Small nodules and tabular bodies of practically pure micro-crystalline magnesite up to a few centimetres across occur in late Cainozoic calcrete around Gosses Bluff. The calcrete underlies an area of about 15 km^2 and the magnesite constitutes somewhat less than 1 per cent of the outcrop. The bedrock in the area of calcrete is impact breccia of the Gosses Bluff structure. The superficial weathered products of these breccias constituted a suitable host rock for the calcrete because of their inherent permeability and their position in topographically low areas. The source of magnesium carbonate was most likely in the Late Proterozoic and Palaeozoic dolomites and magnesian limestones exposed to the north in the MacDonnell Ranges. Petrographic evidence from the random samples of calcrete collected at Gosses Bluff suggests that the history of the deposit commenced with invasion of magnesium carbonate rich waters and formation of small bodies, probably of a metastable hydrate of magnesite which later reverted to the stable phase. A period of sub-aerial dessication ensued and caused disintegration of the nodules and production of cracks and voids. Phreatic water enriched in calcium carbonate invaded the deposit; voids were lined with fibrous calcite, smaller cracks were completely filled, and at the same time the magnesite was replaced, in part, by calcite. The larger voids were subsequently filled by calcrete which consisted of a mixture of disintegrated breccia fragments, chiefly quartz, feldspar and quartzite grains, with the addition of a minor proportion of magnesite grains incorporated from the nodules; and an abundant matrix of calcite. There is scant reference to magnesite in calcretes elsewhere in Australia, possibly because magnesite is not readily distinguished in some fine-grained calcretes.
