Dolomitization and the genesis of the Woodcutters lead-zinc prospect, Northern Territory, Australia

Abstract

The Woodcutters L. 5 lead-zinc prospect in the Northern Territory, Australia, occurs in the Golden Dyke Formation, a sequence of carbonaceous siltstone, dolomite, and greywacke forming part of the Lower Proterozoic Goodparla Group, which was deposited on an Archaean granitic basement. An attempt has been made to show how those factors which are considered to have been significant in the formation of dolomite were also important in the genesis of the Woodcutters deposit. These factors are: 1. An evaporitic environment which favoured dolomite formation concentrated lead and zinc in the overlying solutions. 2. The base metals were further concentrated, and fixed in the sediments, by co-precipitation with the precursors of dolomite, Mg-calcite and/or aragonite. 3. The formation of dolomite during diagenesis resulted in either a structural change if the precursor was aragonite, or an ordering if the precursor was Mg-calcite. The dolomite could not accommodate the relatively large amount of base metal associated with its precursors, and as a consequence, during dolomitization these were released to the pore solutions. The metals in the pore solutions possibly complexed with organic materials such as those from the degradation of algal protein, and so remained in solution during lithification. During folding, the metal-enriched solutions were transported to fractures, and metal sulphides precipitated when the organic complexes became unstable. After lithification the carbonate-quartz-sulphide veins were zones of weakness along which shearing took place, probably over a considerable period of time. This shearing, as well as slight rise in temperature, resulted in fracturing, recrystallization, and reaction between the first-formed simple sulphides to produce the ore in its present form.