ORE MICROSCOPY OF A NEW BORNITE POD AT THE CASTEEL MINE, VIBURNUM TREND, SOUTHEAST MISSOURI LEAD DISTRICT

Bornite ores are unique to the Viburnum Trend in the Southeast Missouri Lead District. The bornite ores formed very early in the paragenetic sequence. Sphalerite and galena of the main lead-zinc periods of deposition formed later and partially replace the earlier bornite. The bornite ores occur in small pods and lenses that are nearly conformable with the bedding of the host Cambrian Bonneterre Formation. They commonly occur in shaly portions of the Bonneterre Formation. The shaly character may be due to clay representing an insoluble residue left from the intense hydrothermal replacement by these massive bornite ores. Alternatively the shaly character may be an original aspect of early paleokarst structures. Fine-grained massive steel galena occurs nearby in the same stratigraphic horizon, but they are separated from each other.

The new bornite ores discovered at Casteel represent the ninth occurrence of bornite ores in the Viburnum Trend. Although all of the bornite occurrences have similar mineralogy, their textures vary significantly. The ores are massive, exceedingly fine grained, and require microscopic examination to distinguish their mineralogy and textural character.

Ore microscopic study of the new bornite occurrence is texturally unlike most previously studied bornite ores in the trend. Rather than bornite-chalcopyrite spheroids, these 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 in which the individual crystals appear to be euhedral doubly terminated prisms or even long plates. SEM-EDS indicates that they are indeed bornite, and that there is no microscopic evidence suggesting they have derived their highly unusual morphology by the replacement of non-isometric minerals.

The bornite ores 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.