Christoph A. Heinrich

Porphyry-related mineral deposits are giant geochemical anomalies in the Earth’s crust with orders-of-magnitude differences in the content and proportion of the three main ore metals Cu, Au, and Mo. Deposit formation a few kilometers below surface is the product of a chain of geologic processes operating at different scales in space and time. This paper explores each process in this chain with regard to optimizing the chances of forming these rare anomalies. On the lithosphere scale, deposits with distinct metal ratios occur in provinces that formed during brief times of change in plate motions. Similar metal ratios of several deposits in such provinces compared with global rock reservoirs suggest preceding enrichment of Au or Mo in lithospheric regions giving rise to distinct ore provinces. The largest Cu-dominated deposits and provinces are traditionally explained by selective removal of Au during generation or subsequent evolution of mantle magmas, but the possibility of selective Cu pre-enrichment of lithosphere regions by long-term subduction cannot be dismissed, even though its mechanism remains speculative. Evolution of hydrous basaltic melts to fertile magmas forming porphyry Cu deposits requires fractionation toward more H₂O-rich magmas in the lower crust, as shown by their adakite-like trace element composition. The prevailing interpretation that this fractionation leads to significant loss of chalcophile ore metals by saturation and removal of magmatic sulfide might be inverted to a metal enrichment step, if the saturating sulfides are physically entrained with the melt fraction of rapidly ascending magmas. Ascent of fertile magma delivers a large mass of H₂O-rich ore fluid to the upper crust, along points of weakness in an overall compressive stress regime, within a limited duration as required by mass and heat balance constraints. Two mechanisms of rapid magma ascent are in debate: (1) wholesale emplacement of highly fractionated and volatile-rich granitic melt into a massive transcrustal channelway, from which fluids are exsolved by decompression starting in the lower crust, or (2) partly fractionated magmas filling a large upper crustal magma chamber, from which fluids are expelled by cooling and crystallization. Transfer of ore-forming components to a hydrothermal ore fluid is optimized if the first saturating fluid is dense and Cl rich. This can be achieved by fluid saturation at high pressure, or after a moderately H₂O rich intermediate-composition melt further crystallizes in an upper crustal reservoir before reaching fluid saturation. In either case, metals and S (needed for later hydrothermal sulfide precipitation) are transferred to the fluid together, no matter whether ore components are extracted from the silicate melt or liberated to the ore fluid by decomposition of magmatic sulfides. Production and physical focusing of fluids in a crystallizing upper crustal magma chamber are controlled by the rate of heat loss to surrounding rocks. Fluid focusing, requiring large-scale lateral flow, spontaneously occurs in mushy magma because high water content and intermediate melt/crystal ratio support a network of interconnected tubes at the scale of mineral grains. Calculated cooling times of such fluid-producing magma reservoirs agree with the duration of hydrothermal ore formation measured by high-precision zircon geochronology, and both relate to the size of ore deposits. Ore mineral precipitation requires controlled flow of S- and metal-rich fluids through a vein network, as shown by fluid inclusion studies. The degree of hydrothermal metal enrichment is optimized by the balance between fluid advection and the efficiency of cooling of the magmatic fluid plume by heat loss to convecting meteoric water. The depth of fluid production below surface controls the pressure-temperature (P-T) evolution along the upflow path of magmatic fluids. Different evolution paths controlling density, salinity, and phase state of fluids contribute to selective metal precipitation: porphyry Au deposits can form at shallow subvolcanic levels from extremely saline brine or salt melt; high-grade Au-Cu coprecipitation from coexisting and possibly rehomogenizing brine and vapor is most efficient at a depth of a few kilometers; whereas fluids cooling at greater depth tend to precipitate Cu ± Mo but transport Au selectively to shallower epithermal levels. Exhumation and secondary oxidation and enrichment by groundwater finally determine the economics of a deposit, as well as the global potential of undiscovered metal resources available for future mining.

Stokes calculations with assumed model compositions; thermodynamic modeling method with data sources and sensitivity tests; comments on component diffusion in silicate melts; a mass balance estimate for Cu, Ag transfer by entrained sulfide melt; and supplemental references on methods and material properties.

