Gold Fineness in the Jiangnan Orogenic Belt of South China: Correlations with Geological Characteristics and Fluid Conditions

Abstract A large number of Au deposits in the Jiangnan Orogenic Belt (JOB) are characterized by producing the bonanza-style gold with extremely high gold fineness (GF) in orogenic Au deposits and intrusion-related Au deposits. However, there has been limited research on the correlation between GF and geological characteristics as well as fluid conditions, which is crucial for understanding Au mineralization in the JOB. This study systematically summarized characteristics of deposit geology and ore-forming fluids in fifty representative Au deposits and compared those characteristics with their GF. A comparative analysis of the GF in two types of Au deposits across eight province-scale areas of the JOB reveals lower GF of the orogenic Au deposits from the western part of JOB and larger fineness ranges of the intrusion-related Au deposits. This study identifies that the geological characteristics, including Au/Ag ratios of host rocks, orebody depths, Au grades, are positively correlated with the GF. Additionally, the fluid conditions such as temperature and salinity also exhibit positive correlations with the GF. To reveal the controlling factor of GF, this study constructs 3D thermodynamic models of gold solubility and GF based on the summarized geological and fluid data. The results indicate that the high Au/Ag ratio of fluid is the key factor contributing to the extremely high GF in orogenic Au deposits, leading to their narrow ranges of high fineness under varying fluid conditions. In contrast, intrusion-related Au deposits exhibit a larger fineness range due to low Au/Ag ratios in fluids and decreasing in temperature and salinity. This study provides an example for exploring the correlation between GF and geological factors and offers insights into the thermodynamic simulation of GF as an indicator.