Characteristics and controls of the largest porphyry copper‐gold and epithermal gold deposits in the circum‐Pacific region

Eleven gold‐rich porphyry copper and 14 epithermal gold deposits around the Pacific rim contain > 200 t (‐7 million oz) of gold. These large porphyry‐type deposits conform to a single overall model, whereas the large epithermal gold deposits are varied in both genetic type and mineralisation style. Most regional and local characteristics of the largest porphyry and epithermal deposits fail to explain convincingly their extremely high gold contents. Nevertheless, a number of hypothetical processes operative alone, or in combination, in the mantle, in upper crustal magma chambers and at the sites of gold accumulation are believed to maximise the likelihood of exceptional gold concentrations. Partial melting of the upper parts of stalled lithospheric slabs in the mantle, immediately following collision or arc migration, promotes oxidation of mantle sulfides and the release of gold. These tectonic scenarios may also result in rapid cooling and uplift‐induced depressurisation of upper crustal magma chambers, thereby accelerating the release of gold‐bearing magmatic fluids. Upper crustal magma mixing and passive degassing of the resultant SO2 are also considered to favour gold availability. Rheological and permeability contrasts at sites of gold deposition are important controls on the size and tenor of gold deposits. The summation of these mechanisms tends to result in short‐lived, areally restricted gold‐forming events, commonly as an end‐stage of arc development. These hypothetical processes translate into several practical criteria of potential use to explorationists. Both large gold‐rich porphyry and epithermal deposits seem to be more common in atypical arc settings and in association with unusual, especially highly potassic, igneous rocks. During the search for gold‐rich porphyry deposits, high hydrothermal magnetite contents, very young arcs in the tropical environment and impermeable host rocks, especially limestones, deserve emphasis. In contrast, large epithermal gold deposits are commonly controlled by marked lithologic differences and associated with flow‐dome and/or maar‐diatreme systems.