Precious metals (gold and silver), mineral resources, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
J W Arthurs and G Earls
As displays of gold artefacts in Ireland’s museums testify, gold occurrences have been known in Ireland since prehistoric times. The earliest historical reference to gold in Northern Ireland is by Gerald Boate in 1652, which referred to alluvial gold in the Moyola River . Nevertheless, although there was clear evidence of potential, there were no workings in historical times and gold was not explored for in Northern Ireland until the early 1980s. The catalysts that promoted increased gold exploration at this time were a dramatic rise in the gold price and the application locally of the then recently developed plate tectonic theory. As a result of this surge in exploration, at least two potentially economic deposits of gold have been discovered at Curraghinalt and at Cavanacaw in Co. Tyrone.
Gold (Au) always occurs alloyed with silver (Ag) and other elements. As deposits of silver ore alone are not known in Northern Ireland, gold and silver are discussed together. Four styles of gold mineralisation occur in Northern Ireland.
Mesothermal quartz vein-hosted gold
Gold occurs in quartz veins ranging from a few mm to several metres in width and is often associated with pyrite and base metals. The veins were formed by high temperature mineralising fluids which filled space created by tectonism. Gold-bearing quartz veins have been found in two geological settings in Northern Ireland, in the Central Highlands (Grampian) Terrane and in the Southern Uplands-Down-Longford Terrane.
Central Highlands (Grampian) Terrane (Dalradian Supergroup)
Auriferous veins occur in the Neoproterozoic Dalradian Supergroup. It comprises a thick succession of metagreywacke and pelite, with minor carbonate, quartzite, graphite and basic igneous rocks that were deformed in the Grampian and Caledonian orogenies (see Central Highlands (Grampian) Terrane - metamorphic basement article) and (see Midland Valley Terrane article). Although gold-bearing quartz veins occur throughout the Dalradian outcrop, most occurrences have been discovered close to the southern boundary, the Omagh Thrust Fault, on which Dalradian rocks are thrust southeastwards over the Ordovician Tyrone Igneous Complex (P948093).
The two main gold deposits in the Dalradian are at Curraghinalt [H 570 860] and Cavanacaw [H 401 708] in Co. Tyrone. The multi-vein deposit at Curraghinalt, about 5km east of Gortin, Co. Tyrone, was discovered in 1983 as a result of prospecting around a shallow soil arsenic anomaly . The resource has been calculated at 470 000t at a grade of 17g/t Au (P948049) and (P948050).
The Curraghinalt deposit is interpreted to overlie a positive relief feature or ‘ramp’ in the Ordovician footwall of the Omagh Thrust Fault. Accommodation structures in the Dalradian overlying the ramp are believed to control the locus of quartz veining and gold mineralisation. The deposit consists of a swarm of WNW-trending, northeast-dipping veins in pelites, semi-pelites and psammites (P947896). Veins vary in width from a few mm up to 3 m. Two K-Ar model ages on gouge from mineralised shear zones give dates of 315.5±6.5 and 325±6.7Ma. These dates indicate that the last structural movement associated with the veins occurred during the Variscan Orogenic Cycle .
Measurements of liquid-gas inclusions in the quartz veins, allied with studies of quartz growth, indicates that four distinct fluids (Q1-Q4) contributed to the formation of the vein system at Curraghinalt (P947897), (P948051). Q1 is interpreted as a metamorphic fluid and Q2 is probably related to intrusive intermediate igneous activity. Q3 has characteristics associated with derivation from surface and probably represents connate or formational water, while Q4 is basinal brine that may have been derived from the inversion of the Carboniferous Newtownstewart Basin to the west. Tectonism associated with the Q2 and the Q4 fluid phases brecciated earlier quartz and probably reflect the Caledonian Orogenic and Variscan Orogenic cycles respectively.
Gold mineralisation at Curraghinalt is related to the Q2 and Q4 fluid phases and tectonic events. Gold associated with the Q2 phase of mineralisation occurs within pyrite and is typically in the order of 90% Au and 10% Ag (P948052). The second phase of gold mineralisation associated with the Q4 brine is of lower fineness, typically containing about 80% Au and 20% Ag (P948053). However, although reducing the fineness of the gold present, the Q4 phase locally increases the gold grade in areas of increased fluid flow in the deposit. Gold at Curraghinalt occurs as electrum, in Au-bearing telluride (hessite) and as trace substitutions in pyrite. Associated sulphides in the veins include pyrite, arsenopyrite, chalcopyrite, galena, sphalerite and sulpharsenides. Bismuth (Bi) and antimony (Sb) also occur. The gangue is dominantly quartz with minor calcite, dolomite, siderite and barite.
Gold-bearing quartz veins at Golan Burn [H 524 873], 2 km northeast of Gortin, Co. Tyrone, have also been explored and may represent the northwestern strike extent of the Curraghinalt vein swarm .
The Cavanacaw deposit occurs in Dalradian rocks of the Lack Inlier (P947791) , west of Omagh. Since the discovery by prospecting in 1985 of outcropping mineralisation a geological resource of 2 million tonnes grading 6.9g/t Au has been quoted by the company . Several auriferous quartz veins have been identified, all of which are steeply dipping and trend generally north-south (P947898). Cavanacaw, and other occurrences of veins with this trend are interpreted as being related to a deep crustal structure, the Omagh Lineament . The veins at Cavanacaw are internally complex and heavily fractured and it has not been possible to determine the fluid history of the deposit. Cavanacaw is broadly mineralogically similar to Curraghinalt, although the relative abundance of certain elements differ. Where Curraghinalt is Cu dominant, Cavanacaw is Pb-Zn dominant and Au:Ag ratios are lower.
