Porphyry Cu systems host some of the most widely distributed mineralization types at convergent plate boundaries, including porphyry deposits centered on intrusions; skarn, carbonate-replacement, and sedimenthosted Au deposits in increasingly peripheral locations; and superjacent high-and intermediate-sulfidation epithermal deposits. The systems commonly define linear belts, some many hundreds of kilometers long, as well as occurring less commonly in apparent isolation. The systems are closely related to underlying composite plutons, at paleodepths of 5 to 15 km, which represent the supply chambers for the magmas and fluids that formed the vertically elongate (>3 km) stocks or dike swarms and associated mineralization. The plutons may erupt volcanic rocks, but generally prior to initiation of the systems. Commonly, several discrete stocks are emplaced in and above the pluton roof zones, resulting in either clusters or structurally controlled alignments of porphyry Cu systems. The rheology and composition of the host rocks may strongly influence the size, grade, and type of mineralization generated in porphyry Cu systems. Individual systems have life spans of ~100,000 to several million years, whereas deposit clusters or alignments as well as entire belts may remain active for 10 m.y. or longer. The alteration and mineralization in porphyry Cu systems, occupying many cubic kilometers of rock, are zoned outward from the stocks or dike swarms, which typically comprise several generations of intermediate to felsic porphyry intrusions. Porphyry Cu ± Au ± Mo deposits are centered on the intrusions, whereas carbonate wall rocks commonly host proximal Cu-Au skarns, less common distal Zn-Pb and/or Au skarns, and, beyond the skarn front, carbonate-replacement Cu and/or Zn-Pb-Ag ± Au deposits, and/or sediment-hosted (distal-disseminated) Au deposits. Peripheral mineralization is less conspicuous in noncarbonate wall rocks but may include base metal-or Au-bearing veins and mantos. High-sulfidation epithermal deposits may occur in lithocaps above porphyry Cu deposits, where massive sulfide lodes tend to develop in deeper feeder structures and Au ± Ag-rich, disseminated deposits within the uppermost 500 m or so. Less commonly, intermediatesulfidation epithermal mineralization, chiefly veins, may develop on the peripheries of the lithocaps. The alteration-mineralization in the porphyry Cu deposits is zoned upward from barren, early sodic-calcic through potentially ore-grade potassic, chlorite-sericite, and sericitic, to advanced argillic, the last of these constituting the lithocaps, which may attain >1 km in thickness if unaffected by significant erosion. Low sulfidation-state chalcopyrite ± bornite assemblages are characteristic of potassic zones, whereas higher sulfidation-state sulfides are generated progressively upward in concert with temperature decline and the concomitant greater degrees of hydrolytic alteration, culminating in pyrite ± enargite ± covellite in the shallow parts of the lithocaps. The porphyry Cu minerali...

