1. Geological setting

The study area is located W of Tyndrum, Perthshire, in the Grampian Highlands of Scotland (see box, Fig. 2). Three glacially sculpted valleys, Glen Orchy, Glen Lochy and Cononish Glen, dissect this mountainous tract, with the highest peaks being Beinn Chuirn (880 m) and Beinn Udlaidh (840 m) (Figs 3 and 4). A thick cover of glacial till and moraine obscures bedrock in the valley bottoms and on the lower slopes, and in many places access is made difficult by mature pine forest.

Figure 2

Simplified geological map showing the location of the study area (boxed), and the line of cross-section A–B (Fig. 3).

Figure 3

Simplified true-scale N–S geological cross-section across the study area (see Fig. 2 for location) (after Tanner & Thomas Reference Tanner and Thomas2010, fig. 14). This figure shows the location of the Cononish gold mine on the right-way-up (south) limb of the Beinn Chuirn Anticline, and the relationship of this fold to the Orchy Dome.

Figure 4

Geological setting of the Cononish gold mine, showing the axial trace of the D2 Beinn Chuirn Anticline (in black). For clarity, the traces of the other major D2 folds are omitted (see Tanner & Thomas Reference Tanner and Thomas2010, fig. 5, for these). A–E are referred to in the text.

The network of major quartz veins described here occurs in rocks belonging to the Dalradian Supergroup that had already undergone polyphase deformation (D1–D4) during the ∼470 Ma Grampian Orogeny (Oliver Reference Oliver2001; Baxter et al. Reference Baxter, Ague and Depaolo2002). All of the Dalradian rocks in the study area have been regionally metamorphosed to garnet grade (lower amphibolite facies), and most of them have been affected by widespread retrogression, resulting in the alteration of biotite and garnet to chlorite.

The Dalradian succession ranges in age from Neoproterozoic to Early Ordovician (Tanner & Sutherland Reference Tanner and Sutherland2007), and is divided, from the base upwards, into the Grampian, Appin, Argyll, Southern Highland and Trossachs groups. The rocks in the Tyndrum–Dalmally area span the conformable boundary between the Grampian and Appin groups, with the Appin Group rocks being separated from those of the overlying Argyll Group by the Iltay Boundary Slide (Bailey Reference Bailey1922; Roberts & Treagus Reference Roberts and Treagus1975) (Figs 2, 3). The latter was considered to be a major structural break, but is now recognised as a regional stratigraphical disconformity (see Tanner & Thomas Reference Tanner and Thomas2010), and referred to as the ‘Boundary Slide’. The Dalradian sequence that lies beneath it comprises three formations: the Meall Garbh Psammitic Formation (Grampian Group); the Beinn Udlaidh Quartzite Formation; and the Coire Daimh Pelite Formation (Leven Schist). The two latter units belong to the Lochaber Subgroup (Appin Group) (Fig. 4).

Bailey & Macgregor (Reference Bailey and Macgregor1912) established the tripartite stratigraphical sequence and recognised that the overall structure consists of two major isoclinal folds, now referred to as the Beinn Chuirn Anticline and Glen Orchy Syncline (now Beinn Udlaidh Syncline (Tanner & Thomas Reference Tanner and Thomas2010)), folded over a late antiformal structure. Thomas & Treagus (Reference Thomas and Treagus1968) deduced the presence of a further major isocline, the Glen Lochy Anticline (Fig. 3). Recent work has clarified the tectonic interpretation, and the structure is now seen as a stack of three major, SE-facing isoclines of D2 age folded by a large-scale E–W-trending periclinal structure termed the Orchy Dome (Fig. 3) (Tanner & Thomas Reference Tanner and Thomas2010).

Originally named the ‘Orchy Anticline’ by Bailey & Macgregor (Reference Bailey and Macgregor1912), the true shape of the dome was only revealed recently by structure contours drawn on the quartzite–pelite boundary (shown in blue on Figure 5), on the upper limb of the isoclinal Beinn Udlaidh Syncline (Tanner & Thomas Reference Tanner and Thomas2010). It is slightly elongated, with an average trend of 087°, a southerly limb dipping at 15°, and periclinal closures to the east and west that plunge around 10–15° to the E & W, respectively. A further set of structure contours (in red, Fig. 5; see inset) has been drawn on the same interface on the top limb of the synformal Beinn Chuirn Anticline, that shows that this limb dips at around 25°S. In order to link these two sets of contours, trend lines for bedding/foliation have been drawn (pecked lines, Fig. 5). This combination of structure contours and trend lines reveals the regional scale of the Dome.

Figure 5

Simplified geological map of the Tyndrum–Dalmally area and its extension westwards. The shape of the Orchy Dome is outlined by structure contours drawn on the boundary between the Beinn Udlaidh Quartzite and the Coire Daimh Pelite: those on the lower limb of the Beinn Chuirn Anticline are shown in blue, and on the upper limb in red. The bold black arrows are accompanied by calculated dip values. The inset 3-D sketch shows the relationship between the two sets of structure contours on the limbs of the curvilinear Beinn Chuirn Anticline. For A, see text.

Numerous sills, dykes and irregular bodies of the lamprophyre–appinite suite, mainly vogesite, were emplaced into the Dalradian rocks following D4.

The lamprophyres were accompanied by explosion breccia pipes, which are roughly circular in cross-section and range up to 120 m in diameter. They are filled with well-cemented fragments of Dalradian country rock, with collapse textures (Fig. 1D), and at several localities early pipes are cut by the lamprophyric intrusions. As noted by Tanner & Thomas (Reference Tanner and Thomas2010), the main bodies of lamprophyre are clustered around Meall Garbh and lie astride, but slightly offset from, the axial trace of the Orchy Dome, suggesting that their intrusion was spatially influenced by this structure (Fig. 5). Most of the pipes occur in the two areas outlined by pale green shading on Figure 5.

