New clues to origin of porphyry-type ore deposits

The prominence of porphyry Cu-Au-Mo deposits above active subduction zones at continental margins, as in the Andes, has long encouraged ore geologists to suggest that they form as part of continental arc magmatism. Typically they occupy cupolas above large, intermediate to felsic, subvolcanic magma chambers that source the ore-forming fluid and most of the metals. Most show evidence of the influence of explosive fluid boiling that shatters the host porphyry mass during late stage hydrothermal activity thereby producing myriad cracks that become mineralised as a stockwork. One of the largest, among the longest worked and most investigated porphyry deposits is that at Bingham Canyon in Utah, USA. New isotope geochemistry bucks the accepted wisdom about porphyry-type mineralisation, in particular the source of the contained metals (Pettke, T. et al. 2010. The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions. Earth and Planetary Science Letters, v. 296, p. 267–277).

The Bingham Canyon ores and host intrusion are Cenozoic in age (~38 Ma). However, isotopes of lead in fluid inclusions within the ore zone reveal a much more ancient metal endowment of the mantle underlying continental crust, around 1800 Ma ago, probably by metasomatism during the accretion of Palaeoproterozoic island arcs. Magmatism in the late Eocene, presaging the evolution of the Basin and Range extensional province drew in Cu and Au from the mantle and Mo from assimilated continental crust; i.e. Bingham Canyon and other huge porphyry deposits of the Western USA inherited metal enrichment from long beforehand, unlike those of active continental arcs. The intrinsic importance of the discovery is that given intermediate to felsic magmatism of any age, if it is sourced in relics of earlier arc-related igneous events then there is a chance that more recent activity may spawn rich porphyry deposits; more or less anywhere, given a metal endowed infrastructure. That opens up exploration possibilities to hitherto unexplored ground above ancient subduction zones.

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