IRON ORE CAVES IN BRAZIL: LONG TERM SPELEOGENESIS AND MICROBIALLY MEDIATED PROCESSES

The recent expansion of mining operations in Brazil has led to the discovery and study of over 3,000 caves in iron ore. The abundance of iron ore caves (IOCs) is enigmatic, because the host rocks are very resistant to weathering. Alternating bands of silica (sometimes carbonate) and iron characterize the ancient (> 2.5 billion years old) original Banded Iron Formation (BIF) in Brazil. Long term silica/carbonate removal and iron mobilization within the BIF lead to the formation of a highly heterogeneous suite of rocks, including friable high grade iron ore, compact hematite-rich rocks (known as hematitite), and an iron conglomerate cemented by a ferruginous matrix known as canga. All these rocks contain caves, but the contact between the surficial canga and the underlying iron ore tends to concentrate the largest number of IOCs.

Chemical removal of silica leads to both an increase in rock porosity and the concentration of iron, resulting in an association of voids with high-grade ore. Various Fe reducing microbes have been identified in the IOC host rock, and are responsible for converting insoluble Fe⁺³ into soluble Fe⁺², thereby allowing for Fe removal by ground water. These geochemical processes appear to be active at great depth, given the common interception of voids during drilling operations. These originally entranceless caves bear similarities with carbonate hypogene caves, such as an irregular pattern at meso and micro scales, presence of pendants and pillars, lack of water flow marks, etc. IOCs appear to be the product of a complex and still little known suite of speleogenetic mechanisms operative over timescales of several million years, including chemical and microbiological reactions, and both deep (hypogenic?) and surface-related processes.