IS THE LIESEGANG PHENOMENON TYPICAL OF DIAGENETIC IRON OXIDE MINERALIZATION?

Among the most distinctive forms of mineralization are the bands of pore-filling iron oxide that are typically called Liesegang bands. Although the Liesegang phenomenon has been the subject of numerous published studies in the chemical literature, it is important to note that laboratory-based Liesegang systems typically have four features that may not be duplicated in systems that appear in the rock record. 1)The lab-based systems are typically 1 or 2-dimensional systems. Typical Liesegang experiments examine the formation of bands in cylindrical glass tubes (1-D) or in petri dishes (2-D). 2) Solute transport in lab-based systems is typically diffusive. 3)Laboratory-based Liesegang systems are typically the product of fluid-fluid interactions. 4)The laboratory-based systems are typically abiotic. We have examined iron oxide mineralization in several formations that have been referred to as Liesegang. Many features considered to be Liesegang bands are actually sheet-like and pipe-like concretions. The three-dimensional geometry of these features is lost when viewed in the two dimensions of an outcrop face. The mineralization in these features is produced by oxidation of reduced iron and subsequent precipitation of iron oxide. We interpret the source of the reduced iron to have been ferrous iron mineral precursors; typically siderite in clastic rocks. Many features referred to as Liesegang are, therefore, three-dimensional systems produced by the flow of oxygenated groundwater into rocks that contain ferrous minerals. Iron is endogenous to the host rock and experiences cm-scale transport by diffusion whereas oxygen is exogenous to the host rock and transported by advection. Many geological systems that are considered to be Liesegang actually have relatively little in common with the Liesegang phenomenon as studied in the laboratory.