Metallogenesis of a B-F-metasomatic skarn tin deposit, Gejiu, Yunnan Province

Abstract

Recently, a new type of skarn tin ore formed by B-F-metasomatism has been found in the central part of the Malage ore field at Gejiu, Yunnan Province, which contains 1–7% Sn. The principal tin minerals are nordenskiol dine, schoenfliesite, varlamoffite, etc., accounting for about 4%. Cassiterite is rarely seen (less than 0.2%). Outward from endoskarns are distinguished four alteration zones, ore fluids were rich in Al and Si in the early stage of skarnization, and became enriched in Fe, Mg, Sn, B, F, H2O, etc. in the later stages. Two episodes of hydrothermal metasomatism of B and F took place during post-skarnization, and the different alteration zones may be due to reaction between B-and F-bearing hydrothermal solutions and country rocks (calcareous carbonates). The stability of tin minerals is controlled by μB2O3, μCO2,μF2O-1 and μF(OH)-1. It has been concluded from the discussion in terms of the system CaO-SnO2-SiO2-H2O-B2O3-CO2-F2O-1 proposed by Burt (1978) and the topological diagrams constructed by the author that nordenskioldine was formed in the depletion of Si at about 400°C in the skarn environment, where metallogenetic conditions are characterized by rich oxygen but poor sulfur, and high boron but low fluorine, while schoenfliesite and varlamoffite were formed at the lower temperature (about 150°C) in response to hydrothermal metasomatism of fluorine. In comparison with the metallogenetic conditions for adjacent endoskarn tin ores and other tin ore deposits at Gejiu, the role of the characteristic elemental association Sn-B-F is emphasized in this paper.