Carbonate-hosted base-metal mineralization

Bornite and chalcocite. Gortdrum, Ireland

Bornite (pink-brown) is complexly intergrown with white copper sulphide, chalcocite (centre). Anhedral quartz (dark grey, top right) and euhedral dolomite (light grey, centre) are the main gangue phases. Dolomite rhombs show bireflectance from lighter grey (centre) to darker grey (top centre), but have higher reflectance than quartz. Black areas are polishing pits.

Polished block, plane polarized light, x 80, air

Bornite and chalcocite. Gortdrum, Ireland

Bornite (brown-pink) and white copper sulphide, chalcocite, show a typical fine-scale symplectite-like intergrowth. The gangue is trigonal carbonate showing bireflectance from lighter to darker grey (top right) and minor quartz (dark grey, centre left).

Polished block, plane polarized light, x 80, air

Chalcocite, bornite and TiO₂ mineral. Gortdrum, Ireland

Bornite (brown-pink) and white copper sulphide, chalcocite, show a coarse symplectite-like intergrowth. Veinlets of a TiO₂ mineral (light grey, bottom), associated with fine-grained bornite and chalcocite, cross-cut the gangue. This comprises rhombic dolomite, showing bireflectance from lighter to dark grey (centre, centre left). Black areas are polishing pits.

Polished block, plane polarized light, x 80, air

Pyrite, jamesonite, sphalerite and galena. Ireland

Pyrite (pale yellow) is rhythmically banded with the growth bands picked out by very fine gangue inclusions. Pyrite has nucleated on sphalerite (light grey, top right) and on galena (blue-white, bottom left). Pyrite has been fractured and recemented by calcite (dark grey, bottom left), which shows multiple twinning (top centre) and faint light internal reflections. A blue-white acicular to tabular sulphosalt (centre) has a similar reflectance to galena, but is better polished and is highly anisotropic; it has the optical properties of jamesonite.

Polished block, plane polarized light, x 110, air

Uraninite, chalcocite, bornite and chalcopyrite. Gortdrum, Ireland

Rhythmically banded uraninite (light grey, centre) is extensively replaced by bornite (pink, centre left) and blue-white copper sulphide, chalcocite (centre bottom). The replacement has proceeded along radiating fractures and selected growth bands within the uraninite (centre). Silicate (bottom right) shows light coloured internal reflections and carries small intergrown chalcopyrite bornite (orange-yellow, bottom right) inclusions.

Polished block, plane polarized light. x 180, oil

Sphalerite, pyrite and jamesonite. Ireland

Concentrically banded pyrite (yellow, centre) is intergrown with sphalerite (light grey, bottom left). Acicular crystals of jamesonite (blue-grey, centre top) are found within calcite (dark grey) which shows bireflectance (top right).

Polished block. plane polarized light, x 40, air