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ple arrow). Ideal compositions of kaolinite, illite, smectite, greenalite and cronstedtite are marked by brown rectangles along the atomic proportion axes. Fields of clinochlore, chamosite, greenalite-like chlorite and cronstedtite-like chlorite are indicated by colour-coded areas joining the atomic proportion axes of Al—Fe and Si-Fe Fig.6 Al—Fe-Si ternary diagram for Fe-silicate minerals (cf., May- nard 1983). a Synthesised products during reaction of clay and metal- lic iron: product | in oxic synthesis (Rivard et al. 2013), product 2 in anoxic synthesis (Rivard et al. 2013), product 3 at 50 °C (Pignatelli et al. 2014), and Fe-rich product 4 (Rivard et al. 2015). b Samples of the Chilpi Group, defining the iron-enrichment/depletion trend (pur-

Figure 6 ple arrow). Ideal compositions of kaolinite, illite, smectite, greenalite and cronstedtite are marked by brown rectangles along the atomic proportion axes. Fields of clinochlore, chamosite, greenalite-like chlorite and cronstedtite-like chlorite are indicated by colour-coded areas joining the atomic proportion axes of Al—Fe and Si-Fe Fig.6 Al—Fe-Si ternary diagram for Fe-silicate minerals (cf., May- nard 1983). a Synthesised products during reaction of clay and metal- lic iron: product | in oxic synthesis (Rivard et al. 2013), product 2 in anoxic synthesis (Rivard et al. 2013), product 3 at 50 °C (Pignatelli et al. 2014), and Fe-rich product 4 (Rivard et al. 2015). b Samples of the Chilpi Group, defining the iron-enrichment/depletion trend (pur-

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The northwestern part of the craton has a succession of volcanosedimentary rocks of Neoarchean-Paleoproterozoic Era, constituting the Dongargarh Supergroup (S arkar 1957). The Amgaon Gneiss, having 3396+47 Ma (Sm—-Nd whole rock age; Ahmad et al. 2009) Paleoarchean components, is considered to be stratigraphic equivalent o f the Sukma Gneiss (Mohanty 2015, 2021); it constitutes the basement of the Neoarchean-Paleoproterozoic successions in the northwestern part of the craton (Fig. 1). The northwestern boundary of the Amgaon Gneiss is marked by a~ ENE- WSW striking regional shear zone (the Central Indian Shear), which defines the boundary between t he Southern Province of the Bastar Craton and the Satpura Orogenic/ Mountain Belt (SMB) on the margin of the Northern Prov- ince (Jain et al. 1991). The Neoarchean succession of the Dongargarh Supergroup is known as the Nandgaon Group, consisting of the Bijli Rhyolite Formation (2663 + 14 U-Pb zircon age; Manikyamba et al. 2016) at the base, overlained by the Pitepani Andesite Formation. Granite plutons intrud- ing the Nandgaon Group occur in a linear zone with ~ N-S
The northwestern part of the craton has a succession of volcanosedimentary rocks of Neoarchean-Paleoproterozoic Era, constituting the Dongargarh Supergroup (S arkar 1957). The Amgaon Gneiss, having 3396+47 Ma (Sm—-Nd whole rock age; Ahmad et al. 2009) Paleoarchean components, is considered to be stratigraphic equivalent o f the Sukma Gneiss (Mohanty 2015, 2021); it constitutes the basement of the Neoarchean-Paleoproterozoic successions in the northwestern part of the craton (Fig. 1). The northwestern boundary of the Amgaon Gneiss is marked by a~ ENE- WSW striking regional shear zone (the Central Indian Shear), which defines the boundary between t he Southern Province of the Bastar Craton and the Satpura Orogenic/ Mountain Belt (SMB) on the margin of the Northern Prov- ince (Jain et al. 1991). The Neoarchean succession of the Dongargarh Supergroup is known as the Nandgaon Group, consisting of the Bijli Rhyolite Formation (2663 + 14 U-Pb zircon age; Manikyamba et al. 2016) at the base, overlained by the Pitepani Andesite Formation. Granite plutons intrud- ing the Nandgaon Group occur in a linear zone with ~ N-S
Materials and methods Field mapping was carried out in the Chilpi Basin in the area west of Lohara. About 55 samples were collected from freshly broken surface outcrops (Fig. 2) of the chem- ogenic sediments (iron-rich rocks of BIF and carbonate unit). Petrographic, XRD and EPMA studies were car- ried out on selected samples to identify different minerals present and their textural relationship (Figs. 3, 4, 5). Bulk rock geochemical analyses were carried out on a set of samples to infer the depositional condition in the basin (Mishra and Mohanty 2021; Mohanty and Mishra 2023).
