U–Pb Zircon Age Constraining the Source and Provenance of Gem‐Bearing Late Cenozoic Detrital Deposits, Mamfe Basin, SW Cameroon

Abstract

Zircons and other heavy minerals (corundum, rutile, ilmenite, magnetite, sillimanite) are identified in the Nsanaragati gem corundum placer deposit, in the western part of the Mamfe sedimentary basin, SW Cameroon. These alluvial minerals have different morphological characteristics and zircons, in particular, vary mostly in colour and shape. They are reddish, brownish, yellowish, pink or colourless. These minerals form rounded and sub-rounded alluvial grains, prismatic, pyramidal or dipyramidal crystals. Reddish zircons retain their original crystallographic shape. Trace element and U-Pb isotopic geochemical analyses of these reddish zircons, using the LA-ICP-MS method give significant Hf (4576-6334 ppm), Th (46-1565 ppm) and U (66-687 ppm) contents, with Th/U ratio ranging from 0.6 to 3.0. The 206 Pb/ 238 U corrected mean age gave 12.39 Ϯ 0.55 Ma, which characterizes an Upper Cenozoic (Serravallian) magmatic event. The zircons are probably sourced from a magmatic field in the South eastern boundary of the Cross River Formation. The Cameroon Volcanic Line of basaltic and alkaline lavas and intrusions which lie east of the Mamfe Basin mostly range in age from 37 Ma to <1 Ma. The zircons may also relate to the Mount Bambouto plateau lavas which lie northeast of the Mamfe sedimentary basin and have an eruptive age range of 21-14 Ma. The oldest Nsanaragati reddish zircon ages overlap within error with the end stages of the Bambouto eruptions. This eruptive or a related episode provides a potential source for megacrystic reddish zircons within the Nsanagarati placer deposit.

U-Pb Zircon Age: Preliminary Data Evaluating the Earth History Recorded by Two Basement Rocks (Granitic Pegmatite and Mica-Schist) in Mamfe Basin (Sw Cameroon, Central Africa)

“…Bab: zircon from Babi mica-schist; c: core; r: rim; Prop_2SE: propagated 2 standard error Table 2: Quantified trace element abundance and U-Pb age for zircon crystals from Babi mica-schist in the west of the Mamfe Basin. [4,16,32,35,36,53]. The relative low U (16 ppm) and Th (18.3 ppm) in Ot3-r cannot relate the crystallization of this crystal to mantle, as these elements content in Ot3-c of this same crystal, although relatively low, is more than values found mantle source zircon.…”

“…The crystallization age of zircons grains ranges from 11 to 1959 Ma with their sedimentation periods suggested being post-Serravallian, postEocene, post-Oligocene, and post-Miocene, post-Lower-Cretaceous, post-Neoproterozoic, and post-Paleoproterozoic [32,35]. The source of those clasts are suggested to be around the Cameroon Volcanic Line, Benue Trough, and part of the basement rock of the Mamfe Basin [24,32,35].…”

“…The lithified sediments are locally overlain by recent sediments sorted from volcanic, sedimentary, plutonic, and/or metamorphic rocks [24]. A fraction of these sediments notably those in the western part of the basin encloses economic viable heavy minerals (e.g., zircon, rutile, chrysoberyl, amethyst, corundum) from diverse origins [24,[32][33][34][35][36]. Chrysoberyl, zircon, and part of the corundum were crystallized in magmatic plutonic rocks [32,[34][35][36], whereas rutile grains were mainly grown in metapelitic metamorphic protoliths [33].…”

“…A fraction of these sediments notably those in the western part of the basin encloses economic viable heavy minerals (e.g., zircon, rutile, chrysoberyl, amethyst, corundum) from diverse origins [24,[32][33][34][35][36]. Chrysoberyl, zircon, and part of the corundum were crystallized in magmatic plutonic rocks [32,[34][35][36], whereas rutile grains were mainly grown in metapelitic metamorphic protoliths [33]. The crystallization age of zircons grains ranges from 11 to 1959 Ma with their sedimentation periods suggested being post-Serravallian, postEocene, post-Oligocene, and post-Miocene, post-Lower-Cretaceous, post-Neoproterozoic, and post-Paleoproterozoic [32,35].…”

Greyish-Black Rutile Megaclasts from the Nsanaragati Gem Placer, SW Cameroon: Geochemical Features and Genesis

“…Mineralogical studies of the Nsanaragati gem corundum placer in the south western region of Cameroon, show rutile, zircon and ilmenite enrichment within the heavy minerals (Kanouo et al, 2012). No detailed study has been performed in the rutile grains.…”