 6. High-gold zones are shown in dark pink, and low gold in light pink. One high gold zone occurs near the surface and another is associated with the fragmental rock that contains pyrite-quartz stringers. The high gold zones are surrounded by a low gold zone that ends at the contact of the pyrite zone with metasedimentary rocks. The high base-metal zone in the clastic rocks adjacent to the porphyry is probably the local base-metal mineralization that does not contain gold. High silver values are associated with both the gold and base-metal mineralization. The most abundant barite in this hole is in the low gold zone, but quartz-barite veins also cut metasedimentary rocks. SLIDE 25 Whole thin section of pyritic zone. Judy Back has begun to do detailed petrographic studies on the Barite Hill rocks. This is the entire thin section in plane light in which we searched for gold using a scanning electron microscope. This thin section is from a rock with abundant pyrite stringers in the Rainsford South zone. Several distinct areas can be defined by textures. The area numbered 1 is least affected by shearing and silicification, and may have been a lithic clast. The area numbered 2 shows a well developed shear schistosity. SLIDE 26 Area 2~Polarized light. This is the fabric of area 2 under polarized light, showing the strong preferred orientation defined by aggregates of pyrite (black) and muscovite. The field of view is about 2.2 mm in the long dimension. SLIDE 27 Area 2 Reflected light. This is the same area in reflected light, showing elongation of pyrite along schistosity and muscovite cutting pyrite grains, suggesting that this pyrite was pre- to syntectonic. Repeat of SLIDE 25. The areas numbered 3 are coarser grained segregations or veinlets that contain more abundant quartz that invades and engulfs the pyritemuscovite schistosity. The red square outlines the area of the next slide, which is near the end of an area 3 veinlet. SLIDE 28 SEM. This is an SEM photo of the area in the square. The shades are the opposite here as on the previous slide. The black is quartz, the dark gray blades are muscovite, white is pyrite, and the small bright spot in the square contains Au. The next slide will be a closer view of the area in the square. SLIDE 29 SEM Closer view. This is the gold-bearing grain, surrounded by quartz. And now, zooming in on the area in the red square, SLIDE 30 SEM Closest. A mottled texture can be seen. The grain is a Au-Ag telluride, probably sylvanite, with Au only in the larger and slightly brighter end of the grain. SLIDE 31 Area 3 with gold grain crossed nicols. This is the area that contained the gold viewed under crossed nicols, 2.2 mm in the long dimension, showing at the tip of
the arrow, the spot identified as gold on SEM, which is at the boundary of quartz grains. SLIDE 32 Cu mineral, SEM. In other parts of the slide we found Ag, Pb, and Cubearing minerals generally in quartz-pyrite rich areas. This SEM photo shows typical relationships. The light gray mineral is a Cu-Fe-As-S mineral with minor zinc and antimony that has crystallized in intergranular area of subhedral pyrite crystals. SLIDE 33 Mineralogy. This is a list of minerals from Barite Hill that were reported by Gunter and Padgett. They found gold in its native state and in sylvanite and electrum. Silver is present as native silver and in electrum, argentite, galena, hessite, and complex selenides and tellurides. Other minerals include base-metal sulfides and Cu-Pb-Bi tellurides and selenides. Repeat of SLIDE 25. There also quartz overgrowths that show up here as white spots and also occur in area 3 overprinting the strained quartz that is associated with the gold mineralization. SLIDE 34 Late barite, thin section. In rocks with coarse-grained barite, barite crystals engulf gold-bearing quartz and pyrite suggesting that barite crystallized after gold. This slide is 2.2 mm in the long direction. SLIDE 35 Supergene barite. Barite also formed much later as crystals in cavities in the oxidized zone along with iron oxides, suggesting supergene dissolution and reprecipitation. SLIDE 36 Conclusions. This is a summary of some tentative interpretations of the features we have described. Mineral assemblages and alteration are consistent with an epithermal environment. The gold that we have seen is most commonly in veinlets cutting hydrothermally altered breccias, with veinlets so abundant in some places that they obliterate the protolith. The gold is generally associated with quartz and pyrite that show the effects of tectonism, plus or minus barite, which is later in the paragenesis. Gold is most commonly in intergranular areas or microfractures. The sequence of tectonic events that we are beginning to unravel, suggests an order of events in which volcanism is followed by deformation that produced penetrative schistosity and shearing. This was followed by precipitation of gold from hydrothermal fluids either as a late phase of the major schistosity-producing event, or as a later event, which was followed by movement along shears and high-angle faults.
