Paper No. 7
Presentation Time: 4:00 PM
OCCURRENCE OF CALCAREOUS BAUXITE-LIKE CLAY IN A CAVE IN THE SHENANDOAH VALLEY, VIRGINIA
Calcareous bauxite-like clay was discovered along cave passages within an abandoned quarry at Linville, Virginia. Linville Quarry Cave is formed within the Middle Ordovician New Market Limestone. The morphologic characteristics of the passages indicate that the cave is entirely phreatic (hypogenic?) in origin. Dark reddish-brown pisolitic clay is cemented with calcite in an indurated reddish-orange crust that forms a patchy lining along the ceilings and walls of the cave passages. Preliminary x-ray diffraction (XRD) analysis of a bulk powdered sample shows the mineralogy is dominated by calcite (~80%), followed by quartz, clays, and a trace amount of goethite. XRD analysis after 10% HCl digestion of the bulk sample shows the insoluble residue to be sub-equal amounts of quartz and clays (illite and kaolinite in a 4:1 ratio, respectively), along with trace amounts of feldspar and goethite. Viewed in thin section, the clay pisoliths (1 to 20 mm in diameter) are dispersed in a calcite cement matrix; calcite crystals grew to invade the clay pisoliths, and scalenohedral calcite crystals fill small vugs. Wavelength dispersive x-ray fluorescence by electron microprobe reveals that the mineralogy also includes detrital apatite grains with ragged overgrowths of apatite and monazite, iron and manganese oxy-hydroxides, and pyrite framboids altered to iron hydroxide.
The Linville Quarry Cave deposit is classified as calcareous bauxite-like clay according to Bárdossy (1982). The cave and its bauxite-like deposit were evidently not exposed prior to limestone quarrying. Several surface exposures of true bauxite were mined near Spottswood, Virginia about 70 km to the southeast of Linville. These bauxites were interpreted to be the result of intense surficial weathering of transported sediments trapped in sinkholes in limestone of the Ordovician Beekmantown Group (Warren and others, USGS Bulletin 1199K, 1965). The Linville Quarry Cave deposit indicates that bauxite may form via in-situ subsurface weathering of cave (residual?) sediments. The weathered sediment was subsequently overprinted with late phreatic calcite cement, possibly of hydrothermal origin as indicated by the scalenohedral habit of vuggy calcite.