Supergene enrichment of copper ores
Digenite, bornite, haematite and chalcopyrite. English Lake District, Britain
Digenite (blue, top left) shows minor replacement by covelline (deep blue) along cleavage and small fractures. More extensive replacement is shown by bornite (brown-pink, centre) which contains relict digenite. Minor amounts of chalcopyrite (yellow, right centre) occur on the edge of bornite but are difficult to see. Two distinct generations of haematite are present. Haematite laths (light blue, hard, centre bottom) occur within digenite and bornite, whereas most haematite (green-grey) is very fine-grained and replaces bornite along grain edges (bottom centre). Quartz is dark grey.
Polished block, plane polarized light, x40, air
Digenite (blue) is replaced along cleavage and fractures by covelline (deep blue, centre left) and about its margin by bornite (brown-pink, centre) and wittichenite (cream-white, centre right). Much of the wittichenite is associated with fine-grained haematite (green-grey, centre top), with fractures cutting digenite having a central haematite core and wittichenite margins (centre left). A small intergrowth of chalcopyrite (yellow) and bornite (brown) is crystallographically controlled (top centre). Quartz is dark grey, black areas are polishing pits.
Polished block, plane polarized light, x 40, air
Digenite (light blue, top) is replaced along cleavage by covelline (dark blue, centre left) and along grain boundaries by bornite (orange-brown, centre). A vein cross-cutting digenite is infilled with wittichenite (white, top left). Fine-grained haematite (green-grey) replaces copper and copper-iron sulphides (especially bottom right). Very fine-grained banded haematite (green-grey, low reflectance, centre) is associated with quartz (dark grey). Minor amounts of chalcopyrite (yellow, centre bottom) are surrounded by a haematite rim. Black areas are polishing pits. Quartz (dark grey) is the main gangue phase.
Polished block, plane polarized light, x 80, air
Covelline, haematite, bornite, digenite and wittichenite. English Lake District, Britain
Digenite (light blue) is extensively altered about its margins to covelline (dark blue), some of which shows reflection pleochroism from dark to light blue (bottom left), and only the cores of digenite crystals remain unaltered (centre). Digenite is also replaced by bornite (brown, top left), by wittichenite (cream-white, top left) and by fine-grained haematite (green-grey, right), which also replaces all the other sulphides. Dark grey areas are quartz. Black areas are polishing pits.
Polished block, plane polarized light, x80, air
Covelline, bornite, haematite, wittichenite and arsenopyrite. English Lake District, Britain
Digenite has been totally replaced by fine-grained covelline which shows reflection pleochroism from dark to light blue (centre). Only the cleavage of digenite shows its former presence. Bornite (orange-brown) and minor wittichenite (cream, top centre) surround covelline and are rimmed by fine-grained haematite (green-grey, top left). A euhedral crystal of arsenopyrite (white, high reflectance, centre) occurs within covelline. Dark grey areas are quartz crystals (top left). Black areas are polishing pits.
Polished block. plane polarized light. x80, air
Pyrite, bornite, haematite and covelline. Cornwall, Britain
Pyrite (pale yellow-white, bottom) is intergrown with bornite (brown-orange, left). This has altered along its grain edges to covelline (blue), which shows reflection pleochroism from light to dark blue (right centre). Rhythmically banded fine-grained haematite (green-grey, centre) surrounds the sulphides and shows slight differences in reflectance due to grain size variations (centre right). Quartz is dark grey.
Polished block, plane polarized light, x 80, air