The geology of the Agnew nickel deposit, Western Australia

CIM Bulletin, Vol. 82, No. 929, 1989

M.J. Gole, S.J. Barnes and R.E.T. Hill, Commonwealth Scientific and Industrial Research Organisation, Division of Exploration Geoscience, Wembley, Western Australia

The Agnew (also known as the Perseverance) nickel deposit and its northern extension, the Rocky's Reward deposit, form one of the world's largest accumulations of nickel sulphides hosted by komatiites. Agnew was previously classified as an "intrusive dunite-associated" deposit. It has now been shown that the sulphides are hosted by komatiite flows which underlie an adcumulate-textured dunite body that crystallized in a large lava channel. The rocks of the mine area have been subjected to a meta-morphic temperature of 550 °C and pressure of up to 3 kb. The komatiites have mostly been reconstituted to olivine-amphibole-chlorite assemblages. There is a zoned sequence of enstatite-, anthophylite-, talc- and antigorite-bearing assemblages inwards from the western margin of the ultramatic reflecting variations in the CO2/H2O ratio of metamorphic fluids. Facing directions are determined from variations in the proportions of metamorphic minerals and in rock compositions which reflect asymmetric layering in the flows. Olivine-sulphides matrix ore is hosted in a single 50 m thick, east-facing flow that is overlain by flows containing massive sulphides. Much of the massive sulphide has been physically remobilized into fold noses, faults and shear zones. Chemical diffusion of ore components has upgraded a sulphide-rich metasedimentary unit where it is in structural contact with massive ore. To the north, the mineralized flow sequence is separated from a stratigraphically higher komatiite sequence, the Perseverance ultramafic complex, by approximately 150 m of felsic meta-volcanic and metasedimentary rocks. Southward, the Perseverance ultramafic grades laterally into an olivine adcumulate body and its lower contact cuts progressively down stratigraphy until it lies directly on top of olivine-sulphide ore of the mineralized flow sequence. This discordant contact is interpreted as the base of a thermal erosion channel formed by a voluminous komatiite lava flow. This contact greatly influenced later deformation of the mineralized flow sequence.

Keywords: Geology, Nickel deposits, Agnew nickel deposit, Rocky Reward deposit, Sulphides, Ore mineralogy