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

Epithermal deposits are important sources of gold and silver that form at 20 wt percent NaCl equiv for Ag-Pb-Zn deposits. Stable isotope data indicate that hydrothermal solutions were composed mostly of deeply circulated meteoric water, with a nil to small and variable component of magmatic water. Epithermal deposits associated with quartz + alunite ± pyrophyllite ± dickite ± kaolinite assemblages contain Au ± Ag ± Cu ores. Native gold and electrum are the main ore-bearing minerals, with variable amounts of pyrite, Cu-bearing sulfides and sulfosalts such as enargite, luzonite, covellite, tetrahedrite, and tennantite, plus sphalerite and telluride minerals; enargite dominates the Cu sulfides and indicates a high-sulfidation state. Quartz (both massive and vuggy) and alunite are the main gangue minerals with kandite minerals (dickite and/or kaolinite) and/or pyrophyllite. Concentric patterns of hydrothermal alteration envelop the zone of vuggy and massive quartz alteration, which hosts ore. Outward, these comprise zones of quartz and alunite, dickite ± kaolinite or pyrophyllite, and illite or smectite alteration, surrounded by regional propylitic alteration. Zones of illite or pyrophyllite alteration occur in the roots beneath some deposits. Fluid inclusion data indicate that salinities are typically 30 wt percent NaCl equiv. Stable isotope data indicate that the altering fluids are composed mostly of magmatic fluids with a minor to moderate component of meteoric water. Critical genetic factors include: (1) at several-kilometers depth, the development of oxidized and acidic versus reduced and near-neutral pH solutions, controlled by the proportions of magmatic and meteoric components in solution, and the amount of subsequent water-rock interaction during ascent to the epithermal envi

Geological characteristics of epithermal precious and base metal deposits

Magmatic to hydrothermal metal fluxes in convergent and collided margins

Phanerozoic continental growth and gold metallogeny of Asia

The iron oxide copper-gold (IOCG) group of deposits, initially defined following discovery of the giant Olympic Dam Cu-U-Au deposit, has progressively become too-embracing when associated deposits and potential end members or analogs are included. The broader group includes several low Ti iron oxide-associated deposits that include iron oxide (P-rich), iron oxide (F- and REE-rich), Fe or Cu-Au skarn, high-grade iron oxide-hosted Au ± Cu, carbonatite-hosted (Cu-, REE-, and F-rich), and IOCG sensu stricto deposits. Consideration of this broad group as a whole obscures the critical features of the IOCG sensu stricto deposits, such as their temporal distribution and tectonic environment, thus leading to difficulties in developing a robust exploration model. The IOCG sensu stricto deposits are magmatic-hydrothermal deposits that contain economic Cu and Au grades, are structurally controlled, commonly contain significant volumes of breccia, are commonly associated with presulfide sodic or sodic-calcic alteration, have alteration and/or brecciation zones on a large, commonly regional, scale relative to economic mineralization, have abundant low Ti iron oxides and/or iron silicates intimately associated with, but generally paragenetically older than, Fe-Cu sulfides, have LREE enrichment and low S sulfides (lack of abundant pyrite), lack widespread quartz veins or silicification, and show a clear temporal, but not close spatial, relationship to major magmatic intrusions. These intrusions, where identified, are commonly alkaline to subalkaline, mixed mafic (even ultramafic) to felsic in composition, with evidence for mantle derivation of at least the mafic end members of the suite. The giant size of many of the deposits and surrounding alteration zones, the highly saline ore fluids, and the available stable and radiogenic isotope data indicate release of deep, volatile-rich magmatic fluids through devolatization of causative, mantle-derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional-scale fluid flow paths. Precambrian deposits are the dominant members of the IOCG group in terms of both copper and gold resources. The 12 IOCG deposits with >100 tonnes (t) resources are located in intracratonic settings within about 100 km of the margins of Archean or Paleoproterozoic cratons or other lithospheric boundaries, and formed 100 to 200 m.y. after supercontinent assembly. Their tectonic setting at formation was most likely anorogenic, with magmatism and associated hydrothermal activity driven by mantle underplating and/or plumes. Limited amounts of partial melting of volatile-rich and possibly metal-enriched metasomatized early Precambrian subcontinental lithospheric mantle (SCLM), fertilized during earlier subduction, probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts, with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the IOCG deposits. Preservation of near-surface deposits, such as Olympic Dam, is probably due to their formation above buoyant and refractory SCLM, which resisted delamination and associated uplift. Most Precambrian iron oxide (P-rich) or magnetite-apatite (Kiruna-type) deposits have a different temporal distribution, apparently forming in convergent margin settings prior to or following supercontinent assembly. It is only in the Phanerozoic that IOCG and magnetite-apatite deposits are roughly penecontemporaneous in convergent margin settings. The Phanerozoic IOCG deposits, such as Candelaria, Chile, occur in anomalous extensional to transtensional zones in the Coastal Cordillera, which are also the sites of mantle-derived mafic to felsic intrusions that are anomalous in an Andean context. This implies that special conditions, possibly detached slabs of metasomatized SCLM, may be required in convergent margin settings to generate world-class IOCG deposits. It is likely that formation of giant IOCG deposits was mainly a Precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized SCLM. Generally smaller and rarer Phanerozoic IOCG deposits formed in tectonic settings where conditions similar to those in the Precambrian were replicated.

Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits

Porphyry Deposits: Characteristics and Origin of Hypogene Features

The Bajo de la Alumbrera porphyry Cu-Au deposit, Argentina, is in the eastern Andes, near the north edge of a region of reverse fault-bound basement uplifts that overlie a low-angle segment of the subduction zone. Alumbrera, now above the transition from steep to flat subduction, formed at ~7 Ma in the Farallon Negro volcanic field, which was active as volcanism was waning regionally above the flattening subduction zone. Reconstruction of volcanic structure suggests that the top of the exposed orebody was emplaced beneath about 2.5 km of andesite and dacite but not directly beneath the vent of a stratovolcano. Production plus remaining resources are 605 million metric (Mt) tons of ore that averages 0.54 percent Cu and 0.64 g/t Au. The deposit is centered on a closely spaced cluster of small felsic porphyry stocks and dikes, emplaced into andesites during seven phases of intrusion. Dikes of several phases define a radial pattern. Most of the porphyries are very similar to one another, with phenocrysts of plagioclase, hornblende, biotite, and quartz, in a matrix of fine-grained quartz, K-feldspar, and minor plagioclase, biotite, and magnetite. Individual porphyries are distinguished mainly on the basis of intrusive contact relationships. Highest Cu-Au grades are associated with abundant quartz veins, secondary K-feldspar, ±magnetite, ±biotite, ±anhydrite, in the earliest porphyry (P2), and adjacent andesite. P2-related mineralization is truncated by porphyries of the second phase of ore-related intrusions (Early P3 and Quartz-eye porphyry), which contain similar but generally less intense mineralization and alteration. Porphyries of the next phase (Late P3) truncate mineralization associated with earlier phases and are weakly mineralized with Cu-Au, sparse quartz veins, and secondary biotite. The still later Northwest porphyries truncate most Cu-Au, quartz veins, and potassic alteration, and themselves contain only traces of such mineralization and partially biotitized hornblende. Postmineral porphyries, the youngest, truncate all such mineralization and alteration, and none of their hornblende is biotitized. Los Amarillos porphyry and igneous breccia, along the western periphery of the porphyry cluster, is between P2 and Early P3 in age but shows little relationship to mineralization. Zones of secondary K-feldspar associated with the earlier porphyries are surrounded by a larger zone of secondary biotite. All significant Cu-Au lies within these potassic zones. The biotite zone is surrounded by epidote-chlorite alteration lacking significant sulfides. Like potassic alteration, epidote-chlorite alteration is also truncated by Postmineral porphyries. Strong feldspar destructive alteration, consisting mostly of veinlet-controlled sericite-quartz-pyrite, is younger than all secondary K-feldspar, biotite, and epidote-chlorite and occurs in a shell in the outer parts of the biotite zone. Weaker feldspar destructive alteration occurs inside and outside this shell. Pyrite veins with sericite-quartz-pyrite alteration cut Postmineral porphyries. In the earliest secondary K-feldspar assemblage, which is usually barren of Cu sulfides, biotite is altered to magnetite plus K-feldspar. Most Cu sulfides are associated with slightly later K-feldspar-biotite ± magnetite assemblages. Where feldspars and biotite are not overprinted by later feldspar destructive or chloritic alteration, Cu minerals are bornite and chalcopyrite, coexisting with magnetite. Barren as well as Cu sulfide-bearing assemblages are associated with early veinlets, including A-type quartz, which are truncated by the next later porphyry. Deposition of Cu-Au during or between emplacement of closely related porphyries suggests high temperatures and magmatic fluids, and the assemblage bornite-chalcopyrite-magnetite indicates a relatively low sulfidation state, and along with the assemblage K-feldspar-biotite ± magnetite ± anhydrite a relatively high oxidation state. Cu-Au distribution is not related to feldspar destructive zones nor to the interface between sericitic and potassic zones. Much Cu-Au mineralization, however, has been overprinted by late alteration, resulting in partial destruction of feldspars, chloritization of mafics, and sulfidation of bornite-chalcopyrite-magnetite to chalcopyrite-pyrite ± relict magnetite. This probably took place at significantly lower temperature. A low-grade core zone consists in large part of barren K-feldspar-magnetite alteration and quartz veins in Early P3 porphyry, and in part consists of later barren porphyry, so is mostly younger than the Cu-Au deposited with P2 porphyry. The youngest features at Alumbrera include small postore normal faults, gypsum veins due to hydration and mobilization of anhydrite, local dissolution of gypsum veins, and locally developed thin zones of near-surface oxidation, leaching, and secondary enrichment.

