Greenstone Occurrence

Occurrence, Inactive

Commodities and mineralogy

Main commodities Cu
Other commodities Ag; Au
Ore minerals bornite; chalcocite; chalcopyrite; malachite
Gangue minerals epidote; quartz

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale A-5
Latitude 63.2255
Longitude -146.4105
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Greenstone Occurrence claims are on a flat-topped ridge on the east side of upper Boulder Creek at an elevation of about 3,900 feet and about 750 feet east of the center of section 33, T. 19 S., R. 7 E., Fairbanks Meridian (Stout, 1976, plate 1). The site corresponds to locality A22 of Kurtak and others (1992), locality 29 in figure 4 of Cobb (1979 [OFR 79-238]), and locality 39 in table 2 of Nokleberg and others (1991). The location is accurate within 1,000 feet.

Geologic setting

Geologic description

The rock at the Greenstone Occurrence claims is propylitized metabasalt of the Nikolai Greenstone of Late Triassic age (Nokleberg and others, 1991); this unit was previously mapped with the Boulder Creek Volcanics by Stout (1976).
Mineral deposits are copper-bearing quartz-epidote veins. The quartz veins occur in an en echelon set, strike about N 25 E, and are steeply dipping. They are exposed intermittently for about 300 feet on strike and for about 120 feet vertically (Kurtak and others, 1992, p. 131).
Veins range from a few inches to 1 foot thick. They are mainly composed of quartz; epidote occurs in the veins or on their margins. Copper-bearing minerals identified at the site are chalcocite, chalcopyrite, bornite, and malachite. Copper content of the veins ranges from less than 0.1 percent to 2.15 percent (Kurtak and others, 1992, table A22). Gold is enriched to a maximum of 410 parts per billion (ppb) (sample 1409). The copper-bearing vein that contains 2.15 percent copper assayed 0.18 ounce per ton silver and 145 ppb gold (sample 1669). No other trace elements significantly exceed background.
According to the U.S. Bureau of Mines, the veins are too small to be an economic source of copper and do not contain significant values of silver (Kurtak and others, 1992).
Geologic map unit (-146.41275979009, 63.2250709017046)
Mineral deposit model Basaltic Cu (Cox and Singer, 1986; model 23).
Mineral deposit model number 23
Age of mineralization The mineralization is post-Late Triassic and also postdates a folding event that occurred before the Late Jurassic (Stout, 1976); it probably occurred in Cretaceous time, during regional metamorphism and granitic plutonism (Nokleberg and others, 1986) or in the early Tertiary at about the time of emplacement of dacite porphyry dikes (Stout, 1976).
Alteration of deposit Propylitic alteration of basaltic bedrock.

Production and reserves

Workings or exploration Two hundred and eighty-six claims were staked in the area between 1966 and 1976, but there are no significant exploration workings.
Indication of production None

References