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Magmatic-Hydrothermal BIF-Hosted Iron Ore in the SE São Francisco Craton, Brazil
Silveira Braga FC, Rosière CA, Santos JOS, Pack A & Hagemann SG
Silveira Braga FC, Rosière CA, Santos JOS, Pack A & Hagemann SG (2020) Goldschmidt Abstracts, 2020 2387
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04f: Room 1, View in program
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Hello Flávia, thank you for your nice talk! What a complex system you have been unraveling, impressive. I'm curious about how much Fe you think was redistributed versus introduced in stage 2. You show very nicely that the kenomagnetite from stage 2 was geochemically distinct from stage 1, but could the Fe have been redistributed from the stage 1 IF during stage 2? Or did the anatectic pegmatite transport more/new Fe into the deposit? Is this pegmatite a relative of the enigmatic magmas parental to IOCG-type deposits? My colleagues at the Denver USGS and I have observed that Fe-oxide discrimination diagrams can be problematic due to hydrothermal modification of magnetite, how did you sort out the later overprinting by stage 3? Are there other minerals less prone to hydrothermal modification that support the Fe-oxide story (e.g., apatite)? Do you have any advice on how to approach such a complex system with hindsight from your hard work? Thanks! -Celeste
Hello Celeste, thank you for your interest > We have no evidence yet that pegmatite has transported new Fe into the deposit . The pegmatites that crosscut other country-rock have iron oxides only as accessory minerals and exhibit considerable content of iron oxide only at the contact with IF. None IOCG-type deposit occurs in the region of these pegmatite intrusions. > > The overprinting of stage 3 is noted mainly by the textural characteristics. It is common a transitional fabric between the kenomagnetite domain towards the granular hematite domain. > > I agree with you about the Fe-oxide discrimination diagrams. They have to be used very critically and are not by themselves reliable for genetic conclusions but the position of texturally and chemically different magnetite in different fields support their distinct origin > > Yes, we have apatite in the high-grade domain and pegmatite. The other high-grade ore accessory minerals are ilmenite, quartz, carbonates (calcite and dolomite), chlorite, cummingtonite-grunerite, tremolite-actinolite, biotite, talc and muscovite. > > This work was also supported by geochronological work and Zr-Hf studies to date the succession of hydrothermal events (already published). However, detailed field mapping in the open pit and petrographic work helped me a lot to construct the mineralization model and are essential as a criterion for subsequent geochemical work. Another helpful tool was the geological implicit modeling, where I could recognise the close relationship between iron enrichment and pegmatite in 3D. After these two steps was possible to better choose the samples for chemical, microchemical, and isotopic analysis.
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