J.S. Mason - Central Wales Orefield: Breccia textures


The Central Wales ore deposits are hosted by fault-fracture systems in which small to moderate normal and dextral wrench-movements may be demonstrated. Most of the fractures trend ENE with a minor suite trending NW-SE.

The fact that many breccia samples show radial growth of minerals about clasts led to the realisation that brecciation and mineral precipitation must have been virtually simultaneous. These are not classic fault-breccias but "hydraulic breccias" and their mode of formation was eloquently explained by the late W. J. Phillips in 1972.

Phillips demonstrated that hydrothermal fluids are capable of causing fracture propagation by hydraulic means. Anyone who has used a trolley-jack will know that half a pint of hydraulic fluid, under compression, can lift an object weighing two or more tons off the ground. Compressed fluid is a force to be reckoned with!

In Phillips' scenario, the pressure on a hydrothermal fluid occupying a fracture (exerted by the fissure walls) would encourage fracture tip propagation (in addition to the regional tensile stresses that would create the initial conditions for fracture development). In other words, the highly-compressed fluids at the fracture-tip would jack the walls apart! In each episode of fracture propagation upwards. the pressurised fluids would rush into the open space newly created. This would cause a sudden depressurising of the fluid: this change would cause the wallrocks, under considerable lithostatic and pore-water pressures, to explode outwards into the newly-formed relatively low-pressure zone. The process is not dissimilar to the rockbursts which are an ever-present hazard in the world's deeper mines.

The exploded rock-fragments would have a seeding effect in the depressurised hydrothermal fluid, so that minerals would nucleate on the clasts and the cementation process would commence. In some lodes, multiple episodes of brecciation have occurred, in which previously-formed mineral assemblages have been rebrecciated and cemented by later assemblages.

Some of the later assemblages depict a transition from this violent process to more passive, open-fissure filling. The images below are some representative samples.


Phillips, W.J. (1972) Hydraulic fracturing and mineralisation. Journal of the Geological Society of London, 128, 337-359.

Early (A1) breccia: part of a cut and polished slab from the Darren mine, Central Wales, actual size.

Clasts of pale greenish-grey mudstone (belonging to the Cwmsymlog Formation) are cemented by a matrix of tough milky quartz, with bands of fine-grained galena, chalcopyrite and tetrahedrite. A1 breccias are commonly matrix-supported. They occasionally contain vugs lined with prismatic waterclear quartz crystals.
Late (A2) breccia: part of a cut and polished slab from Penycefn mine, Central Wales, half actual size.

Clasts of grey mudstone (belonging to the Borth Mudstones Formation) are closely-spaced and cemented by a matrix of clear quartz, sphalerite and galena. Vugs in A2 breccias are common and are lined with noticeably squat quartz crystals.
Late (A2) vein sample from Pandy mine, Central Wales, half actual size.

Here we have two features: firstly a thin vein containing brecciated rock clasts (below) and secondly open fissure-fill (above) in which mineralising fluids have precipitated quartz on open vein walls during repeated openings. Each new fracture opening has added a new vein. The vein-rock interface is delineated by a thin dark grey parting: some shattering of an older vein has resulted in the partings being disrupted as in the diagonal dark line on the R.

Open fissure-fill is a strong feature of the later (A2) assemblages.