Ferropyrosmalite-bearing fluid inclusions in the North Patagonian Andes metasedimentary basement, Argentina: A record of regional metasomatism

GRACIELA SOSA; SEBASTIÁN ORIOLO; ALFONS VAN DER KERKHOF; PABLO DIEGO GONZÁLEZ; EZEQUIEL OLAIZOLA; FLORENCIA BECHIS

AMERICAN MINERALOGIST

MINERALOGICAL SOC AMER

Año: 2021 vol. 106 p. 1172 - 1182

0003-004X

Quartz segregations in paragneisses from the Paleozoic basement of the North Patagonian Andes contain primary highly saline multiphase fluid inclusions with the rare daughter mineral ferropyrosmalite detected by Raman analysis, besides halite, sylvite, hematite and/or magnetite. During heating experiments, L-V homogenization occurs (256-515 °C), followed by halite dissolution (287-556 °C) and the dissolution of ferropyrosmalite at 550-581 °C. The latter phase transition triggers the growth of clinopyroxene or amphibole crystals. During the heating experiment, hematite (if present) transforms to magnetite by the uptake of H2, whereas inclusions without Fe-oxides contain traces of H2 after reaction (2), as demonstrated by Raman analysis. These reactions show that ferropyrosmalite might resulted from the retrograde reequilibration of these minerals with the brine, implying the uptake of Fe2+, Cl- and H2O, and the enrichment of Ca2+ in the brine. Pervasive fluid flow and fluid-assisted diffusion are recorded by channelway microstructures, healed microfractures and dissolution-precipitation phenomena, as demonstrated by cathodoluminescence microscopy. These high salinity brines derived from magmatic sources, possibly of Mesozoic age, and were related to a regional metasomatic process during retrograde metamorphism, coeval with widespread granitoid activity.