Nature of the physical processes in the generation of vein haloes in fault-vein systems: implications for hydrothermal systems

Abstract

Understanding the key fundamental physical processes governing vein-halo formation in fault-vein systems is valuable for comprehending a wide range of geological processes. In particular, hydrothermal ore deposits formation, where hot aqueous fluids interact with the wallrock. Understanding how fluids flow forming halo-type veins is essential to assess the potential of higher-grade zones within ore systems. Haloes exhibit evidence of the passage of fluids through rock fractures and mass transfer processes at various scales. Furthermore, haloes mimic damage zones during fracture propagation (e.g. Faulkner et al. 2011), contributing to the identification of scaling relations between halo width and fault displacement.In this work, we examine vein-wallrock geometry, composition, and contact, encompassing halo development. Field samples of well-exposed fault-vein networks were selected. Vein length and width and halo width were measured directly at the outcrop and later under the optical microscope. We performed qualitative textural analyses of the distribution and morphology of the various haloes, alongside SEM analysis to determine element distributions within and around the haloes. By measuring these parameters, we aim to establish the scaling relationships between halo width and vein width and provide information on the main variables controlling the nature of fluid-rock interaction. We hypothesize that the halo width to vein width ratio is related to the mechanism by which the halo is generated. I.e., whether the halo formed by elements transferred from the vein fluid to the wallrock or vice versa.We analyzed veins and haloes formed as extensional, extensional-shear, and shear fractures in different host rocks. Currently we are studying twenty vein-containing samples with distinct halo-wallrock contacts from IOCG deposits in the Chilean Atacama Desert and from the Chinese Cathaysia tectonic block, showing a roughly linear halo widths and vein widths ratio.Such ratios and scaling have potential implications on a more reliable estimation of ore grade variations away from high-grade mineralized veins to the relatively lower grade surrounding wallrock volumes. Mesoscopic veinlets and haloes might upscale to district scales, where there is incomplete ore deposit exposure.