Faja del Potro and Cordón de la Brea Ore Deposits, La Rioja and San Juan.

Abstract

Two elongate belts of rocks containing large hydrothermal alteration zones with indications of porphyry gold and copper-gold mineralization, as well as potential for epithermal gold-silver deposits, were examined near the border of Argentina with Chile in southern La Rioja and northern San Juan Provinces. On the west along the height of land a zone consists predominantly of Tertiary volcanic rocks and is referred to as the 'Faja de Potro'. This region is roughly midway between the wellmineralized porphyry gold- epithermal gold-silver Maricunga district in Chile to the north and the large, newly discovered high-sulphidation epithermal goldsilver Pascua-Veladero district that straddles the Argentinean-Chilean border to the south. To the east this belt of economic interest, the Cordón de la Brea, is made up of mainly Paleozoic felsic volcanic and sedimentary units and Permo-Triassic granitic and volcanic rocks. There the exploration interest has been both the base metal and gold potential. The Tertiary volcanic rocks in the study area consist of a basal unit of back-arc rocks of predominately andesitic composition that form eroded fie1ds of coalescing lava flow and pyroclastic units; a number of sma11, dark-coloured domes in this unit are made of basal t. Overlying volcanic units that comprise the bulk of the Cordillera consist of dacitic to andesitic are rocks, and less commonly quartz-phyric rocks ('rhyolite'), that give rise to pyroclastic accumulations with locally abundant flow domes. Ali Tertiary volcanic accumulations include locally derived volcanigenic sed iments. Previously ali the older volcanic units in this region were regarded to be equivalent to, and correlated with, the late Oligocene to early Miocene (26 - 21 Ma) Doña Ana Group . The term 'Group' is used in Chile whereas the equi valent rocks in Argentina are commonly referred to 'Formations'. The overlying successions of dacitic to andesititc rocks, with minor basalt units, are equivalent to the middle to late Miocene Cerro de Las Tortolas Group (17 to 1 O Ma). In this study we have recognized that the o1dest volcanics are late Eocene to 0Iigocene in age. They crop out as andesite and Iesser basaltic units in the Rio Peña Negra area and to the east of Ria Blanco at the Sapitos prospect. In this report we present resu1ts of new Ar-Ar dating results frorn these, and the other map units. These new data better constrain regional corre1ation of the Tertiary volcanic units. We also present new petrochemical data that characterize the vo1canics and allow a refined interpretation of their tectonic setting. This work can lead to the construction of a regional stratigraphic framework that is constrained by radiometric dating in which the regional metallogeny can be considered. The o1der rocks of the Cajón de la Brea belt, in addition to copper rnineralization, locally have significant gold enrichment. This appears to be associated in both granitic and rhyolitic hostrocks with quartz-feldspar porphyry dykes containing distinctive bipyrimidal, 'smoky' grey quartz phenocrysts. A single Ar-Ar date from K-feldspar from one of these dykes gives an Eocene age (52 Ma). This suggests that a Tertiary precious metal mineralizing episode occurred in this older Cordilleran terrane. Visits to a number of the alteration zones identified in the 1993 N orwest study, and a few other areas, have provided a large, well-documented sample collection füí those interested in details about the various alteration zones. This information is presented in Appendix C. In addition to whole rock petrochemistry sorne alteration minerals have been analyzed by infra-red spectrometry (PIMA), and there are a number of exploration geochemical sample analyses (assays).