The Orefield is a polyphase mineralised terrain. The mineralisation is epigenetic vein-type and at least twelve episodes of fracturing and mineralisation, represented by successive crosscutting mineral assemblages, have been identified. The mineralisation is entirely hosted by Upper Ordovician to Lower Silurian clastic marine sediments.

The veins are all post-Acadian in age, with a range of isotopic ages suggesting emplacement from Devonian through to Permian times. The mineralisation is therefore very different from the pre-tectonic metallogenic province known as the Dolgellau Gold-belt, only a short distance to the north. The Gold-belt, however, is hosted by a highly altered suite of Cambrian sediments and intrusives: the paragenetic and alteration patterns are completely different and the veins themselves have been folded and boudinaged.

The veins of Central Wales are dominated by quartz with two major generations of carbonate, the early one being ferroan dolomite and the later one calcite. Baryte and witherite are late-stage components in the eastern part of the orefield. Fluorite is absent so far as we know.

Sulphides are locally abundant and may form large masses or ribs. Galena and sphalerite are broadly speaking the most abundant, with lesser chalcopyrite and tetrahedrite and locally important very late marcasite. A wide range of accessory ore minerals, particularly cobalt and nickel-bearing species, occur in certain assemblages.

The paragenetic determinations that allowed the assemblages to be determined as set out in the table below were based on basic principles involving fracturing and rebrecciation. A frequent feature in Central Wales is to find one assemblage occurring in a vein crosscutting another, or to find one assemblage rebrecciated and cemented by another. In such cases it is straightforward to say that one came first, the other later. Moreover regional patterns, in which such observations repeat themselves time and again, tend to build up as more sites are studied.

Above: a view of part of Cwmystwyth mine, one of the larger mines of Central Wales, where extraction of ores has intermittently occurred since the Bronze Age. Several of the twelve Central Wales mineral assemblages may be identified here.


The twelve assemblages I have divided (in my MPhil thesis & later publications) into Early (A1) and late (A2). The suggested ages are based on a lead isotope study using paragenetically selected galena samples that I did with geochemists at the British Geological Survey in the late 1980s. The model isotopic ages were certainly consistent with the "paragenetic relative ages" of the assemblages. The "paragenetically last" Central Wales assemblage - the A2-f marcasite assemblage, has not been studied isotopically - no galena! If the Pb-Pb dates are correct this would pont to a major mineralising event occurring in post-Permian times.

Interestingly some of the later Central Wales assemblages with consistent Permian Pb-Pb dates are texturally very similar to the MVT assemblages of the UK Pennine Orefields, and we know from evidence exposed at Ogmore in South Wales that the hydrothermal activity responsible for these MVT deposits continued at least into Lower Jurassic times and quite possibly beyond. For example, at Witches Point near Southerndown on the Bristol Channel coast, there is a thick iron sulphide-calcite vein which has undergone cataclasis during the mid-Tertiary Alpine earth-movements: the smashed pyrite is crosscut by veins of undeformed galena and sphalerite indicating geologically very young Pb-Zn mineralisation. As ever, further work is required!





Above: Central Wales features, in addition to the more typical Pb-Zn-Cu ores, a number of unusual metalliferous minerals, and particularly several species containing nickel and cobalt. This is a sample, magnified x1.5, of pinkish grey metallic siegenite (Ni,Co)₃S₄ associated with brassy yellow chalcopyrite in quartz-cemented breccia.


The table below gives details of the primary paragenesis. The assemblages are listed in order of deposition as are the minerals in each assemblage. Species underlined are major economic components, those in italic only occur in trace amounts; the rest are minor phases occurring at more than two localities.

IMAGE MENU:

The images in the menu above cover some of the more interesting ore minerals from Central Wales. For the uninitiated, apart from the breccias page, these images are taken through a high-power microscope which views the samples in reflected light. Each sample is a block of opaque ore minerals that has been painstakingly polished to an optical mirror-finish using diamond pastes down to 1 micron in size.

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