A fluid inclusion reconnaissance study of the Huanuni tin deposit (Bolivia), using LA-ICP-MS micro-analysis

Abstract

Fluid inclusions have been studied in selected quartz-cassiterite veins of the Huanuni tin deposit, Bolivia. Fluid inclusion assemblages were found in quartz that bracket at least one stage of cassiterite deposition, and some were found in quartz crystallised during a later sulphide mineralisation stage. All fluid inclusions consisted of two phases only (liquid+vapour), which homogenised to a liquid upon heating and were classified to be of pseudosecondary or secondary in origin. Salinities in fluid inclusions related to the cassiterite deposition were up to 22 wt% NaCl equiv., and homogenisation temperatures of low and high salinity inclusions overlap in the range of 370 to 390 °C. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine element concentrations in selected fluid inclusions. The high salinity fluid inclusions show elevated contents of Li, B, Na, K, Mn, Fe, Zn, Ge, As, Rb, Sn, Sb, Cs, La and Pb compared with the lower salinity samples (wt% NaCl equiv. lower than 6%). High concentrations of incompatible lithophile elements including Li, B and Cs are characteristic for fluids derived from highly fractionated granitic systems. Fe, Zn, Pb, and Ge are the most important heavy metals transported with tin in the aqueous fluid. Sn, Fe, Zn, Pb are probably transported as chloride complexes, and the initial fluid-chemical data for successive fluid-inclusion generations indicates that SnO2 precipitation occurred by injection of a hot saline magmatic fluid into heated low-salinity waters of possible meteoric origin.