Samples 1291-1293 were collected at the Donimalai iron mine, in the southeastern part of the Archean Sandur schist (greenstone) belt, in Karnataka (formerly Mysore) state, on 17 November 1990. One of the ore types at the mine is a grey supergene-enriched hematite-rich rock. It is a pale, steel grey, white-dusted lithology with high density, banded on a 1-5 mm scale, and moderately magnetic in hand specimen. The figured samples are the protolith, a banded iron formation (BIF) with even layering. An attractive grey and red banded rock, the BIF displays 2-8 mm red jasper and grey oxide layers in roughly 50:50 proportions. This material is NOT ore in the mine: supergene leaching of jasper is needed to double the circa 30 percent Fe content of the original BIF. Enrichment to ore grade occurs down to a depth of about 170 m. Some of the layers in the BIF have undergone "chocolate tablet boudinage". The BIF is distinctly magnetic, when measured quantitatively, with a magnetic susceptibility of 21x10^-3 SI units (mean of 5 measurements).

"Rock of the Month # 98, posted for August 2009" ---

Samples of banded iron formation, composed of alternate layers dominated by red cherty silica (jasper) and iron oxides (hematite and magnetite). In general, the strata in the Sandur belt young to the northeast, and they were deposited in mainly shallow marine settings (Chadwick et al., 1996, 1997). The rocks in the mine area are dominated by shales with tuffaceous units, BIF and younger diabase (dolerite) intrusions (Murthy and Chatterjee, 1995). The mixed oxide-silicate facies BIF occur as thin bands in the Eastern Volcanic Block of the greenstone belt, and the principal BIF minerals are amphibole (cummingtonite- grunerite), hematite, magnetite, and chert / quartz. The local rocks contain up to 25% Mn and 46% Fe. The mineral species in the sediments include Mg chlorite, sericite, ripidolite, Mn siderite, kutnahorite, dolomite, ferroan dolomite, psilomelane, pyrolusite, cryptomelane, hematite and magnetite (Manikyamba and Naqvi, 1995; Manikyamba, 1998).

The Fe deposits of the Sandur greenstone belt form part of the Archean Dharwar craton of the southern Indian state of Karnataka (Mishra, 1970), and are part of the rich mineral endowment of that state (Mukhopadhyay and Matin, 1993; Murthy and Chatterjee, 1995; Radhakrishna, 1996; Sawkar, 1999). Shiva Kumar and Naganna (1975) recognized seven categories of Fe ores in the area: 1) hard, banded, silica-poor primary magnetite ore, 2) hard, massive hematite ore, 3) hard banded-to-brecciated hematite ore, 4) brittle to hard laminated hematite ore, 5) soft powdery hematite ore (`blue dust'), 6) highly ferruginous quartzites and shales, and 7) laterites.

References

Chadwick,B, Vasudev,VN and Ahmed,N (1996) The Sandur schist belt and its adjacent plutonic rocks: implications for late Archaean crustal evolution in Karnataka. J.Geol.Soc.India 47, 37-57.

Chadwick,B, Vasudev,VN and Ahmed,N (1997) The Sandur schist belt and its adjacent plutonic rocks: implications for late Archaean crustal evolution in Karnataka - reply. J.Geol.Soc.India 49, 461-464.

Manikyamba,C (1998) Petrology and geochemistry of mixed oxide-silicate facies banded iron formations from Sandur schist belt, India. J.Geol.Soc.India 52, 651-661.

Manikyamba,C and Naqvi,SM (1995) Geochemistry of Fe-Mn formations of the Archaean Sandur schist belt, India - mixing of clastic and chemical processes at a shallow shelf. Precambrian Research 72, 69-95.

Mishra,RN (1970) Iron ores of Sandur hills - their exploration and exploitation. Indian Minerals 24 no.1, 20-27.

Mukhopadhyay,D and Matin,A (1993) The structural anatomy of the Sandur schist belt - a greenstone belt in the Dharwar craton of South India. J.Struct.Geol. 15, 309-322.

Murthy,PSN and Chatterjee,AK (1995) The origin of the iron ore deposits of Donimalai area of Sandur schist belt, Karnataka state, India. J.Geol.Soc.India 45, 19-31.

Radhakrishna,BP (1996) Mineral Resources of Karnataka. Geol.Soc.India, Bangalore, Text Book 3, 471pp.

Sawkar,RH (1999) Rational utilisation of low grade iron ore fines and slimes. J.Geol.Soc.India 54, 438-441.

Shiva Kumar,BS and Naganna,C (1975) Study of the iron ores of Sandur schist belt, Bellary district, Karnataka state. In `Studies in Precambrians' (Naganna,C editor), Bangalore University, 291pp., 232-242.