Hydrothermal Dolomite in the Upper Sinian (Upper Proterozoic) Dengying Formation, East Sichuan Basin, China

Hydrothermal Dolomite (HTD) is present in the Upper Sinian (Upper Proterozoic) Dengying Formation, east Sichuan Basin, China. The strata are comprised by primary dolomite. The HTD has various textures, including zebra dolomite, subhorizontal sheet-like cavities filled by saddle dolomite and breccias cemented by saddle dolomites as well occur as a fill of veins and fractures. Also co-occur MVT type lead-zinc ores in the study area. The C-13 and O-18 isotopes of HTD in the Upper Sinian Dengying Formation are lighter than those of the host rocks, while Sr-87/Sr-87 is higher. The apparent difference in carbon, oxygen and strontium isotopes, especially the large difference in Sr-87/Sr-86 isotopes ratio indicate crystallization from hot basinal and/or hydrothermal fluids. Saddle dolomite was precipitated at temperatures of 270-320 degrees C. The diagenetic parasequences of mineral assemblage deposited in the Dengying Formation are: (1) dolomite host rock sphalerite-galena-barite-fluorite; (2) dolomite host rock saddle dolomite quartz; (3) dolomite host rock saddle dolomite bitumen; (4) dolomite host rock saddle dolomite barite. The mean chemical composition of the host dolomite matrix and HTD didn't change much during hydrothermal process. The fluids forming the HTDs in the Dengying Formation were mixtures of freshwater from the unconformity at the top of Sinian, fluids from diagenetic compaction and hydrocarbon generation & expulsion from the Lower Cambrian Niutitang Formation mudstones or the Doushantuo Formation silty mudstones, and hydrothermal fluids from the basement The hydrocarbon reservoirs associated with the HTD were mostly controlled by the basement faults and fractures and karsting processes at the unconformity separating Sinian and Cambrian strata. The hydrocarbon storage spaces of HTD included dissolved cavities and intercrystalline pores. Dissolution cavities are extensive at the top of Dengying Formation, up to about 46m below the unconformity between Sinian and Cambrian and were generated mainly during karstification. Hydrothermal alteration enhanced the reservoir property of the Dengying Formation dolomites with 3%-5% increase in porosity. No agreement has been reached why zebra dolomite occurs only in the Upper Sinian strata, which would indicate that HTD mineralization occurred during two different periods, each of them related to major extensional tectonic event. The early one related to the Xingkai taphrogenesis (Z(2)-Euro(1)) and the later one to the Emei taphrogenesis (D-2-T-2). But, all the data from saddle dolomite suggest that the predominant crystallization occurred during the latter event.