The Insizwa lobe of the Mount Ayliff Intrusion, South Africa, is part of an extensive intrusive body that reaches up to 1 km in thickness. and has a variably developed ultramafic basal facies, with minor Ni-Cu-PGE sulfide mineralization. An interpretation of gravity data indicated the existence of a deep graben structure in which was preserved a thick succession of ultramafic rocks. This hypothesis has been confirmed by the drilling of a borehole into the center of the Insizwa lobe that revealed an abnormally thick ultramafic section (400 m). Detailed geochemical data on whole rocks and olivine along this borehole suggest that the olivine-rich rocks formed from a magma with up to 14% MgO. The highest Fo content of olivine is 87%, and does not change systematically or markedly upward, indicating throughflow of large Volumes of magma, rather than in situ differentiation. The initial Sr-87/Sr-86 (R-o) values at 183 Ma for the entire body show wide variations (0.705 to 0.711). Based on the combined isotopic. mineral and whole-rock chemical information, five periods of magma injection are identified. The first magma was a low-Mg basalt that was contaminated by crustal rocks at the base (but which did not cause sulfide separation). The second magma was a high-Mg liquid with an anomalously high initial Sr isotope ratio (R-o > 0.710). Subsequent magmas are harder to define chemically, but two further isotopically identifiable events are recognized in the ultramafic succession. Most of the upper half of the body is gabbronorite, derived from a low-Mg magma with an initial Sr isotope ratio of 0.705. No significant sulfide mineralization was intersected, but two intervals of Ni-depletion in the ultramafic section suggest that an immiscible sulfide liquid has formed. These intervals of Ni-depletion are not vertically extensive, suggesting that large volumes of magma have not experienced such sulfide segregation. In the gabbronorite are three olivine-rich layers, the lower two of which contain sulfides. A thin sill in the footwall contains quenched glass with over 10% MgO. Compositions of hopper and equant olivine grains range from Fo(86) to Fo(74). This sill has a low Ni content, indicating prior separation of sulfide.
