Mississippi Valley-style lead-zinc deposits

Sphalerite, chalcopyrite and gersdorffite. Nenthead, North Pennines, Britain

Sphalerite (light grey, top) is inclusion-free and is intergrown with coarse-grained quartz (dark grey) showing faint internal reflections (centre left). Sphalerite (centre) is intergrown with chalcopyrite (yellow, centre right) and euhedral gersdorffite (white, high reflectance, centre left). Black areas are polishing pits.

Polished block, plane polarized light, x 80, air

Sphalerite, chalcopyrite, pyrite and gersdorffite. Nenthead, North Pennines, Britain

Sphalerite (light grey, right) encloses a euhedral gersdorffite (white, top right). Larger euhedral gersdorffite crystals (centre right) occur within quartz (dark grey, centre). Chalcopyrite (yellow, bottom left) is inclusion-free, as is pyrite (light yellow-white, top left), which has a slightly lower reflectance than gersdorffite., Black areas are polishing pits.

Polished block, plane polarized light, x80, air

Sphalerite, chalcopyrite, pyrite, galena and marcasite. Nenthead, North Pennines, Britain

Sphalerite (light grey, bottom left) and chalcopyrite (yellow, top right) are the major sulphides. Pyrite (light yellow, centre) is euhedral to subhedral and is intergrown with minor marcasite (white, centre right), which is difficult to see, and with chalcopyrite, sphalerite and galena (blue-white, centre left). Chalcopyrite (centre) contains a small lath-shaped inclusion of galena. Differences in reflectance in the galena are due to different amounts of tarnish (centre, centre left). Quartz (dark grey) is euhedral (top left) and shows faint internal reflections (top centre). Black areas are polishing pits.

Polished block, plane polarized light, x80, air

Sphalerite and gersdorffite. Nenthead, North Pennines, Britain

Small gersdorffite crystals (white, right) occur within zoned sphalerite. Zoning in the sphalerite is just visible as blue-grey (centre) and brown-grey (bottom right) areas. Black areas are polishing pits.

Doubly polished thin section, plane polarized light, x40, air

Sphalerite (and gersdorffite). Nenthead, North Pennines, Britain

This is the same field of view as 58d but in transmitted light. The fine scale of the growth banding in sphalerite is very clear. In thin section or polished thin section, much of the fine detail would be lost. The intensity of the colours are due to variations in the trace element content of the growth bands, most importantly the iron content.

Doubly polished thin section, transmitted plane polarized light, x 40, air

Xenotime, monazite and pyrite. Groverake, North Pennines, Britain

Xenotime (darker grey, centre left) forms the core to later monazite (lighter grey, centre right) and both occur within quartz (grey), the main mineral. Very fine-grained pyrite (white, centre) infills later fractures. Black areas are polishing pits.

Polished block, plane polarized light. x 160, air