Multiple proxies indicating methane seepage as the origin of Devonian large barite deposit in Zhenning-Ziyun, Guizhou, SW China

Abstract

Bedded barite deposits are found in Cambrian and Devonian strata in China. In this paper, we report the occurrence of Late Paleozoic methane seeps from bedded barite in one of the largest barite deposits in China. The ore bodies are stratiform and lenticular in shape and are hosted within Upper Devonian cherts. The barite deposits usually have laminar, banded, fragmental, massive and rosette textures, and radial and spheroidal structures were observed in the barite ores. Nodular limestones are associated with the barite deposits; they consist of clotted micrites and framboidal pyrites. The internal structure of framboidal pyrite is simple, in which the sun-flower structure has been observed, similar to modern seep carbonates. The δ13CV-PDB values range from −8.82‰ to −10.27‰, and δ18OV-PDB ranges from −8.10‰ to −8.20‰ in Xiyahe and from +0.03‰ to −3.31‰ in Luocheng. The 13C depletion suggests that these carbonates formed in a methane seep through the anaerobic oxidation of methane (AOM). This evidence lends strong support to the interpretation that methane hydrate contributed to the barite deposition. In addition, the δ13CV-PDB values decrease gradually from Xiyahe to Luocheng, which indicates the gradually weakening strength of the methane seep activities. The δ30SiNBS-28 values of the cherts range from −0.3‰ to +0.5‰, except for two samples (LJ-24: δ30SiNBS-28 values=+0.9‰; ZL18: δ30SiNBS-28 values=+1.1‰). The lowest δ30SiNBS-28 values reflect silicon derived from hydrothermal fluids ascribed to intense tectonic activity, which consequently caused the rapid maturity of hydrocarbons. The 87Sr/86Sr values (87Sr/86Sr=0.70863–0.70898, avg. 0.70877; n=11) of barite are higher than the average value of contemporary seawater, showing that hydrothermal processes played a minor role in barite mineralization.