Olympic Dam Copper-Gold-Uranium Ores, Roxby Downs

Two bornite (Bo; tan) grains contained in a matrix of hematite (Hm; white). The grains appear to be partly crudely rounded and may represent fragments of earlier deposited sulfides that were redistributed by subsequent volcanic eruptions. Olympic Dam copper-gold-uranium mine, Roxby Downs, Australia. Ore microscopy, reflected light, moderate magnification (150X in original slide), oil immersion.



Crudely rounded grain of chalcopyrite (Cp; yellow) with coating of bornite (Bo; tan) followed by a partial coating of later chalcopyrite. Subsequent hematite has preferentially replaced parts of the bornite layer to form a discontinuous ring of fine-grained hematite within the chalcopyrite-bornite grain. Larger grains of hematite (Hm; bluish gray) outside the sulfide grain appear to older than the sulfides and locally (upper left corner) are partially replaced (caries texture) by bornite. Olympic Dam copper-gold-uranium mine, Roxby Downs, Australia. Ore microscopy, reflected light, moderate magnification (150X in original slide), oil immersion.



Grain of bornite (Bo; purple) and chalcocite (Cc; grey) showing an intergrowth that may have formed by chalcocite replacement of bornite, or may have formed by exsolution. The smooth boundaries of the bornite grains against the chalcocite suggest the latter. The margin of the bornite-chalcocite grains has been slightly replaced by hematite. Hematite also occurs in larger angular grains, especially in the upper left corner of the photomicrograph. Two single crystal of specular hematite occur within the bornite-chalcocite grain. The identification of the copper sulfide mineral as chalcocite has been made by optical means. However, it is difficult to distinguish the four members of the chalcocite by optical means alone. Digenite (Cu~1.8S) has a distinct bluish tint and a lower reflectance (23) than the reflectance (32) of chalcocite (Cu2S) . Djurleite (Cu1.96S) is closest to chalcocite in color and reflectance; anilite (Cu1.75S) is closest to digenite in optical properties. X-ray or electron microprobe analyses are required for precise determination of the chalcocite-group copper sulfides. Olympic Dam copper-gold-uranium mine, Roxby Downs, Australia. Ore microscopy, reflected light, moderate magnification (150X in original slide), oil immersion.



Large crystal (60% core) and angular fragments of original magnetite that has been subsequently oxidized mostly to martite (Mar; grey). Martite is hematite that is pseudomorphic after magnetite, and it is characterized by its fine-grained polycrystalline nature. Small remnants of magnetite remain in most of the martite grains, especially the grain in the lower central portion of the photomicrograph, that attest to their original magnetite character. Bornite (Bo; tan) has formed veins that traverse the martite and occur along the grain boundaries of the martite grains. It is not clear whether the bornite veins are pre- or post-martitization. Chalcopyrite (Cp; white) occurs in a manner similar to that of bornite. Finer grained hematite, much of which has not taken a polish, occurs in the groundmass between the larger martite grains. Olympic Dam copper-gold-uranium mine, Roxby Downs, Australia. Ore microscopy, reflected light, moderate magnification (150X in original slide), oil immersion.



Exsolution intergrowth between bornite (Bo; tan) and copper sulfide (blue to grey) showing smooth boundaries between the two minerals that are typical for such exsolution intergrowth. The chalcocite-group mineral marked "Cc:" can be separated into two phases: perhaps anilite (blue; darker reflectance) and djurleite (grey; bright reflectance). Hematite (Hm; white; hard) occurs especially along the margins of the sulfide grain and probably occurs as a partial replacement of the sulfide grain. The fact that the hematite grains have rather broad black margins indicates that they have strong relief about the enclosing sulfides and therefore are much harder. Olympic Dam copper-gold-uranium mine, Roxby Downs, Australia. Ore microscopy, reflected light, high magnification (500X in original slide), oil immersion.



A bornite (Bo; tan) grain that has been partially replaced by subsequent uranium-bearing fluids. Thin NW-trending exsolution lamellae of chalcopyrite are present throughout the bornite, and they can be recognized in the photomicrograph with difficulty since they are at the limit of resolution (1 µm) of the microscope. Coffinite Cof; bluish grey) is the most common uranium mineral replacing the bornite. It appears to have a bluish tint due to the presence of very fine-grained covellite that is disseminated throughout the coffinite. Locally, sufficient amounts of covellite are present that they can be perceived as small blue grains in the coffinite. Anatase (Ats; grey) is an abundant phase, especially in the upper right corner of the photomicrograph. Its recognition can best be confirmed by crossing the polars and observing it intense white internal reflections (not shown here). Brannerite (Bran; grey), a titanium-bearing uranium mineral, is locally formed in association with the anatase. Although the uranium mineralization shown here has formed at a late time, the fact that uranium was present in the early ore fluids is indicated by the presence of trace amounts of uranium in Olympic Dam hematite. Olympic Dam copper-gold-uranium mine, Roxby Downs, Australia. Ore microscopy, reflected light, high magnification (500X in original slide), oil immersion.