Origin of Minette-Type Ironstones
Agemar Siehl, Jean Thein
Minette-type ironstones containing clasts, pisoids, and ooids of silica-rich, aluminum-substituted ferric oxides, ferrous silicates, or a combination of both, in various detrital associations, occur in different sedimentary environments: shallow marine to deltaic, lacustrine, fluvial, and pedogenic. Formational and depositional environments cannot always be distinguished, especially where goethite or berthierine/chamosite is the primary mineral formed. Most marine and fluvial minettes consist of reworked, coated grains deposited in agitated water, but structural features also indicate in-situ formation in a supporting medium in lateritic or other hydromorphic environments. In the zone of oscillating ground-water table, repeated leaching and subsequent concretionary precip tation of hydrated ferric oxides occur, according to the prevailing Eh/pH conditions and microbial activity. The low aluminum substitution of goethite corresponds to the observed range in oolitic ironstones. Therefore, we assume erosion, reworking, and fluviomarine redeposition of soil-derived ooids to be the major processes generating minette-type ironstones, where uncoated ferricrete clasts of the same grain size are always present.
Postdepositional diagenetic changes may convert the ferric oxides into berthierine/chamosite if the chemical bulk composition of the primary goethite is similar. Slight variations of the iron-aluminum-silicon proportions could enhance or hamper the transformation to ferrous silicates and may result in ooids with alternating ferrous/ferric crusts. Since any aquatic milieu with appropriate fluctuations of Eh and pH can produce ferruginous coated grains, marine iron ooids associated with hardgrounds and areas of low sediment input can also occur. But there, release of ferrous iron, transport in saline interstitial waters, and fixation of ferric oxides occur on a much smaller scale, and cannot generate the huge iron accumulations of minette-type ore deposits. Also, oscillation amplitude o the redox interfaces is much smaller than that in deeply weathered rocks of subtropical regions with pronounced alternations of humid and dry seasons, where lateral migration of fresh ground water carries iron in various dissolved states. Ultrastructure scanning electron microscopy investigations of ferruginous ooids and pisoids from various environments reveal that size, shape, and orientation of constituent particles highly depend on diagenetic recrystallization and usually do not provide diagnostic means to determine the origin.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.