JOHN S. MASON - CONSERVATION OF WALES' MINERAL HERITAGE:
ROAD CUTTINGS AND QUARRIES

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The construction of new roads or tracks in upland areas frequently exposes much bedrock and it follows that there is a reasonable prospect of interesting mineralisation coming to light. Some important discoveries have been made along track sections in Wales, including the only decent exposures of the Coed Y Brenin porphyry-copper orebody, which are now protected as SSSI's.

Working quarries offer a challenge in conservation terms but mineral suites may be conserved by bulk removal ("rescue-collecting") of material to Museum geology departments, where it can be preserved and studied at leisure. When a quarry closes, if there is still important mineralisation on show, its preservation can where safely practical be written in to the post-closure plan.
Here is a prime example of a roadside exposure of metallogenic interest. This is a forest road in Coed Y Brenin. It has been cut through Cambrian sedimentary rocks and intrusions. Here, however, is a boss-like area of harder rock...
Closer examination reveals an abundance of sintery quartz and fine-grained pyrite...
It is a pipelike mass of brecciated rock cemented by silica and iron sulphides. It is enriched in base-metals, gold, silver, arsenic and antimony.
Here is a close-up. Repeated brecciation is evident and the mineralisation is texturally complex even under high magnification.

The pipe has been interpreted as a fossil fumarolic system, representing the altered and mineralised zone through which superheated waters passed upwards to exist as geysers upon reaching the surface. The heat was supplied by the magma chamber which underlay the early Ordovician Rhobell volcano, which at the time of formation would have been somewhere overhead! This site is now a SSSI.
Blasting at a quarry in Carboniferous Limestone, NE Wales. This area contains many metal mines, the majority of which are so overgrown that their mineralogical interest is difficult to assess without substantial mechanised excavation. However, the working quarries freshly expose the same mineralisation on a regular basis, making the collection of unweathered sample suites a more realistic possibility. This quarry was producing some fine azurite specimens in 1998 and many are now preserved in the National Museum of Wales' mineral collection.
Azurite (dark blue) and the copper arsenate tyrolite (light blue) in situ at Dolyhir Quarry, on the Welsh Border, 1996. The quarry works Wenlock Reef Limestone and the underlying Longmyndian (Precambrian) sedimentary rocks. A series of Cu-Pb-As-Ba dominated veins cuts through the sequence.

This short-lived exposure was studied and sampled extensively and produced a great diversity of different minerals, both primary and secondary. Prior to the area's removal by quarrying, many tens of kilogrammes of samples were preserved. The mineralisation is highly complex and studies will be ongoing for some time to come.


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