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

At Henderson, Colorado, 12 rhyolitic stocks of Oligocene age crystallized in three intrusive centers whose tops are 1 km below the surface of Red Mountain: Henderson (oldest), Seriate, and Vasquez (deepest and youngest). A variety of fluorine-rich hydrothermal mineral assemblages are grouped according to their temperatures of formation, either high-, moderately high, moderate-, or low-temperature (the latter three groups are termed lower temperature assemblages). The groupings were based on relative age relationships in areas free of cyclical events, and absolute temperature estimates based on fluid inclusion study later were made for many mineral assemblages, as detailed in a companion paper. Numerous crosscutting contacts of individual stocks combined with limits of associatd high-temperature assemblages can be interpreted as spatially extensive time lines, thereby enabling temporal correlation of spatially separated events. High-temperature silicic (quartz-fluorite) and intense potassic (quartz-K-feldspar-molybdenite) assemblages developed in numerous cycles, each corresponding to emplacement of a stock and deposition of molybdenite. Molybdenite occurs without pyrite mostly in the high-temperature assemblages. In contrast, lower temperature assemblages broadly envelop intrusive centers, rather than single stocks. Some of the moderately high temperature, less intense potassic assemblages (including mottled K-feldspar-quartz and magnetite-K-feldspar) also formed during multiple cycles. Moderate-temperature sericitic assemblages (including topaz with pyrite or magnetite and sericite with pyrite or magnetite) only locally exhibit evidence for multiple cycles, and low-temperature intermediate argillic assemblages (pyrite-clay and several assemblages containing base metal sulfides, F-bearing, Mn-rich garnet, and rhodochrosite) were deposited in only a single event. These lower temperature assemblages terminate 700 m below the present surface. A zone of relatively fresh rocks extends upward and outward from this termination, separating the lower temperature assemblages from the first appearance of propylitic alteration in a more peripheral position, beyond the area of study. At no time did an advanced argillic assemblage form.

Lower temperature assemblages are subdivided into two suites on the basis of position: above intrusive centers, formed from fluids that flowed upward out the top of mineralizing stocks from several intrusive centers; and on the flanks of the Seriate center, formed from fluids released from the apex of the Seriate stock and injected laterally and downward along the flanks of the stock. Na- and Na-K-feldspars (± K-feldspar ± topaz ± micas) are members of some assemblages of the second suite.

The upper and lower limits of the hydrothermal system, for much of its history, are preserved within the exposures studied. The upper limit of the hydrothermal system rose to 250 m above the apices of the stocks for the first time during development of low-temperature assemblages, which extended higher above Henderson than Seriate. Prior to that time, the region where thermally and chemically evolving, saline hydrothermal fluid was reacting with wall rocks was confined to a vertical interval of approximately 0.5 km. The base of the system rose as moderately high and moderate-temperature assemblages formed but fell rapidly in elevation upon development of low-temperature assemblages. There is no evidence for system-scale convective circulation of fluids during development of the assemblages studied. Sericitic alteration did not begin to form anywhere in the system until after intense potassic alteration had terminated in the vicinity of stocks in both the Henderson and Seriate centers; proximal to stocks, the multiplicity of intrusive events repeatedly reversed the overall trend toward lower temperatures. Sericitic alteration was followed by a late cycle of intense potassic alteration and molybdenite deposition associated with emplacement and crystallization of the Vasquez stock prior to formation of low-temperature assemblages.

The hydrothermal system that formed the Henderson orebody can be viewed as a series of thermally retrograding cycles, all except the last of which was truncated by a reversal in the general trend of declining temperatures. Hence, the evolutionary style of Henderson is termed variably cyclical, in contrast to the nearly perfectly cyclical style embodied in the original Climax model for porphyry molybdenum deposits and the nearly perfectly unidirectional style envisaged for many porphyry copper deposits. An evaluation of evolutionary style can lead to geologic tests for geochronologic interpretations and proposed genetic origins of deposits.

