Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, November 12, 2004
Formation of Fibrous Banded Veins: Observations from the Core of the Ouachita Orogen, Arkansas
We studied tectonic fibrous quartz and quartz-calcite veins from two localities in the Ouachita orogen, Arkansas, to test hypotheses for vein formation. The first set of veins is contained in the Lower Ordovician Mazarn Formation. Veins range from 0.1 to 5 m in length and from 0.1 to 20 cm in width and are either lensatic or thin and elongated in shape. Veins are not texturally homogeneous. At the tip: (a) fibers are made only of quartz, (b) their widths range between 20 and 120 ?m, (c) some fibers show host bands parallel to the vein edge, and (d) the vein edge is irregular. At the center: (a) the fibers are made of calcite and quartz, (b) quartz fiber widths range between 30 and 300 ?m, (c) wide fibers (>100 ?m) show inclusion trails parallel to the fiber long axis, (d) 10% of the fibers span the width of the vein and, (f) the vein edge is well defined. These observations suggest that the mechanisms of growth at the tips and central part of the vein are different. Thin veinlets between 5 and 25 ?m wide are observed parallel to the vein edge throughout the vein length, separated by 15-?m-wide host segments. Their number increase in fine host laminae and disappear in coarse host laminae. These observations suggest that vein material was derived locally from coarse laminations.
By contrast, fluid advection is most likely the primary transport mechanism in the second set of veins we have studied rom shales and coarse calcareous sands of the Ordovician Womble Shale, Benton uplift, Arkansas. All veins are composed of calcite with minor quartz and trace feldspar and dolomite or high-Mg calcite in the coarser veins. All host lithologies have a pressure-solution cleavage, more closely spaced in the fine-grained shale beds. The vein internal fabrics are coarsely to finely fibered, with a strong host-rock grain size control on fiber width. The finest fibers are in veins with shale host and the coarsest in the coarse-grained calcareous sandstone. Fiber aspect ratio is inversely proportional to host grain size; more equant vein grains are found in the veins hosted in the coarse host fraction. Within one outcrop, the ?13C and ?18O compositions of the host lithologies range from 1.5 to –3.0 and 7.5 to –14.0 per mil (PDB), respectively. By contrast, the ?18O composition of the veins is remarkably constant (-13.5 per mil) among veins of starkly different fabrics. This composition is identical to that of the coarse calcareous sandstone lithology in the outcrop. No CL or stable isotope zoning was observed in the veins. In addition, there were no gradients in Ca or Si in the vicinity of the veins, suggesting either that the host did not contribute these elements or that diffusion was not the rate-limiting step to vein formation. In any case, the wide variety of veins was probably formed from meter-scale migration of fluid derived from local calcite-rich layers in calcareous sandstone.
Taken together, observations on both sets of veins indicate, 1) strong host-rock control on grain size, 2) host rock control on vein composition in the more permeable fractions, 3) modification of vein fibers after initial formation.