Materials and methods Field mapping was carried out in the Chilpi Basin in the area west of Lohara. About 55 samples were collected from freshly broken surface outcrops (Fig. 2) of the chem- ogenic sediments (iron-rich rocks of BIF and carbonate unit). Petrographic, XRD and EPMA studies were car- ried out on selected samples to identify different minerals present and their textural relationship (Figs. 3, 4, 5). Bulk rock geochemical analyses were carried out on a set of samples to infer the depositional condition in the basin (Mishra and Mohanty 2021; Mohanty and Mishra 2023).
(0.5%). The d-value of the peaks are labelled for the identified peak (having common annotation on the lower panel), alongwith the (hkl) value of crystal faces of chamosite (after Brindley 1951). Abbre- viations used for identified minerals: Chm chamosite, Gre greenalite, Hem hematite, /it illite, Mag magnetite, and Qz quartz. Note: 3.34 A peak (of quartz) in K-60 has been clipped Fig.3 XRD patterns of samples containing chamosite and greenalite. The sample numbers are annotated on the right-hand side of the XRD spectrum. K-60: quartz (65.2%), hematite (14.8%), greenalite (10.4%) and chamosite (9.6%); K-43: hematite (63.4%), magnetite (16.8%) and greenalite (19.8%); K-45-1: magnetite (51.1%), hematite (10.5%), greenalite (38.0%) and quartz (0.4%); and K-51: hematite (52.2%), magnetite (33.5%), chamosite (13.0%), greenalite (0.8%) and quartz
(0.5%). The d-value of the peaks are labelled for the identified peak (having common annotation on the lower panel), alongwith the (hkl) value of crystal faces of chamosite (after Brindley 1951). Abbre- viations used for identified minerals: Chm chamosite, Gre greenalite, Hem hematite, /it illite, Mag magnetite, and Qz quartz. Note: 3.34 A peak (of quartz) in K-60 has been clipped Fig.3 XRD patterns of samples containing chamosite and greenalite. The sample numbers are annotated on the right-hand side of the XRD spectrum. K-60: quartz (65.2%), hematite (14.8%), greenalite (10.4%) and chamosite (9.6%); K-43: hematite (63.4%), magnetite (16.8%) and greenalite (19.8%); K-45-1: magnetite (51.1%), hematite (10.5%), greenalite (38.0%) and quartz (0.4%); and K-51: hematite (52.2%), magnetite (33.5%), chamosite (13.0%), greenalite (0.8%) and quartz
netite, cronstedtite and greenalite under dual mode microscopy of K-27. d An iron-rich rock K-45, preserving an oolite containing acicular grains of greenalite and magnetite, and micro-platy cronst- edtite viewed under dual mode microscopy. e A banded iron-rich rock K-30, showing disrupted bands and preservation of green—brown cha- mosite in dual mode microscopy. f An oolite of magnetite (dark gray) surrounded by hematite (light gray) in K-27, indicating oxidation/ replacement at the margin viewed in dual mode microscopy; cham- osite oolites are present around the grain Fig.4 Photomicrographs of selected analyzed samples. a A carbon- ate rock (dolomite), M-41, showing pisolites of fine-grained dolo- mite bordered by a quartz-rich laminae and a medium-grained matrix between the pisolites under plane polarized light. b Oolites of cham- osite and fine-grained magnetite with a border laminae of Fe-silicate phase within a fine-grained matrix of hematite-magnetite and cham- osite in K-17 seen under dual mode microscopy (combined reflection and transmitted modes). ¢ Retention of oolites within a fine-grained mixture of magnetite-hematite, showing fibro-radial grains of mag-