The lamprophyre suite was followed by the intrusion of microdiorite dykes (s.l.) and Late Carboniferous quartz-dolerites (Fig. 4). The Tyndrum–Dalmally area is bound to the SE by the Tyndrum Fault, a major strike-slip fault with a left–lateral displacement of ∼4 km, and a vertical downthrow to the SE of some 2 km (Treagus Reference Treagus1991). Similarly, the Ericht–Laidon Fault (Fig. 2), which marks the NW limit of this crustal compartment, has estimated left-lateral displacement of 5·5 km and a downthrow of 0·2 km SE (Treagus Reference Treagus1991). The geometry and tectonic significance of the major transcurrent faults in the surrounding area was discussed by Jacques & Reavy (Reference Jacques and Reavy1994), Treagus et al. (Reference Treagus, Pattrick and Curtis1999) and Dewey & Strachan (Reference Dewey and Strachan2003).

1.1. Mineralisation

Lead mining had been carried out in the Tydrum district since 1741, mainly at Eas Anie and at Tyndrum Mine itself, but work ceased in 1925. Gold was discovered in-situ in a quartz vein at Cononish in 1984 and, as a result, Ennex International constructed an exploration adit there in 1986. The Cononish mine, which lies on the upper limb of the Beinn Chuirn Anticline (Fig. 4), is set to become Scotland's first commercial gold mine, with a total metal inventory of 163000 oz Au and 596000 oz Ag, at a Au cut-off grade of 3·5 g/t (Scotgold 2010). Details of the history of mining and exploration at Cononish are given by Earls et al. (Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992) and Dominy et al. (Reference Dominy, Platten, Xie, Sangster and Dominy2009).

The quartz veins in the Tyndrum–Dalmally area contain either precious- or base-metal mineralisation, or are barren. Gold occurs in the Cononish Vein both as electrum (Au_x Ag_100-x), intergrown with pyrite or galena, and, to a lesser extent, as native gold in grains <0·2 mm across. It is always accompanied by silver, either as tellurides or as native silver (Parker et al. Reference Parker, Clifford and Meldrum1989; Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992; Treagus et al. Reference Treagus, Pattrick and Curtis1999), and in places by galena. Pyrite is the main sulphide mineral, with galena, sphalerite and chalcopyrite as minor constituents of the ore. Regarding mineral exploration, pyrite is the pathfinder for gold mineralisation, especially where it is finely disseminated in quartz, and accompanied by pink to reddish-coloured metasomatic alteration of the country rock at the vein margin, that is probably due to the introduction of K-feldspar. The only evident structural control for the Au–Ag mineralisation is that it occurs along 042°- and 326°-trending major faults of the Cononish and River Vein sets (see Section 6). The former are slightly curved to sigmoidal along strike, possibly giving rise to local releasing bends where mineralisation might be concentrated, in contrast to the generally non-mineralised Barren and Mother Vein sets, which consist essentially of planar veins. The following sequence of mineralisation episodes has been recognised:

1. 1. Gold–silver mineralisation in major quartz veins, and in adjacent metasomatic haloes. Referred to here as the Au–Ag mineralisation (the ‘A-min’ at Cononish of Earls et al. (Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992)).

2. 2. Base-metal, Pb–Zn mineralisation (B-min) generally spatially associated with the Au–Ag-bearing veins (for example, the Lower Adit Vein at Cononish) but also forming a major orebody in the Tyndrum Fault zone at Tyndrum Mine. The locations of small Pb workings are shown on Figure 6.

3. 3. Emplacement of the so-called ‘barren’ veins, some of which, such as the Mother Vein, have been found subsequently to be weakly mineralised (Au and Ag).

4. 4. Emplacement of a rare set of veins at Tyndrum containing a variety of uranium- and tellurium-bearing minerals (Pattrick Reference Pattrick1985).

Figure 6

Geological map showing the spatial distribution, orientations and thicknesses of bodies of quartz breccia and massive quartz (both shown in blue) in the Tyndrum–Dalmally area. For A and B, see text.

In an endeavour to find further gold reserves, Scotgold have recently investigated a number of explosion breccia pipes in the area (Scotgold 2010). One 12 m core yielded 1·45 g/t Au and 8·9 g/t Ag, and a further six pipes yielded 1–2 g/t Au over 3–14 m. However, the related, and more widely distributed, sills, dykes and bosses of lamprophyre gave low Au values of <1 g/t.

2. The quartz-breccia veins

The veins range in thickness from a few cm to >20 m, exceptionally 50 m, and in length from decimetres, to >3 km. Most of the major veins form wall-like features in the landscape (Fig. 1A), and the larger ones can be seen from several km away. In this paper, the term ‘quartz breccia’ is used in a descriptive, non-genetic sense to describe veins that consist of massive grey or white quartz with abundant, angular fragments of the local country rock, generally up to 10 cm across (Fig. 1C). These quartz-breccia veins are invariably located along fault planes, show little evidence of having been affected internally by subsequent fault movement, and take their name from the parent fault. All known veins >1 m thick are shown on Figure 6. A consistent feature of the large quartz veins in the Tyndrum–Dalmally area is that they are discontinuous, and consist of a number of discrete, almost perfectly aligned (‘in-line’) segments.

Early workers (Kynaston & Hill Reference Kynaston and Hill1908; Bailey & Macgregor Reference Bailey and Macgregor1912; Thomas & Treagus Reference Thomas and Treagus1968; Roberts & Treagus Reference Roberts and Treagus1975) made no reference to these veins. The Creagan Ghlas vein complex is the sole exception: it was shown as a ‘Quartz Vein’ on the 1897 edition of the BGS Sheet 45 (Oban) but reduced to the status of a fault on the subsequent edition (Dalmally Sheet 45E, published 1992).

Although some of the faults give rise to marked topographic features, the amount of displacement on them is small, typically less than a few tens of metres (Tanner & Thomas Reference Tanner and Thomas2010, fig. 7). They appear to be normal faults, but as they have a steeper dip than is the norm for this type of fault, some may be oblique-slip faults. Fault breccia and gouge are associated with these faults, but cataclasite and mylonite are absent.

Several major faults clearly cut and displace members of the lamprophyre suite, for example at locality A (Fig. 4), and 0·6 km to the west of it. In addition, the quartz breccia veins cut lamprophyre bodies at a number of places including B (Fig. 4), and also locally contain angular fragments of lamprophyre as at A and B on Figure 6. However, the outcrop patterns of the lamprophyres suggest that, in some cases (e.g. Fig. 4B and C), they may have been intruded along pre-existing fault planes (Tanner & Thomas Reference Tanner and Thomas2010). If this is correct (the rock exposure is not good enough to be absolutely certain of this), the above relationships, when taken together, suggest that faulting and lamprophyre emplacement were broadly contemporaneous. In this scenario, the earliest lamprophyre intrusions were emplaced into barren fault planes, brecciated during fault reactivation and, together with the existing fault breccia, were repeatedly injected by pulses of silica-rich fluid to form quartz breccia.