Numerous other vein-related bedrock occurrences of gold in the Dalradian have been recorded in Counties Londonderry and Tyrone by exploration programmes.
Southern Uplands-Down-Longford Terrane (Leadhills Supergroup)
Ordovician and Silurian rocks of the Leadhills Supergroup in Northern Ireland are an extension of the Southern Uplands Terrane of Scotland into Ireland (see Southern Uplands-Down-Longford Terrane article). Gold in quartz veins has long been known at Clontibret, Co. Monaghan, in the Republic of Ireland, close to the border with Co. Armagh. The Clontibret mineralisation includes stibnite and is part of the same vein swarm that includes the lead veins of the South Armagh-Monaghan Mining District .
Gold occurrences have been identified in two areas of Co. Armagh in the Southern Uplands-Down-Longford Terrane. At Cargalisgorran [H 803 333], drilling has proved vein widths of 3.03 m grading 2.16g/t Au. Initial interpretations indicate that the auriferous veins are associated with subsidiary fracturing along the northeast-trending Orlock Bridge Fault, a major shear zone extending through the entire terrane.
Stratabound gold occurrences
Stratabound gold mineralisation occurs in the Glengawna Formation (P947914) of the Dalradian Supergroup in the Sperrin Mountains. Throughout the formation numerous grab samples contain gold in the 20–100ppb range. However, the most notable occurrence is at Glenlark Lodge [H 595 901] where semi-massive and disseminated iron, zinc and lead sulphides are associated with significant precious metal grades. The host rock is chloritic-talc schist, typical of that found throughout the Glengawna Formation, which may represent a hydrothermally altered protolith. The widest interval sampled contained 2.55g/t Au over 21 m including a section of 2 m grading 7g/t Au and 42g/t Ag. The gold mineralisation outcrops as massive and disseminated sulphides, (sphalerite, galena and pyrite), lying parallel to the transposed foliation of the schists and was deformed in the same tectonic event as the host rock. The significance of this occurrence is that, being stratabound and possibly originally stratiform, it may represent a syngenetic or syn-diagentic style of mineralisation. There are also small cross-cutting veinlets of coarse-grained massive sulphide which are manifestly post-tectonic.
Volcanogenic gold occurrences
The Tyrone Igneous Complex lies to the south of, and structurally beneath, the Dalradian rocks of the Sperrin Mountains (see Midland Valley Terrane article). The complex comprises a core of sillimanite grade gneissic basement, dated at 640Ma, which is overlain by metabasic igneous rocks, interpreted as an early Ordovician obducted ophiolite dated at 472±4Ma (Arenig-Llanvirn) . Overlying the ophiolite and beneath the Omagh Thrust Fault is the Tyrone Volcanic Group. Igneous rocks within the group range in composition from basaltic andesite to rhyolite and are interpreted as having formed in a subduction-related island arc environment. The volcanic rocks are cut by intrusions of calc-alkaline granite and tonalite.
The Tyrone Igneous Complex is highly prospective for gold and base metals. The Cashel Rock prospect [H 589 802] in Co. Tyrone comprises a silicified rhyolite with disseminated gold in intervals of up to 3.63 m grading 30.1g/t Au and 43.3g/t Ag (including 1.23 m at 86.2g/t Au and 122.4g/t Ag). Geochemical base metal anomalies and siliceous ironstone (of probable exhalative origin) occur in the Tyrone Volcanic Group and suggest the presence of a volcanogenic style of mineralisation. This area of Co. Tyrone represents the eastern extension of the island arc system that hosts the Buchans volcanogenic massive sulphide deposits in Newfoundland.
Gold prospecting in the 1980s, in the southern part of the Fintona Block, north of the Clogher Valley showed that Devonian volcanic rocks of andesitic composition (see Devonian article) contain minor gold occurrences associated with wallrock alteration. Little is known about the geology and genesis of this style of gold mineralisation.
Alluvial gold is found in streambeds as native metal alloyed with silver. Gold is eroded from weathered bedrock occurrences and dispersed into streams. Owing to their very high specific gravity, gold grains (P948054) become differentiated from other mineral species and are trapped in areas of reduced stream flow. Although alluvial gold is unlikely ever to become an economic source of gold in Northern Ireland, it does provide physical evidence of the presence of gold in bedrock. Insofar as alluvial gold is common in areas where vein gold is not presently known, the occurrence of gold in bedrock may be much more widespread.
Alluvial gold occurrences are scattered throughout Counties Armagh and Down and therefore suggest that other gold occurrences remain to be discovered. One of the most spectacular finds in recent years is the Clay Lake nugget, now exhibited in the Ulster Museum (P948055). Gold can also be panned in the Hilltown area of Co. Down .
A note on the potential for other styles of gold mineralisation
There are several potential gold exploration targets that have yet to be examined in detail in Northern Ireland. As research has strongly suggested that Carboniferous basinal brines have remobilised gold, there is a possibility that receptive horizons and structures older than the Carboniferous fluids could act as traps for gold mineralisation. Such lithologies in the Dalradian Supergroup include the graphitic and impure limestone associated with the Dungiven Limestone Formation in the Sperrin Mountains and the Torr Head Limestone Formation in northeast Co. Antrim.
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