Abstract

The Hamersley province of northwest Western Australia is one of the world’s premier iron ore regions with high-grade martite-microplaty hematite iron ore deposits mostly hosted within banded iron formation (BIF) sequences of the Brockman Iron Formations of the Hamersley Group. These high-grade iron ores contain between 60 and 68 wt percent Fe, and formed by the multistage interaction of hydrothermal fluids with the host BIF formation.

The oxygen isotope compositions of magnetite and hematite from BIF, hydrothermal alteration assemblages, and high-grade iron ore were analyzed from the Mount Tom Price, Paraburdoo, and Channar iron ore deposits. The δ18O values of magnetite and hematite from hydrothermal alteration assemblages and high-grade iron ore range from −9.0 to −2.9 per mil, a depletion of 5 to 15 per mil relative to the host BIF. The δ18O values are spatially controlled by faults within the deposits, a response to higher fluid flux and larger influence on the isotopic compositions by the hydrothermal fluids. The oxygen isotope composition of hydrothermal fluids (δ18Ofluid) indicates that the decrease in the 18O content of iron oxides was due to the interaction of both basinal brines and meteoric fluids with the original BIF. Late-stage talc-bearing ore at the Mount Tom Price deposit formed in the presence of a pulse of 18O-enriched basinal brine, indicating that hydrothermal fluids may have repeatedly interacted with the BIFs during the Paleoproterozoic.

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