Platinum deposits of the Goodnews Bay district, Alaska

Abstract

Platinum placers were discovered in 1926 in a small area south of Goodnews Bay, in southwestern Alaska. Beginning in 1927, the placers were worked for 7 years by small-scale mining methods; in later years dragline excavators and a dredge were utilized. These deposits are important, not only because they are of high grade but because they are the only commercial source of platinum metals in the United States. The bedded rocks of this area are sedimentary and volcanic rocks of late Paleozoic(?) age that have been intruded by a variety of ultrabasic rocks. The platinum metals of the placers have been derived from a mass of dunite and related rocks that constitute the bedrock of Red Mountain in the upper valley of the Salmon River. The western headwater tributaries of this stream are the fluvial conduits that have produced all the placers in the valley of the river. The principal placers lie in two pay streaks, one in the valley floor of the Salmon River and the other in an ancient stream channel along the east side of this valley. The stream-channel deposit, called the bench pay streak, was formed in early Pleistocene time; its alluvial materials consist largely of clay derived from an ancient moraine believed to be of Nebraskan age. The valley-floor deposit, called the pay streak of the valley floor, consists of alluvial materials of fluvial and glaciofluvial origin whose ages range from Yarmouth to Holocene. Both channels contain high-grade placers that have yielded a large volume of platinum metals. The platinum metals of these deposits are contained mainly in two alloys intergrown in a pseudoeutectic fabric. The major alloy is mainly platinum, with a small amount of iridium, still smaller amounts of rhodium and palladium, and probably some osmium and ruthenium. The minor alloy is dominantly iridium and osmium with less platinum and still less ruthenium and rhodium. The compositions of these two alloys are somewhat variable but tend to approach constant mean values. A small amount of free gold is recovered with the platinum metals. In addition to these two alloys, minute amounts of five platinum minerals have been identified. The weighted mean percentages of platinum, iridium, osmium, ruthenium, rhodium, palladium, and gold, as mined from 1936 to 1972, are respectively 82.25, 11.32, 2.15, 0.17, 1.30, 0.38, and 2.43. The lodes from which these placers have been derived are, or have been, localized in the Red Mountain ridge, where the principal rocks are dunite and serpentinite. No lodes have been recognized, either because the platinum metals are sparsely distributed or because a large part of the platiniferous rock has been eroded. It is known, however, that the amounts of iridium, osmium, and ruthenium, or of osmiridium, decrease from south to north. Other generalizations regarding the composition and granularity of these metals have been deduced. The size and shape of the platinum lodes may have ranged from diffuse disseminations to high-grade concentrations in small loci. Under certain assumptions, the tenor of platinum metals in the dunite can be roughly approximated. Utilizing two totally different methods of computation, the mean tenor has been estimated to lie between 0.19 and 0.27 grain of platinum metals per cubic yard of dunite, or 0.014 to 0.023 gram per stere. No large low-grade deposits of commercial value are likely to be found, but it is possible that some small high-grade concentrations occur.