Formation Environment and Mechanism of Siderite Concretions in the Longtan Formation, Anhui Province (Eastern China)

ABSTRACT Siderite concretions formed in different depositional settings exhibit distinct mineral paragenesis and can offer important clues to paleoenvironmental conditions. In this study, we conducted PLM, XRD, XRF and SEM–EDS analyses on the nucleus, mantle and wallrock of concretions found at Longtan Formation in the Chaohu area (Eastern China), with the aim of establishing a formation model for the concretions. The concretions are composed of a black nucleus, a grey mantle, white stockwork and a brown annular vein. The nucleus is primarily composed of apatite and the mantle is siderite‐rich. The stockwork comprises mainly ankerite and the annular vein consists primarily of pyrite. The apatite nucleus is formed through the combination of phosphate ions with calcium ions. The siderite mantle developed subsequently, resulting from the association of carbonate ions with ferrous ions. The ankerite stockwork is probably related to the increased alkalinity of the pore waters induced by the oxidation of organic matter. The pyrite annular vein is interpreted as a product of sulphate‐reduction zones. Comparative analysis indicates that the siderite concretions were mainly formed around bivalve fossils in the suboxic zone within sediments, while the sediments of the Longtan Formation (at Guibeitan, Chaohu) were formed under oxic conditions in a marginal marine–terrestrial transitional environment.