Chemical composition of gemstones and characterization of their host pegmatites and country rocks from Chumar Bakhoor, Gilgit-Baltistan, Pakistan: implications for the source of gem-forming fluids

This paper presents petrographic details and geochemical data on gemstone-producing pegmatites and the rocks enclosing them, Sumayar pluton exposed in the vicinity, and chemical composition of a variety of gemstones to elucidate the nature and source of gemstone-forming fluids in the Chumar Bakhoor area of Gilgit-Baltistan, northern Pakistan. The pegmatites occur as patches, pods, lenses, and dykes in calc-silicate rocks and amphibolite belonging to the southern Karakoram Metamorphic Complex (KMC) as well as in the intrusive Sumayar pluton. A close spatial association suggests the possibility of a strong genetic relationship between the Sumayar pluton and the gemstone-hosting granitic pegmatites. Hence, the latter most likely represent pegmatitic phase of a granitic system that in its orthomagmatic stage produced the Sumayar pluton. The pegmatites thus solidified from pockets of volatile-rich melt residues after the granitic magma that solidified as Sumayar pluton. The mineralogical make-up of the pegmatite, the gemstone assemblage, and chemical characteristics of the individual gemstones suggest that the pegmatite-forming melt was highly enriched in Be, B, F, Cl, and H 2 O, and depleted in Li, REE, and Ta relative to its parent granitic magma. Owing to a strong likelihood of generation by localized partial melting of metapelitic rocks at shallow depth in a post-collisional tectonic setting, the magma parental to the Sumayar pluton had the potential to evolve through the process of differentiation into residues containing gemstone-forming fluids. The calc-silicate rocks, which host the gemstone-producing pegmatites, were formed by isochemical metamorphism of calcareous mudstones rather than metasomatism of carbonate rocks, although they show some mineralogical similarities with skarn-type assemblages. However, the calc-silicate rocks neither played any role in the formation of gemstones nor caused any change in the gemstone chemistry.