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.