Original uraninite crystals in the pegmatite at the Ruggles mine in New Hampshire have been altered by supergene fluids to a succession of secondary uranium minerals. In contrast to normal brightfield ore microscopy, these ore minerals are distinguished best under darkfield ore microscopy. Darkfield requires the insertion of a block into the path of most of the light and allows only light travelling along the margins of the field to impinge the polished section. Among other properties, darkfield enhances the internal reflections in ore minerals. Only small remnants of the original uraninite remain (black), and the original uraninite crystals have been altered mostly to fourmairite (Four; orange internal reflections) with rims of vandendreisshite (Van; yellow to yellowish white internal reflections). Soddyite (Sod; white internal reflections) occurs between the original uraninite crystals. Ore microscopy, reflected light, darkfield, low magnification (25X for original slide).



Original uraninite crystal further altered mostly to kasolite (brown internal reflections). Kasolite is veined by B-uranophane (Uran; white internal reflections). Kaesolite replaces fourmairite (Four; yellow) and a small amount remains at the top of the image. Soddyite (white internal reflections) occurs at the left side of the photomicrograph. Ruggles pegmatite mine, New Hampshire. Ore microscopy, reflected light, darkfield, low magnification (25X for original slide).



Rare earth mineral-rich magnetite-hematite iron ores from Bayan Obo, Inner Mongolia, China. The recovery of by-product bastnaesite and monazite from these iron ores makes Bayan Obo the largest producer of rare earths in the world. A few grains of magnetite (Mt; bright brownish white), partly oxidized to hematite are present at the bottom center portion of the photomicrograph. Bastnaesite and monazite are much darker but can be distinguished by their differences in reflectance. Bastnaesite (Bas; light grey; locally prismatic) is the brighter of the two rare earth minerals (R~9), and monazite (Mon; medium grey) is slightly darker (R~8). Fluorite (Fl; dark grey) is abundant and recognized by its very low reflectance (R~4). Ore microscopy, reflected light, moderate magnification (100X for original slide).



Another rare earth mineral-rich magnetite-hematite iron ore. Magnetite (Mt; bright brownish white) grains, partly oxidized to hematite are more abundant. Bastnaesite (Bas; light grey; R~9; locally prismatic) is the most abundant of the two rare earth minerals. A few grains of monazite (Mon; R~8; medium grey) are present in the center of the photomicrograph. Some fluorite (dark grey; R~4) is present especially in the lower left corner. Bayan Obo, Inner Mongolia, China. Ore microscopy, reflected light, moderate magnification (100X for original slide).



Renierite (Ren; brown), tennantite (Tn; blue), galena (Gn; white), and pyrite (Py; yellow) in the ores from Tsumeb, North-Central Namibia. The Tsumeb copper-lead-zinc deposit is a vertical pipe that includes three zones of oxidized ores. By-product germanium and gallium minerals previously provided the world's largest supply of germanium. Holes in the surface of the polished section are black. Ore microscopy, reflected light, oil immersion moderate magnification.



Sphalerite (Sl; dark grey) that contained sufficient gallium to exsolve a separate phase, gallite. The gallite lamellae have been exsolved along the crystal planes of the sphalerite. Tennantite (Tn; light blue) surrounds and has partly replaced the margins of the sphalerite. Small amounts of renierite (Ren; orange) and pyrite (Py; yellow) are present. Tsumeb copper-lead-zinc mine, North-Central Namibia. Ore microscopy, reflected light, oil immersion moderate magnification.



Umangite (Cu3Se2)(Um; strong red and greyish pink reflective pleochroism) partly altered to klockmannite (CuSe)(Kl; strong dark blue and bluish white reflective pleochroism). Malachite (Mal; dark grey) is present in the upper left portion of the photomicrograph. Beaverlodge ores, Saskatchewan. Ore microscopy, reflected light, oil immersion, high magnification (400X in original slide).



The Homestake gold ores consist of arsenopyrite (Asp; white), pyrrhotite (Po; brown), and small amounts of native gold (Au, deep gold yellow). This is an exceptionally rich gold specimen from Homestake. For many year the Homestake district in South Dakota had been the largest producer of gold in the United States. The ores were previously interpreted to have formed as hydrothemal replacements with the metals coming from Tertiary magmatic intrusions. Thin veinlets of gold, and less commonly pyrrhotite, traverse the arsenopyite; some gold veins cut pyrrhotite. Earlier ore microscopic interpretations of the textures shown in the above photomicrograph were used to support that genetic model. It was interpreted that arsenopyrite was deposited from the early ore fluids, and it was then brecciated during a period of deformation. Pyrrhotite and subsequently gold were introduced into the fractures in arsenopyrite by subsequent ore fluids. More recently, the genesis of the Homestake ores has been interpreted to involve volcanic exhalations on the Precambrian sea floor, and and that those sedimentary sulfides subsequently subjected to high-grade regional amphibolite facies metamorphism. Current ore microscopic interpretations of the above texture view all three minerals being involved in metamorphism but that arsenopyrite behaved in a brittle manner and formed cracks whereas both pyrrhotite and gold behaved in a ductile manner flowing into the cracks in arsenopyrite during deformational metamorphism. Ore microscopy, reflected light, moderate magnification (100X in original slide).