A new salinity-based model for Cryogenian Mn-carbonate deposits

Abstract

The genesis of manganese (Mn)-carbonate deposits in the Cryogenian Datangpo Formation (∼662.9–654.5 Ma), Nanhua Basin, South China remains controversial. Here, we combine new proxy data (B/Ga) for watermass salinity with existing data (major- and trace-element concentrations, bulk-rock Sr and Nd isotopes, pyrite sulfur isotopes, and organic and inorganic carbon isotopes) for redox and other environmental parameters to gain new insights into the conditions under which these deposits formed. Our analysis focuses on the Mn-carbonates of the 1st Member of the Datangpo Formation, which were deposited at the termination of the Sturtian Ice Age, with an emphasis on understanding their environment of formation, Mn sources, and microbial processes. Close relationships between Mn content and salinity (B/Ga), redox (Corg/P, CuEF), and carbon-cycle (δ13Ccarb, δ13Corg) proxies reveal a dominant role of salinity in the development of these Mn-rich deposits. These relationships document Mn accumulation in a watermass that fluctuated between brackish, euxinic conditions (Mn-shale beds) and saline, ferruginous conditions (Mn-carbonate beds). Significant correlations between Mn content and hydrothermal proxies (Eu/Eu*, 87Sr/86Sr and εNd(i)), as well as covariation of (87Sr/86Sr)i vs εNd(i) and Fe/Ti vs Al/(Al + Fe + Mn), suggest that Mn was sourced mainly from hydrothermal vents in the deep Nanhua Basin. Episodic hydrothermal activity also provided nutrients that boosted primary productivity and organic matter accumulation rates in the form of mineralized biomats. Partial oxidation of the organic carbon coupled to dissimilatory Mn(IV) reduction promoted Mn(II)-carbonate formation in conjunction with high levels of glacially generated alkalinity. A similar confluence of factors (i.e., high background alkalinity, hydrothermal inputs of Mn and nutrients, and microbial activity) may have played a role in the formation of large-scale Mn-ore deposits during other geologic epochs.