Geological study of sapphire occurrences in Mayo Kila and Mbiame areas (North-West Region, Cameroon)

Abstract

Mayo Kila and Mbiame as well as other parts of Mount Oku are known to host alluvial sapphire occurrences. These deposits have been characterized in order to trace the source rock(s) of their host materials within geological formations in the local and regional settings. Alluvial and geological prospections were carried out in the areas, followed by laboratory analyses such as particle size distribution for alluvial materials, LA-ICP-MS for sapphire grains, and ICP-AES and ICP-MS for surrounding rocks. Alluvial prospection showed that, the Bewende and Mbven Rivers in Mayo Kila and Mbiame respectively are the main alluvial sapphire concentration areas. These sapphires occur in association with other minerals such as zircon, apatite, tourmaline, hornblende and magnetite. Particle size distribution analysis show that the detrital materials are made up of fine to coarse sand and cobble, pebble gravel sizes. The sediments are poorly to very poorly sorted, indicative of a close source. Sapphires from these placers are mostly blue, with sizes that range from 2 to 5 mm. They are dominantly euhedral to sub-hedral with angular to sub rounded grains. Those from Mbiame are euhedral with few sub-rounded to rounded polished lustrous grains, features acquired during moderate to short distance transport from a proximal source. Primary relicts of some crystals are displayed by platy and barrel shaped grains. Their surfaces show irregular corrosion patterns, indicative of disequilibrium with their carrier magma.Geochemical analyses of the sapphires by LA-ICP-MS show trace element concentrations such as Fe (2,208 - 14,473 ppm), Ti (82 - 1,783 ppm), Ga (77- 512 ppm), Mg (upto 264.9 ppm), Cr (upto 168 ppm) and V (upto 82 ppm). Calculated ratios show extreme variations, with Fe/Mg (43 -3,043), Fe/Ti (2 - 76), Ti/Mg (1 - 328), and Ga/Mg (0.4 - 363) values that express affinity to magmatic corundum. Solid inclusions in these sapphires are limited to rutile and zircon. Their zircon inclusions are characterized by hafnium (13,354 – 26,238 ppm), thurium (4,018 – 45,584 ppm), uranium (7,825 –17,175 ppm) and Th/U ratios between 0.39 and 2.65. These values are similar to those of zircons crystallized in a crustal environment, in highly evolved melts close to syenitic or granitic compositions. Cenozoic age (mean of 30.78 ± 0.28 Ma) obtained from these zircon inclusions is within the range of ages obtained from Mount Oku volcanic rocks. Both areas are characterized by plutonic and volcanic rocks. The plutonic rocks in Mayo Kila are two mica granites, biotite granites, diorites and monzonite. Their composition is typical of High-K calc-alkaline to shoshonitic and an alkali-calc to calc-alkalic series. Most of the samples are metaluminous with affinity to I-type granites while only one biotite granites sample is peraluminous with affinity to S-type granite. They are formed by partial melting from different protoliths at crustal level. These protoliths are metapelites, metagreywacks and metatonalites to metaamphibolites. These rocks were formed in a syn- to post-collisional tectonic setting. The mafic volcanic rocks in both areas express an alkaline affinity with magmas that portray a residual mantle source, formed from the partial melting of garnet peridotite, and are similar to alkaline basalts from an intracontinental rift. The above data show that the sapphires of the study area are magmatic, with parent rock close to syenitic or monzonitic compositions, probably the Ntumbaw syenites, formed at crustal level. They were transported to the surface by mantle formed alkaline basaltic magma through the numerous fractures that characterize the Cameroon Line and the Central Cameroon Shear Zone.