Geologic description
The Drenchwater deposits were found in 1975 by the U.S. Geological Survey and several detailed studies were done by the USGS and the U.S. Bureau of Mines from 1977 to 1992 that included detailed mapping, geochemical and geophysical surveys, and much sampling (Jansons and Baggs, 1980; Jansons, 1982; Nokleberg and Winkler, 1978 and 1982; Kurtak and others, 1995). Anaconda Exploration explored the deposits in 1980 and in 1993 the Arctic Slope Regional Corporation and Kennecott Exploration applied for a permit to drill. The permit was turned down by the Bureau of Land Management because the area was part of the National Petroleum Reserve that is closed to exploration and mining.
The Drenchwater-West deposit is one of three similar deposits that form a belt about 1.2 miles long (see also Drenchwater Creek (HW004) and Drenchwater-East (HW002)). The deposits are in the Drenchwater Fenster where the Key Creek sequence consists of the Kuna, Siksikpuk, Otuk and Okpkkruak Formations of Mississippian to Cretaceous age. The structure of the area is dominated by a series of south-dipping thrust faults that intensely deformed the rocks. The stratiform mineralization is mostly in carbonaceous shale and silicified mudstone, associated with altered volcanic rocks, mainly submarine felsic tuffs, with lesser mafic volcanics including trachyte, trachyandesite, and basalt. The mineralization is associated with silicification of the shale, mudstone, and volcanic rocks. There are four types of mineralization: semi-massive; disseminated; layered diagenetic; and breccia-cemented. Sphalerite is the dominant sulfide; variable amounts of pyrite, marcasite, and galena occur; there is rare fluorite and barite. Anomalous silver, arsenic, and antimony show up in assays of the ore. Samples contained up to 23 percent zinc, 5.1 percent lead, 1,150 parts per million (ppm) copper and 15 ppm silver. Biotite from volcanic rocks interbedded with the mineralization has K-Ar ages of 319+10 and 330+17 Ma; a sample of galena has a Pb-isotope model age of 200 Ma. Most of those who have studied the deposits interpret them as volcanogenic massive-sulfide deposits. |