The close association of topaz with cassiterite

The Massangana granitic complex, located in the central region of the State of Rondônia, State, southwestern of the Amazon craton, Brazil, is an important example of the Rondônia Tin Province. The complex represents a batholitic body with conspiscuous ring structures related to successive intrusion events hosting Sn, W, Nb, Ta, topaz and beryl mineralizations associated to pegmatite. In this paper we determine the chemical composition and the minimal conditions of capture of the fluids in topaz from the Massangana granitic complex. The blue topaz has gemological quality and presents five groups of H₂O-NaCl fluid inclusions, with salinity between 3,4 to 11,7% weight % NaCl equivalent, density about 0,75g/cm³and homogenization temperature between 320 to 350ºC. The close association of topaz with cassiterite, wolframite and columbite-tantalite in pegmatites suggests the same minimal homogenization temperature for the crystallization of the mentroned metalic minerals. The pegmatitic bodies in the Massangana Complex are associated with the last magmatic phases and their probable emplacement depth is less than 3 km corresponding to an estimated pressure between 1 to 1,5 kbar.

Keywords : Massangana granitic complex; gemological topaz; fluid inclusions; Roraima.