Hydrothermal Controls on Metal Distribution in Porphyry Cu (-Mo-Au) Systems
Kalin Kouzmanov, Gleb S. Pokrovski, 2012. "Hydrothermal Controls on Metal Distribution in Porphyry Cu (-Mo-Au) Systems", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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Extensive research during the 20th century on porphyry Cu (-Mo-Au) deposits has revealed the following major geodynamic, petrological, mineralogical, and geochemical features that characterize these deposits: (1) these systems commonly occur in continental and oceanic magmatic arcs or in collisional orogenic belts; (2) they have spatial and genetic relationships to basaltic-to-felsic magmas emplaced in the upper 10 km of the crust; (3) lateral and vertical alteration-mineralization zoning consists of a Cu (± Mo ± Au) ore shell in the shallow portion of a potassic alteration zone, produced by magmatic fluids; this can be overprinted by phyllic alteration, also largely magmatic in signature, that in turn may be overprinted by argillic alteration, with a dominantly meteoric signature; (4) associated deposits such as skarns, Cordilleran vein, and high and intermediate sulfidation epithermal deposits may occur above or adjacent to porphyry orebodies; (5) porphyry systems form from S- and metal-rich, single-phase aqueous fluid of moderate salinity (2–10 wt % NaCl equiv) exsolved from magmas; during its ascent toward the surface this fluid undergoes a variety of processes that can cause metal precipitation, including decompression, phase separation, cooling, interaction with host rocks, and mixing.
In the last 20 years, novel microanalytical techniques for in situ characterization of individual fluid inclusions have provided direct evidence for the chemical and phase composition plus metal content of ore-forming fluids in porphyry systems. In this contribution, we compile a large dataset of published fluid inclusion compositions from more than 30 deposits of the porphyry-skarn-epithermal suite. Four main types of fluid inclusions are identified, based on their origin and phase composition at the time of entrapment: (1) single-phase, intermediate-density inclusions, regarded as equivalent to the primary single-phase fluid exsolved from the magma, (2) vapor-rich and (3) hypersaline liquid inclusions, both resulting from phase separation of the single-phase fluid, and (4) low to intermediate salinity aqueous liquid inclusions. The first three fluid types are characteristic of porphyry and skarn environments at elevated temperatures and depths, whereas the last is present during the retrograde stage, both in skarn and porphyry deposits as well as in the shallow epithermal environment.
Absolute concentrations of ore-related metals in the pristine single-phase magmatic fluids are typically one to three orders of magnitude higher than their average crustal abundances, demonstrating the ability of magmatic-hydrothermal fluid to concentrate and transport metals. Decompression-induced phase separation of this magmatic fluid upon ascent and intersection of the two-phase vapor-liquid boundary of the water-salt system results in metal fractionation, as evidenced by coexisting vapor and hypersaline liquid inclusions. The hypersaline liquid is largely enriched in metals such as Zn, Pb, Fe, Mn, and Ag, whereas Au, As, S and, to a lesser and uncertain extent, Mo may partition into the vapor phase. Copper is likely to have a partitioning behavior intermediate between these groups of elements; however, its true vapor-liquid distribution may be obscured by post-entrapment diffusion processes which lead to an apparent enrichment in Cu in natural S-rich vapor and single-phase fluid inclusions. These metal fractionation trends are quantitatively explained by recent experimental data on vapor-liquid partitioning that show a preferential affinity of Au (and partly Cu) for reduced sulfur and that of other metals for chloride, and by physical-chemical models involving the fluid density. Single-phase, vapor-rich, and hypersaline liquid inclusions from giant porphyry deposits at Bingham, El Teniente, Bajo de la Alumbrera, Questa, and Butte present a characteristic Zn/Pb ratio, ranging from 1 to 6 in the order of the listed deposits, which is constant for a given deposit and is not affected by phase separation of the input magmatic fluid or Cu-Au-Mo precipitation in the porphyry environment, thus implying differences in the Zn/Pb ratio of the parental magmas.
Recent experimental studies on metal speciation and ore mineral solubilities under hydrothermal conditions coupled with thermodynamic modeling allow the reported metal contents in natural inclusion fluids to be interpreted. Modeling shows that cooling of a magmatic fluid, accompanied by water-rock interaction, is likely to be the major cause of most metal deposition, as well as the cause of spatial separation between Cu-Mo and Zn-Pb-Ag mineralization in porphyry systems. Changes in sulfur speciation on cooling lead to SO2 disproportionation; this is likely to control the observed fractionation of Au from Cu and other base metals during fluid evolution in the transition from the porphyry to epithermal environment. Fluid neutralization by wall-rock reaction appears to be the main driving force for Zn, Pb, Ag, and partially Au deposition in more distal portions of the porphyry system. Fluid immiscibility in the porphyry regime mostly affects Au and to a lesser extent Cu and Mo behavior by enabling a significant fraction of these metals to be transported by the vapor phase. Mixing with external waters is uncommon and not directly involved in ore formation in the porphyry environment. These predicted tendencies agree with the commonly observed metal zoning pattern in porphyry systems, and may provide useful clues for specific metal prospecting if the major fluid evolution events can be identified from fluid inclusion, mineralogical, or stable isotope analyses.
Comparison between mineral solubility calculations and metal contents in natural fluids shows that for some metals, such as Cu, Ag, Fe, Zn, and Pb, there is a good consistency. In contrast, thermodynamic predictions for Au and, particularly, Mo commonly underestimate their contents compared to natural fluid compositions. This requires reassessment of existing speciation models for these metals and consideration of recently discovered sulfur species that could potentially be important as metal transporting agents. Further development of microanalytical and in situ experimental approaches in hydrothermal geochemistry may provide new predictive tools in mineral exploration; coupled with physical hydrology models, this will allow the generation of integrated reactive transport models of fluid evolution and three-dimensional ore distribution in magmatic-hydrothermal systems, thus contributing to better exploration strategies.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.