Origin of iron oxide spherules in the banded iron formation of the Bababudan Group, Dharwar Craton, Southern India

Abstract

The banded iron formation of the Bababudan Group (Western Dhawar Craton, India) is composed of millimetric to centimetric alternating quartz and grey to red Fe-oxide bands.Major phases are quartz and martite (hematized magnetite) with minor Fe-sulfides and CaMg-Fe-carbonates. Micrometric Fe-oxide spherules fill cavities in discontinuous micrometric layers of Fe-oxides that occur in the massive quartz layers and at the interface of massive Feoxide and quartz layers. The spherules are composed of micrometric radial plates of hematite intergrown with nanometric magnetite. These spherules contain carbonaceous matter (CM) with nanometric Fe-particles and have low N contents (~ 900 ppm; CM1). The spherule formation is attributed to a low temperature hydrothermal process (150-200°C) at around 2.52 Ga, possibly favored by the presence of CM. These hydrothermal fluids dissolved diagenetic interstitial sulfides or carbonates creating cavities which, provided space for the spherule precipitation. Carbonaceous matter of semi-anthracite maturity is encapsulated in quartz grains adjacent to the Fe-oxide spherules (CM2) and it is thus concluded that CM1 and CM2 are most likely contemporaneous and of the same origin, either incorporated at the time of BIF formation or during the hydrothermal event at 2.52 Ga from the underlying phyllitised black shales. Carbonaceous matter (CM3) was also found around the Fe-oxide spherules and the martite grains. CM3 has much higher N contents (> 5000 ppm) and is of a lower maturity than CM1 and CM2 and is related to weathering, also indicated by the presence of goethite and kaolinite. The  13 C of all CMs varies from -19.4 to -24.7 ‰, similar to values measured in the underlying phyllitised black shales and likely reflect denitrifying microbial activity.

Micro- to nano-scale characterization of martite from a banded iron formation in India and a lateritic soil in Brazil

“…This interpretation is consistent with the geological environment: the Indian martitehosting BIF was affected by greenschist facies metamorphism related to the emplacement of the post-kinematic Chitradurga granite (2.60 ± 0.02 ga) and to gold mineralization during compressional tectonics at 2.52 ga (Taylor et al 1984;Kolb et al 2004;Jayananda et al 2006;Sarma et al 2011). These oxidizing hydrothermal fluids dissolved carbonate minerals and precipitated hematitemagnetite spherules in cavities (Orberger et al 2012). The transformation from magnetite into hematite and the trellis formation are thus attributed to this hydrothermal event, as proposed by Beukes et al (2008) for the origin of porous martite from the Archean BIFs at Noamundi, NE India.…”

“…The BIF chemically precipitated from a mixture of hydrothermal and seawater fluids (Kumar and Das Sharma 1998;Srinivasan and Ojakangas 1986;Orberger et al 2012) and experienced greenschist facies metamorphism (Taylor et al 1984;Jayananda et al 2006;Sarma et al 2011). The Indian sample is characterized by alternating millimetric layers of gray Fe-oxide and white quartz (Fig.…”

“…We studied two samples from different geological environments. The first belongs to chemically precipitated BIFs, which were affected by lower greenschist facies metamorphism (Bababudan group; 2.9 ga, Mid-Archean Western Dharwar Craton, Southern India; Orberger et al 2012). The second sample comes from a lateritic soil, which developed on siliciclastic and volcanic rocks previously affected by metamorphic fluids (MidProterozoic Espinhaço Supergroup, Minas gerais, Brazil;Cabral et al 2011Cabral et al , 2012 and references therein).…”

Mineralogical and microfabric characteristics of magnetite in the Wuyang Precambrian BIFs, southern North China Craton: Implications for genesis and depositional processes of the associated BIFs

“…Moreover, the occasional existence of siliceous magnetite in BIFs implies biologic activity during their formation process (Huberty et al, 2012). Additionally, some iron oxide spherules in BIFs were also attributed to biogenetic activities (Orberger et al, 2012). The aforementioned studies denote the close relationship between BIFs and biological activity.…”

“…The BIFs appear to be concurrent with various geological events, such as the rapid crustal accretion (Rasmussen et al, 2012), the tectonic plates convergence (Dobson and Brodholt, 2005) and the activity of mantle plumes (Barley et al, 1998;Konhauser et al, 2002;Wang et al, 2009). They can also help us to decipher the evolution of the Earth's atmosphere (Klein, 2005;Babinski et al, 2013) and understand complex mechanisms controlling important geobiological activities (e.g., Orberger et al, 2012;Li et al, 2013c).…”