Vein formation essentially involves two steps:

A. transport of vein forming material (nutrients) to the growing vein,

B. precipitation of the vein forming mineral(s) in the growing vein.

As veins were defined as forming from precipitation from an aqueous fluid (possibly containing CO2, but excluding melt), it is clear that material transport involves transport in solution and that precipitation involves supersaturation of a fluid. This section deals with how and why material can get transported in solution to veins and why at some stage the fluid may get supersaturated and precipitate vein forming mineral(s). Transport and the cause for precipitation are by no means always independent processes, but a division is attempted here for clarity.

3.1. transport mechanisms

3.1.1. Transport through a fluid: diffusional flow

Diffusional flow is the first of two basic transport mechanisms. This transport mechanism does not necessarily involve movement of a fluid: even in a completely stagnant fluid, there can be a net flux of dissolved material through the fluid if there is a gradient in chemical potential of that dissolved material and the fluid provides a connected pathway. Diffusion is a geologically very important transport mechanism. It is the primary transport 'vehicle' for dissolution-precipitation creep (Durney & Ramsay 1973, Durney 1976, Raj 1982, Rutter 1983) and metamorphic reactions. Although diffusion is a very effective transport mechanism on the small scale (< cm-dm), it is relatively ineffective for transport over larger distances.

3.1.2. Transport with a fluid: fluid flow

The second basic transport mechanism is fluid flow. When a fluid flows, it takes with it its solute. Aqueous fluid have a very low viscosity (compared to rocks) and can therefore move easily and quickly over large distances through rocks. Fluid flow is therefore the only effective mechanism for transport of dissolved material over large distances (>m-km) through rocks (see Jamtveit & Yardley (1997) for a recent review). Again, we can distinguish two types of fluid flow (Appendix A):

1) fluid flow through channels (e.g. fractures) or a permeable medium (Darcian or advective flow);

2) fluid flow with its containing fracture (mobile hydrofractures).

3.1.3. Darcian or advective fluid flow

In the first case, Darcian or advective flow, fluid flows down a gradient in hydraulic head, through interconnected pathways. These pathways can be distinct macroscopic channels, such as fractures, or the pores inside a solid permeable rock (pervasive flow). With localised or channellised flow, the fluid by-passes most of the rock volume, whereas with pervasive flow, the fluid comes in close contact with most of the rock. This of course has significant implications for the chemical interaction between fluid and rock (Rye & Bradbury 1988).

3.1.4. Mobile hydrofractures

In the second case, fluid is contained as a unit in a fracture and both fracture and fluid move at the same time and the same rate. This transport mechanism is known as hydrofracture mobility (Weertman 1971, Secor & Pollard 1975, Pollard 1977, Takada 1990) and is invoked to explain the rapid ascent of magmas (Clemens & Mawer 1992, Clemens 1998), but has received relatively little attention in metamorphic hydrology or hard-rock structural geology. Transport rates in case of hydrofracturing are not determined by Darcy's law. Transport is very rapid (in the order of metres per second), but intermittent: short bursts of mobility are followed by possibly long periods of stagnation (or other types of flow). Hydrofracture mobility is a mechanism for rapid long distance transport of fluids, without much interaction with the wall rock during transport (Davies 1999, Bons in press b)

The difference in rate controlling factors between Darcian flow and flow by mobile hydrofractures can be illustrated by the analogy of a gardener watering his plants. The Darcian flow case would be the case where the gardener uses a hose. The rate at which water reaches the plants is determined by the water pressure at the tap, the diameter of the hose and the length of the hose. The gardener using a bucket to carry the water would be analogous to mobile hydrofracture transport. The rate is now determined by the rate at which the bucket can be filled, the size of the bucket and the distance and walking speed of the gardener.