Genetic and metamorphic conditions constrained by fluid inclusions from Paleoproterozoic (c. 1.8 Ga) magnesite ore deposits, NE Brazil

摘要

literatura especializada sobre metamorfismo de rochas carbonáticas. As características físico-químicas, composicáo e distribuicáo suge-rem aumento de temperatura aparentemente ligado ao metamorfismo amplamente reconhecido dessas rochas. Na magnesita de grao medio nota-se a contaminação parcial de inclusões ricas em CO_2 por material carbónico reliquiar (betume, hidrocarboneto?) o que favorece, embora nao confirme, a hipótese singenética sedimentar e é também responsável pelo rebaixamento da temperatura de fusáo do CO_2 das inclusões aquo-carbónicas. Portanto, embora existam evidencias texturais relacionadas a um modelo genético sedimentar-diagenético, as inclusões fluidas registram as condições do evento metamórfico.%Precambrian magnesite occurrences hosted by metadolomites from the Oros belt, Ceara, Brazil, are part of a greenschist-amphibolite, metavolcano-sedimentary terrain, dated at 1.8 Ga, cut by Meso- to Neoproterozoic Brasiliano granites and Neoproterozoic basic sills. These rocks were affected by a shear zone between 580 and 500 Ma. The magnesite-bearing marbles can be grouped as medium-grained (1-9 mm) at the Riacho Fundo ore deposit or sparry magnesite (1-15 cm) at the Cabeca de Negro ore deposit. The sparry magnesite shows textural characteristics related to original sedimentary structures. Both types of magnesite-bearing marbles contain aqueous and aqueous-carbonic fluid inclusions that yield homogenization temperatures between 170 and 370 ℃. Applying a pressure correction, these temperatures are compatible with the evolution from greenschist to amphibolite facies metamorphic conditions, as described in previous work on the Oros region. It also agrees with data in specialized literature on the metamorphism of carbonate rocks. Fluid inclusion distribution, composition, and physical-chemical characteristics suggest temperature increase, probably related to metamorphism on these rocks. The medium-grained magnesite records partial contamination of CO_2-rich inclusions by relict carbonaceous material (bitumen, hydrocarbons?) that favors, but does not confirm, a syngenetic sedimentary origin and could have caused the lowering of CO_2 melting point in these inclusions. Therefore, though textural evidence points to a sedimentary-diagenetic model, fluid inclusions record conditions of a metamorphic event.