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Henderson Porphyry Molybdenum System, Colorado: I. Sequence and Abundance of Hydrothermal Mineral Assemblages, Flow Paths of Evolving Fluids, and Evolutionary Style

Fluorine-rich hydrothermal mineral assemblages developed at Henderson, Colorado, as 12 rhyolitic stocks of Oligocene age were emplaced, crystallized, and cooled inside a much larger and slightly older, rhyolitic intrusion, forming a Climax-type molybdenum deposit. The stocks are grouped into three intrusive centers, and the mineral assemblages, including ore minerals, are grouped according to their temperatures of formation as either high-, moderately high, moderate-, or low-temperature assemblages. The latter three groups are termed lower temperature assemblages, which are subdivided into two suites on the basis of position: above intrusive centers and on the flanks of the Seriate center. Integration of the mineralogy, sequence, and spatial positions of mineral assemblages defines the geochemical evolution of hydrothermal fluids, places qualitative constraints on mass transfer, and provides insight into the development of the geochemical halos that are used in exploration for porphyry orebodies.

A reconnaissance fluid inclusion study targeted the lower temperature assemblages, and a geologic assessment of results indicates that we were unable to identify fluid inclusions from several assemblages. After pooling meaningful results, the ranges of temperatures of formation for each group of lower temperature assemblages are moderately high, 600° to 460°C; moderate, 530° to 310°C; and low, 390° to 200°C, with significant differences in salinity between the two suites of assemblages. The suite formed above intrusive centers was deposited by more saline fluids (salinity commonly 28–65 wt % NaCl ± KCl equiv) than the suite formed on the flanks of the Seriate center (mostly <29 wt % NaCl + KCl equiv). The lower temperature assemblages described here do not overlap in space with higher level propylitic alteration, but fluid inclusions in one sample of propylitized rock indicate temperatures of 320° to 210°C and salinity <29 wt percent NaCl + KCl equiv.

Phase equilibria suggest that, at comparable temperatures, the suite of assemblages that formed above intrusive centers formed at higher f S2, f O2, and a K+/ a Na+ and slightly lower a K+/ a H+ than the suite of assemblages that formed on the flanks of the Seriate center. From inception of potassic alteration (~600°C) to termination of sericitic alteration (~300°C), values of log a K+/ a H+ of fluids remained essentially constant (~3), but the a F– ⋅ a H+ of fluids increased. Fluids were undersaturated with respect to topaz during high-temperature silicic and intense potassic alteration, but topaz precipitated in both suites of assemblages after further fluid evolution. Topaz formed in certain less intense potassic, transitional potassic-sodic, and sericitic assemblages as the a F- ⋅ a H+ of fluids increased with decline in temperature and probably pressure, before the fluid again became undersaturated with respect to topaz. Because fluids associated with sericite-pyrite alteration at Henderson were unusually saline (mostly 29–36 wt % NaCl equiv), cooling of evolved, magmatic fluids (rather than dilution by meteoric water) likely drove sericitic alteration by cooling through the K-feldspar-sericite buffer at nearly constant a K+/ a H+. Indeed, halite-bearing fluid inclusions persist into intermediate argillic assemblages that precipitated sphalerite and F-bearing, Mn-rich garnet.

Fluorine was continuously added to wall rocks by hydrothermal fluids along the entire evolutionary paths of both suites of assemblages. The alkalis underwent a complex history of mass transfer, with the behavior of potassium fluctuating between periods of strong leaching and strong addition to the rock. Iron was leached at high to moderately high temperatures and then fixed in the rock at lower temperatures, first mainly as magnetite and then as pyrite and minor pyrrhotite and specular hematite. Molybdenum and tungsten show no evidence of any period of significant leaching or recycling.

Each mineralizing intrusion introduced juvenile magmatic hydrothermal fluids containing metals and other components, but the deposition of metals and other components was staggered over time and a wide temperature range. The order of deposition was Mo to W to Pb-Zn to Mn. The spatial zoning of metals from proximal to distal around any intrusive center generally—but not always—corresponds to the temporal sequence early to late. Moreover, the metals that were deposited relatively late tended to be deposited at about the same time throughout the system, so that Pb-Zn and Mn introduced with the earliest intrusions were not deposited until much later, after other intrusions and hydrothermal fluids were introduced. Hence, there was progressively greater decoupling between the times of introduction and deposition of metals in the order Mo to W to Pb-Zn to Mn, and this time delay lengthened for any given component introduced by an earlier intrusion compared to the same component introduced by a later intrusion. The degree of decoupling of introduction and deposition of components may reconcile discrepancies between differing interpretations of when the majority of Cu is deposited in porphyry copper deposits.

