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 Yilgarn craton is one of the world’s major nickel provinces, containing 31.5 million tonnes (Mt) of Ni metal with an in situ value of about $350 billion on a pre-mining basis, amounting to 13.6 percent of the world’s currently known Ni resources. This entire resource inventory has been discovered since 1966. This chapter presents an analysis of the 40-year discovery history, which is ideal as a province-scale case study in mineral exploration. The province experienced a major peak in exploration activity between 1966 and 1971, the “nickel boom,” which accounted for more than half of all NiS deposits, and all of the giant (>1 Mt Ni) NiS deposits so far discovered. Almost 70 percent of those discoveries were related to direct surface prospecting methods, commonly based on recognition of magnetic ultramafic rocks as favorable hosts. Since the end of the nickel boom, the dominant discovery method has been follow-up exploration around significant known mineralization. From about the mid-1990s, electromagnetic (EM) surveying, which had been considered ineffective during the nickel boom phase, became a demonstrably successful technique for the detection of sulfide deposits. An improved understanding of geologic processes and controls has played an important role in sustaining exploration success since the end of the early, surface-prospecting phase of the nickel boom. Most nickel laterite deposits were first found during the nickel boom but not considered at that time to be economically significant. A large surge in exploration activity, much of it focused on resource delineation rather than true green-fields exploration, occurred between 1996 and 2001, triggered by the advent of the Pressure Acid Leach (PAL) technology in the mid-1990s. The discovery record of the Yilgarn province exhibits many patterns typical of an exploration province: early discovery of both the largest deposits and most of the metal and generally increasing discovery costs as the province matures. The average discovery costs for nickel in the Yilgarn have been 5.2 c/lb for sulfide nickel and 0. 6 c/lb for laterite nickel. The Yilgarn province offers two examples of exploration expenditure booms arising from the coincidence of an upturn in commodity price with the opening up of new exploration parameter spaces. These are the initial discovery of komatiite-hosted nickel sulfide at Kambalda in 1966, and the mid-1990s recognition of the potential of PAL technology to treat laterite ore deposits.

Abstract Nickel sulfide deposits of the Yilgarn craton are almost exclusively associated with komatiites, a distinctive suite of ultramafic volcanic rocks characterized by very high MgO contents implying extreme eruption temperatures. Komatiites have a remarkable array of igneous textures, including spectacular dendritic “spinifex” olivine and pyroxene textures as well as a spectrum of cumulate textural types. Characteristic assemblages of olivine textural types and distinctive spatial arrangements of corresponding rock types have been used to define a range of volcanic facies and subfacies which form the components of large komatiite flow fields. The emplacement history of these flow fields can be interpreted in the light of recent developments in understanding modern basaltic lavas. Feeder pathways in komatiite fields are marked by very high proportions of low-porosity, often coarse-grained, olivine cumulates, which are the products of high magma fluxes and prolonged crystallization against hot substrates. These pathways are the main hosts to mineralization. The spectrum of komatiite rock types is converted into a complex assemblage of metamorphic rock types through the process of hydration, talc-carbonate reconstitution, dehydration, and decarbonation over the range of metamorphic conditions encountered in the history of the craton. In extreme cases primary igneous features are entirely obliterated. Identification of primary rock types in highly metamorphosed terranes requires an understanding of the resulting metamorphic assemblages as a function of temperature and fluid composition. The geochemistry of komatiite suites is dominated by variations due to the fractionation and accumulation of olivine, with additional complexity due to superimposed effects of alteration. Notwithstanding, primary igneous signatures of komatiite suites are commonly preserved, and in some case are important lithogeochemical indicators for mineralized environments.

