Koksetna, Chilikat East

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu; Pb; Zn
Other commodities As; F; Sb; Sn
Ore minerals arsenopyrite; chalcopyrite; galena; pyrite; tetrahedrite
Gangue minerals kaolinite; quartz; sericite; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale LC
Quadrangle map, 1:63,360-scale B-5
Latitude 60.4713
Longitude -154.8076
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Koksetna prospect is located in the hills between the Koksetna and Chilikadrotna Rivers approximately 20 miles northwest of Lake Clark. The center of the prospect is on the northeast side of the valley above 2500 foot elevation at the headwaters of a tributary of the Koksetna River in the northeast 1/4 of section 3, T. 4 N., R. 32 W. of the Seward Meridian. The location is accurate within 200 meters.

Geologic setting

Geologic description

The area between the Koksetna and Chilikadrotna Rivers is underlain by undivided Kuskokwim Group sedimentary rocks and the Koksetna River sequence of Wallace and others (1989) that have been intruded by Cretaceous to Tertiary quartz monzonite plutons and rhyolite to dacite plugs, sills, dikes and flows (Wilson and others, 2006).
The Koksetna prospect is an altered and mineralized pipe-like breccia zone approximately 750 meters diameter cutting siltstones of the Koksetna River sequence. Several siliceous breccia units and a quartz-eye rhyolite porphyry along with wall rock clasts have been noted in the breccia pipe (Scott and Ellis, 1982).
Hydrothermal alteration at Koksetna is related to the intrusive rocks as well as a final stage of supergene leaching (Scott and Ellis, 1982). Prospect alteration is laterally restricted to the composite pipe and a 200 foot peripheral zone of spotty propylitiztion in the wall rock. This is due to the non-reactive nature of the siltstones, and their low permeability. The first alteration event accompanied the intrusion of quartz-eye rhyolite porphyry which has a plug-like geometry. The vast majority of the exposed porphyry is argillically altered to kaolinite with minor sericite and montmorillonite. Argillization is generally restricted to the porphyry groundmass with occasional replacement of sanidine phenocrysts by kaolinite. Small zones of quartz-sericite alteration were observed in central portions of the porphyry. Here the porphyry texture is totally obliterated and replaced by a hard, silica-rich skeleton. Crystal vugs are commonly lined with orange-yellow jarosite, probably from the weathering of sulfide. Intrusion of the rhyolite porphyry also propylitized the wall rock siltstone, andesite, and greywacke, creating localized conversion to chlorite, epidote, illite, and secondary biotite (Ellis and others, 1985).
At least two and possibly three separate pulses of mineralization have been identified at Koksetna. An early pulse of 1 to 5 percent sulfide with a high silver to copper ratio is believed to have been originally disseminated in the quartz-eye rhyolite porphyry. A possible second pulse of minor sulfide mineralization accompanied the intrusion of silica breccia. The third and final pulse of sulfide mineralization is represented by fracture-filling veins that average 2 to 5 millimeters in thickness and appear to cut all lithologies visible within the near periphery of the Koksetna breccia pipe. The quartz veins contain iron oxides, rare chalcopyrite and tetrahedrite. Local stockwork fracture zones, frequently healed by quartz, tourmaline, or sulfide veinlets, occur within the wall rock along a 100 meter margin of the intrusive contact (Ellis and others, 1985).
Geologic map unit (, )
Mineral deposit model Polymetallic veins (Cox and Singer, 1986; model 22c).
Mineral deposit model number 22c
Age of mineralization Mineralization is probably related to 60.5 to 61.6 Ma intrusions dated in the area (Eakins and others, 1978).
Alteration of deposit
Hydrothermal alteration at Koksetna is related to the intrusive rocks as well as a final stage of supergene leaching (Scott and Ellis, 1982). Prospect alteration is laterally restricted to the composite pipe and a 200 foot peripheral zone of spotty propylitiztion in the wall rock. This is due to the non-reactive nature of the siltstones and to their low permeability.
The first alteration event accompanied the intrusion of quartz-eye rhyolite porphyry in a plug-like geometry. The vast majority of the exposed porphyry is argillicaly altered to kaolinite with minor sericite and montmorillonite. Argillization is generally restricted to the porphyry groundmass with occasional replacement of sanidine phenocrysts by kaolinite. Small zones of quartz-sericite alteration were observed in central portions of the porphyry where the porphyry texture is totally obliterated and replaced by a hard, silica-rich skeleton. Crystal vugs are commonly lined with orange-yellow jarosite, probably from the weathering of sulfide. Intrusion of the rhyolite porphyry also propylitized the wall rock siltstone, andesite, and greywacke, creating localized conversion to chlorite, epidote, illite, and secondary biotite (Ellis and others, 1985).

Production and reserves

Workings or exploration
In 1982 the geophysical crew of Anaconda Minerals Company (Anaconda) conducted a induced polarization (IP)-resistivity survey over the center of the rhyolite breccia pipe. The objectives of this survey were to evaluate the quality of the IP-resistivity data gathered in 1975 by Von Blaricom Geophysical Services, and to test below 1,000 feet depth for a hypothetical supergene enrichment zone (Scott and Ellis, 1982). Three-hundred rock samples were collected from a grid over the Koksetna prospect with each sample representing a 50 foot composite zone along the grid lines. Select samples of mineralized veins and breccias contained anomalous geochemistry as follows: copper 0.15 to 4 percent, silver 0.5 to 16 ounces per ton (oz/ton), lead 100 to 4100 parts per million (ppm), zinc 200 to 2700 ppm, antimony 20 to 480 ppm, tin 20 to 550 ppm, arsenic 300 to 17300 ppm, and 5 samples had anomalous gold from 50 to 1250 parts per billion (ppb) (Scott and Ellis, 1982).
In 1983 Anaconda collected 10 check samples and 7 vein samples at the Koksetna prospect. The geochemistry from these samples verified that anomalous mineralization in the periphery of the feeder pipe was strongest in the northern contact zone. Geochemical values were the following: silver 1 ppm to 4.05 oz/ton, copper 125 to 3000 ppm, lead 100 to 3500 ppm, zinc 300 to 4080 ppm, fluorine 300 to 1350 ppm, arsenic 200 to over 1000 ppm, tin 30 to 340 ppm, and gold 30 to 565 ppb (Ellis and others, 1985).
In 2007 Andover Ventures Inc. conducted a ridge and spur rock and soil sampling survey peripheral to the multiphase intrusive pipe. An IP-resisivity survey was attempted but was unable to get sufficient current into the poorly developed soils to get meaningful results (Ellis and Hoffman, 2008).
Indication of production None

References

References

Reporters W.T. Ellis (Alaska Earth Sciences)
Last report date 3/15/2016