U-Pb ages for zircon grains from Nsanaragati alluvial gem placers: its correlation to the source rocks

The Mamfe Basin is located in the SW of Cameroon and is an extension of the much bigger Benue Trough in the SE of Nigeria. Along the Asenem River and its tributaries in the western part of the Mamfe Basin (close to the border with Nigeria) gem placers yielding big zircon grains were found in recent river sediments close to Nsanaragati. In order to determine the source area and to establish a possible correlation between the zircons found in the Nsanaragati placers and rocks surrounding the Mamfe Basin 56 detrital zircon grains were analysed regarding their U–Pb ages and selected trace element contents by LA-ICP–MS techniques. Possible source areas are rocks from the Benue Trough in the West and from the Cameroon Volcanic Line (CVL) in the SE of the study area. Based on microscopic analyses it was possible to distinguish two groups of zircons: reddish and non-reddish ones, where the latter group comprises color variations from brown to orange, yellow to even colorless. In general, the detrital zircons show high hafnium values (4576 to 12565 ppm) and very variable thorium (7.8 to 1565 ppm) and uranium values (13.4 to 687 ppm). The Th/U ratios vary from 0.4 to 2.3, allowing correlations for some zircon grains to kimberlitic, granitic or syenitic affinities. It was also possible to distinguish zircon grains crystallised in mafic mantle-derived melts from those crystallised in felsic melts, e.g. from continental rift-related magmatic systems. In general, the U–Pb zircon ages obtained range from 11.7 to 1949 Ma. All zircons of the reddish group yielded almost similar ages resulting in a Concordia age of 12.4 Ma (Serravallian), an age unknown from the Mamfe Basin so far. The group of non-reddish zircons showed various ages ranging from Serravallian to Orosirian. It was possible to correlate the youngest ages with rocks known from intrusions along the CVL, dated with K/Ar or Ar/Ar methods. The most probable sources were Mount Bamenda and Mount Bambouto in the east of the Mamfe Basin. Cretaceous ages are interpreted as re-recycled clastic sediments whose original source had been rocks in the south and the north of the Benue Trough and who had been eroded and deposited within the catchment area of the Asenem river system in the Mamfe Basin in post-Cretaceous times. The oldest ages are assumed to represent the pan-African and pre-pan-African basement of the Mamfe basin.