MINERAL DISSOLUTION AND NEOCRYSTALLIZATION IN LIMESTONE SOLUTION STRUCTURES: THE CONTROL OF FLUID CHEMISTRY DURING DEFORMATION

Grain-to-grain and stylolitic solution structures in a central Appalachian Siluro-Devonian limestone macroscale fold were examined by optical reflectance microscopy and SEM-EDS in order to characterize their mineralogy and mineral morphology. The structures range in thickness from <10 µm (grain-to-grain) to >500 µm (stylolitic), and each accounts for 10s to 1000s of µm of volume loss normal to the structure. The host rock mineral assemblage is dominantly ferroan calcite ± chlorite ± illite ± authigenic quartz ± authigenic dolomite ± pyrite. The solution structures however, have three distinct mineral assemblages that are characterized by the dominant iron phase present. 1) chlorite ± illite ± magnetite ± calcite ± quartz; where magnetite occurs as clusters of neocrystallized(?) euhedral <0.1 to 0.8 µm crystals within chlorite-illite folia. More rarely magnetite occurs as pseudoframboids that exhibit grain-to-grain dissolution. Where magnetite is present within the solution structures, pyrite is not found. 2) chlorite ± illite ± pyrite ± calcite ± quartz ± TiO₂ ± magnetite, where pyrite occurs as <1 to 20 µm euhedral to subhedral grains. These grains are similar to those found in the rock matrix and are interpreted to be passively concentrated during solution. Occasionally, pyrite grains that are partially altered to magnetite are found in these solution structures. 3) chlorite ± illite ± goethite ± calcite ± quartz ± TiO₂, where the neocrystallized(?) goethite is characterized by radiating <0.5 µm long needles. In all solution structures, calcite and quartz exhibit significant dissolution and show grain shape alignment parallel to the chlorite-illite folia. Based on the mineral assemblages, the primary control on solution structure mineralogy is the redox conditions present in the solution structures during deformation. The redox conditions may have been controlled by the presence-absence of hydrocarbons and organic acids within the formation fluids, and/or the influx of externally derived fluids during the folding process. The evidence for the growth of new magnetite in the solution structures has significant implications for paleomagnetic applications, and use of AARM and AMS fabrics.