Effects of Magmatic Processes, Serpentinization, and Talc-Carbonate Alteration on Sulfide Mineralogy and Ore Textures in the Black Swan Disseminated Nickel Sulfide Deposit, Yilgarn Craton

The Black Swan disseminated orebody occurs predominantly within the partially carbonated serpentinite core of a zoned serpentinite–talc-carbonate ultramafic cumulate body of komatiitic parentage. The Black Swan disseminated sulfides depart significantly from the characteristic pattern observed in otherwise similar deposits in Western Australia and Canada, which display magnetite-bearing pentlandite-rich sulfide aggregates (“blebs”) in serpentinites, and S-rich millerite-bearing assemblages in talc carbonates. The Black Swan disseminated ores in contrast are dominated by millerite-pyrite–(polydymite) assemblages in both serpentinites and talc carbonates, with no evident relationship to the progression of carbonation. Sulfides show three types of intergrowth with gangue minerals, which can occur separately or all together: intergrowth with magnetite, at least some of which appear to be primary magmatic in origin; fine intergrowth with interlocking lath-textured antigorite; and intergrowth with carbonate minerals. Carbonate commonly replaces the core of sulfide blebs while retaining the primary magmatic outline. Carbonate replacement of sulfide roughly parallels the overall progression of carbonation of the whole rock, but no systematic relationship of the other intergrowth types to carbonate content is observed in the serpentinites. Serpentine mineralogy, determined by microbeam Raman spectroscopy, comprises lizardite, which occurs in pseudomorphic mesh textures after olivine, and antigorite, formed by variable degrees of overprinting of original lizardite mesh textures, ranging to a completely overprinting interlocking texture with no relic igneous texture. This progression is interpreted as the result of mild thermal metamorphism of original lizardite serpentinite. This progression is completely independent of carbonate content and is also apparently independent of extent of sulfide-antigorite intergrowth, which is interpreted as being early, and related to volume expansion during initial sea-floor serpentinization. Talc is almost completely absent from serpentinites, while carbonate content varies widely. Whole-rock Ni and S data are very strongly correlated in millerite-poor, pentlandite-rich samples and poorly correlated in samples where millerite is the dominant Ni-bearing sulfide. On this basis we conclude that the pentlandite assemblages are slightly modified survivors of the original magmatic event, while the millerite-pyrite assemblages formed during the earliest stages of serpentinization. Overprinting of the early serpentinization event by talc carbonate had essentially no effect on sulfide mineralogy, other than inducing intergrowth with carbonate. A number of distinctive features, such as magnetite-rich bleb cores, the association of sulfides with segregation vesicles and chromite “shells,” the subspherical morphology of the blebs, and preferential replacement of bleb cores, are considered to be inherited from the magmatic phase and are attributed to an original high but variable oxygen and halogen content of the sulfide melt component. Exsolution of this component during solidification of the blebs in some cases produced sulfide-centered bubbles now preserved as segregation vesicles, while in other cases oxygen was retained within the bleb forming magnetite-sulfide intergrowths. Chromite shells formed as a result of exsolved oxygen interacting with komatiite magma. Volatile-rich blebs solidified in some cases with hollow or porous cores, which were subsequently exploited by carbonate replacement.