鄂西宜昌地区页岩气勘探发现对MVT铅锌矿成矿的指示意义

Abstract

The coupling relationship between hydrocarbon accumulation and the involvement of organic matter in metal mineralization has been a frontier scientific question in recent years. On the basis of the petrography and Raman spectral analysis of fluid inclusions, high-density methane inclusions were discovered in the samples of quartz and calcite veins from Sinian Doushantuo shales, Lower Cambrian Niutitang shales, and MVT (Mississippi Valley type) lead-zinc deposit of Sinian Dengying Formation in Yichang area. The Raman scatter peak v of methane inclusions was applied to calculate the density of pure methane inclusions, and the ore-forming ages of MVT lead-zinc deposit were determined by using Rb-Sr and Sm-Nd isochron dating. The density of methane inclusions trapped in the quartz veins within the Doushantuo shales of well EYY 1 mainly ranges from 0.237 to 0.278 g/cm, and the density of methane inclusions trapped in the calcite veins within the Hejiaping MVT lead-zinc deposit mainly ranges from 0.213 to 0.271 g/cm, which signifies methane inclusions of high density. Paragenetic mineral association sphalerite and galena within the Hejiaping lead-zinc deposit have a Rb-Sr isochron age of 189.1±1.8 Ma, and calcite within the Hejiaping lead-zinc deposit yields a Sm-Nd isochron age of 189.9±2.0 Ma, which indicates is the mineralization of Hejiaping lead-zinc deposit was closely related to the Yanshanian tectonic compressions. The Sr/Sr values of the paragenetic mineral association (0.711 92) and calcite (0.712 03-0.712 27) indicate that the ore-forming fluids of the Hejiaping lead-zinc deposit were derived largely from shale sources. The high-density methane trapped in fluid inclusions within the Hejiaping lead-zinc deposit was most likely derived from the high-density overpressure methane generated by the Doushantuo shales and/or Niutitang shales. The discovery of high-density methane inclusion in the shale gas layer and MVT lead-zinc deposit, and the determination of ore-forming ages of MVT lead-zinc deposit provide new evidence for the study of organic matter participation in the mineralization of MVT lead-zinc deposit.