1. Barite Hill 2. Ridgeway 3. Halle 4. Brewer KINGS MOUNTAIN1 BELT KIOKEE BELT SLIDE 2 SLIDE 1 SUMMARY BACKGROUND University of Georgia, Athens (1977-82) Robert Carpenter Gilles Allard Martin Fay Thomas Biggs Verner Guthrie Travis Paris Amselco (1985-88) Douglas Duskin William Gunter Joel Padgett David Watkins
Present Study (1989-present) SLIDE 3 Reserves -- 1.658 MT of 1.3 g/t Au and 6.2 g/t Ag Host rocks -- Lower Cambrian felsic metavolcanics Greenschist facies metamorphism Steep dips to NW, overturned Lenticular mineralized zones Early high base metal-low gold stage followed by cross-cutting high gold-low base metal stage Au zone preferentially in fragmental rocks Au is intergranular, associated with pyrite, quartz ± barite
Au and Ag -- native, in electrum, sylvanite, tellurides, selenides i, Mineralized zone cut by shear and faults SLIDE 4
N ALBEMARLE-DENTON- ASHEBORO AREA, N.C. 33M5 War 'Graves Mountat L Upper sedimentary sequence Felsic pyroclastic sequence
1 High-grade metamorphic rock tt Mine t Anticlinai axis i Synclinal axis / '
,- Adapted from Carpenter, 1972; Secor, 1987; and Butler and Secor, 1991 fcLIDE 5 Albemarle Group Yadkin Graywacke McManus Fm. Title ry Fm. laminated argllllte Jwharne Fm. felsic crystal, llthic tuffs, welded lulls, Hows LINCOLNTON, GA.,- C( McCORMICK, S.C.
AREA - (Sedimentary Sequence Felsic .Sequence Rb-Sr 553 unnri" U-Pb 580 (2) Metadacite Rb'Sr 554 ± 20 1 J From Carpenter a DLUMBIA, S.C AREA lichtex FrrH Persimmon Branch Fm. "Wiidfiorse Branch nd othera, 1982 Bedding parallel to foliation BARITE HILL-MAIN PIT AREA SLIDE 7 Quartz-barlte rock Feldspar porphyry Mafic to intermediate volcanic rocks Sedimentary rocks Felsic volcanic rocks From W.L. Gunter, 1986, unpublished mapping Quartz-barite rock Sulfide-rich rock Mafic dike or sill Feldspar porphyry Sedimentary rock Felsic volcanic rock unpublished mapping by Joel P.dgett and David Watklnt (1988) SLIDE 8
SLIDE 9 BARITE HILL-MAIN PIT AREA Quartz-barite rock Sulfide-rich rock Mafic dike or sill Feldspar porphyry Sedimentary rock Felslc volcanic rock °g°° Fragmental rock /// Shear zone 100 M From unpublished mapping by Joei Padgett and David Watkins (1988) SLIDE 10
BARITE HILL-MAIN PIT AREA High Au (av ppm) Quartz-barite rock Sulfide-rich rock Mafic dike or sill Feldspar porphyry Sedimentary rock SLIDE 11 J? Fragmental rock /// Shear zone 100 M From unpublished mapping by Joel Padgett and David Watkins (1988)
BARITE HILL-RAINSFORD SOUTH 50 M Sulfide-rich rock Mafic dike or sill Feldspar porphyry Sedimentary rock Felsic volcanic rock 0° Fragmental rock Shear zone <j§)High Au (av >1ppm) From unpublished mapping by Joel Padgett and David Watkins (1988) SLIDE 12
STAGES OF MINERALIZATION Early -High base metal-low gold Pyrite and locally base-metal sulfides with quartz + sericite ± barite Late -- High gold-low base metal Locally intense quartz-barite alteration Complex cross cutting veinlets with pyrte and volumetrically minor base-metal sulfides, tellurides, and sulfosalts BHD6 100ooooo o O o ° ooooo 2 0 o oooo ,0 0.000 o & p op tOC? 01 ® 00 O O O O ,0 O .O O e o O O, o Q n O Q O' a o o P c SLIDE 14 Pyritic 'stringers Fragmental felsic volcanics Quartzbarite veins Pyrite zone Sedimentary Feldspar porphyry
Co Ljj Q
Co Co Co Co
Slide 19 Slide 20
Co Oj
PARTS PER MILLION BHD6 Feldspar porphyry Sedimentary rock
LOGIC vein Pyrite zone Felsic volcanic rock '# Fra9m*ntal rock //// Shear zone GRAPHS High gold (Av 1 ppm) Low gold (Av~.3 ppm) SLIDE 24
Slide 25
Slide 26 Slide 27
Slide 28 Slide 30 Slide 31 Slide 29
SLIDE 32 MINERALOGY Au Native gold Au Sylvanite (Au,Ag) Te4 Electrum AuAg Cu Chalcopyrite CuFeS2 Chalcocite Cu2S Bornite Cu-Pb-Bi tellurides and selenides Ag Native silver Ag Electrum AuAg Argentite Ag2S Galena PbS Hessite Ag2Te Complex selenides and tellurides Pb Galena PbS . Zn Sphalerite ZnS From Gunter and Padgett, 1988 SLIDE 33
SLIDE 34 CONCLUSIONS A. Epigenetic environment B. Location of gold Veinlets cutting hydrothermally altered breccias Associated with tectonic quartz, pyrite ± later barite Intergranular and in fractures Relative timing Volcanism Penetrative schistosity 3. Au-bearing hydrothermal fluids and tectonism Faulting SLIDE LIDE 35