Unnamed (southwest Gilbert Peninsula)

Occurrences, Inactive

Commodities and mineralogy

Main commodities Cu; Mo
Other commodities Ag; Bi; Zn
Ore minerals chalcopyrite; molybdenite; pyrite; sphalerite
Gangue minerals quartz and alteration products of quartz diorite and alaskite

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale D-2
Latitude 58.775
Longitude -136.561
Nearby scientific data Find additional scientific data near this location
Location and accuracy Numerous occurrences of copper and molybdenum minerals are near the southwest tip of Gilbert Peninsula and on an unnamed island south of the southwest tip of the peninsula. The coordinates are the approximate center of these occurrences on the southwest part of Gilbert Peninsula at an elevation of about 100 feet. The location includes location numbers 44 and 45 of MacKevett and others (1971) amd locations 35 and 36 of Cobb (1972-MF-436), and location number 54 of Kimball and others (1978). Kimball and others also reported nearby occurrences (locations 53 and 55) respectively across Hugh Miller Inlet (on Gilbert Peninsula) and across Scidmore Inlet.

Geologic setting

Geologic description

Southern Gilbert Peninsula and nearby areas are mainly underlain by granitic rock of Cretaceous age containing local inliers of metamorphosed hornfels and marble of Paleozoic age (Brew and others, 1978). On the southwest tip of Gilbert Peninsula, stockworks of quartz veins in altered quartz diorite, cut by light colored aplite (alaskitic) dikes, occur in a silicified zone at least 50 feet wide and several hundred feet long. The quartz veins contain minor amounts of chalcopyrite and molybdenite close to vein selvages. The mineralogy and intrusive setting suggests a porphyry copper-molybdenum model. Pyritic stockworks and somewhat younger crosscutting pyritic veins also occur to the east across Hugh Miller Inlet and west across Scidmore Inlet. Selected vein samples from location 45 contained as much as 10 ppm silver, 7000 ppm copper and 2000 ppm molybdenum (MacKevett and others, 1971); chip samples collected across 52- and 100-foot long lines were nearly barren. Samples collected in pyritic stockwork vein areas on the west side of Scidmore Inlet contained a trace of molybdenum and as much as 70 ppm bismuth, and 1500 ppm zinc (Kimball and others, 1978, location 55).
Geologic map unit (-136.562848015985, 58.7746088915009)
Mineral deposit model Porphyry copper-molybdenum (Cox and Singer, 1986; model 21a).
Mineral deposit model number 21a
Age of mineralization Tertiary.
Alteration of deposit Silicification in stockwork zones.

Production and reserves

Workings or exploration The mineralized area was described briefly by Rossman (1963, B 1121-K) and investigated in 1966 in conjunction with the Geological Survey's study of Glacier Bay (MacKevett and others, 1971). There are no mine workings. A 0.5 foot quartz vein collected on the southwest tip of Gilbert Peninsula contained 1000 ppm copper; a selected high-grade piece of quartz float contained 10 ppm silver, 7000 ppm copper and 2000 ppm molybdenum (MacKevett and others, 1971, p. 50-51, table 9). To the west, across Scidmore Inlet, a selected sample contained 1500 ppm zinc and 70 ppb bismuth. The average grade of exposed materials is very low, as long chip samples contained only near background amounts of valuable metals.
Indication of production None

Additional comments

Fairly large but very low grade quartz vein and stockwork system. It is in Glacier Bay National Park and Preserve.

References