Ore-forming fluids of vein-type fluorite deposits of the Cerro Aspero batholith, southern Cordoba Province, Argentina

Vein-type fluorite deposits in the southern part of thr Sierras Pampeanas. Cordoba Province, Argentina, occur mainly hosted by calc-alkaline porphyrit ic biotite granites, which belong to the Paleozoic, post-tectonic Cerro Asp ero batholith. The fluorite veins, of Cretaceous age, occupy steeply dippin g, strike-slip regional fault zones, and are composed of fluorite and chalc edony, locally with subordinate amounts of pyrite and, in some cases, coffi nite and pitchblende. These veins show typical open-space-filling textures and are closely related to pervasive silicic and argillic alteration of the host granite. Three successive stages of mineralization were distinguished on the basis o f vein chronology, REE data, and fluid-inclusion study in fluorite ores. Th ese stages generally display slightly fractionated REE patterns (La/Yb = 1. 4 to 14). with REE behavior given by a relatively stronger LREE fractionati on with respect to HREE. The REE composition of the fluids responsible for fluorite deposition was largely controlled by differential mobility of the REE during; the silicic or argillic alteration of the host granite. Prefere ntial leaching of HREE over LREE occurred during both alteration types, hut in the argillic alteration the LREE were practically not removed. The tota l homogenization of primary-like aqueous inclusions took place invariably i n the liquid phase at temperatures ranging from 187 degrees C to 103 degree s C, with concentrations of values around 160 degrees C, 136 degrees C, and 116 degrees C (stages I, II, and III, respectively), defining a clear tren d of fluid cooling. This cooling is accompanied by large changes in the f(o 2) of the fluid, from oxidizing to reducing, as inferred from the Eu/Eu* ra tios anti the mineral assemblage (pyrite, pitchblende. and coffinite). The three stages of fluorite deposition exhibit temperatures of ice melting within the interval from -0.3 degrees C to +0.4 degrees C. indicating that the mineralizing fluids were exclusively aqueous anti highly dilute. No ev idence of fluid mixing or boiling was found. The fluid-inclusion data sugge st that the proposed three stages of mineralization probably were the resul t of a single hydrothermal event, and strongly support a single, uniform fl uid reservoir for the ore-forming solutions; evidently, the latter were hea ted meteoric waters rather than fluids generated in deep-seated environment s within the crust.