Rex

Prospect, Inactive

Commodities and mineralogy

Main commodities Au; Cu
Other commodities Mo; Pb; Zn
Ore minerals chalcopyrite; galena; gold; molybdenite; pyrite; sphalerite
Gangue minerals barite; quartz

Geographic location

Quadrangle map, 1:250,000-scale UG
Quadrangle map, 1:63,360-scale B-4
Latitude 57.255
Longitude -157.084
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Rex prospect is located approximately 8 miles northeast of Mother Goose Lake in sec. 12, T. 34 S., R. 49 W., of the Seward Meridian (MacKevett and Holloway, 1977, locality 2; Nokleberg and others, 1987, locality AP 13; Church and others, 1989: MF-1539I, locality 2). Site location is accurate to within 1/2 mile.

Geologic setting

Geologic description

At the Rex prospect, sedimentary rocks of the Cretaceous Chignik and Eocene Tolstoi Formations are cut by an Oligocene porphyritic quartz diorite stock (Church and others, 1989: B-1858). The site is marked by a color anomaly and by copper, molybdenum, lead, zinc, silver, gold and tungsten stream-sediment anomalies. Panned concentrates taken in the vicinity of the prospect contain free gold.
Chalcopyrite and molybdenite are present mostly in a zone of potassic alteration which occurs over an area of 1 mile square within the stock (Church and others, 1989: B-1858). The chalcopyrite occurs as disseminations and in stockwork fractures, whereas the molybdenite is locally present only in the stockwork fractures. Sampling by Bear Creek Mining Company indicated a small area containing 1350 to 3900 ppm copper and 26 to 56 ppm molybdenum (Hedderly-Smith, 1977). In a separate(?) area he describes a 500-foot wide zone containing 0.2 to 0.25 percent copper. Elsewhere values of 500 to 1000 ppm copper were found sporadically. Gold values, up to 0.37 ppm, were coincident with the higher copper values. Float samples of intrusive with secondary potassium feldspar and tourmaline contained 0.2 to 0.25 percent copper. Scattered galena and sphalerite are present in the altered rocks peripheral to the copper mineralization. Occasional anomalies of 40 to 60 ppm lead and 100 to 300 ppm zinc have been detected (Hedderly-Smith, 1977. Pyrite is present in the mineralized zone and in the surrounding altered sedimentary rocks in amounts up to 10 percent.
Beyond the potassic mineralized zone, defined by the presence of secondary biotite, some phyllic alteration has developed. The dominant alteration at this prospect is propylitic and is present in the sedimentary rocks surrounding the stock.
Geologic map unit (-157.086070564502, 57.2542622592836)
Mineral deposit model Porphyry copper-gold (Cox and Singer, 1986; model 20c)
Mineral deposit model number 20c
Age of mineralization Oligocene or younger.
Alteration of deposit This prospect exhibits zoned potassic, phyllic, and propylitic alteration.

Production and reserves

Workings or exploration
This prospect was originally identified in 1975 by Resource Associates of Alaska in the basis of the color anomaly and a stream-sediment survey (Hedderly-Smith, 1977).
Bear Creek Mining Company did extensive mapping and sampling at the Rex prospect in 1977 (Church and others, 1989: B-1858). A small area in the intrusive was found to contain 1350 to 3900 ppm copper and 26 to 56 ppm molybdenum (Hedderly-Smith, 1977). In a separate(?) 500-foot wide zone, copper values of 0.2 to 0.25 percent were reported. Elsewhere in the intrusive, sporadic copper values of 500 to 1000 ppm were indicated. Gold values up to 0.37 ppm were found to be coincident with the higher copper values. The average copper, gold, and molybdenum values are not available.
In the 1980s the U.S. Geological Survey did a silt and rock geochemical survey which included the area around this prospect (Church and others, 1989: B-1858).
Indication of production None

Additional comments

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

References

MRDS Number A012457

References

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