gems, beryl, emerald, Pakistan, Swat, emerald chemistry

Emerald deposits in Swat, northwestern Pakistan, occurring in talc-magnesite and quartz-magnesite assemblages, have been investigated through stable isotope studies. Isotopic analyses were performed on a total of seven emeralds, associated quartz (seven samples), fuchsite (three samples) and tourmaline (two samples) from the Mingora emerald mines. The oxygen isotopic composition (δ 18O SMOW) of emeralds shows a strong enrichment in18O and is remarkably uniform at + 15.6 ± 0.4‰ (1σ,n = 7). Each of the two components of water in emerald (channel and inclusion) has a different range of hydrogen isotopic composition: the channel waters being distinctly isotopically heavier (δD = −51 to −32‰ SMOW) than the other inclusion waters (δD = −96 to −70‰ SMOW). Similarly the oxygen isotopic compositions of tourmaline and fuchsite are relatively constant (δ 18O = + 13 to + 14‰ SMOW) and show enrichment in18O. Theδ 18O values of quartz, ranging from + 15.1 to + 19.1‰ SMOW, are also high (+ 16.9 ± 1.4‰ 1σ, n = 7). The meanδD of channel waters measured from emerald (−42 ± 6.6‰ SMOW) and that of fluid calculated from hydrous mineralsδDcalculated (−47 ± 7.1‰ SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measuredδD values of the hydroxyl hydrogen in fuchsite (−74 to −6‰ SMOW) and tourmaline (−84 and −69‰ 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. The variation in the carbon and oxygen isotopic composition of magnesite, locally associated with emerald mineralization, is also very restricted (δ 13 ∼ −3.2 ± 0.7%, PDB;δ 18O ∼ + 17.9 ± 1.27‰ SMOW). On the basis of the isotopic composition of fluid (δ 13C ≈ −1.8 ± 0.7‰ PDB;δ 18O ≈ + 13.6 ± 1.2‰ SMOW calculated for the 250-550°C temperature), it is proposed that the Swat magnesites formed due to the carbonation of previously serpentinized ultramafic rocks by a CO2-bearing fluid of metamorphic origin.