Petrography and origin of inversely graded manganese pisolite from Groote Eylandt, Australia

Abstract

The sedimentology and petrography of graded manganese pisolite from the mid-Cretaceous sedimentary ore body at Groote Eylandt, Australia form the basis for a model describing the depositional environment and mode of origin of pisoliths. Pisolitic manganese deposits elsewhere may also be included in such a model. Grading in manganese oxide pisolite/oolite ores is interpreted to have formed as a result of variations in basin dynamics, chemistry and water depth. Specifically, inverse grading results primarily from shoaling (regression) with consequent progressive increase in sediment movement and decrease in availability of sites for Mn oxide nucleation, whereas normal grading requires progressive deepening of water (transgression), decreasing grain agitation and an increase in rate of nucleation. Manganese oxide ooliths and pisoliths show evidence of primary accretionary origin in a shallow-water (<50 m), relatively low-energy marine environment. Precipitation of Mn oxide is thought to occur, initially, via catalytic oxidation on a suitable reactive surface (typically particulate organic matter), and then through autocatalytic processes. The presence of concentric structure in the cortex is interpreted as indicating sudden short-term changes in both the chemistry and energy of the aqueous environment at the time of deposition. Comparative studies of Mn pisolith petrography and occurrence at Groote Eylandt and similar pisolitic deposits elsewhere indicate similarities with regard to environment of deposition and genesis of pisoliths.