Geologic and geochemical evidence of possible bedded barite deposits in Devonian rocks of the Valley and Ridge Province, Appalachian Mountains

Mo. Dep. Nat. Resour., Div. Geol. and Land Surv., Rolla, MO, United States
Univ. Mo.-Columbia, United States

Middle Devonian rocks in the Appalachian basin valley and ridge province offer new ground for exploration of economic bedded barite deposits. The depositional strike length of documented barite exposures extends from central Pennsylvania through eastern West Virginia and southern Virginia. Appalachian barite has features similar to other sediment-hosted barite deposits: (1) deposition in restricted marine conditions, (2) deposition in a trough or geosynclinal setting, (3) insignificant amounts of volcanic rocks in the stratigraphic succession, (4) subjection to structural deformation, and (5) delta ³⁴ S values similar to nodular barite from western Canada and Nevada.Barite is restricted to the Needmore Shale Formation, the Marcellus-Millboro Shale Formations, and the Tully Limestone. The latter two stratigraphic horizons offer the best potential for hosting economic barite deposits. The Purcell Limestone Member of the Marcellus-Millboro Formations hosts many exposures of barite and should be the primary target of an exploration program.Syngenetic to early diagenetic barite includes barite nodules and incipient barite beds. Diagenetic barite includes baritized ammonoids, barite-filled syneresis cracks in barite nodules and limestone concretions, and small barite nodules which fringe zones of abundant barite nodules. Epigenetic barite includes vein barite in rock fractures, weathering-produced barite rinds, and acicular crystals in limestone which appear to be replacement structures.Minor and trace element contents show no apparent enrichment in metals relative to similar rock types from other stratigraphic settings. An exception is that barite-bearing rock units appear enriched in barium which becomes more concentrated toward zones of abundant barite nodules.The delta ³⁴ S values (28.0-47.4ppm) and delta ¹⁸ O values (19.3-30.4ppm) show that barite nodules are considerably enriched in these constituents relative to coeval seawater. Systematic decreases of delta ³⁴ S values updip from zones of abundant nodules combined with a slight decrease of ³⁴ S content downdip suggest two styles of deposition: deposition of smaller nodules updip because of increasing distances from the critical redox boundary or by remobilization during diagenesis, and deposition from a warm Ba-rich spring system subject to biogenic alteration.Sulfur isotope values of vein- and vug-filling barite from shrinkage cracks in barite nodules are generally heavier than those of the host nodule. This fact coupled with elevated temperature (72 degrees -142 degrees C) fluid inclusions, some of which are organic bearing, suggest that barite enriched in ³⁴ S was remobilized during diagenesis.We speculate that these sediment-hosted barite occurrences represent the margins of larger, yet undiscovered, bedded barite deposits. We feel that barium-bearing fluids were derived from submarine springs developed during the initial stages of the Acadian orogeny and deposited barite in minor submarine euxinic basins.

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