Differential deposition of gold in mafic-hosted and ultramafic-hosted hydrothermal systems on the mid-ocean ridge

The global mid-ocean ridge, where the gold-bearing hydrothermal circulation occurs by tectonic and magmatic processes, is the largest mountain chain on the Earth. It consists predominantly of ultramafic and maficd rocks. A significant amount of gold is initially leached out from the two types of source rocks, and then move by similar physical-chemical processes of transportation and precipitation, and finally incorporate into the massive sulfide deposits. After formation, the sulfide deposits will suffer from extensive dissolution, remobilization and reprecipitation. It is observed that the gold ratios between the sulfide deposits and the source rocks in the ultramafic-hosted hydrothermal systems are much greater than those in the mafic-hosted hydrothermal systems. Such differences in the two types of hydrothermal systems suggest that they have significant distinctions in the occurrence, evolution and enrichment of gold in the hydrothermal circulation on the mid-ocean ridge. Four factors in favor of such distinctions in the ultramafic-hosted hydrothermal systems have been identified: 1) the nature of the surrounding rocks; 2) abiotic organic compounds and gaseous species in the vent fluids; 3) permeability of the hydrothermal fluids upflowing zones; 4) incidence of the volcanic event. For the gold deposits on the mid-ocean ridge, in-situ and micro-area analyses of high-precision for the composition, species and properties in different components of the hydrothermal systems together with the experimental and thermodynamic modelling are critical to revealing quantitatively the extent of the source contributions and enrichment or depletion of gold in different evolutionary stages of gold, and provide significant insight into the enrichment process of gold on the mid-ocean ridge under the intervention of mankind.