Ore containing platinum group metals in general and palladium in particular have an igneous history that originated in the earth’s crust far below the surface. PGM containing ore is found in intrusive complexes, that is, magma that cooled before reaching the surface, and in a magma type termed mafic (or ultramafic). Mafic is a type of igneous rock that is dark in color from the minerals that it contains, and is rich in iron and/or magnesium. The word mafic itself originated as a combination of magnesium and ferrous. (The converse of this rock type is felsic, which is light colored and forms, for example, the great granite walls and domes of Yosemite).
This metal rich magma is thought to have fractional crystallization, a process in which molten rocks solidify by the crystallization of a series of different minerals. As the minerals crystallize, they are removed from the melt and no longer interact with it; this results in rock layers of different chemical or mineralogical composition. Large layered mafic intrusions are important for a number of reasons and in our context here, they are major sources for the platinum group metals (PGMs). The manner of the crystallization process is still the subject of research and uncertaintly.
The hardened mafic intrusion reached the earth’s surface through continental scale fractures that split the crust. Economic deposits result only when a large intrusive complex was concentrated with substantial amounts of metal prior to the hardening of the magma. The ratios of palladium to platinum vary considerably in the deposits where PGMs are found. In the Stillwater Complex, the ore contains a palladium to platinum ratio of 3:1. In South African PGM ores, the ratio of palladium to platinum is roughly 1:2.
The alluvial deposits of the original South American finds were washed there through erosion action and were not native to the local where found.
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