USGS Professional Paper 1662, Chapter 8

Studies by the U.S. Geological Survey in Alaska, 2000 U.S. Geological Survey Professional Paper 1662 Environmental-Geochemical Study of the Slate Creek Antimony Deposit, Kantishna Hills, Denali National Park and Preserve, Alaska By Robert G. Eppinger, Paul H. Briggs, James G. Crock, Allen L. Meier, Stephen J. Sutley, and Peter M. Theodorakos Abstract TDS contents. Waters in Slate Creek immediately below the mine exceed drinking-water standards for As, Fe, Mn, and We undertook a reconnaissance environmental-geochemi- Sb contents. At the mouth of Slate Creek, about 1.8 mi (2.9 cal investigation in 1998 at the Slate Creek antimony deposit km) downstream from the mine, only Mn and Sb contents in Denali National Park and Preserve, Alaska. This cooperative exceed drinking-water standards. Interestingly, stream- and study between the U.S. Geological Survey and the National spring-water samples collected from areas unaffected by the Park Service focused on identifying and characterizing envi- minesite also exceed drinking-water standards for pH value 2− ronmental geochemical signatures associated with this histori- and SO4 , As, Fe, Mn, Sb, and TDS contents. Aquatic-life cal mining area. The deposit, which is hosted by quartzite, standards are exceeded only by minesite waters for As, Cd, quartz-mica schist, marble, and greenstone, is a locally brec- Cu, Ni, and Zn contents. ciated fissure quartz vein system containing stibnite and lesser Elements measured in highly anomalous concentrations in pyrite, stibiconite, kermesite, boulangerite, cervantite, and stream-sediment, rock, and soil samples collected outside the arsenopyrite. About 800,000 lb (362,400 kg) of Sb was pro- minesite include As, Mn, and Sb. Mine waste and mineralized duced from 1905 through 1983. outcrops at the minesite have highly anomalous As and Sb Samples collected include water, stream-sediment, rock, contents. Stream-sediment samples downstream from the min- and soil samples from 13 sites upstream of, downstream eralized area have highly anomalous As, Mn, and Sb contents. from, and at the mineralized area. Stream-water samples The low pH value and predominance of sulfate in collected outside the disturbed mining area were bicarbon- mine-related waters are due largely to the dissolution of ate dominant and had near-neutral pH values, conductivities pyrite. Stream waters downstream from the minesite have ranging from 130 to 300 μS/cm, alkalinities ranging from major-anion compositions that reflect mixing of bicarbonate- 60 to 130 mg/L, and total-dissolved-solids (TDS) contents dominant, near-neutral pH waters with mineralized, acidic, of as much as 316 mg/L. Two samples of naturally mineral- sulfate-dominant waters. Metallic ions abundant in the mine- ized spring water collected outside the disturbed mining area related waters are quickly coprecipitated with Fe oxide flocs had pH values of 5.6 and 7.7, conductivities of 300 and 750 upon mixing with bicarbonate waters, resulting in heavily Fe μS/cm, alkalinities of 57 and 180 mg/L, and TDS contents of oxide-stained alluvium downstream from the deposit. Ionic 230 and 544 mg/L. One of the springs was sulfate dominant, species in high concentrations that remain mobile at higher 2− and the other mixed sulfate-bicarbonate dominant. Surface pH values, such as Mn, Sb, SO4 , and Zn, remain in solution waters draining the minesite proper were sulfate dominant downstream. and had pH values ranging from 2.7 to 5.8, conductivities The presence of natural springs within the mineralized ranging from 770 to 1,700 μS/cm, alkalinities ranging from 0 area but outside the disturbed mining area suggests premin- to 110 mg/L, and TDS contents ranging from 671 to 993 mg/ ing spring waters that naturally ranged from acidic to neutral L. Stream waters from Slate Creek below the mine and min- in pH value, had relatively high conductivities and varyingly eralized area were mixed sulfate-bicarbonate dominant and high TDS contents, were Fe rich, and were metalliferous. Red- had near-neutral pH values, conductivities ranging from 260 stained alluvial gravel along Slate Creek, natural Fe oxide-rich to 280 μS/cm, alkalinities ranging from 63 to 78 mg/L, and springs near the head of the Slate Creek basin, and ferricrete TDS contents of less than 250 mg/L. Metallic-ion concentra- deposits probably were present before, and aided prospectors tions in stream waters all steadily decreased with distance in, the discovery of the Slate