MULTICOMPONENT ELEMENTAL AND ISOTOPIC MIXING IN Ni Cu (PGE) ORES AT KAMBALDA, WESTERN AUSTRALIA

Abstract

Most magmatic Ni-Cu-(PGE) deposits are considered to have formed from sulfide-undersaturated silicate magmas and to contain a significant component of crustal sulfur that was derived via wholesale melting, partial melting, or devolatilization of wall rocks. Under such circumstances, the system may comprise a silicate magma and a sulfide magma, with or without crystalline solids, undissolved wallrock-derived xenoliths, an unmixed silicate xenomelt, or an undissolved xenovolatile phase, each of which may contain distinct chalcophile and lithophile components. Because traditional two-component (silicate magma -sulfide magma) mass-balance models do not accurately model such systems, we have developed a series of multicomponent elemental and isotopic mass-balance equations to model batch equilibration in magmatic Ni-Cu-(PGE) systems. We have applied them to the type examples of komatiite-associated Ni-Cu-(PGE) deposits at Kambalda, Western Australia. The calculations indicate that the elemental and isotopic compositions of the various components in a multicomponent system will vary considerably as a function of the relative abundances of the components, and that different metals and isotopic systems may decouple from each other, yielding apparently conflicting information regarding the sources of the components. The results suggest that the S isotopic and Zn compositions of the ores are more sensitive indicators of contamination than the Os isotopes, and support a sedimentmelting model for Kambalda.

Contrasting mechanisms for crustal sulphur contamination of mafic magma: evidence from dyke and sill complexes from the British Palaeogene Igneous Province

“…Noril'sk-Talnakh Region (Ripley et al 2003;Ripley & Li 2013); Duluth (Arcuri et al 1998); Voisey's Bay (Ripley et al 1999); Kambalda (Lesher & Burnham 2001); and others (see Naldrett 2004Naldrett , 2011, and references therein)). A scenario in which crustal sulphur is available for magma contamination is therefore often an important factor in Ni-Cu-PGE exploration.…”

Origin of PGE-Depleted Ni-Cu Sulfide Mineralization in the Triassic Hongqiling No. 7 Orthopyroxenite Intrusion, Central Asian Orogenic Belt, Northeastern China

“…bulk assimilation of the upper crust but can be well explained by selective assimilation of crustal sulfides. Osmium is mainly hosted in the sulfides of the continental crust (e.g., Ripley et al, 1999Ripley et al, , 2001) so that if crustal sulfides are selectively assimilated by mantle-derived magma, the 187 Os/ 188 Os ratios in the contaminated magmas will become elevated (Ripley et al, 1999;Lambert et al, 2000;Lesher and Burnham, 2001). Since crustal sulfides are not the main carriers of Nd, Sr, and REE, addition of crustal sulfides to mantle-derived magma will not significantly change eNd(t) values and REE patterns.…”

“…Chalcophile elements have been widely used to study the origin of magmatic sulfide deposits because they have very high partition coefficients between sulfide liquid and magma (Bezmen et al, 1994;Fleet et al, 1999) and are therefore good indicators for sulfide segregation. Re-Os isotopes are useful in detecting crustal contamination of mantle-derived magma during ore formation (e.g., Walker et al, 1994;Sproule et al, 1999;Lambert et al, 2000;Lesher and Burnham, 2001). A combination of lithophile-chalcophile trace elements and Sr-Nd-Os isotopes is widely used in assessing the role of fractional crystallization and crustal contamination in the formation of magmatic sulfide deposits.…”

Heterogeneous Os isotope compositions in the Kalatongke sulfide deposit, NW China: the role of crustal contamination

“…Therefore, it is possible that Os isotope composition heterogeneities in sulfidebearing mafic magma systems may be related to crustal contamination and sulfide segregation under different R factors. γ Os values and Re/Os ratios of sulfides are similar to those of the contaminant at low R factor, whereas γ Os values and Re/Os ratios of sulfides approach those of the magma at high R factor (Lesher and Burnham 2001). However, modeling of the R factor suggests that sulfides in Kalatongke were not formed with a single R factor process (Fig.…”

“…Assimilation of S-rich country rocks has been proposed to explain the origin of magmatic sulfide ore deposits (Lesher and Burnham 2001), and selective crustal contamination has been invoked to explain the behavior of different isotope systems during the interaction between mantle-derived magmas and crustal rocks (Watson 1982;Walker et al 1994;Ripley et al 1999Ripley et al , 2001Lambert et al 2000). In an AFC process, if the systems are variably contaminated, the Re-Os data will plot around the mixing lines between parental magmas and contaminants and form "pseudoisochrons" (DePaolo 1981).…”