Magmatic processes, fluid-rock interaction and provenance of Nb-Ta and REE mineralizations in NW Argentina
Deep-seated processes in the Andean crust, now exposed by tectonics and erosion in the foreland, are integral parts of the resource-forming systems under investigation. After recycling of continental crust by partial melting and magmatism, hydrothermal fluids of magmatic and/or meteoric origin are key agents for a range of processes that form and modify metallic ore deposits. This project will investigate a range of granite-pegmatite related rare earth element (REE) and Sn-Ta-Nb-Li mineralization types in the San Luis range of NW Argentina. The goals are to assess the relative influence of magma composition, particularly the S-type vs. A-type granite composition, in the metals enrichment, and to determine the role of fluids in mineralization. We will concentrate on well-characterised metamorphic complexes and their intruding granitoids of the San-Luis range (Eastern Sierras Pampeanas), where compositionally distinct pegmatites occur: predominantly intragranitic mostly Ordovician Nb-Y-F pegmatites related to water-poor and fluorine-rich melts and mostly Ordovician Li-Cs-Ta pegmatites related to water-rich peraluminous melts and emplaced either in the metamorphic complexes or in their intruding granitoids. These will be compared with similar occurrences further north in Sierras Pampeanas. A small but significant part of the work will follow the alteration of the NYF pegmatites to obtain proxies for provenance studies of critical elements (source to sink) and the research will also help constraining the sources of granite magmas and the role of the interplaying between recycling of continental crust and juvenile additions.
Provenance and transport of Sn (Nb-Ta) and REE from the pre-Andean crustal basement: primary mineralization and secondary enrichment processes
This project will investigate the primary source rocks for Sn (and Nb-Ta) and rare earth elements (REE) in the pre-Andean basement rocks (granites, pegmatites, metamorphic complexes) and the enrichment/mobilization processes related to chemical (hydrothermal alteration or weathering) and mechanical processes (ductile or brittle deformation in fractures and ductile shear zones). The investigation will focus on selected field occurrences of Palaeozoic granites, pegmatites, and their highly deformed metamorphic equivalents of the pre-Andean crustal basement, especially of the Salta Province. For the REE minerals especially (carbonates, phosphates, fluor-carbonates, oxides), there is substantial redistribution related to solubility-reprecipitation processes in the deep crust (e.g. lode deposits in shear zones) as well as surface adsorption in weathered exposures (e.g. heavy REE enrichment in clays: Longnan-type deposits). Because tin minerals and coltan (Nb-Ta ore) from pegmatite fields are commonly mined in alluvium (placers), it is critically important to understand how secondary processes (alteration, chemical and mechanical weathering) affect the criteria used for provenance identification. Therefore, the major aim of the project will be to analyse the origin and enrichment processes of the rare elements in primary (magmatic) as well as in secondary (fluid- and structurally controlled) environments. Emphasis will fall on studies in Salta province, because of the geological situation, including rare element pegmatites, and the direct links with project [G 2.2] working on the reactivation of fault zones and project [G 3.5], which addresses the role of basement composition for metal enrichment in near-surface volcano-tectonic complexes. Close collaboration with project [G 4.2] and the Argentine metallogenesis projects will extend the scope of investigation to places best suited to study the relevant processes (e.g., shear zones, alteration/weathering profiles, etc.). The research will combine modern techniques of mineralogical, microstructural and (isotope) geochemical analysis with field campaigns to examine key localities best suited to study the chemical and mechanical alteration of primary mineralization. The relevance of this study is in the field of mineral exploration primarily, i.e., for orogenic structurally-controlled REE deposits, and also for the growing field of provenance certification to control conflict ores.