Study on the thermal persistence of silver acetate: Implication for the role of organic acids in metallic mineralization

Abstract

Interaction between organic and inorganic fluids can facilitate formation of mineral deposits. Organic acids are common in oilfield brines, and their decomposition can generate H₂O, CO₂ and CH₄ that are abundant in fluid inclusions from hydrothermal mineral deposits. However, little research has been performed on the interaction between organic acid-bearing solution and metals. This study insights into the role of organic acids in metal transportation by examining the thermal persistence of silver acetate at high temperatures and pressures using a hydrothermal diamond anvil cell. The experimental results indicate that silver acetate becomes more persistent at higher pressure at the same temperature. The presence of pyrite facilitates the decomposition of silver acetate at temperatures of 150–330 °C under geothermal gradient of 25 °C/km. Our thermodynamic modeling results also show that organic matter could generate silver acetate and acetic acid at high temperature and pressure, furthermore, silver acetate remain stable and maintain higher concentrations under a lower geothermal gradient with temperatures below 500 °C and pressure above 500 MPa. These observations accord with the geologic fact that large-scale orogenic-type mineralization of Ag, Au and Cu generally occurred in the post-orogenic thermal extension. They also consist with the numerous observations of CO₂ − H₂O inclusions, the carbonate and hydroxylic alterations in the orogenic-type mineral deposits, and the widespread presence of pyrite formed during earlier mineralization stages. This research provides new understanding of organic–inorganic interaction during hydrothermal mineralization, orogeny and thermal evolution of sedimentary basins.