Stability of copper acetate at high P-T and the role of organic acids and CO 2 in metallic mineralization.

Many metal deposits were formed by carbonic fluids (rich in CO ₂ ) as indicated by fluid inclusions in minerals, but the precise role of CO ₂ in metal mineralization remains unclear. The main components in fluid inclusions, i.e. H ₂ O and CO ₂ , correspond to the decomposed products of organic acids, which lead us to consider that in the mineralization process the organic acids transport and then discharge metals when they are stable and unstable, respectively. Here we show that the thermal stability of copper acetate solution at 15-350 °C (0.1-830 MPa) provides insight as to the role of organic acids in metal transport. Results show that the copper acetate solution is stable at high P-T conditions under low geothermal gradient of <19 °C/km, with an isochore of P = 1.89 T + 128.58, verifying the possibility of copper transportation as acetate solution. Increasing geothermal gradient leads to thermal dissociation of copper acetate in the way of 4Cu(CH ₃ COO) ₂ + 2H ₂ O = 4Cu + 2CO ₂ + 7CH ₃ COOH. The experimental results and inferences in this contribution agree well with the frequently observed fluid inclusions and wall-rock alterations of carbonate, sericite and quartz in hydrothermal deposits, and provide a new dimension in the understanding of the role of CO ₂ during mineralization.