Fernando D'Eramo

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 the Sierras Pampeanas, Córdoba Province, Argentina, occur mainly hosted by calc-alkaline porphyritic biotite granites, which belong to the Paleozoic, post-tectonic Cerro Aspero batholith. The fluorite veins, of Cretaceous age, occupy steeply dipping, strike-slip regional fault zones, and are composed of fluorite and chalcedony, locally with subordinate amounts of pyrite and, in some cases, coffinite 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 of vein chronology, REE data, and fluid-inclusion study in fluorite ores. These stages generally display slightly fractionated REE patterns (La/Yb = 1.4 to 14), with REE behavior given by a relatively stronger LREE fractionation 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. Preferential leaching of HREE over LREE occurred during both alteration types, but in the argillic alteration the LREE were practically not removed. The total homogenization of primary-like aqueous inclusions took place invariably in the liquid phase at temperatures ranging from 187°C to 103°C, with concentrations of values around 160°C, 136°C, and 116°C (stages I, II, and III, respectively), defining a clear trend of fluid cooling. This cooling is accompanied by large changes in the fO2 of the fluid, from oxidizing to reducing, as inferred from the Eu/Eu∗ ratios and the mineral assemblage (pyrite, pitchblende, and coffinite). The three stages of fluorite deposition exhibit temperatures of ice melting within the interval from −0.3°C to +0.4° C, indicating that the mineralizing fluids were exclusively aqueous and highly dilute. No evidence of fluid mixing or boiling was found. The fluid-inclusion data suggest that the proposed three stages of mineralization probably were the result of a single hydrothermal event, and strongly support a single, uniform fluid reservoir for the ore-forming solutions; evidently, the latter were heated meteoric waters rather than fluids generated in deep-seated environments within the crust.