Diagenetic barite deposits in the Yurtus Formation in Tarim Basin, NW China: Implications for barium and sulfur cycling in the earliest Cambrian

Abstract

Barite concretions and bands are widely distributed in black shale–chert horizons in the Yurtus Formation of Lower Cambrian in Aksu area, northwestern Tarim Basin, NW China. They mainly consist of coarse-grained anhedral to euhedral barite crystals with minor dolomites and pyrites. Petrological features indicate these concretions grew from the porewater in unconsolidated sediments at shallow burial below sediment-water interface. The slight deviation of 87Sr/86Sr ratios (0.7083 to 0.7090) and significant elevated δ34S values (56.8–76.4‰ CDT) of barite samples with respect to those of the Early Cambrian seawater further support that barite deposits precipitated from the enclosed porewater in sediment column, which evolved from the penecontemporaneous seawater with weak interaction with the host fine-grained siliciclastic sediments and highly-depleted sulfate in response to prolonged strong bacterial sulfate reduction without necessary renewal. The abundant organic matters in the basal Yurtus Formation should have facilitated developing sulfate-depleted methanogenesis zone and sulfate–methane transition zone (SMTZ) slightly after deposition. Therefore, barite deposits in the Yurtus Formation most likely resulted from diagenetic barium cycling and persistently grew from the porewater in the static SMTZ with a low sedimentation rate in the Early Cambrian. In comparison with the distribution of sedimentary barites in geological records, we tentatively proposed that a transition in diagenetic barium cycling and associated mineralization may have occurred from the Precambrian to Cambrian periods; this scenario may be causally linked to the changes in marine ecology (the advent of mesozooplankton and associated faecal pellet) and geochemistry (the increase of seawater sulfate concentration). Thus, the occurrence of diagenetic barite deposits in the Yurtus Formation implies that diagenetic barium cycling and more effective scavenging of barium from CH4- and Ba-rich porewaters within sediments might have become an nonnegligible process in continental margin areas, at least, since the earliest Cambrian, which could have significantly impacted the marine barium cycling.