Academia.eduAcademia.edu

Figure 14 – uploaded by Beate Orberger

See full PDFdownloadDownload figure
Fig. 11. Graph of Ce/Ce* against Pr/Pr* following the approach of Bau and Dulski (1996) to discriminate between a true negative Ce and positive La anomalies. Four carbonate itabirites and the dolomitic limestone are located in the negative Ce anomaly field. All other samples are found in the positive La field.

Figure 11 Graph of Ce/Ce* against Pr/Pr* following the approach of Bau and Dulski (1996) to discriminate between a true negative Ce and positive La anomalies. Four carbonate itabirites and the dolomitic limestone are located in the negative Ce anomaly field. All other samples are found in the positive La field.

Related Figures (21)

Fig. 2. (A) Stratigraphic profile of drillcore SC-18 from the Itabira Group with depths of samples indicated for the QI, shales and carbonates (modified after Pimenta da Silva et al., 2009). (B) 5'3C and 8!80 isotope data for drillcore SC-18 tied to the stratigraphy.
Fig. 2. (A) Stratigraphic profile of drillcore SC-18 from the Itabira Group with depths of samples indicated for the QI, shales and carbonates (modified after Pimenta da Silva et al., 2009). (B) 5'3C and 8!80 isotope data for drillcore SC-18 tied to the stratigraphy.
Fig. 3. Quartz itabirites. (A) Thin section showing alternating, folded bands of chert and magnetite with hydrothermal specular hematite (spec) crosscutting the banding (B) partial martisation (mrt) of magnetite (mag) within a quartz (qtz) matrix (reflected light); (C) anhedral, globular magnetite (mag) with droplets of quartz (qtz) inclusion in a matrix of quartz and carbonates (carb). Also specular hematite (reflected light); (D) euhedral micro-hematite inclusions (inset) in the quartz and a hydrothermal veii containing specular hematite, magnetite, talc and biotite cutting (reflected light); (E) porous intergrown massive ankerite (ank) and euhedral sideroplesite (sdp) with globula magnetite (mag). Subhedral porous apatite (ap) to right of image (SEM-BSE); (F) close association between the magnetite bands and the carbonates (SEM-BSE); G) Micrometri inclusion of chalcopyrite (chp) in a magnetite grain (SEM-BSE).
Fig. 3. Quartz itabirites. (A) Thin section showing alternating, folded bands of chert and magnetite with hydrothermal specular hematite (spec) crosscutting the banding (B) partial martisation (mrt) of magnetite (mag) within a quartz (qtz) matrix (reflected light); (C) anhedral, globular magnetite (mag) with droplets of quartz (qtz) inclusion in a matrix of quartz and carbonates (carb). Also specular hematite (reflected light); (D) euhedral micro-hematite inclusions (inset) in the quartz and a hydrothermal veii containing specular hematite, magnetite, talc and biotite cutting (reflected light); (E) porous intergrown massive ankerite (ank) and euhedral sideroplesite (sdp) with globula magnetite (mag). Subhedral porous apatite (ap) to right of image (SEM-BSE); (F) close association between the magnetite bands and the carbonates (SEM-BSE); G) Micrometri inclusion of chalcopyrite (chp) in a magnetite grain (SEM-BSE).
Fig. 4. (Continued).
Fig. 4. (Continued).
Fig. 5. Carbonates. (A) Hand specimen of massive grey dolomitic limestone with a pure dolomite vein (top right); (B) dolomite (dol) matrix with a vein of intergrown talc (tlc) and chlorite (chl; reflected light); (C) porous dolomite (dol) being replaced by calcite (cal), both associated with granular quartz (qtz) and globular magnetite (mag) with biotite (bio) cutting (SEM-BSE); (D) biotite and talc laths orientated parallel to the faint banding due to magnetite grains (normal light).
