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Abstract

Porphyry Cu ± Mo ± Au deposits (porphyry) and iron oxide copper-gold deposits formed by magmatic-hydrothermal systems (MH-IOCG) share many features, most notably enrichments in Fe, Cu, and Au, as well as an association with calc-alkaline to mildly alkaline magmas generated in orogenic (subduction to collision) and postorogenic tectonic settings. Differences include the predominance of Fe-sulfide minerals and widespread late-stage acidic alteration in porphyry deposits, compared with Fe-oxide minerals and more extensive near-neutral pH alteration in MH-IOCG deposits. They are also separated by temporal distribution, with MH-IOCG deposits predominantly forming in the Precambrian era and porphyry deposits predominantly forming in the Phanerozoic eon, with only rare examples in older rocks.

We propose that these differences between otherwise quite similar deposit types can be explained primarily by a difference in magmatic sulfur content, specifically oxidized sulfur (SO42-). Hydrous, sulfate-rich magmas exsolve SO2-rich aqueous fluids upon upper crustal emplacement and cooling. Further cooling of these fluids causes disproportionation of SO2 to H2S and H2SO4, resulting in the precipitation of Fe ± Cu ± Au-rich sulfide minerals (and sulfates) and the formation of progressively more acidic alteration styles. In contrast, fluids exsolved from hydrous but sulfate-poor magmas precipitate the bulk of transported Fe as oxide phases, with minor (but potentially economically important) Cu ± Au-rich sulfide phases and more restricted development of late-stage acidic alteration styles.

We further propose that a broad temporal division between sulfate-poor and sulfate-rich magmas at the end of the Precambrian era reflects the global oxygenation of the deep oceans, which lagged the buildup of oxygen in the atmosphere by almost 2 billion years. Deep seawater sulfate concentrations increased rapidly following the Neoproterozoic Oxygenation Event and would have resulted in the voluminous fixation of sulfate in sea-floor-altered oceanic crust for the first time in Earth’s history. Release of sulfate-bearing fluids during prograde metamorphism of subducted oceanic crust is thought to contribute the bulk of sulfur to the metasomatized mantle source of Phanerozoic arc magmas, but this source of sulfate would not have been widely available during subduction of Precambrian oceanic lithosphere. Consequently, S-rich porphyry deposits would rarely have formed from Precambrian arc magmas, in contrast to S-poor MH-IOCG-type deposits, and vice versa in the Phanerozoic eon. Exceptions may arise: locally oxidizing conditions in the Proterozoic eon (e.g., in shallow subduction zones involving oxidized, upper ocean waters) may explain the rare occurrence of porphyry deposits in older rocks; and locally S-poor source rocks, such as the hydrous lithospheric residues of earlier arc magmatism, or sulfate-poor conditions generated during oceanic anoxia events, may explain the occurrence of locally important MH-IOCG deposits in the Phanerozoic eon.

Other differences, such as the greater extent of high-temperature alteration zones and the occurrence of lithophile-element enrichments (e.g., rare earth elements [REEs] and U) in some MH-IOCG deposits, may reflect higher geothermal gradients such as were common in the Precambrian era or formation, in high-heat flow extensional environments and/or in proximity to batholiths. Some deposits show a spatial (if not genetic) association with high-heat-producing granites of predominantly crustal origin. Regardless of the cause, extensive high-temperature alteration can result in a greater degree of crustal metal and/or fluid fluxing.

The greatest overlap between porphyry and MH-IOCG deposits occurs for postsubduction Au-rich porphyry systems, whose mildly alkaline magmas are generated by partial melting of hydrous amphibole-rich residues of earlier arc magmatism. Such magmas are S poor relative to arc magmas, because the flux of new sulfur from the subduction zone is no longer present. Small amounts of Au-enriched residual sulfide in the metasomatized lithospheric source readily dissolve in these S-undersaturated second-stage melts, rendering the magmas fertile for subsequent porphyry Cu-Au deposit formation. Such mildly alkaline porphyries are typically magnetite rich, with alteration styles similar to those encountered in MH-IOCG deposits.

Thus, the key link between porphyry and MH-IOCG deposits is in the introduction of fluid-mobile components from subduction zones to the upper plate lithosphere. Deposits may form in direct association with arc magmatism or may be generated at some later time by remelting of deep lithospheric residues of prior arc magmatism. Under S-rich conditions in either setting, derivative magmas may go on to form porphyry Cu ± Mo ± Au deposits, whereas under S-poor conditions MH-IOCG deposit formation is more likely.

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