The structural setting and hydrothermal alteration of BIF-hosted high-grade iron ore deposits, the Hamersley province, Western Australia

[Truncated] The Hamersley province of northwest Western Australia is one of the world’s premier iron ore regions. The high-grade iron ore deposits are mostly hosted within banded iron formation (BIF) sequences of the Brockman and Marra Mamba Iron Formations of the Hamersley Group. The two main types of bedded iron deposits (BID) are: martite-microplaty hematite containing between 60 and 68 wt.% Fe, and martite-goethite containing between 56 and 63 wt.%Fe. Examples of martite-microplaty hematite deposits include Mount Whaleback, Mount Tom Price, Channar and Paraburdoo. The martite-microplaty hematite deposits are spatially associated with normal faults that formed during D2 deformation (~2200 Ma), and have later been affected by D3 deformation (~1650 Ma) and formed by the multistage interaction of hydrothermal fluids with the host BIF formation. Microplaty hematite within martite-microplaty hematite deposits display a variety of sizes, ranging from 20 to 300μm, and textures, ranging from platy to tabular. Microplaty hematite is commonly associated with supergene-modified hydrothermal deposits but can also form in the hydration zone of supergene deposits.

Analysis of the oxygen isotope compositions of magnetite and hematite from BIF, hydrothermal alteration assemblages, and martite-microplaty iron ore was conducted on samples from the Mount Tom Price, Paraburdoo, and Channar iron ore deposits. The results showed that 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 (δ¹⁸O_fluid) 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.

At the Paraburdoo deposit, high-grade iron ore (>63 wt. % Fe) of the 4EE orebody of the 4E deposit (>200Mt @ 63.5% Fe) occurs as a southerly-dipping sheet within banded iron formation (BIF) of the Early Proterozoic Dales Gorge and Joffre Members. Structural reconstruction of the 4E deposit established that reactivation of the 18E fault and the development of the NW-striking, steep SW dipping 4E and 4EE normal faults resulted in the preservation of the 4EE orebody below the 4E deposit, and 400m below the modern topographic surface.