Cassiterite, zircon and thorites in greisenized granite

Sphalerite, cassiterite, thorite and zircon. Greisenized granite. Ririwai, Nigeria

Sphalerite (light-grey, left) is inclusion-free and has a reflectance slightly greater than cassiterite (light-grey, centre right). Two irregular thorite crystals (dark grey, centre bottom, one with a central polishing pit, bottom centre) have low reflectance; higher reflectance areas are due to fine-grained haematite inclusions. Zircon crystals are euhedral (bottom left) and have a reflectance greater than thorite but less than cassiterite. Quartz (dark grey, well polished, bottom left) and micas (lower reflectance and poorly polished) are the main silicates. Black areas are polishing pits, some of which contain grinding grit (centre top). At this magnification the identification of the accessory phases is difficult.

Polished thin section, plane polarized light, x40, air

Cassiterite, thorite and zircon. Greisenized granite. Ririwai, Nigeria

Cassiterite crystals (light grey) show very faint bireflectance variations due to twinning (top centre). Small zircon crystals (medium grey) are enclosed within, and surround, cassiterite. Discrete zircons (centre, centre top left) show zoning, with thin lower reflectance zones about a higher reflectance core. Thorite (bottom centre) has low reflectance with haematite-rich areas showing slightly higher reflectance. Quartz is the main silicate with some feldspar (bottom right) showing red internal reflections due to very finely disseminated haematite. Much of the mica is poorly polished and shows parallel striations due to microprobe damage. Black areas are polishing pits, often infilled with grinding grit (top right and left). The angular shape, low reflectance that is similar to rutile and restricted size distribution of the grit are all characteristic. Brass (yellow, high reflectance, centre) is present in a triangular-shaped pit.

Polished thin section, plane polarized light, x80, air

Cassiterite, zircon and thorite. Greisenized granite. Ririwai, Nigeria

Cassiterite crystals (light grey, right) show bireflectance along twin planes (top centre). Enclosed zircons (medium grey, centre and bottom right) show strong zoning with a lower reflectance zone about a higher reflectance core. Discrete euhedral zircons are also strongly zoned (bottom left). Thorite crystals (centre, centre bottom) have lower reflectance than the zircon cores and are characterized by faint red internal reflections due to very fine-grained haematite inclusions. They occur about the main cassiterite aggregate. Brass (yellow, high reflectance, centre) and polishing pits (black) are present, some of which carry angular grinding grit (top left).

Polished thin section, plane polarized light, x 160, air

Cassiterite, zircon and thorite. Greisenized granite. Ririwai, Nigeria

Under oil immersion the twinning of cassiterite is distinct (top centre), as are faint brown-orange internal reflections (centre right). Although the light-coloured internal reflections of zircon are clear, its zoning is less apparent than in air (left centre). It is very difficult to see any surface features of the thorite but its presence is indicated by its strong orange-red internal reflections (centre), due to fine-grained haematite within it. The zircon above the upper thorite (centre top) encloses haematite (reflectance higher than that of cassiterite). The polishing pit (upper left) has trapped an air bubble (dark circle) in the immersion oil. Yellow high reflectance areas arc brass (bottom centre).

Polished thin section, plane polarized light. x 160, oil

Thorite, cassiterite and zircon. Greisenized granite, Ririwai, Nigeria

Two aggregates of thorite crystals have very strong red-orange internal reflections due to very fine-grained haematite inclusions. This is the characteristic feature of thorite from these rocks. Although few surface features are discernible, the core (oriented cast-west) of the right hand aggregate is relict zircon. Cassiterite (light greys) is euhedral to subhedral and twinned (top centre). The light yellow, poorly polished area (bottom right) is brass within a polishing pit. A zircon with strong internal reflections lies below it.

Polished thin section, plane polarized light, x 160, oil