This contribution provides an overview of available experimental, thermodynamic, and molecular data on Au aqueous speciation, solubility, and partitioning in major types of geological fluids in the Earth's crust, from low-temperature aqueous solution to supercritical hydrothermal- magmatic fluids, vapours, and silicate melts. Critical revisions of these data allow generation of a set of thermodynamic properties of the AuOH, AuCl 2 , AuHS, and Au(HS) 2 complexes dominant in aqueous hydrothermal solutions; however, other complexes involving different sulphur forms, chloride, and alkali metals may operate in high-temperature sulphur-rich fluids, vapours, and melts. The large affinity of Au for reduced sulphur is responsible for Au enrichment in S-rich vapours and sulphide melts, which are important gold sources for hydrothermal deposits. Thermo- dynamic, speciation, and partitioning data, and their comparison with Au and S contents in natural fluid inclusions from magmatic-hydrothermal gold deposits, provide new constraints on the major physical-chemical parameters (temperature, pressure, salinity, acidity, redox) and ubiquitous fluid components (sulphur, carbon dioxide, arsenic) affecting Au concentration, transport, precipitation, and fractionation from other metals in the crust. The availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations.

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Gold speciation and transport in geological fluids: insights from experiments and physical-chemical modelling

Mineral textures and fluid inclusion petrography of the epithermal Ag-Au deposits at Guanajuato, Mexico: Application to exploration

Geological aqueous fluids operate in a wide range of temperatures (from 0 to 1000 °C) and depths (from Earth surface to ~10s km), over which the physical-chemical properties of water and water-salt-gas systems and, consequently, their capacities to dissolve minerals and to transport chemical elements are very different. The principal types of geological fluids are illustrated in Figure 1, showing the domains of the liquid, vapor, and supercritical fluid phases in the water phase diagram as a function of temperatures ( T ) and pressures ( P ) typical of Earth’s crust. Understanding the impact of these different fluid phases on geological processes requires knowledge of mineral solubility and metal speciation and partitioning among the different fluid phases. The degree of this knowledge also significantly varies across this T-P range, as roughly illustrated by the three fields identified in Figure 1. In the domain of moderate-temperature aqueous solutions (light-gray field, labeled 1) a large amount of data exists about the nature and stability of major metal complexes, and thermodynamic models have been developed over ~50 years for predicting ore mineral solubilities. By contrast, it is only recently that new insights were obtained into the speciation and transport of economic metals in low-density vapor and fluid phases typical of hydrothermal-magmatic deposits (white field, labeled 2 in Fig. 1), but a number of questions still remain concerning the role of the vapor phase in metal transfers. Similarly, our current knowledge is still insufficient in the domain of high-pressure fluids typical of subduction zones (dark-gray field, labeled 3 in Fig. 1), for which very little is known about the effect of the major ligands like chloride, sulfur, carbon or silica on metal mobility. These lacks hamper our understanding of the geological impact of all these fluids in the continuity of metal and volatile transfers through …

Speciation and Transport of Metals and Metalloids in Geological Vapors

A model of boiling for fluid inclusion studies: Application to the Bolaños Ag-Au-Pb-Zn epithermal deposit, Western Mexico

Predicting gold-rich epithermal and porphyry systems in the central Andes with a continental-scale metallogenic GIS

The Au-Ag (±Pb-Zn) Apacheta deposit is located in the Shila district, 600 km southeast of Lima in the Cordillera Occidental of Arequipa Province, southern Peru. The vein mineralization is found in Early to Middle Miocene calc-alkaline lava flows and volcanic breccias. Both gangue and sulfide mineralization express a typical low-sulfidation system; assay data show element zoning with base metals enriched at depth and higher concentrations of precious metals in the upper part of the veins. Three main deposition stages are observed: (1) early pyrite and base-metal sulfides with minor electrum 1 and acanthite; (2) brecciation of this mineral assemblage and cross-cutting veinlets with subhedral quartz crystals, Mn-bearing calcite and rhombic adularia crystals; and finally (3) veinlets and geodal filling of an assemblage of tennantite/tetrahedrite + colorless sphalerite 2 + galena + chalcopyrite + electrum 2. Fluid inclusions in the mineralized veins display two distinct types: aqueous-carbonic liquid-rich Lw-c inclusions, and aqueous-carbonic vapor-rich Vw-c inclusions. Microthermometric data indicate that the ore minerals were deposited between 300 and 225 °C from relatively dilute hydrothermal fluids (0.6–3.4 wt% NaCl). The physical and chemical characteristics of the hydrothermal fluids show a vertical evolution, with in particular a drop in temperature and a loss of H2S. The presence of adularia and platy calcite and of co-existing liquid-rich and vapor-rich inclusions in the ore-stage indicates a boiling event. Strong H2S enrichment in the Vw-c inclusions observed at –200 m, the abundance of platy calcite, and the occurrence of hydrothermal breccia at this level may indicate a zone of intense boiling. The vertical element zoning observed in the Apacheta deposit thus seems to be directly related to the vertical evolution of hydrothermal-fluid characteristics. Precious-metal deposition mainly occurred above the 200-m level below the present-day surface, in response to a liquid/vapor phase separation due to an upward boiling front.

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Erratum to Boiling and vertical mineralization zoning: a case study from the Apacheta low-sulfidation epithermal gold-silver deposit, southern Peru

The Au–Ag epithermal mineralization of the Shila Cordillera is dated at about 10.7 Ma (K/Ar on adularia). The vein system is characterized by the association of a major ≈east–west vein and N120–135°E secondary fractures. The strike-slip faults controlling the veins indicate an initial NE–SW to ENE–WSW shortening direction, which is compatible with that generally accepted for this period. These structures were reopened during a second phase and channelized mineralizing fluids, the circulation of which may have began at the end of stage 1.

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Structural control and K/Ar dating of the Au–Ag epithermal veins in the Shila Cordillera, southern Peru

Les mineralisations epithermales a Au-Ag de la Cordillera Shila sont datees a environ 10,7 Ma (K/Ar sur adulaire). Le systeme de veines est caracterise par l'association entre une veine principale sensiblement est-ouest et des fractures satellites N120 a N135°E. Les decrochements controlant les veines indiquent une direction de raccourcissement initiale NE-SW a ENE-WSW, compatible avec celle generalement admise pour cette periode. Dans un deuxieme stade, ces structures sont re-ouvertes pour servir de receptacle aux fluides mineralisateurs, dont la circulation debute probablement des la fin du stade 1.

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Fernando Llosa

Papers

Erratum to Boiling and vertical mineralization zoning: a case study from the Apacheta low-sulfidation epithermal gold-silver deposit, southern Peru

Structural control and K/Ar dating of the Au–Ag epithermal veins in the Shila Cordillera, southern Peru

Âge K/Ar et contrôle structural de mise en place des veines épithermales à Au-Ag de la Cordillera Shila, Sud Pérou.