Origin and formation of Ordovician sedimentary phosphates at Phosphate Hill, Mansfield, southern Australia

Abstract

Ordovician sedimentary phosphatic rocks were mined for fertilizer use in the 1920’s from a small, highly deformed belt of Ordovician rocks at Phosphate Hill, near Mansfield (Warburton, 1:250,000 topographical map). The formation of the phosphate and the environmental conditions at the time of its formation have not been previously discussed in detail. It is an example of a sedimentary phosphate deposit that displays the results of both primary phosphogenesis and secondary, reworking processes. The black shale lithologies are host to the primary phosphate formation in the form of concretionary phosphates, derived from the high phosphorous content of bottom waters and the shells of the phyllocarid crustacean Caryocaris. Caryocaris has been misidentified in previous studies and their importance as the dominant bioclast at Phosphate Hill, has not been realised until now. Phosphorous enrichment within the depositional environment resulted from the extremely high organic contents within marine waters, which is illustrated by the bituminous character of some of the black shale lithologies. The depositional environment was relatively low energy, with low sedimentation rates, in isolated and stagnant marine waters. Thin alternating shale and phosphatic sand lithologies were derived from changes in the energy levels of the depositional environment which led to small turbidity currents reworking the concretionary phosphates and forming clastic phosphatic grains. Phosphate Hill has been extensively folded, fractured and faulted, with near-surface exposure resulting in the episodic precipitation of alumino-phosphatic, hydrous mineral phases. Phosphate Hill is compositionally similar to the Cheshunt sedimentary phosphate occurrence in Victoria, reflecting a similarity in environmental conditions within the Ordovician depositional marine environment.