Gold and base metal vein association

Pyrite, native gold and galena. Clogau Mine, Wales, Britain

Pyrite (light yellow-white, centre) is associated with native gold (yellow, high reflectance, centre left) and galena (grey-white, low reflectance, top right, left centre). The perceived surface colour of pyrite and galena is unaffected by the presence of the gold. Quartz is the main gangue. The gold is 800 fine.

Polished block, plane polarized light. x80, air

Pyrite, native gold, sphalerite and carbonaceous matter. Dolgellau Gold Belt, Wales, Britain

Pyrite (light pink-blue, centre) contains abundant native gold (yellow, centre) inclusions and minor sphalerite (light grey, centre bottom) on its margin. A larger inclusion-free sphalerite (left) is present. A curved lath of carbonaceous matter (medium grey, lower reflectance than sphalerite, centre right) rims a void (black). Quartz, the main gangue, shows faint internal reflections (top left). Black areas are voids and polishing pits. The gold is 800 fine. Specimen number BM077R44. British Museum Collection.

Polished block. plane polarized light, x80, air

Pyrite and native gold. Dolgellau Gold Belt, Wales, Britain

Pyrite (light pink-blue, top left) contains abundant inclusions of native gold (yellow, high reflectance) some in a symplectite like intergrowth (bottom left). The gold is replacing pyrite. Quartz is black or shows strong internal reflections (bottom right). The gold is 780 fine. Specimen number BM077R44. British Museum Collection. The perceived anomalous colour of the pyrite is due to the strong colour and high reflectance of native gold.

Polished block, plane polarized light, x 160. oil

Pyrite, native gold and galena. Clogau Mine, Wales, Britain

A pyrite crystal (green-white) has an outer growth zone of poorly crystalline pyrite with lower reflectance itself overgrown by later euhedral pyrite (centre right). Pyrite carries inclusions of native gold (yellow, high reflectance, top left) and galena (blue-grey, centre). Black areas are quartz. The presence of the gold alters the perceived surface colour of the other minerals and lowers their apparent reflectance. The gold is 800 fine.

Polished block, plane polarized light, x 80, air

Galena, native gold and pyrrhotite. Clogau Mine, Wales, Britain

Galena (blue, centre top) is intergrown with native gold (yellow, high reflectance, centre bottom). Pyrrhotite (brown, hard, centre) crystals are enclosed within galena. Quartz (black) is the main gangue The apparent surface colour and reflectance of galena, and to a lesser extent pyrrhotite are altered by the optical properties of the gold, which is 950 fine. The gold shows fine scratches and minute immersion oil droplets (grey), which almost look like crystallographically controlled fine-grained inclusions.

Polished block, plane polarized light, x80, air

Sphalerite, native gold, pyrrhotite and chalcopyrite. Pamour Mine, Ontario, Canada

Sphalerite (grey-brown, left) encloses a small crystal of pyrrhotite (light brown, centre), chalcopyrite (yellow, centre left) and native gold (yellow, centre bottom). It is very difficult to distinguish the chalcopyrite and gold at this magnification, but to the eye the gold grain has a stronger surface colour. Coarse-grained native gold (yellow, high reflectance, bottom right) is scratched and grows around the sphalerite or infills fractures in the silicates (dark grey).

Polished block, plane polarized light, x 160, air