| Site Description |
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 | | Dickson’s Island, Castleward, Co.Down, June 1997. |
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| Highlights: |
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| Hydrothermal vein minerals including galena and sphalerite. |
| Introduction: |
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Dickson's Island comprises a peninsula, c.60m long and 30m wide at high tide, which projects into Castleward Bay 300 metres northeast of the gate at Old Castleward. Historical accounts, and lead-bearing stones strewn on the shore, attest to the former presence of an underground mine here.
 | | Dickson’s Island, Castleward, Co.Down, June 1997: boat house. |
The boathouse on the peninsula is an attractive building with crenellated gables, dating from the late nineteenth century. The variety of stone from which the boathouse was constructed, together with internal features on its southern wall, suggest that it was built from the engine house which previously stood at the site. The engine house with its smoking chimney can be seen in an isometrical sketch of the Castleward Lead Mine dated March 1864, held in the Castle Ward Library. This isometrical sketch, commissioned by the company directors to promote the sale of the workings, indicates that ore was extracted from a number of galleries extending out from the central shaft. The vein is thought to be oriented N10°W. |
| Description: |
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| The 3 metre square outline of the infilled mine shaft can be found, under thick grass, some 12 metres south of the boathouse. The eroded remains of a quay may be discerned on the north side of Dickson’s Island, and across the bay some 100 metres to the north is a sturdy stone-built shed known as ‘the powder house’ where the explosives would have been stored. Sixty metres west of this building, another infilled shaft can be found, the site now occupied by a trailer van used for storage by the Yacht Club. It is not clear whether the ore occurred in a single vein or in several, or whether both shafts were sunk to intersect the same structure. |
| These are the remains of a mining venture which started in 1855 and finished, after intermittent production, ten years later when the Castleward United Mining Co. Ltd. was liquidated and the mining sett and machinery were put up for sale by auction. It was probable that no sale materialised, and that the company directors’ glowing reports of rich ore underground were unfounded. The mine was excavated to a depth of 60 metres and yielded no more than 110 tons of lead ore, the value of which represents a fraction of the working expenses incurred. Further ‘strings’ of lead ore (galena) are reported in Audleystown Townland to the north, occurring along cleavage planes and joint intersections in the greywacke and shale bedrock. |
 | | Dickson’s Island, Castleward, Co.Down, June 1997: spoil. |
The peninsula is largely composed of spoil material comprising greywacke veined with galena, sphalerite, dolomite and calcite. The galena typically forms coarse (cm-sized) crystals within thin (c.5mm) veinlets. Paragenetically early barren quartz veins are cut by milky quartz - dolomite gangue containing 2-6mm crystals of galena and ferroan sphalerite. Coarsely crystalline calcite infills cavities in the breccias but there is no evidence of further brecciation. In contrast to Whitespots (Key Site 529), neither chalcedony nor barite were observed in the spoil at Castleward. Moles et al. (1997) report fluid inclusion homogenisation temperatures that were obtained for a mineralized quartz vein from Castleward. Most inclusions homogenised at temperatures in the range 150-160°C, however about one third of inclusions homogenised at higher temperatures, up to 260°C. |
| Importance: |
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| In contrast to Whitespots, Castleward was not a major, economically viable underground mine. However, the Dickson’s Island site is important as a representative of the many small lead-bearing vein occurrences in the Lower Palaeozoic rocks of the north of Ireland. As noted above, there are similarities between these occurrences that link them as a metallogenic set. |
| Interpretation: |
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| Several workers (e.g. Parnell, 1995) have suggested that Pb-Zn vein deposits hosted by the Lower Palaeozoic basement in the north of Ireland and south of Scotland, and the Irish Carboniferous carbonate-hosted stratabound Pb-Zn deposits, are coeval and genetically related. This view is supported by fluid inclusion results from Castleward (Moles et al., 1997). Ore deposition is regarded as a result of mixing of high temperature, low salinity, deep-seated hydrothermal fluids with hypersaline connate brines. Structural reactivation and multiple fluid pulses were responsible for the larger base metal vein deposits whereas minor vein occurrences such as Castleward and Tullyratty (Key Site 531) record just some of these events. |
| Conclusions: |
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| Dickson’s Island, Castleward, is an important site for designation under the Mineralogy/Metallogenesis Block. Key features are the presence of mineralized spoil, the survival of mine-associated buildings and historical documentation, and accessibility of the site. |
| Notes: |
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| No Notes |