Mineralogy, geochemistry and stable isotope studies of the ultramafic rocks from the Swat Valley Ophiolite, northwestern Pakistan: Implications for the genesis of emerald and nickeliferous opaque phases.

The ulttamafic and related lithologies of the Swat valley ophiolite lie along the Main Mantle Thrust (MMT) between the Kohistan island arc and Indo-Pakistan plate. The textural and mineralogical characteristics of the ultramafic rocks seem to be the result of a complex interplay of a number of processes including partial melting, serpentinization, metamorphism, carbonation and late-stage hydrothermal alteration. The modal mineralogy of the least modified varieties (non-carbonated serpentinized) of the ultramafic rocks indicate that the majority of them them are harzburgites. The chemical composition of the primaiy phases (chrome spinel and pyroxenes) and major, minor as well as trace element contents of the non-carbonated ultramafic rocks suggest that the major bulk of the rocks originated as the upper mantle residue left after an intermediate degree of partial melting. Pervasive serpentinization of the rocks, evident from the almost common occurrence of serpentine and orthopyroxene bastites, was followed by a low to medium grade regional metamorphism converting the original low temperature serpentine (lizardite) to antigorite, forming olivine and, rarely, diopside. The trace amounts of Ni-rich opaque phases in the rocks largely formed by the modification of primary sulphides during serpentinization. Locally, alteration by a CO2-bearing fluid resulted in the carbonation of the formerly serpentinized ultramafic rocks forming quartz- and/ or talc-carbonate lithologies. The oxygen and carbon isotope composition (delta18Ocalculated = +13.6 1.2 ‰ SMOW; delta13Ccalculated= -1.8 0.7 ‰ PDB) indicates that the fluids responsible for carbonation were probably derived from the subduction-related metamorphism of the carbonate-bearing sedimentary rocks of the Indo-Pakistan plate. Late-stage hydrothermal activity by Be, B and K-bearing fluids, apparently related to quartz veins and stockworks, has affected the ultramafic rocks mainly the carbonated types because of their highly fissile and permeable nature. This process has produced emerald, Cr-tourmaline and fuchsite. The chromium in these phases was extracted from the host ultramafic rocks. The oxygen isotope composition of parental waters (delta18Ocalculated = +14 1.7 ‰ SMOW), calculated from the measured values of emerald, quartz, tourmaline and fuchsite, is high in the delta18O component. This high delta18Ocalculated well as the range of 5D of channel waters from emerald (-42 6.6‰ vs SMOW) and that of fluid calculated (-47 7.1 ‰ vs SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measured 5D values of the hydroxyl waters in fuchsite (-74 to -61‰ vs SMOW) and tourmaline (-84 and -69 %o vs SMOW) with pegmatitic muscovite and tourmaline suggests that the mineralization was probably caused by modified (18O-enriched) hydrothermal solutions derived from an S-type granitic magma