Sequential vein growth with fault displacement: An example from the Austin Chalk Formation, Texas

Abstract

To determine the opening and precipitation history and characteristics of vein‐forming fluids, analyses of oxygen and carbon isotopes and trace elements were carried out on multilayered crack‐seal calcite veins in the Austin Chalk Formation near San Antonio, Texas. The veins developed within the normal fault zones possessing unique chemical and textural characteristics which indicate sequential vein development. They are composed of alternating millimeter‐ to submillimeter‐thick calcite veinlets and host lithons, occasionally crosscut by coarse, equant‐grained secondary calcite veins. Regular changes in δ 18 O (e.g., −2.6 to −5.6‰, Pee Dee belemnite (PDB)) of the calcite veinlets along the length of veins suggest that the individual calcite veinlets were sequentially developed. A systematic δ 18 O decrease in the vein opening direction primarily resulted from a continuous increase in temperature of the ascending fluids delivered to the Austin Chalk. Relatively constant δ 13 C (approximately +1.4±0.4‰, PDB) for the multilayered veins and most secondary veins indicates that the composition of fluids from which the calcite veins precipitated was initially buffered by the bulk chalk. There is no spatial variation in trace element composition of the calcite veinlets along the length of veins. Low Sr concentrations in both calcite veinlets and secondary veins relative to those of the host chalk reflect a low partition coefficient of Sr in calcite during vein formation. Normal faults in the Cretaceous Austin Chalk were conduits to upwardly mobile vein‐forming fluids.