Title: Characterizing orogenic vein systems to promote gold exploration across Irish terranes
Researchers: James Stratford, Dr Sean McClenaghan
Research purpose and background
This is a microanalytical project focused on a re-examination of the genesis of orogeny-associated gold-bearing hydrothermal systems, hosted by the Dalradian stratigraphy of Ireland and Scotland, through textural analysis of sulphide phases and geochemical analysis of trace concentrations of gold and associated elements.
Dalradian host rocks - The Dalradian Supergroup is a succession of sedimentary and volcanic rocks which was deposited onto the margin of Laurentia between the Neoproterozoic and the Cambrian (between ~800 Ma and ~510 Ma), and was subsequently uplifted and deformed in a series of collisional events referred to collectively as the Caledonian Orogeny (~475-405 Ma). The sequence is predominantly constrained within the Grampian tectonostratigraphic terrane which extends along an approximate SW-NE trend and is encountered extensively in both Ireland and Scotland. The southernmost boundary of this terrane is referred to as the Fair Head - Clew Bay Line (FHCBL) in Ireland, and the Highland Boundary Fault (HBF) in Scotland.
Metamorphism and metal transport - Temperatures within the terrane exceeded 400℃ in the north of Ireland (greenschist facies conditions) and 700℃ in the far west (amphibolite facies) at peak-metamorphism coincident with D3 regional peak-deformation (~470-465 Ma). Under these conditions, significant amounts of fluid can be released from hydrous minerals via metamorphic devolatisation reactions, which flow through the rock driven by heat generated by the orogen itself and the proximal island arc, transporting metals via fractures and along favourable horizons into physical and chemical traps where they were concentrated as mineral deposits. It has also been suggested by previous workers that magmatic processes may also have a role in the mobilisation of fluids.
Gold exploration across the Dalradian - The Dalradian is currently the focus of a resurgence in the exploration and appraisal of gold resources after a hiatus which followed their initial discovery in the 1980s. This has been most effective in County Tyrone, close to the Omagh Thrust (a segment of the FHCBL), though vein-hosted gold with economic potential has been found at various localities from Croagh Patrick in the West of Ireland to Aberdeenshire in eastern Scotland. Should favourable economic, political and social conditions prevail, it is considered likely that Ireland will become a significant European producer in the near future. The timing of the pursuit of research into the genesis of these systems and the distribution of gold is therefore considered optimal and depending on results may have a significant implications for exploration.
Gold and associated sulphide minerals in hydrothermal systems - Gold across the whole of Ireland, to the extent of our current understanding, is contained in two main types of mineral systems: alluvials, whereby gold is transported and trapped through surface processes following uplift and erosion (for example in County Wicklow to the south), and hydrothermal vein systems within the bedrock itself (from which alluvial deposits are likely derived). It is the latter which is the focus of current exploration activity within the Dalradian and of this research. Gold in hydrothermal systems is typically associated with pyrite (FeS), occuring along with other sulphides such as arsenopyrite and chalcopyrite.
Central field locality: the >6Moz Curraghinalt Au-Ag Deposit - The largest gold resource on the island of Ireland is Curraghinalt, 17km NE of Omagh in County Tyrone. A structurally controlled deposit hosted within a swarm of WNW-ESE trending quartz-carbonate veins which dip at approximately 70 degrees to the NE, resource estimates are currently over 6 million ounces and average grades in inferred to measured confidence categories of between 11 and 22 grams per tonne. At this deposit research has delivered a degree of understanding of the microscopic visible gold (VG) which occurs in the form of gold-silver amalgam, or electrum, and comprises the majority of this resource. The timing of mineralisation within a 10 Myr interval post-peak Caledonian metamorphism is considered well contrained, and models of structural framework have been developed. Little research has been conducted to date however, into the trace concentrations of gold and associated elements within the sulphide minerals themselves. The deposit is also a short distance (<30km) from Cavancaw, a smaller producing gold mine with a N-S trending vein swarm and a distinctive characteristic Pb signature not observed at Curraghinalt, raising questions regarding the relationship of these deposits, and why they are so different. It is considered that there are structural controls on these deposits and faults being exploited by fluids are different and work has been conducted on this, however research into elemental abundances from sulphides may reveal important information into the genetic relationship suggested by previous workers.
Research into trace concentrations of gold in deposits globally - Trace concentrations of gold, known as 'invisible gold', within pyrite in orogenic deposits tends to be associated with other elements including As, Ni, Pb, Zn, Ag, Mo, Te, V and Se. Research into the trace abundances of these and other elements within sulphide minerals at orogenic gold deposits such as Sukhoi Log (Russia), Spanish Mountain (United States), Bendigo (Australia) and in the El Callao District of Venezuela have revealed important insights into the source of gold, along with the sequence of mineralisation events allowing researchers to model a well constrained paragenesis. Such work has enabled focused targeting of specific phases and the development of exploration vectors.
Aims and Objectives -- The aims and objectives of this project as per the original proposal are: (a) to generate a detailed geochemical database for vein Au systems, focussing on the microscopic distribution of precious metals and associated trace elements in host sulphide phases; (b) to contextualise geochemical data using in-situ age dating methods (Re/Os, U/Pb), fluid inclusions, and detailed growth histories of veins (SEM-CL of quartz/carbonate), with the goal of elucidating regional variations along Lower Palaeozoic and Dalradian trends; (c) to define the basic structural control of gold mineralisation and associated vein systems; and (d) to develop and/or modify existing exploration models for orogenic and intrusion-related vein systems along prominent Au-bearing terranes in Ireland.