In situ S isotope analysis and source tracing of pyrite from lacustrine hydrothermal sedimentary rocks: the Chang 7-3 sub-member, Triassic Yanchang Formation, Ordos Basin

Abstract

The Ordos Basin developed a large depression-type lake basin in the Late Triassic. Here, we report on pyrites from lacustrine hydrothermal sedimentary rocks that were found at the bottom of a black shale layer from the Chang 7-3 sub-member of the Triassic Yanchang Formation in Tongchuan and Yaoqu and Well Z9 sections, on the southern margin of the Ordos Basin. Observations of field outcrops and underground cores were used to identify vein and brecciated pyrites that are distributed within water-explosive dolomite breccias or sandwiched in a textured layer of hydrothermal sedimentary rock. Based on previous work, scanning electron microscopy observations, electronic probe microanalysis, nano-SIMS trace-element mapping and in situ sulfur isotope analyses were completed. The crystal form of pyrites is mainly cubic with surfaces mostly smooth and clean. Pyrites contain trace elements, such as As, Au, Co, Cu, Cr and Zn. The pyrite sulfur isotope values are enriched in 34S (δ34SV-CDT = 7.31–10.05‰; average 8.49‰) and show that the pyrite is related to hydrothermal deposition. Late Triassic hydrothermal activity at the bottom of the lake provided sulfate for pyrite formation, and hydrothermal sulfide may have also been an important sulfur source. KEY POINTS Pyrites from lacustrine hydrothermal sediments were formed at the bottom of a black shale layer from the Chang 7-3 sub-member of the Triassic Yanchang Formation on the southern margin of the Ordos Basin. The pyrite contains trace elements such as As, Au, Co and Ni. In situ pyrite sulfur isotope values are enriched in 34S and show that the pyrite is related to hydrothermal deposition with sulfur derived from sulfate and hydrothermal sulfide