Hydrothermal mineral assemblages that formed above intrusive centers at Henderson resemble assemblages in other Climax-type porphyry molybdenum deposits associated with high-silica rhyolites. In contrast, the suite of assemblages that formed on the flanks of the Seriate center is similar to alteration-mineralization products formed in porphyry molybdenum deposits related to alkali syenites and in porphyry tungsten-molybdenum deposits associated with rhyolites and granites. Hydrothermal assemblages in granite-related molybdenum deposits of the differentiated monzogranite suite and granite-related molybdenum-copper deposits have little in common with either suite of assemblages at Henderson.

Henderson porphyry molybdenum system, Colorado: II. Decoupling of introduction and deposition of metals during geochemical evolution of hydrothermal fluids

The root zone of a porphyry system is a specific region beneath a porphyry orebody that was a site of focused fluid flow, as evidenced by abundant quartz veins, widespread wall-rock alteration, or porphyry dikes merging downward into a porphyritic granite cupola. These zones constitute an important source region of ore fluids and other components, and in certain geologic terrains the characteristics of root zones may point to previously undiscovered deposits.

The root zones of four Laramide porphyry copper systems in Arizona recently have been characterized at a reconnaissance level: the Miami Inspiration system associated with the Schultze Granite, the Sierrita-Esperanza system associated with the Ruby Star Granodiorite, the Ray system associated with the Granite Mountain pluton, and the Kelvin-Riverside system associated with the Tea Cup pluton. The two well-studied root zones related to the Jurassic Yerington batholith in Nevada, and associated with the Yerington mine and the Ann-Mason deposit, provide a basis of comparison. All six systems occur in areas with unusually large exposures in both lateral and vertical paleodirections, locally to paleodepths of >10 km, because of postore extensional faulting and associated tilting. No two systems are alike, but many share the presence of the following hydrothermal characteristics: quartz veins and potassic alteration, sodic-calcic and sodic alteration, calcic alteration, and relatively coarse grained muscovite-quartz (greisen). Quartz veins and potassic alteration are focused centrally, directly above related cupolas; sodic-calcic and sodic alteration, calcic alteration, and evidence for leaching of silica are observed on the deep flanks of certain systems; and greisen occurs directly beneath ore within and beneath coeval cupolas in many systems. Certain systems exhibit evidence of multiple cycles of release of magmatic fluid followed by incursion of saline ground waters, which are analogous to the biological cycle of exhale-inhale, respectively.

The characteristics of the root zones provide important constraints on the exsolution and transport of the magmatic aqueous phase that leads to ore formation, the variable incursion of external fluids into the hydrothermal system, and the degassing of magmatic volatiles that may not be related directly to porphyry ore formation. The most robust conclusions are drawn from the localities that offer the greatest quality of exposure and degree of continuity (including compelling structural reconstructions) between the roots and the ore deposit, from the studies that identify timelines linking processes in the roots with those in the mineral deposit, and from systems in which the deposit itself is well characterized.

Root Zones of Porphyry Systems: Extending the Porphyry Model to Depth

Multiple lines of evidence, including new and published geologic mapping and paleomagnetic and geobarometric determinations, demonstrate that the rocks and large porphyry copper systems of the Sierrita Mountains in southern Arizona were dismembered and tilted 50° to 60° to the south by Tertiary normal faulting. Repetition of geologic features and geobarometry indicate that the area is segmented into at least three major structural blocks, and the present surface corresponds to oblique sections through the Laramide plutonic-hydrothermal complex, ranging in paleodepth from ~1 to ~12 km. These results add to an evolving view of a north-south extensional domain at high angles to much extension in the southern Basin and Range, contrast with earlier interpretations that the Laramide systems are largely upright and dismembered by thrust faults, highlight the necessity of restoring Tertiary rotations before interpreting Laramide structural and hydrothermal features, and add to the broader understanding of pluton emplacement and evolution of porphyry copper systems.

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Eric Seedorff

Papers

Porphyry Deposits: Characteristics and Origin of Hypogene Features

Henderson Porphyry Molybdenum System, Colorado: I. Sequence and Abundance of Hydrothermal Mineral Assemblages, Flow Paths of Evolving Fluids, and Evolutionary Style

Henderson porphyry molybdenum system, Colorado: II. Decoupling of introduction and deposition of metals during geochemical evolution of hydrothermal fluids

Root Zones of Porphyry Systems: Extending the Porphyry Model to Depth

Tertiary tilting and dismemberment of the laramide arc and related hydrothermal systems, sierrita mountains, arizona