The high-grade hematite deposits of Minas Gerais, Brazil, are those averaging more than 66% Fe and less than 1.5% H20+. They occur in the Caue Itabirite, a metamorphosed oxide-facies iron formation of Pre-cambrian age. This formation was intricately folded during an orogeny that metamorphosed pelitic rocks to the greenschist and almandine-amphi-bole fades, accompanied by the formation of granitic rocks. The folding produced much plastic rock flowage toward the axes of many folds, thickness of the itabirite ranging from a few to more than 1,400 m. Of 40 ore bodies on which enough information for judgments is available, 11 probably contain more than 100 million tons, 22 between 10 and 100 million tons, and 7 less than 10 million tons. At least two are thought to be larger than 400 million tons. The great majority of these ore bodies are associated with folded structures in the host itabirite, a few are associated with preore thrust faults, and some with abrupt steepening of regional linear structure. Most of the ore occurs in the lower, more siliceous two-thirds of the itabirite; some occurs in the upper third, which is locally dolomitic. Some of the ore is foliated, some unfoliated; the unfoliated cuts the foliated and heals ore breccias. The ore mineral is dominantly hematite. Ore was formed during the metamorphic cycle, after the folding. It is localized in relatively low-pressure, hence higher permeability, parts of folds. Ore replaced pre-existing itabirite, preserving structures. Regional metamorphism indicates that the metasomatizing fluids were at high temperature and under high confining pressure. Increasing solubility of quartz with gradually rising temperature caused metasomatic replacement of quartz by hematite picked up from iron formation by throughpassing fluids during the rising temperature cycle of metamorphism. Source of fluids is thought to have been first the metasedimentary rocks, then granitic gneiss and intrusive granite; the source of heat to have been the metasomatizing fluids and intrusive granite. The unfoliated iron ore healing brecciated foliated ore was deposited during the waning metamorphic cycle. Gold ore with associated tourmaline and cassiterite is found in porous but not massive iron ore. The abundant soft high-grade hematite ore formed from the more porous parts of the hard ore bodies, where metasomatism had not been complete, by supergene leaching of very minor quantities of hematite from crystal boundaries.
|Publication Subtype||Journal Article|
|Title||Nature and origin of the high-grade hematite ores of Minas Gerais, Brazil|
|Series title||Economic Geology|
|Publisher||Society of Economic Geologists|
|Google Analytic Metrics||Metrics page|