Abstract Most of the komatiite-hosted sulfide deposits in the Yilgarn craton exemplify the two major types: type 1, sulfide-rich accumulations at the base of magma pathways, and type 2, disseminated sulfides in the center of very olivine rich cumulate bodies. The Yilgarn komatiite resource consists of small, high-grade type 1 deposits, along with a number of much larger but lower grade type 1 and type 2 deposits. The largest deposits, Perseverance and Mount Keith, and the Kambalda Camp as a whole, are genuinely world-class deposits comparable in metal content to giant deposits elsewhere in the world. Type 1 deposits are almost exclusively hosted by bodies of olivine cumulate at least several tens of meters thick, and in some cases hundreds of meters thick. Immediately adjacent host rocks range from olivine orthocumulates to olivine adcumulates. Spinifex-textured rocks are only rarely found in contact with ores, but are more common in flanking rocks tens to hundreds of meters away. There is a close association between mineralization and the compound cumulate-rich flow facies. Komatiite-hosted ores in general have high Ni tenors and low Cu tenors, bulk ore compositions being controlled largely by parent magma compositions and magma channel dynamics. Superimposed variations are due to internal magmatic differentiation and hydrothermal effects. The ores of the Black Swan area illustrate primary magmatic features, including partially molten footwall inclusions in ores and host rocks. The Kambalda deposits show a wide spectrum of superimposed effects due to deformation, and there is an unresolved controversy surrounding the origin of linear troughs which evidently host the ores. Orebodies at Perseverance, Honeymoon Well, and elsewhere also demonstrate substantial structural modification but retain important magmatic signatures. The body of evidence supports the substrate erosion model, whereby sulfide is derived by thermomechanical erosion of sulfidic rocks in the floors of major magma pathways or channels. To some degree the geochemistry of the host rocks reflects this process, but the signal varies greatly according to the immediate environment. All the deposits are invariably deformed to some degree, and massive sulfide orebodies have had a profound influence on localizing the deformation. There is a wide spectrum from almost intact deposits to those which have undergone intense deformation and remobilization into entirely shear hosted type 5 deposits.

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

Abstract The late discovery (1966) of significant nickel sulfide deposits in the Yilgarn craton was due in part to the effects of widespread intense weathering that obscured their surface expression. Even in eroded areas where host rocks are mostly fresh or only slightly weathered, the sulfides are oxidized to considerable depth and crop out as strongly leached ferruginous gossans or silicified saprolite. Massive and matrix nickel sulfides are conductors and weather electrochemically. The upper part of the sulfide body, near the water table, acts as a cathode and deeper parts act as anodes. The process is driven by access to oxygen in the ground water and may continue to great depths. Primary pentlandite, pyrrhotite, and millerite initially oxidize to a violarite-pyrite assemblage, which in turn alters to goethite and hematite to form gossan. The reactions are pseudomorphic, so that the primary fabrics may be retained and recognized in the gossan. Disseminated sulfides are surrounded by nonconducting silicates and weather in a similar manner to the ultramafic host rock; where these are adcumulates, the saprolite is commonly strongly silicified and resistant to erosion and may form an outcropping cap rock. The regolith expression of nickel sulfide mineralization is described in terms of geochemical exploration models based on relict, erosional, and depositional landform situations, illustrated with appropriate case studies. Strong leaching during sulfide weathering, minor lateral dispersion, and lateritic enrichment of nickel even over barren ultramafic rocks, all greatly reduce geochemical contrast in surface sample media. Discrimination of mineralized systems in weathered material is particularly difficult in less eroded and depositional terrain. Pathfinder element geochemistry, principally copper and platinum group elements, is widely used but has limited success in many cases.

Abstract The primary geophysical methods currently used for nickel exploration in the Yilgarn are magnetic and electromagnetic surveys and, to a lesser extent, induced polarization surveys. This chapter describes the basic principles, optimal strategies, and data interpretation techniques for each method, along with a compilation of the essential petrophysical properties such as conductivity, magnetic susceptibility, and polarizability for the host rocks and ores. A number of detailed discovery case studies are given for deposits and camps including the Kambalda Dome, the Black Swan area, the Cosmos and Waterloo-Amorac areas of the Norseman-Wiluna greenstone belt; the Maggie Hays-Emily Ann area of the Lake Johnston greenstone belt; and the Flying Fox-New Morning area of the Forrestania belt. Early discoveries in the Kambalda-Widgiemooltha area drew heavily on aeromagnetic data to delineate belts of komatiitic olivine cumulates. in more recent times, electromagnetic surveys have played an important role in the discovery of new deposits such as Maggie Hays North, Emily Ann, Cosmos, Waterloo, and Amorac. Surface and downhole transient electromagnetic method (TEM) (also called time domain electromagnetic method, or TDEM) studies have been the most successful methods by far. Rapidly continuing development in instrumentation and interpretation software will only improve the success rate of TEM techniques. Downhole electromagnetic surveys are now the most important methods in locating extensions and new lodes at depth.

