Can Au be deposited in brine fluid?
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Gold (Au) Deposition in Brine Fluids: Insights from Hydrothermal Systems
Introduction to Gold Deposition in Brine Fluids
Gold (Au) deposition in brine fluids is a significant process in the formation of various hydrothermal ore deposits. Understanding the conditions and mechanisms that facilitate this deposition is crucial for mineral exploration and economic geology. This article synthesizes findings from multiple studies to elucidate how gold can be deposited in brine fluids.
High-Temperature Brine Inclusions and Gold Deposition
Porphyry Cu-Au Systems
In porphyry Cu-Au deposits, such as the Bajo de la Alumbrera in Argentina, high-temperature brine inclusions have been identified as key carriers of gold. These brine inclusions, which can reach temperatures up to 750°C and pressures above 1 kbar, are characterized by high salinity (50-60 wt% NaCl equivalent) and contain significant concentrations of gold (0.55 ppm) and copper (0.33 wt%). The cooling and decompression of these brines lead to the precipitation of gold and copper minerals, such as chalcopyrite and native gold, particularly as the temperature drops from 400°C to 305°C.
Ernest Henry Deposit
Similarly, the Ernest Henry iron oxide-copper-gold (IOCG) deposit in Australia shows evidence of ultra-high-salinity brine inclusions (30-70 wt% NaCl equivalent) that played a crucial role in the mineralization process. These brines, derived from magmatic fluids and halite dissolution, facilitated the transport and deposition of gold at depths of 6-10 km.
Metal Fractionation and Fluid Phase Separation
Brine and Vapor Partitioning
The behavior of gold during fluid phase separation, or "boiling," in high-temperature saline systems has been extensively studied. It has been observed that gold tends to partition into the vapor phase rather than the brine phase, likely as HS complexes. This selective partitioning is a significant factor in the chemical differentiation and enrichment of gold in magmatic-hydrothermal systems.
Naruo Porphyry Cu-Au Deposit
In the Naruo porphyry Cu-Au deposit in Tibet, high-salinity brine inclusions (30.1-61.5 wt% NaCl equivalent) were found to be the primary carriers of gold. The high oxidation state of these fluids facilitated the scavenging of sulfur and chalcophile metals, promoting the precipitation of gold and copper minerals at temperatures between 440°C and 300°C.
Experimental Studies on Gold Solubility in Brines
Synthetic Fluid Inclusions
Experimental studies using synthetic fluid inclusions have provided valuable insights into the solubility of gold in NaCl brines. These studies have shown that gold concentrations in brines can reach up to 103 ppm under high-temperature and high-pressure conditions (600-800°C, 1.5 kbar). The solubility of gold decreases with increasing salinity, suggesting that high-salinity brines can dissolve and transport significant amounts of gold across the magmatic-hydrothermal transition.
Conclusion
Gold can indeed be deposited in brine fluids under specific high-temperature and high-salinity conditions. The deposition process is influenced by factors such as fluid phase separation, oxidation state, and the presence of sulfur and other chalcophile elements. Understanding these mechanisms is essential for exploring and exploiting hydrothermal ore deposits. The insights gained from studies on porphyry Cu-Au systems, IOCG deposits, and experimental fluid inclusions provide a comprehensive understanding of gold deposition in brine fluids.
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