The NNE- trending faults are intruded by microdiorite (s.l.) dykes, for example, at D (Fig. 4). Quartz-dolerite dykes either cross the faults without deflection, or terminate against them, as at E (Fig. 4).

Contacts between the veins and the country rock are either planar and sharply-defined, possibly due to later fault movements along the contact, or are gradational on a metre scale (Fig. 1B), with ‘ghost bedding’ being preserved within the vein along one or both margins. In places, there is a transition zone up to a few metres wide, with abundant centrimetre-scale quartz veins marking the vein/country rock contact.

At many localities, bedding and foliation in the country rock have a similar orientation on both sides of the vein, but localised brittle folding or brecciation of the country rock can also be present at either contact. In several cases, such as in the Beinn Udlaidh and Mother veins, the main quartz breccia is flanked by a narrow, deeply-weathered, sulphide-rich band, with field relationships showing that the sulphide-rich mineralisation predated formation of the barren quartz vein (D. Catterall, pers comm. 2011), indicating an extended event history for these faults/fractures/veins.

2.1. Field relationships of individual quartz-breccia veins

2.1.1. Cononish Vein

The Cononish Vein (Scotgold 2007), previously named the Eas Anie Vein, or Eas Anie structure (Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992) hosts the Cononish Au–Ag mineral deposit (Fig. 7A). It lies along the Cononish Fault in Cononish Glen 1 km SE of the summit of Beinn Chuirn (Fig. 6) but unlike most of the other major veins, it lacks topographic expression. This is clearly seen in Figure 8A, which shows the geographical and geological setting of the Cononish gold mine. The Vein is cut by the 15 m-thick Eas Anie quartz-dolerite dyke, and affected by several minor faults, none of which has a displacement >9 m (Dominy et al. Reference Dominy, Platten, Xie, Sangster and Dominy2009).

Figure 7

A series of geological maps (A–E) showing the spatial distribution of selected major quartz-breccia veins, and their relationship to the local stratigraphy and structure of the Dalradian rocks. Note that although true N is the same for all of the maps, it is not in its normal orientation; also, the maps are at different scales. A–D on map D are referred to in the text.

Figure 8

Geological setting of the Cononish gold mine with transverse (A) and longitudinal (B) sections through the Cononish mineral deposit: (A) location of the mine on the flank of Beinn Chuirn viewed looking NE. The dotted line (Q) shows the approximate location of the outcrop of the Beinn Udlaidh Quartzite, separating the stratigraphically-older Grampian Group (GG) below, from the Coire Daimh Pelite (CDP), above; (B) vertical cross-section A–B (see location on Fig. 7A), showing the lensoid shape of the Cononish Vein (after Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992). It cuts the longitudinal section shown on (C), along D–C–E; (C) contoured Au-values (g/t=gram per m) drawn on the surface of the Cononish Vein, showing the existing adits, raises, and faults. The dotted line (LAP) marks the inferred position of the Lower Adit Psammite (Beinn Udlaidh Quartzite), and corresponds to that shown in (A) (after Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992).

The Vein is very poorly exposed, and surface outcrop was only obtained in 1984 when Ennex International excavated trenches normal to strike at intervals of 10–20 m. The trenches were back-filled and the Vein is now only exposed in a few places. Almost all of the information on the Cononish Vein comes from diamond drilling, and from continuous exposure in a 1·2 km-long exploration adit driven SW from the gold mine portal at [29122 28575]. This adit is at 400 m OD and runs parallel to, or within, the Vein (Fig. 7A). The following account of the Cononish Vein is based largely on published information, as the mine has been held on a care-and-maintenance basis since 1995; for safety reasons, access to the adit was not permitted during the present study.

The Cononish Vein cuts the right-way-up limb of the Beinn Chuirn Anticline that, in the area around the adit, has a mean orientation of strike 089°, dip 29°S (N=26) giving a line of intersection with the Cononish Vein of 22° to 223°. This line is seen as the subcrop of the Lower Adit Psammite on the surface of the Vein on Figure 8, where it has a pitch of 17°SW.

The host rocks at the adit portal are psammites and semipelites of the Grampian Group (Meall Garbh Formation; Tanner & Thomas Reference Tanner and Thomas2010), and the adit passes SW through a thin band of the Lower Adit Psammite (Beinn Udlaidh Quartzite), into a banded garnet-bearing pelitic unit, the Coire Daimh Pelite (the local equivalent of the Leven Schist) (Fig. 7A). The apparent vertical displacement across the Cononish Vein is <5 m (Parker et al. Reference Parker, Clifford and Meldrum1989).

The Cononish Vein has a mean trend, measured over >1 km, of 041°, and a calculated dip in the central portion of 89°SE. In detail, the dip in the adit varies from 70°SE, through the vertical, to 70°NW as it is traced from the portal area SW to the end of the adit (Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992). The Vein has a slightly sinuous outcrop (Fig. 7A), varies markedly in thickness along strike from a few cm to 6 m, but is generally <4 m thick. The longest continuous section of the Vein is 90 m long and the longest break in continuity is 4–10 m (Dominy et al. Reference Dominy, Platten, Xie, Sangster and Dominy2009). Similar variations in thickness are also shown in the only published vertical cross-section through the deposit (Fig. 8B), based on the results of early diamond drilling, which when combined with the outcrop pattern, suggests that the body comprises several, large, oblate or pancake-shaped segments.

