Kolossal

Prospect, Inactive

Alternative names

Black Hill

Commodities and mineralogy

Main commodities Ag; Au; Cu
Other commodities As; F
Ore minerals arsenopyrite; chalcopyrite; pyrite; pyrrhotite; scorodite
Gangue minerals calcite; fluorite; quartz; sericite; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale LC
Quadrangle map, 1:63,360-scale B-6
Latitude 60.2827
Longitude -155.2481
Nearby scientific data Find additional scientific data near this location
Location and accuracy Located near the headwaters of Black Creek, a tributary of the Koksetna River about 25 miles west of Lake Clark. The prospect lies on the northeast flank of VABM Black in the northeast 1/4 of section 7, T. 2 N., R. 34 W. of the Seward Meridian.

Geologic setting

Geologic description

The Kolossal prospect is underlain by sedimentary rocks of the Koksetna River sequence of Wallace and others (1989) that have been intruded by Cretaceous to Tertiary quartz monozonite plutons and rhyolite to dacite plugs, sills, dikes and flows. The Koksetna sequence at the prospect consists of dominantly fine grained silty sandstones and sandy siltstones. Local zones of thin bedded to laminated siltstones and poorly sorted medium-grained greywacke are interbedded on the eastern portion of the prospect area (Wilson and others, 2006).
At Kolossal the sedimentary rocks have been extensively hornfelsed, becoming pinkish-gray in color and commonly having a conchoidal fracture (Scott and Ellis, 1982). Locally the rocks have been highly tourmalinized and tourmaline veins with radiating crystals and quartz with tourmaline cores are common. The hornfelsed sedimentary rocks commonly contain from 1 to 2 percent finely disseminated pyrrhotite.
A leucocratic fine-grained equigranular, locally porphyritic, granite occurs in the northwest portion of the Kolossal prospect. Ten to twenty percent potassium feldspar phenocrysts 2 to 3 millimeters in diameter occur in portions of the intrusive. One to three percent chloritized mafics (after hornblende and biotite) are generally present except where leaching has left iron-oxide stained vugs. Locally miarolitic cavities 1 to 3 millimeters in diameter occur and are commonly lined with euhedral quartz and feldspar crystals (Ellis and others, 1985). The equigranular groundmass is composed of approximately 70 percent potassium feldspar plus plagioclase along with about 30 percent quartz and 1 to 3 percent chloritized mafics (biotite and hornblende). This granite was emplaced as a high level hypabyssal intrusion.
Greisen occurs sporadically in float in a 200 meter by 100 meter zone approximately 300 meters due north of VABM Black (Scott and Ellis, 1982). It consists of 65 to 90 percent granular quartz, 5 to 15 percent white mica (sericite) which is commonly in fractures and along boundaries of quartz grains, and 5 to 10 percent hornblende pseudomorphs which contain traces of unidentified opaques. Local zones of 1 to 3 percent purple fluorite occur. The greisens are locally mineralized with 1 to 3 percent arsenopyrite or its oxidation product, scorodite. Two occurrences of tourmaline quartz breccia have been located as float in the greisen area. The breccia consists of 1 to 10 centimeter fragments of greisenized granite and fine-grained 40 percent tourmaline and 60 percent quartz matrix (Ellis and others, 1985).
Five to ten meter wide aplite dikes occur sporadically in the prospect area. They are typically fine-grained (locally porphyritic) and are composed dominantly of quartz and feldspar. Near Black Creek the aplite dikes contain 1 to 2 percent pyrite and quartz carbonate veins also located along the creek contain limonite stains and replacements, possibly after pyrite and chalcopyrite. On the northeast side of VABM Black a quartz latite dike is spatially associated with fracture controlled pyrite and chalcopyrite and local tourmalinization (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 59.5 intrusions dated in the area (Eakins and others, 1978).
Alteration of deposit
At Black Hill the sedimentary rocks have been extensively hornfelsed, becoming pinkish-gray in color and commonly having a conchoidal fracture (Scott and Ellis, 1982). Locally the rocks are highly tourmalinized and tourmaline veins with radiating crystals and quartz with tourmaline cores are common.
Greisen occurs sporadically in float in a 200 meter by 100 meter zone approximately 300 meters due north of VABM Black (Scott and Ellis, 1982). It consists of 65 to 90 percent granular quartz, 5 to 15 percent white mica (sericite) which is commonly in fracture 1 to 3 percent purple fluorite occur. The greisens are locally mineralized with 1 to 3 percent arsenopyrite or its oxidation product, scorodite.

Production and reserves

Workings or exploration
The Kolossal prospect was discovered by Anaconda Minerals Company (Anaconda) following up on aeromagnetic anomalies in 1982. In 1983 Anaconda conducted geologic mapping, grid rock sampling, and gravity and ground magnetic surveys (Ellis and others, 1985).
During the 1982-1983 Anaconda programs five types of mineralization were identified at Kolossal: 1) arsenopyrite-bearing greisen, 2) quartz latite dike spatially associated with fracture controlled pyrite and chalcopyrite, 3) disseminated pyrrhotite in horfelsed sediments, 4) aplite dikes with pyrite, and 5)quartz carbonate veins with limonite stain and replacement. Rock samples from the mineralized quartz latite dike on Black Hill had fracture controlled pyrite and chalcopyrite which returned gold values from 0.3 to 1.7 parts per million (ppm) and silver values from 2 to 28 ppm. The most anomalous grab sample taken adjacent to an aplite dike contained 0.15 percent copper, 28 ppm silver, and 1 ppm gold (Ellis and others, 1985). The most anomalous tin value in a rock sample was 225 ppm.
Indication of production None

References

References

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