Mississippi Valley lead-zinc deposits

Sphalerite and galena. Shullsburg, Wisconsin, USA

Botryoidal sphalerite (light grey) comprises growth bands of different grain sizes; the coarser crystals (left) are well polished and overgrow earlier finer crystalline bands (centre left) with many polishing pits. Galena (white) is both crystalline and euhedral (centre), or botryoidal and poorly crystalline (centre left). Black areas are polishing pits, dark grey areas are carbonate (centre left) grains.

Polished block. plane polarized light, x80, air

Galena and sphalerite. Shullsburg, Wisconsin, USA

Sphalerite (light grey) occurs as radiating aggregates of different grain sizes. Very fine-grained sphalerite is poorly polished and shows reddish-brown or light-coloured internal reflections (top and bottom right). The central bands of sphalerite have grown into a vug and hence have euhedral crystal terminations. Galena (white) has infilled most of this central vug. Black areas are polishing pits.

Polished block, plane polarized light, x 80, air

Galena and sphalerite. Shullsburg, Wisconsin, USA

Well crystallized galena is white (top right) and is intergrown with poorly polished fine-grained galena (pitted areas, centre). Both are interbanded with sphalerite (light grey, centre left). Black areas are voids.

Polished block, plane polarized light. x 80, air

Sphalerite, pyrite, galena and marcasite. Shullsburg, Wisconsin, USA

Sphalerite (light grey) shows faint internal reflections (centre left) and encloses lath-shaped pyrite (yellow-white, centre left) and a partially replaced rhomb of marcasite (white, high reflectance, Centre). Pyrite crystals are replaced by galena (blue-white, top). Dark grey area is carbonate, black areas are polishing pits. In crossed polars the relict marcasite shows optical continuity, showing it to have been one crystal.

Polished block, plane polarized light, x 80, air

Galena, sphalerite and pyrite. Shullsburg, Wisconsin, USA

Pyrite (yellow-white, centre) as lath-shaped crystals has been extensively replaced by galena (blue-white, centre) and minor sphalerite (light grey, centre right). This replacement is crystallographically controlled. Inclusion-free galena (centre right, bottom centre) is intergrown with replaced pyrite and with sphalerite. Sphalerite (light grey, bottom right and left) is inclusion-free. Dark grey areas are carbonate (top) grains.

Polished block, plane polarized light, x80, air

Pyrite, marcasite, galena and sphalerite. Shullsburg, Wisconsin, USA

A euhedral pyrite (yellow-white, right) crystal has a pyrite overgrowth forming a rim (bottom right) that has a slightly higher reflectance. Pyrite is replaced along its cleavage by galena (blue-white, centre) and minor sphalerite (dark grey, Centre). A tabular crystal of marcasite (white-yellow, centre bottom) has higher reflectance than pyrite, and also is extensively replaced by galena and sphalerite which exactly pseudomorph the original marcasite crystal. Black area is carbonate gangue.

Polished block, plane polarized light. x 160, oil