Host rock characteristics and source of chromium and beryllium for emerald mineralization in the ophiolitic rocks of the Indus Suture Zone in Swat, NW Pakistan

Bodies of magnesite-rich rocks (magnesite ± talc ± quartz ± dolomite), locally containing emerald deposits, occur within the Swat Valley. These rocks, part of the Indus suture mélange group, are distributed mostly along contacts of serpentinized ultramafic rocks with carbonate ± graphite-bearing metasedimentary rocks. Their field association, petrographic details, mineralogical composition and geochemical characteristics show that they likely formed due to carbonate alteration of previously serpentinized ultramafic rocks by CO2-bearing fluids released as a result of metamorphism of spatially associated, originally sedimentary rocks of the Indo-Pakistan plate. Locally, late-stage hydrothermal activity affected these highly fissile magnesite-rich rocks to produce veins and stockworks of quartz as well as emerald, Cr-rich tourmaline and Cr, Ni-rich muscovite. Detailed petrographic and mineral chemical investigations suggest that all three Cr-bearing silicates are genetically related and their Cr, as well as Ni and Mg found in Cr-rich muscovite and Cr-rich tourmaline, was derived from the original ultramafic protoliths. Detailed geochemical comparison reveals that relative to non-mineralized sections, magnesite-rich rocks in mineralized zones show significant enrichment in B and Be as well as As, Pb, Zr, Rb, Ba, W, Sn, Sr and Y. Boron and Be enrichment in these rocks appears to be structurally controlled. More extreme B and Be enrichment is evident in small granitic dikes cutting granite gneisses and in Miocene leucogranitic stocks at Malakand 45 km southwest. These geochemical features argue strongly for a leucogranite-associated source for Be-transporting fluids to the emerald's host rocks.