Tin-niobium mineralization in granites

Pyrite, rutile, columbite and zircon. Greisenized granite. Ririwai, Nigeria

Rutile (pale blue-grey, centre right) encloses slightly lower reflectance columbite (brown-grey, centre right) cores in a characteristic association. The blue surface colour of rutile is associated with its deep blue internal reflections. Subhedral to euhedral pyrite (yellow, high reflectance, centre) surrounds rutile and shows alteration to limonite (poorly polished areas of pyrite, bottom right). Zircon (medium grey, slightly higher reflectance than quartz) is rounded to subhedral and two grains lie within rutile crystals (centre right) or discretely between pyrite and rutile (centre bottom). Although the reflectance of zircon is lower than columbite, it is higher than the silicates. The main silicate is quartz (left) accompanied by phyllosilicates (poorly polished, lower reflectance, centre top). Black areas are polishing pits.

Polished block, plane polarized light. x 80, air

Columbite, pyrite and zircon. Greisenized granite. Ririwai, Nigeria

Two columbite crystals (brown, centre left and right) have small zircon crystals (medium grey) growing around them. A discrete zircon (centre top) with a thin pyrite rim lies between the columbites. Fine-grained secondary pyrite (yellow, high reflectance, centre) infills voids in fluorite (dark grey with polishing pits, centre). Chlorite (green-grey, bottom centre) and quartz (medium grey, right) are the main silicates. Finely disseminated haematite is responsible for the 'high' reflectance of chlorite. Black areas are polishing pits.

Polished block, plane polarized light, x 160, air

Cassiterite and zircon. Microclinized granite. Ririwai, Nigeria

Cassiterite crystals (light grey, highest reflectance, many polishing pits) are twinned (centre right) as shown by their bireflectance variations. Cassiterite is intergrown with subhedral to euhedral zircon (medium grey), which is zoned with thin lower reflectance outer rims (centre left, left). A little pyrite (pale yellow, high reflectance, bottom) is present. The main silicates are quartz (right) and feldspar (bottom left). Black areas are polishing pits.

Polished block. plane polarized light, x 80, air

Columbite, rutile, zircon and xenotime. Greisenized granite. Ririwai, Nigeria

Columbite (brown-grey, centre) is enclosed within a higher reflectance rutile (light grey, with fractures). A subhedral zircon (dark grey, centre bottom) is intergrown with both phases. Discrete, euhedral to subhedral rutile and zircon which shows internal reflections (top right) have grown within lath-shaped chloritized mica (blue-grey, top right). A euhedral zircon (right centre) has a rhombic euhedral xenotime growing from it into chlorite, but the slight differences in reflectance (xenotime is lower) are difficult to see at this magnification. Quartz and chlorite (grey-green, bottom left) are the main silicates. Black areas are polishing pits.

Polished block. plane polarized light. x 160, air

Zircon, thorite and xenotime. Albitite. Ririwai, Nigeria

The core of a large zircon crystal (light grey, highest reflectance, centre left) or aggregate of zircon crystals has been extensively replaced by mica (centre). The relict rim is both well crystalline (left) and vuggy (centre right). Thorite crystals (medium grey, reflectance higher than the silicates, faint internal reflections) have grown upon the relict zircon (top right corner of zircon and bottom right comer). A single rhombic-shaped euhedral crystal of xenotime (bottom right corner of zircon) has a similar reflectance to thorite and also shows internal reflections. Albite and mica (poorly polished, top left) are the main silicates. Black areas are polishing pits.

Polished block. plane polarized light, x 160, air

Zircon, rutile and columbite. Greisenized granite. Ririwai, Nigeria

A typical association of ore and accessory minerals in altered mica. Zircon crystals (dark grey, centre left, right centre) are euhedral to subhedral and show faint internal reflections and zoning with lower reflectance outer zones. Rutile grains (light grey, highest reflectance) enclose lower reflectance cores of rounded columbite (brown-grey, centre top). An aggregate (centre top left) comprises two rounded zircon crystals (grey, low reflectance) surrounded by rutile on their left and columbite on their right. The main matrix is altered brown mica. Black areas are polishing pits.

Polished block, plane polarized light, x 160, air