Preliminary fluid inclusion study of the Casposo epithermal gold-silver deposit (Cordillera Frontal, San Juan, Argentina)

SOTARELLO, GUSTAVO; BELVIDERI, IRMA; TOWNLEY, BRIAN; CASTRO DE MACHUCA, BRÍGIDA; HODGKIN, ANDREW; MACHUCA, EDUARDO; MC GUINTY, WILLIAM

San Salvador de Jujuy

Congreso; XVII Congreso Geológico Argentino - Simposio de Metalogenia; 2008

Asociación Geológica Argentina

The Casposo deposit in the Cordillera Frontal, San Juan province, Argentina, is an epithermal Au-Ag vein deposit of quartz ± calcite ± adularia ± illite type, formed from a hydrothermal system associated with Early Permian-Triassic subaerial volcanism. The vein system covers an area of approximately 20 km2 and it is hosted in a variably pyritized and propylitically altered bimodal pyroclastic-volcanic sequence of the Choiyoi Group. The deposits main quartz veins pass laterally into low grade and subeconomic stockwork quartz veins and veinlets within the upper rhyolitic unit and penetrate an underlying andesite volcanic package with a less pronounced stockwork halo. Although the main vein extensions remain open at depth they generally taper down dip. The deposit (368,200 oz Au and 10,551,000 ozAg) consists of subvertical to steeply W-SW dipping quartz veins ranging from 30 to over 150 m long and 1 to 15 m wide. In detail, intersections, deflections, horsetails, splays, jogs, and linkages are common. Kinematic data indicate that the veins formed in an extensional environment and opened predominantly by normal dip-slip movement. There are three major vein systems that strike northwest (B vein, Inca and Mercado veins in Kamila-Mercado and Western Kamila), north (Aztec vein in the Kamila ore-shoot) and east (Cerro Norte and Oveja Negra areas). Vein filling records a complex multistage history and a great variety of vein textures are present including banded (crustiform and colloform); massive quartz; microbotroydal, breccia and cockade, zonal and feathery quartz; quartz pseudomorph after platy lattice-bladed calcite etc. Crustiform-colloform veins made of multiple quartz laminations and polymictic breccia veins with fragments of wall rocks and vein material cemented by quartz are the most common. In addition to quartz, some veins contain adularia, calcite and illite as gangue minerals. The deposit has relatively low Au/Ag ratios with highest contents encountered at 250 g/t Au and 1500 g/t Ag. Electrum and lesser native gold and silver are the main Au/Ag-bearing minerals and acanthite is the main Ag sulphide, although stromeyerite, sulphosalts (argentotennantite, low Ag-bearing tetrahedrite), Se and Te-bearing minerals (naumannite, hessite, cervelleite) and iodargyrite are also important. Scarce sulphides, mostly pyrite and lesser chalcopyrite, sphalerite, galena, arsenopyrite, digenite and luzonite occur locally. Fluid inclusion (FI) data of 12 samples from Mercado and Kamila quartz vein systems were obtained to constrain the temperature and composition of the mineralizing hydrothermal fluids and to assess emplacement depths of epithermal mineralization. The analyses included FI petrography and microthermometry. In the studied samples most of the observed FI are two phase and vapor-rich(vapor phase content between 10-50% in volume), indicating trapping of a two-phase (boiling) fluid. The results show homogenization temperatures (Th) that range between 190 and 370ºC (average 299ºC). Fluid inclusion ice-melting temperature (Tm) measurements indicate that fluids associated with precious metal deposition had salinities