Gold values plotted in the plane of the vein show a nodal distribution (Fig. 8C). The linear elements of this pattern plunge steeply SW, and are interpreted as ore-shoots (Dominy et al. Reference Dominy, Platten, Xie, Sangster and Dominy2009). This pattern is linked to the variations in vein thickness noted above, the higher Au values coinciding with the thickest part of the Cononish Vein. However, this is not necessarily a straightforward relationship, because the Vein consists essentially of two main components: a massive quartz breccia, which acts as a marker and is only relatively weakly mineralized, and the ore zone, which varies in thickness from 20 cm to 100 cm. Thus the zonal variation in Au values (Fig. 8C) could be related to variations in thickness of the ore zone, and be independent of the 3-D shape of the quartz breccia. A similar anomaly pattern at a comparable scale, but in this case for Ag, was reported by Paez et al. (Reference Paez, Ruiz, Guido, Jovic and Schalamuk2011) from a longitudinal section of the Martha epithermal silver mine in Argentina. From detailed underground mapping, they correlated this pattern of second-order, steeply-plunging ore-shoots, with the presence of dilatational jogs and step-overs. A similar interpretation may hold for Cononish gold mine.

The Vein varies considerably in character along strike, from a few weakly mineralised quartz veins contained within a 5 m-wide bleached-looking zone in the portal area, to a massive quartz breccia 6 m thick at its maximum development some 400 m SW along the adit (Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992). After the first 400 m, where it crosses Grampian Group rocks, there is a 500 m-long section in which the Vein is thicker and shows higher gold values. This section, shown by diamond drilling to extend for at least 500 m vertically, comprises the Cononish Au–Ag deposit (Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992) (Fig. 7A).

Arrays of sub-parallel minor quartz veins, and a several m-wide zone of characteristic pink to red metasomatised country rock, mark one or both margins of the Vein. The pink colouration is due to the introduction of K-feldspar and this zone passes out into sericitised and chloritised country rock (Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992).

Apart from the quartz breccia, the other constituents of the Vein form a complex pattern of discontinuous lenses on a 1–50 m-scale. These lenses are cut by both minor shear zones, the ‘pyritic shears’ of Earls et al. (Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992), and in places by shear zones that may be traced for >100 m (Dominy et al. Reference Dominy, Platten, Xie, Sangster and Dominy2009), but no shear sense was recorded by these workers.

The continuation of the Vein outside of the area covered by Figure 7A is uncertain. Southwest from the portal, the vein passes laterally into a zone of metasomatised country rock before it meets the Barren Vein, and this may mark the termination of the quartz breccia body. However, the Cononish Fault, which hosts the deposit, can be followed along strike to two disused Pb mines above the Allt an Rund (Fig. 6), and there may be further Au–Ag mineralisation in this section. A 1·6 m-thick quartz breccia vein, which was formerly worked on both banks of the Allt an Rund, strikes at 040° and dips at 73°SE, an orientation very close to that of the Cononish Vein farther NE.

Northeast of the adit portal, the last known exposure of the Cononish Fault occurs 40 m before its inferred intersection with the Mother Vein, on the SW bank of the burn (Allt Eas Anie) at [NN 2995 2898], where there is a 7 m-long trench exposing pink metasomatised psammite with thin quartz veins.

2.1.2. Beinn Udlaidh Vein

The outcrop of this quartz-breccia vein, which approaches within 150 m of the summit of Beinn Udlaidh, is partially obscured by glacial deposits. The parent Beinn Udlaidh Fault brings rocks of the Grampian Group, on the SE side, into contact with lamprophyre sills and the Beinn Udlaidh Quartzite to the NW (Fig. 7B). However, the displacement on the fault is small because the locality is close to the crest of the Glen Orchy Dome, where the rocks dip at <15° and only a small vertical movement was necessary to give the present outcrop pattern. The Beinn Udlaidh Vein can be traced for >1·4 km and consists of at least four segments (Fig. 7B). It trends at 043° and varies in dip from 70°SE, at the SW end, through the vertical, to ∼80°NW, at the NE end. The mean orientation of the quartz breccia bodies is strike 044°, dip 88°SE (N=5). The thickness of the vein has not been measured consistently but averages around 4 m.

Ennex International investigated the economic potential of the Beinn Udlaidh Vein in 1985 by channel samples taken across the full width of the Vein and by diamond drilling. This operation showed that significant Au–Ag mineralisation was present (Au: 11·1–167 g/t over 1 m), and recent diamond drilling has yielded, for example, 2·8 g/t Au and 359 g/t Ag over 1 m (Scotgold 2010).

2.1.3. Coire Daimh Vein

This well-exposed, apparently non-mineralised vein crosses the ridge that runs north from Beinn Udlaidh and can be traced for 1·2 km. It is divided into four segments, of which most of the terminations are clearly seen, and two overlap in plan view (Figs 7C and 9A). The vein is up to 16 m thick and consists of a stockwork of quartz veins with units of massive white quartz (Fig. 9B). At either end of its outcrop, the vein passes into a zone of brecciated country rock, with the host fault continuing to the SE across Coire Daimh, and to the NE to the Allt Ghamhnain (Fig. 6). The vein has a calculated strike of 026° and a dip of 80°SE; the mean orientation for the margins of the quartz breccia is: strike 027°; dip 76°SE (N=7).

Figure 9

(A) The segmented nature of the Coire Daimh Vein (Q 1–4 on Fig. 7C) as seen on the steep valley side above the Allt Ghamhnain; the photograph, taken from a light aircraft, was provided by Peter Thomas. (B) Cross-sectional view of part of the Coire Daimh Vein, viewed to the SSW, showing the random orientation of the quartz veins and lenses, and their irregular nature. (C) A boxwork of quartz veins in the country rock, adjacent to the margin of the Mother Vein SW of Meall Odhar. (D) Major segments of quartz-breccia, with massive white quartz constituting the Creagan Ghlas Vein complex in Glen Orchy, viewed to the NE. (E) Field sketch of the relationship between the quartz breccia and the country rock on Creagan Ghlas (location on Fig. 7E). (F) Massive quartz and quartz breccia (Creagan Dubha Vein) invading flat-lying Dalradian psammite/semi-pelite on the N side of Glen Lochy at [NN 2822 3217] (Fig. 6).

For most of its length, the Coire Daimh Vein cuts across and displaces the upper, gently dipping and inverted limb of the D2 Beinn Udlaidh Syncline, whose axis plunges at 06° to 064° and axial plane dips at 20°N. This Vein, although apparently lacking mineralisation, is important in that it sheds light on the geometry of major segmented veins. For example, a photograph taken of the most northeasterly segment (Fig. 9A, location on Reference Browne, Smith and AitkenFig. 7C) shows that it terminates vertically downwards, rather than becoming thicker, or being connected to a parent vein at depth.