Creek antimony deposit. How- from the deposit. Parameters measured in waters draining ever, the naturally degraded waters that were likely present, as the minesite that exceeded established Alaska drinking-water well as the extent of iron staining along Slate Creek, have cer- 2− standards include pH value and SO4 , As, Fe, Mn, Ni, Sb, and tainly both been worsened by historical mining activities. Environmental-Geochemical Study of the Slate Creek Antimony Deposit, Kantishna Hills, Denali National Park and Preserve, Alaska 123 Introduction The fieldwork was conducted over a 1-week period from August 9 to 15, 1998. Summer 1998 was an unusually wet We undertook a reconnaissance environmental-geochemi- period, with numerous rainstorms occurring throughout park cal investigation in the Slate Creek drainage of Denali National during the season (Ken Karle, oral commun., 1998). As a Park and Preserve in 1998. Surface-water, bedload-stream-sedi- result, springs issued from numerous points in and around the ment, rock, and mine-waste samples were collected from the Slate Creek antimony deposit, and surface flow was observed Slate Creek antimony deposit near the head of the drainage and in all intermittent drainages. at intervals downstream along Slate Creek to its confluence with The Slate Creek antimony deposit is located in the Eldorado Creek, about 1.8 mi (2.9 km) downstream. Histori- northwestern part of Denali National Park and Preserve, cally, the Slate Creek antimony deposit has also been referred about 5 mi (8 km) southwest of Kantishna and about 6 to as the “Antimony” or “Taylor mine” (Capps, 1918; Cobb, mi (10 km) due west of Wonder Lake (fig. 1). Low, roll- 1972; Dames & Moore, 1992). The purpose of this cooperative ing, heavily vegetated hills typify the topography of the study with the National Park Service (NPS) was to characterize Kantishna Hills in the vicinity of the Slate Creek antimony the geochemistry of the mineralized area, to identify potential deposit (fig. 2A). Elevations range from about 2,100 ft (640 environmental-geochemical hazards, and to determine baseline m), at the confluence of Slate and Eldorado Creeks, to 3,774 levels for selected elements. The focus of this report is on the ft (1,150 m), on Brooker Mountain immediately north of the hydrogeochemical media, the principal avenue for movement of deposit. Access to the area is by foot along an old tractor metals from rocks and sediment into the aquatic biota. trail from Kantishna. �� Figure 1. Denali National Park and Preserve, south-central Alaska, showing location of the Slate Creek antimony deposit 124 Studies by the U.S. Geological Survey in Alaska, 2000 Environmental-Geochemical Study of the Slate Creek Antimony Deposit, Kantishna Hills, Denali National Park and Preserve, Alaska 125 In 1997, the NPS recontoured slopes at the Slate Creek antimony deposit, filling in pits and open cuts with mine waste. Just before this study in 1998, the NPS installed rubber dams within the recontoured fill to confine and redirect ground-water flow to a plastic drainpipe (fig. 2B). Ground water from the drainpipe was intended to flow into a buried anoxic carbonate-filtration mechanism to neutralize the acidic water. At the time of this study, the carbonate-filtration mechanism was under construction, and the drainpipe was not yet connected to it (Ken Karle, oral commun., 1998). Thus, the water samples collected at the minesite for this study were pretreatment samples. Geologic Setting A The earliest geologic and mineral-deposit descriptions of the Kantishna region date from during and immediately after the early 1900s’ Gold Rush to interior Alaska (Prindle, 1907; Brooks and others, 1909, p. 56; 1910, p. 44; 1912, p. 38; 1913, p. 45; 1914, p. 68; Brooks, 1916). More extensive studies of the region were conducted by Capps (1918), Davis (1922), Moffit (1933), and Ebbley and Wright (1948). The most extensive recent studies of the geology and mineral deposits of the Kantishna Hills area were by Bundtzen (1978, 1981, 1983, 1994) and Bundtzen and Turner (1978). Mineral-resource reports by the U.S. Bureau of Mines include those by Hawley (1978), Jeske (1984), Fechner and Hoekzema (1986), and White and others (1986). Basement rocks in the Kantishna Hills area are regionally metamorphosed, range in age from Precambrian to late Paleo- zoic, and consist of four principal rock units. The oldest and most aerially extensive rocks are Precambrian

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