Unnamed (on Threesome Mountain)

Prospect, Inactive

Commodities and mineralogy

Main commodities Mo; W
Other commodities Ag; Cu
Ore minerals chalcopyrite; molybdenite; scheelite; silver mineral
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale MF
Quadrangle map, 1:63,360-scale C-2
Latitude 58.5319
Longitude -136.5658
Nearby scientific data Find additional scientific data near this location
Location and accuracy The deposit is on the east flank of Threesome Mountain, about 1.5 miles northeast of the head of Abyss Lake. Elevation of the deposit ranges from about 2000 to 2800 feet. The deposit is about one-half mile long, elongated east-west. The coordinate location is the approximate mid-point of the deposit, and is accurate to about 0.1 mile.

Geologic setting

Geologic description

This molybdenum-tungsten prospect is in a terrane composed mainly of Paleozoic sedimentary rocks. The prospect is east of a major fault that separates the Paleozoic strata from granitic rock of Mesozoic age. The fault is concealed by Brady Glacier and is inferred to control the course of Taylor Bay to the south (Brew and others, 1978). The deposit is of porphyry-type. Molybdenite and scheelite, with minor chalcopyrite, occur in the roof-zone of a Tertiary granodiorite stock that intrudes volcanic and sedimentary rocks (limestone) of Devonian age. The molybdenite and scheelite occur in quartz veins and as fracture coatings on joints in the intrusion. Molybdenum-bearing vein density measured by the U.S. Bureau of Mines (Kimball and others, 1978, p. C-253-C256) was about eight veins per 100 feet. The maximum observed vein thickness was 0.2 foot, with an average of about 0.08 foot. Mineralized veins and fractures strike east-northeast and dip from 25 to 76 degrees northwest. Selected vein samples contained as much as 2000 ppm molybdenum, 4917 ppm tungsten, 7 ppm silver, and 330 ppm copper. Iron-stained hornfels near the granitic stock contains as much as 200 ppm copper and 50 ppm tungsten.
Richer tactite deposits could occur in nearby contact zones between the stock and the limestone.
Geologic map unit (-136.567635666065, 58.5315290838696)
Mineral deposit model Porphyry molybdenum-tungsten deposit, similar to W skarn and porphyry Mo of Cox and Singer (1986; model 14a and 21b). Associated tactite. Porphyry deposit is mainly mineralized fracture surface type.
Mineral deposit model number 14a, 21b
Age of mineralization Tertiary.
Alteration of deposit Local iron staining.

Production and reserves

Workings or exploration The deposit was discovered by Alvenco, Inc. in 1968, who staked 24 claims to cover it. Alvenco drilled three diamond drill holes to evaluate the deposit. The grades were in the low range molybdenum equivalent. Selected samples collected by the Bureau of Mines contained as much as 2000 ppm molybdenum and more than 4000 ppm tungsten (Kimball and others, 1968).
Indication of production None

Additional comments

The deposit is in Glacier Bay National Park and Preserve.

References