Geochemistry of Iron Ores

Abstract

Geochemical process of selective concentration of elements is controlled by the dynamic tectonic evolution of earth in the geological time scale of million of years. Iron crystallizes out from magmatic melt if the evolved basal-tic melt is highly enriched with Fe (high chemical activ-ity of Fe) and suitable pressure temperature (thermo-dynamic) condition to stabilize spinel magnetite and ilmen-ite. Iron is soluble in atmospheric EhpH conditions and is amenable to precipitate as hydroxide, oxyhydroxide, carb-onate and sulphide in localized change in Eh-pH. The sili-cate minerals weather to release Fe which precipitates as goethite, chamosite, siderite, pyrite in sedimentary geochemical environment depending upon the low-temperature thermodynamics prevailing in the depositional site. The phase transition to magnetite andhematite is also possi-ble during metamorphism and martitisation. So, concen-tration of iron in geological set up can occur in widely varied conditions from lacustrine to marine, even magmatic to metamorphic.While sedimentary iron deposit is assoc-iated with sedimentary rocks and to their metamorphic equivalent in case the terrain is metamorphosed, the mag-matic iron deposit is associated with basalt and meta-basalt. It is also suggested that hydrothermal action can selectively leach out carbonate-silicate metasediments with supergene enrichment of Fe. Majority of the iron ore deposits (Fig.- 5.1) are confined to Banded iron formation (BIF) of Proterozoic volcano-sedimentary sequence (supra-crustal) of age 2.5-2.4Ga, 2.2 - 2.06Ga and 2.0Ga. Thus, Complex genetic process has a control over the mineral-ogical and geochemical attribute of the iron ore, associ-ated rocks and their host rocks. The associated rocks contribute to the gangue component, which is the interest of Mineral Engineering to get rid of. The geochemical data also helps to understand the ore, its' utility and recov-ery of valuable metals if any.