Structural Control, Hydrothermal Alteration Zonation, and Fluid Chemistry of the Concealed, High-Grade 4EE Iron Orebody at the Paraburdoo 4E Deposit, Hamersley Province, Western Australia

摘要

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 la, 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 equivwt % CaCla) basinal brine interacted with magnetite-chert layers, transforming them into magnetite-quartz-dolomite-stilpnomelane-hematite-pyriteBIF.