Metamorphism and Mineralisation, San Juan, Argentina

The regional geology and tectonic evolution of the Andean and Pampean Ranges have been discussed extensively by Ramos (1988a,b, and references therein): Recently; Sims et al. (1997) have made extensive mapping of the Sierras Pampeanas and discussed the tectonic and metallogenic evolution of the region. The peterogenesis of gabbroic intrusions in northwestern Argentina (Sierras Pampeanas) has been documented by DeBari (1997 and references therein) and in the San Luis Province - Las Aguilas, Virorco and El Fierro areas - by Mogessie et al. (2000). The La Huerta Valle Fertil ranges and particularly their eastern margin are considered as part of the Famatinian orogen. These ranges have been studied by Castro de Machuca (1996) and Vujovich (1996) among others. According to observations made by Castro de Machuca and co-workers the rocks of this magmatic arc were affected by both metamorphism and deformation. However, the spatial and temporal relationships between the magmatic units are not clear. There are no electron microprobe and theromobarometric data for the magmatic complex and the host metamorphic rocks. From a metallogenic point of view nothing has been done on the mafic- ultramafic ranges of La Huerta and Valle Fertil. Although a large amount of data is available on the regional geology and several platetectonic models have been suggested, detailed petrological, geochemical, isotopic, and economic geological studies are lacking in the proposed project area. In this project, in cooperation with our Argentinian colleagues, a detailed geological- structural, geochronology, petrographic, phase petrologic, geochemical and geophysical work will be undertaken on the basement and the associated mafic-ultramafic rocks of the La Huerta and Valle Fertil ranges, San Juan Province, NW Argentina.

This project dealt with the study of the metamorphism and mineralization of the basement and mafic-ultramafic intrusive rocks. Colleagues from the University of Bahia Blanca and University of San Juan were involved in the project in addition to those from the Institute of Earth Sciences, University of Graz and the University of Vienna. Our aim was to understand the Pressue-Temperature-time relations of the rocks outcropping in the area. The tectono-stratigraphic position of the granulite facies basement is ascribed to a subduction related magmatic arc (with associated mafic-ultramafic intrusions (490-330 my)), that developed on the overriding plate at the time when an allochtonous Laurantia derived terrain approached the western margin of Gondwana. The rocks we identified were metasediments (migmatitic gneisses), mafic rocks (gabbro-norites), metacarbonates and calc-silicates. Each of these rock types have mineral parageneses which tell the story of how the rocks formed and how they came to be emplaced in such an area. In order to understand the geological and geodynamic processes our research work concentrated on a detailed field observation and sampling (2006 and 2007) of the different rocks based on already available geological maps. These samples were then shipped to the University of Graz for further studies. This involved: 1) a detailed microscopic investigation in order to determine the mineral parageneses, 2) followed by electron microprobe chemical analyses of the mineral phases and scanning electron microscope observation of the textural relations of the minerals, 3) The mineral chemical data and textural relationships of the phases were then used in understanding the processes which have led to their formation by employing thermodynamic calculations to determine the Pressure and Temperature of formation. 4) Age dating of the rocks have also been conducted (Univ. Vienna) and a P-T-time path reconstructed. The overall result shows that the rocks were metamorphosed to a T of > 800C and P of 6.5 Kb (ca. 20 Kms depth) in the granulite facies. These metamorphic imprints have been detected in all rock types. In order to understand the tectonic position of the intrusive mafic rocks a geochemical analyses of representative samples have been conducted. Combined with the field, mineralogical and petrological data it is concluded that the rocks were formed at a back arc.tectonic position. In addition, we made a detailed study of the economic mineral deposits of old abandoned Lead-Zinc mines in the area and found the first occurrence of Cadmium Sulphide (CdS) associated with rare sulphides in this part of Argentina. Most of the results have been presented in national and international scientific conferences and published as abstracts and in several peer reviewed journals as manuscripts or are being reviewed.