ORE MICROSCOPY OF BORNITE PODS IN THE VIBURNUM TREND, SOUTHEAST MISSOURI LEAD DISTRICT

Bornite ores are unusual in the Viburnum Trend of the Southeast Missouri Lead District. They formed very early in the paragenetic sequence, they are massive ores uncharacteristic of the typical ores in the district, and they are restricted to small known occurrences. Recently discovered bornite ores at the Casteel mine represent the ninth occurrence of bornite ores in the Viburnum Trend. 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.

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.