MINERALOGY AND TEXTURES OF BORNITE ORES IN THE SWEETWATER MINE, VIBURNUM TREND, SOUTHEAST MISSOURI

The bornite pods in the Viburnum Trend of the southeast Missouri Pb-Zn district are unique in their mineralogy and textures with respect to typical Mississippi Valley-type ores in that district and elsewhere. The pods are small bodies that are typically less than 5 meters across. They consist of massive sulfide ores that contain very little gangue material. The massive ores are comprised of very fine-grained sulfides that require ore microscopic study to ascertain their mineralogical compositions and textures. They have formed prior to the deposition of the subsequent Pb-Zn ores and they are totally devoid of sphalerite and galena. The fluids from which the bornite pods were deposited appear to have had little communication between pods and each pod has its own distinctive mineralogy and texture. Recent mining of a bornite pod in the northern portion of the Sweetwater mine has provided an opportunity to examine the mineralogy and textures of that pod and to compare its character with that of other bornite pods previously studied in the district. The pod consists predominately of bornite (usually more than 90%) mostly in the form of spheroids. The spheroids are coalesced and cemented by later bornite to form the massive character of the ore. Chalcopyrite typically comprises 5-10% of the ores but locally can form as much as 35%. It occurs mostly as grains but locally forms exsolution laths within the bornite. Compared to other bornite pods in the district, the Sweetwater bornite ores are especially rich in carrollite (1-12%). Carrollite was deposited as early euhedral isometric crystals that exhibit all degrees of replacement by subsequent chalcopyrite and bornite. Smaller amounts of gersdorffite, tennantite, and pyrite also are locally present in these bornite ores. The typical presence of spheroidal textures and exceptionally fine-grained textures suggests that the ores in the Viburnum Trend bornite pods were rapidly deposited. They were deposited from ore fluids that entered the district very early in its history, and the fluids were introduced along paths that differed for each bornite pod. The ores were probably deposited at a somewhat higher temperature (perhaps 200-150oC) than that for the subsequent Pb-Zn ores.