Abstract

High-grade iron ore of the 4EE orebody of the 4E deposit (>200 Mt at 63.5 wt % Fe) occurs as a southerly dipping sheet within banded iron formation (BIF) of the Paleoproterozoic Dales Gorge and Joffre members of the Brockman Iron Formation. Structural reconstruction of the 4E deposit shows that reactivation of the 18E fault and development of the NW-striking, steeply SW dipping 4E and 4EE normal faults resulted in preservation of the 4EE orebody below the 4E deposit, and 400 m below the modern topographic surface.

Three hypogene alteration zones between low-grade BIF and high-grade iron ore are observed: (1) distal magnetite-quartz-dolomite-stilpnomelane-hematite ± pyrite, (2) intermediate magnetite-dolomite-hematite-chlorite-quartz-stilpnomelane, and (3) proximal hematite-dolomite-chlorite ± pyrite ± magnetite. Hydrothermal alteration is temporally and spatially constrained by NW-trending dolerite dikes that intruded the 4E and 4EE faults prior to hypogene alteration. Six vein types (V1–V6) are recognized at the 4E deposit. The veins both cut and parallel the primary BIF layers and were emplaced contemporaneously with the hydrothermal alteration zones that record the transformation of low-grade BIF to high-grade iron ore.

Our integrated structural-hydrothermal alteration and fluid flow model proposes that during early stage 1a, hypogene fluid flow in the 4E orebody occurred during a period of continental extension and enhanced heat flow within sedimentary basins to the south of the Paraburdoo Range. Heated basinal brines were focused by the NW-striking, steeply SW dipping 4E and 4EE normal faults and reacted with BIF of the Dales Gorge and Joffre members. The warm to hot (160°–255°C), Ca-rich (26.6–31.9 equiv wt % CaCl2) basinal brine interacted with magnetite-chert layers, transforming them into magnetite-quartz-dolomite-stilpnomelane-hematite-pyrite BIF. The iron-rich brine (up to 2.8 wt % Fe) likely originated from evaporated seawater that had lost Mg and Na and gained Li and Ca through fluid-rock reactions with volcaniclastic rocks and carbonate successions within the Wittenoom Formation. The first incursion of deeply circulating, low-salinity (5.8–9.5 wt % NaCl equiv), heated (106°–201°C) modified meteoric water is recorded in late stage 1a minerals. This modified meteoric water had lost some of its Na through wall rock interaction with plagioclase, possibly by interaction with dolerite of the Weeli Wooli Formation that directly overlies the Joffre and Dales Gorge members.

Stage 1b involved continuing reactions between the hydrothermal fluids and the magnetite-quartz-dolomite-stilpnomelane-hematite-pyrite BIF, and produced both the intermediate magnetite-dolomite-hematite-chlorite-pyrite and the proximal hematite-dolomite-magnetite-stilpnomelane alteration assemblages. Microplaty (10–80 μm), platy (100–250 μm), and anhedral hematite increasingly replace magnetite in the intermediate alteration zone, forming the proximal alteration zones that consist of microplaty, platy, anhedral hematite and magnetite. The intermediate and proximal alteration zones represent the mixing of a hot (250°–400°C), high-salinity, Ca-rich (30–40 wt % CaCl2 equiv), Sr-rich basinal brine with low-temperature and low-salinity (~5 wt % NaCl equiv) modified meteoric water that was heated (~100°–200°C) during its descent into the upper crust. Heterogeneous mixing of the two end-member fluids resulted in the trapping of primary fluid inclusion assemblages containing a wide range of trapping temperatures (up to 200°C) and salinities (up to 25 wt % NaCl equiv).

Stage 1c of the hypogene hydrothermal fluid is characterized by low-temperature (<110°C), low-salinity (~5 wt % NaCl) meteoric water that interacted with the proximal hematite-dolomite-magnetite-stilpnomelane–altered BIF, leaving a porous, hematite-apatite high-grade ore. Supergene alteration affected the orebody since the Cretaceous and produced a hematite-goethite alteration assemblage, resulting in destruction of the hypogene alteration zones that are only preserved below the depth of modern weathering.

Discovery of the concealed 4EE orebody of the 4E deposit demonstrates that structural geology plays a critical role in the exploration for high-grade iron orebodies. Structural reconstruction should be considered a critical exploration activity in structurally complex terranes where concealed orebodies may exist.

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