Post River Pluton Prospect, McGrath Mining District, Bethel Census Area, Alaska, USAi

Regional Level Types
Post River Pluton Prospect	Prospect
McGrath Mining District	Mining District
Bethel Census Area	Census Area
Alaska	State
USA	Country

Location: The Post River Pluton prospect is located in a saddle 10 kilometers southwest of Post Lake at an elevation of 5,400 feet (1,646 m) in the NW1/4 sec. 15, T. 23 N., R. 24 W., of the Seward Meridian. The prospect corresponds to map number 6a, b in Bundtzen and others (1988) and map number 28 in Bundtzen, Harris, and Gilbert (1997). The reporter investigated the site in 1982.
Geology: The Post River Pluton prospect is a chalcopyrite-scheelite-quartz vein system that cuts sheared, thermally altered shale of the Lower Ordovician to Lower Silurian Post River Formation; the unit is intruded by an early Tertiary quartz monzonite pluton (Bundtzen and others, 1988). The largest vein, which reaches a maximum one meter wide, trends across the saddle in a N40E direction for about 84 meters. Smaller en echelon veins and veinlets also trend in a northeasterly direction. Lack of good bedrock control prevented any systematic dip or size measurements. A strong secondary biotite overprint occurs in the thermally altered shales. Principal mineralization consists of disseminated scheelite-chalcopyrite aggregates in quartz-calcite veins, and pyrite in hornfels. Bundtzen and others (1988) report values of 98.0 grams/tonne silver, 1.15 percent copper, 0.18 percent lead, 275 ppm tungsten, and 890 ppm tin from five chip-channel samples. Tin and lead minerals were not identified. One sample contained 900 ppb gold. A one meter wide zone of pyrite-rich hornfels contained 759 ppm copper and 0.8 grams/tonne silver. A pan concentrate from a steep ravine draining the mineralized area contained 59.0 grams/tonne silver and abundant visible scheelite.
Workings: T.K. Bundtzen and M.S. Lockwood found the mineralized area during geologic mapping for the Alaska Division of Geological and Geophysical Surveys in 1982. Bundtzen and others (1988) report values of 98.0 grams/tonne silver, 1.15 percent copper, 0.18 percent lead, 275 ppm tungsten, and 890 ppm tin from five chip-channel samples. Tin and lead minerals were not identified. One sample contained 900 ppb gold. A one meter wide zone of pyrite-rich hornfels contained 759 ppm copper and 0.8 grams/tonne silver. A pan concentrate from a steep ravine draining the mineralized area contained abundant visible scheelite and 59.0 grams/tonne silver.
Age: Undated; nearby quartz monzonite pluton yielded K-Ar age of 60.4 Ma (Solie and others, 1991).
Alteration: Secondary biotite (potassic).
Reserves: None.

Commodities (Major) - Ag, Cu, Sn, W; (Minor) - Au, Pb
Development Status: No
Deposit Model: Polymetallic vein (Cox and Singer, 1986; model 22c),

Other Databases

Link to USGS - Alaska:	MG070

Other Regions, Features and Areas containing this locality

References