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Natural exposures of mineralisation were the predecessors to the myriad mineworkings that are scattered throughout the Welsh hills and have attracted human attention from Bronze Age times onwards. Consequently the number of unworked occurrences of well-exposed mineralisation are limited to those without obvious economic interest. But in some cases the scientific importance is nevertheless considerable. Here are some examples.

This is a view looking south from Fairbourne beach in Gwynedd, North Wales, towards the cliff-section known as Friog. The rocks are Middle Cambrian sediments with associated minor intermediate intrusives (known locally as "greenstones").

The Friog section lies at the SW end of the Dolgellau Gold-belt and exposes a series of quartz-sulphide lodes ("Gold-belt veins") which are beautifully folded and boudinaged. Here then is the critical exposure which demonstrates that these lodes pre-date the end-Caledonian (Acadian) regional deformation. Prior to the discovery at Friog, the Gold-belt veins had been assumed to be Upper Palaeozoic in age, a date seemingly confirmed by K-Ar isotopic dating of wallrock micas. However, it is now clear that the isotopic age was re-set by Acadian and possibly later deformation!

The thinner veins of suitable orientation (ie. normal to the maximum compressive stress or WNW-ESE trending best show the folding effects: this one is only 3cm wide. Boudinage is a feature of the commoner and frequently thicker ENE-trending veins. The section comprises cliff-bases and wavecut platforms polished by sand and sea over the years.

The veins carry quartz, carbonates, chlorite, sericite, chalcopyrite, pyrrhotite, galena, sphalerite, arsenopyrite and pyrite. The host strata, vein mineralogy and textural features are identical to those of the Gold-belt veins inland. This was one of the most important discoveries of the Minescan project and the site is well worth a visit armed not with a hammer but with a camera to record the beautiful folds. Access is possible over low tide in good weather, best in Spring before heavy weed growth obscures things. You can always email me for tidal information, in return for a pint!

In much younger rocks, the transition from terrestrial desert to marine conditions is superbly displayed at Lavernock point near Cardiff. In cliff exposures here, the red Keuper Marls are overlain by brown Rhaetic beds, which pass up into the Lower Lias - in other words this is the Triassic-Jurassic boundary.

Beds of gypsum, with spots of malachite, are common in the marls and reflect the hypersaline conditions in which evaporite deposits are formed. Remobilisation of the gypsum has occurred so that the mineral occupies thin veins formed during tensile fracturing. Larger fracture-veins in the overlying Rhaetic contain calcite, celestine and strontiantite. Conservation of this site is straightforward as the vein-exposures are impossible to access (crumbling steep cliffs) and samples are only obtained after winter rockfalls.
Not far along the Glamorgan coast, the Ogmore-on-Sea sections reveal eroded Carboniferous Limestone overlain unconformably by the basal Lower Lias. This is a critical metallogenic section. Mississippi Valley Type (MVT) veins occur in the Carboniferous limestone and carry calcite, baryte, galena and sphalerite (L). As they reach the erosion surface the mineralisation takes on exhalative ("black-smoker") characteristics so that the same minerals fill sedimentary structures (like geopetal voids) in the Lower Lias. The mineralisation in the Lias indicates that hydrothermal fluids were expelled onto the sea-floor and into pre- or part-lithified Lower Jurassic sediments - thus indicating that the MVT mineralisation of South Wales is at least partly of Lower Jurassic age.

This demonstrates the core philosophy behind Minescan: a site needs to demonstrate a major point, such as an age relationship, regional process, important mineral assemblage or in many of the best sites two or more of these to warrant SSSI status. Clearly the relationship between MVT vein mineralisation and the Lower Jurassic sea-bed at Ogmore makes it a prime candidate.
Just to the west of Ogmore is Witches Point, near Southerndown. More typical Lias facies, higher in the succession, are exposed along the beach here and consist of interbedded shales and limestones. Close to Witches Point, a fault with severe drag-folding has cut the Lias: its major movement is thought to be related to the Mid-Tertiary Alpine earth-movements. It is mineralised by a thick calcite-pyrite vein which has undergone cataclasis during the major movement: however the pyrite is locally cut by thin veinlets of fresh, undeformed galena and sphalerite. This indicates that the Pb-Zn mineralisation post-dates the pyrite cataclasis: if this major movement is indeed Mid-Tertiary then here we have an example of geologically young metalliferous primary mineralisation by Welsh standards.

Interesting natural exposures of mineralisation are not limited to sea-cliffs. High on the S flank of Snowdon a locality known as Shadow Gully is of considerable interest for its Sn/W enriched mineralisation. The gully is an eroded fault-line and it is floored with mineralised breccia cemented by haematite and magnetite with minor sulphides. The mineralisation is fine-grained and its study involves the use of the petrological microscope.

Here is a rib of magnetite-cemented breccia in the floor of Shadow Gully. The breccia contains fine-grained iron oxides in which microscopic grains of cassitterite and scheelite are present. The mineralisation is related to the Mid-Ordovician Snowdon Caldera and is but one aspect of this important metallogenic mini-province. There is also a suite of quartz-sulphide (Cu-Fe-Pb-Zn-As-Bi) veins, which were extensively mined for copper in the 18th and 19th Centuries. The link between caldera development, magmatism, fracturing and metalliferous mineralisation is well-demonstrated in this area.