This study deals with the oxygen isotope composition of hematite-rich ore bodies in the Iron Quadrangle, Brazil. The area studied can be divided into two different regions: a western (W) zone of greenschist facies assemblages and an eastern (E) zone of amphibolite facies with transitions into granulite facies assemblages.
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 between +6 and 20‰ except one value near +23‰, whereas the iron oxides fall between −4 and +10‰, with nearly 80% of the iron oxide values between −0.5 and 4.0‰. The regional distribution of the δ18O values is as follows: in the W-region 85% of the quartz are >12‰, whereas in the E-region only 46% fall in this range, In contrast to quartz the iron oxides do not show any regional differences.
The variation of oxygen isotope fractionations between quartz and iron oxides is obviously related to the complex deformation history of the iron ores. Samples with a primary schistosity (S1) only represent peak metamorphic conditions. In the E-region the (S1) high temperatures >700° C seem to correspond to orogenic events in the Archaen basement 2,700 m.y. ago. In the W-region S1-temperatures between 460° and 560° C seem to represent peak metamorphic conditions of the Proterozoic Minas metamorphism 2,000 m.y. ago. Iron ores which have been overprinted by later deformation events 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 associated with these hematite-rich ore bodies appear to be sedimentary-metamorphic rather than metasomatic processes. Furthermore, there is no evidence for secondary leaching by weathering solutions.