Magmatic Pt-Au Systems
Boula Style Moderate Grade Open Cut Pt-Au Deposits
The geological setting of the mineralisation is similar to that of the Bushveld Complex in South Africa, in this case the style of mineralisation is more comparable with the palladium rich Lac des Iles deposit in Canada, which likely formed following the upward movement of metal-bearing hydrothermal fluids along steeply inclined structurally weak zones.
Abundant PGE-bearing sulphide mineralisation is dispersed throughout both the matrix and the breccia fragments, together with massive and disseminated ferrian chromite formed by the hydrothermal alteration of the original chromite layer. Chalcopyrite, pyrrhotite and millerite are the most common sulphide minerals, with pyrite, cobaltite-gersdorffite, pentlandite, bornite and various other metallic sulphides in minor or trace amounts. The total sulphide content varies up to as much as 40% by volume, and the PGE content tends to increase with increasing sulphides. The sulphide mineralisation is not restricted to the brecciated zone and has been identified in all the adjacent host rocks.
The PGEs in the Mineralised Breccia Zone comprise mainly platinum, palladium, rhodium and ruthenium, with platinum and palladium being dominant. They occur in a wide variety of complex sulphides, arsenides, antimonides and silicates, sometimes within the mineral lattices and at other times as small (10-20 microns maximum 250 microns) inclusions. PGEs are also found in relatively unaltered rocks near the borders of the breccia zone, where they occur as 1-2 micron size particles within the original silicate minerals. The mineralisation zonation indicates the system to be more Pd rich in the north and more Pt rich towards the south.
The zone of brecciation and PGE mineralisation is thought to result from fluid movement along a structurally weak zone associated with the intrusion of the coarse grained Bangur Gabbro which makes up the matrix of the Mineralised Breccia Zone. The intrusion and influx of high volumes of metal-bearing fluids at the time of dilation, resulted in implosive brecciation of the chromite lode to form a mega-breccia. Extensive fracturing and brecciation of the adjacent rocks types resulted in an extensive zone of mineralisation characterised by net-vein complexes and injection-vein breccias some 75m wide and the full width of the system may be considerably large.