INFLUENCE OF FAULTS IN VEIN FORMATION. A CASE FROM THE OUACHITAS, ARKANSAS

Abundant fibrous quartz and quartz-calcite veins of typical 'crack seal' texture" occur at or near faults in the Lake Ouachita area, Arkansas. The veins are contained in the Lower Ordovician Mazarn Formation (banded shale and fine grained sandstone with micritic limestone laminations) that was deposited in a submarine fan. The main feature is a N60E trending thrust fault dipping south without associated quartz mineralization along the fault plane. Veins appear to be severed by the fault, range from 0.1 to 5 m long and from 0.1 to 20 cm wide, are lensatic in shape and contain mixed quartz and calcite fibers. Aspect ratios (length/width) of fibers range between 3 and 12. No median line is observed; some fibers span the width of the vein. Vein tips show microveins (10 to 30 mm wide) that extend up to 10’s of cm away from the tip oriented sub-parallel to the vein-host interface. Up to 7 microveins were observed sub-parallel to the main body of one vein, separated from each other by 15 mm wide host segment. Inclusions bands in the fibers show similar widths as microvein separation which suggests that veins grow by accretion of microveins to the main body. Preliminary oxygen stable isotope analyses show that the d¹⁸O values of vein quartz range from 19.1‰ to 19.5‰ and host-rock values  from 16.6‰ to 17.1‰ (coarse laminations) and 18.2‰ (fine lamina).  The host rock is nearly pure quartz, indicating that the higher d¹⁸O values of the veins are not locally derived.