A SHAKE-UP IN THE PORPHYRY WORLD?

摘要

Porphyry Cu deposits form in the shallow crustal parts of arc magmatic systems, which root in the mantle wedge, evolve in lower crustal MASH (melting, assimilation, storage, homogenization) zones and lower to midcrustal hot zones, and accumulate in mid- to upper crustal batholiths at depths of 5 to 10 km. A small proportion of the magma and most of the volatile load rises toward the surface due to buoyancy, and may erupt as volcanic or fumarolic emissions. Low levels of volcanism and fumarolic activity, as well as subsurface hydrothermal flow and alteration, are normal and semicontinuous features of active arc magmatic systems, which may operate for millions of years. Porphyry Cu deposits, on the other hand, form rarely (typically = 1 per batholith) and rapidly ( = 100,000 years) in the subsurface (2-5 km depth), where hydrous volatiles exsolved from the underlying batholith are channeled into structurally controlled cupola zones and cool before reaching the surface. The explosively brecciated character of early mineralization stages (breccia pipes and stockworks) suggests that the initiation of fluid flow may be essentially instantaneous and catastrophic, with the longer total duration of hydrothermal activity reflecting slower kinetically controlled fluid exsolution processes, or draining of deeper parts of the system. These fluids generate intense subsurface hydrothermal alteration, and may precipitate economic concentrations of Cu-sulfide minerals in potassic alteration zones as they cool between similar to 400 degrees and 300 degrees C.