Internal structure and patterns of distribution of mineral types of rare metal pegmatites in the Darai Pech valley (Kunar province, Afghanistan)

Abstract

The granite pegmatites of Afghanistan are known for deposits of rare metals: Li, Be, Cs, Ta, Nb, Rb, Sn, etc. Ores in the Darai Pech valley of the Kunar province contain crystals of beryl, spodumene, pollucite, and tantalite and are of interest to industry. The article discusses the petrographic features of pegmatite bodies, their internal structure, and the morphological parameters of ore bodies. Particular attention is paid to changes in the mineralogical composition of pegmatite bodies depending on the vertical distance from the parent intrusive rock. Over 20 pegmatite bodies have been studied. Four types of pegmatite veins are identified with distance from the granite intrusion: 1) essentially plagioclase-microcline, biotite-muscovite pegmatites with rare beryl, schorl, and garnet; 2) albitized microcline and microcline-albite pegmatites with coarse-crystalline beryl, tantalite-columbite, schorl, kunzite, and polychrome tourmaline; 3) albite pegmatites with rare spodumene, finely disseminated beryl, and columbite-tantalite; 4) albite-spodumene pegmatites with columbite-tantalite, cassiterite, clevelandite, lepidolite, kunzite, and polychrome tourmaline. Simultaneously, content of biotite decreases and those of muscovite and clevelandite increase, which positively correlates with contents of rare metal mineralization, piezo-optical raw materials, and precious stones. Individual pegmatite veins have asymmetrical zoning: the footwall has relatively fine- and medium-grained rocks and the hanging wall – coarse-grained rocks.