EVOLUTION OF FLUID CHEMISTRY DURING SYNTECTONIC VEIN FORMATION

Calcite veins contain an abundance of isotopic and trace element information. By integrating textural observations and microchemical analyses of hydrothermal veins, information is revealed about vein growth history, changes in fluid chemistry, and the evolution of fluid flow pathways during vein growth.

In this study, we examined veins from an extensive fault-fracture network hosted within a limestone-shale sequence from the Lachlan Orogen, southeast Australia. The vein swarm probably formed at depths of 5-10 km (~100-200 ^oC) during late Devonian crustal shortening.

Our study reveals abrupt changes in ⁸⁷Sr/⁸⁶Sr, δ¹⁸O, δ¹³C, Ce/Ce*, Eu/Eu* and trace element concentrations in different growth regions of fibrous, crustiform and laminated veins. The results indicate that during the growth of individual veins, stepwise changes in fluid flow pathways and consequent variations in fluid chemistry occurred. Variations in δ¹³C, Ce/Ce* and Eu/Eu* are interpreted in terms of variable fluid oxidation state and fluid-rock reaction.

This information aids our understanding of the dynamics of crustal permeability and fluid flow in hydrothermal systems. Taken together, our findings are consistent with externally sourced fluids migrating along episodically changing, fracture-controlled, flow pathways. s information aids our understanding of the dynamics of crustal permeability, and fluid flow in hydrothermal systems. In addition, this research may provide information on the chemical conditions conducive to high-grade ore mineralization.