Hydrothermal breccias in vein-type ore deposits: A review of mechanisms, morphology and size distribution

Abstract

Breccias are among the most widely distributed rock textures found in hydrothermal vein-type deposits. Previous studies have mainly been interested in developing qualitative descriptive approaches, leading to a confusing profusion of terms. Brecciation originates in numerous ways, resulting in highly complex classification systems and frequent misinterpretations of facies. Field observations are difficult to reconcile with physical theories of fragmentation, partly due to the fact that few satisfactory quantitative tools have been developed. A review of the main brecciation processes occurring in hydrothermal vein-type deposits allows for the discrimination between chemical and physical mechanisms, including tectonic comminution, wear abrasion, two types of fluid-assisted brecciation (hydraulic and critical), volume expansion or reduction, impact and collapse. Each of these mechanisms can be distinguished using nonscalar parameters that describe breccia geometry, including fragment morphology, size distribution of the fragments, fabric, and dilation ratio. The first two parameters are especially important because: (1) the morphology of the fragments allows chemical and physical (mechanical) breccias to be distinguished, and (2) the particle size distribution (PSD) is a function of the energy input during breccia formation. The slope of the cumulative PSD (fractal dimension) ranges from high values for high-energy brecciation processes, to low values for low energy processes indicated by an isometric distribution. The evolution of a vein system can be divided into three stages: propagation, wear and dilation. These stages are separated by one threshold of mechanical discontinuity and one of hydraulic continuity. These two thresholds also mark the transition between different types of brecciation. Mineralization occurs during all three stages and may display different textures due to pressure variations. The use of quantitative parameters in fault-related hydrothermal breccias allows a better understanding of the physical parameters related to a vein environment, including structural setting and crustal level, as well as fluid–rock interactions. Recognition of the different breccia types could also be important during the early stages of mineral exploration.