Nature and origin of central Idaho blacksands

Abstract

Fifty or more heavy minerals occur in blacksand placers adjacent to the Cretaceous Idaho batholith. They contain Nb (columbium), Ta, Y, Zr, Hf, U, Th, Ti, and the rare earths. These minerals are of present or future potential value as sources of metals characterized by versatility, including strength and resistance to heat and chemical corrosion. Natural concentrations of blacksand involve mineral hardness, texture, chemical composition, and specific gravity; mineral size, shape, and degree of subroundness; and major alterations in stream regimens affecting competency, capacity, and interruptions of transportation. The latter include climatic changes, lava damming, crustal warping, faulting, and mass-wasting phenomena. Local deep disintegration of bedrock is an important adjunct to blacksand concentration. The original source of heavy mineral elements is believed to be Precambrian placer accumulations. These concentrations were later partially dispersed by metasomatic reconversion during diastrophic maxima, and indistinct swarms of accessory minerals were produced. New placers were formed again later through the operation of erosional and depositional agents. Recent blacksand placers derived their heavy minerals from quartz monzonite, granodiorite, diorite, and metamorphic remnants of original country rock commonly found in adjacent exposures of the Idaho batholith. The batholith may appear deceptively normal in these areas. When present, pegmatite and aplite dikes provide an additional source of heavy detrital minerals. The domination of one or more minerals over others in the more recent placers, appears to be related to the domination of similar elemental constituents in earlier original placer deposits. Thus recent data on both the petrography of the Idaho batholith and the nature and origin of blacksand placers lead to the following conclusions: erosional processes concentrated the first placer ore deposits in the Precambrian; and subsequent events both geomorphic and metasomatic probably played more important roles in the genesis of all types of ore deposits, including placers, than previously has been supposed.