163-A 4.1

Processes of the magmatic-hydrothermal transition and metal transport in late stage magmatic and hydrothermal fluids and its structurally controlled redeposition will be addressed through four leading topics: a) Fluid migration controlled by thermal gradients and /or tectonic processes b) Role of hydrothermal fluids in concentrating specific metals and c) F, B, and P role in the transport of rare elements in hydrothermal fluid; d) sources of the fluids. This general aim will be developed by the study of the Sn mineralization of a selected group of LCT pegmatites of the Pringles Metamorphic Complex of the Sierra de San Luis with emphasis on the emplacement and genetic relationships of pegmatites with the Ordovician magmatism, the source of Sn, and finally the economic potential of the pegmatite district for specific metals. The PMC hosts LCT-class (lithium-cesium-tantalum) pegmatites of the spodumene subtype with Nb-Ta minerals as well as Sn-bearing pegmatites which mainly occur in a separate NNESSW trending belt close to the western border of the Complex. Although basic geology/mineralogy of the San Luis pegmatites are well known, but it is not understood what controls the contrasting metals association. Our working hypothesis is that the degree of differentiation of the pegmatitic melt exerts a strong control on the final mineral paragenesis, but a higher order control may be the composition of the magma source and the influence of secondary mobilisation by the fluid/rock interaction.The research will involve detailed field examination and sampling, conventional geochemical analysis of rock and mineral samples, and then specialised geochemical and petrologic investigation using fluid and melt inclusion analyses, geochronology and tracer isotopic studies for testing the working hypothesis and developing an scenario to differentiate the roles of magmatic, hydrothermal and tectonic/metamorphic processes in ore formation and to explain the regional distribution of Nb-Ta and Sn-bearing pegmatites. The economic relevance of this work is to improve exploration for primary resources of both metallic and non-metallic minerals associated with these rocks. Primary and secondary processes for mineral enrichment are at the focus of the companion project G4.1 working mainly in similar basement in NW Argentina, and the two projects will benefit from comparison and collaboration in the two regions.