Origin of microbial–hydrothermal bedded dolomites in the Permian Lucaogou Formation lacustrine shales, Junggar Basin, NW China

Abstract

Dolomite is widely developed in the shale sequences of saline lakes and has significance in paleoenvironmental, paleoclimatic, and petroleum geology research. In this study, a new end-member type of dolomite is discovered from the Permian Lucaogou Formation in the Junggar Basin, northwest China, along with its geological significance. The microbial–hydrothermal origin of the bedded dolomite is evident from the development of hydrothermal minerals (including barite and cubic pyrite) and methanogenic micro-organisms. In terms of geochemistry, the bedded dolomite has high (Ba/La)N ratios (~2.8) and Sr contents (~594.9 ppm) and positive Sr anomalies (Sr/Sr* ~3.8). Chondrite-normalized rare earth element (REE) patterns exhibit light REE enrichment, heavy REE depletion, slightly positive Ce anomalies, and negative Eu anomalies. The bedded dolomite has relatively low 87Sr/86Sr ratios (~0.705283), high δ13C values (~6.63‰ ), and low δ18O values (~−9.92‰ ). All these indicate that the dolomitizing fluid is a mixed source of hydrothermal fluids and lake waters, and the hydrothermal fluids and microbial methanogenesis provided the Mg2+ and C, respectively. The combination of microbial and hydrothermal activity overcame the limitation of dolomitization hydration shells around Mg2+. In terms of geological significance, the studied Lucaogou Formation was deposited in an extensional tectonic setting under hot–arid climatic conditions, which controlled the high-frequency interbedding and meter-scale cyclicity of sediment deposition of dolomites and shales. The combined effects of tectonism and paleoclimate may be important factors for the formation of bedded dolomite, enrichment of organic matter and hydrocarbons in saline lacustrine shales.