Summary:
Banded iron formations of the Iron Ore Group (Archean greenstone belts) of Jharkhand-Orissa region, India host a good number of large iron ore deposits (Fe wt %> 62). Iron ore mineralization of Gandhamardan hill is one of them where iron ores occur in two stratigraphic horizons. One is strictly confined within banded iron formation (stratabound mineralization) with irregular geometry, and show fracture filling and replacement vein-type mineralization along the fringes of hard massive ores of the core. This type of mineralization is exposed along the western slope of the hill.
Hard massive and laminated ores dominate this mineralization. The other type occurs as low dipping sheet like body above banded iron formation and covered by laterites forming the top of the hill. Flaky ores dominate this mineralization with formation of hard goethitic crust near the top. Both the mineralizations contain mineralized banded iron formation core-stones surrounded by hard massive or flaky iron ores. Hard massive ores are entirely represented by martite-microplaty hematite mineralogy. Hard laminated ores contain microplaty hematite and few martite grains representing early magnetites of the banded iron formation. Flaky ores are high porosity ores produced by leaching of silica, martite and microplaty hematite. Hard goethitic ores are developed due to replacement of martite and microplaty hematite or precipitation of goethite in the pore spaces.
Formation of iron ores within banded iron formation horizon, hydrothermal brecciation and replacement of jaspery blocks by iron ores, presence of mineralized banded iron formation core-stones within the massive hard ores and martite-microplaty hematite rich mineralogy of the ores strongly advocate in favour of hydrothermal origin for the Gandhamardan iron ore mineralization. Iron bearing reducing and alkaline basinal or deeply circulated meteoric water may be held responsible for the hydrothermal mineralization. The ore fluid became oxidizing with cooling. Such hydrothermal ores recorded further leaching and goethitization under supergene environment.
Iron Ore Mineralization And Ore Types
There are two types of iron ore mineralizations in Gandhamardan Hill. One is strictly confined within banded iron formation (stratabound mineralization) with irregular geometry, and show fracture filling and replacement vein-type mineralization along the fringes of hard massive ores of the core, and is exposed along the western slope of the hill. This vein-type replacement ore mineralizations are commonly refered to as 'slope ores' by the local miners. The other is a low easterly dipping sheet like ore body that occurs above the banded iron formation and covered by a thick laterite horizon forming the top of the hill.
The ore bodies exposed along the western slope of the Gandhamardan Hill are being mined at four places, viz., Donra, Jharna, Jagar and Putul Pani respectively. On the mine faces these ore bodies exhibit an irregular shape and show a gradual change from hard massive iron ore in the core to the unmineralized banded iron formation in all directions. The maximum length and width of the minable portion of the ore bodies (with Fe > 63%) ranges between 70 to 180m and 15 to 40m respectively.
Presence of undigested or caught-up patches of banded iron formation (core-stones) of different sizes and shape is frequent within these iron ore bodies. The core-stones are partly mineralized and show sharp or gradational contact with massive ores. Near the fringe zone of this type of iron ore mineralization development of hydrothermal breccias are common. The breccia zones are layer parallel and with irregular margin. The voids between the irregular jasper fragments in such breccias are filled up with iron ore veins. Mismatching boundaries of jasper fragments within these breccia zones, indicate their transportations along with migrating iron rich hydrothermal fluid. Locally in thick veins (more than 15cm) these jasper fragments show normal grading, indicating settlement of jasper fragments from the flowing fluid during capacity failure. Within these veins, at places, the contact between jasper fragments and iron ore is diffused, indicating variable degree of replacement of jasper by iron ore.
Locally the whole banded iron formation is transformed to iron ore through replacement of jaspery bands, maintaining their delicate laminated fabric. Further away from the ore mineralization, thin veinlets of iron oxides are emplaced along the laminations in banded iron formation and along fractures across laminations. In few mine sections the iron ores grade to unindurated dusty ore (blue dust) upward before the banded iron formation. The dusty iron ore maintain the overall banded habit of the original banded iron formation. Hence, there is a visible zonation from massive iron ore to intact banded iron formation through a zone of mineralized banded iron formation or an intervening zone of blue dust.
In the top ore, massive iron ores are overlain by soft or friable ores, hard goethitic ore and laterite. The massive ores are banded at places. The soft friable ores retain original banded habit of the iron formation protolith, and also contain undigested iron formation protoliths (core-stones). Ball-like massive hard ores are present within the soft flaky ores. At places hard goethitic ores (duricrust) mantel the soft friable ores before that passes into lateritic top.
Mineralogy of the ores is represented by hematite of different paragenetic types (euhedral, anhedral, cryptoplaty and microplaty), martite (after magnetite), goethite and magnetite. Considering mineralogy and physical attributes like hardness, porosity and friability, the iron ores of Gandhamardan Hill can be grouped into four fundamental types, viz. hard martite-hematite ore, soft friable/flaky martite-hematite ore, hard goethite- hematite ore and unindurated martite-hematite blue dust.
The high grade hard hematite ores are dominantly composed of different generations of hematite, mainly microplaty (5 to 35µm) and cryptoplaty (<5µm) with subordinate martite and magnetite, and are massive or laminated. Massive hard hematite ores essentially consist of martite and variable amount of microplaty hematite. Loss of laminated habit of the iron formation protolith in this ore is due to fabric-destructive replacement of silica by iron oxides. Laminations in the hard hematite ore, where preserved, are due to development of alternating mosaics of microplaty hematite of different dimensions, and the replacement of early silica by iron oxide phase was not fabric-destructive. Magnetite, where present, occurs as patches within equant martite grains. Flaky ores contain thin flakes (few mm to cm) of massive ore in a matrix of microplaty hematite. This ore type is highly porous because of open framework of microplaty hematite and martite. The hard goethitic ores are produced by replacement of early martite or microplaty hematite by goethite. Porosity in this ore type is partially reduced due to filling of pore spaces by late goethite. The blue dust is completely unindurated and composed of loose grains of microplaty, microcrystalline hematite and martite.