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This document is with a copy of the following article published by the Mining Heritage Trust of Ireland. It is provided for non- commercial research and educational use. The Mining Heritage Trust of Ireland formally ceased its existence in 2019 but has provided a continuing website of resources with free access for those interested in the activities of the organisation in its various formats from 1996-2019, and in Irish mining heritage in a broader sense. McArdle, P. (2007) ‘Geological Setting of the Lead-Bearing Veins the Glendalough-Glendasan District, County Wicklow’ Journal of the Mining Heritage Trust of Ireland, 7, pp.3-7 Copyright of this article remains with the Mining Heritage Trust of Ireland whose archives, intellectual assets and library have been transferred to the Natural History Division of the National Museum of Ireland. Please contact [email protected] for any enquiries relating to the MHTI. This cover page must be included as an integral part of any copies of this document. Please visit www.mhti.com for more information. GEOLOGICAL SETTING OF THE LEAD-BEARING VEINS IN THE GLENDALOUGH-GLENDASAN DISTRICT, COUNTY WICKLOW By Peadar McArdle Abstract: The lead-bearing veins in the Glendalough-Glendasan district of County Wicklow occur along major fractures on the eastern margin of the Leinster Granite. The veins are typically 1-6m thick and are composed of galena-sphalerite- pyrite in a quartz-calcite gangue. The host granite has been brecciated and hydrothermally altered. The geological context and development of the veins is described in this paper. Journal of the Mining Heritage Trust of Ireland, 7, 2007, 3-7. INTRODUCTION Granite was emplaced and cooled down. Over the period of this evolution the area of Wicklow migrated from the high southern Prior to 1960, Wicklow was the premier mining county on the latitudes to about 30°S. More of this fascinating history can be island of Ireland. This was based in part on the substantial pro- found in references such as Cocks and Torsvik (2006). duction of copper and pyrite from the Avoca district. It also reflected the lead output from several veins along the eastern SLATES AND SCHISTS margins of the Wicklow Mountains and centred around Glendalough. In these circumstances it is not surprising that The rock sequence of County Wicklow occurs in the one of the earliest county geological maps completed in Ireland Caledonian terrain and is composed of Cambrian to Silurian was for County Wicklow. marine sediments which contain some developments of vol- canic rocks (Figure 1). The mountainous core of the county is That map was published in 1801 by Robert Fraser and it shows dominated by the early Devonian Leinster Granite. To the west the distribution of rock types across County Wicklow. It colour the sediments are unconformably overlain by, or faulted codes the various types of rocks wherever they are exposed. against, post-Caledonian sandstones and limestones of The pink areas are granite, principally forming the mountains. Devonian and Carboniferous age. The low ground on either side is composed of slaty sediment, shown in blue colour. The Avoca mining ("metalliferous") dis- Four major rock groups are recognised in the Cambrian- trict, now known to be underlain by volcanic rocks, extends Silurian rock sequence of this district. The oldest, the Bray from Avondale to Croghan Kinshelagh Mountain at the county group of Cambrian age, comprises greywackes, slates and boundary and is shown in golden shade. The quartzites around quartzites which formed in a marine environment. The Bray Head feature in brown colour. The lowest ground, espe- greywackes are green sandstones with varied amounts of cially in the west, has a greenish tint and represents boulder muddy matrix (well exposed on the N11 north of the Rathnew clays derived specifically from the limestones of the Irish mid- exit). They, along with much of the other sediments, formed lands. The main rock types as we know them today are all rep- from turbidity currents, sediment-charged bodies of seawater resented although they are not shown in the same resolution as generated by storms and earthquakes which surged down even today's modern geological maps (for example, Mc Connell et the gentlest of slopes. The greywacke was formed from mix- al. 1994). tures of sand and mud, the quartzite from sand and mudstone from mud. The distinctive fossil Oldhamia, consisting of ani- The rock sequence containing the lead-bearing veins of County mal traces in muddy sediment, is found at Bray Head. Wicklow is entirely at least 400 million years old and it records the plate tectonic history of the Earth's crust over the period The succeeding Ribband Group rocks are the most widespread 535-400 million years ago (Max et al.1990). This history com- and are Cambrian to Ordovician in age. They are closely asso- prises the events of the Caledonian Orogeny: this involved the ciated with the lead-bearing veins that were