2.5-D GRAVITY MODEL OF THE NI-CU-PGM MINERALIZED MOUNT AYLIFF INTRUSION (INSIZWA COMPLEX), SOUTH-AFRICA

The Mount Ayliff Intrusion is the largest and thickest Karoo-aged sill in South Africa. It contains a small Ni-Cu-platinum-group metal (PGM) sulphide deposit, Waterfall Gorge, at the base of its largest lobe ca lled Insizwa. The deposit was once mined for its Cu. In September 1990 , a 900-m-deep, vertical diamond drill exploration borehole was drille d through a thick, central portion of the intrusion. This new geologic al control confirms an earlier gravity survey-based hypothesis that th e intrusion has a thick, hidden keel of ultramafic rock with finite de pth extent below the centre of the Insizwa lobe. Along with other new geological evidence, the borehole log affords a unique opportunity to further constrain models for three regional gravity profiles and to ar rive at a new, 3-dimensional model for the Insizwa lobe. The backgroun d residual terrain-corrected Bouguer gravity anomaly of the Mount Ayli ff Intrusion reaches up to +9 mgal. This results from an igneous slab of 0-150 m of picrite overlain by about 600 m of gabbronorite. Superim posed on this anomaly over the central and northwestern Insizwa lobe i s a 10-km-wide gravity anomaly with an amplitude of +8 to +17 mgal and having steep edges. This is interpreted to represent thickening of pi crite to 400-800 m in a major, hidden basin bordered by two NW-strikin g fault lineaments that are believed to mark a hidden graben structure with major geological transgressions at the base of the Insizwa lobe. At least three discrete, thinner picrite basins are interpreted to ex tend above and beyond both flanks of the graben. Picrite in these basi ns is modelled to be less than 200 m in thickness. The basins are sepa rated either by the major NW-striking geological transgressions, or by domes in the footwall of the Insizwa lobe, above which no picrite has developed. Locations of several hidden, narrow, discontinuous feeder dykes below the Mount Ayliff Intrusion are postulated. The role of pon ding of picrite at the mostly hidden base of the intrusion emerges as being particularly significant. In view of the existence of Ni-Cu-PGM mineralization at Waterfall Gorge where exposed picrite is only 30 m t hick, the recognition of much thicker picrite elsewhere at the base of the intrusion presents exciting exploration targets.