Oil-source rock analysis and metallogenic significance of the palaeo-oil reservoir in the Qinglong antimony deposit, South China

Abstract

The Qinglong antimony deposit is located in the Nanpanjiang (Youjiang) basin, South China. The role of organic matter in antimony mineralisation is often discussed due to hydrocarbon gases within inclusions of antimony deposits, which have been detected since the 1990s. However, as no macroscopic or mass organic material has been discovered, the origin of such hydrocarbon gases and their potential relation to mineralisation remain unclear. The discovery of the Qinglong palaeo-oil reservoir has provided a new opportunity for further study of the relationship between organic matter and antimony mineralisation. Five samples of bitumen from the palaeo-oil reservoir and seven samples of Devonian source rocks from the northern part of an antimony mine were analysed for organic elements, total organic carbon, chloroform bitumen ‘A’ and its group compositions, bitumen/vitrinite reflectance (Rb/Ro), biomarkers, carbon isotopes, and Rock-Eval pyrolysis. Kerogen microscopy and rare earth elements (REEs) analysis were also performed on the Devonian source rocks samples. Finally, seven samples of pyrite coexisting with bitumen in the palaeo-oil reservoir were analysed using stable sulphur isotope analysis.Results show that the Devonian source rocks are rich in organic matter, yet it is well known that few residual organic matters are capable of continuing hydrocarbon generation after strong thermal evolution. Both the palaeo-oil reservoir and source rocks are found to have the same source of parent material and belong to the same mixed type of kerogen, and have reached the over-mature stage. The ratio of pristane/phytane (Pr/Ph) and the gammacerane index (Ga/C30H) indicate that the source rocks were formed in a reductive brackish water sedimentary environment, with a parameter nearly the same as that of the palaeo-oil reservoir. The palaeo-oil reservoir samples are highly consistent with the source rocks in terms of the source and type of organic matter, as well as the sedimentary environment, thermal maturity, normal alkanes, key fingerprint parameters, and grey relevancy degree of REEs. In addition, the carbon isotopes (δ13C) of bitumen range from − 28.75‰ to − 30.23‰, with an average of − 29.45‰, while those of the source rocks vary from − 25.20‰ to − 28.56‰, with an average of − 27.06‰, which are consistent with each other in the range of carbon isotope fractionation generated by the thermal evolution of organic matter. Therefore, it is determined that the palaeo-oil reservoir was derived from the Devonian oil-source rocks.The sulphur isotopes (δ34S) of pyrite associated with bitumen in the palaeo-oil reservoir have a relatively narrow range (−1.9‰ to 3.1‰, average 0.97‰), which are very close to those of sulphides in the mineralisation period of the gold and antimony deposits. We believe that the sulphur needed for mineralisation was mainly derived from hydrocarbon fluids in the basin and speculate that the palaeo-oil and hydrocarbon gases formed by thermal cracking and then served as a reducing agent to convert SO42− in the oilfield brine into reduced sulphur (S2−/HS−) through the thermochemical sulphate reduction (TSR) reaction. It is found that although the Qinglong palaeo-oil reservoir did not contribute Sb to the deposit (existing Sb was instead found to be from the Dachang member and its overlying basalt), it did, however, provided a reductive metallogenic environment and contributed a relatively large amount of reduced sulphur (S2−/HS−) to participate the mineralisation.