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Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, Swiss Federal Institute of Technology, ETH Zentrum NO, CH-8092 Zürich, Switzerland
heinrich{at}erdw.ethz.ch
This paper summarizes our current understanding of the formation of porphyrystyle Cu ± Au ± Mo deposits, in the light of data obtained by direct analysis of the ore metals in individual fluid and melt inclusions using laser-ablation ICP mass spectrometry. An integrated study of the evolution of the calcalkaline Farallón Negro Volcanic Complex hosting the Bajo de la Alumbrera porphyry Cu-Au deposit (Argentina), and supplementary fluid-chemical data from Bingham (Utah) and other examples, permit a quantitative re-assessment of the fundamental processes controlling the key economic parameters of porhyry-style ore deposits. Deposit size (total metal content) is optimized by exsolution of a relatively dense (>0.3 g cm–3) single-phase fluid or a two-phase brine + vapour mixture from a moderately large hydrous pluton, possibly with an intermediate step involving the scavenging of the ore-forming elements in a magmatic sulphide melt. Emplacement mechanism, magma-chamber dynamics and possibly an additional source of sulphur are probably more decisive for the formation of a large deposit than sheer pluton volume and elevated Cu contents in the melts. Primary bulk ore grade is determined by temperature-controlled precipitation of ore minerals, which is optimized where a large magmatic fluid flux is cooled through 420–320 °C over a restricted vertical flow distance. Bulk metal ratios of the deposits, exemplified by the economically important Au/Cu ratio in the ore, are primarily controlled by the magmatic source defining the composition of the fluids before they reach the deposit site, although selective precipitation may contribute to metal zoning within orebodies.
