Granitic pegmatite chrysoberyl in a shear zone of the Achala batholith, Córdoba, Argentina

Abstract

Chrysoberyl of granitic pegmatite affiliation was found in the Ethel Mary mine, at coordinates S 31º 42´ 24.8´´ and W 64º 57´ 25.1´´, in the western margin of the Sierra Grande, San Alberto Department, Córdoba province, Argentina. Up to date, the sole published finding of chrysoberyl in Argentina was reported near Virorco, Sierra de San Luis. At this site, chrysoberyl occurs as a subordinate phase in the assemblage dumortierite-tourmaline-kyanite, in zoned veins or dikes of uncertain origin, emplaced in norite and hornblende gabbro. This contribution describes the geological setting and mineralogical peculiarities of this first finding of granitic pegmatite chrysoberyl in the country, in an attempt to constrain its formation environment. The lonely occurrence of chrysoberyl in the Ethel Mary mine, is interpreted as a by-product of the breakdown reaction of beryl, i.e., beryl ==>chrysoberyl+3quartz+2BeO, or beryl+2 aluminosilicate ==> 3chrysoberyl+8quartz. Its location in a reverse fault zone and textural evidence that shearing was active during pegmatite infill/replacement stage, suggest that chrysoberyl formation was favored by its confinement to a regional shear-stress environment, like other worldwide known examples. Strongly fractured and partially replaced chrysoberyl by muscovite aggregates suggest that a hydrothermal fluid was channelled through the fault zone during post-deformation stage.