OXYGEN AND HYDROGEN ISOTOPIC STUDIES OF THE UITKOMST NI-CU SULFIDE DEPOSIT, SOUTH AFRICA: EVIDENCE OF SELECTIVE CRUSTAL CONTAMINATION, MULTIPLE MAGMA INJECTION, AND HYDROTHERMAL ALTERATION

The Uitkomst Complex is believed to be a satellite intrusion of the Bushveld Complex. It is a tabular shaped intrusive body with an exposed area of around 6.4 square km. located in the Mpumalanga province of South Africa. This deposit is a 12 km long layered mafic intrusion emplaced within the Transvaal Supergroup, which is composed of the Pretoria group, Chuniespoort group and Black reef quartzite. The Complex consists of five major lithologic units, represented by upper gabbro, gabbronorite, pyroxenite, harzburgite and basal gabbro from top to bottom.

Previous S isotopic studies suggest that selective S assimilation from country rocks was an essential process for ore genesis. We have measured oxygen isotopic composition of primary silicate minerals within the various rock units to gain additional information on the extent of contamination. The ranges of δ ¹⁸O pyroxene and plagioclase values (between 6.9 and 10.1‰ for pyroxene and 7.9 and 10.8‰ for plagioclase) of the upper portion of the gabbronorite are quite different than those of the pyroxenite and harzburgite (5.3 to 6.3‰ for pyroxene and 5.9 to 6.5‰ for plagioclase). A simple two component mixing calculation involving a crustal component (assumed δ ¹⁸O = 18‰) and magmatic component (δ ¹⁸O = 6‰) suggests that 10 to 20% assimilation is required to produce magma δ ¹⁸O values between 7 and 10‰. In contrast, the δ ¹⁸O values of olivine in the main Harzburgite show magmatic signatures between 5.2 and 5.5‰.The variation in oxygen isotopic signatures suggests an open system which experienced multiple intrusive events.

In addition, oxygen and hydrogen isotopic data, in conjunction with detailed mineralogical studies, of the serpentines from the harzburgite unit have been undertaken to assess the role of fluid in the alteration and modification of primary assemblages. O and H isotopic values of serpentine range from 5.9 to 6.7 ‰ (VSMOW) and -69 to -83‰ (VSMOW) respectively. At a T of 300°C, the fluid in equilibrium with serpentine would be characterized by δ ¹⁸O values between 5.5 and 6.5 ‰ and δD values between -79 and -93‰. These values suggest that the fluid involved in the serpentinization process may have been of meteoric water derivation, but isotopic exchange occurred over a prolonged time at relatively low water/ rock ratios.