netite, cronstedtite and greenalite under dual mode microscopy of K-27. d An iron-rich rock K-45, preserving an oolite containing acicular grains of greenalite and magnetite, and micro-platy cronst- edtite viewed under dual mode microscopy. e A banded iron-rich rock K-30, showing disrupted bands and preservation of green—brown cha- mosite in dual mode microscopy. f An oolite of magnetite (dark gray) surrounded by hematite (light gray) in K-27, indicating oxidation/ replacement at the margin viewed in dual mode microscopy; cham- osite oolites are present around the grain Fig.4 Photomicrographs of selected analyzed samples. a A carbon- ate rock (dolomite), M-41, showing pisolites of fine-grained dolo- mite bordered by a quartz-rich laminae and a medium-grained matrix between the pisolites under plane polarized light. b Oolites of cham- osite and fine-grained magnetite with a border laminae of Fe-silicate phase within a fine-grained matrix of hematite-magnetite and cham- osite in K-17 seen under dual mode microscopy (combined reflection and transmitted modes). ¢ Retention of oolites within a fine-grained mixture of magnetite-hematite, showing fibro-radial grains of mag-
*All the data are in weight %. ** Interpreted in Table 2 Table 1 EPMA data for composition* of different chlorite phases** analyzed from carbonate and iron-rich rocks of the Chilpi Grouy
*All the data are in weight %. ** Interpreted in Table 2 Table 1 EPMA data for composition* of different chlorite phases** analyzed from carbonate and iron-rich rocks of the Chilpi Grouy
Fig.5 BSE image of the textural context of the grains analyzed. a Carbonate rock M-41, showing micro-oolites of dolomite with cha- mosite inclusion with quartz encrustation and chamosite, ankerite and quartz in matrix. b Carbonate rock M-51, showing platy clinochlo- res within massive dolomite and radiation haloes around few grains. ¢ Iron-rich rock K-45, showing needle-shaped grains of greenalite and magnetite within an oolite. d Iron-rich rock K-17, showing an elliptical oolite containing sudoite core surrounded by chamosite and maghemite bands; a hemi-spherical oolite of magnetite surrounded by chamosite is present on the lower part; radiation haloes around few grains may be noted. e A banded rock K-43 having magnet- ite — maghemite — greenalite bands containing platy greenalite. f A iron-rich rock K-27, showing elliptical oolites/pisolites with internal cracks and powdery grains of cronstedtite
Fig.5 BSE image of the textural context of the grains analyzed. a Carbonate rock M-41, showing micro-oolites of dolomite with cha- mosite inclusion with quartz encrustation and chamosite, ankerite and quartz in matrix. b Carbonate rock M-51, showing platy clinochlo- res within massive dolomite and radiation haloes around few grains. ¢ Iron-rich rock K-45, showing needle-shaped grains of greenalite and magnetite within an oolite. d Iron-rich rock K-17, showing an elliptical oolite containing sudoite core surrounded by chamosite and maghemite bands; a hemi-spherical oolite of magnetite surrounded by chamosite is present on the lower part; radiation haloes around few grains may be noted. e A banded rock K-43 having magnet- ite — maghemite — greenalite bands containing platy greenalite. f A iron-rich rock K-27, showing elliptical oolites/pisolites with internal cracks and powdery grains of cronstedtite
*Bourdelle et al. (2013) method, **Inoue et al. (2009) method, ***Inoue et al. (2018) method, #Walshe (1986) method for temperature in ***, and $ calculated for 25 ‘ Table 2. Composition of chlorite phases, and their geothermometry and redox state in the Chilpi Group
*Bourdelle et al. (2013) method, **Inoue et al. (2009) method, ***Inoue et al. (2018) method, #Walshe (1986) method for temperature in ***, and $ calculated for 25 ‘ Table 2. Composition of chlorite phases, and their geothermometry and redox state in the Chilpi Group
Table 3 Free energy for formation of different minerals/substances associated with iron-rich rocks at 298.15°K and 1 bar (AGyog ;5 in kCal/mol NH,-S-O, system for a;,.= 107, 10°, 10“ and 10°; Po9,= 107! bar; Ayya’ =Aso4° =107, having Fe-siderite-magnetite-goethite-ammo- niojarosite as solid substances (after Gonzalez et al. 2015). Blue lines mark the limit of stability for water. A scale of pE+ pH has been pro- vided in (c) Fig. 7 Pourbaix diagram for iron-water system at 25 °C and 1 bar: a for az,= 10°, 10°, 107 and 10°, with no sulphur, CO, and NH, activities, having Fe, Fe(OH), and Fe(OH); as solid substances (after Pourbaix 1966), b for same conditions as in (a), having Fe, Fe;O, and Fe,O; as solid substances (after Pourbaix 1966), and ¢ Fe-H,0-CO,-
