Abstract

Volcanic peridotitc-associated nickel sulfide deposits occur in Archean komatiitic flow sequences around the Widgiemooltha dome in the Eastern Goldfields region of Western Australia. The deposits are typically small shoots (< 4.5 million metric tons (t) of 1-3% Ni) of disseminated to more massive sulfides which lie at or near the base of the altered ultramafic sequences. The shoots are localized in embayments and associated tectonic structures in the underlying metabasalt contacts and have undergone dynamothermal metamorphism to amphibolite facies grade.The basal host units comprise thick bodies of altered dunite and olivine peridotitc (commonly with MgO > 40% volatile free, Al 2 O 3 < 3%, total FeO ca., 8%, Ni 0.3-0.4%, and S 0.1-0.3%). These are generally overlain by thinner, less magnesian flows with intercalated metasediment layers. The 'ultramafic host rocks from the various deposits show different alteration effects related to the varying CO 2 content of metamorphic fluids and the timing of fluid access to the rocks. Along the eastern side of the Widgiemooltha dome, host rocks have undergone serpentinization and variable but generally extensive prograde talc-carbonate alteration (e.g., Redross, Dordie Rocks North, and Widgiemooltha 3 deposits). In the north (Munda deposit) they have experienced serpentinization with minor carbonate alteration and, in addition, prograde metamorphic reconstitution to forsteritc-talc rich assemblages. Dunitic units in the west (Wannaway and Wannaway North deposits) escaped prograde serpentinization, talc-carbonate alteration, and significant metamorphic reconstitution but have been retrogressively serpentinized. Sulfides from the different alteration environments show systematic mineralogical and compositional differences.At all deposits, peak metamorphic temperatures (550 degrees -600 degrees C) resulted in reversion of sulfides to a dominant Fe-Ni-Co-Cu-bearing monosulfide solid solution, accompanied by pentlanditc or Fe-Cu intermediate solid solution in some cases. Most of the present ore mineralogies and textures relate to exsolution of low temperature sulfides from metamorphic monosulfide solid solution, with reequilibration and recrystallization during slow postmetamorphic cooling.Deformation produced a tectonite fabric in the ores characterized by preferred orientation of sulfide grains and mineral layering. Breccia ores were formed in response to brittle deformation in host rocks and continued ductile behavior in the sulfides. Deformation also affected the overall configuration of the ore shoots and led to significant sulfide mobilization and ore shoot distension.After allowing for the metamorphic imprint, a number of primary features of the ore environment can be isolated, and there is strong evidence for the prior existence of magmatic sulfides. The association of ores with early-formed, largely dunitic units, similar bulk Ni/Cu and Ni/Co ratios for ores from different deposits, precious metal data, low S/Se ratios, and distinctive magmatic spinels in the sulfides all point to a magmatic control on the ores. Strong positive correlation between Ni and MgO in host units, and similar Ni/MgO ratios in both mineralized and unmineralized units, suggest that sulfide liquids formed and equilibrated with the parent magma at depth, before being extruded as part of olivine-rich crystal mushes.

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