Hydrothermal Mineral Assemblages of Calcite and Dolomite–analcime–pyrite in Permian Lacustrine Lucaogou Mudstones, Eastern Junggar Basin, Northwest China

The eastern Junggar Basin, controlled by continental extension (rift), was deposited by lacustrine dominated sediments during middle Permian Lucaogou period. An unusual porphyritic-like texture was observed in Lucaogou/Pingdiquan dark fine-grained organic-rich sediments in two sub-tectonic units in the basin. The “phenocrysts” are composed of two types of mineral assemblages. The first is a coarse euhedral calcite assemblage in the Jimusar Sag, and the second consists of dolomite, analcime, and pyrite in the Shishugou Sag. The lithological and mineralogical features indicate a hydrothermal origin for these phenocryst-like minerals. The chondrite-normalized rare earth element patterns show flat or positive Ce anomalies and negative Eu anomalies, which reflect a suboxic to anoxic, off-axis site from the center of the fault system, where the temperature of the hydrothermal fluid might be less than 250 °C. The high ratios of Ba N /La N at 1.6–65.5, strongly positive Sr anomalies at Sr/Sr* = 5.54–39.9, and relatively low 87 Sr/ 86 Sr isotopes at 0.705002–0.705776 in the coarse calcite suggest an origin of mixed sources of lake water, underlying biogenetic sediments, and deep magmatic water. However, the low 87 Sr/ 86 Sr ratios of 0.705321–0.705968 in the dolomite and δ 34 S V-CDT of 10.8‰–12.3‰ in the pyrite indicate that water–underlying-rock interaction and the abiotic thermochemical sulfate reduction of lake water or organic matter might have participated together resulting in the precipitation of the dolomite–analcime–pyrite assemblages in the Shishugou Sag.