Corundum-bearing metasomatic rocks in the Central Pamirs

Abstract

The Muzkol metamorphic complex in the Central Pamirs contains widespread occurrences of corundum mineralization, sometimes with gem-quality corundum. These occurrences are spatially related to zones of metasomatic alterations in calcite and dolomite marbles and crystalline schists. The calcite marbles contain corundum together with muscovite, scapolite, and biotite; the dolomite marbles contain corundum in association with biotite; and the schists bear this mineral coexisting with biotite and chlorite. All these rocks additionally contain tourmaline, apatite, rutile, and pyrite. The biotite is typically highly aluminous (up to 1.9 f.u. Al), and the scapolite is rich in the marialite end member (60–75 mol %). The crystallization parameters of corundum were estimated using mineral assemblages at T = 600–650°C, P = 4–6 kbar, X CO 2 = 0.2–0.5 at elevated alkalinity of the fluid. The Sr concentration in the calcite and dolomite marbles is low (345–460 and 62–110 ppm, respectively), as is typical of recrystallized sedimentary carbonates. The variations in the 87Sr/86Sr ratio in the calcite and dolomite marbles (0.70852–0.70999 and 0.70902–0.71021, respectively) were controlled by the introduction of radiogenic 87Sr during the metasomatic transformations of the rocks. The isotopic-geochemical characteristics obtained for the rocks and the results of numerical simulations of the fluid-rock interactions indicate that the corundum-bearing metasomatic rocks developed after originally sedimentary Phanerozoic carbonate rocks, with the desilication of the terrigenous material contained in them. This process was a manifestation of regional alkaline metasomatism during the closing stages of Alpine metamorphism. In the course of transformations in the carbonate reservoir, the juvenile fluid flow became undersaturated with respect to silica, which was a necessary prerequisite for the formation of corundum.