Specificity of Ore Generation (Tin, Pegmatites, Gems) in Trans-Porphyry Deposits

During the magmatic stage, base and rarer metals segregate from silicate melts to form ore deposits. The usual case is the porphyry (PD) type (Cu, Mo, W) above subduction zones. The metal grade increases from some ppb or ppm up to percent levels. A new type of trans-porphyry (TPD) deposits (Sn, Ta, Nb, gems) results from large-scale shear between cratons within continental plates, internal decoupling and vertical motion. The bulk ore generation process develops along three stages, from magma generation; emplacement; and formation of an immiscible magmatic phase (MIP), fluids and melt. However, in TPD, metals segregate from the crust during melting below 800 °C, breakdown of biotites, and the melt remains below the critical point (731 °C). Fluids advection competes with chemical diffusion yielding the required enrichment. The subcritical MIP splits into a silicate-rich and an aqueous rich phase, both incompatible with each other. Granite, pegmatites and greisen coexist in the magma chamber. Their respective extraction from a composite mush involves electrons exchanges between charges, or orbitals, yielding metal oxides through chemical diffusion. In contrast, metals (Nb, Ta) observed in pegmatites, and also in gems, electrons rearrange their electronic cloud through its polarizability. At last, gems independently grow under the influence of the extremely hard fluids (Li, Be, B). Magma generation, involving the lower crust (garnet, pyroxene) result in melts that form the two observed pegmatites groups (NYF, LCT)., each being associated with alkaline (A-type) or continental (S-type) granitic melts.