Abstract
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 BaN/LaN at 1.6–65.5, strongly positive Sr anomalies at Sr/Sr* = 5.54–39.9, and relatively low 87Sr/86Sr 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 87Sr/ 86Sr ratios of 0.705321–0.705968 in the dolomite and δ34SV-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.
Highlights
Modern global ocean explorations have revealed hundreds of submarine hydrothermal venting sites spread across the ocean floors, which continue to increase in number (Baker et al 2016; Baker 2017)
They consist of two compositional textures of the “phenocrysts” and “matrix,” which is similar to the porphyritic texture observed in volcanic rocks (ESM; Fig. S1)
There is a lack of rare earth element (REE) information on ancient lake water in the Jimusar Sag, normalized REE patterns of Jimusar calcite partly coincide with those of carbonate-rich microbial mats which are formed in shallow lake water with mixed rain, seawater, and CO2-dominated hydrothermal fluids in Venere Lake, Pantelleria Island (Fig. 6 a, c, Censi et al 2015)
Summary
Modern global ocean explorations have revealed hundreds of submarine hydrothermal venting sites spread across the ocean floors, which continue to increase in number (Baker et al 2016; Baker 2017). Some sparry calcite and pyrite in Lucaogou dark mudstones in the Santanghu and Junggar basins have been interpreted as evaporative gypsum pseudomorphs with a suspected hydrothermal origin (Hackley et al 2016; Qiu et al 2016a; Wu et al 2016). These cases indicate that many uncertainties and conflicting opinions remain on the identification of hydrothermal minerals and the explanation of their origins. Additional research is needed to explore the detailed processes of ancient sublacustrine hydrothermal activities to evaluate their potential economic and scientific significance
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