AN UNUSUAL OCCURRENCE OF COVELLITE WITHIN IMMISCIBLE SULFIDE DROPLETS IN THE SHEPHERD MOUNTAIN GABBRO, PROTEROZOIC ST. FRANCOIS MOUNTAINS TERRANE, MISSOURI

The Shepherd Mountain Gabbro is a 400 foot thick near-horizontal dike within the Proterozoic St. Francois Mountains Terrane in southeast Missouri. The dike is principally known from intersections in five surviving core holes drilled into the Pilot Knob Magnetite deposit beneath Pilot Knob, Missouri (two complete intersections and three partial). Rounded sulfide blebs are immiscible magmatic droplets and are composed principally of pyrrhotite with lesser chalcopyrite, pyrite, covellite, and sphalerite. Some of the sulfide blebs have a similar mineralogy but have an irregular shape caused by being located within the interstices of the gabbro minerals plagioclase, pyroxene, olivine, plus titanomagnetite and ilmenite. In copper ores covellite is most commonly found as a supergene mineral within the zone of sulfide enrichment. In addition, primary hypogene hydrothermal covellite has been reported with the best known occurrence being at Butte, Montana. Localities where magmatic covellite has been reported in the literature include the Skaegaard intrusion, Greenland, the Tellnes ilmenite deposit, Norway, and the Kiglapait layered intrusion, Labrador. In the Shepherd Mountain gabbro, in addition to the sulfide droplets, there are similar rounded oxide blebs composed of titaniferous magnetite and ilmenite. The origin of the rounded oxide crystals is uncertain, but the occasional euhedral crystal within them suggests that they are not immiscible droplets, but instead may have been rounded by partial resorption of previously crystallized magnetite-ilmenite.