Chemistry and origin of the Mayo Kila sapphires, NW region Cameroon (Central Africa): Their possible relationship with the Cameroon volcanic line

Abstract

Mineralogical, chemical and geochronological studies constrained the origin of sapphires from Mayo Kila, Northwest Cameroon. The sapphires are mostly blue, with sizes ranging from 2 to 5 mm. The pale blue grains are transparent, whereas, other corundums are transparent to translucent and/or opaque. The sapphires are dominantly euhedral to sub-hedral with few polished lustrous grains, acquired features during moderate to short distance transport from a proximal source rock. Solid inclusions are limited to rutile and zircon.Trace element analysis of sapphires shows significant concentration (in ppm) in some elements: Fe (2208–14,473), Ti (82–1783), Ga (77–512), Mg (0.9–264.9), Cr (b.d.l −168) and V (1.3–82). The other elements (e.g. Sn, Nb, Ta, Th, Zr, Ni, Ce) are generally below 10 ppm. The calculated ratios for some of the selected elements show an extreme variation: Fe/Mg (43–3043), Fe/Ti (2–76), Ti/Mg (1–328), and Ga/Mg (0.4–363). They are dominantly corundum crystallized in alkaline magma (s) with few from metamorphic source (s).Trace elemental features with Hf (13,354–26,238 ppm), Th (4018–45,584 ppm) and U (7825–17,175 ppm), and Th/U (0.39–2.65) found in zircon inclusions are compatible with quantified values in magmatic crystallized zircons. The Cenozoic age (mean of 30.78 ± 0.28 Ma) obtained for zircon inclusions is close to the age of some igneous rocks found within the Cameroon Volcanic Line (e.g. rocks of the Mount Oku: 31–22 Ma), showing the same period of formation. The most probable source of the zircon host sapphires is the Oku Mountain located SW of Mayo Kila.