Neoproterozoic Sedimentary Environment of Shilu Area, Hainan Island, South China

Abstract

The paucity of BIFs deposition during the “boring billion” (1.8–0.8 Ga), precludes a robust understanding of their sedimentary environment and Fe precipitation mechanism. Recently, a unique large-scale early Neoproterozoic (0.96–0.83 Ga) Shilu hematite-rich BIF was identified in South China. The Shilu BIF provides an opportunity into the understanding of the depositional conditions for the BIFs formed in the interval between 1.8 and 0.8 Ga. Here, we use a combination of elemental and isotopic data on the Shilu Fe ores and the overlying dolostones to reconstruct the depositional environment. The dolostones demonstrate geochemical features that resemble those of modern ocean water with PAAS-normalized negative Ce anomaly, super-chondritic Y/Ho ratios, as well as C and O isotopes of typical marine origin, except for mild depletion in LREE relative to HREE and small positive Eu anomaly. These features, together with high 87Sr/86Sr ratios and low εNd(t) values, support a shallow-water restricted epicontinental basin with exchange to the open-ocean, as well as riverine and low-T hydrothermal input. The Shilu Fe ores display strongly positive to weakly negative Eu anomalies, positive to slightly negative Ce anomalies, and variable but consistently positive δ56Fe values. These suggest that the deposition of the Shilu BIF took place along the chemocline with repetitive upwelling of deep, suboxic to anoxic, Fe-rich, high- and low-T hydrothermal solutions, mixed with shallow oxidized water. In combination with the co-existence of barite and hematite, the S isotope signatures as well as highly radiogenic Sr and non-radiogenic Nd isotopic ratios of the barite, we infer an Fe-rich system without quantitative sulfate drawdown at Shilu.The thermodynamic modeling based on Ce/Ceanom data in the dolostones and a global model for Fe isotope geochemical cycle on the basis of δ56Fe values from the Fe ores, indicate that the dissolved O2 concentrations continued to decrease with water depth. This feature is in concert with the increased positive Ce and Eu anomalies from the shallower to deeper waters. In such conditions, the abiotic oxidation of Fe2+ by the free O2 is the major means of Fe3+ precipitation, whereas the anoxygenic photosynthesis Fe2+ oxidation is minor. Moreover, although the oxidation of Fe2+ to Fe3+ occurred, the oxidation of Ce3+ to Ce4+ and Eu2+ to Eu3+ was not pervasive at Shilu.