OLIVINE AND SULFUR ISOTOPIC COMPOSITIONS OF THE UITKOMST SULFIDE-BEARING INTRUSION, SOUTH AFRICA: RECORDS FOR MULTIPLE FLOWS OF MAGMAS AND THEIR ROLES IN ORE FORMATION IN A MAGMA CONDUIT

The Uitkomst intrusion that hosts a primary Ni-Cu sulfide deposit in South Africa has age and parental magma composition similar to that of the Bushveld Complex. The intrusion has previously been interpreted to be a satellite body of the Bushveld Complex, apparently exploiting the bedding planes of its footwall sediments including shale, quartzite and dolomite. The intrusion has a tabular shape, and consists of stratiformed mafic and ultramafic units. From the base to top, these are basal gabbro, sulfide-mineralized harzburgite, sulfide-poor harzburgite, pyroxenite, gabbronorite, and upper gabbro units. The gabbroic units are early sills formed by fractionated magma (the upper gabbro unit), or by magmas highly contaminated with ironstone, chert or quartzite (the gabbronorite and basal gabbro units). There are abrupt changes in the Ni contents of olivine from different units, which can not be explained by normal course of olivine fractional crystallization from a single magma, but is consistent with different flows of magmas with different Ni contents. The abrupt change of Ni contents in olivine between the sulfide-mineralized harzburgite and sulfide-poor harzburgite units is coupled by significant negative departure of d³⁴S values in the mineralized unit. Such departure is consistent with contamination of crustal sulfur from the dolomite that was intruded and engulfed by the magma that eventually formed the sulfide-mineralized harzburgite unit. Sulfide saturation and segregation are likely to have resulted from the contamination process. Immiscible sulfide droplets were carried with cumulus olivine by magma as it pushed through the channel. The immiscible sulfide droplets became more concentrated to the base of the channel when the flow of magma eased. New fresh magma that formed the sulfide-poor harzburgite then entered the channel, pushing away much of the early magma in the upper part of the channel, and eroding the hangingwall gabbronorite to form a hybrid unit of pyroxenite.