Fossil placer
Pyrite and zircon. Vaal Reef, Witwatersrand, Republic of South Africa
Rounded pyrite (yellow-white) grains comprise single crystals, grains with poorly crystalline pyrite cores (top centre), framboidal grains (centre bottom right) and recrystallized framboids (centre top right). Many pyrite grains are fractured. Two rounded zoned zircons (light grey, centre) are present. Galena, arsenopyrite, rutile and altered uraninite are also present but cannot be distinguished at this magnification.
Rounded pyrite grains (yellow-white) are heavily fractured and carry pyrrhotite and chalcopyrite inclusions (top left) which cannot be distinguished from each other at this magnification. A rounded chromite (light grey, bottom centre right) has a zircon (dark grey, lower reflectance than chromite, bottom centre) lying between it and the adjacent shattered pyrite. Lying above two small chromite crystals (bottom right) is an extensively altered uraninite grain (diffuse lighter grey patch) containing very fine-grained chalcopyrite and galena (higher reflectance 'spots'). Small rutile crystals (light grey) occur within a pyrite grain or surround it (centre top). Quartz is dark grey.
Rounded pyrite grains (light yellow-white) are heavily fractured (centre) and contain small chalcopyrite (yellow, bottom centre) inclusions. One pyrite grain (top left) comprises recrystallized framboids and above it a rounded detrital grain has an (?)authigenic pyrite overgrowth separated by a thin silicate-rich zone. Uraniferous carbon, also known as thucholite (brown-grey, centre right), contains small square shaped uraninite (light grey) crystals in its centre. The carbon shows faint reflection pleochroism and bireflectance. The carbon is surrounded by a coffinite-rich area (light grey, centre top) enclosing pyrite. Acicular crystals of TiO2 (light grey, bottom centre) form a leucoxene aggregate and have higher reflectance than coffinite and zircon (dark grey, bottom right) which is also fractured. Two euhedral crystals of pyrrhotite (brown-yellow, bottom left) have nucleated upon a detrital pyrite grain.
Pyrite, arsenopyrite, TiO2 minerals. Vaal Reef, Witwatersrand, Republic of South Africa
Pyrite (light yellow, bottom right) has a lower reflectance than arsenopyrite (white, top left, centre right), one grain of which shows reflection pleochroism from pink-white to white (centre right). Both pyrite and arsenopyrite are rounded grains. Two aggregates of fine-grained TiO2 (blue-grey, bottom left and top right) are pseudomorphs after iron-titanium oxide minerals. Quartz is dark grey, softer gangue phases have plucked out.
Polished block, plane polarized light, x 80, air
Pyrite, chalcopyrite, pyrrhotite, native gold. Vaal Reef, Witwatersrand, Republic of South Africa
Pyrite (light yellow) encloses small inclusions of chalcopyrite (yellow, centre right), pyrrhotite (brown, centre left) and a mixed chalcopyrite-pyrrhotite (centre top) inclusion. A small native gold grain (golden yellow, high reflectance, centre right) is also present and has a higher reflectance than chalcopyrite. Quartz is dark grey.
Polished block, plane polarized light, x 160, oil
Pyrite (light yellow, right centre) and fractured arsenopyrite (light blue-white, top centre) are cemented by pyrrhotite (brown, bottom centre). A gold grain (golden-yellow, very high reflectance, centre top) is close to a single crystal of a TiO2 mineral (grey, centre top). Other small crystals of pyrite, arsenopyrite and TiO2 occur within pyrrhotite. Quartz (left) has internal reflections. The colour and reflectance of gold in this photomicrograph closely approach its appearance under the microscope.
Polished block, plane polarized light, x 160, oil