2.1.4. Mother Vein

Previously called the ‘Mother Reef’ (Treagus et al. Reference Treagus, Pattrick and Curtis1999), it is best exposed on Meall Odhar (Fig. 6) where it comprises over eight distinct segments, each 150–250 m long, arranged in-line (Fig. 7D). These segments were previously reported to form an en-echelon array (Treagus et al. Reference Treagus, Pattrick and Curtis1999), but field measurements and examination of aerial photographs fail to support this interpretation.

The Mother Vein consists mainly of quartz breccia and massive white quartz; it is 4–7 m thick, but locally reaches 19 m. Although the vein is generally considered to be non-mineralised, electrum was reported by Pattrick (Reference Pattrick1985). Treagus et al. (Reference Treagus, Pattrick and Curtis1999) found Pb–Zn mineralisation at the western, faulted margin of the vein, and Scotgold have reported low but consistent Au and Ag values from a number of localities along the Vein (Scotgold 2010).

Distant bearings taken along strike of the Vein on Meall Odhar, together with clinometer readings, and GPS mapping of the line of the fault for >3 km, all combine to show that it has a strike of 028°, and a dip of 87°SE. This orientation is in agreement with those recorded from the margins of the quartz breccia vein whose mean value is strike 027°, dip 89°SE; (N=27).

The combined fault–vein structure can be followed for over 5 km, but there is a lack of stratigraphical markers or boundaries to indicate the amount or sense of displacement across it. However, where the Mother Vein crosses the Boundary Slide it causes little displacement (Fig. 4).

The contact between the Vein and the country rock varies between a sharp faulted margin, and a gradational boundary across which the foliation in the Dalradian rocks can be traced without deflection, and is indicative of in-situ replacement. Locally, the bedding/foliation is preserved as a ghost fabric in the Vein. West of the Vein, bedding and foliation in the country rock generally retain their regional orientation up to the contact, but this is seldom the case on the east side, where drag folds occur. The observation of Treagus et al. (Reference Treagus, Pattrick and Curtis1999, fig. 3d) that drag folds generated by fault displacement (amount unknown) occur on both sides of the vein, and can be used to determine the sense of displacement, could not be substantiated.

Areas of silicified country rock, which in places preserve minor D2 folds, occur along the margins and between the ends of both adjoining, and overlapping, segments. These silicified rocks are generally separated from the unaffected host rock by a complex of thin (cm-scale) quartz veins (Fig. 9C) (shown in blue on Fig. 7D). Certain orientations of the veins are dominant, e.g. a random selection from locality A, Figure 7D gives a mean of strike 201°; dip 84°NW (N=11), but the veins at a single locality cannot be grouped into clearly defined sets, as suggested by Treagus et al. (Reference Treagus, Pattrick and Curtis1999).

A late, brittle fault runs along part of the western margin of the Vein and is marked in places by a zone of banded cataclasite and fault gouge (Fig. 7D, loc. B). The adjoining quartz breccia is crushed and progressively transformed into a fault rock with relict isolated angular fragments of quartz breccia. At locality C (Fig. 7D), the fault gouge is itself crossed obliquely by a band of quartz breccia. Highly fractured portions of a quartz-dolerite dyke, up to 4 m thick, are found within the fault zone 18 m farther SW, and probably correlate with the dyke to the west (Fig. 4). Hence, at least localised fault movement clearly took place after the Late Carboniferous.

Farther SW, near Eas Anie, at [NN 2913 2837], the Mother Vein has a different character and comprises two dyke-like bodies of quartz breccia, 6 m and 9 m thick with 31 m of brecciated country rock sandwiched between them.

2.1.5. Significant features of other major veins

Creagan Ghlas Vein complex

This vein system is located on the NW side of Glen Orchy, near Catnish (Fig. 5). In the SW it consists of a plexus of three faults, each marked by a 1–3 m-thick quartz breccia dyke, but farther NE, three bodies of quartz rock tens of metres thick lie between the faults (Figs 5, 7E and 9D). Due to a lack of rock exposure around these large bodies, it is uncertain how some of the faults link up. The complex has been studied in reconnaissance detail here, but merits further work.

The approximate orientation of the quartz-breccia/massive quartz–country rock contact, on the NW side of the complex, is strike 036°, dip 76°SE (N=3), and is apparently intermediate between the Cononish and the Coire Daimh veins. Excellent examples of vein/country rock relationships are seen in places along the margins of the massive quartz lenses. For example, at the locality labelled on Figure 7E, ‘tongues’ of quartz appear to have been introduced passively into the country rock, in part preserving the main foliation (Fig. 9E). The thicker bodies in the complex consist mainly of barren, massive, white quartz.

Halliday's Vein

This quartz breccia vein, which trends at 045°, was reported by Halliday (1962; in Pattrick et al. Reference Pattrick, Boyce and MacIntyre1988). It crops out in a burn a few tens of metres NNE of, and aligned with, the shaft of the Tyndrum Pb–Zn mine (Fig. 6). Earlier reports that it is gold-bearing have recently been confirmed by Scotgold (2010), with Au=57 g/t. However, it is not aligned with the Cononish Vein, and no extension of it has been found elsewhere along strike.

Lower Adit Vein

Named after the Lower Adit on Cononish cliff (Figs 7A and 8C), this Pb–Zn vein follows approximately the line of the Cononish Vein underground, but diverges considerably from it in places. It is probably the source for the Pb formerly mined along the extrapolated line of the Cononish Fault to the Allt an Rund.

Creaghan Dubha Vein

This vein consists of a number of parallel pendant-like lenses of vuggy quartz. The near-horizontal trace of the adjacent bedding and concordant D2 foliation continues undeflected from side-to-side of the Vein, and is preserved locally within it (Fig. 9F).

Creaghan Soilleir Vein

comprises a number of N-trending quartz lenses, which have a laterally offset arrangement and give the impression from a distance that the composite vein is curved and trends approximately NE (Fig. 6). Evidence of small-scale mining of this vein for Pb–Zn mineralisation is seen at several places.

