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

Magmatic fluids, both vapour and hypersaline liquid, are a primary source of many components in hydrothermal ore deposits formed in volcanic arcs. These components, including metals and their ligands, become concentrated in magmas in various ways from various sources, including subducted oceanic crust. Leaching of rocks also contributes components to the hydrothermal fluid—a process enhanced where acid magmatic vapours are absorbed by deeply circulating meteoric waters. Advances in understanding the hydrothermal systems that formed these ore deposits have come from the study of their active equivalents, represented at the surface by hot springs and volcanic fumaroles.

The role of magmas in the formation of hydrothermal ore deposits

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

More than half of the 25 largest known porphyry copper deposits, defined in terms of contained copper
metal, formed during three time periods: the Paleocene to Eocene, Eocene to Oligocene, and middle Miocene
to Pliocene. These giant deposits are clustered within three provinces, central Chile, northern Chile, and
southwest Arizona-northern Mexico. Other giant deposits occur in Montana, Utah, Panama, Peru, Argentina,
Irian Jaya, Mongolia, and Iran. Compressive tectonic environments, thickened continental crust, and active uplift
and erosion were associated with the formation of many of these deposits. Calc-alkalic magmas are most favorable
for the formation of giant porphyry copper deposits, although several of the largest systems are associated
with high K calc-alkalic intrusions.
The 25 largest gold-rich porphyry deposits are concentrated in the southwest Pacific and South America,
with other occurrences in Eurasia, British Columbia, Alaska, and New South Wales. Many of the deposits
formed in the last 13 m.y. The largest of the deposits are associated with high K calc-alkalic intrusions. Many
calc-alkalic porphyritic intrusions have also produced giant gold-rich porphyries.
In the last 20 m.y., the formation of giant porphyry copper-molybdenum and copper-gold deposits in the circum-Pacific region has been closely associated with subduction of aseismic ridges, seamount chains, and
oceanic plateaus beneath oceanic island and continental arcs. In several examples, these tectonic perturbations
have promoted flat-slab subduction, crustal thickening, uplift and erosion, and adakitic magmatism coeval with
the formation of well-endowed porphyry and/or epithermal mineral provinces. Similar tectonic features are inferred
to be associated with the giant porphyry copper-molybdenum provinces of northern Chile (Eocene-Oligocene) and southwest United States (Cretaceous-Paleocene).
Topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry
ore formation. Other factors, such as sutures in the overriding plate, permeability architecture of the upper
crust, efficient processes of ore transport and deposition, and, in some cases, formation and preservation of supergene
enrichment blankets are also vital for the development of high-grade giant ore deposits. A low-grade
geochemical anomaly may be the final product of mineralization, if ore-forming processes do not operate efficiently, even in the most favorable geodynamic settings.

Giant Porphyry Deposits: Characteristics, Distribution, and Tectonic Controls

Porphyry Deposits: Characteristics and Origin of Hypogene Features

Geological characteristics and tectonic setting of proterozoic iron oxide (CuUAuREE) deposits

$KARN deposits occur throughout a broad range of geologic environments from Precambrian to late Tertiary age. Most deposits of economic importance are relatively young, however, and are related to magmatic-hydrothermal activity associated with dioritic to granitic plutonism in orogenic belts. The feature that sets skarn deposits apart from other types of mineral deposits is the gangue--a coarse-grained, generally iron-rich, mixture of Ca-Mg-Fe-A1 silicates, formed by metasomatic processes at relatively high temperature and termed skarn. Since skarns are becoming increasingly important as sources of certain metals and as subjects for scientific investigation, the editors felt that a call for papers would result in a timely publication of data and ideas that could guide new or on-going exploration and research programs. Thus, the present issue of Economic Geology is devoted to papers dealing with the geology and geochemistry of skarn deposits, based on research being conducted in North America and Japan. The result is a major new descriptive and analytical data base for two dozen skarn deposits in Canada, the United States, Mexico, Japan, and Korea (Fig. 1, Table 1). Recent experimental studies on the stabilities of calc-silicates and a historical bibliography of skarn research round out the issue. Other topics that the editors planned to include, but which did not materialize, were reviews of metasomatic processes (e.g., Frantz and Mao, 1979; Walther and Helgeson, 1980), computational approaches to mass transport and water-rock reactions in carbonate environments (e.g., Norton et al., 1975), and light stable isotopes and fluid inclusions (e.g., Patterson et al., 1981). Some of these techniques are only now beginning to be applied to the study of skarn deposits--we hope that the present issue will provide an incentive for further studies.

