|Quadrangle map, 1:250,000-scale||KC|
|Quadrangle map, 1:63,360-scale||C-6|
|Nearby scientific data||Find additional scientific data near this location|
|Location and accuracy||This site, in section 12, T. 72 S., R. 87 E., of the Copper River Meridian, represents three prospects on the southwest side of Helm Bay, about 5 miles from the mouth of the bay. The Snowstorm and Last Chance (Helm Bay) prospects are at or near sea level, and the Beulah prospect is at an elevation of about 150 feet (Maas and others, 1995, fig. 46). The map site is at the approximate center of the 0.25-square-mile area that contains these prospects and is accurate within 0.1 mile.|
The country rocks near this site are marine, andesitic and basaltic metavolcanic rocks that are gradationally interbedded with flyschlike metasedimentary rocks (Berg and others, 1988, p. 18). The strata were regionally metamorphosed to greenschist-grade phyllite and semischist in Late Cretaceous time (Brew, 1996, p. 27). The depositional age of the strata is uncertain. Berg and others (1998, p. 17) report that they closely resemble Upper Jurassic to mid-Cretaceous marine strata nearby on Gravina Island.
According to Maas and others (1995, p. 183-184 and fig. 46), the deposits at the Snowstorm, Beulah, and, presumably, the Last Chance (Helm Bay) prospects consist of quartz vein swarms or stringer lodes in hornblende metadiorite. The veins carry auriferous pyrite, which in places is also disseminated in bleached wallrock adjacent to the veins. Maas and others (1995, table 25) report the following average metal contents in their samples from these deposits. Snowstorm: 1.20 ppm Au, 0.06 ppm Ag, 33 ppm Cu, 3.1 ppm Pb, and 9 ppm Zn. Beulah: 0.62 ppm Au, 0.16 ppm Ag, 30 ppm Cu, 6.3 ppm Pb, and 25 ppm Zn. Au and Cu values at the last Chance respectively were up to 16 ppb and 101 ppm.Fluid inclusion studies of quartz vein material from several of the Helm Bay lodes suggest that the veins formed at temperatures and pressures consistent with conditions during the Late Cretaceous greenschist-grade regional metamorphism (Maas and others, 1995, p. 184).
|Geologic map unit||(-131.995683884606, 55.6446372317812)|
|Mineral deposit model||Low-sulfide gold-quartz veins (Cox and Singer, 1986; model 36a)|
|Mineral deposit model number||36a|
|Age of mineralization||Late Cretaceous.|
|Alteration of deposit||Locally, the wallrocks adjacent to the veins are bleached and pyritized.|
|Workings or exploration||There are two adits, six feet long and three feet long, on the Snowstorm prospect; a 63-foot adit on the Last Chance prospect; and a 63-foot adit in the Beulah prospect (Maas and others, 1995, table 26). All of the workings date from the early 1900s.|
|Indication of production||None|
Berg, H.C., Elliott, R.L., and Koch, R.D., 1988, Geologic map of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Mineral Investigations Series Map I-1807, 27 p., scale 1:250,000.
Brew, D.A., 1996, Geologic map of the Craig, Dixon Entrance, and parts of the Ketchikan and Prince Rupert quadrangles, southeastern Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2319, 53 p., 1 sheet, scale 1:250,000.
|Reporters||H.C. Berg, USGS|
|Last report date||6/30/1999|