Abstract
Most high-grade banded iron formation (BIF)-hosted iron ores are composed almost exclusively of microplaty haematite and martite-textured haematite, i.e. pseudomorphs of haematite after magnetite. Goethite abounds only where haematite-rich iron ores are affected by geologically recent lateritic weathering. Magnetite and kenomagnetite occur usually only as small, irregular remnants enclosed in martite. Although low-grade magnetite–carbonate ores have been discovered in spatial association with high-grade iron ore bodies, high-grade BIF-hosted iron ores composed predominantly of magnetite have as yet only been reported from the Iron Quadrangle in Minas Gerais (Brazil). In this contribution, the authors describe such magnetite-rich high-grade BIF-hosted iron ores from the Gorumahishani deposit in the state of Orissa, India. The high-grade magnetite ores occur as stratabound bodies hosted by a single prominent BIF unit of the Mesoarchaean Iron Ore Group (IOG). The ores are composed of an interlocking xenotopic mosaic of fine-grained magnetite, accompanied by trace amounts of Fe-rich talc, sulphides and carbonaceous matter. The occurrence of the magnetite ores is very similar to that of economically significant microplaty haematite–martite ore bodies hosted by the same BIF from other parts of the IOG; both are regarded as being of hydrothermal–metasomatic origin, formed at the expense of the iron formation protolith. The abundance of magnetite, combined with the presence of sulphides, carbonaceous matter, and a marked enrichment of Ni, Co and Zn are attributes that differentiate the magnetite-rich ores from haematite–martite counterparts. These differences are tentatively attributed to differences in hydrothermal fluid composition. Based on the available evidence, it is suggested that magmatic–hydrothermal fluids derived from closely associated mafic intrusives were instrumental in the formation of the magnetite orebodies of the Gorumahishani deposit. The abundance of magnetite and the presence of sulphides and carbonaceous matter indicate that the hydrothermal fluid was mildly reducing, rather than oxidising as proposed for haematite-rich assemblages. The abundance of martite in the coexisting haematite–martite orebodies of other parts of the IOG may be used to speculate that the formation of massive magnetite bodies may have been an essential intermediate stage during the hydrothermal formation of high-grade iron ore deposits of the IOG.