Abstract. Retrograde textural and chemical changes in oxide minerals from the Proterozoic Serrote da Laje deposit, northeastern Brazil, have been investigated. The deposit is situated in a mafic-ultramafic layered sill. Oxidation and cooling leading to successively decreasing diffusion rates resulted in disequilibrium on the microscale. Pleonaste in particular shows a rapid change in composition between (a) coarse grains in a granoblastic magnetite host, indicating metamorphic peak conditions, (b) coarse lamellae in magnetite, indicating commencement of exsolution, and (c) composite pleonaste -ilmenite lamellae in magnetite, which indicate oxidation exsolution. Barren rock layers cooled under more oxidized conditions compared with oxide-rich layers. Formation of pleonaste-and ilmenite lamellae in magnetite and ilmenite -hematite relations are discussed.An understanding of the mechanisms behind chemical and textural interactions in complex systems is essential for the interpretation of metamorphic processes. Reactions in different mineral systems may differ enormously kinetically, but there are reasons to believe that the basic mechanisms are essentially the same. In this paper certain textural and chemical changes in oxide minerals of a metamorphosed, intrusive rock-suite from Serrote da Laje, northeastern Brasil, are discussed in relation to cooling and to oxidation. Some observations are compared with textures, formed during experimental reactions among phases in the Cu-Fe-S system. Reaction rates between Cu-rich sulphides are many orders of magnitude more rapid than between oxide minerals. Comparison with the "rapid" experimental system in particularly advantageous in interpretation of textures from natural "sluggish" systems, since experimental conditions can be controlled with respect to reaction time, temperature, and chemical composition, in order to follow the reactions step by step.It should be stressed that the present paper is concerned with textural relations which are common in metamorphosed Fe-Ti oxide-rich rocks all over the world. Data concerning ore formation and regional geology will be published elsewhere, hence only essential geological information is given here, The discussion is based on microscopic observations and microprobe analysis.
General characteristicsThe Proterozoic Serrote da Laje deposit, which was discovered by DOCEGEO in 1982, is situated about 400 km NE of Salvador in Alagoas State, northeastern Brazil (9~176 General geology, lithology and metamorphism are discussed by Horbach and Marimon (1988). It comprises a reserve of approximately 60 M tons with a tenor of 0.8% Cu and 0.22 ppm Au and contains both oxides and sulphides. It is confined to a strongly deformed, 100-400 m thick, differentiated Fe-rich, maficultramafic sill, with layered hypersthenites, hypersthenebearing biotite schists, amphibolites, and metagabbro. Intercalations of noritic and anorthositic layers are essentially devoid of oxide concentrations. The country rocks are dominanted by metasedimentary gneisses and ...