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

This paper aims to resolve two main problems related to the formation of qu artz veins: (1) the predominance of quartz veins at shallow crustal levels and not deeper in the crust, close to the source of metamorphic fluids wher e the temperature sensitivity of quartz solubility is much higher than at l ower, upper-crustal temperatures and (2) the formation of very large 100-10 00 in scale quartz veins that would require huge amounts of fluid flow in c urrent 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 hyd rofractures are fluid-filled fractures that propagate at the upper tip and simultaneously close at the bottom end. As such, the fluid moves with its c ontaining fracture. Mobile hydrofractures can attain larger sizes than pred icted from current theory, due to a combination of channelling and accumula tion at obstacles. The very fast ascent does not allow for significant cool ing and precipitation of dissolved material during ascent and thus brings h ot fluids with high concentrations of dissolved minerals to hi-h 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 emplacem ent structures, that only indirectly record the most important mode of flui d flow. The theory is applied to a case study on abundant and sometimes very large (> 100 in) quartz veins at Poolamacca Station, western New South Wales, Aus tralia. (C) 2001 Elsevier Science B.V. All rights reserved.