Moth Bay

Prospect, Undetermined

Alternative names

Black Jack
Bonanza King
Lone Jack
Sulphide
Youzinka

Commodities and mineralogy

Main commodities Pb; Zn
Other commodities Ag; Au; Cu
Ore minerals bornite; chalcopyrite; covellite; galena; pyrite; pyrrhotite; sphalerite
Gangue minerals calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale KC
Quadrangle map, 1:63,360-scale B-5
Latitude 55.296
Longitude -131.343
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Moth Bay prospect is at an elevation of 300-350 feet, 0.55 mile north of the head of Moth Bay. The site is in section 7, T. 76 S., R. 93 E., of the Copper River Meridian. It corresponds to loc. 83 in Elliott and others (1978), and to loc. 307 in Maas and others (1995). The location is accurate within a few hundred feet.

Geologic setting

Geologic description

The principal country rock in this part of Revillagigedo Island is a stock of Cretaceous quartz diorite (Berg and others, 1988). On the north and south, the stock intrudes diverse metamorphosed Mesozoic or Paleozoic sedimentary, volcanic, and intrusive rocks. In part, the southern boundary of the stock also is a moderately northeast-dipping thrust fault and blastomylonite zone (Berg, 1982). The outcrop area of the stock includes roof pendants and inclusions of dioritic metaplutonic rocks and pelitic metasedimentary rocks. Berg and others (1988) assigned these rocks a Mesozoic or Paleozoic premetamorphic age. One such pelitic roof pendant, 0.5 mile long and 0.25 mile wide, hosts the Moth Bay deposit.
The Moth Bay deposit consists of layers of biotite schist and underlying muscovite schist that are partly replaced by pyrite, pyrrhotite, sphalerite, chalcopyrite, galena, and minor bornite and covellite, accompanied by interstitial quartz and calcite (Robinson and Twenhofel, 1953, p. 59-71). Locally, the sulfide minerals form isolated, podlike masses of ore. The deposit crops out in an area at least 1500 feet long and 700 feet wide. The ore beds are as much as 15 feet thick, but generally no more than 5 feet thick. The principal ore bed ranges from 2-17 feet thick; it has been traced on the surface for 600 feet and in underground workings for 440 feet. The orebody has a vertical extent of 200 feet and a downdip length of 140 feet. A second ore bed averages 4 feet thick for a length of 80 feet. A third ore bed of unknown length is up to 3 feet thick and occurs on the crest of a southeast-plunging anticlinal drag fold in the schist hostrock. The order of abundance of the sulfide minerals is: 25% pyrite, up to 10% pyrrhotite, 3-15% sphalerite, 1-5% chalcopyrite, and trace galena. Gold and silver have been reported only in some assays of the ore.
The deposit was explored between about 1911 and 1931 by trenches, pits, and about 900 feet of underground workings, including an 800-foot adit and short crosscuts, and a 75-foot adit and 100-foot inclined winze (Robinson and Twenhofel, 1953; Maas and others, 1995, p. 221). The deposit was drilled by the U.S. Bureau of Mines in 1950 (see Reserves).
Geologic map unit (-131.344688505776, 55.2956566918142)
Mineral deposit model Stratiform massive sulfide replacement body. Possibly sedimentary exhalative Zn-Pb (Cox and Singer, 1986; model 31a)
Mineral deposit model number 31a?

Production and reserves

Workings or exploration The deposit was explored between about 1911 and 1931 by trenches, pits, and about 900 feet of underground workings, including an 800-foot adit and short crosscuts, and a 75-foot adit and 100-foot inclined winze (Robinson and Twenhofel, 1953; Maas and others, 1995, p. 221). The deposit was drilled by the U.S. Bureau of Mines in 1950 (see Reserves).
Indication of production None
Reserve estimates Drilling of the Moth Bay deposit by the U.S. Bureau of Mines in 1950 indicated: (1) about 100,000 tons of ore containing 7.5% Zn, 1% Cu, and 0.02 ounce of Au and 0.20 ounce of Ag per ton; (2) 10,000 tons of ore containing 3% Cu; and (3) an additional 100,000 tons of lower-grade ore (Robinson and Twenhofel, 1953; Cobb and Elliott, 1980, p. 77; Maas and others, 1995, p. 221).
Production notes Despite fairly extensive underground workings, there is no public record of any ore shipments from the Moth Bay prospect.

Additional comments

Robinson and Twenhofel (1953) provide detailed drill core and assay data for this deposit. At various times, the property has been called Black Jack, Bonanza King, Lone Jack, Sulphide, and Youzinka (Cobb and Elliott, 1980, p. 144-146).

References