ON THE HETEROGENEITY OF FLUIDS FORMING BEDDING-PARALLEL VEINS

Geochemical studies of veins are often used to constrain the origin, transport, and storage of ancient fluids in fracture networks. Along with structural information on the timing, geometry, and distribution of the conductive fractures, geochemical data allow researchers to better understand both modern and ancient, fracture-controlled hydrogeologic systems. For regional-scale systems, however, restrictions of exposure, outcrop accessibility, time, and analysis costs often limit the sampling density and consequently the ability of researchers to resolve meter-, decameter-, and in some cases kilometer-scale variability in the paleohydrologic system. In such cases, researchers often implicitly assume that the properties of sampled veins are representative of veins throughout an outcrop, in a given vein set, or throughout a region. Though this assumption is rarely tested, its violation can lead to overly simplistic interpretations that are based on the collection and analysis of only a few samples that are spaced too far apart to accurately capture the true variability of the studied area.

We tested the assumptions of vein homogeneity by analyzing the variability of isotopic, petrographic, and microthermometric properties of two bedding-parallel, calcite veins that are exposed almost continuously for nearly 500 m in Cretaceous mudstones of the La Popa Basin, northeastern Mexico. Analysis of samples collected every 50 to 100 m along each vein show that δ¹⁸O of the veins and host rocks is similar, and varies between 20 to 25‰ (SMOW); δ¹³C of the veins and host rocks varies by less than 2‰ (PDB), and ranges from -3 to -5‰; ⁸⁷Sr/⁸⁶Sr isotopic ratios in the vein calcite range from 0.707213 to 0.707700, and falls slightly below the expected values for similar aged marine rocks. Pseudosecondary fluid inclusions in the veins uniformly yielded salinities less than 6 wt% NaCl equiv., yet their homogenization temperatures ranged from 90 to 180˚C. In the absence of 35 to 45˚C paleo-geothermal gradients, the geochemical analyses suggest the veins formed from warm, upward-migrating brines that mixed with downward-migrating meteoric water. The veins were derived from a large reservoir of fluids that were spatially homogeneous on scales of hundreds of meters to kilometers, and temporally homogeneous for the life of the veins.