Ore-Forming Fluids of Vein-Type Fluorite Deposits of the Cerro Aspero Batholith, Shouthern Cordoba Province, Argentina

CONIGLIO JORGE; PINOTTI LUCIO PEDRO; D'ERAMO FERNANDO

INTERNATIONAL GEOLOGY REVIEW

TAYLOR & FRANCIS INC

Lugar: Londres; Año: 2000 vol. 12 p. 368 - 383

0020-6814

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, eventually, coffinite and pitchblende. These veins show typical open-space-filling textures and are closely related with pervasive silicic and argillic alteration of the hosted granite. Three successive stages of mineralization were distinguished on the basis of vein chronology, REE data and fluid inclusions study in fluorite ores. These stages generally display slightly fractionated REE patterns (La/Yb=1.4-14), with REE behavior given by a relatively stronger LREE fractionation with respect to HREE. Thus, it is suggested that the REE composition of the fluids responsible for fluorite deposition was largely controlled by a different 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 temperatures of primary-like aqueous inclusions took place invariably in the liquid phase at temperatures ranging from 187°C to 103°C, with concentration of values around 160°C, 136°C and 116°C (stage I, II and III, respectively), defining a clear trend of fluid cooling. This cooling is accompanied by large changes in the 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 depositions exhibit temperature of ice melting within the interval -0,3°C - +0,4°C, indicating that the mineralizing fluids were exclusively aqueous and highly diluted. No evidence of fluid mixture or boiling was found. The fluid inclusion data suggest that the proposed three stages of mineralization was probably the result of a single hydrothermal event and strongly support a single and uniform fluid reservoir for the ore-forming solutions, probably heated meteoric waters rather than fluids generated in deep-seated environments within the crust.