Syndepositional and postdepositional features of the manganese ore deposits of the Proterozoic Penganga group, Adilabad district, Andhra Pradesh, India

Abstract

The Proterozoic Penganga Group consisting of terrigenous and orthochemical sediments including a manganese orebody is well developed in the northwestern part of the Adilabad district, Andhra Pradesh. The manganese orebody of unmetamorphosed and undeformed, interbanded manganese oxide ore, chert, and minor calcareous shale has retained excellent syndepositional and postdepositional features both on the macro-and microscales. The primary depositional features include meso- and microbands of manganese oxide and silica of different descriptions, scour-and-fill structures, and Mn oxide micronodules. Spherical siliceous μm-sized structures and other features of biogenic origin have been observed. Diagenetic features such as fabric changes, syneresis cracks, concretionary pods, and Mn oxide nodules have been recorded. They are accompanied by penecontemporaneous deformation structures such as pinch-and-swell structures, gravity-density features, brecciation, and folding and faulting of various kinds. All these features suggest that the manganese orebody was formed in a shallow-marine environment on a stable shelf possibly behind a barrier bar and subsequently underwent diagenetic reorganization and penecontemporaneous deformation when the sediments were still in a hydroplastic state.