Geochemistry of fluid inclusions in the Vazante zinc deposit, Minas Gerais, Brazil

Abstract

The Vazante deposit in Minas Gerais, southeast-central Brazil is the world’s largest known occurrence of hypogene non-sulfide zinc (HNSZ) mineralization, originally consisting of about 28.5 million tonnes of ore at an average grade of 18% zinc. The zinc mineralization consists mainly of willemite with lesser, broadly coeval sphalerite and is hosted by sheared Proterozoic slaty dolomites. The Vazante deposit lies at the southern end of the 120 km long Vazante-Unaí trend, the largest district of Zn-Pb mineralization in Brazil, which consists of both hypogene non-sulfide and sulfide deposits. All of the Vazante-Unaí mineralization lies within the Brasília fold belt in the western part of the Bambuí foreland basin and is thought to be a product of fluids mobilized by the 630-610 Ma Brasiliano orogeny. Little is known about the nature of the fluids that form HNSZ deposits beyond fluid inclusion bulk salinities and homogenization temperatures. The present study of the Vazante deposit is the first to measure the elemental composition of potential mineralizing fluids in a HNSZ deposit, and was carried out using LA-ICP-MS and microthermometry. The analyses were all performed on fluid inclusions hosted by sphalerite, as no primary fluid inclusions were identified in willemite. Most of the primary fluid inclusions in sphalerite were found to contain aqueous liquid and vapor in highly variable proportions. Some primary fluid inclusions (~20%) consist entirely of aqueous liquid. Most secondary fluid inclusions were found to consist of a single aqueous liquid phase with about 10% consisting of aqueous liquid + vapor. A few fluid inclusions were observed to contain rhombohedral crystals, which are interpreted to be accidental carbonate mineral inclusions rather than daughter minerals based on their inconsistent occurrence within fluid inclusion assemblages, variable sizes in proportion to their host fluid inclusion sizes, and the abundance of carbonate mineral inclusions in the sphalerite matrix. Microthermometry was performed only on the liquid-vapor primary fluid inclusions and yielded highly variable homogenization temperatures ranging between 47 and 352° C. This high degree of variability is interpreted to be a product either of the deformation and low grade metamorphism that affected the Vazante-Unaí trend during the Brasiliano orogeny, or of necking down during cooling. Salinities were also found to be highly variable, ranging from 0.9 to 22 equivalent weight percent NaCl, and are interpreted to reflect mixing of a saline fluid with a dilute fluid. No correlation between salinity and homogenization temperature was observed. LA-ICP-MS analyses were preformed after the microthermometry analyses to determine the elemental compositions of the fluid inclusions. Absolute elemental concentrations varied widely in keeping with the wide range of fluid inclusion salinities, but the atomic ratios of elements with respect to Na were relatively constant as a function of salinity, consistent with the pattern expected from the dilution of a saline fluid. Average atomic ratios were K/Na = 0.36, Mg/Na = 0.078, Ca/Na = 0.12, and Sr/Na = 0.001. Ba was consistently detected in the fluid inclusions at concentrations of 10’s to 100’s of ppm. Pb was quantifiable in about 14% of the fluid inclusions in concentrations of 100’s to 1000’s of ppm. Vazante fluid inclusion compositions obtained from the present study were compared to Morro Agudo fluid inclusion compositions obtained from previous studies. Vazante and Morro Agudo fluid inclusions were found to have similar K/Na and Ba/Na ratios but Vazante fluid inclusions were found to have significantly lower Mg/Na, Ca/Na, and Sr/Na ratios and to be overall more dilute. Also, none of the Morro Agudo fluid inclusions contained any detectable Pb. Thus, the Vazante and Morro Agudo deposits and their corresponding different styles of mineralization appear to have been formed by different fluids. The compositions of Vazante fluid inclusions were also compared to the compositions of fluid inclusions in Mississippi Valley-type (MVT) deposits in the central United States and in the Irish deposits. Vazante fluid inclusions were found to have higher Ba/Na, K/Na, and Mg/Na but lower Ca/Na and Sr/Na atomic ratios than Ozark MVT fluid inclusions. Vazante fluid inclusions were also found to be distinct from Irish fluid inclusions, having higher Ca/Na, K/Na, and Mg/Na ratios but lower Ba/Na and Sr/Na ratios than Irish fluid inclusions. The fluid inclusion microthermometry and LA-ICP-MS data suggest that the Vazante mineralization formed as a result of dilution of a brine. Willemite and sphalerite solubility are identically sensitive to salinity, but willemite solubility decreases more strongly than sphalerite solubility with respect to pH increase. If the diluting fluid had a higher pH than the brine, then the higher fraction of dilute fluid present in Vazante fluid inclusions compared to Morro Agudo fluid inclusions may mean that the pH of the ore fluid mixture at Vazante was higher than at Morro Agudo, which would favor willemite precipitation and explain its predominance at Vazante. This hypothesis was tested using reaction path modeling, in which potential end member ore fluids were extracted from the fluid inclusion data and allowed to mix. The results confirmed that willemite precipitation should predominate over sphalerite when a cool, dilute, basic, oxidizing fluid mixes with a hotter, saline, acidic, reducing fluid, and produce an overall ore mineral assemblage that resembles what is observed in the field.