Dr McArdle, who recently retired as Director of the Geological Survey of Ireland, has had a long term interest in the history and origin of the Goldmine River gold deposits.
Text copyright Dr Peadar McArdle 2011.
Pt 11 – Pocket history of Avoca’s volcanic history
We cannot identify the specific volcano responsible for the region’s volcanic rocks: it is either buried deep underground and out of sight or it was destroyed by subsequent geological events. But we do know it dominantly spewed out ashes, because plentiful evidence of them remains in the rocks around Avoca. Low viscosity magma like basalt will usually give rise to lavas that flow downhill with the speed and consistency of thick broth. Rhyolitic and similar high-viscosity magma, on the other hand, is generally very gas-rich and erupts explosively. Avoca’s magma was predominantly of rhyolitic composition and so gave rise to ashes. Ash clouds would have billowed skywards from the volcanic crater and spread out over the surrounding seas, dropping their ash loads onto the sea floor. The ashes became incorporated into seabed-hugging and sediment-charged water currents called turbidity currents, which surged rapidly downslope and lost their sediment load when they reached flat-lying seafloor in water depths of about 200 metres. Thus developed the sediments that now form the bulk of the Avoca rock sequence.
So where would Avoca’s volcano fall on the scale of recent volcanic eruptions? Probably somewhere in the middle, outshone by examples such as Mount St Helen’s and Krakatoa. Think of the 1990’s Montserrat eruptions in the Caribbean in order to imagine the overall scale and setting.
But where are the metals for which Avoca (and indeed Croghan Kinshelagh) is famous? We know that mining took place in the vicinity of the Vale of Avoca since at least 1720. The suggestions of earlier outputs of metals extend back to Ptolemy’s time but alas are based on speculation, however tantalising. The recorded Avoca production amounts to 16 million tonnes of copper ore containing 0.6 percent copper and 5 percent sulphur. Even more prolonged, but on a smaller scale, was the output of iron ore from the Ballycoog-Moneyteige ridge on the northwest flank of the Goldmine River Valley. These workings are believed to have been operated by the Vikings and I have a suspicion their output may have been used in the Dublin of their era.
The Avoca mine frequently had but marginal profitability, especially since it re-opened in 1958, and it required Government support in its final years. Most ore was extracted from the intensive workings of West Avoca, which reached a depth of 300m below surface, but the environmental impact was greater in East Avoca where shallower workings were more extensive. The main open pits, which contributed over 30 percent, were in East Avoca. At least one of the open pits, Cronebane, gave rise to extensive waste which formed a significant dump that was subsequently used to partially backfill the pit itself. The Ballymurtagh Open Pit in West Avoca became a landfill facility for County Wicklow in the 1980’s before finally closing some years ago...
To be continued…
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