Generation of porphyry copper deposits by gas–brine reaction in volcanic arcs
Date
2015-02-09
Authors
Blundy, J.
Mavrogenes, J.
Tattitch, B.
Sparks, S.
Gilmer, A.
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Nature Publishing Group
Abstract
Porphyry copper deposits, that is, copper ore associated with hydrothermal fluids rising from a magma chamber, supply 75%
of the world’s copper. They are typically associated with intrusions of magma in the crust above subduction zones, indicating
a primary role for magmatism in driving mineralization. However, it is not clear that a single, copper-rich magmatic fluid could
trigger both copper enrichment and the subsequent precipitation of sulphide ore minerals within a zone of hydrothermally
altered rock. Here we draw on observations of modern subduction zone volcanism to propose an alternative process for
porphyry copper formation. We suggest that copper enrichment initially involves metalliferous, magmatic hyper-saline liquids,
or brines, that exsolve from large, magmatic intrusions assembled in the shallow crust over tens to hundreds of thousands of
years. In a subsequent step, sulphide ore precipitation is triggered by the interaction of the accumulated brines with sulphurrich
gases, liberated in short-lived bursts from the underlying mafic magmas. We use high-temperature and high-pressure
laboratory experiments to simulate such gas–brine interactions. The experiments yield copper–iron sulphide minerals and
hydrogen chloride gas at magmatic temperatures of 700–800 ◦C, with textural and chemical characteristics that resemble
those in porphyry copper deposits. We therefore conclude that porphyry copper ore forms in a two-stage process of brine
enrichment followed by gas-induced precipitation.
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Nature Geoscience
Type
Journal article