Summary:
Dalradian’s Northern Ireland property has potential to host two distinct deposit types. Licences DG1, DG3,DG4, DG5 and DG 6 which includes the Curraghinalt gold deposits, has potential to host orogenic gold deposits. Licence DG2, underlain by the Tyrone Igneous Complex, has potential to host volcanic massive sulphide mineralization (VMS), as well as porphyry copper-gold, and irongold exhalites (Hollis et al., 2014; Hollis et al., 2015; British Geological Survey, 2016).
Orogenic Gold Deposits
Rice et al. (2016) noted that the timing of gold mineralization at Curraghinalt (ca. 462.7 – 452.8 Ma) closely followed peak metamorphism associated with the Grampian event of the Caledonian Orogeny. It is temporally linked with an extensional setting following orogenic uplift and collapse. Rice et al. (2016) concluded that Curraghinalt is more likely an orogenic (rather than intrusion related) gold deposit. Thus an orogenic gold deposit model best describes the Curraghinalt vein system.
Volcanic Massive Sulphide Deposit Model
The volcanic stratigraphy of the Tyrone underlying licence DG2 is a potential host to volcanic massive sulphide deposits. VMS deposits are syngenetic, stratabound, and in part stratiform accumulations of massive to semi-massive sulphide that form seafloor hydrothermal systems at or near the seafloor (Gibson et al., 2007; Galley et al., 2007). The deposits consist of two parts: a concordant massive sulphide lens (>60 percent sulphide minerals), and discordant vein-type sulphide mineralization, commonly called the stringer or stockwork zone, located within an envelope of altered footwall volcanic and or sedimentary rocks (Gibson et al., 2007).
The Curraghinalt Gold Deposit
High-grade gold mineralization occurs as a series of west-northwest trending, moderately to steeply dipping, subparallel stacked veins and arrays of narrow extension veinlets. These veins are hosted by the Neoproterozoic Dalradian rocks in the central section of the Sperrin Mountains, and represents the largest known gold deposit in the United Kingdom.
The mineral resource model discussed herein focusses on a set of 21 prominent gold-bearing quartz veins that occur mainly within psammites, semi-pelites, and pelites of the Dalradian Argyll Group, within the Mullaghcarn Formation. Auriferous quartz veins exist between the main modelled veins, but their continuity is difficult to demonstrate at the current drill spacing. The quartz vein system was investigated by core drilling and is partly exposed in underground workings. Surface exposures of the vein system are limited to the Curraghinalt and Attagh Burns (creeks), as well as a variety of surface trenching excavations completed in 2003 and in the late 1980s. The veins range from a few centimetres to five metres wide.
The vein swarm has been modelled along strike for approximately 2,300 metres, across strike for approximately 800 metres and down dip for over 1,200 metres by prospecting, trenching, and drilling. Though the modelled veins extend 2,300 metres along strike, the vein system is traceable with similar strike aligned veins occurring over approximately four kilometres from Alwories in the east to Scotchtown in the west. The vein system remains open along strike and at depth. On average, the quartz veins dip between 50 degrees and 75 degrees to the north northeast.
The gold-bearing vein system in the Curraghinalt deposit is crosscut by several generations of structures with different characteristics and orientations. The two most prominent structures in the deposit are the Kiln and Crow shear zones. These shear zones are east-west trending, either south-dipping or north-dipping. The Kiln shear shows evidence of brittle reactivation as indicated by the presence of gouge zones along the contact between the highly strained ductile rocks within the shear zone and the Dalradian metasedimentary wall rocks. The Kiln shear disrupts and displaces the vein zones (D veins) with observed oblique dextral-normal kinematics.
Vein zones are entrained within the Kiln Shear and previous workers (Boland, 1997) have suggested that the shears have controlled vein emplacement or at least served to produce wider mineralized segments.
In addition, other shear zones and faults with gouge development crosscut the Curraghinalt deposit. These structures trend northeast and dip moderately northwest. Fault thicknesses and characteristics vary based on the country rock lithology. Faults and shear zones within the pelite, north of the No.1 vein, tend to be wider (typically 0.5 to1.0 metre wide), either crosscut or disrupt the pre-existing regional foliation in the pelite, and in some cases, have measurable offsets (e.g., four metre offset of the T17 vein).
Limited information is available for the kinematics of the crosscutting fault systems. However, in 3D the Kiln shear zone is currently modelled to offset the auriferous vein system with a dextral strikeseparation. Underground mapping by Dalradian shows sinistral strike-separation of the vein system along northeast-trending faults, particularly where faults intersect the western section of veins (e.g., T17 west, eight metre offset by FZ_107). However, the Dalradian mapping also shows the dextral strike-separation of veins where faults intersect the eastern section of veins, for example the V75 vein east is offset by three metres. This fault contains steep- plunging slickenlines, suggesting the movement direction was dominantly vertical. This is possibly related to dominantly normal movement creating sinistral or dextral separations given the dip of the faults.
Petrographic work by Clarke (2004) has documented that the gold mineralization at Curraghinalt occurs in quartz-pyrite-carbonate veins and is associated with variable abundances of carbonate, chalcopyrite, and tennantite-tetrahedrite. Gold is commonly in the form of native gold and more rarely as electrum (>20 weight percent silver), and occurs primarily along fractures in pyrite, as inclusions in pyrite, and at pyrite grain contacts with carbonate and quartz. Most native gold grains are associated paragenetically with carbonate, chalcopyrite, tennantite-tetrahedrite, and telluride minerals infilling fractures in pyrite. The seven veins studied at the time have similar mineralogy. Native gold was observed in samples from all veins and grains range in size from 2 micrometres to 150 micrometres.