ISSN:
1432-1866
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract. 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 (δ18O SMOW) of emeralds shows a strong enrichment in 18O 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 (δ18O=+13 to +14‰ SMOW) and show enrichment in 18O. The δ18O values of quartz, ranging from +15.1 to +19.1‰ SMOW, are also high (+16.9±1.4‰; 1σ, n=7). The mean δD of channel waters measured from emerald (−42±6.6‰ SMOW) and that of fluid calculated from hydrous minerals δDcalculated (−47±7.1‰ SMOW) are consistent with both metamorphic and magmatic origin. However, the close similarity between the measured δD values of the hydroxyl hydrogen in fuchsite (−74 to −61‰ 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 (δ13C∼−3.2±0.7‰ PDB; δ18O∼ +17.9±1.2‰ SMOW). On the basis of the isotopic composition of fluid (δ13C≈−1.8±0.7‰ PDB; δ18O≈+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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s001260050032
