Geochemistry of the sediment-hosted disseminated gold deposits in southwestern Ghizhou Province, China

Zhiwei Bao, Thèse de Doctorat

Projet de Doctorat en ressources minérales sous la supervision du Dr Jayanta Guha.

Description du projet

The sediment-hosted disseminated gold (SHDG) deposits in southwest Guizhou Province, China, are located near the buried edge of the Yangtse craton. Although some investigations have been carried out on their geological and geochemical characteristics, in general, most of the studies are based on limited database. The source of the gold in the deposits and the relationship between gold and the organic matter in the ores are still not clear. Hence, the role of organic matter and the source of Au in SHDG mineralization have been the major objectives of this study. Three deposits, the Lannigou, Getang and Zimudang were chosen for this study. The deposits are hosted by rocks of distinct sedimentary environment and were formed at low temperatures (120Å`240°C) from solutions of low salinity, weak to neutral acidity, and in a reducing environment. In southwest Guizhou Province, the SHDG deposits are commonly hosted by the same fold crests, usually containing a remarkable amount of hydrocarbon material that is common to SHDG deposits. From the crosscutting relationship of bitumen and quartz veins, the bitumen migration was likely to have taken place before the gold mineralization. However, the possibility that some of the hydrocarbons were brought in by the ore forming solution cannot be ruled out. Group analysis of the dichloromethane extractable organic component of the ores and host rocks shows that the degree of maturation of the ores is slightly higher than the host rocks. However, the compositions of their alkanes, steranes and terpenes, which serve as biomarkers, are quite similar; this suggests that the organic matter found in the ores and host rocks has a common source. Finally, organic matter most probably contributed significantly to the preconcentration of Au in the host rocks. However, its role in the primary and secondary generation of crude oil and its migration apparently predate mineralization in most SHDG deposits. Hence, crude oil was not a likely potential agent of Au transportation. In conclusion, organic matter was an active component in the SHDG mineralization under study. It contributed to the thermal reduction of sulfate and thus pyritization. Hydrocarbons in the solution may have increased the potential of the hydrothermal solution to transport Au. The hydrocarbons were mainly derived from the host rocks while the gold might have been derived from a mixed source including host rocks, underlying crustal rocks, basaltic rocks and possibly ultramafic rocks. Our data suggests that the SHDG deposits in southwest Guizhou are most likely amagmatic in origin, which is supported by the complete lack of igneous intrusives in the vicinity of the deposits, the very weak metamorphic grade of the crustal rocks and lack of metamorphic event within the time range of the SHDG mineralization, and the coincidence of the time frame of mineralization with the regional extension.