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 Certain aspects of the genesis of Archean epigenetic gold deposits remain controversial, in particular the source of the auriferous fluids, which are arguably magmatic, metamorphic, or mantle derived. In an attempt to constrain the fluid source, it is essential to consider Archean gold mineralization in terms of the tectonic, magmatic, and metamorphic history of greenstone terranes. Asymmetries in the distribution of volcanic, sedimentary, and plutonic rock types, the pattern of deformation, and the rapid evolution of the greenstone sequences within the Norseman-Wiluna belt in the eastern Yilgarn block are akin to those of younger orogenic belts at obliquely convergent continental plate boundaries. Archean gold deposits show many similarities to younger, cordilleran-style gold deposits (e.g., the Mother Lode) which occur in a similar tectonic setting, particularly in terms of their strong dependence on structural controls and the composition of the ore fluids. In the eastern Yilgarn block there is a coincidence of lode gold mineralization, calc-alkaline porphyry, and lamprophyre dike swarms and craton-scale oblique-slip faults with their attendent mantle-derived carbonation. With no compelling evidence for direct derivation of ore fluids from felsic magmas, gold mineralization is best viewed as the upper crustal expression of a deep-seated tectono-thermal event with mantle-crustal outgassing, occurring in response to a deep mantle heat source, related to convergent tectonics. In all probability the ore fluid contained magmatic, metamorphic, and mantle components, but it is impossible at this stage to determine with which component the gold was predominantly associated.

Abstract Extremely intimate space-time associations between caic-alkaline (shoshonitic) lamprophyres and mesothermal gold deposits are now confirmed worldwide and from Archean to Tertiary times. They include the late Archean gold deposits of the Superior province (Canada) and the Norseman-Wiluna belt (Western Australia), which hosts the world’s most golden square mile (Kalgoorlie) and almost certainly one of the richest gold deposits outside South Africa, (Porgera, Papua New Guinea). In an increasing number of areas, lamprophyres are found to be the only igneous rocks emplaced at the same time as the gold, and economic status has been found to correlate quantitatively with the presence of lamprophyres. Many lamprophyric rocks appear to be enriched in Au relative to other igneous rocks, with contents of tens of ppb Au being common. Evidence that such enrichments could be primary includes the following, although this will remain impossible to prove for lamprophyres in major gold fields: (1) the persistence of high Au contents in calc-alkaline lamprophyres which lie outside the alteration halos of large-scale gold systems (e.g., British Caledonides) and more especially in lamprophyric rocks which are not associated with gold deposits at all (e.g., lamproites); (2) the plausible explanation for Au enrichment that exists in the lamprophyres’ exceptionally deep origins in presumed Au-rich regions of the earth (>150 km), high F, K, Ba, and Rb, moderate S contents, and H 2 O/(H 2 O + CO 2 ratios, and fluidized condition, which make them uniquely similar to auriferous ore fluids in their element abundances and possibly in their physical state, and thus, well suited to transporting gold into the crust; and (3) detailed statistical analysis of Au data, notably for the Superior province, comparing fresh versus altered and proximal versus remote samples from gold mineralization to rule out any overall addition of Au to these lamprophyres from external sources. The widespread presence of lamprophyres suggests a much more significant role in gold deposition for large-scale crust-mantle events and for certain (e.g., oblique subduction) tectonic regimes than has hitherto been generally recognized. A part-genetic, part-structural interpretation is preferred, in which lamprophyres may contribute at least some Au or fluids from deep sources into mesothermal systems, which then redeposit the Au according to the metamorphic model in its broadest sense. This is in accord with stable isotope evidence, which argues against a direct relationship between lamprophyric and gold-depositing fluids but by no means precludes an indirect one.