Massive volcanogenic sulphides

Chalcopyrite, pyrite and sphalerite. Neves-Corvo, Portugal

Poorly crystalline collomorphic pyrite (brown-yellow, centre) is intergrown with well crystalline pyrite (light yellow-white, left) and both are extensively replaced by chalcopyrite (yellow), which is intergrown with sphalerite (light grey, bottom centre). The variations in reflectance and body colour of the pyrite are due to differences in grain size and degree of chalcopyrite replacement. The fine-grained pyrite has been replaced more extensively than coarse-grained pyrite, and hence has a lower reflectance.

Polished block, plane polarized light, x80, air

Chalcopyrite, pyrite and sphalerite. Neves-Corvo, Portugal

A relict of banded poorly crystalline (brown-yellow, lower reflectance, centre) and well crystalline (light yellow-white, centre top) pyrite has been fractured and extensively replaced by chalcopyrite (yellow), which carries rare small sphalerite (light grey, bottom right) inclusions. The brownish poorly crystalline pyrite is the so-called melnikovite pyrite. Black areas are polishing pits. Banding within the relict pyrite (centre left) shows little sign of disruption, suggesting that replacement was not accompanied by significant fracturing.

Polished block, plane polarized light, x 80, air

Pyrite, chalcopyrite, tetrahedrite group mineral, sphalerite and arsenopyrite. Neves-Corvo, Portugal

Coarse crystals of pyrite (light yellow-white, centre) and poorly crystalline pyrite (lower reflectance, light brown-yellow, centre right) have been replaced by chalcopyrite (yellow). Sphalerite (medium grey, left centre and right centre) and tetrahedrite (blue-grey, higher reflectance, centre top) are intergrown with each other and with chalcopyrite. Small euhedral rhombs of arsenopyrite (white, high reflectance, centre top left, centre top) are rare but occur within tetrahedrite. Dark grey area is resin, black areas are polishing pits.

Polished block, plane polarized light, x80, air

Chalcopyrite, pyrite, stannite and sphalerite. Neves-Corvo, Portugal

Euhedral to subhedral pyrite crystals (light yellow, high reflectance, top right) occur within chalcopyrite (yellow). Sphalerite (light grey, top left, bottom right) has lower reflectance and less surface colour than stannite (light brown, centre). Both are intergrown with chalcopyrite and locally with each other (centre left). Dark grey areas are silicate.

Polished block, plane polarized light. x80, air

Chalcopyrite, pyrite, sphalerite and stannite. Neves-Corvo, Portugal

Subhedral to euhedral pyrite (light yellow-white, high reflectance, centre top) is partially replaced by chalcopyrite (bottom left). Chalcopyrite (yellow) is intergrown with sphalerite (light grey, centre) and stannite (brown, centre, left). Intergrowths between sphalerite and stannite are complex but much stannite forms rims between sphalerite and chalcopyrite (centre). Dark greys are rhombic carbonates (right centre) and silicate (darker grey, centre bottom). Black areas are polishing pits.

Polished block, plane polarized light, x 160, air

Chalcopyrite, pyrite, sphalerite and tetrahedrite group mineral. Neves-Corvo, Portugal

Pyrite (light yellow, high reflectance) forms euhedral crystals (bottom right) and fine-grained aggregates of relict pyrite (poorly polished, centre left) within chalcopyrite (yellow). Tetrahedrite (light blue-grey, centre) is enclosed within sphalerite (light grey, centre) rims.

Polished block. plane polarized light, x80, air