Famous British Mineral Localities:
Greystone Quarry, Lezant,
Cornwall

Nick Elton
David Green
Jeremy Hooper
Stefan Weiss

At Greystone Quarry, lead-copper-zinc veins cut a structurally complex metasedimentary sequence which is pervasively intruded by dolerite sills. The veins contain major primary quartz, with calcite, chalcopyrite, dolomite, galena and sphalerite. Supergene alteration has produced a diverse assemblage of secondary minerals including anglesite, aurichalcite, caledonite, cerussite, dundasite, hemimorphite, hydrocerussite, leadhillite, linarite, malachite, mimetite, plumbojarosite, pyromorphite, rosasite, vanadinite, and wulfenite many of which are rare in southwest England. The supergene mineral suite contains the only Cornish descloizite, crocoite and schmeiderite and the first British vauquelinite. The east-west veins at Greystone Quarry are mineralogically and structurally distinct from the north-south lead and zinc bearing cross-courses which are well known in southwest England. They are probably associated with late-stage Variscan deformation.

Left: Tabular slightly corroded leadhillite crystals 6 mm across with minor linarite in quartz matrix. Peter Golley specimen, photo David Green. Centre: A typical deep blue translucent prismatic linarite crystal 1.0 mm tall from the Greystone Lead Lode. Neil Hubbard specimen, photo Julie Ballard. Right: An orange wulfenite crystal 0.3 mm across from Greystone Quarry. Photo David Green.

The Petrography of the Zinc-Lead-Copper Ores at
Crow Island, Killarney, Ireland

Rob Ixer

Crow Island, close to Killarney, Ireland is the smallest and least tectonized of three related polymetallic deposits emplaced in the Carboniferous Ballysteen Limestone Formation that were mined in the eighteenth and nineteenth centuries. The mineralization comprises sphalerite-galena-pyrite-chalcopyrite-arsenopyrite-tennantite with trace amounts of electrum. The ore records an early metasomatic replacement phase of mineralization one that is all but obliterated in the ores from the other two deposits at Ross Island and Muckross Mine.

The Development of Supergene Lead Mineralisation
in Central Wales

John Mason

The in situ alteration of hypogene vein galena to cerussite, pyromorphite and wulfenite is of widespread occurrence in the formerly important lead-mining district of Central Wales. Detailed fieldwork over many years has shown that this supergene lead mineralisation can be divided into two texturally and paragenetically distinct assemblages. Of relatively restricted occurrence is a highly evolved, coarsely crystalline assemblage that is developed in particularly permeable lode systems. This assemblage is commonly associated with extensive bleaching of wallrock and the complete oxidation of primary sulphides. Of more widespread occurrence is a microcrystalline assemblage associated with partly oxidised sulphides in relatively unbleached wallrock. The cited evidence is used to advance a theory for the genesis of supergene lead mineralisation in Central Wales. The coarsely crystalline assemblages are thought to be the surviving remnants of pre-glacial oxidation zones, produced by deep weathering during Mesozoic to Tertiary times and now largely removed by glacial erosion. The microcrystalline assemblages are more probably the product of geologically recent alteration, post-dating the last glaciation.

Left: Massive coarsely crystalline cerussite from the dumps at Frongoch Mine. Steve Rust specimen. Centre: Coarsely crystalline pyromorphite on quartz from the main pipe, Bwlchglas Mine. Specimen is 55 mm tall. Right: Early coarse cerussite overgrown by later microcrystalline pyromorphite with minor bipyramidal orange wulfenite. Specimen from Bwlchglas mine, John Mason collection. Photos David Green.

Apatite Crystals from the Coal Measures
of Northern England

Tim Neall