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

Abstract

Quartz segregations in paragneisses from the Paleozoic basement of the North Patagonian Andes contain 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 550-565 °C. The latter phase transition triggers the growth of clinopyroxene or amphibole crystals according to the following idealized reactions, written for the Fe-end-members: (1) 𝐹𝑒8𝑆𝑖6𝑂15[(𝑂𝐻)6𝐶𝑙4] + 3𝐶𝑎2+ (𝑎𝑞. ) ↔ 3𝐶𝑎𝐹𝑒𝑆𝑖2𝑂6 + 5𝐹𝑒2+(𝑎𝑞. ) + 4𝐶𝑙−(𝑎𝑞. ) + 3𝐻2𝑂 (2) 4Fe8Si6O15[(OH)6Cl4] + 6Ca2+ (aq.) ↔ 3Ca2Fe5Si8O22(OH)2 + 17Fe2+ (aq.) + 16 Cl- (aq.) + 12OH- + 3H2 Ferropyrosmalite Ferroactinolite During the heating experiment, hematite (when 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 result from the retrograde reequilibration of these minerals with the brine, implying the uptake of Fe2+, Cl- and H2O, and the enrichment of Ca2+ 38 in the brine. Pervasive fluid flow and fluid-assisted diffusion are recorded by channelway microstructures, healed microfractures and dissolution-reprecipitation phenomena, as demonstrated by cathodoluminescence microscopy. These high salinity brines derived from magmatic sources, possibly of Mesozoic age, were related to a regional metasomatic process, coeval with a widespread granitoid activity.