Unnamed Occurrence (ARDF - HE058; divide between Christy and Copeland creeks), Valdez Creek District, Matanuska-Susitna Borough, Alaska, USA

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Location: This occurrence is at an elevation of about 5,200 feet, about midway between the middle reaches of Christy and Copeland Creeks. the map site is about 1.3 mile north of VABM 5048 (Copeland), in the NE1/4 of sec. 1, T. 21 S., R. 12 W., of the Fairbanks Meridian. The location is accurate to within 0.5 mile. This is location 22 of Cobb (1978: OFR 78-1062), and location E-29 of Balen (1990: OFR 34-90).
Geology: The country rocks in the area of this occurrence are a tectonically mixed assembleges of Upper Devonian serpentinite, basalt, chert, and gabbro. The occurrence consists of talus blocks of serpentinite(?) containing pods of massive chromite. The blocks occur in an area underlain by serpentinite altered to quartz-carbonate rock containing disseminated pyrite and stained with garnierite. Samples comprising 95% chromite contain 39.5% Cr2O3 and a chromium/iron ratio of 3.1:1, along with traces of platinum group metals (principally rhenium) and 1,000 ppm nickel (Hawley and others, 1969). The platinum values in eight samples were at the lower limit of detection (Balen, 1990: OFR 34-90).
Workings: There has been surface sampling only. Samples comprising 95% chromite contain 39.5% Cr2O3 and a chromium/iron ratio of 3.1:1, along with traces of platinum group metals (principally rhenium) and 1,000 ppm nickel (Hawley and others, 1969). The platinum values in eight samples were at the lower limit of detection (Balen, 1990: OFR 34-90).
Age: Chromite probably was a magmatic segregation in Upper Devonian ultramafic intrusive rocks.
Alteration: Serpentinization of ultramafic intrusive rocks. The serpentinite is altered to quartz-carbonate rock containing disseminated pyrite and stained with garrnierite.

Commodities (Major) - Cr; (Minor) - Ni, Pt
Development Status: None
Deposit Model: Podiform chromite (Cox and Singer, 1986; model 8a)

References

Balen, M.D., 1990, Geochemical sampling results from the Bureau of Mines investigations in the Valdez Creek mining district, Alaska: U.S. Bureau of Mines Open-File Report 34-90, 218 p., 2 plates, scale 1:250,000. Cobb, E.H., 1978, Summary of references to mineral occurrences (other than mineral fuels and construction materials) in the Healy quadrangle, Alaska: U.S. Geological Survey Open-File Report 78-1062, 113 p. Hawley, C.C., and Clark, A.L., 1974 (1975), Geology and mineral deposits of the upper Chulitna district, Alaska: U.S. Geological Survey Professional Paper 758-B, p. B1-B47, 2 plates, scale 1:12,000 and 1:48,000. Hawley, C.C., Clark, A.L., Herdrick, M.A., and Clark, S.H.B., 1969, Results of geological and geochemical investigations in an area northwest of the Chulitna River, central Alaska Range: U.S. Geological Survey Circular 617, 19 p.

Mineral List

Chromite

'Garnierite'

Pyrite

'Serpentine Group'

4 entries listed. 2 valid minerals.