Sapphire-ruby characteristics, West Pailin, Cambodia : clues to their origin based on trace element and O isotope analysis

Trace element (Particle-induced X-ray emission) and oxygen isotope (ß18O) analyses on West Pailin, Cambodian sapphires and rubies further constrain the geochernical characteristics of this basalt-derived, bimodal (two corundum types) gem deposit. The results confirm a clear distinction between magmatic origin sapphires (Fe 2300-8000ppm, Ti 15-1800ppm, Cr below detection, Ga 160-260ppm; Cr/Ga < 0.01; ß18O) 7.0-7.8 0/00) and metamorphic-origin sapphires and rubies (Fe 3300-9130ppm, Ti 60-3600ppm, Cr 160-5300ppm, Ga 28-46ppm; Cr/Ga 3.6-155; ß18O 4.0-4.9 0/00). Comparisons with a similar Australian bimodal corundum deposit from the Barrington Tops, basaltic gem field show some common features, although the West Pailin magmatic suite is more depleted in Fe and noticeably higher in ß18O values. The metamorphic suite is more enriched in Ti and Cr and distinctly lower in its ß18O range than its Barrington Tops counterpart. This allows geographic distinctions to be drawn between these bimodal corundum suites and indicates differences in the geological processes by which corundum crystallised under the two regions.