Coire Seileach Vein

This N-trending vein (Fig. 6) provides a good example of a quartz breccia body with overlapping segments.

Barren Vein

The precise orientation of this vein, which passes close to the summit of Beinn Chuirn (Fig. 6), has been determined from structure contours as strike 155°, dip 69°SW, and provides the standard for the NNW vein set.

River Vein

This gold-bearing vein is located in the River Orchy (Fig. 6) and was discovered by Scotgold (Scotgold 2010). Other veins belonging to this set are distinctive in that they have bright red metasomatic selvedges along their margins (especially in psammite) and trend in an unusual direction: 300–330°.

The River Vein is cut by a number of mineralised fractures and thin veins that trend NE and displaying their igneous parentage by the presence of abundant molybdenite (D. Catterall, pers. comm. 2012). Similar Mo-bearing veins are found at Blackmount close to the Moor of Rannoch Granite. Curtis et al. (Reference Curtis, Pattrick, Jenkin, Fallick, Boyce and Treagus1993) and Lowry et al. (Reference Lowry, Boyce, Fallick and Stephens1995) showed that they have a magmatic isotopic signature.

3. Segmentation of the quartz-breccia veins

Segmentation is a fundamental feature of veins, major fractures and faults, but the factors that control the relationship between the thickness of a vein, width of a fault zone etc., and the length and separation of the segments, have yet to be quantified.

All of the major quartz veins in the area are divided into a string of ‘in-line’ segments that taper at their ends. For example, the Mother Vein on Meall Odhar consists of five segments, up to 250 m in length, that are parallel to the mean orientation of the vein for >1·2 km (Fig. 7D). With the exception of the Barren and Coire Seileach Track veins (Fig. 6), the individual segments neither meet, nor overlap.

The Coire Daimh Vein, which is partly exposed on flat ground, and partly on the steeply inclined N-facing valley side in Coire Ghamhnain (Fig. 7C), provides a rare opportunity to examine the 3-D geometry of the vein segments. An along-strike view of this vein from the NNE (Fig. 9A) shows that the segments terminate downwards as well as horizontally, suggesting that they may have a circular or elliptical shape in plan view. The Mother Vein shows similar features, but less clearly.

4. Internal morphology of the quartz-breccia veins

Each quartz-breccia body, regardless of its size or orientation, consists largely of a complex network of vuggy quartz veins, individually up to a few cm in thickness, that wrap around, and isolate, angular fragments of the country rock. The breccia contains vugs lined with euhedral quartz crystals, which are lightly dusted with haematite, and preserve internal crystal surfaces outlined similarly. In many of the major veins, rusty-looking films (? of haematite) are seen on fracture surfaces within massive quartz or quartz breccia, giving the rocks a pink hue.

Only the Cononish Vein (Parker et al. Reference Parker, Clifford and Meldrum1989; Earls et al. Reference Earls, Parker, Clifford, Meldrum, Bowden, Earls, O'Connor and Pyne1992; Dominy et al. Reference Dominy, Platten, Xie, Sangster and Dominy2009) and the Mother Vein (Treagus et al. Reference Treagus, Pattrick and Curtis1999) have been described previously. Most of the early workers considered them to be ‘hydrothermal breccias’ (Pattrick Reference Pattrick1985), but gave no specific details of their morphology. There is only passing mention in the literature of occasional vuggy and crustiform structures in some of the major veins, possibly because freshly-cut surfaces or ‘clean’ fractures do not generally show details of the internal structure of the quartz breccia (Fig. 1C). However, even these surfaces do reveal some clues as to the origin of the quartz matrix, such as radiating aggregates of quartz crystals and relict vugs. Crack-seal quartz fabrics and ‘stretched’ quartz crystals are absent from even the thinnest veins associated with the breccias. Representative types of quartz-breccia vein are now described in more detail, with reference to Figures 10 and 11. The latter is an idealised, composite, vertical cross-section, designed to show the relationship between the different features seen in these quartz-breccia veins. It is stressed that no single major vein displays all of these features: some consist mainly of massive quartz and others of quartz breccia. The veins are either symmetrical with a gradational boundary on either side, or have one faulted contact.

Figure 10

Representative examples of open-space quartz veins in the major quartz-breccia bodies of the Tyndrum–Dalmally area. In all cases except E the scale is 5 cm long: (A) a network of hydrothermal quartz-breccia veins in the country rock psammite adjacent to Creagan Ghlas Vein complex; (B) angular fragments of country rock wrapped by crack-fill veins with vuggy central zones, representing an early stage of cockade formation; (C) cockade texture consisting of breccia fragments encased in several generations of crack-fill veins; (D) internal structure of comb-textured quartz veins in a cockade structure as revealed by weathering; (E) scanned image showing the internal structure of a crack-fill vein nucleated upon a rock fragment, which shows the internal development of minute vugs (0·50 mm across); (F) the morphology of face-controlled, comb textured quartz veins seen in 3-D. Terminology based on Vearncombe (Reference Vearncombe1993).

Figure 11

Conceptual model of a typical quartz-breccia vein, incorporating features (1)–(11) (see text) characteristic of such bodies in the Tyndrum–Dalmally area.

As the major vein is approached, sets of widely spaced, narrow, rectilinear quartz veins, with differing orientations, appear in the country rock along the outer margins of some of the major veins (Fig. 10A). The small veins have a vuggy texture and enclose numerous angular portions of host rock and are a representation, in miniature, of the parent body. At the contact, the country rock contains either a few thin quartz veins (Fig. 11, 1), or develops minor vein complexes (Fig. 11, 3). Minor D2 fold structures (Fig. 11, 2) are preserved in the silicified zone adjacent to the main vein and generally have the same vergence and orientation on the opposing side of the vein.

