Role of carbonaceous material in gold precipitation for orogenic gold deposits: A case study of the Bangbu gold deposit in southern Tibet, China

Abstract

Carbonaceous material (CM) is widely distributed at the Bangbu orogenic gold deposit in vein-hosted and disseminated ores, as well as wall rocks. Despite being closely ore-related, the CM nature and genesis, as well as its possible role in gold metallogeny, remain unclear. Hence, we investigate the role of five ore-related CM types (CM1-4 and methane, which coexist with three pyrite generations of Py1-Py3) by integrating petrographic observation, Raman spectroscopy, LA-ICP-MS, fluid inclusion microthermometry, and thermodynamic modeling. CM1 only occurs in disseminated ores and is overprinted by large euhedral Py1 with median 5.28 ppm Au. Raman spectroscopy analysis shows that CM1 has undergone high-temperature regional metamorphism (∼510 °C). Geochemical modeling shows that Au precipitation in Py1 can be attributed to the fluid-rock interactions of CM1 and Au-bearing ore fluids, which future leads to the host rock alteration and CM1 consumption. CM2 occurs with large subhedral Py2 and hydrothermal sericite in quartz veins, and gold exists as native gold and as Au+ in pyrite. Py2 consists of Py2a with median 0.44 ppm Au and Py2b with median 49.80 ppm Au. Raman spectroscopy of CM2 experienced a temperature of ∼455 °C, much higher than the main ore stage fluid inclusion homogenization temperature (167–247 °C). Meanwhile, methane is commonly found in CO2-bearing fluid inclusions in the quartz closely associated with Py2 and CM2. Geochemical modeling shows that methane can be a more efficient reductant than CM2 and is capable of reducing Au-bisulfide to native gold. CM3 intergrows with fine Au-poor subhedral Py3 (median 1.00 ppm Au) in randomly-oriented “black sinuous” veins within vein-type ores. The CM3 formation temperature is calculated as ∼290 °C. During the sulfidation processes, H2S and CO2 from the ore fluids may have reacted with Fe-bearing minerals, precipitating CM3 and Py3. The loss of aqueous H2S would destabilize Au-bisulfide complexes, which is the secondary gold precipitation mechanism proposed for this deposit. CM4 occurs in the wall rocks and have similar structural features with structure-bound semi-graphite observed by petrology, indicating that the formation process is different from that of other CM types. This is supported by the Raman geothermometric results of CM4 (formation temperature ∼388 °C). Additionally, no spatial association was observed between CM4, auriferous sulfides, and native gold. In conclusion, our study proposed the most likely roles of CM in gold precipitation in this deposit, which bears important implications for better understanding of the processes that lead to the formation of high-grade ores.