GEOCHEMICAL AND FLUID DYNAMIC MODELING OF COMPOSITIONAL VARIATIONS IN ARCHEAN KOMATIITE-HOSTED NICKEL SULFIDE ORES IN WESTERN-AUSTRALIA

Stratigraphic, volcanological, petrological, and fluid dynamic studies suggest that Archean komatiite-associated nickel sulfide deposits in Western Australia formed by thermal erosion of unconsolidated sulfidic sediments beneath channelized lava flows. Geochemical and Nd isotope data for the type area at Kambalda suggest that mineralized lava channels are less contaminated than barren flanking segments of the flow, owing to high discharge rates and continuous replenishment within the channel. This interpretation has several implications for the mode of emplacement of sulfides, their degree of equilibration with the host komatiite, and the compositions of sulfide ores: (1) the solubility of sulfur in komatiite is relatively low, so most of the sulfide in eroded sediments will be melted and remain as a dense layer at the base of the flow or be incorporated into the turbulently flowing komatiite as immiscible sulfide droplets; (2) sulfide ores will not necessarily be in equilibrium with overlying komatiites but will record equilibration with variably contaminated lavas during initial stages of emplacement and crystallization; and (3) ore compositions will vary from channel to channel (shoot to shoot) depending on (i) the composition of the hybridized komatiite, (ii) the fluid dynamics of the system and the effective magma-sulfide ratio (R factor), and (iii) the sulfide/silicate partition coefficients under the prevailing conditions of temperature and f(O2)/f(S2). The size of immiscible droplets and the interface between two immiscible layers are affected by inertial forces during turbulent flow. Turbulence causes coalescence of small droplets and breakup of large droplets, leading to an equilibrium droplet size. If the average size of the droplets is small and/or the flowage rate is high, they may be carried in suspension in the flowing komatiite; droplets have a large surface area to volume ratio and will be effective in scavenging chalcophile elements since they are carried by the turbulent motion of the flowing komatiite. If the average size of the droplets is large or if the flowage rate is low, they will settle and collect at the base of the komatiite; komatiite and sulfide liquids may continue to flow, but the sulfide layer will have a low surface area to volume ratio and will be ineffective in extracting chalcophile elements from the overlying komatiite layer. Thus, the initial stages of emplacement, when sulfides are initially melted and are most likely to be in suspension (if at all), control the effective R factor and therefore the tenor of the sulfide ores. Numerical modeling of the observed variations in ore compositions, constrained by known komatiite compositions and experimentally or empirically determined distribution constants, indicates that the observed range of ore compositions in Western Australian komatiite-associated nickel sulfide deposits can be accounted for primarily by variations in the effective sulfide/silicate ratio (500 > R > 100), although required variations in the partition coefficient for Ni (200 > D(Ni) > 100) suggest that f(O2)/f(S2) may also have varied systematically. Assimilation of H2O-FeS-rich sediments should increase f(O2)/f(S2), because of the relative abundances of the two components in the magma and unconsolidated sediments. If low tenor ores represent equilibration at lower R and higher f(O2)/f(S2), and high tenor ores represent equilibration at higher R and lower f(O2)/f(S2), as suggested by the modeling, then D(Ni) appears to decrease with increasing f(O2)/f(S2), as suggested by Doyle and Naldrett (1987). The range of calculated R values is relatively low, suggesting that low tenor sulfides were not entrained into the komatiite lava, but remained as a segregated layer at the base of the flow, and that high tenor sulfides were not transported long distances in suspension, but settled rapidly to the base of the flow. Magma compositions vary only slightly from an inferred parental komatiite; low tenor ores appear to have equilibrated with only slightly more evolved (contaminated and fractionated) lavas.