Table 3 Free energy for formation of different minerals/substances associated with iron-rich rocks at 298.15°K and 1 bar (AGyog ;5 in kCal/mol NH,-S-O, system for a;,.= 107, 10°, 10“ and 10°; Po9,= 107! bar; Ayya’ =Aso4° =107, having Fe-siderite-magnetite-goethite-ammo- niojarosite as solid substances (after Gonzalez et al. 2015). Blue lines mark the limit of stability for water. A scale of pE+ pH has been pro- vided in (c) Fig. 7 Pourbaix diagram for iron-water system at 25 °C and 1 bar: a for az,= 10°, 10°, 107 and 10°, with no sulphur, CO, and NH, activities, having Fe, Fe(OH), and Fe(OH); as solid substances (after Pourbaix 1966), b for same conditions as in (a), having Fe, Fe;O, and Fe,O; as solid substances (after Pourbaix 1966), and ¢ Fe-H,0-CO,-
(Yalgintas et al. 2016); Fe(OH), and siderite — Pe9)= 107! atm and SO4 (ota) = 108 M (Cravotta 2008); Fe(OH); — K,,= 10°"! (Lang- muir 1997); AI(OH)3, goethite and kaolinite — SO, (ota) = 10" M and H,SiO,=10°°* M (Cravotta 2008); gibbsite - K,,=10~°° (Langmuir 1997); calcite and dolomite — closed to atmospheric CO, (Somasundaran et al. 1985) Fig. 8 Effect of pH on saturation of Si, Al, Fe*+, Fe?+, Ca and Mg for precipitation of selected minerals/phases, based on solubility stud- ies at 25 °C and 1 bar: a iron minerals/phases and b non-iron min- erals/phases. Solubility conditions: greenalite — ajgio2, aq/= 1077 and 107% M (Klein and Bricker 1977); magnetite (cr) calcu- lated for pe+pH,,=4 (Kobayashi et al. 2013) and pe+pH,,=7
(Yalgintas et al. 2016); Fe(OH), and siderite — Pe9)= 107! atm and SO4 (ota) = 108 M (Cravotta 2008); Fe(OH); — K,,= 10°"! (Lang- muir 1997); AI(OH)3, goethite and kaolinite — SO, (ota) = 10" M and H,SiO,=10°°* M (Cravotta 2008); gibbsite - K,,=10~°° (Langmuir 1997); calcite and dolomite — closed to atmospheric CO, (Somasundaran et al. 1985) Fig. 8 Effect of pH on saturation of Si, Al, Fe*+, Fe?+, Ca and Mg for precipitation of selected minerals/phases, based on solubility stud- ies at 25 °C and 1 bar: a iron minerals/phases and b non-iron min- erals/phases. Solubility conditions: greenalite — ajgio2, aq/= 1077 and 107% M (Klein and Bricker 1977); magnetite (cr) calcu- lated for pe+pH,,=4 (Kobayashi et al. 2013) and pe+pH,,=7
Fig.9 Controls of selected variables on the stability fields of different Al-Si-minerals and Fe-mineral associations in Fe-Si-water systems at 25 °C. a Activities of dissolved silica (H,SiO,) and total Al on Al- Si-minerals and chert association (after Maynard 1983); b activities of dissolved silica (H4SiO,) and Al complexes on greenalite-cham- osite-chert precipitation (after Maynard 1983), showing the region of higher activities derived from metalliferous hot geothermal waters and brines (from Damyanov and Vassileva 2001); ¢ activity of dis- solved silica (H,SiO,) and CO, partial pressure on chert-greenalite- (stilpnomelane)-siderite assemblage (from Klein and Bricker 1977); d CO, partial pressure and pH controls for kaolinite-berthierine-siderite association (from Maynard 1986); e partial pressures of CO, and O, on precipitation of chamosite-siderite-Fe(OH), (after Maynard 1983); and f Peo, and Po, relationship for greenalite-magnetite-dolomite- siderite-Fe(OH), stability (from Klein and Bricker 1977). Note: Figures b-f have dyp.o4., =O. Activities in average present-day seawater have a{Mg7*] =l0-177 (Thompson 1966), ajcy4) =10-*7 (Thompson and Ross 1966), IAL, total = 10°” (Sackett and Arrre- nius 1962) and Anasion = 10-27 (Goldberg 1965; Klein and Bricker 1977). a(sio2, aql =10~*" is saturation with respect to amorphous silica (Siever 1962, 1992), implying precipitation of chert
