Origin of Precambrian iron-formations in the Lake Superior region

Abstract

The Precambrian banded, cherty iron-formations in the Lake Superior region were deposited on broad continental shelves, and deposits of the supratidal, intertidal, and subtidal zones can be recognized in the well-known facies of the iron-formations. The supratidal deposits were primarily calcitic, aragonitic, and sideritic muds. Other minerals, in order of decreasing abundance, may have been gypsum, anhydrite, ankerite, dolomite, quartz, apatite, and halite. Intraclasts were produced by desiccation-granulation, and some of this material was redeposited in the intertidal and subtidal zones. The intertidal zone was characterized by the development of extensive algal reefs and digital-sized and larger stromatolitic mounds. Desiccation-granulation and tidal rip of mineralized stromatolites further added to the accumulation of intraclasts. The intertidal zone was the domain of granules that were derived by abrasion and rounding of intraclasts. The original mineralogy of the intertidal deposits was similar to that of the supratidal zone; however, because of photosynthesis, the sea water was oxygenated. Lozenge-shaped grains are considered to be secondary, silicified, and iron-oxide replicates of original gypsum crystals, and, by comparison with recent evaporites, some anhydrite and halite may also have been present. The deposits of the intertidal and subtidal zones are considered to be comparable to the present-day Abu Dhabi complex in the Persian Gulf. Spherical microstructures are abundant and well preserved in the chert of the subtidal deposits. Pyrite, which characterizes these deposits, probably was formed largely by the bacterial reduction of sulfate derived from the intertidal zone. The laminations in the iron-formations resulted from several causes that were controlled by chemical and mechanical processes, such as the rates and localization of the precipitation of calcitic, aragonitic, and sideritic muds and the mechanical breakup and redistribution of these sediments as sand, silt, and mud. These processes resulted in compositional and textural laminations. The diversity of the banded, cherty iron-formations, recognized in the different facies, then, can be explained largely in the diagenetic changes. The large-scale pervasive diagenetic changes were the replacement of calcite and aragonite by chert and the formation of iron silicates by the reactions between ionic silica and siderite and iron-bearing dolomite. Vein-like structures and the development of magnetite along stylolites indicate mobility of iron in late diagenetic stages, and iron-organic chelates are suggested to account for the mobility. Depending on the composition of the laminae and the chemical environment, silicification during diagenesis produced the hematite, magnetite, siderite, pyrite, and mixed-mineral facies.