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 sediment-hosted 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, intermediate-sulfidation 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 litho-caps. The porphyry Cu mineralization occurs in a distinctive sequence of quartz-bearing veinlets as well as in disseminated form in the altered rock between them. Magmatic-hydrothermal breccias may form during porphyry intrusion, with some of them containing high-grade mineralization because of their intrinsic permeability. In contrast, most phreatomagmatic breccias, constituting maar-diatreme systems, are poorly mineralized at both the porphyry Cu and lithocap levels, mainly because many of them formed late in the evolution of systems.

Porphyry Cu systems are initiated by injection of oxidized magma saturated with S- and metal-rich, aqueous fluids from cupolas on the tops of the subjacent parental plutons. The sequence of alteration-mineralization events charted above is principally a consequence of progressive rock and fluid cooling, from >700° to <250°C, caused by solidification of the underlying parental plutons and downward propagation of the lithostatic-hydrostatic transition. Once the plutonic magmas stagnate, the high-temperature, generally two-phase hyper-saline liquid and vapor responsible for the potassic alteration and contained mineralization at depth and early overlying advanced argillic alteration, respectively, gives way, at <350°C, to a single-phase, low- to moderate-salinity liquid that causes the sericite-chlorite and sericitic alteration and associated mineralization. This same liquid also causes mineralization of the peripheral parts of systems, including the overlying lithocaps. The progressive thermal decline of the systems combined with synmineral paleosurface degradation results in the characteristic overprinting (telescoping) and partial to total reconstitution of older by younger alteration-mineralization types. Meteoric water is not required for formation of this alteration-mineralization sequence although its late ingress is commonplace.

Many features of porphyry Cu systems at all scales need to be taken into account during planning and execution of base and precious metal exploration programs in magmatic arc settings. At the regional and district scales, the occurrence of many deposits in belts, within which clusters and alignments are prominent, is a powerful exploration concept once one or more systems are known. At the deposit scale, particularly in the porphyry Cu environment, early-formed features commonly, but by no means always, give rise to the best ore-bodies. Late-stage alteration overprints may cause partial depletion or complete removal of Cu and Au, but metal concentration may also result. Recognition of single ore deposit types, whether economic or not, in porphyry Cu systems may be directly employed in combination with alteration and metal zoning concepts to search for other related deposit types, although not all those permitted by the model are likely to be present in most systems. Erosion level is a cogent control on the deposit types that may be preserved and, by the same token, on those that may be anticipated at depth. The most distal deposit types at all levels of the systems tend to be visually the most subtle, which may result in their being missed due to overshadowing by more prominent alteration-mineralization.