In the outer part of the quartz breccia, the minor veins become thicker, branch, and sub-divide (Fig. 10B). Bedding and S2 commonly retain their original orientation in blocks within the damage zone (Fig. 11, 4). Farther into the breccia body, angular fragments of protolith become subdivided and mantled by thicker and more complex sets of crack-fill veins (Fig. 11, 5), to give a cockade texture (Fig. 10C), as defined by Genna et al. (Reference Genna, Jébrak, Marcoux and Milési1996). As the core of the original fault zone is approached, successive generations of vuggy or open-space quartz veins have crystallised along newly-formed fractures that split the wall-rock breccia fragments into progressively smaller units (Fig. 11, 6), resulting in them being mantled by multiple sets of encrusting veins (Fig. 10C), and developing crustiform textures. In the more quartz-rich portions of the parent breccia, this process has resulted in a chessboard pattern of vugs alternating with wallrock fragments, as quartz crystals growing out from the fragments met those rooted on their neighbours. Vuggy zones developed between them, in which the quartz crystals grew in a fluid-filled cavity, and developed rhombohedral terminations (Fig. 10D). The ‘core’ consists either of massive quartz, or quartz breccia with widely separated, matrix-supported, angular rock fragments (a cryptic cockade texture) (Fig. 11, 7; Fig. 1C). Bodies of massive quartz or quartz containing only a few rock fragments are present in the damage zone (Fig. 11, 8), and units of wallrock breccia fragments identical to that in the inner part of the damage zone occur within the core (Fig. 11, 9). Some of the fragments in the core possibly preserve a pre-silicification shear fabric. A (?) late fault, with a highly polished slickenside and accompanying cataclasite, defines the margin of some of the breccia bodies (Fig. 11, 10 and 11).

Thin sections of the cockade veins illustrate (i) the growth pattern of the prismatic quartz crystals that developed orthogonal to the vein margin; and (ii) the presence of minute vugs inside the wallrock fragments (Fig. 10E). Deeply weathered and frost-shattered examples of these veins show clearly the 3-D internal arrangement of the quartz prisms in the cockade structure (Fig. 10F), and the contrast in morphology between the two sides of the vein.

The cockade texture annotated in Figure 12 indicates how the space problem resulting from the synchronous growth of crystals from all of the breccia fragments is resolved. This Figure shows discontinuities (highlighted in red) between the quartz veins mantling adjacent rock fragments (R), which, although they give the impression that pressure dissolution has occurred, are almost certainly ‘compromise boundaries’ between different sets of actively growing quartz crystals.

Figure 12

Analysis of the growth pattern shown by cockade-textured quartz veins mantling rock fragments (R), with prismatic quartz growing into vugs (V) in a quartz-breccia vein. The growth directions of the quartz crystals are shown by red arrows and a compromise boundary in the growth pattern is marked by a red line.

The effects of simple shear on such textures is illustrated by Genna et al. (Reference Genna, Jébrak, Marcoux and Milési1996) and involves the deformation and rotation of individual cockades and the development of duplex-like sets of dislocations, none of which are seen in the vein complexes studied here.

5. Geometry of the fault–vein array

The majority of faults in the Tyndrum–Dalmally area trend between NE–SW and NW–SE (Figs 4 and 6), and it is significant that four NE-trending faults, namely, the Tyndrum, Cononish, Beinn Udlaidh and Arichastlich faults, are noticeably discordant to the general conjugate pattern. It is also evident from Figure 6 that, in many cases, individual NNE- and NNW-trending faults change direction and become N-trending structures, suggesting that these particular faults were contemporaneous and formed a linked system. Most of the faults in the Tyndrum–Dalmally area dip at a high angle, with 80% having a dip of >65°, which is atypical for normal, dip-slip faults.

In order to determine the sequence of events, it is necessary first to establish the timing of the fault movements, and their relationship to the Orchy Dome, which has an axial trace trending at 087°, and limbs dipping to north and south at <25°. Examining first the mineralised veins, the Beinn Udlaidh Vein is near vertical, and cuts the Dome close to its highest point, and the Cononish Vein has a dip that, though ranging from 75°SE to 75°NW, is also essentially vertical. These simple relationships suggest strongly that the Orchy Dome formed before the faulting took place. If the converse were true, and both veins were sub-vertical prior to doming, the Beinn Udlaidh Vein, on the crest of the structure would have retained its original orientation, but the Cononish Vein, which cuts the southerly dipping limb of the Dome, would have been rotated to dip NW, instead of steeply SE, as at present. Had it been rotated to its present orientation during the development of the Dome, the Cononish Vein would have had an initial dip of ∼50°SE and been markedly oblique to the Beinn Udlaidh Vein; an unlikely configuration. Similar reasoning applies to the other near-vertical veins and faults that cut the Orchy Dome. It seems likely that they also formed in the late stages of, or following, the formation of the Dome.

In the absence of clear cross-cutting relationships between the Cononish Vein Set and the other major vein sets, it is difficult to be certain of the order in which they formed. However, individual veins belonging to the Cononish set all appear to terminate just before, or against, through-going veins/faults belonging to other sets and it seems most likely that they pre-dated the other vein sets but had some influence on their spatial development.

5.1. Stereographic analysis

In the Tyndrum–Dalmally area, the orientations of the faults unaccompanied by quartz breccia, and traceable for >10 m, are shown on Figure 13A. There are clear similarities between this scatter plot and that for the faults accompanied by quartz breccia veins (Fig. 13B), and especially for minor quartz veins from the quartz breccia/country rock transition zone (Fig. 13C). Although these comparisons are not statistically valid, because the sampling was not random, and some quartz breccia veins are over-represented, the plots suggest that it is valid to sub-divide the data into sets represented by a mean value (Table 1). The rare steeply dipping set of E–W faults, which are associated with minor quartz veins, but not quartz breccia or massive quartz bodies, are excluded, as they are probably late fractures associated with the emplacement of the quartz-dolerite dykes (Fig. 4).

Figure 13

Equal-area stereographic projections showing the relationships between the major quartz-breccia veins (either individually or in sets), faults, and minor quartz veins from the Tyndrum–Dalmally area: (A) scatter plot showing poles to fault planes (see text for further explanation); (B) scatter plot showing poles to quartz-breccia dykes, divided into sets by inspection according to their orientation; (C) poles to minor quartz veins (see text); (D) great circle plot showing the calculated measured mean orientations of veins making up the conjugate Mother Vein and Barren Vein sets; the plane that bisects the acute angle between them; the derived σ₁ direction; and the mean orientation of the Creagan Dubha set; (E) great circle plot showing the orientations of the individual Mother and Barren veins; the plane that bisects the acute angle between them; and the derived σ₁ direction.