Abstract The commercial interest in nickel laterites in the Yilgarn craton dates from the late 1990s and is driven by a rejuvenation of interest in high-pressure sulfuric acid leaching (HPAL) technology. A large volume of nickel- rich, weathered ultramafic rock was identified during exploration for sulfide deposits, and has subsequently been identified as a resource of over 25 million tonnes (Mt) of contained Ni metal. The laterite deposits are the product of chemical and physical weathering of dunites and peridotites of komatiitic affinity. Prolonged weathering over 200 m.y. gave rise to complex profiles with variable mineralogy depending on the major factors of protolith, topography, drainage, and tectonics. Stable tectonic conditions precluded the development of topographic relief and efficient drainage that promotes supergene enrichment and high nickel grades as found elsewhere in the world—for example, New Caledonia. instead, a deep weathering profile with low to moderate nickel grades has developed over ultramafic rocks, characterized by retention of silica in the profile either as free silica in oxide-silica type laterites or in smectite clays. Oxide-silica laterites are developed at Cawse-Siberia, Goongarrie, Ravensthorpe, Marshall Pool, and Weld Range, where the ultramafic rock is primarily adcumulate in composition, and hence very low in alumina. Smectite clay laterites are formed over predominantly mesocumulate and orthocumulate rocks at Murrin Murrin, Bulong, and Kalpini. Deposit descriptions are given for Murrin Murrin, Cawse, Goongarrie, Bulong, and Ravensthorpe. Mining strategies require definition of hard, ferruginous cap rocks that require blasting, and also delineation of the Mg discontinuity. The efficiency of the HPAL process is critically dependent upon the Mg content of the ore, through its effect on acid consumption. Beneficiation by mechanical screening of silica from Ni-bearing goethite can substantially improve the head grade of silica-oxide laterite ores.

Abstract The Yilgarn craton is one of the world's major nickel provinces, containing 31.5 million tonnes (Mt) of Nimetal with an in situ value of about $350 billion on a pre-mining basis, amounting to 13.6 percent of the world'scurrently known Ni resources. This entire resource inventory has been discovered since 1966. This chapterpresents an analysis of the 40-year discovery history, which isideal as a province-scale case study in mineralexploration. The province experienced a major peak in exploration activity between 1966 and 1971, the "nickel boom," which accounted for more than half of all NiS deposits, and all of the giant (>1 Mt Ni) NiS deposits so far discovered. Almost 70 percent of those discoveries were related to direct surface prospecting methods, commonly based on recognition of magnetic ultramafic rocks as favorable hosts. Since the end of the nickel boom, the dominant discovery method has been follow-up exploration around significant known mineralization. From about the mid-1990s, electromagnetic (EM) surveying, which had been considered ineffective during the nickel boom phase, became a demonstrably successful technique for the detection of sulfide deposits. An improved understanding of geologic processes and controls has played an important role in sustaining exploration success since the end of the early, surface-prospecting phase of the nickel boom. Most nickel laterite deposits were first found during the nickel boom but not considered at that time to be economically significant. A large surge in exploration activity, much of it focused on resource delineation rather than true green-fields exploration, occurred between 1996 and 2001, triggered by the advent of the Pressure Acid Leach (PAL) technology in the mid-1990s. The discovery record of the Yilgarn province exhibits many patterns typical of an exploration province: early discovery of both the largest deposits and most of the metal and generally increasing discovery costs as the province matures. The average discovery costs for nickel in the Yilgarn have been 5.2 c/lb for sulfide nickel and 0.6 c/lb for laterite nickel. The Yilgarn province offers two examples of exploration expenditure booms arising from the coincidence of an upturn in commodity price with the opening up of new exploration parameter spaces. These are the initial discovery of komatiite-hosted nickel sulfide at Kambalda in 1966, and the mid-1990s recognition of the potential of PAL technology to treat laterite ore deposits.

