Pseudomorphism of alkali feldspar to biotite from the alkaline nana complex granite in Cameroon

Alkaline complexes all over the world generally are enriched in high–field strength elements and rare earth elements. This enrichment can be accounted for by magmatic processes, but, many of these complexes display evidence of hydrothermal alteration. In Cameroon, several alkaline complexes have been the subject of numerous studies, none of which focused on hydrothermalism, hydrothermal alteration or REE mineralization. One of these is the Nana plutonic-volcanic complex, situated in the Tikar plain, at the Central part of the Cameroon Line in Central Africa. Granitic rocks widely dominate over other plutonic and volcanic rocks. The granites display granophyric and micrographic textures, and under optical microscope and SEM show mineralogical evidence of hydrothermal processes. They have been affected by alkali feldspar albitization and argillization processes, known to be associated to the late – to post – magmatic stages of alkaline rocks. In addition, an unusual transformation process, marked by the partial to total replacement of alkali feldspar by biotite of annite and silicic annite composition (XFe = 1 ; Si = 5.6–6.7 apfu…) was observed and a reaction describing the replacement has been proposed. The above characteristics are compounded with the low variation of SiO2 (74–76 wt%) opposed to the large variation of high-field strength elements (HFSE; Y: 36.2–1540 ppm, Zr: 181–1070 ppm) and LREE (La : 58–2210 ppm; Ce: 90–2114 ppm; Nd: 41–1810 ppm) of whole rock chemical compositions; the high content of Fe2O3 (up to 14.5 wt%) of some alkali feldspars ; and the accessory minerals (apatite, zircon, monazite, fluorite and other REE-minerals) filling microcracks of preexisting magmatic minerals, are consistent with the major role played by fluids circulation in the studied granites. According to the feldspar alteration features, the quantitative analyses of accessory minerals and secondary biotite, we found that the circulating fluids contain at least OH−, Cl−, F− and SO2− acting as ligands for the transport of Zn, Ca, Na, Si, Zr, P, Y and REE (La, Ce, Nd). We attribute the presence of Cl and F to the chloride and fluorite salt composition of the circulating fluids. Isotopic and fluid inclusions studies are needed to better constrain the nature of the circulating fluid.