mined in Wicklow. opening of an extensive Iapetus Ocean in the period to about They consist mainly of fine grained sedimentary rocks, slates, 480 million years ago and then its contraction and elimination siltstones and sandstones which were deposited as silt and mud down to about 430 million years ago. In all of this time, from turbidity currents in deep seawater. These included the Wicklow was situated on the southern (Avalonian) margin of manganese-rich sediments called coticules. Deposition took this ocean. Following its destruction, opposing plates collided place on a deeper and more remote seafloor than in the case of with each other, leading to the deformation and metamorphism the Bray Group. The tectonic plate movements which caused of the rocks of both margins as well as the emplacement of this evolution in the Iapetus Ocean also gave rise to volcanic granites such as the Leinster Granite. The bedrock history of activity, and basalts and andesites are preserved as relatively Wicklow effectively finished around 400 Ma when the Leinster small developments in several areas. 3 ties to the greywackes of the older Bray Group. Sedimentation ceased when opposing sides of the Iapetus Ocean collided with each other about 430 million years ago and the seafloor was entirely con- sumed by subduc- tion. Continued collision led to one plate overrid- ing the other, leading to moun- tain building and the climax of the Caledonian Orogeny. The sediments were compressed and a new cleavage imposed on the muddy ones (due to the realignment and recrystallisa- tion of mud flakes). The rocks were also exten- sively crumpled and faulted on all scales. The base of the overridden plate sank to a depth where par- tial melting occurred and liq- uid granite Figure 1. Geological setting of mineral deposits in southeast magma ascended in the crust to cool and form granite bodies. Ireland. From Kennan et al. (1986). The movement of magma was often facilitated by the presence of major fault zones. The heat from the granite baked the slates The next group, the Duncannon Group, is Ordovician in age it intruded and metamorphosed them to schists containing and composed of a variety of volcanic rocks with only minor newly formed minerals such as andalusite and garnet. This, and sediments. These are, as in the case of Ribband Group volcanic the related formation of lead-bearing veins, was the final act of rocks, the products of continuing subduction and destruction of the Caledonian Orogeny. Iapetus Ocean. They originated from chains of volcanoes, or volcanic arcs, which were deposited beneath seawater. These GRANITE are hosts to the economically important copper deposits which Granite is a very distinctive and familiar rock type to many peo- were mined extensively around Avoca. Sediments of the ple because it is commonly used in construction. Several quar- Duncannon and Ribband Groups locally contain preserved fos- ries exist in the Leinster Granite, situated especially in South sils which have been used to date them and these consist espe- Dublin and North Wicklow, although few are still actually pro- cially of graptolites, brachiopods and trilobites. ducing. Although the granite forms an upland area its rounded hills have few exposures but the deeply incised valleys and cor- The youngest group of rocks, the Kilcullen Group, is Silurian in ries are locally well exposed. age and is exposed to the west of the Leinster Granite. The rocks are dominantly of green greywackes. They bear similari- 4 The Leinster Granite stretches from the coastline at Dun Granite does contain areas of sheeted granite which would sup- Laoghaire to the vicinity of New Ross, covering an area in port this alternative view of its development. The granites of excess of 1500 sq km. It is the largest granite body in this part Donegal have received much attention in recent decades, of Europe and comprises five individual units, or plutons. Each involving this recent interpretation, and the interested reader is is circular or oval in shape and traditionally was viewed as a referred to the seminal work of Pitcher (for example, Pitcher cross-section through a deep-going body with the shape of an and Hutton 2003). inverted teardrop. The individual plutons are contiguous to each other and arranged in a northeast-southwest direction. They MINERAL DEPOSITS have been emplaced along the cleavage of the surrounding slates during a period of extensive shearing. Intrusion has been There is a variety of metallic mineral deposits in the Caledonide dated as 405+/- 2 million years ago. There are a series of north- rocks of southeast Ireland (Figure 1). Only that at Avoca has west-trending fractures cutting the granite at regular intervals supported modern mine operations although historical produc- some of which may have controlled granite intrusion and also tion on a small scale is reported from many localities (Cole influenced the location of lead-bearing veins.

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