WP(4): Metallogenesis of strategic deposits in felsic magmas at different crustal levels
A major proportion of the metallic mineral resources in the Andean orogen and foreland of Argentina is associated in time and space with felsic magmas. This region host several world class deposits (Aguilar Ag-Pb-Zn, Mina Pirquitas Ag-Sn-Zn; Chinchillas Ag-Zn-Pb) linked to magmatism and many moderate to small Cu porphyries (Inca Viejo, Taca Taca, Las Burras-Pancho Arias) that are currently under exploration. Despite the clear association with magmatic centers, the role of magmatism in mineralization is obscure because the economic deposits are mostly hydrothermal in nature and show a strong tectonic control. Thus, the processes of metal transport and enrichment are complex and multi-stage, and their understanding involves many of the same aspects of deformation and heat/fluid flow that are relevant for basin formation and evolution. With regard to the role of magmas, the main debate concerns the primary metal endowment in the melting source and the late-stage concentration by magmatic-hydrothermal processes at the emplacement level1-4. For the world-class Cu-Au and Sn-Ag mineral deposits of the Central Andes, there are contrasting metallogenetic models that differ mostly in the source of metals. One model suggests a twostage evolution that relates to episodes of shallow and steep subduction5, where the ultimate metal source is the subducted slab. In this model, shallow subduction prevents magma formation and allows hydration of the lower crust by slab-derived fluids containing metals like Cu, Zn, Au and Pb. During a later episode of slab steepening and mantle melting, the lower crust undergoes dehydration melting and the metals are transported by magma to shallow depths. An alternative model considers the metals to be sourced in the middle or upper crust, from which they are moblized by fluid-rock interaction; in this scenario, magma is seen as a heat engine, but not a direct source of metals. A special feature of the Central Andes mineral deposits is their arrangement in two orogen-parallel belts with contrasting metal associations (Fe, Cu-Au to the west, vs. polymetallic, Sn-Ag to the east). One objective of our studies will be to determine the controlling factors for contrasting metal associations in contemporaneous magmatic centers, including metal source inheritance vs magmatic differentiation as predominant processes, and the role of basement composition and/or structural control.
Post-magmatic metal transport and enrichment relate to geochemical processes (rock alteration, leaching and chemical weathering), which may be linked to tectonics and rock deformation (enhanced permeability and fluid focus by faults). In many orogens, including the Andes and pre-Andean basement (Sierras Pampeanas), crustal-scale fault zones, including both upper crustal structures6 and ductile shear zones, have played a decisive role in mineralization. Thus, crustal deformation in brittle and ductile fault zones is not only important for topographic evolution and basin formation, but also enhanced metamorphic reactions and fluid/heat flow, which play a major role in material transport and deposition.
The advantage of our coupled orogen-foreland study region is that examples of both deep and shallow-level mineralizing systems are well exposed and can be studied together. At deeper crustal levels, ore formation in felsic magmas is related to Paleozoic granites and granitic pegmatites in the foreland. More than 95% of granitic pegmatites in Argentina with economic potential for metal and non-metallic resources are located in central and NW Argentina7. These pegmatite fields encompass large compositional ranges and differ in their metal specialization, the controls on which are not well understood, particularly with respect to the influence of source composition versus magmatic differentiation on metal concentration. The main targets for this research will be rare-metal enrichments in Sn (Nb-Ta-Li) in pegmatites developed during the Pampean (~550-520 Ma) and Famatinian (~500-450 Ma) orogenies, as well as REE enrichment in hybrid Devonian (400-360 Ma) granitoids and pegmatites8,9. We will address the role of shear zones for emplacement of granites and pegmatites, as well as secondary features of alteration and deformation of the ore minerals that are relevant for understanding the mobilization of metals from the basement into shallow-level systems.
1 Sillitoe, 1972, 1976; 2 Kay et al., 1999; 3 Lehmann et al., 1990; 4 Wilkinson, 2013; 5 Kay et al., 1999; 6 Chernicoff et al., 2002; 7 Galliski, 1994; 8 López de Luchi et al, 2007, 9 Montenegro and Sosa, 2003