GROWTH OF FIBROUS VEINS IN SHALE, MT. IDA, ARKANSAS

Veins in the Ordovician Womble Shale west ofMt. Ida, Arkansas, are composed of calcite with minor quartz and tracefeldspar. Although all veins are fibered to some degree, the veins with thefinest and coarsest fibers are hosted in shale and calcareous sandstones,respectively. The d¹³C andd¹⁸O compositions of the host lithologies rangefrom 1.5 to -3.0 and 7.5 to -14.0 per mil (PDB), respectively, in hostlithologies whereas the d¹⁸O composition of the veins is remarkablyconstant (-13.5 per mil) among veins of starkly different fabrics. Thiscomposition is identical to that of the coarse calcareous sandstone lithologyin the outcrop. The veins in shale can be traced from a distal axial zone ofincreased quartz grain size in the host to a thin symmetical quartz vein andfinally a quartz-rimmed but largely calcite filled vein along the veinÕslength. This suggests a sequence of events that corresponds to the degree ofopenness of the rock to fluids. Because there is no calcite in the shale host,the quartz-then-calcite precipitation sequence suggests an increase in opennessto fluids in equilibrium with the calcite sandstones as vein developmentprogressed.