High-temperature tin-tungsten-bismuth mineralization
Arsenopyrite (white, high reflectance, top right) is the main phase; some is extensively fractured and cemented by chalcopyrite (yellow, centre left), which is altered to covelline and rimmed by brown stannite. Two euhedral cassiterite crystals .(grey, centre) are surrounded by stannite (brown-grey, centre) and covelline (blue, centre, left). Sphalerite (pale grey, right) is intergrown with arsenopyrite. Black areas are polishing pits.
Polished block, plane polarized light, x 80, air
Arsenopyrite and native bismuth. Cornwall, Britain
Arsenopyrite (white, left) is fractured and contains lath-shaped inclusions of native bismuth (cream, high reflectance, centre). Quartz (grey, right) gangue is pitted. To the eye, the reflectance of bismuth is higher than the photomicrograph would suggest.
Polished block, plane polarized light, x80, air
Arsenopyrite, wolframite and covelline. Cligga Head, Cornwall, Britain
Euhedral arsenopyrite (white, high reflectance, centre) is extensively altered to covelline (blue, bottom left). Wolframite (brown-grey, top right) is poorly polished with many polishing pits (black) and shows faint zoning with blue-grey cores and brown-grey margins (right centre); this is difficult to see. Dark grey areas are quartz.
Polished block, plane polarized light, x80, air
Chalcopyrite (yellow, right) is rimmed by a stannite group mineral (pink-brown, left) containing abundant chalcopyrite inclusions (left), but is also altered to covelline (deep blue, top right) along fractures and crystal edges. Small subhedral crystals of cassiterite (dark grey, bottom centre) are enclosed within sphalerite (light grey, bottom centre) and inclusion free stannite (brown-grey, bottom centre). Smaller areas of cassiterite within stannite itself within chalcopyrite are also present (centre right). Anhedral arsenopyrite (white, high reflectance, centre left) has been replaced by sulphides. Sphalerite with chalcopyrite inclusions (centre right) is intergrown with chalcopyrite. Black area is quartz (bottom left).
Chalcopyrite, bornite, chalcocite, haematite and cassiterite. Wheal Jane, Cornwall, Britain
Equant and prismatic cassiterite (dark grey-brown, well polished, centre) is intergrown with fine-grained haematite (light blue-grey, pitted, centre bottom). Chalcopyrite (yellow) is veined by bornite (brown, top right) and chalcocite (light blue, top left). Chalcocite also forms a rim around the oxide minerals. Black areas are polishing pits. A single crystal of cassiterite (top right) is present within the copper-iron sulphides. The cross-cutting relationships show that the alteration sequence is chalcopyrite to bornite to chalcocite.
Polished block, plane polarized light. x80, air
Euhedral cassiterite (light grey, centre left) occurs within euhedral quartz (dark grey, left), which is rimmed by chalcocite and bornite. Chalcopyrite (yellow) is veined by bornite (brown, centre) and by chalcocite (light blue, centre). Chalcocite cross-cuts bornite (bottom centre) and therefore is later. Stannite (grey-green, right centre) is intergrown with chalcopyrite. Black areas are fractures and polishing pits.
Polished block, plane polarized light, x80, air