Prospect, Inactive

Commodities and mineralogy

Main commodities Mo
Other commodities Cu; Pb; Zn
Ore minerals chalcopyrite; galena; hematite; magnetite; molybdenite; pyrite; sphalerite
Gangue minerals barite; fluorite; quartz; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale UG
Quadrangle map, 1:63,360-scale A-4
Latitude 57.054
Longitude -157.235
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Mike prospect is located approximately 9 miles south of Mother Goose Lake in sec. 24, T. 36 S., R. 50 W., of the Seward Meridian (Church and others, 1989: MF-1593I, locality 1; Nokleberg and others, 1987, locality AP 14; MacKevett and Holloway, 1977, locality 1). Site location is accurate to within 1/2 mile.

Geologic setting

Geologic description

The Mike prospect is marked by a large color anomaly and by a large copper-molybdenum anomaly shown by stream sediment analyses (Church and others, 1989: MF-1539I). The area of this site consists of dikes, sills, and stocks which have intruded sandstones and siltstones of the Jurassic Naknek Formation (Detterman and others, 1987). The main area of mineralization occurs in a zone of silica flooding around and within a rhyolite porphyry stock. The stock is bordered to the north and west by numerous dacitic dikes. A small quartz monzonite stock has intruded the rhyolite stock on its east side and another cuts sedimentary rocks approximately 1 mile north-northeast of the rhyolite. A breccia pipe measuring 400 by 700 feet occurs near the latter quartz monzonite stock. Late-stage quartz veins trending northeast and northwest cut the rhyolite stock. Some if these are traceable for up to 10,000 feet. Potassium-argon dating of the intrusives indicates they are of Pliocene age (Wilson and Shew, 1992).
Sulfides (mostly pyrite) are present in amounts of 5 to 10 percent over an area of 1 square mile. Sulfide content grades outward to 1 to 2 percent over an area of 3 square miles. Northwest of the porphyry stock magnetite is present in veins, breccias, and as disseminations in quartzite and siltstone (Hedderly-Smith, 1977).
Within the rhyolite porphyry, fine-grained molybdenite occurs within a stockwork of small quartz-filled fractures. The mineralized area measures approximately 3600 by 4300 feet as defined by values of greater than 40 ppm molybdenum. Locally, molybdenite may occur in larger, coarsely crystalline quartz veins. Lesser amounts of it are present sporadically in the hornfelsed sedimentary rocks surrounding the stock. The average molybdenum content of surface rock samples from the main mineralized area averaged 130 ppm (Church and others, 1989: B-1858). Grades of up to 0.21 percent molybdenum have been reported (Eakins and others, 1984). Within the mineralized zone copper (10-50 ppm), lead, zinc, and silver values are generally low. This zone also contains some tin and tungsten anomalies and has a fluorine content averaging 710 ppm with values up to 2100 ppm. Fluorine and molybdenum values show a strong correlation.
In 1977 Bear Creek Mining Company drilled 3 holes for a total of 258 feet. The drill cut silicified rhyolite stock, rhyolitic sill, and silicified sedimentary rock. Average molybdenum values for the holes ranged from 10 to 125 ppm (Church and others, 1989: B-1858). The bottom 7 feet of hole 77-3 averaged 253 ppm molybdenum.
A second stockwork occurs on the east side of the rhyolite porphyry, apparently in the quartz monzonite (Hedderly-Smith, 1977). Chalcopyrite is present as disseminations and fracture fillings. Samples of mineralized quartz monzonite are reported to contain 500 to 1000 ppm copper and 10 to 20 ppm molybdenum (Church and others, 1989: B1858). A sample of skarn adjacent to a dacite dike contained 3450 ppm copper and a grab sample of the breccia pipe contained 160 ppm molybdenum.
The molybdenum stockwork and area of silica flooding is surrounded in part and overlapped by intense sericitic and phyllic alteration. Beyond this zone the rocks have been propyliticized. The alteration has been dated as Pliocene (Wilson and Shew, 1992).
Geologic map unit (-157.237063708678, 57.0532561871433)
Mineral deposit model Porphyry Mo (Cox and Singer, 1986; model 21b)
Mineral deposit model number 21b
Age of mineralization Pliocene or younger.
Alteration of deposit The silica-flooded zone in the rhyolite stock is in part surrounded and overlapped by intense sericitic and phyllic alteration. Beyond this zone the rocks have been altered propylitically.

Production and reserves

Workings or exploration This prospect was originally identified in 1975 by Resource Associates of Alaska on the basis of the color anomaly and a stream-sediment survey (Hedderly-Smith, 1977). In 1977 Bear Creek Mining Company did detailed mapping and sampling and drilled 3 short holes at this prospect. Average molybdenum values for the holes ranged from 10 to 125 ppm with the lower 7 feet of one hole containing 253 ppm. An average value of the surface of the main mineralized zone is stated at 130 ppm molybdenum (Church and others, 1989: B-1858). Grades of up to 0.21 percent molybdenum have been reported (Eakins and others, 1984). In the 1980s the U.S. Geological Survey did geochemical sampling of the prospect and of the surrounding area (Church and others, 1989: B-1858).
Indication of production None

Additional comments

This prospect is located on land selected by the Koniag Native Corporation.


MRDS Number A012456


Hedderly-Smith, D.A., 1977, 1977 Annual report, Alaska search, Koniag Inc. region: Bear Creek Mining Company, Spokane Office, 23 p. (Report held by Kennecott Alaska, Anchorage).
Reporters S.H. Pilcher (Anchorage)
Last report date 2/1/2000