End of the rainbow - gold exploration in Ireland
Michael Forrest talks to Kevin McNulty, Senior Geologist at Conroy Diamonds and Gold plc, Dublin, about gold exploration in the Republic of Ireland.
Gold is well documented in the Celtic fringe of Britain and Ireland, but most discoveries (or rediscoveries) have been small high-grade deposits that could be mined with minimal equipment. Investment requires large reserves – one million ounces of defined resources is a good starting place. To support investment at the gold grades currently mined world-wide this value has recently been reported by Conroy Diamonds and Gold plc (CDG) at its Clontibret property in County Monaghan in the Republic of Ireland. The rocks that hold these gold resources are part of the Longford-Down Massif in the eastern part of the country straddling the UK-Irish border.
Conroy Diamonds and Gold holds over 1,500km2 of licenced area over the Longford-Down Massif, a place identified by the company’s Chairman, Professor Richard Conroy, as an area of outstanding potential. ‘Professor Conroy founded Conroy Petroleum and Natural Resources plc in 1980, then went on to discover the Galmoy lead-zinc deposit in County Kilkenny. The company was a founding member of the consortium that discovered the Pogo gold field in Alaska,’ says Kevin McNulty, Senior Geologist from CDG. By the mid-1990s Galmoy was well on the way to becoming a major lead-zinc producer. By the time CDG had listed on OFEX in 1997 and on AIM in 2000, promising exploration results in the Longford-Down Massif led the company to extend its licences to cover the entire area.
For an explorationist, the selection of a target area has to fulfil a number of criteria, most notably setting on a regional scale. Most large-scale mineralisation is related to plate tectonics, where movements can produce significant mineral-forming environments. In the case of the Longford-Down Massif, all the components are there, says McNulty. ‘The area is underlain by Ordovician/Silurian sediments which were laid down in an accretionary prism comprising continental slope, trench and ocean floor sediments generated by north/north west subduction, with the north-west volcanic arc represented by lower Ordovician volcanics stretching from Mayo to Tyrone (Tyrone Igneous Complex).’
The subduction (where the ocean floor passes beneath a continent, as in the Pacific-Andean coast today) represents the closure of an ocean between America and northern Europe. Evidence for this can be seen in fossils found within sediments where the northern margin in Ordovician and early Silurian times represent two distinct populations, who by late Silurian times had merged. This difference is best accounted for by a significant ocean that at first isolated the two palaeontological communities and later closed, allowing a merger of the groups. The ocean is described as a ‘proto-Atlantic’ (the present Atlantic developed in Mesozoic times some 200Ma later) and is named Lapetus. The significance of this event is in the heat and fluid movements associated with closure and the suture line that marks the point of collision.
The Longford-Down Massif represents the north-west sedimentary arc of the Caledonian Lapetus Ocean, with the Ordovician/Silurian rocks of the Leinster zone representing the southeast arc of the Lapetus. The two zones are separated by the Orlock Bridge Fault, which is believed to be the structural suture marking the line of closure of Lapetus.
In the Longford-Down Massif outcrop, the Ordovician and Silurian rocks (see map below, left, the purple and olive green area on the south side of the fault line marked in brown) is mainly greywacke facies formed in deep water by submarine slumps and turbidity currents. Structurally, the Longford-Down Massif features wrench faults of up to one kilometre displacement, while south of the Orlock Bridge Fault, several major faults each parallel to the regional strike, and with a large stratigraphic displacement, have been recognised. This structural profile is similar to that of a modern subduction complex, where continuous underthrusting of the inner trench wall by a descending oceanic lithosphere produces a sequence of sedimentary slices.
Within the northwest dipping sedimentary sequence at Clontibret, two types of disseminated gold mineralisation are identified – that associated with sulphides in a stockwork zone largely within an arenite sequence. Lying both above and below the stockwork are zones of higher-grade lode-style mineralisation that cross-cut argillite and arenite sequences. Exploration began in the late 1990s with stream sediment sampling, trench sampling and geological mapping over the licence area. By 2000, two drill holes had tested the Tullybuck/Lisglassan geochemical anomalies to reveal 10g/t of gold over a width of 1.6m at approximately two metres depth. A second hole intersected the same lode vein with 14g/t over nearly three metres at 46m below surface.
Drilling in 2002 included an intersection of a 67m mineralised zone at Tullybuck/Lisglassan. This broad zone of gold mineralisation is wider than any previous intersection within this deposit and has important implications to demonstrate an economic resource for future mining.
Later exploration widened over the entire Armagh-Monaghan gold district and included collection of over 1,000 soil samples that produced values in the range of 178-739 parts per billion (ppb) gold. Normal background levels are around four ppb. These zones extended up to 500m in length.
Another anomaly was also identified at Slieve Glah, some 35km to the southwest in County Cavan, where the Orlock Bridge Fault suffered a major strike swing that may have created a dilation zone. Such zones are significant elsewhere in hosting substantial gold mineralisation.
Although high-value anomalies are welcome in exploration, the objective of any would-be producer is to establish a resource estimate that will lead to project development. According to CDG, the company had to ‘go back to basics’ to construct the mineralisation model in the Clontibret area.
The narrow lode veins, typically one to two metres wide, contain sporadic gold values of 10-20g/t and also feature a wider low-grade halo around the central structure. These haloes may be tens of metres in width and appear to be more laterally persistent than the high-grade mineralised fault zones they encompass. Low-grade haloes can be traced for 100-200m both laterally and down dip. Conroy Diamonds and Gold’s latest geological model is based, not on correlating narrow high-grade veins, but on correlating the low-grade haloes from section to section laterally and to depth. Thirty-four mineralised lode structures or zones have been identified with a strike direction of 330-335º.
In 2006 the company commissioned CSA, the international mining consultants, to calculate a series of inferred and indicated resource scenarios based on one closely-drilled area covering less than 20% of the Clontibret target anomaly. These JORC-compliant estimates show an inferred resource of 500,000oz contained gold (12Mt grading 1.3g/t using a one gramme per tonne cut-off). This includes an indicated resource of 1.3Mt grading 1.4g/t (64,000oz contained) at one gramme per tonne cut-off.
More recently, CDG revised its estimates at Clontibret to an indicated resource of 10.9Mt averaging 1.24g/t (440,000oz), and an inferred resource of 14.0Mt averaging 1.32g/t (590,000oz) for a total of 1.03Moz. This calculation is based on earlier work, upgraded using a 0.75g/t cut-off limit. With a US$875/oz gold price, this cut-off translates to a gross rock value cut-off of US$21/t, supporting the move to the lower limit. The gross rock value of CDG’s defined resource is US$36.25/t. This is based on an area 600x500m, that represents no more than 20% of the Clontibret target.
Recognition of persistent low-grade mineralisation adds a new context to gold mineralisation. Although high-grade gold and antimony at Clontibret had been known historically, the gold is intimately correlated with arsenic both in old drill data and new exploration. Arsenic values of 100ppm can be used to determine the edges of these haloes, which can be traced for up to 100-200m laterally and down dip, along structures open in both these directions.
Work is continuing to refine the model and confirm gold resources. Some success has been achieved at the Glenish and Clay Lake. Soil geochemistry has revealed anomalous gold concentrations at levels exceeding those at Clontibret. However, the source of the large nugget found at Clay Lake remains to be discovered.
Further information: Conroy Diamonds and Gold