Introduction; terminology, classification, and composition of skarn deposits

Surface exposures in the Singatse Range at the Ann-Mason deposit yield a nearly complete vertical cross section, from 1 to 6 km in paleodepth, through one of three porphyry Cu deposits genetically tied to the Middle Jurassic Yerington batholith. Detailed field mapping of wall-rock alteration, veinlets, and sulfides, combined with petrographic, mineral composition, and fluid inclusion studies, has led to an assessment of temperatures, salinities, origins and flow paths of hydrothermal fluids through time and space. Ann-Mason is associated with a granite porphyry dike swarm emanating from a deep granite cupola eraplaced into earlier quartz monzodiorite. On the basis of crosscutting relations between different veins and age relative to porphyry dikes, hydrothermal alteration-mineralization can be divided into premain stage (endoskarn), main stage (propylitic, sodic-calcic, and potassic) and late stage (sodic, chloritic, and sericitic). Advanced argillic alteration in the nearby Buckskin Range represents the paleosurface environment of porphyry systems exposed in the Singatse Range.Pre-main-stage endoskarn (plagioclase + salite + or - garnet) in quartz monzodiorite is localized adjacent to metasedimentary wall rocks at paleodepths of 3 to >6 km. Main-stage propylitic alteration (albite + epidote + actinolite + chlorite) formed at paleodepths above 4 km; sodic-calcic alteration formed at greater paleodepths of 3.5 to >6 km along the granite-quartz monzodiorite contact and in the deep portion of the porphyry dike swarm. The most intense sodic-calcic alteration lacks sulfide and formed oligoclase-actinolite-sphene by addition of Na and leaching of K, Fe, and Cu. Potassic alteration, characterized by replacement of hornblende by biotite + or - K feldspar, formed in a vertical zone 4 km high and 1.2 km in diameter extending upward from the granite cupola along the axis of the dike swarm. The hypogene Cu orebody (495 Mt of 0.4 wt % Cu) lies within the area of most intense potassic alteration at a 2.5- to 4.0-km paleodepth, containing abundant hydrothermal biotite and quartz-K feldspar veins. Sulfide assemblages include chalcopyrite + or - bornite + or - molybdenite, and chalcopyrite > pyrite. Much of the Cu is found in later sulfide epidote veins.Late-stage alteration affected all porphyry dikes and formed a funnel-shaped zone, rooted in the Cu orebody at a palcodepth of 4 km. At a 1-km paleodepth, the late-stage alteration zone is 3 km wide. Late-stage alteration assemblages include early sodic and chloritic and late sericitic ones. Sodic alteration is zoned upward from albite-chlorite-vermiculite to albite-sericite-pyrite; albite-chlorite alteration leached Cu from the center of the ore zone, and both sodic assemblages contain albitized K feldspar. Sodic assemblages grade laterally into chloritic assemblages in which mafic minerals are replaced by chlorite + or - sericite. Sodic and chloritic assemblages are cut by pyrite veins with quartz-sericite-pyrite halos and quartz-tourmaline breccias at paleodepths above 2 km.Hydrothermal evolution can be modeled in terms of four fluids of different origins reacting with rocks by alkali exchange and H (super +) metasomatism. Phase equilibria and fluid inclusion studies constrain temperatures and salinities of hydrothermal fluids to the following values: (1) garnet endoskarn fluids at >500 degrees C, low salinity, X (sub CO 2 ) 400 degrees C, 31 to 41 wt percent NaCl equiv; (3) potassic, ore-forming fluids at 700 degrees to <400 degrees C, 32 to 62 wt percent NaCl equiv; and (4) late-stage fluids at 240 degrees to 200 degrees C, 2 to 13 wt percent NaCl equiv. Endoskarn mostly predated porphyry dike eraplacement and resuited from local influx of Ca-bearing fluids from carbonate wall rocks; these fluids are not linked to the ore-forming process. Sodic-calcic alteration was due to prograding, saline, nonmagmatic fluids (deep formation waters?) that flowed from the wall-rock contact laterally 3 km into the batholith at depths of 4 to 6 km. As they heated, these fluids altered K feldspar to oligoclase and leached Fe and Cu from approximately 10 km 3 of quartz monzodiorite. In the dike swarm, these fluids mixed with upwelling, cooling, saline fluids of magmatic origin and may have contributed some K, Fe, and up to 30 wt percent of the Cu in the ore zone. Magmatic and nonmagmatic, main-stage hydrothermal fluids flowed in convective cells coincident with emplacement of multiple dikes; maximum thermal gradients, fluid fluxes, and metasomatism occurred in the orebody at 500 degrees to 350 degrees C. Dilute, late-stage fluids of nonmagmatic (shallow meteoric?) origin initially exchanged sodium for potassium and leached copper along warming paths within the ore zone, but as they cooled and acidified, they caused widespread sericitic alteration near the paleosurface. The zone of maximum fluid flux moved progressively upward and major metasomatism ceased at about 200 degrees C.The volume and intensity of Na metasomatism that characterizes the Ann-Mason deposit is recognized in only a few other porphyry Cu districts; the processes related to Na metasomatism are unlikely to be necessary for the formation of such deposits. Na metasomatism does occur in numerous (nonporphyry-related) mineralized and barren plutonic-volcanic terranes around the world and is the expected result of heating (e.g., prograde) saline fluids which circulate near igneous heat sources. Where such fluids are involved in porphyry Cu systems, they may both add components to orthomagmatic ores and redistribute components from earlier concentrations.

