Porphyry-style tin-tungsten-molybdenum ores
Pyrite, arsenopyrite, sphalerite, marcasite, wolframite and rutile. Mount Pleasant, New Brunswick, Canada
Subhedral rhombs of arsenopyrite (white, centre, bottom) have higher reflectance than pyrite (pale yellow, top left). Pyrite forms coarse-grained crystals (top left) but much of it is finely intergrown with marcasite (blue-yellow, slightly higher reflectance, right bottom). Sphalerite (medium grey, bottom left) contains abundant fine-grained and submicroscopic chalcopyrite inclusions which give the sphalerite a yellow-brown colour, especially in the cores of the crystals. A single tabular crystal of wolframite (brown-grey, bottom centre) is present. Rutile (light grey, top centre) forms anhedral aggregates and acicular crystals.
Rutile, pyrite, marcasite, wolframite, cassiterite and arsenopyrite. Mount Pleasant, New Brunswick, Canada
Rutile crystals (light grey, centre) show bireflectance (light to darker grey) and are rimmed by wolframite (brown-grey, bottom left) and fractured cassiterite (dark grey, top left, centre top) which has a markedly lower reflectance than rutile or wolframite. Euhedral cassiterite crystals occur within the gangue between rutile and pyrite-marcasite aggregates (centre top) and show internal reflections (top centre). Pyrite (light yellow-white, bottom right) and marcasite (blue-green white, centre top right) are intimately intergrown (right) and have replaced pyrrhotite. A euhedral crystal of arsenopyrite (white, high reflectance, top left) is present in the silicate gangue (black).
Pyrite, marcasite, wolframite, arsenopyrite, molybdenite, rutile and cassiterite. Mount Pleasant, New Brunswick, Canada
Pyrite (light yellow-white, centre top left) forms small euhedral crystals within marcasite (blue-green white, higher reflectance, centre bottom right). This is typical of pyrrhotite replacement/alteration by iron disulphides. Rhombic arsenopyrite (white, high reflectance, bottom centre) is present. Prismatic wolframite (brown-grey, centre and left centre) is enclosed within a pseudomorph after pyrrhotite (left centre) but mainly occurs within the gangue (black). Rutile (light grey, centre) is rimmed by cassiterite (dark grey, lower reflectance than wolframite, centre). Curved laths of molybdenite (light to dark grey, bottom centre, centre right) show strong bireflectance and reflection pleochroism.
Arsenopyrite, galena, sphalerite, chalcopyrite, molybdenite and rutile. Mount Pleasant, New Brunswick, Canada
Euhedral arsenopyrite (white, high reflectance, left) is intergrown with galena (light blue-white with triangular cleavage pits, centre), chalcopyrite (yellow, centre) and sphalerite (light grey, centre right), with fine chalcopyrite inclusions (top left) or submicroscopic chalcopyrite (grey to brown-grey, centre right). A lath of poorly polished molybdenite (light grey, centre) is enclosed within chalcopyrite and galena and mother has partially rimmed arsenopyrite (bottom right). Minor amounts of rutile (light grey) form acicular crystals within the gangue (right centre). Black areas are polishing pits.
Polished block, plane polarized light, x 160, oil
Molybdenite and native bismuth. Mount Pleasant, New Brunswick, Canada
Molybdenite laths (greys, centre) show bireflectance and reflection pleochroism and are well to poorly polished, with the degree of polishing dependent upon orientation. Discrete crystals of native bismuth (cream-white, top left) are soft (scratches) and show high reflectance. To the eye, the reflectance of bismuth is higher than the photomicrograph would suggest. The gangue comprises carbonates (lighter greys, centre left) and silicates (dark greys).
Polished block. plane polarized light, x80, air
Cassiterite, arsenopyrite, tennantite, sphalerite, chalcopyrite and covelline. Mount Pleasant, New Brunswick, Canada
Euhedral rhombic arsenopyrite (white, high reflectance, top centre) is partially intergrown with euhedral equant crystals of cassiterite (grey, bottom centre) which show faint bireflectance (bottom centre). Sphalerite (medium brown-grey, centre) has very fine chalcopyrite inclusions and shows lower reflectance than tennantite (blue-grey, right), which has a small chalcopyrite (yellow) crystal on its margin. A small crystal of chalcopyrite (top left) has altered to covelline (deep blue, top left). The main gangue is silicate (dark grey) with polishing pits (black). An accessory mineral, zircon, with higher reflectance than the silicates (left centre), lies close to one of the pits.
Polished block, plane polarized light, x 80, air