Fig. 5. Carbonates. (A) Hand specimen of massive grey dolomitic limestone with a pure dolomite vein (top right); (B) dolomite (dol) matrix with a vein of intergrown talc (tlc) and chlorite (chl; reflected light); (C) porous dolomite (dol) being replaced by calcite (cal), both associated with granular quartz (qtz) and globular magnetite (mag) with biotite (bio) cutting (SEM-BSE); (D) biotite and talc laths orientated parallel to the faint banding due to magnetite grains (normal light).
Fig. 6. Extramil quarry limestone. (A) Thin section scan of hematite nodules clustered in close proximity to millimetric bands of hematite and overprinting bedding; (B) hematite nodule composed of aggregated euhedral, tabular hematite grains with accessory quartz and dolomite (SEM-BSE); (C) dolomite nodule in calcite matrix with minot hematite (SEM-BSE); (D) distortion of hematite band by hematite nodules (polarised normal light); (E) laths of talc (tlc) cutting the calcite (cal) and dolomite (dol) grains in the matrix, which are themselves cut but subhedral hematite grains (hem; SEM-BSE).
Fig. 6. Extramil quarry limestone. (A) Thin section scan of hematite nodules clustered in close proximity to millimetric bands of hematite and overprinting bedding; (B) hematite nodule composed of aggregated euhedral, tabular hematite grains with accessory quartz and dolomite (SEM-BSE); (C) dolomite nodule in calcite matrix with minot hematite (SEM-BSE); (D) distortion of hematite band by hematite nodules (polarised normal light); (E) laths of talc (tlc) cutting the calcite (cal) and dolomite (dol) grains in the matrix, which are themselves cut but subhedral hematite grains (hem; SEM-BSE).
Average electron microprobe data for all carbonate varieties in each sample. Table 2
Average electron microprobe data for all carbonate varieties in each sample. Table 2
Fig. 7. Shales. (A) Hand specimen of the carbonatic shale showing banding between darker and lighter bands of more carbonatic and siliceous-rich bands respectively; (B) contrast between carbonatic and siliceous-rich bands. In the carbonatic-rich bands is heterogeneous ankerite (ank) matrix (due to variations in iron content) and homogeneous ankerite nodules. The siliceous-rich bands contain quartz (qtz) and elongated, subhedral sideroplesite (sdp) grains as well as ankerite (SEM-BSE); (C) jagged and disjointed bands of pyrite (pyr) within the siliceous-rich bands surrounded by ankerite (SEM-BSE); (D) quartz-rich shale with bands of elongated, anhedral pyrite and associated elongated ankerite and sideroplesite (SEM-BSE).
Fig. 7. Shales. (A) Hand specimen of the carbonatic shale showing banding between darker and lighter bands of more carbonatic and siliceous-rich bands respectively; (B) contrast between carbonatic and siliceous-rich bands. In the carbonatic-rich bands is heterogeneous ankerite (ank) matrix (due to variations in iron content) and homogeneous ankerite nodules. The siliceous-rich bands contain quartz (qtz) and elongated, subhedral sideroplesite (sdp) grains as well as ankerite (SEM-BSE); (C) jagged and disjointed bands of pyrite (pyr) within the siliceous-rich bands surrounded by ankerite (SEM-BSE); (D) quartz-rich shale with bands of elongated, anhedral pyrite and associated elongated ankerite and sideroplesite (SEM-BSE).
Fig. 8. The shales and QI of the Itabira Group have ankerites and sideroplesites of similar compositions despite being separated by lithologies with different carbonat compositions (i.e. dolomite). Mn is richest in these lithologies also.
Fig. 8. The shales and QI of the Itabira Group have ankerites and sideroplesites of similar compositions despite being separated by lithologies with different carbonat compositions (i.e. dolomite). Mn is richest in these lithologies also.
Major, minor and trace element compositions for all studied samples. All data obtained by ICP-AES for major and minor elements, and ICP-MS for trace elements at SARM-CRPG (Nancy, France).