Stokes calculations with assumed model compositions; thermodynamic modeling method with data sources and sensitivity tests; comments on component diffusion in silicate melts; a mass balance estimate for Cu, Ag transfer by entrained sulfide melt; and supplemental references on methods and material properties.

Silver isotopes are a potential tracer for understanding the geochemical enrichment of gold, which is monoisotopic but occurs in metallic form with silver as a stable alloy component. This study applies high-precision measurements of silver isotope ratios to two of the oldest world-class hydrothermal gold vein deposits from the Archean (3.55–3.10 Ga) Barberton greenstone belt of the Kaapvaal craton in South Africa. Published structural-geologic and geochronological results show that the hydrothermal gold deposits formed late in the tectono-metamorphic history of the greenstone belt, well after major crustal deformation and regional metamorphism, probably postdating even postorogenic granitic intrusions. Gold is concentrated in extensional brittle-ductile structures, indicating a common extensional stress field during mineralization. Orebodies occur close to the stratigraphic contact between older (ultra-)mafic submarine lavas (the Onverwacht Group) and overlying carbonaceous siltstones and graywackes (the Fig Tree Group), marked by a variably tectonized former chert. Observations at New Consort (higher temperature, close to granitoid contact), Fairview, and Sheba (lower temperature in the interior of the greenstone belt) confirm the structurally late introduction of gold into veins and sulfide replacement orebodies. Chemical and isotopic analyses were performed on native gold from Barberton, on two other lode gold deposits and two carbon leader-type samples from the Witwatersrand, and on native silver from various epithermal silver deposits. All native gold samples contain significant (1–12 wt %) Ag but only trace levels of other metals such as Cu or Hg. Stepwise leaching experiments show that native gold is the main carrier of silver even in sulfide-rich ores. Different gold samples from Barberton yield ε¹⁰⁹Ag values (differences in ¹⁰⁹Ag/¹⁰⁷Ag relative to NIST SRM 978a in parts per 10,000) from −4.2 ± 0.4 to 3.6 ± 0.4, encompassing almost the entire range of reported terrestrial Ag isotope variations and all hydrothermal silver samples. Gold samples from New Consort have negative ε¹⁰⁹Ag values, whereas samples from Sheba have variably positive ε¹⁰⁹Ag values. Isotopically different metal sources of the structurally coeval deposits at Barberton are less likely than a process of isotopic fractionation during transport and ore deposition. The observed isotopic difference can be explained by reduction of dissolved Ag⁺ complexes in the ore fluid to native Ag⁰ in the gold alloy. Assuming that the oxidized Ag⁺_(aq) is enriched in the heavy isotope compared with the reduced metallic form, as in other heavy metal systems, the variation in ε¹⁰⁹Ag between the two deposits can be explained by fractional precipitation of precious metals from a fluid that enters the greenstone belt from an external source, in line with published geologic arguments. Following a Rayleigh distillation model, a first fraction of silver that precipitated close to the contact of the greenstones against surrounding granitoids is isotopically light, as observed at New Consort. Residual silver reaching the interior of the greenstone belt eventually precipitates with a high ε¹⁰⁹Ag, as seen at Sheba.

Measuring water contents of magmas is fundamental to resolving a number of geological questions, such as the mechanisms of silicic magma evolution, the triggering of volcanic eruptions, and the formation of porphyry copper deposits. This study focuses on the correlation between apparent deviations from stoichiometry of plagioclase crystals and high water concentration in the magmatic melt from which they grew. We considered this relationship as a potential geo-hygrometer (water activity indicator). To test and potentially calibrate this new technique, a range of natural and experimental plagioclase crystals were analysed, with particular care taken to identify and avoid analytical bias and artefacts. In contrast to recently published material, we found no systematic aluminium excess in plagioclase, irrespective of the water concentration of the silicate melt it crystallised from. This suggests that aluminium excess in plagioclase cannot serve as a geo-hygrometer. The high likelihood of misinterpreting analytical artefacts (due to alkali migration and imprecise standardisation) as small deviations from stoichiometry, also requires its application as a mineral exploration tool to be treated with caution.