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The Mineralogy of Dolyhir Quarry, Old Radnor, Powys, Wales [UK Journal of Mines and Minerals]
Abstract and Figures
Dolyhir Quarry near Old Radnor in Powys is developed in rocks of the Old Radnor Inlier, a kilometre-scale fault bounded block that lies within the Church Stretton Fault Zone. The quarry exposes Silurian algal limestones of the Dolyhir Limestone Formation, which unconformably overlie Neoproterozoic clastic sediments of the Yat Wood and Strinds formations. Mineralisation includes a limestone hosted lead-copper vein containing major primary tennantite, galena and baryte, with minor greenockite and enargite. Oxidation of this assemblage has produced supergene minerals including anglesite, azurite, cerussite,
malachite, olivenite and tyrolite and rare arsentsumebite, gartrellite, otavite, segnitite and zincolivenite. Supergene enrichment processes have deposited a variety of copper sulphide minerals in the limestone including chalcopyrite, covellite, djurleite, roxbyite, spionkopite, yarrowite and probable geerite. Rich azurite and malachite commonly surround the copper sulphides. Realgar occurs as disseminations in fractures and joints in black shale of the Yat Wood Formation and in Silurian conglomerate. Vein hosted barium carbonate mineralisation comprises alstonite, barytocalcite, paralstonite and witherite. Fracture assemblages in the basement rocks contain anatase, baryte, calcite, edingtonite, harmotome, quartz,
synchysite-(Ce) and ewaldite. Ore minerals are uncommon in the basement rocks, they include chalcopyrite, enargite, galena, luzonite, pyrite, sphalerite, tennantite and rarely proustite, wurtzite and xanthoconite. The vein assemblage in the Dolyhir Limestone is probably of the Mississippi Valley Type, but it is unlike any nearby locality. The realgar deposit is unique in Britain. The limestone hosted copper mineralisation records complex supergene enrichment and is unusually species-rich. The barium mineralisation is also complex and includes unusual paragenetic sequences. The mineralisation that has developed in fractures in the basement rocks is influenced by the local lithology and is diverse in comparison to other British localities. Dolyhir Quarry is the first British locality for ewaldite, geerite, roxbyite and xanthoconite and the first Welsh locality for arsentsumebite, gartrellite, luzonite, otavite, paralstonite, proustite, realgar, segnitite, wurtzite, yarrowite and zincolivenite. These rare species and complex parageneses make it one of the most important mineralogical sites in Britain.
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... At Old Radnor, an upper contact has only previously been inferred from downfaulted blocks and isolated patches of Coalbrookdale Formation within the main mass of the Dolyhir and Nash Scar Limestone inlier [5]. However, ongoing workings at Strinds and Dolyhir quarries have exposed the top of the Dolyhir and Nash Scar Limestone as an irregular limestone surface overlain by brownish-grey mudstones containing septarian nodules mineralized by calcite, quartz, and occasionally barite [66]. At Dolyhir Quarry (S5), the transition between the formations has been investigated. ...
Quarries between Old Radnor and Presteigne, Welsh Borderlands, expose a Silurian nearshore succession, which onlaps a rocky palaeotopography of the Neoproterozoic basement that had been uplifted along the Church Stretton Fault Zone. The succession documents the Aeronian to Sheinwoodian transgression of an island or islands, with the following sequence of events: deposition of shallow marine sandstones (Folly Sandstone Formation), regional uplift, preservation of a rocky shoreline and associated deposits (Dolyhir Rudite Member), deposition of limestones characterized by a profusion of coralline algae and the abundant remains of reefs (Dolyhir and Nash Scar Limestone Formation), and finally deposition of trilobitic silty mudstones (basal Coalbrookdale Formation). Facies analysis, carbon isotope (δ13Ccarb) values, sequence stratigraphy, and collections of bryozoans, conodonts, thelodonts, and trilobites have been used here as a means of refining our stratigraphic understanding of this unique succession. The revised stratigraphy demonstrates many similarities with the adjoining Midland Platform and the wider Silurian world. Notable features include the globally recognized early Sheinwoodian carbon isotope excursion and sea-level changes of regional and global extent. As one of the best examples of its kind, the palaeoshoreline and nearshore succession of Old Radnor and Presteigne acts as a depositional model for ancient rocky shores worldwide.
The rare mineral ewaldite has been identified in mineralised tension fractures in Precambrian sedimentary rocks of the Yat Wood Formation at Dolyhir Quarry, Old Radnor, Powys. It crystallises as isolated pale yellow-brown translucent striated hexagonal pyramids in association with harmotome, witherite, calcite and quartz. This is the first record of ewaldite in the British Isles.
The rare barium-calcium carbonate paralstonite has been identified in mineralised tension fractures in dolerite and Precambrian sedimentary rocks of the Yat Wood Formation at Dolyhir Quarry, Old Radnor, Powys. It forms inconspicuous crusts of minute dipyramidal crystals in association with harmotome, alstonite, ewaldite, calcite and quartz. This is the first report of paralstonite in the British Isles.
