Likely gold transportation via organic complexes in sediment-hosted orogenic gold deposits in southern Tibet: From the perspective of thermodynamic modelling

Abstract

Carbon dioxide and hydrocarbons, typically considered to be end products of organic matter (OM) in metasedimentary rocks and are commonly observed in the sediment-hosted orogenic gold deposits. It has been suggested that gold can be possibly transported via Au-organic complexes during gold mineralization, which, however, has been overlooked. However, whether gold can be transported via Au-organic complexes during gold mineralization has not been verified. In this study, we took the sediment-hosted orogenic gold deposits in southern Tibet as an example and applied thermodynamic modelling to test this hypothesis. The thermodynamic data for gold acetate is the only comprehensive set available for Au-organic complexes in current common thermodynamic datasets. Therefore, we examined the thermal stability of gold acetate in metamorphic ore-forming fluids at temperatures ranging from 150 to 350 °C at 800–5000 bar and 600 °C at 30000–70000 bar, which are the conditions of gold transportation and deposition in southern Tibet. Our thermodynamic modeling results show that OM begins to decomposition as pressure and temperature rise at 350 °C and 3000 bar. Acetate (CH3COO–) and acetic acid (CH3COOH) remain stable and form in large quantities under a lower geothermal gradient with temperatures below 450 °C and above 3000 bar. We additionally demonstrate that the organic compounds have strong stability in the lower crustal even the upper mantle rocks (pressures > 30000 bar). Simultaneously, gold can combine with acetate to form Au-organic complexes, such as Au(CH3COO) and Au(CH3COO)2- in the ore-forming fluids. As temperature increases at constant pressure, Au(CH3COO) and Au(CH3COO)2- gradually decompose, leading to an increase in CO2 and CH4 content. The findings suggest that organic matter (OM) can transport gold during mineralization processes in southern Tibet, indicating its potential for gold transportation under specific conditions.