Veins and hydrothermal fluid flow in the Mt. Whaleback Iron Ore District, eastern Hamersley Province, Western Australia

With the advent of recent hydrothermal models for genesis of giant microplaty hematite ore deposits of the Hamersley Province, we have undertaken regional scale analysis of veins and rocks surrounding the Mt. Whaleback deposit with an aim to characterize the nature of fluid flow before, during and after ore formation. At least five groups of veins formed in the Brockman Iron Formation in deformational events associated with the Paleoproterozoic interaction between the Yilgarn and Pilbara cratons. In a ∼600 km2 area around the Mt. Whaleback microplaty hematite orebody, the five main veining events most likely correspond to the collisional (D2) and extensional collapse (D3) phases of the ca. ∼2300–2200 Ma Ophthalmian Orogeny, with one or two later phases probably associated with the waning stages of the ca. 1700–1650 Ma Capricorn Orogeny (D4) and/or later Proterozoic events. The abundance of veins associated with D2 contraction and D3 extensional fabrics, and the composition (up to 15 wt.% NaCl equivalent) and pressure-corrected homogenisation temperatures of primary fluid inclusions (200–400 °C), indicate that the temperature, salinity and likely volume of fluid reached a peak during the last stages of the Ophthalmian Orogeny and the first stages of post-Ophthalmian extension. Paragenesis of veins relative to the deformation history, and localization of highest vein densities in the Dales Gorge Member, provides circumstantial evidence that the peak of hydrothermal activity in the district was synchronous with the main phase of microplaty hematite ore formation at Mt. Whaleback. We propose that the veins record part of the fluid flow system related to the genesis of this giant ore deposit.