The formation of large quartz veins by rapid ascent of fluids in mobile hydrofractures

Abstract

This paper aims to resolve two main problems related to the formation of quartz veins: (1) the predominance of quartz veins at shallow crustal levels and not deeper in the crust, close to the source of metamorphic fluids where the temperature sensitivity of quartz solubility is much higher than at lower, upper-crustal temperatures and (2) the formation of very large 100–1000m scale quartz veins that would require huge amounts of fluid flow in current models of vein formation.It is proposed here that these problems are resolved by the recognition of very fast (m/s) mobile hydrofracture ascent of batches of fluid. Mobile hydrofractures are fluid-filled fractures that propagate at the upper tip and simultaneously close at the bottom end. As such, the fluid moves with its containing fracture. Mobile hydrofractures can attain larger sizes than predicted from current theory, due to a combination of channelling and accumulation at obstacles. The very fast ascent does not allow for significant cooling and precipitation of dissolved material during ascent and thus brings hot fluids with high concentrations of dissolved minerals to high levels in the crust. Precipitation of the dissolved material takes place immediately upon arrest, leading to possibly large mineral deposits at a single site (e.g. huge quartz veins) and, in some cases, extensive wall rock alteration. The vein and fracture structures that result from this process are emplacement structures, that only indirectly record the most important mode of fluid flow.The theory is applied to a case study on abundant and sometimes very large (>100m) quartz veins at Poolamacca Station, western New South Wales, Australia.