Uranium deposits at the Jomac mine, White Canyon area, San Juan County, Utah

Abstract

The Jomac mine is in the White Canyon area. San Juan County, Utah, about 13 miles northeast of the town of White Canyon, Utah. The mine is owned by the Ellihill Mining Company, White Canyon, Utah. Mine workings consist pf two adits connected by a crosscut. Two hundred feet of exploratory drifting and 2,983.5 feet of exploratory core drilling were completed during 1953 by the owners with Defense Minerals Exploration Administration assistance. Sedimentary rocks exposed in the area of the Jomac mine are of Permian to Late Triassic age, having a combined thickness of more than 1,700 feet. An ancient channel, from 200 to 400 feet wide and about 4 feet deep, enters the mine area from the southwest, swinging abruptly northwest near the mine workings and continuing to the northern tip of the Jomac Hillo This channel was cut into the upper beds of the Moenkopi formation and filled in part by Chinle and in part by Shinarump sediments. This channel is marked by depressions that apparently were scoured into its floor; a tributary channel may have joined it from the southeast at a point near the mine workings. Chinle beds Intertongue with Shinarump beds along the southwestern part of the channel. After the main channel was partly filled by siltstone of the Chinle formation, the stream was apparently diverted into the tributary channel, and scours were cut into the Chinle siltstone and filled by Shina'rump sandstone, conglomerate, and siltstone. Statistical study of wood orientation in the beds of the Shinarump conglomerate further indicates a channel trend of about N. 23° W. Basal siltstoned-pebble conglomerates appear to mark the edge of channels and scours. Jomac Hill is on the crest of a southwest-plunging fold that is on the west flank of a larger syncline. The area surrounding the hill is broken by intense faulting, but no faults were noted in the vicinity of the mine. The major fractures in the mine workings strike N. 70° to 80° E. and are steeply dipping. Secondary steeply dipping fractures strike N. 40° to 60 °E., and N. 10° E. to N. 10° W. The fractures are believed to be related to the anticlinal structure rather than the faults. Most of the uranium is contained in coa!5 associated with jarosite and gypsum in sandstone, conglomerate, or sandy siltstone near the base of the Shinarump conglomerate. Uranium occurs ill a fibrous green secondary mineral, metazeunerite, an unknown fibrous yellow mineral, and an unknown massive yellow mineral. Secondary copper minerals including malachite azurite, and chalcanthite occur locally with the uranium minerals. Principal ore guides at the Jomac mine are channels, and scours at the bottom of these channels coal-bearing sandstone or conglomerate at the base of the Shinarump conglomerate, coal, and jarosite.