ORE MICROSCOPY OF BORNITE PODS IN THE VIBURNUM TREND, SOUTHEAST MISSOURI LEAD DISTRICT
Although all of the bornite occurrences have similar mineralogy, their textures vary significantly. The ores are exceedingly fine grained, and require microscopic examination to distinguish their mineralogy and textural character.
Ore microscopic study of the new bornite occurrence at Casteel shows that it is texturally unlike most previously studied bornite ores in the trend. Rather than bornite-chalcopyrite spheroids, those ores are characterized 1-2 mm diameter colloform bodies, some of which exhibit interior rhythmically deposited layers of bornite and chalcopyrite. The colloform bodies consist of very fine-grained (about 1 µm) rapidly deposited sulfides. In addition, thin bands (1/2 cm) of earlier fine-grained (1-15 µm; average 6 µm) evenly disseminated sulfides occur locally between the larger bands of colloform sulfides.
Locally the Casteel bornite ores consist of disseminated bornite ores in which the individual crystals are euhedral doubly terminated prisms and long plates. Euhedral bornite crystals are rare occurrences for any type of copper ores. SEM-EDS confirms that the crystals are bornite. There is no microscopic evidence to suggest the crystals have derived their morphology by the pseudomorphic replacement of non-isometric minerals.
The bornite ores of the Viburnum Trend owe their origin to early hydrothermal fluids that traveled through the Lamotte formation and deposited copper prior to the introduction of subsequent lead-zinc fluids.