Stratiform siderites from the Mesoproterozoic Xiamaling Formation in North China: Genesis and environmental implications

Abstract

Stratiform or layered siderites are much less common than siderite concretions in the stratigraphic record. In the middle part of the Mesoproterozoic Xiamaling Formation (ca. 1.40–1.35Ga), layered siderites are widespread across the North China Platform. Facies analysis indicates that the Xiamaling siderites were formed in shallow-water environments above the storm wave base. High Fe/Al and V/Al ratios and the lack of pyrites in siderite-bearing strata indicate ferruginous bottom-water conditions during the formation of siderites. The low thermal maturity of organic matter and clay mineral structures suggest that the layered siderites were formed at burial temperatures ≤90°C. The negative δ13C values (−24‰ to −12‰) of siderites and the presence of μm-scale hematite inclusions in siderite grains suggest that dissimilatory iron reduction (DIR) during early diagenesis has played critical roles in siderite formation. The source of Fe-oxyhydroxides required for DIR could be from Fe(II) oxidation in shallow seawater above the redoxcline in response to enhanced hydrothermal activities or, alternatively, directly from continental Fe-oxide input. The positive cerium (Ce) and europium (Eu) anomalies of clay minerals in siderite-bearing strata and their positive correlation with total Fe contents (TFe2O3) seem to favor a hydrothermal origin of iron. Syndepositional hyalophane concretions, intermittent tuff layers, and remarkable enrichment of Cu and Zn in some of the siderite-hosting rocks provide further support to hydrothermally sourced iron. The enhanced hydrothermal activity for the development of stratiform siderites in the Xiamaling Formation probably related to the final breakup of the North China Craton from the Columbia supercontinent, during which active hydrothermal systems provided sufficient Fe(II) for Fe-redox recycling and formation of layered siderites.