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Although it is now widely accepted that porphyry copper deposits consist of zonally arranged shells of alteration and mineralization centered on high-level, calc-alkaline stocks, the nature of their uneconomic upward and downward extensions remains undocumented. This paper attempts to characterize these upward and downward extensions and to integrate the resulting concepts into a hypothetical model for complete porphyry copper systems. Examples from Chile, Argentina, and elsewhere are used to aid in the substantiation of the model. Programs of exploration for porphyry ore deposits can clearly benefit from the application of a model of this sort.A typical porphyry copper-bearing stock is inferred to grade downward into stockwork mineralization and potassium silicate alteration in a phaneritic intrusive, which in turn is transitional downward to an essentially unaltered pluton of considerably larger dimensions than the stock. Porphyry copper deposits are normally located in the basement beneath a comagmatic volcanic pile, which is transected by a column of hydrothermal alteration representing the upper parts of the porphyry copper system. This alteration consists of propylitic and argillic types with localized patches of silicification and advanced argillic alteration. The volcanic pile is thought to constitute a stratovolcano which possesses large native sulfur deposits and small quantities of base metals, particularly copper, in sublimates at high-temperature fumaroles in the vicinity of its central vent; these surficial deposits are considered as the effluent products of active porphyry copper systems.The available evidence favors the emplacement of the tops of typical porphyry copper deposits at depths of 1.5-3 km beneath the summits of stratovolcanoes and suggests that entire porphyry copper systems possess vertical extensions as great as 8 km.At Chuquicamata, Chile, a major high-angle fault may have cut the porphyry copper deposit, and subsequent erosion has removed the portion of the deposit that was situated in the upthrown block. The hydrothermal alteration pattern in the remaining part of the ore body is incomplete and terminates abruptly against the fault. The unaltered, phaneritic granodiorite, containing minor veins and pegmatitic bodies, in the upthrown block is interpreted as the root zone of the Chuquicamata porphyry copper system. The low•er most, rMneralized part of a porphyry system is believed to be exposed at Los Loros, Chile. There a zone of molybdenum-rich and copper-poor potassium silicate alteration carrying abundant K-feldspar occupies an area in the interior of a relatively large pluton of phaneritic granite.At Faral16n Negro, northwest Argentina, several small porphyry copper deposits pierce the infrastructure of a temporally related, andesitic stratovolcano. This unusual locus of the deposits above the subvolcanic basement enables it to be determined that porphyry copper emplacement was a late event in the construction of the stratovolcano, succeeded only by the formation of minor rhyol...

The theory of lithosphere plate tectonics, embodying the concepts of sea-floor spreading, transform faulting, and underthrusting at continental margins and island arcs, is employed as a basis for an actualistic, though speculative, model for the origin and space-time distribution of porphyry copper and porphyry molybdenum deposits.Porphyry ore deposits, occurring in the western Americas, southwest Pacific and Alpide orogenic belts, are thought to constitute a normal facet of calc-alkaline magmatism. Chemical and isotopic data cited are consistent with the generation of the components of calc-alkaline igneous rocks and porphyry ore deposits by partial melting of oceanic crustal rocks on underlying subduction zones at the elongate compressire junctures between lithospheric plates.It is proposed that the metals contained in porphyry ore deposits were derived from the mantle at divergent plate junctures, the ocean rises, as associates of basic magmatism, and transported laterally to subduction zones as components of basaltic-gabbroic oceanic crust and small amounts of suprajacent pelagic sediments; evidence supporting the presence of significant amounts of metals in the oceanic crust is listed.It is suggested that the temporal and spatial distribution of porphyry ore deposits is dependent on two principal factors, namely the erosion level of an intrusive-volcanic chain, and the time and location of magma generation, and the availability of metals, on an underlying subduction zone. The erosion factor is beli.eved to offer an explanation for the paucity of porphyry ore deposits in pre-Mesozoic orogenic belts, and for the relative abundance of exposed porphyry deposits of Upper Cretaceous-Paleogene age in post-Paleozoic orogens. Provinces with a high concentration of porphyry copper deposits, such as southern Peru-northern Chile and the southwest United States, may be interpreted as regions beneath which anomalously copper-rich oceanic crust was subducted at the time of porphyry copper emplacement; one possible explanation for the episodic formation of volumes of copper-rich oceanic crust is the presence of a heterogeneous distribution of metals in the low velocity zone of the upper mantle. Porphyry ore deposits seem to have formed during a series of relatively short, discrete pulses, perhaps correlable with changes in the relative rates and directions of motion of lithospheric plates. In some regions, such as Chile, porphyry ore deposits are arranged in parallel, linear belts, which may be explicable in terms of shifting loci of magma and included metal generation on a subduction zone, and which seem to be largely independent of control by tectonic lineament intersections. The time intervals during which the formation of porphyry deposits took place are shown to be broadly coincident with periods of lithosphere plate convergence, and porphyry deposits may still be forming above currently active subduction zones.A number of potential regions for the discovery of porphyry ore deposits are suggested, and the importance to...

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