Abstract The Singhbhum Craton preserves large low-grade tracts of an extensive stratigraphic period in the Precambrian and therefore is of prime importance for studying the Earth’s early evolutionary processes. An early ( c. 3.1 Ga) crustal stabilization followed by a long period ( c. 500 Ma) of high freeboard conditions has been postulated from the terrane in recent times. Tectonostratigraphic analyses of the supracrustal successions, carried out in the present study, from the west-northwestern margin of the Singhbhum Granite body in the craton identify a hitherto undetected Mesoarchaean shelf sequence among these supracrustal successions. In contrast to current thinking, the observations imply immediate development of a passive margin setting following the craton’s early stabilization. The cratonic margin later succumbed to a major compression, resulting in successive emplacement of thrust sheets from the northern hinterland side that produced an intermingling of thrust slices of basement rocks and the deformed shelf and rift sequences. This later compressive episode not only involved a part of the Mesoproterozoic Kolhan Basin, but its effects are also manifest as a second deformation throughout the western Iron Ore Group belt. Involvement of the Kolhan Group in the deformation milieu constrains the timing of this orogeny to the Grenvillian ( c. 1.0 Ga).

Abstract The whole-rock geochemistry of banded iron formation-hosted high-grade iron ores has long been ignored as a possible source of constraints on the physicochemical conditions of ore formation. In this contribution, available geochemical data, including major, trace, and rare earth element concentrations, from a selected number of high-grade hematite-martite deposits that represent supergene and hypogene ore-forming environments are collated. Geochemical data for high-grade iron ores are evaluated against the average composition of the BIF protolith, to gauge important trends of enrichment and depletion. Results reveal a generally very similar distribution of major and minor elements, irrespective of deposit type. The marked enrichment of iron is in all cases attributable to the effective removal of SiO 2 , MgO, CaO, as well as CO 2 . The often invoked immobility or even introduction of iron during high-grade iron ore formation is called into question by the observation that the increase in concentration of Al 2 O 3 exceeds that of iron in almost all deposits. Furthermore, the distribution of redox-sensitive elements, such as Mn and V, suggests that during the transformation from BIF to high-grade hematite-martite ore f o2 remained effectively buffered by the oxidation of magnetite to hematite. Distinct enrichment of certain trace elements holds the promise to establish geochemical fingerprints to distinguish high-grade iron ore deposit types of different origin. This applies in particular to supergene high-grade hematite-martite ores, which are characterized by distinctly elevated concentrations of Sr and Ba and the efficient fractionation of LREE from HREE. Hydrothermal, magmatic-hydrothermal and supergene-modified hydrothermal deposits, on the other hand, appear not to have unique geochemical fingerprints. Enrichment of trace metals is usually restricted to single deposits but nevertheless provides an indication that more thorough studies may yield meaningful geochemical signatures to also distinguish different types of hypogene hematite-martite deposits.

Abstract With a current annual production of about 170 million tons (Mt), India is the sixth largest producer of high-grade iron ore (>60 wt % of Fe) in the world. The greater part of the high-grade iron ores of India (>5 billion tons reserve base) are hosted by voluminous banded iron formations of major Archean greenstone belt successions. Major deposit districts include (1) the Noamundi-Koira Valley of the Singhbhum craton of eastern India, (2) the Bailadila-Dalli-Rajhara deposits of the Bastar craton in central India, (3) the Donimalai-Hospet deposits, as well as (4) the Goa deposits of the Eastern and Western Dharwar cratons, respectively, in southern India. The present investigation was carried out to compile, augment and interpret information on the geologic setting, mineralogy, petrography, and geochemistry of the most important districts. Results reveal that high-grade iron ore deposits in the major Indian ore districts have characteristics that are similar to many other high-grade BIF-hosted iron ore deposits worldwide. Close correspondence exists in particular with iron ore deposits regarded to be of supergene-modified hydrothermal origin, in particular those of the giant Serra dos Carajás district (Brazil). Hard ores rich in hematite and martite in most of the Indian deposits are believed to have formed during early hydrothermal events. Chemical weathering in wet tropical humid-monsoonal climate resulted in extensive supergene modification of these hydrothermally upgraded iron ores and surrounding BIF to soft saprolitic hematite-martite ores, as well as the development of surficial goethitic ores. The proposed genetic model leads to the conclusion that currently known high-grade hard hematite-martite ore deposits in India might persist to greater depth than currently envisaged, and that deposits of soft and friable hematite-martite ore might, at depth, be underlain by high-grade magnetite-rich hard ore or, alternatively, by hydrothermally altered BIF.