Sulfate-limited euxinic seawater facilitated Paleozoic massively bedded barite deposition

Abstract

Existing models for massively bedded barite (MBB) deposits (e.g., sedimentary exhalative and diagenetic/ cold-seep) satisfy some geological and geochemical observations, but none explain the Paleozoic clustering of MBB deposits in Earth history. Here we bring seawater redox history into the picture and propose a sulfate-limited euxinic seawater (SLES) model where hydrothermally sourced Ba2+ accumulates as dissolved ion while other metal ions precipitate as insoluble metal sulfides. A subsequent encounter of this Ba2+-rich water mass with a sulfate-bearing one results in the deposition of MBB, thus physically separated from the accompanied metal sulfide deposits. We tested the SLES model using early Cambrian MBB deposits in South China through petrographic and isotope analyses. Syngenetic sphalerite and barite layers show a clear separation. A wide range of 87Sr/86Sr values (0.7082 to 0.7120) of the MBB supports a mixing of seawater and hydrothermal sources. A large range of δ34S values (32.2 to 61.1‰) of the MBB and the occurrence of mineral hyalophane supports an overall sulfate-limited but sulfate-concentration temporally and spatially heterogeneous ocean. Our model established the genetic link between MBB deposits and the accompanied metal sulfide deposits in the Paleozoic. The dearth of MBB deposits before and after the Paleozoic is due to widespread ferruginous oceans with little sulfate and complete oxygenated oceans with too much sulfate, respectively. The Paleozoic clustering of the MBB deposits is a consequence of a critical redox transition in Earth history.