BAÑADO DE MEDINA BENTONITE DEPOSIT, MELO, URUGUAI. GEOLOGY, MINERALOGY AND TECHNOLOGICAL UTILIZATION

Abstract

The bentonite deposit found in the region of Bañado de Medina, 18 km southwest of the locality of Melo, in Uruguay, is the first one of commercial potential in that region. The aim of this work is to present mineralogy, chemical composition, types of contaminants and factors that control the deposits and technological properties of the bentonite. A promising exploration block was defined based on 31 boreholes, resulting in the delimitation of a bentonite bed that is locally 6.6 m thick. Field studies and borehole analysis have enabled the recognition of a bentonite deposit enclosed by sandstone beds in the sedimentary sequence of the Rio do Rasto Formation (Upper Permian of Paraná Basin). Such deposit was protected against subsequent erosion in tectonically depressed blocks, defined regionally by old basement, NE- and NW-trending tectonic structures. The reserve measured in the 9.5 ha area of the prospected block was estimated to be 580,000 tons, formed by a continuous bentonite bed with 1.60 m to 11.60 m thick sandstone/siltstone cover. The Bañado de Medina bentonite represents the final events of the Serrinha Member from the Rio do Rasto Formation (Vila Viñoles Member from the Yaguari Formation, in the equivalent Uruguayan nomenclature). According to texture, sedimentary structures and color criteria, the bentonite bed was divided in four units. Mineralogy evaluated by X-ray diffraction shows the predominance of dioctahedral calcium smectite followed by quartz and feldspar, sometimes as trace minerals. The chemical analyses indicate a smectite of low Fe³⁺ content and 76% of interfoliar charge, mainly associated to octahedral replacement of Al³⁺ by Mg^2+, classified as Al-montmorillonite. The cation exchange capacity (measured values between 101 and 116 meq/100g) was evaluated in relation to 2EG, 1EG and 0EG domains. The montmorillonite is predominantly of the low charge type (2EG). Technological uses for foundry and drilling fluid were tested. The results indicate adequate thermal stability, good swelling properties and green strength when the bentonite is treated with 5% sodium carbonate.