The role of deformation in the formation of banded iron formation-hosted high-grade iron ore deposits, Hamersley Province (Australia)

Abstract

The Hamersley Province (Western Australia) hosts some of the world’s largest iron ore deposits but despite decades of research, their genesis is still extensively debated. Many iron ore deposits are hosted in complexly deformed Archean to Paleoproterozoic banded iron formations, comprising thin chert and iron oxide bands interlayered with silicate-rich shales and carbonates. Current iron ore genesis models have identified a strong structural control on ore formation linked to extensive hypogene and supergene fluid circulation along fault structures. These fluid pathways facilitate the removal of vast amounts of gangue minerals, leading to enrichment of the iron oxide residue to iron ore. However, the evolution of the associated structures has not yet been considered as a key element in ore genesis.Here we show through multiscale structural analyses that deformation not only forms suitable fluid channels, but that folding and shearing also result in significant synkinematic removal of gangue minerals. Our multidisciplinary investigation of the structural evolution of the Mount Tom Price deposit combines microtectonic, field geology and 3D implicit modelling techniques to establish a link between deformation structures at various scales. Microscale shear bands and outcrop-scale asymmetric parasitic folds share striking similarities in their evolution and their controlling mechanisms. Both features record substantial non-coaxial deformation accompanied by volume changes due to stress-induced silica remobilisation. The closely spaced layering of rheologically different lithologies within Hamersley Province strata plays a crucial role in complex multilayer deformation, which resulted in extensive strain partitioning.Our study suggests that deformation was of major significance in the upgrading of banded iron formation to iron ore and was active from the early stages of banded iron formation during diagenesis. Deformation structures also established a micro- to deposit-scale lateral and vertical fluid network, which enabled infiltration by hypogene and supergene fluids during or after deformation. These new insights have important implications for iron ore genesis models, structural applications in the mine environment, and for understanding complex multilayer deformation with volume loss.