Platypus Mountain Occurrence, Valdez-Cordova Borough, Alaska, USA

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Arehart (1982) reported numerous copper occurrences in intermediate and mafic volcanics near the Platypus Mountain occurrence. Additional information can be obtained from Ahtna Minerals in Anchorage, Alaska.
Location: This occurrence is on a rounded mountain labeled '4666' on the USGS 1:63,360-scale, topographic map; it is in a range locally called the Alphabet Hills. The occurrence is about 2.5 miles north-northeast of the point where Keg Creek bends from southwest to south. The occurrence is on the southwest flank of the mountain at an elevation of about 4100 feet. It is in the NW1/4NW1/4, section 5, T. 12 N., R. 6 W., Copper River Meridian. The prospect is known accurately within 500 feet.
Geology: The Platypus Mountain occurrence is in a thin unit of felsic tuff and exhalite within a 5000-meter-thick section dominated by mafic volcanic flows, tuffs, and breccia. The section also contains subordinate, weakly metamorphosed siltstone, claystone, and marble. The rocks are probable late Paleozoic in age. They are possibly correlative with the Pennsylvanian to Permian, Tetelna Formation and probably are correlative with the country rocks at the KFC occurrence (GU001). Prospecting in the area is handicapped by extensive Quaternary gravel which mantles many of the hills. Chalcopyrite, partly oxidized to azurite and malachite, occurs in stringers within a chloritic, rhyolite(?) flow-breccia at this occurrence (Arehart, 1982). Samples of the breccia contain as much as 4,100 ppm copper, 1.5 ppm silver, and 620 ppb gold. Semi-massive pyrite occurs in a cherty exhalite unit adjacent to the flow-breccia. Lenses and stringers of chalcocite, partly oxidized to malachite and azurite, are associated with the predominant mafic rocks. The pyritic, cherty, exhalite unit which hosts the flow-breccia extends east-west for about two miles.
Workings: The Platypus Mountain occurrence was discovered in 1981 by a Noranda Exploration company team (Arehart, 1982). A gridded, soil survey and a CEM geophysical survey were completed on the prospect. The lands are mainly covered by selections of Ahtna Inc., a regional Native corporation.
Age: Diagenetic or epigenetic remobilization of copper and associated metals in a volcanic pile of Pennsylvanian and Permian age. Remobilization of copper may have occurred during green-schist metamorphism in the Jurassic(?).
Alteration: Chloritization of felsic tuff and exhalite.

Commodities (Major) - Ag, Au, Cu
Development Status: None
Deposit Model: Cyprus massive sulfide (Cox and Singer, 1986, model 24a)

References

Arehart, G.B., 1982, Reconnaissance geology/geochemistry of the Alphabet Hills, Gulkana quadrangle, Alaska: Noranda Exploration, 4 p., maps and appendices. (Report held by Ahtna Minerals Co., Anchorage, Alaska).

Mineral List

Azurite Chalcocite

Chalcopyrite 'Chlorite Group'

Malachite Pyrite

6 entries listed. 5 valid minerals.