Greisen. Ririwai, Nigeria

The greisen comprises quartz and lithium micas, together with minor amounts of fluorite, chlorite, sericite and topaz.

Major Minerals:

Cassiterite, wolframite, columbite, sphalerite, galena

Minor Minerals:

Zircon, thorite, monazite, molybdenite, chalcopyrite

Trace Minerals:

Include stannite group minerals, marcasite, pyrrhotite, ilmenite, TiO2, minerals, xenotime, REE minerals, coffinite


The accessory minerals form discrete coarse-grained crystals or aggregates comprising intergrowths of cassiterite, zircon, thorite, TiO2 minerals and molybdenite associated with fluorite in altered micas. Zircon is embayed and corroded when in association with fluorite. Zircon has successive overgrowths of thorite and xenotime. Thorite has fine-grained haematite inclusions that give it red internal reflections, and has pyrite or haematite rims. Monazite forms discrete lath-shaped crystals and is altered to bästnasite group minerals. Subhedral columbite crystals form the cores to Nb-rich TiO2. mineral (rutile) overgrowths. Primary sulphides, other than molybdenite, are not associated with the accessory minerals. They comprise separate intergrowths of galena and sphalerite


Ixer et al., 1987; Pointer et al., 1988a,b