Exploration Strategy and Methods for Pegmatite Deposits of Tantalum

Abstract

Economic concentrations of tantalum in granitic pegmatites are restricted to the rare-element class of orogenic pegmatite associations. These pegmatites are particularly abundant in the Kenoran greenstone belts and parametasedimentary gneissic troughs, Hudsonian cratonic margins and rifts, and Variscan fold belts. Abukuma-type metamorphic facies series is the typical metamorphic environment, and a close spatial relationship to major structural breaks plus interformational boundaries is noted. Within the provinces, individual pegmatite fields comprise largely posttectonic intrusions of fertile granites and their pegmatite aureoles, related to fault systems or lithologic boundaries within andalusite-, cordierite- and staurolite-bearing schists of mainly lower-amphibolite facies. Within the fields, individual granite + pegmatite groups may contain tantalum mineralization within the pegmatitic-granite cupolas of the parental granites themselves, but it is usually found only in pegmatites populating intermediate to outer parts of the aureoles. Directions of decreasing metamorphic grade of the host rocks; increasing textural and paragenetic complexity of the pegmatites; progressive changes in their typomorphic minerals; and increasing fractionation point to areas with better tantalum potential. Within individual pegmatitic granites and pegmatite bodies, a variety of geochemical indicators is available to assess the general fractionation level conducive to Ta enrichment relative to Nb, and to absolute concentrations of Ta in the bulk composition. Geochemical and geophysical methods are applicable to searching for, and outlining, pegmatite deposits. Special techniques are available for exploration in deeply weathered terranes and in glaciated regions.