Rhenium-Osmium isotope systematics of an Early Mesoproterozoic SEDEX polymetallic pyrite deposit in the North China Craton: Implications for geological significance and the marine osmium isotopic record

Abstract

The rhenium-osmium (Re-Os) isotopic system preserved in ancient seafloor hydrothermal sulfide deposits can be used to gain insight into the Os composition of ancient seawater. In particular, the age of sedimentary exhalative (SEDEX) deposits can be determined by measuring Re-Os isotopes in syn-depositional pyrite, a common constituent of SEDEX deposits. Here we report on the Gaobanhe sediment-hosted, polymetallic pyrite deposit which is hosted within the Gaoyuzhuang Formation in the central part of the northern margin of the North China Craton (NCC). Re-Os isotopic data from syn-depositional pyrite yield an isochron age of 1505 ± 55 Ma, constraining the timing of primary mineralization, as well as the minimum age of the host sediments to the Mesoproterozoic. Analyzed pyrite grains indicate an initial 187Os/188Os (Osi) value of 0.181 ± 0.066, suggesting that the mineralizing fluids were an admixture of seawater and submarine hydrothermal fluids, with the Os being primarily derived from seawater. The Osi value is slightly higher than that of Paleoproterozoic seawater (~0.13), suggesting that there was a radiogenic riverine flux to the oceans and that atmospheric oxygen must have been sufficiently high at that time to drive this increased, continental radiogenic Os input to the oceans. We propose that the Re-Os isotopic system, as recorded by these SEDEX deposits, may provide key insights when examining changes in the Os isotopic composition of seawater throughout the Precambrian.