Abstract

PONS, María Josefina; FRANCHINI, Marta; MAS, Graciela  and  RIOS, FranciscoJavier. Genetic model of the Vegas PeladasFe-sakarns, Mendoza, Argentina. Rev. Asoc. Geol. Argent. [online]. 2009, vol.64, n.4, pp.615-634. ISSN 0004-4822.

Based on fluid inclusions (FI) studies, theFe skarn as- -sociated with the diorite pluton in Vegas Peladas formedat 3,5 km depth, under lithostatic pressure of 950 bars, at expenses of highsalinity (up to 70 wt. % NaCl eq.) - high temperature fluids (670°-400°C).These NaCl ± KCl ± FeCln ± hematiterich fluids along with vapour may have beenformed by the immiscibility of low salinity (6-8 wt. % NaCl eq.) magmaticfluids. The isotopic data of the water in equilibrium with garnet (δ¹⁸O7.2-8.5‰) and with magnetite (4.8-7.9‰) confirm the magmatic originfor these early fluids. During the interaction with the wall rocks and theformation of the prograde exoskarn, fluids decreased isobarically theirtemperatures (up to ~250°C). The continuum volatile exsolution from the magmaand sealing of conducts by the precipitation of silicates generatedoverpressures, consequent fracturing of the exoskarn, and boiling of thefluids. Under hydrostatic pressures, the increase of permeability allowed theinfiltration of external fluids (formations water± meteoric water?) to thehydrothermal system, their mixing with the magmatic fluids and cooling,promoting early silicate mineral instability and their replacement by hydrousminerals, quartz (with δ¹⁸O values -0.55 a 4.5‰) and the massiveprecipitation of most iron oxides. The FI record indicates fluids with lowersalinities (41.6-23 wt. % NaCl eq), temperatures between 420° and 320°C, andhydrostatic pressure of 325 to 125 bars for this stage. The decrease intemperature and salinity were the main factors that favoured the iron oreprecipitation. FI in calcite from later veins that cut all the prograde andretrograde assemblages indicate fluids with the lowest temperatures (165-315°C)and salinities (8.4 y 13.5 wt. % NaCl eq.). The δ¹⁸O values for the water in equilibrium withthis epidote (-4.66 a 0.19‰) and calcite (-3.9 a 2.68 ‰), suggestmixing and dilution of previous fluids with meteoric water (with a dominance ofthe later) during cooling and collapse of the hydrothermal system. Theintrusion of the granite pluton increased the temperature of the previous skarn(>550°C) and generated saline fluids and vapour by immiscibility that causeddissolution of Fe from previous skarn. These brines carried Fe insolution and, when cooled, precipitated it as iron oxides along with the new skarnminerals around the granite.

Keywords : Fluid evolution; Fe-skarns; Cordillera Principal.

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