Dolaucothi Gold Mines

Geology and Mineralisation

This justly famous mine site is quite geologically unique, for which reason it is a Site of Special Scientific Interest.

The Romans (and, according to recent research, their predecessors) took away the top of this large gold deposit, where it was most weathered. They used water, carried along miles of leats, to wash away the waste rock and collect the gold that, due to its great density, remained behind. We suppose that this must have been the case because the unweathered ore is so difficult to treat, even today, because the gold occurs in a very fine-grained form locked within sulpide crystals embedded in hard white quartz. But near the surface, the sulphides would have weathered fairly extensively, decomposing into oxides and clay, thus liberating the gold and making it retrievable. You can see the unweathered mineralisation in the various sections of the mine now open to the public.

The ore is a mixture of dark grey to black shale and quartz, in both of which occur sulphides. Pyrite (iron sulphide) and arsenopyrite (iron arsenic sulphide) are the principal ones, with minor amounts of copper, lead and zinc minerals that are generally localised along fractures (like the Lead Lode.) Because they cut the gold-bearing mineralisation they must have been formed at a later date. Pyrite occurs as fine-grained disseminations, massive bands and layers of cubic crystals in the shale and as coarse-grained masses within quartz. Arsenopyrite usually forms perfect, silvery rhomb-shaped crystals in both shale and quartz, sometimes bunched together into large solid, heavy masses. Most of the gold is associated with arsenopyrite though a little occurs free in quartz, where it is very rarely visible to the naked eye. The gold associated with the arsenopyrite may be very fine-grained, down to a few microns in size, occurring as discreet particles within the arsenopyrite grains. Standard techniques of separating such gold from its waste by using shaking tables and water do not work efficiently. Imagine powdering a kilo of pyrite up, adding two or three 0.25mm gold grains and then trying to separate them again in a gold-pan! It is very difficult indeed. So what the 19th and 20th century mine operators had to do was to try using various other methods, like amalgamating the gold from the powdered sulphide with mercury, or dissolving it with cyanide. In the end, nobody would treat Dolaucothi ore in Britain, and it was sent away to Antwerp for gold recovery: when Europe fell into war at the end of the 1930s this avenue was closed - and the mine followed suit.

The geology of the site is quite complex, which is why it is such a good training ground for our young geologists. Essentially, the mineralised zone is a NE-trending anticline - an upwards fold - in the middle of which are Ordovician sedimentary rocks, surrounded by younger Silurian sedimentary rocks along its sides. The sedimentary rocks were all deposited in the old sea that we refer to as the Welsh Basin, which was obliterated at the end of the Silurian by earth-movements causing uplift and folding. According to current theories (and there have been many), the gold, pyrite, arsenopyrite, quartz, carbonates and other minerals (including the rare lithium-bearing silicate cookeite) were all deposited from fluids migrating into the fold as it developed, creating big flat-lying quartz veins known as saddle-reefs connected by thin, steep leader-veins. But there are other theories too - and one day one of them may gain acceptance. In geologically complex deposits like Dolaucothi, the mechanism for the formation of the mineralisation is often re-defined many times through years of patient research - that's science!

For many years, MSc students from Cardiff have assisted with the drilling of a borehole and examining and logging the core obtained as part of their course-work. The drilling has shown that further mineralisation similar to that seen at the mine occurs in zones "along-strike" - that is, to the northeast of the mine area. As time goes by, therefore, we will all gain a better and better understanding of this highly complex mineral deposit.