Alpine fissure-veins from the Ffestiniog Slate Belt

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Introduction

WHAT ARE THEY?

Alpine-type or Alpine fissure-veins are bodies of quartz and other minerals occupying brittle tensional fractures in relatively competent rock-units, generally occurring within a less competent host. Named after the famed occurrences in the Alps, from which huge crystals of quartz and other minerals have been produced, they are of widespread occurrence in the Caledonides of North Wales. Hosts include dykes, sills and competent beds such as sandstones and tuffs. They are syn-tectonic veins - that is, they were formed during the regional deformation and strain-related metamorphism that led to the formation of the slate itself.

DEFORMATION AND WELSH SLATE

During the Caledonian deformation, the strata of North Wales were compressed under phenomenal pressure. Under these conditions, the clay-minerals making up the muddier sediments became soluble and readily recrystallised, becoming aligned at right-angles to the direction of maximum pressure. Thus the rocks developed a grain - a cleavage. Due to the flaky nature - like mica - of clay-minerals, rocks largely made up of them developed a particularly strong, pervasive, parallel cleavage, making them easily splittable into thin sheets. Thus were the foundations of the Welsh slate industry laid.

COMPETENT vs INCOMPETENT

When such deformation is ongoing, individual rock units deform according to their intrinsic properties, most importantly their competence. Fine-grained muddy rocks are relatively incompetent and can be squashed, folded and so on with relative ease as explained above. However, competent rocks are less co-operative! So if you fold or compress a competent bed, you also end up creating tensional stresses which cause the bed to pull apart in a series of fractures. Try bending an eraser through 90 degrees. It will bend so far then it will start cracking across the axis of the bend. The same thing happens to competent rocks.

FLUID MIGRATION

If an open fissure suddenly appears amidst all of this pressurised rock, it forms a low-pressure zone. Any nearby pressurised fluids will eagerly make their way into it. The fluids interact with the rock and hence carry various substances in solution which are then deposited as minerals.

MINERAL CRYSTALLISATION

Although Alpine fissure-veins often begin their existence as fibre-veins, where crystals grow in contact with both fissure walls as the fissure slowly opens, rapid fracture widening then leads frequently to open-space filling with well-formed (euhedral) crystals coating the sides of cavities.
Mineral assemblages within Alpine fissure-veins tend to reflect the host rock mineralogy and geochemistry to an extent, a feature that suggests that the fluids responsible were in, or close to, chemical and/or thermal equilibrium with the host rocks. So where did the fluids come from? There are various possibilities, including the release of water during recrystallisation of clay minerals.

THE CURRENT STUDY

This page gives information about some recently-studied Alpine fissure-veins outcropping in the working slate quarries near Blaenau Ffestiniog. These have been studied in collaboration with David Green of Manchester Museum and Tom Cotterell of the National Museum of Wales, over the past 4 years. I would emphasise that the work was undertaken with permission of the quarry operators and the sampling locations are on private land. I would also emphasise that the minerals found are by no means common!

Geology

At both the Gloddfa Ganol and Cwmorthin quarries, Middle Ordovician slates are cut by various basic dykes, either fine-grained or porphyritic. In addition there are several tuff turbidite bands known to the miners as "cherts". The latter are distinctive, pale/dark grey banded rocks with a tough, splintery nature. They were formed by underwater flows of fine-grained debris erupted from nearby volcanoes.

Alpine fissure-veins occur within the dykes, within brecciated slate (at Gloddfa Ganol) and within the tuff-turbidites - particularly at Cwmorthin.

	View of the main surface working face at Cwmorthin, looking NE, in which the tuff-turbidite beds may be seen running diagonally down from R to L, displaced here and there by minor late faulting.
	Zoom-in on a thick (2.5m) tuff-turbidite unit. The cleavage of the slate is readily visible (diagonal R down to L), as are bedding-normal quartz veins in the tuff turbidite. These are the Alpine fissure-veins.
	Digitally enhanced close-up of the above tuff-turbidite showing Alpine fissure-veins and banding within the unit.

Mineralogy

Although quartz may form larger crystals to several centimetres, most of the more exotic minerals from these localities are fine-grained. They are best studied using binocular or scanning electron microscopy. The mineral assemblages at each site are broadly similar but there are subtle differences. At Gloddfa Ganol early fibrous quartz-calcite-titanite is followed by a vuggy assemblage comprising quartz, albite, chlorite, anatase, synchysite, and pyrite with minor apatite, galena, sphalerite and chalcopyrite.

At Cwmorthin, early veining is again of a fibrous nature (quartz-calcite-titanite). Later, thin planar open cavities contain quartz, anatase, synchysite and xenotime with minor albite, apatite, rutile, brookite, galena and pyrite. Some typical images are presented below.

	Synchysite (a calcium cerium fluorocarbonate) with anatase on quartz, Cwmorthin quarry. Here, it forms prismatic crystals to a few millimetres in length. The white outer zone is typical. The habit is also reminiscent of the related mineral parisite and both minerals are difficult to distinguish without much analytical work. Photo: David Green.
	Anatase (titanium dioxide) from Gloddfa Ganol. Crystals here occur embedded in chlorite and are always tabular and blue in colour (compare with the black bipyramids from Cwmorthin, above). SEM Photo: John Mason.
	Xenotime (yttrium phosphate) on quartz, Cwmorthin Quarry. Minute crystals a fraction of a millimetre long! Photo: David Green.
	Xenotime on anatase, Cwmorthin Quarry. Image taken on SEM in Backscatter mode - a way of using the electron microscope to find substances containing elements of high atomic number. They show up as relatively bright areas. Energy-dispersive analysis of the xenotime showed that, as well as yttrium, it contains a number of rare-earth elements including significant dysprosium, which is why it looks so bright in this image, by John Mason.
	Pyrite on synchysite from Gloddfa Ganol quarry. Note the thin tabular plates of synchysite - compared to the preferred prismatic habit at Cwmorthin. SEM photo by John Mason.

Future Research

Why are Alpine fissure-veins so interesting? There are a number of reasons. Firstly the mineralogy itself has attracted worldwide interest for years and continues to do so. Secondly, because they are interesting in structural terms because they were formed as a response to deformation. Thirdly, because the veins formed at the time of the deformation and strain-related metamorphism that created the slate, they are of interest because some of the minerals may contain isotopes which will allow dating to be done.

Future research will look at the structural geology of the veins, will further examine the mineralogy and the fluids that deposited the minerals (fluid inclusion studies) and the geochemistry of the minerals present, possibly with a view to obtaining a radiometric date for the mineralisation. There's a lot to be got out of these small, unassuming veins and the even smaller crystals that they contain!

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