Petrochemical Factors Governing the Metallogeny of the Bolivian Tin Belt

Abstract

Geochemical data both from tin-bearing peraluminous granitic plutons of Triassic and Middle Tertiary age in northern Bolivia and from the peraluminous ash-flow tuffs of late Tertiary age from Macusani, southern Peru, and the Morococala and Los Frailes volcanic fields in the central part of the Bolivian tin belt define systematic tin enrichment trends which are consistent with a magmatic evolution controlled by fractional crystallization at a bulk tin distribution coefficient DSn(xtls/melt) <1. Locally similarly fractionated plutonic and volcanic rocks from outside the tin belt (Western Cordillera and NW Argentina) show no tin enrichment, i.e. DSn ≈1. The metasedimentary source of the igneous systems of the Bolivian tin belt was probably not anomalous in tin. However, it provided the reduced environment which gave rise to the ilmenite-series character of the tin-bearing rocks. This contrasts with the magnetite-series affiliation of the non-tin granites and porphyries of the central Andes. A high degree of fractionation together with a low oxidation state of the igneous system are considered to have been the major controls on the origin of the Bolivian tin belt.