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The importance of diffusion, advection, and host-rock lithology on vein formation: A stable isotope study from the Paleozoic Ouachita orogenic belt, Arkansas and Oklahoma

Ian J. Richards^,1, Jeffrey B. Connelly^,2, Robert T. Gregory^,3 and David R. Gray^,4

¹ Stable Isotope Laboratory, Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA
² Department of Earth Sciences, University of Arkansas, Little Rock, Arkansas 72204, USA
³ Stable Isotope Laboratory, Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275, USA
⁴ School of Earth Sciences, University of Melbourne, Melbourne, Victoria 3010, Australia

More than 600 stable isotope analyses from veins and their metasedimentary host rocks from the Ouachita orogenic belt of Arkansas and Oklahoma provide an opportunity to study fluid-rock interaction processes associated with vein formation during deformation and low-grade regional metamorphism. The ¹⁸O values of vein quartz vary from 16.0 to 26.4, whereas coexisting host rocks have a greater range from 12.9 to 27.4. The oxygen isotopic compositions of quartz vein versus those of the coexisting host rocks follow an array described by ¹⁸O_{vein quartz} ¹⁸O_{whole rock} + , where 8–0.3(¹⁸O_{whole rock}). This relationship emphasizes the dependence of ¹⁸O values of vein quartz on host-rock oxygen isotopic composition. The term empirically monitors the difference between the quartz-water fractionation factor and the compositional dependence of the bulk-rock–water fractionation factor. Vein-quartz–host-rock ¹⁸O fractionations are 0 in chert, novaculite, quartzite, and siliceous shale and typically between 1 and 4 in sandstones and shales. In quartzite and sandstone units that are bounded by shales and associated with significant quartz-crystal deposits, vein-quartz–host-rock fractionations are often unusually large, near 7. Quartz-calcite oxygen isotope geothermometry indicates that veins from the Ouachita Mountains formed over a temperature interval of 100 °C, consistent with fluid-inclusion temperatures previously obtained from quartz crystals. Individual quartz veins are homogeneous, with <0.4 variation, for all vein orientations at all scales, even though vein formation occurred over a temperature interval in which quartz-water fractionation varies by 5. This homogeneity highlights the insensitivity of vein-quartz ¹⁸O values to temperature when veins form under rock-buffered conditions. The similarity between vein and host-rock ¹⁸O values in quartz-rich lithologies, and between vein and host-rock ¹³C values in calcite-bearing rocks, indicates that diffusion was an important mass-transport mechanism. The variability in ¹⁸O values between calcite-bearing veins and host rocks and large vein-quartz–whole-rock fractionations in some sandstones and quartzites indicates that advection also played a major role in mass transport associated with vein formation. This inference leads to the interpretation that veins from the Ouachita Mountains formed by a combined diffusion-advection process, whereby ¹⁸O and ¹³C from the host rock was transported into the veins with the assistance of a rock-buffered fluid on outcrop scales of 10–100 m.

Key Words: Ouachita Mountains • quartz • stable isotopes • veins

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