The Perseverance and Mount Keith Nickel Deposits of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia
Stephen J. Barnes, Marco L. Fiorentini, Paul Duuring, Ben A. Grguric, Caroline S. Perring, 2011. "The Perseverance and Mount Keith Nickel Deposits of the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia", Magmatic Ni-Cu and PGE Deposits: Geology, Geochemistry, and Genesis, Chusi Li, Edward M. Ripley
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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.
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Magmatic sulfide deposits fall into two major groups when considered on the basis of the value of their contained metals, one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the PGE are the most important. The first group includes komatiite- (both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to Ural-Alaskan–type intrusions. PGE deposits are mostly related to large intrusions comprising both an early MgO- and SiO2-rich magma and a later Al2O3-rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical validity of this observation.
A number of key events mark the development of a magmatic sulfide deposit, partial melting of the mantle, ascent into the crust, development of sulfide immisciblity as a result of crustal interaction, ascent of magma + sulfides to higher crustal levels, concentration of the sulfides, their enrichment through interaction with fresh magma (not always the case), cooling and crystallization. Factors governing this development include (1) the solubility of sulfur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallization, (2) the partitioning of chalcophile metals between sulfide and silicate liquids, and how the results of this vary during mantle melting and subsequent crystallization and sulfide immiscibility (degree of melting and crystallization, R factor and subsequent enrichment), (3) how effectively the sulfides become concentrated and the factors controlling this, and (4) processes that occur during the cooling of the sulfide liquid that govern aspects of exploration and mineral beneficiation. These topics are discussed first in general terms and then with specific reference to deposits at Noril’sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modeling is consistent with the very low PGE concentrations in this deposit being the result of some sulfide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the