SUCCESSIVE MAGMATIC PROCESSES LEADING TO TIN AND HIGH STRENGTH FIELD ELEMENTS MINERALIZATIONS

Ore deposits of tin and high strength field elements (Zr, Nb, Hf and Ta) are commonly associated with peraluminous granitic magmas generated in collisional settings. They are also observed with intrusions where the halogen (F, Cl) contents of the vapour phase are quite high. The concepts about generation of such granitic plutons recently changed from a unique magma chamber chemically evolving through differentiation to a succession of different magma pulses, each of them generating temperature disequilibrium within the magma chamber. The effect of the successive magma inputs is examined in terms of the changes that they induce in diffusion, variation of partition coefficients values and changes in the chemistry of the exsolved vapour phase. We have selected the two examples: the Davis Lake Pluton, South Mountain Batholith, Canada and the Podlesi Stock in the Erzgebirge, Germany. Diagrams from the data sets show that the progressive enrichment in Sn is linked to the release of F, whereas Nb and Ta show a distinct behaviour depending on whether the Ti phase (rutile or titanite) is completely consumed. Hf and Zr are also used to constrain the evolution of the magma. The interpretation of these diagrams suggests that these elements behave differently, depending on whether or not they are linked to the release of the fluid phase. The questions of where, when, how and why these elements are concentrated and methodological problems linked with ore genesis are discussed.