Accessory mineralogy of the Ririwai biotite granite, Nigeria, and its albitized and greisenized facies

Abstract

The Ririwai granitic ring complex has suffered a sequence of post-crystallization alteration and mineralizing processes including albitization and, close to the Ririwai Sn–Zn lode, microclinization, greisenization, and quartz-cassiterite-sulphide veining. Within these rock types the opaque and accessory minerals of Zr, Hf, U, Th, Nb, Ta, Ti, Sn, and REE have a number of styles of occurrence. They occur in coarse-grained discrete crystals associated with magmatic quartz or as aggregates of small crystals, which are often aligned along the cleavage of micas, or form complex intergrowths outside the micas. Late stage, possibly magmatic processes within the biotite granite have precipitated Zr, U, Pb, Nb, Ta, Ti as complex intergrowths of uranium-bearing plumbopyrochlore, columbite, ilmenite, TiO2 minerals, and zircon. Minor to trace amounts of monazite, uranothorite, and cassiterite are also present in these granites. Albitization has resulted in the crystallization of coarse-grained, early haematite intergrown with slightly later magnetite, ilmenite, and columbite. Both columbite and ilmenite are highly zoned with respect to their iron and manganese, but not their niobium and tantalum contents. Zircon is often strongly zoned with uranium-rich cores and hafnium-rich rims and shows partial replacement and evergrowth by uranothorite and xenotime. Cassiterite, columbite, zircon, and Fe, Mo, Pb, and Zn sulphides are the main non-silicates developed during greisenization, although thorium too, is mobile, crystallizing mainly within uranothorite. Columbite has a wide range of compositions with tantalum-rich (up to 27 wt% Ta2O5) and tungstenach (up to 14·5 wt% WO3) varieties. Cassiterite is commonly colour zoned and this can be related to its iron and niobium content. Zircon shows replacement by, or enclosure within, uranothorite, which itself has later xenotime margins. The textural and chemical evidence suggests that each alteration process essentially dissolved, or replaced, earlier phases and then reprecipitated them; but with compositions compatible with the new fluids. The dissolution of small accessory phases within, and the alteration of biotite, initially to chlorite, appear to have played important roles in the liberation of rare elements by these fluids.