IRON FORMATIONS: WHY THE MYSTERY PERSISTS

The origin of iron formations is one of the greatest geologic mysteries. Ironstone deposits just 5 Ma remain as enigmatic as the oldest Archean deposits. Iron formations unquestionably record Earth’s evolution, given their decline in production. However, we do not yet know what aspect of evolution they record. Perhaps the biggest impediment to solving the iron-formation mystery is the assurance by some textbook authors that the problem already has been solved. Unfortunately, iron formations cannot simply be attributed to oxygen-deficient seawater because the sulfide of anoxic seawater keeps iron solubility as low as in oxidized seawater. Iron formations cannot simply be attributed to climatic variation or volcanism because, taken collectively, they exhibit no obvious correlation to paleoclimate or volcanic rocks. In an attempt to frame this problem, we have compounded it by creating barriers. One is nationalism, as exemplified by classification based on type deposits, e.g. the Lahn-Dill type in Europe, Algoma type in America, and Xinyu type in China. In practice, these are all volcanic-associated iron formations, but each region hides the volcanic association behind a local name. Another barrier is excessive emphasis on texture, e.g., oolitic texture and lamination (banding), to the exclusion of the more challenging issue of iron supply. Generations of budding geochemists have constructed phase diagrams for the sedimentary iron minerals, cementing the notion that equilibrium was achieved in seawater. However, this concept may be an unnecessary barrier because it is possible that the iron-rich fluids were exhalative and that equilibrium never existed at surficial temperatures. A breakthrough in iron formations is more likely to come from an improved understanding of modern processes than additional analysis of old rocks. Surficial sediment on the continental shelf of Venezuela is extensively enriched in iron silicates. Berthierine ooids will be described from Cabo Mala Pascua, Venezuela.