Interpretation of Mineral Zoning in Submarine Hydrothermal Ore Deposits in Terms of Coupled Fluid Flow-Precipitation Kinetics Model

Abstract

Major minerals (sulfates, sulfides, quartz) are distributed in different parts of submarine hydrothermal ore deposits. For instance, the abundance of barite increases stratigraphically upwards in the massive orebodies of the Kuroko deposits (black and yellow ores), while quartz is abundant in the lower parts (siliceous ore). The different distribution of barite and quartz in the Kuroko deposits can not be accounted for by thermochemical equilibrium calculations based on the precipitation due to mixing of ascending hydrothermal solutions with ambient cold seawater. In the present study, a coupled fluid flow-precipitation kinetics model was used to calculate the amounts of quartz, barite, and anhydrite precipitated from a hydrothermal solution mixed with seawater, assuming reasonable values for temperature, precipitation rate, fluid flow velocity, mineral surface area/fluid mass ratio (A/M), and initial concentrations of hydrothermal solution and seawater before mixing occurred. The results indicate that barite precipitates more efficiently than quartz from discharging fluids with relatively higher flow velocity, lower temperatures and under the condition of lower A/M ratios on the seafloor (black ore), whereas quartz precipitates more effectively from solutions with lower flow velocity, higher temperatures and higher A/M ratios beneath the seafloor (siliceous ore) and in the orebody (barite ore, ferruginous chert ore). Anhydrite precipitates in shallow sub-seafloor environments with lower precipitation rates and higher A/M ratios than barite and higher precipitation rates and lower A/M ratios than quartz. These results explain the observed occurrences of barite, anhydrite, and quartz in the Kuroko deposits. Namely, barite is abundant in black ore and barite ore which formed above the seafloor, anhydrite formed in high-permeability tuff breccias, and quartz formed in low permeability dacite intrusive bodies in the sub-seafloor environment.