The origins of iron ores in the middleback ranges, South Australia

Abstract

The Middleback Ranges consist of tightly folded Precambrian iron-formations flanked by granite-gneisses and feldspathic metasediments. Hematite orebodies occur within the iron-formations and are confined to pitching synforms and steeply dipping fold limbs. At Iron Duke, a large hematite orebody extends down the pitch of a synform into a hidden body of magnetite-hematite-carbonate rock which is surrounded by talcose zones. Various theories including granitisation, primary sedimentation, and supergene leaching have been proposed previously to account for the origin of the hematite orebodies. The magnetite-hematite-carbonate and talcose rocks of Iron Duke have been attributed to carbonate metasomatism affecting the iron-formation. The mineralogy and microtextures of the iron-formation units are described. Mineral assemblages indicate that regional metamorphism reached lower amphibolite fades grade at temperatures and pressures in the order of 450ºC to 550ºC and 2kb to 6kb. Micro-textures suggest that mineral layers within the iron-formations remained as essentially closed systems during metamorphism and there was negligible movement of components other than water and carbon dioxide. The geology of Iron Duke is described in detail, with emphasis on the mineralogy and geochemistry of the magnetite-hematite-carbonate and associated rocks. The ores occur in a tight, pitching synform terminated on the east side by a high angle reverse fault. There is considerable evidence to show that hydrothermal activity has played an important role in the formation of the magnetite-hematite-carbonate ore. Fluid inclusion studies suggest that the hydrothermal solutions were brines at temperatures in the order of 120ºC to 250ºC and a maximum possible pressure of 2-3 kb. The origin of the magnetite-hematite-carbonate ore is discussed. It is proposed that the ore formed by hydrothermal fluid action on an iron-formation unit near the base of the iron-formation sequence, which contained interlayered iron oxides, carbonates, tremolite and quartz. The silicate minerals were replaced by carbonates in a complex process of carbonate meta somatism and silica removal. Ore controls include the pre existing rock mineralogy, the tight synformal structure, and the coincidence of a thick basal carbonate unit with the major reverse fault. Theories of hematite ore genesis in the Middleback Ranges are discussed. It is concluded that the ores formed by supergene leaching and enrichment, and that ore controls include favourable structures, pre-existing rock mineralogy, and the presence of underlying impermeable layers.