Illinois Creek

Mine, Active

Commodities and mineralogy

Geographic location

Quadrangle map, 1:250,000-scale NL
Quadrangle map, 1:63,360-scale A-4
Latitude 64.04
Longitude -157.91
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Illinois Creek mine is in the southern Kaiyuh Mountains, about 9 miles south of the top of Khotol Mountain. It is on hill 810, in the SW1/4 sec. 4, T. 17 S., R 5 E., Kateel River Meridian. The coordinates are for the approximate midpoint of a 6000-foot-long mineralized shear zone that trends east-west. The location is accurate within 500 feet.

Geologic setting

Geologic description

The Illinois Creek deposit is in the southwestern part of the Ruby Geanticline in the Kaiyuh Mountains. The Kaiyuh Mountains are composed of highly deformed Paleozoic (or older?) pelitic schist, along with lesser amounts of quartzite, metacarbonate, and greenstone. K-Ar ages of schist in the Illinois Creek region date the regional deformation and metamorphism of these rocks at about 150 Ma. The Cretaceous Khotol pluton is exposed approximately 12 km north of the Illinois Creek mine; it is composed of granite and granodiorite (Flanigan, 1998).
The Illinois Creek deposit is an oxidized shear zone hosted in metasedimentary rocks. This shear zone trends east-west for at least 6000 feet and dips about 60 degrees south. The deposit is thoroughly oxidized, and contains anomalous to ore-grade concentrations of gold, bismuth, silver, copper, lead, zinc, arsenic, antimony, iron, and manganese (Flanigan, 1998). The eastern part of the deposit contains most of the supergene pyrolusite, hematite psuedomorphs of pyrite, goethite, and rhombohedral limonite. Limestone and dolomite are prominent in this part of the deposit. Gold is concentrated in the central part of the deposit, in association with hydrothermal quartz containing pyrite casts. This quartz is heavily oxidized. The western margin of the deposit contains lesser amounts of iron-stained quartz but greater amounts of massive, supergene iron minerals. Supergene copper mineralization also increases to the west. A secondary gold anomaly is in the western part of the deposit (Flanigan, 1998).
Although the Illinois Creek deposit is deeply oxidized and very few primary sulfides remain, microscopic sulfides can be identified. Pyrite is the most abundant sulfide, and most of the other primary ore minerals occur either as inclusions in pyrite or on pyrite grain boundaries. Other ore minerals include sphalerite, chalcopyrite, arsenopyrite, tetrahedrite, electrum, native bismuth, bismuthinite, stannite, tetrahedrite, stibnite, boulangerite, and galena. Some trench samples revealed zones of relatively unaltered sulfides, including pyrite, chalcopyrite, arsenopyrite, and covellite, as well as some probably supergene minerals such as acanthite and native copper (Flanigan, 1998). Anaconda Minerals Company also reports native silver, argentojarosite, plumbojarosite, arsenobrakenbushite, cornwallite, delafossite, and todorokite (Flanigan, 1998).
The fineness of gold and electrum at Illinois Creek is highly variable and ranges from 800 to greater than 980. The richer electrum grains typically are subrounded and relatively homogeneous, while the electrum grains of medium fineness are usually irregularly shaped and inhomogeneous, and have gold-enriched rims. Gold grains are porous, spongy, irregularly shaped, and have only been identified using secondary electron imaging. These gold grains occur in supergene iron oxides, iron hydroxides, malachite, and azurite. The within-grain variations in fineness and the gold-enriched rims of electrum grains suggest silver leaching and gold-enrichment during weathering and supergene processes (Flanigan, 1998).
Although syngenetic origins for the deposit have been proposed, Flanigan (1998) conclusively demonstrates that a plutonic-related epigenetic model best fits its characteristics. K-Ar and Ar/Ar dates show that the mineralization and the intrusion of the Khotol pluton are contemporaneous (~ 113 Ma), supporting Flanigan's interpretation that the Khotol pluton is probably the source of mineralizing hydrothermal fluids at Illinois Creek.
The Illinois Creek mine began production in 1997. Currently (2001), the American Reclamation Group, LLC, is conducting minor mining while reclaiming the site.
Geologic map unit (-157.912436650126, 64.0393446192752)
Mineral deposit model Plutonic-related, epigenetic Au-Ag
Age of mineralization An Ar/Ar date on sericite from a mineralized vein in the Illinois Creek deposit yielded a mineralization age of 113 Ma, roughly coeval with the 108-133 Ma Khotol pluton (Flanigan, 1998).
Alteration of deposit The sulfides of the Illinois Creek deposit have been highly oxidized, with supergene enrichment of several metals.

Production and reserves

Workings or exploration The Illinois Creek mine was discovered by Anaconda Minerals Company in 1980. By 1989, Goldmor had completed exploration of the area (Bundtzen and others, 1990). From 1991 through 1992, North Pacific Mining Company leased the area and completed over 3000 meters of drilling and extensive trenching (Bundtzen and others, 1992; Swainbank and others, 1993). During 1993, Echo Bay Alaska explored Illinois Creek using ground-based geophysical methods, trenching, and reverse-circulation drilling (Bundtzen and others, 1994). In 1994, USMX began permitting for the Illinois Creek mine and completed a mine feasibility study (Swainbank and others, 1995). The first gold was poured by Dakota Mining Corporation (the parent company of USMX) in June of 1997 (Swainbank and others, 1998). Gold production in 1998 was less than anticipated, and USMX filed for bankruptcy (Szumigala and Swainbank, 1999). There was no production in 1999, although Viceroy Resources drilled 23 drill holes as part of a due-diligence program (Swainbank and others, 2000). Currently, the American Reclamation Group, LLC, is conducting small-scale mining while reclaiming the Illinois Creek mine (Ken Pohle, oral communication, 2001).
Indication of production None
Reserve estimates In 1997, the geologic resource was estimated to be 7,761,000 tons of ore grading 0.063 ounce of gold per ton and 1.38 ounces of silver per ton (Lamborn, 1997).
Production notes In June of 1997, Dakota Mining Corporation poured the first gold from the Illinois Creek mine. Total gold production for 1997 was 20,111 ounces (Swainbank and others, 1998). Production for 1998 was 29,998 ounces of gold and 155,235 ounces of silver. In 1998, the operating company, USMX filed for bankruptcy (Szumigala and Swainbank, 1999). There was no production in 1999 (Swainbank and others, 2000). Currently (2001), the American Reclamation Group, LLC, is conducting small-scale mining while reclaiming the Illinois Creek mine (Ken Pohle, oral communication, 2001).

References

MRDS Number A013472

References

Flanigan, B., 1998, Genesis and mineralization of ore deposits in the Illinois Creek region, west central Alaska: University of Alaska, Fairbanks, M.Sc. thesis, 125 p., 2 plates.
Lamborn, J., 1997, Illinois Creek geology and exploration potential: American Reclamation Group, LLC, internal report, 4 p.
Reporters C.E. Cameron (Northern Associates Inc.)
Last report date 8/7/2001