Fig.9 Controls of selected variables on the stability fields of different Al-Si-minerals and Fe-mineral associations in Fe-Si-water systems at 25 °C. a Activities of dissolved silica (H,SiO,) and total Al on Al- Si-minerals and chert association (after Maynard 1983); b activities of dissolved silica (H4SiO,) and Al complexes on greenalite-cham- osite-chert precipitation (after Maynard 1983), showing the region of higher activities derived from metalliferous hot geothermal waters and brines (from Damyanov and Vassileva 2001); ¢ activity of dis- solved silica (H,SiO,) and CO, partial pressure on chert-greenalite- (stilpnomelane)-siderite assemblage (from Klein and Bricker 1977); d CO, partial pressure and pH controls for kaolinite-berthierine-siderite association (from Maynard 1986); e partial pressures of CO, and O, on precipitation of chamosite-siderite-Fe(OH), (after Maynard 1983); and f Peo, and Po, relationship for greenalite-magnetite-dolomite- siderite-Fe(OH), stability (from Klein and Bricker 1977). Note: Figures b-f have dyp.o4., =O. Activities in average present-day seawater have a{Mg7*] =l0-177 (Thompson 1966), ajcy4) =10-*7 (Thompson and Ross 1966), IAL, total = 10°” (Sackett and Arrre- nius 1962) and Anasion = 10-27 (Goldberg 1965; Klein and Bricker 1977). a(sio2, aql =10~*" is saturation with respect to amorphous silica (Siever 1962, 1992), implying precipitation of chert
Fig. 10 Eh—pH diagrams depicting the stability fields of some min- erals (or their precursors) in the system Fe-H,O-O,-SiO, at 25 °C and | atm total pressure (from Klein and Bricker 1977; Klein 2005). Contours show partial pressure of oxygen (from Walker et al. 1983). Boundaries between aqueous species and solids at ayp., aq = 10°. Anasion = 10-77 represents seawater saturated with respect to amor- phous silica (Siever 1962, 1992) and Anusiog = 104 represents aver- age present-day sea water (Goldberg 1965; Klein and Bricker 1977). AtMgo4j=10-'7” (Thompson 1966) and ajaijotay=!0-°* (Goldberg 1965) represent activities in average present-day seawater. a Phase stability of magnetite, siderite, greenalite, minnesotaite, and stilp- nomelane-like composition (with Fe?*+/Fe*+ =2:1) for two different dissolved C content. b Conditions as in (a) but stilpnomelane not considered. ¢ Greenalite stability for two different activities of Si. d Stability of greenalite having composition representative of natural occurrences. Box with red line in (b-d) indicates mean with standard deviation of the oxygen fugacity range of the studied samples at 25 °C, assuming pH of 7-8
Fig. 10 Eh—pH diagrams depicting the stability fields of some min- erals (or their precursors) in the system Fe-H,O-O,-SiO, at 25 °C and | atm total pressure (from Klein and Bricker 1977; Klein 2005). Contours show partial pressure of oxygen (from Walker et al. 1983). Boundaries between aqueous species and solids at ayp., aq = 10°. Anasion = 10-77 represents seawater saturated with respect to amor- phous silica (Siever 1962, 1992) and Anusiog = 104 represents aver- age present-day sea water (Goldberg 1965; Klein and Bricker 1977). AtMgo4j=10-'7” (Thompson 1966) and ajaijotay=!0-°* (Goldberg 1965) represent activities in average present-day seawater. a Phase stability of magnetite, siderite, greenalite, minnesotaite, and stilp- nomelane-like composition (with Fe?*+/Fe*+ =2:1) for two different dissolved C content. b Conditions as in (a) but stilpnomelane not considered. ¢ Greenalite stability for two different activities of Si. d Stability of greenalite having composition representative of natural occurrences. Box with red line in (b-d) indicates mean with standard deviation of the oxygen fugacity range of the studied samples at 25 °C, assuming pH of 7-8
Related topics:
GeochemistryPrecambrian GeologyOceanic Anoxic EventsBanded Iron FormationGreat Oxidation Event
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