Relationship between Carlin-type gold deposits and paleo-petroleum reservoirs in SW Guizhou, China: Evidence from gas compositions of fluid inclusions and Raman spectroscopic characteristics of bitumen

Carlin-type gold deposits in SW Guizhou Province, China, represent the world's second largest concentrations of this type of deposits and show a close spatial relationship with paleo-petroleum reservoirs. Whether gold genesis is genetically related to the paleo-petroleum reservoirs, however, remains unclear and debated. New compositional data of fluid inclusions in various ore-related minerals and Raman spectroscopic characterization of disseminated bitumen in high grade ores from selected gold deposits are used to better understand the relationship between gold mineralization and hydrocarbon generation in the region. Arsenic, and commonly auriferous pyrite, quartz, calcite, and fluorite, which typically formed in different paragenetic stages of gold mineralization, have comparable species, contents, and variation patterns of hydrocarbons. This observation indicates minimum decomposition or restructuring of organic ionic compounds during the evolution of ore-fluids. It is thus inferred that organic matters had a negligible role in carrying gold in the ore-forming fluids as previously suggested. In the N-2/Ar versus CO2/CH4 diagram, analyses from various minerals show a positive correlation extending from a circulating meteoric water field to magmatic fluid field, indicating that the inclusion fluids most likely represent mixture of gaseous species derived from deep concealed magma chamber and circulating meteoric water. It is also noteworthy that one quartz sample from the Shuiyindong gold deposit deviates significantly from the main trend and approaches the organic component field, reflecting abnormal high contents of organic components in the fluid inclusions trapped during the ore-fluid migration. Bitumen disseminations in high grade ores from the selected gold deposits show consistent Raman spectroscopic patterns and therefore are considered to have a similar origin. Based on the Raman spectroscopic data of bitumen, the early-stage ore fluids have calculated temperatures of 317 similar to 336 degrees C, which are significantly higher than paleogeothermal temperatures of 160 similar to 250 degrees C and homogenization temperatures of fluid inclusions of regional paleo-oil reservoirs (73 similar to 175 degrees C). The abnormally high temperatures of ore fluids suggest that regional paleogeothermal field was insufficient to provide heat source responsible for ore-fluid transportation and gold mineralization. Instead, the heat source was mostly likely provided by deep-seated magma chambers. In conclusion, we suggest that Carlin-type gold deposits in SW Guizhou were distal products of deep-seated magmatic hydrothermal systems, and had no genetic relationship with paleo-oil reservoirs in the region. Gold- and hydrocarbon-bearing fluids were derived from different sources but may have been driven by the same thermal event and shared the same structures during their transportation and deposition.