Fluid inclusions in Sn-bearing pegmatites of the Sierra de San Luis, Argentina

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T. Montenegro
G.M. Sosa


In the Sierra de San Luis, Argentina, Sn-enriched granitic pegmatites represent the extremes of fractional crystallization of Ordovician, syn-kinematic, meta- to peraluminous S-type granites. The pegmatites are concordantly emplaced in quartz-mica schist with N-S general strike and vertical dip. The metamorphic grade varies from green-schist facies to lower-amphibolite facies. Contacts between pegmatite and country rock are sharp and parallel to the metamorphic foliation, suggesting a late-kinematic emplacement. Microthermometric measurements of fluid inclusions in quartz reveal an early complex CO2-H2O-NaCl fluid. Inclusions are distributed in groups or in intragranular trails, and are interpreted as primary or pseudosecondary. The salinity is generally 0-7 wt% NaCl-eq., but rare high-salinity inclusions (29-32 wt% NaCl-eq.) were possibly originated in a magmatic stage. Homogenization temperatures (into liquid or vapor, occasionally critical) range from 227 to 346°C. The fluid inclusion pressures at the homogenization points (minimal trapping pressures) vary between 1500 and 2700 bars. A low salinity aqueous fluid was trapped in secondary inclusions with homogenization temperatures (into liquid) of 210-264°C, and densities of 0.76-0.85 g/cm3. At lower temperature (103 a 183°C), a late fracturing originated secondary fluid inclusions, which contain low salinity water (0-5wt% NaCl-eq.). The wide variation in cavity morphology, the large variation of CO2 proportion and the total homogenization temperatures indicate immiscibility of an early complex fluid, with fluid inclusions reaching up to 100% of CO2 due to the leakage of the aqueous fluid, induced by deformation of the country rock. Occasionally, a recrystallization event trapped part of the aqueous fluid.

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Montenegro, T., & Sosa, G. (2021). Fluid inclusions in Sn-bearing pegmatites of the Sierra de San Luis, Argentina. Revista De La Asociación Geológica Argentina, 58(3), 347-355. Retrieved from https://revista.geologica.org.ar/raga/article/view/1533