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

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

Porphyry Cu-(Mo-Au) deposits are relatively rare but reproducible products of subduction-related magmatism. No unique processes appear to be required for their formation, although additive combinations of common tectono-magmatic processes, or optimization of these processes, can affect the grade and size as well as the location of the resulting deposits. These various contributing processes are reviewed, from partial melting in the mantle wedge overlying the subducting plate, through processes of magma interaction with the lithosphere, to mechanisms for magma emplacement and volatile exsolution in the upper crust. Specific ore-forming processes, such as magmatic-hydrothermal fluid evolution, are not discussed. Hot, hydrous, relatively oxidized, sulfur-rich mafic magmas (predominantly basalts) generated in the metasomatized mantle wedge above a subducting oceanic slab rise buoyantly to the base of the overlying crust where they stall due to density contrasts. Because these magmas are oxidized, sulfur is dominantly present as sulfate, and chalcophile elements such as Cu and Au are incompatible (i.e., they are retained in the melt). As these magmas begin to crystallize they release heat which causes partial melting of crustal rocks. Mixing between crustal- and mantle-derived magmas yields evolved (andesitic to dacitic), volatile-rich, metalliferous, hybrid magmas, which are of sufficiently low density to rise through the crust. Magma ascent is driven primarily by buoyancy forces and is dominantly a fracture-controlled phenomenon. As such, crustal stress and strain patterns play an important role in guiding the ascent of magma from the lower crust. In particular, translithospheric, orogen-parallel, strike-slip structures serve as a primary control on magma emplacement in many volcanic arcs worldwide. A feedback mechanism operates, whereby preexisting faults facilitate magma ascent, the heat from which further weakens the crust and focuses strain. Certain structural geometries, such as fault jogs, step-overs, and fault intersections, offer low-stress extensional volumes during transpressional strain. Such sites represent vertical conduits of relatively high permeability, up which magmas will preferentially ascend. Large upper crustal plutonic complexes may therefore be localized within these structural settings. Having delivered a sufficient volume of evolved, fertile arc magma to a focused position in the upper crust, magmatic fractionation, recharge, and volatile exsolution lead to the development of ore-forming magmatic-hydrothermal systems. To a first approximation, the size of the resulting deposit will be limited by the magma volume delivered to the upper crustal magma chamber. System-specific details such as magmatic-hydrothermal evolution, the nature of the country rocks, and subsequent erosional and weathering history will ultimately control the value of the deposit, but these factors fall outside the scope of this paper.

Tectono-Magmatic Precursors for Porphyry Cu-(Mo-Au) Deposit Formation

Geothermal systems ancient and modern: a geochemical review

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

The largest and best studied concentration of porphyry-type deposits is the Southwest porphyry province in Arizona, southwestern New Mexico, and northern Sonora (Figure 1). Most of these deposits are in the 58 to 72 million-year-old range, with one 163 m.y. date and one 20 m.y. date. Several Mesozoic age porphyry copper deposits occur northwest of the main cluster, and mid-Tertiary age porphyry molybdenum and porphyry copper deposits are present in the Colorado mineral belt and at Bingham Canyon, Utah. A progressive decrease in age occurs from northwest to southeast among the Laramide age Southwest porphyries.Tertiary rocks contemporaneous with and younger than the Laramide mineralization appear to have been formed under conditions of strong deformation and crustal extension. Large displacement, flat, "gravity slide" faults are present in southern Arizona together with evidence of abundant horst-graben structure and rift faulting. A correlation of rock types, structures, and intrusive and extrusive activity suggests that "basin-range"-type extensional structures were formed throughout the Tertiary period and probably through much of the Mesozoic as well.In detail, mineralization tends to favor N. 65 degrees E. and N. 40 degrees W. structures, and the deposits in some instances appear to form belts with these alignments, but the cluster of deposits has an overall oval shape unlike the lineal porphyry provinces in Iran, Chile, and the Philippines.The individual porphyry belts in the Southwest province cross the Texas lineament with no apparent strike-slip displacement. However, current plate tectonic theory suggests that a large right-lateral displacement should exist if this structure corresponds to the Walker Line in Nevada as postulated by Jerome and Cook (1967). There is as yet little definite evidence for the presence of subduction zones in Arizona and west of Arizona during the period of formation of the Southwest porphyries, and evidence for the association of porphyry mineralization with plate tectonic processes as described in the literature is not convincing in the Southwest province. It is particularly unlikely that subducted oceanic crust was the source of metals in the Southwest porphyry deposits.An interpretation of depth of erosion in porphyry deposits can be attempted by comparing the alteration zones and mineral assemblages exposed at the present ground surface with an assumed model of vertical zonation. This suggests that erosion has cut deeply into the columns of Laramide mineralization in deposits in the northwestern part of the general cluster and that deposits to the southeast were left generally intact by post-Laramide erosion.

Regional Characteristics of Porphyry Copper Deposits of the Southwest

Discovery of the orebody, a faulted segment of the San Manuel orebody, was based on recognition of: (1) a zonal pattern in which ore is marginal to a low-grade core with potassic alterations--the zone of highest intensity; (2) evidence for post-mineralization tilting of 45 degrees or more; (3) large-scale mullion structure on the tilted San Manuel fault that indicated the direction of offset.

Geology of the Kalamazoo orebody, San Manuel District, Arizona

The Copper Basin district, Yavapai County, contains a "porphyry Cu" type Cu-Mo deposit in which the bulk of ore mineralization appears to have been controlled by pipe structures. Mineralization is associated with a composite stock of presumed Laramide age which has intruded older Precambrian metamorphosed sediments and intrusive rocks. The Laramide stock is thought to have an elongated "beet" shape with a restricted orifice at depth. Its intrusive units range in composition from diorite to aplite, but mineralization appears to be most closely associated with quartz monzonite and quartz monzonite porphyry units. Mineral deposits of the district have a rough zonal arrangement with Cu-Mo mineralization in the center surrounded by an aureole of Pb-Zn-Ag occurrences. Mineralized breccia pipes are roughly cylindrical, near vertical structures ranging in diameter from 50 to 600 ft. A composite pile is composed of a central core of heterogeneous, rotated, angular to rounded rock fragments surrounded by a zone of nonrotated crackle breccia. The fragments are cemented by quartz and may be mineralized with pyrite, chalcopyrite, and molybdenite. The pipes are thought to have been conduits through which late magmatic fluids, collecting near the restricted root of the stock, passed upward. An individual pipe was initiated by passage of fluid upward along a vertical line such as an intersection of faults or fractures. From this channel, fluid worked into the adjacent fractured rocks and corroded fragments that were eventually loosened and moved in the conduit, gradually enlarging the pipe. Quartz later precipitated, filling all of the open space and choking the conduit. Successive flexures of the stock or recurrent movement on regional faults fractured this quartz in some of the pipes, and later hydrothermal solutions tended to follow these conduits, both because of their location on deep-seated structures and because the fractured quartz provided relatively high permeability. Hydrothermal solutions, preceding and accompanying ore deposition, spread outward from the pipes and formed overlapping aureoles of alteration. Pyrite, chalcopyrite, and molybdenite were deposited along fracture surfaces throughout a large area in Copper Basin, but higher grade mineralization was generally confined to fractured pipe structures.

J. David Lowell

Papers

Lateral and vertical alteration-mineralization zoning in porphyry ore deposits

Regional Characteristics of Porphyry Copper Deposits of the Southwest

Geology of the Kalamazoo orebody, San Manuel District, Arizona

Geology and origin of mineralized breccia pipes in Copper Basin, Arizona

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