Relationship between regional deformation and the emplacement of the Dibilo pegmatites (Liptako, West Niger)

Abstract

The Dibilo pegmatites outcrop in the Paleoproterozoic Tera-Ayorou pluton, in the Niger Liptako area (NE portion of the Man shield of the West African Craton). The main objective of this study is to establish a relationship between the Dibilo pegmatites and their regional deformation context. The methodology implemented is based on a structural analysis of the pegmatites and their enclosing rocks using field data.The geological formations of the Dibilo pegmatites area (granodiorite, migmatite, biotite granite, two-mica granite, two-mica-garnet granite, and muscovite granite) have been affected by two major deformation phases D1 and D2. The first phase D1 includes two episodes of deformation D1a and D1b. The D1 phase is ductile and corresponds to an NW-SE average shortening (N135°). It is responsible for the NE-SW-oriented S1 regional foliation, microfolding, and N–S-oriented sinistral shear zones. The ductile to semi-ductile, NNW-SSE (N160°) shortening D2 phase is associated with the displacement of dextral N100° to N110° shear zones and sinistral N25° to N45° shear zones.The various orientations of Dibilo pegmatites (N25–N45°, N90–N130°, N–S) can be classified into three structural groups: the D1-deformed pegmatites; the D2-deformed pegmatites; and the undeformed pegmatites.The D1 deformed pegmatites are barren (type I), they include folded and boudinaged pegmatites hosted by the granodiorite and migmatite. The pegmatites deformed by D2, are the sheared pegmatites, presenting an early foliation N100–N110° or a schistosity of late fracture N100–N110°. These D2-deformed pegmatites are zoned and mineralized with spodumene and molybdenite and are localized at the foliated biotite granite and are classified as Li, Ta, W pegmatites (type II), Mo, Ta pegmatites (type III), and Mo, Ta, Be, and native Au pegmatites (type IV).Undeformed pegmatites of variable orientation are barren and abundant in migmatitic zones. The barren pegmatites are a mixture of folded, boudinaged, and undeformed pegmatites.The fracture-controlled emplacement mechanism is best suited to explain the emplacement of the Dibilo pegmatites about the displacement of dextral and sinistral shear zones in a simple shear regime.