Boron in the Bolivian tin belt

Tourmaline alteration and high boron contents are typical features of the m agmatic-hydrothermal systems of the Bolivian tin province. The average boro n content in melt inclusions of quartz phenocrysts from tin porphyry system s is 225 ppm (1 sigma-variation range: 110-420 ppm, n = 12) and suggests a magmatic boron input to the hydrothermal tin systems, and not shallow postm agmatic leaching of boron from pelitic country rocks. Boron data from melt inclusions correlate positively with cesium, rubidium and arsenic, and nega tively with lithium. titanium and zirconium, and define magmatic fractionat ion trends. The generally high B, As, Cs and Li contents in melt inclusions suggest involvement of pelitic source lithologies undepleted in these flui d-mobile components, i.e. first-cycle metamorphic rocks. Magmatic fractiona tion modified the trace-element contents within a one-log-unit range. Bulk- rock Nd isotope data (epsilon(Nd) -5 to -10) are in agreement with the domi nantly intracrustal geochemical signature of the Bolivian tin porphyry syst ems, but also imply a variable but minor mantle input. The metallogeny of t he tin belt is likely a consequence of intracrustal melting of Lower Paleoz oic pelitic and slightly carbonaceous source material, combined with an ext ended magmatic evolution. The long-lived thermal preparation of the root zo nes of the silicic systems is provided by mafic magma which also leaves a c hemical imprint in the form of the hybrid dacitic bulk composition of the t in porphyry systems.