Lucky Four

Prospect, Undetermined

Alternative names

Hump Island

Commodities and mineralogy

Main commodities Cu
Other commodities Au
Ore minerals chalcopyrite; molybdenite; pyrite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale C-6
Latitude 55.523
Longitude -131.757
Nearby scientific data Find additional scientific data near this location
Location and accuracy This prospect is on Hump Island, which is in Clover Passage about 13 miles northwest of Ketchikan. The island is about 0.9 mile long and 0.2 mile wide, and is in sections 20, 21, and 29, T. 73 S., R. 90 E., of the Copper River Meridian. The site corresponds to loc. 41 in Elliott and others (1978), and to loc. 264 in Maas and others (1995, p. 277 and fig. 5). The location is accurate within a hundred or so feet.

Geologic setting

Geologic description

The country rocks on and near Hump Island consist chiefly of flyschlike metasedimentary rocks and subordinate andesitic and basaltic metavolcanic rocks that are intruded by Cretaceous feldspar-porphyritic granodiorite stocks, sills, and dikes (Berg and others, 1988). The strata were regionally metamorphosed to greenschist-grade phyllite and semischist in Late Cretaceous time (Brew, 1996, p. 27). Near some of the granodiorite contacts, the phyllite and semischist were subsequently contact metamorphosed to hornblende hornfels (Berg and others, 1988). The premetamorphic age range of the strata is unknown; Berg and others (1988, p. 17) note that they closely resemble Upper Jurassic and mid-Cretaceous marine flysch and volcanic rocks nearby on Gravina Island.
Hump Island consists largely of intensely hydrothermally altered and locally sulfide-bearing rocks. Maas and others (1995, p. 277) describe the island as a small, foliated, Cretaceous quartz diorite or quartz monzonite pluton that intrudes Jurassic or Cretaceous mafic to felsic volcanic rocks and interbedded pelitic sedimentary rocks. Weak porphyry copper mineralization accompanied emplacement of the pluton, along with silicic and argillic alteration of the surrounding country rocks. The east side of the island is underlain by felsic volcanic rocks that have been altered to quartz-sericite-pyrite schist with a high silica content. The west side is intensely argillized pelitic schist.
The deposit consists of pyrite, chalcopyrite, and minor molybdenite that form veinlets and disseminations in the hydrothermally altered rocks. Locally, chalcopyrite, and possibly other sulfides, occur as small masses in quartz veinlets (Maas and others, 1995, p. 278). Assays of samples of sulfide-bearing veins and altered country rocks collected in the early 1990s by the U.S. Bureau of Mines ranged from 806-3080 ppm Cu (average: 2064 ppm Cu) and 6-236 ppm Mo (average: 69 ppm Mo), along with as much as 3.6 ppm Ag, 3837 ppb Au, 172 ppm Bi, and 11 ppm Te (Maas and others, 1995, p. 278 and fig. 73). Other samples of sulfide-bearing altered rocks contained up to 2.0% Cu, 10 ppm Ag, 30 ppm Mo, and 500 ppm Co (Elliott and others, 1978, loc. 41). Maas and others (1995, p. 278) classify the prospect as a metamorphosed porphyry Cu deposit, and note that it is the only one known in the Ketchikan district.
Geologic map unit (-131.758682711212, 55.5226402686418)
Mineral deposit model Metamorphosed porphyry Cu deposit (Cox and Singer, 1986; model 17)
Mineral deposit model number 17
Age of mineralization Probably Late Cretaceous.
Alteration of deposit Locally intense silicification, argillization, and sericitization. Local iron-staining and oxidation of copper minerals.

Production and reserves

Workings or exploration Assays of samples of sulfide-bearing veins and altered country rocks collected in the early 1990s by the U.S. Bureau of Mines ranged from 806-3080 ppm Cu (average: 2064 ppm Cu) and 6-236 ppm Mo (average: 69 ppm Mo), along with as much as 3.6 ppm Ag, 3837 ppb Au, 172 ppm Bi, and 11 ppm Te (Maas and others, 1995, p. 278 and fig. 73). Other samples of sulfide-bearing altered rocks contained up to 2.0% Cu, 10 ppm Ag, 30 ppm Mo, and 500 ppm Co (Elliott and others, 1978, loc. 41).
Indication of production None

References