info:eu-repo/semantics/article
OXYGEN ISOTOPE VARIATIONS IN POLYMETAMORPHIC IRON ORES FROM THE QUADRILÁTERO FERRÍFERO, BRAZIL
OXYGEN ISOTOPE VARIATIONS IN POLYMETAMORPHIC IRON ORES FROM THE QUADRILÁTERO FERRÍFERO, BRAZIL
Autor
MULLER, GEORG
SCHUSTER, ALFRED
HOEFS, JOACHIM
Institución
Resumen
This study deals with the oxygen isotope variations in rich iron ore bodies and itabirites of the Iron Quadrangle (Quadrilátero Ferrífero), Brazil. The δ18O values of 136 quartz-iron oxide pairs have been determined and temperatures of formation have been calculated. The δ18O values of quartz vary distinctly between + 5°‰ and + 20‰ , whereas 80% of the iron oxide values fall in the small range between + 4°‰ and - 0,5‰ in that way the Δ-value variations of the quartz-hematite pairs are dominantly ruled by the quartz. The Quadrilátero Ferrífero can be divided into two different regions: in the large western zone of greenschist facies the quartz is distinctly more enriched in heavy oxygen than that from the smaller eastern amphibolite facies. The variation of oxygen isotope fractionations between quartz and the iron oxides is obviously related to the complex deformation history of the iron ores. Samples with only primary schistosity (S1) represent peak metamorphic conditions. In the E-region these samples yield temperatures of formation > 700°C and seem to correspond to orogenic events in the Archean basement 2,700 Ma ago. In the W-region S1-temperatures between 460°C and 560°C seem to represent peak metamorphic conditions of the Proterozoic Minas metamorphism 2,000 Ma ago. Iron ores which have been overprinted by later deformation events (S2, S3) are selectively reset to lower isotopic temperatures. The more closely spaced the schistosity planes the larger the extent of a temperature lowering. The genetic processes which formed the rich iron ore bodies appear to be sedimentary-metamorphic rather than metasomatic events. Furthermore, there is no evidence for secondary leaching by weathering solutions with respect to the genesis of the blue dust ores. This study deals with the oxygen isotope variations in rich iron ore bodies and itabirites of the Iron Quadrangle (Quadrilátero Ferrífero), Brazil. The δ18O values of 136 quartz-iron oxide pairs have been determined and temperatures of formation have been calculated. The δ18O values of quartz vary distinctly between + 5°‰ and + 20‰ , whereas 80% of the iron oxide values fall in the small range between + 4°‰ and - 0,5‰ in that way the Δ-value variations of the quartz-hematite pairs are dominantly ruled by the quartz. The Quadrilátero Ferrífero can be divided into two different regions: in the large western zone of greenschist facies the quartz is distinctly more enriched in heavy oxygen than that from the smaller eastern amphibolite facies. The variation of oxygen isotope fractionations between quartz and the iron oxides is obviously related to the complex deformation history of the iron ores. Samples with only primary schistosity (S1) represent peak metamorphic conditions. In the E-region these samples yield temperatures of formation > 700°C and seem to correspond to orogenic events in the Archean basement 2,700 Ma ago. In the W-region S1-temperatures between 460°C and 560°C seem to represent peak metamorphic conditions of the Proterozoic Minas metamorphism 2,000 Ma ago. Iron ores which have been overprinted by later deformation events (S2, S3) are selectively reset to lower isotopic temperatures. The more closely spaced the schistosity planes the larger the extent of a temperature lowering. The genetic processes which formed the rich iron ore bodies appear to be sedimentary-metamorphic rather than metasomatic events. Furthermore, there is no evidence for secondary leaching by weathering solutions with respect to the genesis of the blue dust ores.