Metal transport and ore deposition in porphyry copper ± gold ± molybdenum deposits - contrasting behaviour between deep and shallow environments
2015
High-grade zones typically develop in the Ksilicate-
altered centre of porphyry deposits, where copper
ą gold ą molybdenum are localised in and around the apex
of the intrusion. Mineralisation develops in spatial and
temporal association with abundant quartz ą magnetite ą
orthoclase ą biotite ą anhydrite as veins, breccia cement
and alteration minerals. Multiple generations of stockwork
veins ą magmatic-hydrothermal breccias are common and
provide evidence for repeated cycles of fluid release from
the magma reservoir. Unmixing of brines and vapours from
supercritical fluids (deep-level porphyries) and/or direct
exsolution of these two phases from the intrusive complex
(shallow-level porphyries) are essential prerequisites to
ore formation. The brines contain most or all of the oreforming
components, and have densities that restrict their
occurrence to the apex and shoulders of the mineralising
intrusion, where potassic alteration and mineralisation
occurs. The vapour passes through the mineralised centre
of porphyry deposits, ascending rapidly to the nearsurface
environment, where it may form a lithocap.
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