Nickel-Copper Sulfide Mineralization in the Ntaka Hill Ultramafic Complex, Nachingwea Region, Tanzania

Abstract

Abstract The Ntaka Hill Ni-Cu deposit in Tanzania lies within the Mozambique belt, a complex, long-lived Neoproterozoic suture zone that formed during the amalgamation of Gondwana. The deposit is hosted within the moderately deformed Ntaka Ultramafic Complex, which was emplaced at ~660 Ma, around 20 m.y. before the East African orogeny. Due to strain partitioning into amphibolites and metapyroxenites at the margins of the sills, magmatic cumulate textures are very well preserved. The Ntaka Hill mineralization is predominantly disseminated sulfides within orthopyroxenites and harzburgites. Sulfides show typical magmatic mineralogy and occur as isolated blebs and interstitial networks as well as in soft-walled segregation veins that form localized shoot-like high-grade zones. Deformation had relatively little effect on the deposition of sulfides other than minor localization of strain within some preexisting sulfide-dominant veins. Grade shells define elongate tubular channels that coincide broadly with the more magnesian cumulate rocks. Base and precious metal tenors are widely variable, ranging from 1 to 17% Ni and 0.5 to 10% Cu with no particular correlation with host-rock type, although the tenor range in the more magnesian cumulates is more restricted, with the low-tenor component (<3% Ni, <1% Cu) missing. Olivine compositions vary widely, with forsterite contents ranging from Fo84 to Fo89 with up to 0.85% NiO. These unusually high Ni contents are attributed to equilibration with high-Ni-tenor sulfides percolating through the intercumulus pore space. Complex zoning patterns in the Cr content of orthopyroxenes record fluctuating crystallization conditions attributed to wall-rock assimilation. The intrusion emplacement setting is interpreted to be derived from a flow of xenolith-, crystal-, and sulfideladen magma through channelized sills where much of the sulfide was incompletely equilibrated with the host magma, resulting in highly variable R-factors. Despite the widespread presence of graphite in the marginal pyroxenites, olivine-sulfide Fe/Ni partitioning records typical redox conditions that are slightly more oxidized than quartz-fayalite-magnetite (QFM)—further evidence of a lack of extensive large-scale equilibrium within the intrusive complex. The Ntaka Ultramafic Complex preserves an arrested stage of the amalgamation of small sulfide-bearing sill-sediment complex intrusions into larger convecting magma chambers, forming one end of a process continuum including Kevitsa (Finland) as an intermediate stage and Mirabela (Brazil) as the layered intrusion end member.