Porphyry Copper Systems

Geothermal solute equilibria. Derivation of Na-K-Mg-Ca geoindicators

Orogenic gold and geologic time: a global synthesis

Post-collisional strongly peraluminous granites

There are many examples of spatially associated porphyry and epithermal ore deposits; a genetic connection has been suggested for some and argued against for others. Nowhere is this spatial association better demonstrated than in the Mankayan district of northern Luzon, Philippines, where the Lepanto high-sulfidation epithermal Cu-Au deposit is superadjacent to the Far Southeast porphyry Cu-Au orebody; together they contain >3.8 million tons (Mt) Cu and >550 t Au.Quartz diorite porphyry dikes intruded Miocene basement rocks of metavolcanic and volcaniclastic rocks to a 300-m elevation. These intrusions postdate the Pliocene volcanic breccia and dacite porphyry that host much of the epithermal ore. K silicate alteration, consisting of biotite-magnetite and minor K feldspar, is centered on the quartz diorite porphyry. K-Ar ages of the biotite are 1.41 + or - 0.05 Ma (n = 6). Vitreous, anhedral quartz veins are associated with this early alteration and contain vapor-rich and hypersaline liquid inclusions with maximum homogenization temperatures of 450 degrees to 550 degrees C (and 50-55 wt % NaCl equiv salinities). Lithostatic pressure estimates indicate a paleosurface at a > or = 1,500-m elevation. Advanced argillic alteration formed over the top of the porphyry and consists of quartz-alunite, dated at 1.42 + or - 0.08 Ma (n = 5), synchronous with K silicate alteration. The lower limit of extensive quartz-alunite alteration is at a [asymp] 600-m elevation. Similar alteration and a core of leached, silicic alteration extend northwestward >4 km along the basement dacite contact, localized by the Lepanto fault. Chemical and S isotope zoning of alunite along strike indicates progressively lower temperatures away from the porphyry, from 350 degrees to 200 degrees C. K silicate alteration is overprinted by alteration consisting of chlorite plus hematite and/or sericite-illite, with a marginal zone containing pyrophyllite and an outer zone of propylitic alteration. The chlorite-sericite alteration is cut by veins of euhedral quartz that locally fill reopened anhedral quartz veins. The euhedral quartz veins contain anhydrite-white mica-pyrite + or - chalcopyrite + or - bornite and have halos of sericite; illite separated from these halos has ages of 1.30 + or - 0.07 Ma (n = 10). Fluid inclusions provide evidence for boiling on inception of this fracturing event (T h = 350 degrees C, 5 wt % NaCl equiv) and indicate a depth of 1,500 to 2,000 m below the paleowater table. This brittle-fracture event was followed by cooling and dilution of the hydrothermal fluid.The elevation of the enargite Au epithermal ore and its host of silicic alteration increases as the unconformity between the basement and dacite breccia rises from a 700- to 1,200-m elevation with increasing distance from the porphyry. Published data on enargite-hosted fluid inclusions (T h = 295 degrees -200 degrees C, 4-2 wt % NaCl equiv) indicate that the temperature and salinity both decrease with increasing distance from the porphyry. Epithermal ore consists of stage 1 euhedral pyrite-enargite-luzonite, and subsequent stage 2 Au is accompanied by tetrahedrite-chalcopyrite-sphalerite plus telluride and selenide minerals. Anhydrite and barite gangue minerals are followed by late vug-fulling quartz and kandite minerals. The quartz-alunite alteration halo passes outward to "kandite" (kaolinite-nacrite-diclcite) alteration, then to chlorite or montmorillonite, depending on the host rock (basement or dacite, respectively).The dated minerals were also analyzed for their delta 18 O and delta D compositions, and their associated hydrothermal water values were calculated. Water in isotopic equilibrium with biotite averaged +6.3 and -45 per mil, respectively, typical of hypersaline liquid exsolved from felsic magma. The acidic water that deposited the alunite formed when magmatic vapor (+7ppm delta 18 O and -25ppm delta D) was absorbed by local meteoric water (-10ppm delta 18 O and -70ppm delta D) in a proportion of [asymp] 9:1 magmatic to meteoric. Lateral flow to the northwest and progressive mixing with ground water diluted the magmatic component to 1:1 at a distance of 4 km from the porphyry. At the depth of the porphyry, deposit, the later water isotopically stable with sericite was dominantly magmatic (+5.7ppm delta 18 O and -43ppm delta D) in the core. The marginal sericitie alteration (+1.5ppm delta 18 O and -51ppm delta D water values) indicates a maximum 20 to 30 percent component of local meteoric water. Pyrophyllite in both the porphyry and epithermal deposits formed from water with an isotopic composition similar to that which formed the sericitic alteration. The late euhedral quartz veining and sericitic alteration appear to have been associated with the majority of Cu and Au deposition. In addition, mineralogic, paragenetic, isotopic, and fluid inclusion evidence suggests that this water precipitated the enargite and Au within the epithermal deposit.Our results reinforce guidelines for exploration of such deposits. Advanced argillic (quartz-alunite) and K silicate alteration at Lepanto-Far Southeast are coupled in origin and result from vapor and hypersaline liquid separation. Thus, exploration programs for buried porphyry deposits should document carefully the geologic, morphologic, and temporal characteristics of exposed areas of advanced argillic alteration and its origin. Sericitic alteration at Far Southeast is associated with porphyry Cu and Au ore and appears to represent the roots of the main-stage Cu-Au mineralization in the epithermal deposit, hosted by silicic and quartz-alunite alteration that has a lower limit near the top of porphyry Cu-Au ore. In some cases, the sericitic overprint of a porphyry system, particularly where it is related to Cu and Au enrichment, may indicate a potential for nearby epithermal mineralization. Similarly, sericite and/or pyrophyllite underlying or overprinting a zone of hypogene advanced argillic (quartz-alunite) alteration indicates that mineralizing fluid may have ascended to epithermal depths. Epithermal ore at Lepanto-Far Southeast reflects a paleohydrologic regime dominated by lateral fluid flow, with a marked control by intersection of the Lepanto fault and a lithologic unconformity. Recognizing evidence for lateral flow is critical, as paleohydrology controlled the distribution of alteration and mineralization in many high-sulfidation epithermal deposits.

Evolution of an intrusion-centered hydrothermal system; Far Southeast-Lepanto porphyry and epithermal Cu-Au deposits, Philippines

Copper and molybdenum deposits, potassic, phyllic, argillic, and propylitic alteration, emphasis on San Manuel-Kalamazoo deposit of Arizona, tabulated data on characteristics of North and South American deposits

Lateral and vertical alteration-mineralization zoning in porphyry ore deposits

Chloride from fluid inclusions in hydrothermal minerals is found to have variable and distinctive {delta}{sup 37}Cl values spanning the range -1.1 0/{per thousand} to +0.8 {per thousand}. In Mississippi Valley-type deposits of Tennessee, brines of high (>0{per thousand}) and low (near -1{per thousand}) {delta}{sup 37}Cl are present. High {delta}{sup 37}Cl brines may be saline formation waters, but low {delta}{sup 37}Cl brines remain unexplained. In porphyry copper deposits, both high {delta}{sup 37}Cl (0.8{per thousand}, 0.3{per thousand}) and low {delta}{sup 37}Cl (-1.1{per thousand}, 0.7{per thousand}) hypersaline brines of probable magmatic origin occur. High-salinity magmatic brines with low {delta}{sup 37}Cl values contrast isotopically with high {delta}{sup 37}Cl, less concentrated brines responsible for quartz-sericite-pyrite assemblages.

John M. Guilbert

Papers

Lateral and vertical alteration-mineralization zoning in porphyry ore deposits

Preliminary evidence for fractionation of stable chlorine isotopes in ore-forming hydrothermal systems

Distribution of the Radioelements Potassium, Uranium, and Thorium in Selected Porphyry Copper Deposits

Geochemistry and metallogeny of Arizona peraluminous granitoids with reference to Appalachian and European occurrences

Tilting history of the San Manuel-Kalamazoo porphyry system, southeastern Arizona; discussion