Porphyry Deposits: Characteristics and Origin of Hypogene Features

Abstract

Porphyry deposits occur within magmatic belts worldwide and are spatially, temporally, and genetically related to hypabyssal dioritic to granitic intrusions that are porphyritic and that commonly have an aplitic groundmass. The preponderance are Phanerozoic and most typically Cenozoic in age, which reflects the dominance of magmatism related to subduction tectonics and preservation in young rocks. Porphyry deposits are here grouped into five classes based on the economically dominant metal in the deposits: Au, Cu, Mo, W, and Sn. For each porphyry class, the major metal concentration is enriched by a factor of 100 to 1,000 relative to unmineralized rocks of a similar composition. The mass of porphyry deposits ranges over four orders of magnitude, with the mean size of a deposit ordered Cu>Mo approximately Au>Sn>W. Hydrothermal alteration is a guide to ore because it produces a series of mineral assemblages both within the ore zones and extending into a larger volume (>10 km (super 3) ) of adjacent rock. The typically observed temporal evolution in porphyry ores is from early, high-temperature biotite+ or -K-feldspar assemblages (potassic alteration) to muscovite+ or -chlorite assemblages (sericitic alteration) to low-temperature, clay-bearing assemblages (advanced argillic and intermediate argillic alteration), which is consistent with progressively greater acidity and higher fluid-to-rock ratios of fluids, prior to their eventual neutralization. Although advanced argillic alteration is relatively late in the deposits where it is superimposed on ore and potassic alteration, in the deposits where advanced argillic alteration is preserved spatially above ore and commonly extending to the paleosurface, it can form early, broadly contemporaneous with potassic alteration. In contrast, assemblages of Na plagioclase-actinolite and albite-epidote-chlorite-carbonate form from a fluid with low acidity and commonly lack ore minerals. Geologic, fluid inclusion, and isotopic tracer evidence indicate magmatic fluids dominate acidic alteration associated with ore and non-magmatic fluids dominate sodic-calcic and propylitic alteration. Veins contain a large percentage of ore minerals in porphyry deposits and include high-temperature sugary-textured quartz veinlets associated with ore minerals and biotite-feldspar alteration and moderate-temperature pyritic veins with sericitic envelopes. Mineralizing porphyries are intermediate to silicic and their aplitic-textured groundmass represents crystallization as a result of abrupt depressurization of water-rich magma; however, small volumes of ultramafic to intermediate rocks, including lamprophyres, exhibit a close spatial and temporal relationship to porphyry ore formation in some deposits. The understanding of porphyry systems depends critically on determination of the relative ages of events and correlation of ages of events in different locations, which in part depends on exposure. Systems with the greatest degree and continuity of exposure generally have been tilted and dismembered by postmineralization deformation. Most porphyry intrusions associated with ore are small-volume (<0.5 km (super 3) ) dikes and plugs that were emplaced at depths of 1 to 6 km, though some were emplaced deeper. Deposits commonly occur in clusters above one or more cupolas on the roof of an underlying intermediate to silicic intrusion. Altered rocks extend upward toward the paleosurface, downward into the granitoid intrusion from which the porphyry magma and aqueous fluids were generated, and laterally for several kilometers on either side of a deposit. The underlying magma chambers operated as open systems via mafic magma recharge, wall-rock assimilation, crystallization, and intrusion, but mineralizing intrusions did not erupt. Present-day distributions of hydrothermally altered rock and sulfide-oxide ore minerals are time-integrated products of fracture-guided fluid flow. We distinguish three spatial configurations characteristic of all five classes of porphyry deposits, the first of which has two varia ts: (la) sericitic alteration largely lies above and beside potassic alteration in a bell- or hood-shaped volume that narrows upward, as at Chorolque, Henderson, and San Manuel-Kalamazoo; (lb) sericitic alteration is present with advanced argillic alteration, and the latter in some cases forms a broader zone at higher levels in the system, as at Batu Hijau, Cerro Rico, and El Salvador; (2) intense sericitic and local advanced argillic alteration cuts through enclosing potassic alteration near ore but also extends above potassic alteration in an upwardly expanding zone with an overall geometry of a funnel, as at Butte, Chuquicamata, and Resolution; (3) sodic-calcic, in addition to potassic, alteration is widespread in the center of the system and has an inverted cup-shaped volume under potassic alteration, with fingerlike projections of sodic alteration extending up through the overlying orebody, as at Yerington.