Supergene ores, formed by weathering of itabirite (oxide-facies iron formation, averaging 38 percent Fe), are of three intergradational types--enriched itabirite (average 49 percent Fe), intermediate-grade ore (arbitrarily ores with more than 57 percent Fe and less than 7 percent silica), and canga (average 62 percent Fe). Enriched itabirite is typically disaggregated and porous, due to partial solution and removal of quartz; iron oxide is hydrated to varying degree, and enrichment is primarily residual. In intermediate-grade ore, leaching of silica is more complete, secondary enrichment is important, and nearly half of the iron is present as goethite. Canga is a caprock consisting of detrital iron-rich fragments cemented by limonite. This capping protects the soft weathering products below and is essential to supergene ore formation. Climate is also a critical factor, as are grain size and composition of parent itabirite. High-grade hematite ores (more than 66 percent Fe) differ

First Page Preview

First page PDF preview
You do not currently have access to this article.