The geology, geochemistry and mineralization of the north Chilean High Cordillera.
thesisposted on 19.11.2015, 09:02 by Anthony John. Rex
Subduction-related magmatism and associated mineralization in the north Chilean High Cordillera has been studied within the regional tectonic framework of the Andes between latitudes 28° S and 30°30'S. Geochemical studies focus on the Pre-Andean composite plutonic belt. Rb-Sr geochronology defines a 310-280 Ma plutonic complex and a 240-190 Ma plutonic and volcanic complex. Both have a subduction-related geochemical signature and are characterised by initial Sr ratios of 0.706 to 0.711. Magmagenesis involving contamination by Palaeozoic metasediments of mantle-derived magma is advocated. Individual plutonic units within each complex cannot be related by closed-system fractional crystallization, although this is a viable mechanism with which to model intra-suite evolution. Cenozoic magmatism in the High Cordillera conforms to the progressive eastward migration in the locus of Andean magmatism. Palaeocene plutons to the west of the Pre-Andean belt possess low initial Sr ratios (0.7040) compatible with intrusion through attenuated crust. Intra-basement magmatism of mid-Miocene age possesses higher initial Sr ratios (0.7048 to 0.7063). Although there is a progressive easterly enrichment in Sr ratio with time, there are no systematic trace element variations. It is argued that the Cenozoic magmatism possesses a geochemical signature which is dominated by a mantle component and that the Sr isotopic enrichment reflects a progressive sub-continental mantle enrichment. Epithermal precious-metal deposits that occur in extensive zones of hydrothermal alteration are associated with mid-Miocene magmatic activity. Enrichments in K, Sr, Ba +/- Fe, due to alunite/jarosite +/- barytes +/- celestite precipitation, are intrinsically associated with boiling and vapour separation of upwardly-migrating fluids. Precious-metal mineralization and silica precipitation also occur during this process. A geochemical study of vein material reveals that low-salinity fluids were transporting precious-metals as bisulphide complexes. A model involving the relative abundances of CO2 and H2O, and the initial H2O content, in the primary magma is proposed to explain the styles of mineralization associated with any one magmatic event.