A shake-up in the porphyry world?

Abstract

Porphyry Cu deposits form in the shallow crustal parts of arc magmatic systems, which root in the mantle wedge, evolve in lower crustal MASH zones (melting, assimilation, storage, homogenization) and lower-to-mid crustal hot zones, and accumulate in mid-to-upper crustabatholiths at depths of 5–10 km. A small proportion of the magma and most of the volatile load rises due to buoyancy towards the surface, 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 semi-continuous 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 ~400°–300°C. The suddenness and infrequency of these ore-forming hydrothermal events suggests that they are triggered by an external process acting on otherwise normally evolving magmatic systems. Sudden depressurization or agitation of a large, primed, volatile-saturated or supersaturated mid– upper crustal magma chamber could lead to rapid and voluminous volatile exsolution and fluid discharge. This sudden volatile flux could result in either a large explosive volcanic eruption if the surface is breached, or a large magmatic-hydrothermal system that could form a porphyry Cu deposit if fluid flow is restricted to the subsurface. Candidates for triggers of these destabilizing events are catastrophic mass wasting such as volcanic edifice collapse, or mega-earthquakes, the latter possibly causing the former. The frequency of such catastrophic events occurring in proximity to active arc batholiths may approximate the recurrence rate of formation of large porphyry Cu deposits.