Bornite Ores, Viburnum Trend, Southeast Missouri Lead-Zinc District



Exsolution intergrowth between chalcopyrite (Cp; yellow) and bornite (Bo; purple). Smoothly bound spindles of chalcopyrite have exsolved from the bornite during cooling and have been aligned along the octahedral planes in the host bornite. Small amounts of tennantite (Tn; blue) have locally replaced chalcopyrite and especially bornite. The bornite ores occurs as small pods and lenses locally present in some of the mines in the Viburnum Trend. These ores formed early in the sequence of ore deposition, and they were veined and partly replaced by the subsequent lead-zinc ores. Their mineralogy and textures are unusual for Mississippi Valley-type deposits. Fletcher mine, Viburnum Trend, Southeast Missouri Lead District. Ore microscopy, reflected light, high magnification.



A beautifully developed colloform sulfide body showing rythymically deposited bands of bornite and chalcopyrite. Colloform textures consist of very fine-grained sulfides that were rapidly deposited. The last generation of bornite exhibits a porosity that is characteristic of supergene chalcocite deposited in other ore districts. Locally, covellite (blue) has formed small irregularly shaped replacements of bornite. Brushy Creek mine, Viburnum Trend, Southeast Missouri Lead District. Ore microscopy, reflected light, medium magnification.



A vaesite (Vae; light grey) crystal deposited upon and replacing enargite (En; brownish grey). Bornite (Bo; tan) and covellite (Cov; blue to purple) formed after and they locally vein enargite and vaesite. Enargite (Cu3AsS4) formed at the time of introduction of the copper-rich fluids by the reaction with gersdorffite (FeAsS) with copper coming from the fluids and arsenic from gersdorffite. Pyrite (Py; yellow) is locally present. Fletcher mine, Viburnum Trend, Southeast Missouri Lead District. Ore microscopy, reflected light, high magnification.

Porphyry Copper Ores, Arizona



Vein of chalcopyrite (Cp; yellow) that is veined and partially replaced by bornite (Bo; purple), which in turn is subsequently veined and replaced by digenite (Di; bluish grey). Morenci Copper District, Arizona. Ore microscopy, reflected light, medium magnification.



Digenite (Di; blue) replacing galena (Gn; white). Locally, triangular pits, which are characteristic of galena, occur within digenite supporting the interpretation that it has replaced the galena. Bornite (Bo; tan) also is present. Bisbee Copper District, Arizona. Ore microscopy, reflected light, medium magnification.



Bornite (Bo; purple) with local elongate patches of chalcopyrite (Cp; yellow). The elongate chalcopyrite areas appear similar to exsolution spindles, but more likely are veinlet of chalcopyrite following the octahedral planes in the host bornite. Further support for the interpretation of the chalcopyrite areas as veins comes from the fact that digenite (Di; blue) also follows a fracture that probably form the main conduit for the chalcopyrite-depositing fluids. A single galena (white) is present. Holes in the surface of the polished section are black. Bisbee Copper District, Arizona. Ore microscopy, reflected light, medium magnification.