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 that control the generation of vein haloes in fault-vein systems is valuable for comprehending a wide range geological processes. In particular, the formation of hydrothermal ore deposits, 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 a hydrothermal ore system. Haloes exhibit evidence of the passage of externally derived fluids through rock fractures and mass transfer processes from the wallrock into preexisting or newly formed discontinuities at scales from millimeters to kilometers. Furthermore, haloes have been regarded as mimicking damage zones accompanying quasistatic or dynamic fracture propagation (e.g. Faulkner et al. 2011). This, in turn, has allowed the finding of scaling relations between halo width and fault displacement, raising questions on the nature of fault propagation in the crust.In this work, we examine the geometry, composition, and contact relationship between veins and the wallrock, which include halo development (e.g. Vermyle and Scholz 1994). To do this, we selected well-exposed field examples of fault-vein networks, developed on granitic and volcanic rocks. Vein length and width and halo width were measured directly in single veins, both at the outcrop and under the optical microscope. We performed qualitative textural analyses of the distribution and morphology of the various haloes. We also conducted SEM analysis to determine element distributions within and around the haloes. By measuring these parameters, we hope to be able 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. For instance, we hypothesize that the halo width to vein width ratio is related, at least in part, 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. The different wallrock types and vein infills observed allow the quantification of elements gains and losses during vein and halo formation by analyzing mobile and immobile element concentration ratios in the altered and unaltered wallrock.We identify and look into veins and haloes formed as extensional, extensional-shear, and shear fractures in the different host rocks. In particular, we are currently analyzing twenty samples from IOCG mineral deposits in the Atacama Desert, many of which contain multiple sets of veins. To strengthen our database, we also analyzed vein samples from granites of the Cathaysia tectonic block in southeast China. These samples exhibit epidote-dominated veins within a coarse-grained granodioritic host rock. In this case, halo-wallrock contacts are diffuse, and the veins are generally thinner than those of quartz-dominated veins emplaced within fine-grained altered andesite and diorite from Atacama. These veins show sharp halo contacts with the surrounding host rock and display a roughly linear relation between the size of halo widths and vein widths.Halo-width to vein-width ratios and associated scaling have potential implications on amore reliable estimation of ore grade variations away from high-grade mineralized veins to the relatively lower grade surrounding wallrock volumes. For instance, observations of veinlets and haloes at the mesoscopic scale could be upscaled to district scales, especially where there is no complete exposure of the ore deposit.