The genesis of emeralds and their host rocks from Swat, northwestern Pakistan: A stable-isotope investigation

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 (delta(18)O SMOW) of emeralds shows a strong enrichment in O-18 and is remarkably uniform at +15.6 +/- 0.4 parts per thousand, (1 sigma, 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 (delta D = -51 to -32 parts per thousand, SMOW) than the other inclusion waters (delta D = -96 to -70 parts per thousand, SMOW). Similarly the oxygen isotopic compositions of tourmaline and fuchsite are relatively constant (delta(18)O = +13 to +14 parts per thousand, SMOW) and show enrichment in O-18. The delta(18)O values of quartz, ranging from +15.1 to +19.1 parts per thousand, SMOW, are also high (+16.9 +/- 1.4 parts per thousand; 1 sigma, n = 7). The mean delta D of channel waters measured from emerald (-42 +/- 6.6 parts per thousand, SMOW) and that of fluid calculated from hydrous minerals delta D-calculated (-47 +/- 7.1 parts per thousand, SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measured delta D values of the hydroxyl hydrogen in fuchsite (-74 to -61 parts per thousand, SMOW) and tourmaline (-84 and -69 parts per thousand, SMOW) with pegmatitic muscovite and tourmaline suggests that the mineralization was probably caused by modified (O-18-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 (delta(13)C similar to -3.2 +/- 0.7 parts per thousand PDB; delta(18)O similar to +17.9 +/- 1.2 parts per thousand, SMOW). On the basis of the isotopic composition of fluid (delta(13)C approximate to -1.8 +/- 0.7 parts per thousand PDB; delta(18)O approximate to + 13.6 +/- 1.2 parts per thousand, SMOW calculated for the 250-550 degrees 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.