Chemistry of tourmaline from the emerald-hosting rocks in Swat, NW Pakistan
Abstract
The ophiolitic emerald-hosting carbonated ultramafic rocks (magnesite + talc and/ or quartz± chrome spinel) in the Swat valley contain disseminated grains, clusters and veins of tourmaline and/ or fuchsite. These late-stage hydrothermal minerals are especially abundant where the host rocks are traversed by ubiquitous veins and stockworks of quartz. Crystals of emerald, tourmaline and fuchsite also occur in the quartz veins. Besides, the metasedimentary rocks that are spatially associated with the emerald's host rocks also contain tourmaline. In contrast to that in the metasediments, tourmaline in the emerald-hosting altered ultramafic rocks and the invading quartz veins contain variable but distinctly high amounts of Cr2O3, MgO and NiO, and relatively low concentration of TiO2. These chemical features, mode of occurrence and nature of associated phases suggest that the tourmaline may have been deposited from Si-rich, Al-, Be-, B- and K-bearing hydrothermal fluids which passed through and extracted Mg, Cr and Ni contents from the host carbonated ultramafic rocks.
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