Detrital mineral morphology and geochemistry: methods to characterize and constrain the origin of the Nsanaragati blue sapphires, south-western region of Cameroon

Morphological and geochemical analyses of blue sapphires from the Nsanaragati alluvial gem deposits in the south-western region of Cameroon help to characterize these sapphires, and constrain their source parameters. The gem corundums range in color from pale to dark blue, with rare orange brown. They range from transparent to opaque, with most of the pale blue sapphires being transparent and the dark blue and orange-brown sapphires being opaque. The shape of the sapphires ranges from equant sub-platy to sub-prismatic and are sub-rounded to well-rounded. Very few of these sapphires are round and frosted. The angular lustrous sapphires are associated with a more proximal origin, while, the sub-rounded lustrous grains are either from a distal origin or more reworked. The round frosted grains may include wind worn products from eolian erosion. The LA-ICP-MS geochemical analysis of the sapphires shows minor and trace element enrichments among the identified elements. The main elemental contributions include, Fe (2496-4643. ppm), Ti (49-7069. ppm), Ca (360-976. ppm), Si (485-687. ppm) and Ga (149-307. ppm) while V (4-41. ppm) and Mg (2-42. ppm) levels are relatively low, and the remaining element values are less than 9. ppm or below their detection limits. Among element ratios, Fe/Mg (96.8-1106.8), Fe/Ti (0.5-81.5), Ti/Mg (14.5-174.1) and Ga/Mg (6.2-89.3) range into high values, while Cr/Ga (<0.01) is very low. The high Fe and Ga and low Mg and Cr values suggest a magmatic origin for these sapphires, possibly xenocrysts from basaltic magmas. They were potentially crystallized from a Ga- and Fe-enriched aluminous alkaline magma. © 2012 Elsevier Ltd.