The Orefield is a polyphase mineralised terrain, in which 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 apparently all post-Acadian in age, with a range of Pb-Pb isotopic ages suggesting emplacement from Devonian through to Permian times.

The mineralisation is 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 a mesothermal metamorphic gold-quartz vein province hosted by a highly altered suite of Cambrian sediments and intrusives (including some with porphyry-type Cu mineralisation): the paragenetic and alteration patterns are completely different and the veins themselves have, via Acadian deformation, been strongly folded, boudinaged and recrystallised.

Back to Central Wales: in mineralogical terms, the veins 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 marcasite. A wide range of accessory ore minerals, particularly cobalt and nickel-bearing species, occur in certain assemblages.

The paragenetic determinations that allowed the sequence of emplacement of the assemblages to be determined (as set out in the table below) were based on observations of cross-cutting veins and rebrecciation. A frequent feature in Central Wales is to find one assemblage occurring in a vein cutting 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 successive mineral assemblages I deduced in my MPhil research & later publications may conveniently be divided into Early (A1) and late (A2). The original terms ("early-complex" and "late-simple") I now utterly loathe and would discourage their use!

The suggested ages are based on a lead isotope study using paragenetically selected galena samples collected in a joint piece of work 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 - they date material up to the A2-C assemblage, which appears to be of Permian age -  then it appears that mineralisaing events continued, perhaps into 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 evidence exposed at Ogmore in South Wales demonstrates conclusively that the hydrothermal activity responsible for the MVT deposits of South Wales continued at least into Lower Jurassic times.

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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