Experiments were conducted to test whether changes in CO2 partial pressure could have significant effects on the mobility of base metals in low-temperature hydrothermal systems. The results show that CO2 increases the metal content of aqueous fluids, pore water and sedimentary brine, and associated oil. Complexation processes are probably responsible for the increases observed in the aqueous fluids. Other processes, such as viscosity changes, possible reactions, and selective extraction of metal-bearing organics by CO2, may be responsible for the effects observed in the oil tested. In experiments with pure ZnS in water and in one sedimentary brine, acid hydrolysis and bicarbonate complexing do not explain satisfactorily the observed ZnS solubilities. The experimental results suggest the possibility of additional, or stronger than previously believed, complexing reactions involving carbon dioxide. Another possibility is a complexing mechanism involving reduced sulfur, carbonate and metal together as a single species. Such complexes (thiocarbonates) are of potential interest for the transport of metals in reduced sulfur-bearing hydrothermal fluids. The stability of thiocarbonate complexes is known to be pressure controlled, and this particular property could be of importance for the genesis of metal sulfide deposits. These species should be characterized in future studies. Under hydrothermal conditions, in fluids bearing reduced sulfur, CO2-induced metal transport might be considered a significant alternative to chloro complexing.
All Science Journal Classification (ASJC) codes
- Geochemistry and Petrology