Age and origin of gem corundum and zircon megacrysts from the Mercaderes-Rio Mayo area, South-west Colombia, South America

Potential Sources for alluvial gem corundum and Zircon from the Rio Mayo area near Mercaderes Colombia,, are reviewed, based on U-Pb dating of syngenetic and protogenetic mineral inclusions in corundum samples and on a zircon megacryst. Corundum recovered from the region (approx. 99% sapphire, 1% ruby) commonly shows growth banding, includes colour change stones and exhibits overlaps in colour ranges and inclusion characteristics. This suggests a Contiguous genetic suite. The U-Pb dating used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques. Because of the young ages and low-U contents of the dated zircons, the acquired data required a special statistical treatment. The results from zircon, fluorapatite and allanite-(Ce) inclusions provide a corundum crystallization age of 8 to 11 Ma, in relation to northern Andean Miocene uplift and magmatism. The zircon megacryst gave a Younger crystallization age of c. 0.6 Ma, unrelated to the corundum genesis. Geochemical parameters (trace element and 0 isotope ranges) for Corundum samples suggest a metamorphic/metasomatic origin. The age data rules Out Corundum genesis during the Late Cretaceous ophiolitic generation, but leave open possible later metasomatic interactions with this substrate. The Cr/Ga and Ga/Mg ratios and O isotope range for the corundum fall within the known limits for metasomatic, desilicated felsic/ultramafic 'plumasitic' associations, suggesting a possible parental source. Allanite, extremely rare as an inclusion in corundum elsewhere, may Prove a characteristic inclusion for Rio Mayo corundum. (C) 2008 Elsevier B.V. All Fights reserved.