Major, minor and trace element compositions for all studied samples. All data obtained by ICP-AES for major and minor elements, and ICP-MS for trace elements at SARM-CRPG (Nancy, France).
Fig. 9. (A) Sr vs. CaO of the studied samples exhibiting a positive correlation; (B) Zr vs. TiO2 of all studied samples exhibiting a positive correlation; (C) S CREE vs. Al2O3 for all studied samples exhibiting a positive correlation.
Fig. 9. (A) Sr vs. CaO of the studied samples exhibiting a positive correlation; (B) Zr vs. TiO2 of all studied samples exhibiting a positive correlation; (C) S CREE vs. Al2O3 for all studied samples exhibiting a positive correlation.
Rare earth element and yttrium (REEY) data for all studied samples. All data obtained by ICP-MS at SARM-CRPG (Nancy, France).
Rare earth element and yttrium (REEY) data for all studied samples. All data obtained by ICP-MS at SARM-CRPG (Nancy, France).
Fig. 10. Post-Archean Average Australian Shale (PAAS) normalised rare earth element and yttrium (REEY) data (Taylor and McLennan, 1985) Bau and Dulski, 1996; Bolhar et al., 2004; Greene and Slack, 2004; Planavsky et al., 2010). All samples of this study have a positive Eu anomaly (average: 1.48) in support of previous observations (Fig. 10 and Table 4). 7. Discussion 7.1. Depositional environments REE+Y values reflect marine conditions only when there is lit- tle to no terrestrial input (Alexander et al., 2008; Bolhar et al., 2004; Bau and Dulski, 1996; Klein, 2005). The here studied sam- ples (excluding the shales and one CI: AMO7A) have low contents of elements that reside in the terrigenous fraction thus indicating BIFs are widely regarded as marine chemical precipitates and their REE+Y patterns are commonly used to trace the solutions from which they precipitated. Archaean chemical seawater precip- itates typically have positive Eu anomalies, which reflect an input of high-T hydrothermal fluids to the oceans (Alexander et al., 2008;
Fig. 10. Post-Archean Average Australian Shale (PAAS) normalised rare earth element and yttrium (REEY) data (Taylor and McLennan, 1985) Bau and Dulski, 1996; Bolhar et al., 2004; Greene and Slack, 2004; Planavsky et al., 2010). All samples of this study have a positive Eu anomaly (average: 1.48) in support of previous observations (Fig. 10 and Table 4). 7. Discussion 7.1. Depositional environments REE+Y values reflect marine conditions only when there is lit- tle to no terrestrial input (Alexander et al., 2008; Bolhar et al., 2004; Bau and Dulski, 1996; Klein, 2005). The here studied sam- ples (excluding the shales and one CI: AMO7A) have low contents of elements that reside in the terrigenous fraction thus indicating BIFs are widely regarded as marine chemical precipitates and their REE+Y patterns are commonly used to trace the solutions from which they precipitated. Archaean chemical seawater precip- itates typically have positive Eu anomalies, which reflect an input of high-T hydrothermal fluids to the oceans (Alexander et al., 2008;
Fig. 12. Ternary plot of FEO-MgO-Mn0O for the separate samples in the quartz itabirites showing their compositionally distinct carbonate chemistry and thus the compositionally distinct nature of their formational diagenetic fluids despite sam- ples being within close proximity to each other.
Fig. 12. Ternary plot of FEO-MgO-Mn0O for the separate samples in the quartz itabirites showing their compositionally distinct carbonate chemistry and thus the compositionally distinct nature of their formational diagenetic fluids despite sam- ples being within close proximity to each other.
Related topics:
Earth SciencesGeologyGeochemistryMineralogyBanded Iron FormationBanded Iron FormationsGreat Oxidation Event
580 California St., Suite 400
San Francisco, CA, 94104
© 2025 Academia. All rights reserved