Discovery, Geologic Setting, and Controls on Iron Mineralization, South Flank, Western Australia

Abstract

Abstract South Flank is a ~1.8-billion-tonne martite-goethite iron deposit located in the Late Archean to Paleoproterozoic central Hamersley province, Pilbara craton, Western Australia—a district containing multiple giant iron deposits. A combination of detailed mapping, high-precision airborne magnetic and gravity gradiometer data, and resource range analysis, followed up by systematic drilling, was used to discover and fully define iron mineralization at South Flank. Exploration was targeted using a deposit-scale model, based on observed geologic controls on martite-goethite deposits in the South Flank district, combined with a systems approach, which identified key processes in the formation of iron mineralization at the camp scale, namely fluid pathways, controlling structures, potential host rocks, and ore preservation beneath detrital cover. Iron mineralization at South Flank is hosted by the Marra Mamba Iron Formation and occurs as a series of strata-bound tabular orebodies over a strike length of 25 km. Individual ore zones are up to 150 m thick and can extend to depths of 300 m. Martite-goethite-ochreous goethite ore is predominantly hosted by N2 and N3 subunits of the Mount Newman Member and is best developed in E-W–trending, upright to N-verging asymmetric synclines and associated low-angle reverse faults, which have caused substantial thickening of host rocks. Primary textures within banded iron formation are largely preserved within ore zones and can control location and grade of iron mineralization. Both unmineralized iron formations and ore zones are overprinted by recent extensive ferricrete, locally termed “hardcap.” Phosphorous, Al2O3, and volatile contents of ore co-vary with iron, albeit at low absolute abundances, whereas SiO2 is strongly negatively correlated with Fe, reflecting the transition from iron formation (Fe = 30–35 wt %) to iron ore (Fe = 50–65 wt %). Premineralization host-rock composition is an important control on both ore geochemistry and mineralogy. Martite-goethite-ochreous goethite is the dominant style of iron mineralization in the Hamersley province, in terms of overall tonnage and contained Fe, and is also widely developed in iron formations in the Pilbara and Yilgarn cratons and in other major global iron ore districts (e.g., India and Brazil). In each of these regions, martite-goethite and ochreous goethite are commonly developed as a weathering-related supergene overprint of earlier-formed hypogene hematite mineralization. In contrast, South Flank and other major deposits in the central Hamersley province (e.g., Mining Area C, Hope Downs) show no evidence of hypogene iron mineralization and its commonly associated wall-rock alteration. These iron orebodies are characterized by common structural association with synclines and associated reverse faults, preferential host-rock settings within particular units of the Brockman and Marra Mamba iron formations, simple ore mineralogy and geochemistry, and absence of associated wall-rock alteration. The giant martite-goethite deposits in the Hamersley province, of which South Flank is a type example, potentially represent a distinct deposit style. While some of the geologic characteristics of iron mineralization at South Flank are compatible with a supergene origin, many factors relating to ore genesis are unknown or not adequately constrained, including timing and mechanisms of ore formation.