Geology of the Bajo de la Alumbrera Porphyry Copper-Gold Deposit, Argentina

Overview of the Yerington Porphyry Copper District: Magmatic to Nonmagmatic Sources of Hydrothermal Fluids, Their Flow Paths, Alteration Affects on Rocks, and Cu-Mo-Fe-Au Ores

The Ambler district is located along the southern margin of the Brooks Range, Alaska. Mineral exploration since the late 1950s in this remote area has led to the discovery of a number of volcanogenic massive sulfide deposits and several carbonate-hosted copper deposits. The district is composed of two geologic terranes. The Ambler terrane consists of Devonian and older schistose metasedimentary rocks overlain by Middle Devonian to Lower Mississippian phyllitic shales and siltstones. Late Devonian carbonate sediments (Bornite sequence) and mixed carbonate and volcanic rocks (Ambler sequence) are interbedded within this thick metasedimentary succession. A low-angle fault separates the Ambler terrane from the overlying, southerly derived Angayucham terrane which is composed of Devonian to middle -Mesozoic mafic and ultramafic rocks. Both terranes were deformed and metamorphosed during the Jurassic (?) to Cretaceous Brooks Range orogeny. Significant mineralization is restricted to the Ambler and Bornite lithologic successions. The Ambler sequence, which contains a number of volcanogenic massive sulfide deposits, averages 1.5 km in thickness and is composed of approximately 60 percent volcanic and volcaniclastic rocks, 25 percent carbonate sediments, and 15 percent pelitic sediments. The volcanic rocks are bimodal with rhyolite predominating over basalt. Much of the rhyolite was apparently deposited as submarine ash-flow tuff. Two major types of mineralization are recognized: massive sulfide adjacent to major hydrothermal vent areas, and laterally extensive massive sulfide lacking well-defined vent areas. The Bornite carbonate sequence contains significant copper mineralization within large, structurally controlled zones of hydrothermal dolomitization.The bimodal volcanic sequence is suggestive of a rifting-related tectonic environment. This interpretation is consistent with the distribution of carbonate and volcanic sequences on what may be high structural blocks and with possible coeval ocean-floor spreading to the south suggested by the Angayucham mafic volcanic rocks.

John M. Proffett

Papers

Porphyry Deposits: Characteristics and Origin of Hypogene Features

Geology of the Bajo de la Alumbrera Porphyry Copper-Gold Deposit, Argentina

Overview of the Yerington Porphyry Copper District: Magmatic to Nonmagmatic Sources of Hydrothermal Fluids, Their Flow Paths, Alteration Affects on Rocks, and Cu-Mo-Fe-Au Ores

Geology and Mineralization of the Ambler District, Northwestern Alaska

Metallogenesis of the Yerington Batholith, Nevada