L. Winkelmann Boron in the Bolivian tin belt

Abstract

Tourmaline alteration and high boron con- tents are typical features of the magmatic-hydrothermal systems of the Bolivian tin province. The average boron content in melt inclusions of quartz phenocrysts from tin porphyry systems is 225 ppm (1r-variation range: 110± 420 ppm; n 12) and suggests a magmatic boron input to the hydrothermal tin systems, and not shallow post- magmatic leaching of boron from pelitic country rocks. Boron data from melt inclusions correlate positively with cesium, rubidium and arsenic, and negatively with lithium, titanium and zirconium, and de®ne magmatic fractionation trends. The generally high B, As, Cs and Li contents in melt inclusions suggest involvement of pelitic source lithologies undepleted in these ¯uid-mobile components, i.e. ®rst-cycle metamorphic rocks. Mag- matic fractionation modi®ed the trace-element contents within a one-log-unit range. Bulk-rock Nd isotope data (eNd )5 to )10) are in agreement with the dominantly intracrustal geochemical signature of the Bolivian tin porphyry systems, but also imply a variable but minor mantle input. The metallogeny of the tin belt is likely a consequence of intracrustal melting of Lower Paleozoic pelitic and slightly carbonaceous source material, com- bined with an extended magmatic evolution. The long- lived thermal preparation of the root zones of the silicic systems is provided by ma®c magma which also leaves a chemical imprint in the form of the hybrid dacitic bulk composition of the tin porphyry systems.