Abstract

Observations at several porphyry Cu deposits indicate that in those formed above source magma chambers emplaced to relatively shallow levels (roofs ~3–4 km deep), zones of higher Cu grade are associated with the oldest intrusions of a central porphyry cluster, abundant early (A-type) quartz veins, secondary feldspar alteration, and usually high Fe/Fe + Cu, low S/Fe + Cu sulfide-oxide mineral assemblages. In deposits for which source magma chambers were deeper (roofs >5 km), porphyries closest in age to ore are spatially less directly related to mineralization. In these, higher Cu grades in early mineralization comprise swarms of veinlets with alteration halos that include sericite as well as secondary feldspar and biotite. Sulfide ± oxide assemblages are usually of lower Fe/Fe + Cu and higher S/Fe + Cu. These differences in deposit characteristics appear to correspond with a change from two-phase, chloride-bearing, aqueous fluids that would exsolve from magma at shallow depths, to one-phase fluids deeper.

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