Quartz-Rich Fault Rocks as Potential High Purity Quartz Source (I) in a Sequence of the Central Cameroon Shear Zone (Etam Shear Zone): Geology and Structure

The Etam shear zone is the NNE-SSE to NE-SW trending fault zone with coexistence of a sinistral shearing mylonitic corridor and a brecciated quartz vein. The mylonitic zone is evidence of progressive deformation. Primary minerals include Qtz1 + Kfs1 + Pl1 + Bt1. These minerals increasingly and gradually reduce from the wall rock towards the shear zone center. Quartz recrystallization also gradually increases in the same direction. Microstructures observed include: recrystallization, sub-grain boundary, myrmekites and undulose extinction. Mica-chlorite-epidote assemblages define the greenschist assemblage, but the observation of amphibole in the mylonite and the presence of flame perthite in feldspar suggest a transitional greenschist-amphibolite metamorphic condition in the Etam Shear Zone. The brecciated quartz vein has dimensions of about 2.5 × 0.5 km and shows several quartzitic blocks, mainly composed of milky to transparent quartz clasts and a siliceous matrix. Quartz clasts are pure and the observation of thin sections of these quartz under polarized microscope shows grains free of microstructures or displays small micro quartz veins and rare feldspar inclusions. These are some criteria, which can lead to a preliminary assessment of the purity of quartz in Etam area.