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

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. Thermodynamic, 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

Gold mineralisation and orogenic metamorphism in the Lena province of Siberia as assessed from Chertovo Koryto and Sukhoi Log deposits

The Chertovo Koryto gold deposit (80 t Au at 1.84 g/t) in the Lena world-class province, Siberia, is hosted in a metamorphosed sequence of the Paleoproterozoic Mikhailovsk Formation that comprises the oldest black shale strata of the Baikal-Patom region. The mineralisation is confined to the thrust zone complicated with a conjugate anticline fold, zones of shearing and dislocation. The structural position of the mineralisation is similar to that at the giant Sukhoi Log deposit in the neighbouring Mama-Bodaibo zone. In the latter, the isotope age data suggest that Khomolkho black shales, hosts to Sukhoi Log mineralisation, are of Ediacaran age and underwent prograde metamorphism during early Paleozoic. The geochemical composition of the terrigenous rocks that host Sukhoi Log, Chertovo Koryto, and a number of other deposits at the various stratigraphic levels throughout the Proterozoic sequence have much in common. They do not show elevated metal contents above the common black shale abundances, except for Au and As, which is at variance with the accepted view on diagenetic enrichment of black shales in the Lena province. The occurrence of sagenitic rutile in quartz and chlorite pseudomorphs after biotite and other petrographic observations provide evidence on a retrograde nature of the metamorphic mineral assemblages in the Mikhailovsk rocks. The sulphides are pyrrhotite and arsenopyrite with very minor pyrite at Chertovo Koryto, whereas pyrite is the predominant sulphide in the Sukhoi Log ore. Fluid inclusion data on both deposits emphasise a high-temperature nature of the mineralisation albeit revealing great contrast in the fluid composition. Sukhoi Log mineralisation was formed at mixing between low-salinity aqueous solutions and dense gaseous carbonic fluids, which facilitated effective gold scavenging and precipitation, as demonstrated by thermodynamic simulation. The precursory devolatilisation of the Mikhailovsk sediments at the prograde stage results in the paucity of gaseous carbonic fluid during retrograde metamorphism and mineralisation. The similarity in the styles and chemical parameters of mineralisation, and the predominant structural control of ore localisation within the same Precambrian regional tectonic unit support an idea that orogenic gold mineralisation in the Lena province was produced during a single early Paleozoic event