Abstract The Kalgoorlie terrane of the eastern Yilgarn craton is the third largest repository of sulfide nickel ore in the world. The Agnew-Wiluna belt, at the northern end of the Kalgoorlie terrane, contains the bulk of the nickel resource within the province, including the world's two largest known nickel sulfide deposits associated with Archean komatiites, the giant Mount Keith and Perseverance deposits. Both deposits are hosted by lenticular bodies of highly magnesian olivine adcumulates, developed as pods within planar sequences of olivine mesocumulate and orthocumulate rocks. The Perseverance deposit and the satellite Rocky's Reward and Harmony deposits are highly deformed, having been subjected to an early episode of isoclinal folding and associated shearing, resulting in significant mobilization of primary magmatic sulfide ores into axial planar shear zones and subsequently refolding. The bulk of the Perseverance orebody comprises basal accumulation of matrix ores, occupying an arcuate channel feature, with an extensive asymmetric halo of disseminated sulfides. Host rocks display a complex metamorphic history involving multiple episodes of hydration, carbonation, dehydration, decarbonation, and retrograde alteration. The Perseverance Ultramafic Complex is interpreted as a high-flux, flow-through conduit, formed by evolving magmas that became progressively hotter, more primitive, and less Ni depleted with time. There is a pervasive signature of country-rock contamination throughout the complex. The complex is interpreted as either a feeder pathway to a major flow field or a as subvolcanic intrusive conduit; these alternatives are not resolvable given the tectonic overprint. The giant Mount Keith deposit occurs within an extremely olivine rich cumulate unit broadly similar to that at Perseverance but without evidence for flanking flows. On the basis of the presence of apparently crosscutting apophyses in the roof of this unit, and a general absence of spinifex textures, the Mount Keith ultramafic unit is interpreted as an intrusive subvolcanic conduit or chonolith. The degree of penetrative deformation is much less than at Perseverance, but shearing is still evident along contacts. Mineralization is exclusively centrally disposed and disseminated in character and has variable tenors (compositions of the pure sulfide component) spanning the typical range seen in the Kambalda dome deposits. Sulfide mineralogy has been variably modified during hydration and local carbonation of the host rocks, particularly through oxidation of pyrrhotite to magnetite. The mineralogy reflects lower metamorphic grade than at Perseverance and lacks metamorphic olivine. Host-rock geochemistry is broadly similar to Perseverance, although sulfide tenors are considerably higher. Ore formation is attributed to mechanical transport and deposition of sulfide droplets, combined with in situ olivine and sulfide liquid accumulation. Both deposits were emplaced into or onto a felsic volcanic country-rock sequence, from which sulfur has been derived by assimilation, probably during emplacement at the present crustal level. Both are related to strongly focussed flow of komatiite magma and contain components of very primitive melts probably derived directly from the mantle plume source with limited interaction with crustal material. Sulfur assimilation, transport and deposition took place within long-lived feeder conduits that remained as open systems through most of their lifespan. The presence of these high-flux conduits within the Agnew-Wiluna komatiite sequence is attributed to unusually prolonged, high-volume eruptions, emplaced at exceptionally high rates. Deep-seated mantle tapping structures at the edge of an older Archean cratonic block may be the critical link between this style of mineralization and other large magmatic Ni-Cu deposits in younger geologic provinces.

Abstract The Mount Keith and Yakabindie deposits are type examples of large-tonnage, low-grade type 2 nickel sulfide systems. Discovered in 1969, the MKD5 deposit at Mount Keith has been in production since 1994 and is Australia’s largest Ni producer. The Yakabindie deposits (Six Mile Well and Goliath North), discovered over the 1970 to 1971 period, are being evaluated at the time of writing. Both groups of deposits occur within the Archean Agnew-Wiluna greenstone belt and are hosted in giant olivine cumulate ultramafic bodies interpreted to be subsea-floor intrusions in a felsic and/or intermediate volcanic sequence. The bulk of the nickel mineralization is in the form of intercumulus sulfide blebs and is interpreted to have formed by cotectic, olivine sulfide crystallization. The Goliath North deposit shows a Ni grade distribution, which may reflect an element of entrained sulfide (type 1) as well as cotectic mineralization. Following regional metamorphism, all these deposits have been extensively overprinted by retrograde metamorphic fluids, resulting in alteration assemblages dominated by serpentines, magnesite, talc, and hydrotalcite group minerals. D2 structures formed the main fluid conduits during this event resulting in extensive talc-magnesite alteration around these structures.Due to the fact that sulfides are essentially only an accessory component of the ore, modifications to silicate mineralogy during hydrothermal alteration have consequently modified sulfide phase relationships as well.This has resulted in a crude zonation of ore mineral assemblages in the MKD5 deposit and extensive zones of high tenor sulfides such as millerite, godlevskite, and heazlewoodite. The MKD5 deposit is mined using conventional, staged-cutback, open-cut methods and processed to sulfide concentrate using froth flotation. ore blocks for direct processing are selected to avoid deleterious components such as talc, arsenic, and excessive fibers, which negatively affect Ni recoveries and concentrate quality. Using conventional flotation Ni recoveries are typically around 70 percent and are limited by the fact that significant Ni in ore occurs in ultrafine sulfide particles and in silicate form which is not readily recovered. Concentrate Ni grade is generally high (18–30%) due to hydrothermal modification of sulfides, however, low Fe/MgO ratios of the bulk of the concentrates generated means the concentrate is not directly smeltable without blending with concentrates from other sources.Introduction