Gold

Gold is perhaps Wales' most famous metal, and is the only one still actively sought-after today. Nobody knows when the first piece of Welsh gold was discovered. At Dolaucothi, in the southern part of Mid-Wales, the Romans certainly operated a large mining concern, but even here recent research has suggested that they simply took over an existing, much older mine. In North Wales, gold has been mined in the area known as the Dolgellau Gold-belt. The first discovery was officially reported in 1843 - but once again there is a lot of circumstantial evidence to suggest that the existence of gold had been known about for much longer than that.

The Dolgellau Gold-belt has officially produced about 4 tons of gold to date, most of which was mines in the late 19th and early 20th centuries. The yield to the Romans at Dolaucothi will never be known but it must have been substantial judging by the extent of the workings.

Gold is in fact widespread throughout Wales, but except in these two areas the amounts are so small that they are of academic, rather than economic, interest. For example, traces of gold have been found in the silver-lead ores of Central Wales. But only traces!

The two main gold areas are quite different in geological terms. Taking Dolaucothi first, here we have an anticline - an upward-pointing fold - in the local rocks, which are dark grey shales of Lower Silurian age. The folded shales are riddled with quartz veins, from millimetres to metres in thickness and at all angles of repose from flat-lying to vertical. Both the shales and the quartz veins are impregnated with the common sulphide minerals pyrite and arsenopyrite. But here, these minerals both contain gold - as tiny grains enclosed within them.

Such ores can be very difficult to treat, and when the mine was last active in the 1930s, the concentrated sulphides had to be taken to Antwerp to have their gold extracted. Nobody in Britain would touch them! We can only guess that the Romans were working similar ores which, being nearer to surface, had been weathered, a process which would have rotted the sulphides to clay, freeing the gold and allowing it to be extracted by washing and panning. The Dolaucothi deposit is interpreted as being a "Saddle-reef", where mineralising fluids have congregated within a fold as it forms, depositing big flat-lying veins connected by smaller, steep ones. But it is a geologically complex place, and it is possible that this "metallogenic model" will be revised in the future.

In contrast, the Dolgellau gold deposits contain very coarse, visible gold that may be extracted by hand-picking the ore and crushing and washing it. Gold in the Dolgellau area occurs in usually steeply-dipping ribboned quartz-veins which tend to run in an east-west direction. The veins contain other minerals such as sulphides of iron, copper, lead and zinc and additionally there are local concentrations of bismuth tellurides. Tellurium-bearing minerals are in fact a frequent accompaniment to gold in deposits worldwide, and the Dolgellau area is no exception.

The Dolgellau Gold-belt veins are much older than those at Dolaucothi. It has recently been recognised that they have been deformed during the end-Silurian earth-movements known as the Caledonian Orogeny. Modern research has led to the belief that they formed early in the Ordovician Period. At that time, a major volcanic cycle was coming to an end. A great volcano had built up in the area that is now the mountain of Rhobell Fawr, while deep underground magmas had been injected as sheet-like masses into the Cambrian sedimentary rocks. These magmas cooled to form rocks that the Welsh gold-miners now call "Greenstones". With all of this activity going on, conditions below surface were quite hot and the heat allowed fluids to circulate, reacting with the rocks that they passed through and taking gold and other metals into solution. In the closing stages of the volcanic cycle, uplift occurred and the tensions that developed within the rocks caused fissures to open up. The fluids found their way into these, and their payload of metals was deposited, as gold and the other vein-minerals of the area.

Economic gold concentrations only occur locally within the Gold-belt veins, where they form "pockets", perhaps only amounting to a few tons of ore but containing many ounces of gold. The location of such pockets is not easy, which is why Welsh gold mining is a challenging and perplexing activity, requiring the miner to be something of a geologist, and vice-versa! Today, Welsh Gold mining is a cottage industry, run by local people, and the gold is used in jewelery manufacture.