Barytocalcite was first described by Brooke (1824). Whereas no precise details of the locality were given, Blagill mine near Alston is likely to have been the source of the specimens. Although since found at other localities, both in the northern Pennines and elsewhere in the world, barytocalcite remains a rare mineral. The identification of barytocalcite at Rorrington mine reported in this note is the first from Salop (now Shropshire), although Richards (1992, p. 12) hinted at its possible but unconfirmed presence at Snailbeach mine.
Mackelveyite type specimens turn out to be polycrystals consisting of two phases in syntactic intergrowth. One phase, for which the name mackelveyite is retained, is lime yellow, but usually shiny black owing to amorphous carbonaceous inclusions. Optically biaxial, nearly uniaxial, it is hexagonal by x-ray diffraction and has the cell dimensions reported byMilton et al. (1965), but its space group isP31m. The other phase, to be called ewaldite, is the major constituent of the polycrystals studied byDesautels (1967) under the name mackelveyite. It is bluish green and crystallizes inP63mc witha=5.284±7 (1 σ) andc=12.78±1 (1 σ) Å. The intergrown crystals are turned 30° with respect to each other about a commonc direction; their cell dimensions obey the relations 3c
e∼2c
m and 2a
e cos 30°∼a
m, whence 9V
e∼2V
m. Mackelveyite shows twinning by merohedry, twin plane\((2\overline {11} 0)\) and twin symmetry 6′m′ m. Ewaldite has not been observed to twin. The ewaldite formula is$$Ba (Ca_{0.48} RE_{0.20} Na_{0.15} K_{0.11} Sr_{0.03} U_{0.02} \square _{0.01} )(CO_3 )_2 $$. ForZ=2 we obtainDx=3.37 g/cm3, as compared withDm=3.25±5 g/cm3. Electron-probe analysis of polycrystals shows uniform concentration of major elements, thus indicating that we are dealing with two crystalline forms of essentially the same chemical formula. For nearly pure ewaldite we getnE≤1.572,nO=1.646±3. Polycrystals in which ewaldite is the dominant constituent show a morphology that is characteristically that of mackelveyite. Ewaldite is thus pseudomorphous after mackelveyite. X-ray diffuse scattering shows mackelveyite to be the more disordered structure, so that mackelveyite goes to ewaldite by an ordering transformation.
Arsentsumebite, a rare member of the brackebuschite group of minerals, has been identified as drusy crusts of curved blocky mint green crystals up to 0.1 mm on edge at Dolyhir Quarry, Old Radnor, Powys. It is found with anglesite and other lead-bearing secondary minerals in tiny cavities in oxidised lead- and copper-rich veinstone. The crystals are of near end-member composition. This is the first Welsh and second British report of the mineral.
Extreme silver enrichment at the surface of the complex sulphide, tennantite (ideal formula: Cu12As4S13), occurs following exposure to alkaline solutions, and involves the development of an Ag-rich sulphide surface species. The tennantite has a low bulk Ag content of 0.3 at.%, and a percentage surface enrichment of Ag is thirty-six times that of the bulk. The techniques of X-ray photoelectron spectroscopy (XPS) and reflection extended X-ray absorption fine structure spectroscopy show the new phase to be a Ag sulphide species compositionally similar to cupriferous proustite ((Cu,Ag)3AsS3). Solution experiments and XPS depth profiling show that the surface is most depleted in Cu and Zn, and enriched in Ag compared to the bulk tennantite. Selective dissolution and reprecipitation at the tennantite surface cannot explain the enrichment of Ag relative to the bulk. Migration must have occurred and could have been driven by the leaching out of Cu which produces a metal-depleted surface, coupled to the relative incompatibility of Ag in the tennantite lattice. To account for the extreme enrichment at the surface, Ag must have diffused from depths of up to 9 nm, probably via structural weaknesses and vacancies in the tennantite lattice.
Baryte is the most common and abundant Ba-mineral in the orefield; witherite is also widespread. Barytocalcite is recorded from 22 localities, and alstonite only from four. All occur in the Ba-bearing deposits of the outer zone of the orefield. Barytocalcite and alstonite are abundant in a limestone wall-rock and are the products of reaction between the limestone and late-stage mineralizing fluids. Both minerals are commonly well-crystallized. Compact barytocalcite is white, pale grey or pale cream with resinous to greasy lustre and one good cleavage. Lining vugs, the white to honey-coloured crystals are slender monoclinic prisms terminated by prominent (121) faces. Alstonite occurs as colourless to white, acute pseudohexagonal pyramidal or bipyramidal crystals with horizontal striae on the pyramidal faces, with a vertical medial re-entrant line. Identifications were confirmed by XRD. -R.K.H.