Sulfur isotopic characteristics of the Gaobanhe deposit in East Hebei Province and their constraints on the processes of sulfide and Mn mineralization from the Gaoyuzhuang Formation
The Gaobanhe deposit is one of the most typical Mesoproterozoic SEDEX polymetallic sulfide deposits in China.However,the types of its mineralization and the dominant mineralization process of its different ore types and their genetic relationships are still controversial.Here,we focus on the sulfur isotope characteristics of typical ore types,i.e.,the laminated/banded pyrite ores and the massive and vein-type sulfide ores,in the Gaobanhe deposit to study their sulfur isotope compositions.The laminated/banded pyrite ores are closely associated with Mn-rich pyritic black shales,while the massive and vein-type sulfide ores are mainly hosted in dolomite.A bimodal distribution of the sulfur isotope compositions occurred in these two types of ores:the fine-grained pyrites corresponding to the bedding-parallel layers or laminations are significantly enriched in heavy sulfur isotopes(δ34 S=+6.7‰～+27.1‰),while the coarse-grained,anhedral pyrites corresponding to main stage of sulfide mineralization(pyrite mounds,massive mineralization)have significantly lower sulfur isotope compositions(mostly between-10.0‰ and+5.0‰).Unlike the traditional SEDEX model,which emphasizes the sulfate-limited water environment,we suggest that the Gaobanhe sulfide mineralization is a product of the enhanced hydrothermal activity during the transition of the transgressive-regressive sequence.Geological evidence,such as the presence of dolomitic nodules in the lower part of the black shale overlying on the shallow carbonates,indicates that this set of manganese-rich black shale belongs to the product of a large-scale marine transgressive event which led to the formation of a local sulfidic water environment.In this scenario,the complete reduction of seawater sulfate makes the bedding-parallel layers or laminations significantly enriched in heavy sulfur isotopes whose average value is generally consistent with the previously reported sulfur isotopic composition of coeval seawater sulfate.In contrast,the presence of a large number of stromatolites in manganese-bearing carbonate rocks and the"heterochronic"characteristics of the layered ore body indicate a transition of their sedimentary facies.The higher sulfate concentration in the sedimentary-early diagenetic environment of shallow-water carbonates,which underwent thermochemical reduction under locally enhanced hydrothermal activity,accounted for formation of massive and vein ores with lower sulfur isotopes compared to seawater sulfates.In summary,the Gaobanhe deposit is a product of both syn-sedimentary mineralization and hydrothermal replacement,which is accompanied by changes of sedimentary facies during the transition from transgressive to regressive sequences,and that the difference in sulfur isotope composition is mainly caused by the difference in sulfate concentration of seawater at different depths.These results provide important implication for the Gaoyuzhuang sulfide and Mn mineralization.
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