The Role Of Carbonaceous Material In Gold Precipitation

Carbonaceous material（CM）is present in various types of hydrothermal gold deposits worldwide,most people think that CM plays an important role in gold mineralization,but some people hold different views,that CM is not necessarily related to gold mineralization.Therefore,clarifying the relationship between carbonaceous matter and gold mineralization may strengthen the understanding of gold formation mechanisms and improve the efficiency of prospecting.The Jiangnan orogenic belt located in South China is an important gold metallogenic belt.The gold deposits in this area mainly occur in Neoproterozoic metamorphic sedimentary rocks.The Wangu gold deposit,located in the middle of the Jiangnan orogenic belt,has the characteristics of high-grade and large reserves,and mainly occurs in carbonaceous slate and iron-rich altered slate.However,the carbonaceous matter of Wangu Gold Mine has not been studied by predecessors,so this paper takes Wangu Gold Mine as the research object.Based on the detailed geological investigation of the deposit,this paper focuses on the metallogenic mechanism of gold in the carbonaceous slate and iron-rich altered slate of the Wangu deposit using petrography,Raman,electron probe microanalysis（EPMA）,laser denudation plasma mass spectrometry（LA-ICP-MS）,transmission electron microscopy（TEM）and geochemical thermodynamic simulation analysis,and has achieved the following results.The orebodies hosted by CS and IAS in the Wangu deposit have similar Au grades,and gold is mainly present in pyrite and arsenopyrite,with local native gold in ores hosted by CS.Three generations of pyrite（Py1–3）have been recognized in both ores,with Py3 bearing the highest gold concentrations.Each-stage pyrite in ores hosted by CS and IAS shares many similarities,indicating that they were produced by the same ore fluids interacting with different lithologies.Three types of CM（CM1-3）are recognized in the Wangu deposit,and the predominant CM2 generated by pre-ore metamorphism is closely associated with gold mineralization.Moreover,this study reveals that the reduction caused by CM as well as CH₄ generated from CM could only produce the locally occurred native gold in ores hosted by CS.However,the decrease of Au solubility caused by the reduction would be neutralized by the p H buffering at near-neutral conditions through interactions with feldspar and mica.In addition,the general low Au concentrations and high Au solubility due to relatively high S contents in most Au-bearing fluids worldwide indicate that the reduction by CM and CH₄ alone is commonly unable to cause gold mineralization.Instead,the combination of reduction and sulfidation could significantly reduce the solubility of gold and lead to the co-precipitation of sulfide and gold.This paper argues that at the scale of mineral deposits,The ores of the two groups of metasedimentary rocks are produced by similar gold precipitation mechanisms.Consequently,the reducing property of CM in carbonaceous rocks does not make it an efficient trigger for gold deposition under most geological conditions.