Geochemistry of nodular pyrites occurred in the odorous limestone of the Early Permian Qixia Formation from Fenghuang Hill of Chaohu City,Anhui Province
The geochemistry of sedimentary pyrite provides valuable insights into heavy metal sinks,sources of metals for ore deposits,and the chemistry and environmental conditions of ancient oceans. The Pennsylvanian Huanglong Formation and the Lower Permian Qixia Formation are well-exposed in Fenghuang Hill,northern Chaohu City,Anhui Province. Pyrite nodules are commonly found within the odorous limestone of the lower member of the Qixia Formation,particularly in the 2～3 meter thick bituminous limestone layer overlying the carbonaceous shale member. Despite their prevalence,the geochemistry and genesis of these pyrite nodules are still not reported. This study investigates the geochemical characteristics of pyrite nodules,the depositional environment of the odorous limestone,the paleoenvironmental evolution during the Early Permian,and the relationship between the underlying clastic rocks and the odorous limestone. Ten pyrite nodule and limestone samples were collected for analysis. Mineral identification was performed using polarizing microscopy,while major element oxides and trace element concentrations were determined using X-ray fluorescence spectrometry and inductively coupled plasma-mass spectrometry,respectively. Results indicate that the pyrite nodules,widely distributed in the lower Qixia Formation odorous limestone,mainly occur as massive or microcrystalline grains with a distinct boundary between the nodules and the host limestone. The pyrite nodules do not cut or disrupt the surrounding organic-rich laminae,suggesting that it was precipitated from the diagenetic porewater via organisms replacement. The odorous limestone samples exhibit significant enrichment in SO3 and Fe2O3,likely related to the high organic matter content and a similar terrestrial input as the underlying clastic rock member,respectively. Compared to the upper continental crust,the Chaohu odorous limestone samples are enriched in Se,Mo,and Cd,while the pyrite nodule samples show elevated concentrations of Cr,Ni,As,Se,Mo,Cd,Sb,and Hg. The lithological association of clastic rocks and odorous limestone,the distribution of pyrite nodules,and their geochemistry collectively suggest that both the odorous limestone and pyrite nodules formed in a strongly anoxic,reducing marine environment at a passive continental margin with a degree of terrestrial influence.
trace elementsrare earth elementspyrite nodulesQixia Formationodorous limestoneChaohu City
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