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Quartz veins in porphyry copper deposits record the physiochemical evolution of fluids in subvolcanic magmatic-hydrothermal systems. We have combined cathodoluminescence (CL) petrography with fluid-inclusion microthermometry to unravel the growth history of individual quartz veins and to link this history to copper ore formation at Bingham, Utah. Early barren quartz veins with K-feldspar + biotite (potassic) alteration selvages occur throughout the 2 km vertical exposure of quartz monzonite porphyry stock. At depths of 500 m to at least 1350 m below the orebody, fluid inclusions in these barren veins trapped a single-phase CO 2 -bearing fluid containing ∼2-12 wt% NaCl e q u i v . Within and to depths of 500 m below the orebody, early quartz veins contain abundant hypersaline liquid (38-50 wt% NaCl e q u i v ) and vapor-rich inclusions trapped together at temperatures of 560-350 °C and pressures of 550-140 bar, consistent with fluctuations between lithostatic and hydrostatic pressure at paleodepths of 1.4 to 2.1 km. CL petrography shows that bornite and chalcopyrite were deposited together with a later generation of quartz and K-feldspar in microscopic fractures and dissolution vugs in early barren quartz veins and wall rock. This late quartz contains hypersaline liquid (36-46 wt% NaCl e q u i v ) and vapor-rich inclusions trapped at 380-330 °C and at 160-120 bar hydrostatic pressure. We conclude that a single-phase magmatic-hydrothermal fluid underwent phase separation to hypersaline liquid (or brine) and vapor ∼500 m below the base of the orebody at a paleodepth of ∼2.5 km. Brine and vapor continued to ascend and formed multiple generations of barren quartz veins with potassic selvages. Thermal decline to temperatures below 400 °C was the main driving force for copper-iron sulfide deposition, given the lack of evidence of mixing of brines with low-salinity waters, the lack of correspondence of the ore zone with the initiation of phase separation, and no change in wall-rock alteration style.

Marco T. Einaudi

Papers

Porphyry Deposits: Characteristics and Origin of Hypogene Features

Geological characteristics and tectonic setting of proterozoic iron oxide (CuUAuREE) deposits

Introduction; terminology, classification, and composition of skarn deposits

Wall-rock alteration and hydrothermal flow paths about the Ann-Mason porphyry copper deposit, Nevada; a 6-km vertical reconstruction

Copper deposition by fluid cooling in intrusion-centered systems: New insights from the Bingham porphyry ore deposit, Utah