Granites and metal deposits

Abstract

Granitic rocks, spanning a broad range of compositions and silica contents, are related genetically to a spectrum of precious-, chalcophile-and lithophile-metal deposits. The metals present are dictated by the composition, degree of fractionation and redox states of the associated intrusions. The fertile intrusions and associated metal deposits occupy many tectonic settings, but arc and back-arc environments are the most important. With the exception of some rare-metal concentrations in highly evolved granites and related pegmatites, the deposits are formed by precipitation from metal-bearing fluid that separates during magmatic crystallisation. A variety of deposit types is recognised widely. The porphyry type accounts for all of the world's molybdenum and much of the copper. Vein, skarn and carbonate-replacement types supply all the tin and tungsten, with disseminated greisens being the source for much alluvial tin. Pegmatites dominate lithium and tantalum production. Large gold deposits, the main focus of current exploration activity, include porphyry, breccia, skarn, carbonate-replacement and sheeted-stockwork vein types. Gold deposits, unlike those of other metals, appear to be products of two broad intrusive suites with different redox states, as reflected by an association with either chalcophile or lithophile metals.