Platinum Group Metal Deposits

 

There are two types of platinum group metal deposits, endogenetic magmatic and exogenetic eluvial deposits. The first group is characterized by the presence of basic rocks such as dunites and peridotites. The platinum minerals form small vein bodies, pockets, streaks and embedded particles. Less commonly, the platinum minerals are associated in some locations with titanomagnetite or are disseminated through chromite accumulations. In this deposit, the platinum metal minerals are represented by native platinum, iridan platinum, osmiridium and iridosmine. The presence of palladium, rhodium and ruthenium depends on the main metal content in the deposit. The copper nickel sulphide ores are included in this type of platinum group metal deposit and are characterized by their confinement to large layered intrusions of basic rocks. Basically, the platinum and the copper-nickel metallizations are closely related. Pyrrhotite, chalcopyrite, pentlandite and platinum minerals gravitate towards the lower differentiated intrusions where they form disseminated deposits or veined and lenticular deposits. Usually, copper-nickel ores contain more palladium than platinum, and the concentrations of rhodium and ruthenium could exceed those of iridium and osmium. It has been noted that the mineral association in these deposits is quite varied. In addition to native platinum and its ordered varieties, compounds of platinum metals with tin, lead, bismuth, arsenic and antimony are common. The platinum metals in these types of deposits are commonly obtained as a by-product of treatment of copper and nickel concentrates. The largest deposits of copper-nickel sulphide ores are found in the Bushveld Intrusive complex formation, Meresnky; and Sudbury, Canada.

The exogenetic deposits occur with gold deposits and as a result, there are complex gold-platinum placer deposits. Essentially, platiniferous placers are the main source of exogenetic platinum deposits. It has been noted that deposits can present differences in their mineralogical composition and metallic content. According to the geological formation, there are native platinum placers and osmium-iridium deposits. In the first case, is common to find ferroplatinum, native platinum and some alloys. In the second case, the presence of iridosmine and osmiridum is typical. An aspect very interesting is the scarce quantity of palladium in these placer deposits. Deposits which were formed in the weathering zone, by in situ oxidation of the primary platinum-bearing minerals are uncommon. A special location is Ethiopia where high concentrations of platinum are present in the lateritic eluvium of the weathered platiniferous minerals. The varieties enriched with iron are magnetic and this property can be employed to recover the metal by magnetic concentration. Native platinum and ferroplatinum can be found in the form of lumpy and irregular nuggets. In size, they are comparable to gold nuggets. The largest nugget weighed more than nine kilos and was found in Uralian placers.