Peralkalinity in peraluminous granitic pegmatites. I. Evidence from whewellite and hydrogen carbonate in fluid inclusions

Abstract

Abstract Fluid inclusions in pegmatite minerals were studied using Raman spectroscopy to determine the carbon species. Carbon dioxide is very abundant in the aqueous liquid and vapor phases. Occasionally, CH4 was found in the vapor. In the aqueous liquid, HCO3− was detected in fluid inclusions in tantalite-(Mn) from the Morrua Mine and in late-stage quartz from the Muiâne pegmatite and the Naipa Mine, all in the Alto Ligonha District, Mozambique. Moreover, we observed a carbonate (calcite group) in fluid inclusions in garnet from the Naipa Mine and in beryl from the Morrua Mine, both in the Alto Ligonha District, Mozambique, and a calcite-group carbonate and whewellite [CaC2O4∙H2O] in fluid inclusions in topaz from Khoroshiv, Ukraine. The occurrence of oxalate is interpreted to be due to a reaction of some form of carbon (possibly CO or bitumen) with a peralkaline fluid. Our results support the hypothesis that, although counterintuitive, hydrogen carbonate-rich peralkaline fluids may locally be involved in the evolution of peraluminous granitic pegmatites, in which peralkaline minerals are normally absent or very rare.