FRACTURE CONTROL OF FLUID FLOW AND LIESEGANG IRON OXIDE MINERALZATION

In the western and southwestern United States, the term, “wonderstone,” is used to describe rocks with variegated banding produced by iron oxide mineralization and staining. This pattern is typically referred to as Liesegang banding. Following our studies of wonderstone in sedimentary rocks, we have examined wonderstone from outcrops of volcanic and volcaniclastic rocks in Nevada and Utah. Fractures are widespread in the rocks that contain the wonderstone pattern and break the rock into equant to platy fragments that range from a few centimeters to up to 20 cm in length. These fractures contain little to no iron oxide mineralization. Iron oxide mineralization is, however, abundant in the rocks immediately adjacent to the fracture. Moving from the fracture to the center of the fragment, one encounters successive bands of iron oxide mineralization and light-colored rock that has a bleached appearance. A polarity in the bands of iron oxide mineralization is apparent; one side of an iron oxide band will exhibit a sharp contact with bleached rock whereas the other side will exhibit a gradational transition from iron oxide mineralization to bleached rock. The banding patterns are generally radially symmetrical about the center of each fragment. Biotite displays weathering textures. Locally voids that are similar in shape and size to biotite and hornblende are observed. Morphotypes similar to iron oxidizing bacteria were observed within the bands of iron oxide mineralization. We hypothesize that the rock was once saturated with reducing groundwater which facilitated dissolution of biotite and hornblende. Oxidizing groundwaters then invaded along the well-developed fracture network producing redox gradients sub-parallel to the fractures. Aqueous ferrous iron diffused to these redox gradients. Here bacteria oxidized the iron with subsequent precipitation of hematite and other iron oxides. This process repeated until all available labile iron was oxidized. Although short-range (cm-scale) transport of iron was apparently largely diffusive, the iron-oxide banding in these rocks required invasion of oxidizing waters along a well-developed fracture network.