The geology and origin of manganese deposits at Pernatty Lagoon, South Australia

Abstract

The origin of small manganese deposits along the margins of Pernatty Lagoon, a dry salt lake 425 km NNW of Adelaide in South Australia’s Mid-north, has not previously been adequately explained. This region comprises part of the Stuart Shelf, where a condensed sequence of sediments was deposited between the stable Gawler Craton and the subsiding Adelaide Geosyncline, during late Proterozoic times.

After the formation of the Gawler Range Volcanics (and its equivalents) the clastic Pandurra Formation was deposited in a fluvial environment. Horst movements formed elevated structural highs, with the most prominent being the Pernatty Culmination. A further period of volcanism saw the Gairdner Dyke Swarm acting as feeder conduits to the basaltic subaerial Beda Volcanics. All the Stuart Shelf volcanic units are noted for their high concentration of manganese.

An extended period of exposure ensued, during which the sediments of the Stuart Shelf were open to weathering and erosion, with clastic sediments accumulating in the Adelaide Geosyncline. Manganese leached from the volcanics was transported in the aqueous system and precipitated in favourable non-marine environments upon the Stuart Shelf, hence further concentrating manganese. Some of the available manganese may have been disseminated within the clastic sediments of the Adelaide Geosyncline.

During Sturtian times the anoxic nature of the Tapley Hill Formation Sea resulted in the retention of introduced ionic manganese in the Adelaide Geosyncline, where there was the geochemical removal of iron by pyrite formation. Remobilization of manganese from both the Adelaide Geosyncline and accumulations on the Stuart Shelf was achieved by the transgression of the Tapley Hill Formation Sea upon the Stuart Shelf. Transported estuarine currents, the manganese-rich waters were eventually forced to upwell against the Pernatty Culmination. Precipitation of manganese-oxides in shallow, tidal-lagoonal environments against the Pandurra Formation basement high, is associated with the stromatolitic Woolcall Dolomite, which was forming in the same environment. Manganese-rich waters influenced the carbonate sediments, resulting in the high manganese content of the Woolcalla Dolomite. Particulate manganese-oxides are deposited within protected areas and on regression of the Tapley Hill Formation Sea, with later burial by regressive sands and further clastic units.

During the Tertiary, the removal of overburden opened the Woocalla Dolomite to further degradation and perhaps the formation of supergene manganese deposits. However there has been destruction and contamination of existing ore and the formation of secondary iron-rich units, due to the remobilization of iron.

Palaeogeographic analysis reveals further areas of possible manganese deposition at the maximum extent of the Tapley Hill Formation Sea. A large intracratonic basin formed in the Stuart Shelf, which could contain further manganese accumulations associated with the Tapley Hill Formation transgression.