We studied tectonic fibrous quartz and quartz-calcite veins contained in the Lower Ordovician Mazarn Formation (banded shale and fine grained sandstone with micritic limestone laminations, deposited in a submarine fan). Veins range from 0.1 to 5 m long and from 0.1 to 20 cm wide and are either lensatic or thin and elongated in shape. Textural differences are observed within the same vein. At the tip: a) fibers are made only of quartz, b) their widths range between 20 and 120 mm, 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 mm, c) wide fibers (>100 mm) show inclusion trails parallel to the fiber long axis, d) 10% of the fibers span the width of the vein and, d) 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 mm wide are observed parallel to the vein edge throughout the vein length, separated by 15 mm wide host segments. Their number increase in fine host laminae and disappear in coarse host laminae. Preliminary oxygen stable isotope analyses show that the d¹⁸O values of vein quartz range from 19.2‰ to 19.5‰ and host-rock values  from 16.1‰ to 17.1‰ (fine laminations) and 18.2‰ - 19.2‰  (coarse laminae).  These observations suggest that vein material was derived from coarse laminations.