Unnamed (Johns Hopkins Inlet)

Occurrence, Inactive

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Au; Mo; Sn; W
Ore minerals cassiterite; chalcopyrite; molybdenite; pyrrhotite

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale D-4
Latitude 58.8958
Longitude -137.0115
Nearby scientific data Find additional scientific data near this location
Location and accuracy The occurrence is at an elevation of about 850 feet on the east side of Johns Hopkins Inlet on a north-facing point where the inlet turns to the southwest. The location is approximately 0.25 mile south of the common corner of sections 23, 24, 25, and 26, T. 34 S., R. 50 E., of the Copper River Meridian. The location is accurate within 250 feet. It is approximately location number 30 of Kimball and others (1978, fig. C38, p. C172); it is near number 74 of MacKevett and others (1971).

Geologic setting

Geologic description

Interlayered volcanic and sedimentary rocks of Permian age occur west of an approximately concordant granitic intrusion of Cretaceous age that extends northerly across Johns Hopkins Inlet (Brew and others, 1978, plate 1A). Kimball and others (1978) note intermixed diorite and mixed sedimentary and volcanic rocks in the area, and, at this occurrence, there is a conspicuous iron-stained zone about 0.25 mile wide. Grab samples (rock in place) contained from 10 to more than 30 ppm molybdenum (Kimball and others, 1978, fig. C38, p. C147-149). A selected sample (7S044) from a thin (0.2 foot) lens of pyrrhotite in sulfidized volcanics contained 1300 ppm copper, 30 ppm molybdenum, 1.5 ppm silver, and >0.05 ppm gold. A float sample of hornfels (Kimball and others, 1978, location 30, p. C172) from the same area contained 4100 ppm copper, 7 ppm silver, 0.15 ppm gold, 700 ppm tin and 793 ppm tungsten. The sample contained visible chalcopyrite and pyrrhotite.
Geologic map unit (-137.013378754359, 58.8953923594483)
Mineral deposit model Skarn, massive sulfide type.
Age of mineralization Possibly Cretaceous, based on age of granitic plutons in the area.
Alteration of deposit Widespread sulfidation of intrusive and volcanic rocks, now reflected in iron-stained rocks.

Production and reserves

Workings or exploration Surface occurrence only. A Bureau of Mines select sample (7SO44) of a pyrrhotite lens contained 1,300 ppm copper. 30 ppm molybdenite, 1.5 ppm silver and greater than 0.05 ppm gold; also reported from the same area is a float sample of hornfels that contained 0.41 percent copper, 250 ppm zinc, 7 ppm silver, 700 ppm tin and 793 ppm tungsten (Kimball and others, 1978, p. C147, C172). MacKevett and others (1971, table 9, no. 74) found 150 ppm copper and 30 ppm molybdenum in a grab sample of an iron-stained granitic intrusion in the area.
Indication of production None

Additional comments

Moderately large occurrence of iron-stained granitic pluton and hornfelsed volcanic and sedimentary rocks, not adequately prospected. The area is Glacier Bay National Park and Preserve.

References