Abstract Most deposits of platinum group elements (PGE), chromite, and vanadiferous magnetite occur in mafic intrusions. Several represent some of the most laterally continuous and uniform, in terms of grade and thickness, of all orebodies. Such continuity presents one of the major challenges to orebody modeling, in that any proposed process must be able to operate uniformly across enormous areas within a magma chamber. Their size also creates a mass-balance problem, especially for Cr, in that the required source reservoir for some deposits exceeds the size of the currently preserved intrusive body. Whether chromitite layers result from magma addition (with three options for the compositions of the added magma—ultrabasic, plagioclase rich, or siliceous melt), changes in oxygen, water, or total pressure, or some other process has yet to be demonstrated definitely. Podiform chromitite deposits in ophiolites possibly owe their genesis to concentration from basic magma by an aqueous phase. Some models for PGE mineralization mimic those listed for chromitite layers. The mineralization may be primary magmatic, accumulating with the silicate phases, either in some association with sulfide liquid, chromite, or independently, or it may postdate silicate accumulation, having been extracted from very low grade PGE source in vertical sections of footwall cumulates. Identifying definitive criteria to support or negate these processes proves elusive. The abundance of orthopyroxene over clinopyroxene in ultramafic cumulates, and a transition from ultramafic (with Cr-rich pyroxene) to mafic rocks (Cr-poor pyroxene), are two common, but not universal, features of stratiform PGE deposits. Vanadium strongly partitions into magnetite, which forms the major source of this metal. Models for the genesis of magnetitite layers often have a strong analogy with hypotheses about chromitite layers, but fundamental differences do exist, most prominent being the lack of any evidence for magma addition relating to magnetite formation. Density sorting of cumulus minerals is more plausible for magnetite than from chromite deposits. Hypotheses for the origin of PGE, Cr, and Vmineralization have been strongly influenced by observations on the Bushveld Complex that hosts well over half the world’s resources in all these commodities. Such dominance possibly impedes the development of models that might have applicability in other intrusions but where current economic nonviability reduces their apparent significance.

Abstract Komatiite-hosted magmatic nickel sulfide deposits fall into two major categories: sulfide-rich deposits at basal contacts of komatiite flow units (type 1, exemplified by the Kambalda deposits), and diffuse sulfidepoor disseminations within large extrusive bodies of olivine cumulate that are formed in major lava-flow pathways (type 2, exemplified by the giant Mt. Keith deposit in the Agnew-Wiluna belt of Western Australia). Metamorphism of komatiites produces a very wide range of nonsulfide assemblages, despite the relatively restricted compositional range of the rocks; a crucial variable is the of the metamorphic fluid. The sulfide mineralogy of the komatiite-hosted deposits is influenced by the temperature and composition of the metamorphic fluid. Typical greenschist-facies hydration gives rise to serpentinites, hosting assemblages very rich in pentlandite and in some cases heazlewoodite. Reduction reactions associated with the serpentinization front give rise to Ni-Fe alloy-bearing assemblages. Oxidized fluids associated with low-temperature talc-carbonate alteration give rise to vaesite, polydymite, and millerite coexisting with pyrite and hematite. Despite the foregoing commonality, the role of metamorphism in modifying the two types of deposit is very different. In type 1 deposits, the relative modal proportion of sulfide to silicate minerals is generally high, and the bulk of the nickel in the ore resides within sulfide minerals. In this situation, metamorphic reconstitution of the rock has relatively little impact on the overall nickel tenor of the sulfide fraction. However, metamorphism may still have important consequences on mineral grain size and the nature of silicate-sulfide intergrowths, a common consequence being the development of bladed or triangular-textured intergrowths of sulfide with metamorphic olivine. Associated tectonism may result in mobilization of massive sulfide bodies into low strain zones, but there is no evidence for any significant upgrading of sulfide ores by this mechanism. In type 2 deposits, both the sulfide and silicate components of the ore may contain substantial proportions of the total nickel budget. Partitioning of nickel between silicate and sulfide fractions is a sensitive function of the metamorphic mineral assemblage. Under favorable circumstances, metamorphism can give rise to almost complete residence of nickel in sulfide minerals. The economic viability of type 2 deposits rests on the high nickel tenor of the disseminated sulfide component of the ore.