Petrology and Geochemistry of Pb-Zn Mineralization in Upper Sinian Dengying Dolomites in the Central Sichuan Basin, SW China: Implications for the Origin of H2S

Abstract

Recently, a giant sulfur-bearing gas field has been discovered in the Sinian-Cambrian strata in Sichuan Basin, which has varied amount of H₂S (0.10~3.18%). However, the origin of H₂S in Sinian reservoirs remains controversial. Several authors suggested thermochemical sulfate reduction (TSR) origin, while many authors have proposed kerogen-generative origin due to low content of H₂S and lack of sulfate minerals in host dolomites. In order to address this issue, a comprehensive integrated study based on prteological and geochemical data was carried out in this work. The following conclusions can be drawn.

(a) The Pb-Zn mineralization in the Sinian dolomites shares many features with classic Mississippi Valley-Type deposits including geological setting, mineralogy, fluid characteristics and crustal source for metals.

(b) Four discrete mineral suites can be identified in the Dengying Formation, including saddle dolomite-pyrite (Stage I), granular quartz-pyrite mixed with a small amount of galena (Stage II), lath-shaped or rod-like quartz-galena (stage III) and prismatic quartz-sphalerite (stage IV), representing four mineralizing stages. Solid bitumen is commonly associated with mineralizing processes. Heavy sulfur isotope composition of solid bitumens and metal sulfides, as well as light carbon isotope composition of saddle dolomites indicate that in-situ TSR have probably occurred and involved in ore formation. The increase in δ³⁴S from Stage II pyrite through Stage III galena to Stage IV sphalerite suggests Rayleigh fractionation during TSR.

(c) Hydrothermal barites, probably derived from the Sinian Ba-rich basement rock, have been partially replaced by sphalerites and pyrites via incomplete TSR and some nodular barites still remains in solid bitumens, indicating that barite-bearing hydrothermal fluids could react with hydrocarbons. The δ³⁴S values of H₂S have a similar distribution pattern as those of the solid bitumen and metal sulfides, most of which are slightly lighter than those of barites but significantly heavier than those of gypsums, suggesting that most of the sulfur in H₂S was probably derived from the hydrothermal barites.

In summary, most of H₂S was probably generated by in-situ TSR between hydrocarbons and sulfate-bearing hydrothermal fluids rather than diagenetic gypsums in the Dengying Formation.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018