REE geochemistry of altered tectonites in the Huize base-metal district, Yunnan, China

Hydrothermally altered tectonites (e.g. mylonites, cataclasites) occur in ore-controlling fault zones in the Huize base-metal district. Non-altered wallrocks and altered but non-mineralized tectonites have similar REE characteristics indicating seawater origin of the host carbonate rocks but with significant terrigenous input. Calcite gangues in mineralized (argillaceous/barite-bearing) tectonites and tectonite ores contain the highest REE values and exhibit convex-upward and roughly roof-shaped REE patterns with positive Eu anomalies, which are typical of hydrothermal crustal fluids. Negative Ce and negative La anomalies in calcite gangues imply hydrothermal fluids that previously interacted with marine and terrigenous rocks underlying the carbonate host rocks. Similarities in the REE characteristics of mineralized tectonites with those of the non-altered wallrocks and non-mineralized tectonites support earlier propositions that hydrothermal fluids were derived from the Kunyang Group basement rocks. REE characteristics of mineralized (argillaceous/barite-bearing) tectonites indicate fluid-wallrock interactions during migration of hydrothermal fluids along faults. Convex-upward REE patterns of tectonite ores were likely due to remobilization of early-formed base-metal sulphides whereas convex-upward REE patterns of strongly mineralized tectonites (i.e. with disseminated–massive base-metal sulphides) were likely due to recrystallization of early formed calcite gangues. REE characteristics of altered tectonites due to fluid-rock interactions during fluid migration and metal/mineral remobilization/recrystallization are depicted in Y/Ho–La/Ho, Tb/Ca–Tb/La, and Yb/Ca–Yb/La plots of the data. Migration of hydrothermal fluids along ore-controlling faults was likely induced by seismic pumping associated with tectonic deformation, whereas the remobilization/recrystallization process likely occurred in ‘pressure shadows’ along ore-controlling faults.