Table 1

Geometry of the quartz-breccia vein sets. All values are in degrees

¹ Includes proto-Mother Vein.

² Calculated from equal-area stereographic projection.

³ Strike/dip/dip direction.

Inspection of the scatter plot for the quartz breccia veins (Fig. 13B), and of Figure 6, shows that the NNE- and NNW-trending quartz breccia veins form a conjugate pair bisected by the N–S set, with the NE-trending faults forming a separate group.

In order to overcome the sampling bias noted above, the azimuths of the total population of faults, excluding the E–W set mentioned above, were plotted on a rose diagram, against the corresponding length of the fault trace on the map (total length measured=34·8 km at 1:3700 ) (Fig. 14). For this purpose, each fault trace was divided into consecutive segments in which the azimuth was constant to within a couple of degrees: the more curved the trace, the greater the number of segments. This technique is defensible, as the faults are nearly all steeply dipping to vertical, and the fault trace is very little affected by topography. Structure contours were used where this is not the case.

Figure 14

Rose diagram showing the calculated mean azimuths of faults and of the associated quartz-breccia bodies versus their length (total length=34·8 km). The sets are delimited by natural breaks in distribution, as highlighted by pecked lines; mean values are marked by an arrow, with strike direction in degrees.

The rose diagram (Fig. 14) shows that the readings fall naturally into five clearly-defined sectors that correspond to the four main vein sets identified above, together with a sector representing the River Vein set. There is a very close agreement between the means calculated from the stereographic analysis, and from the rose diagram (compare Fig. 14 with Table 1).

As a check on the precision of using the mean values for the orientation of each set, the measured orientations of the best-exposed individual vein in each set, i.e. the Mother Vein and the Barren Vein, are plotted on Figure 13E; they give σ₁=25° to 358°, that is almost identical to σ₁ obtained from the mean values (26° to 360°).

5.2. Application of the modified Coulomb model

This model is based on the Coulomb criterion for brittle failure in rocks and describes the orientation of conjugate shear fractures with respect to σ₁ (the maximum principal stress) in brittle and brittle-ductile shear zones. It has been tested extensively by analogue modelling, notably by Riedel (Reference Riedel1929), Cloos (Reference Cloos1955) and Wilcox et al. (Reference Wilcox, Harding and Seely1973), and by numerous field studies (Sylvester Reference Sylvester1988), and is recognised as one of the tenets of structural geology.

The theoretical angle between the synthetic (R) and antithetic (R′) conjugate Riedel fractures and the direction of σ₁, is given by the equation 45°±φ/2, where φ is the internal angle of friction of the rock, and is typically close to 30° for the average dry, intact, crustal rock. P shears form parallel to the margin of the shear zone, and pure tensional (T) fractures develop at 45° to it. However, when considering these angular relationships, other factors have to be taken into account such as Griffith's cracks, which reduce the strength of the rock, and pore fluid pressure, which reduces the effective stress across the fracture plane and hence the frictional resistance to movement (Hubbert & Rubey Reference Hubbert and Rubey1959). These factors were quantified by Sibson (Reference Sibson2000) who, by combining the Coulomb and Griffith criteria, derived an expression for brittle failure of intact rock in the presence of pore fluid. He obtained an optimal value of 54° for the acute angle subtended by conjugate shear planes under these conditions.

The implications of this model for the geometry of fractures in the Tyndrum–Dalmally area are now examined. The first problem is that the age of the Orchy Dome is not known and it could have formed either in response to a far-field regional stress (pure shear) or to simple shear arising from sinistral movement on the Tyndrum and Etive–Laidon fault pair. In the first case σ₁ would trend at 042°–30°=012° and in the second, the shear couple would result in a secondary maximum principal stress (σ_1′) trending 357°, generating a fold trending 087°, cut symmetrically by conjugate faults (Fig. 15A). Doming may have been caused by a buried igneous intrusion, but this seems unlikely, as the nearest negative gravity anomaly lies well to the north of the Dome (Scotgold 2012).

Figure 15

Comparison of the angular relationships between the sets of quartz-breccia veins and faults a predicted by the Coulomb–Sibson model, with the observed values: (A) geometry of the Mother Vein (R) and Barren Vein (R′) sets as predicted by the Coulomb model, modified after Sibson (Reference Sibson2000), and based on the orientation of the bounding faults. The solid angle between the fractures is 54°; (B) plan view of the present angular relationships between the major vein sets, the Orchy Dome, and the bounding transcurrent faults (for key, see A above); (C) plan view of the geometrical relationships between the proto-Mother Vein (R_A) and the River Vein (R′_A) sets, the Orchy Dome, and the bounding transcurrent faults; (D) plan view of the geometrical relationships between the Barren Vein (R′_B) and Mother Vein (R_B) sets, the Orchy Dome, and the bounding transcurrent faults.

The angular relationships between all of the sets of major fractures, and the Orchy Dome, are shown in Figure 15B. The sets lie almost symmetrically about a N–S line, with the trends of the Mother Vein–Barren Vein pair being in close agreement with those predicted for Riedel shears formed in the same stress field as the Orchy Dome (their bisector is within 6° of being orthogonal to the trace of the Dome). The solid angle between the Mother and Barren vein sets is 50°, compared with the theoretical value of 54–60°. The Creagan Dubha set equates with the extensional T fractures and, although it divides the angle between the other vein sets disproportionately, being at 12° clockwise to the modelled orientation of the T direction (a discrepancy probably due to the small sample size), all three planes intersect in a single point and have a combined σ₁=27° to 002°.

The above relationships clearly favour the simple shear model, and a temporal link between the fractures and the Dome is further indicated by the observation that many of the faults and quartz-breccia bodies occur in the central part of the Dome.

The remaining River Vein set, although of minor importance and under-represented, poses a problem, as it has no obvious partner (the Cononish Vein being parallel to the Tyndrum Fault, the compartment boundary). A possible explanation is that the Mother Vein Fault was active more than once; firstly, as an early mineralised fracture (the proto-Mother Vein, R_A on Fig. 15C) that formed a conjugate pair with the River Vein (R′_A) and, secondly, as the main, non-mineralised component (Mother Vein, R_B) that formed a conjugate pair with the Barren Vein (R^′_B). The River Vein set makes an angle of 75° with the Tyndrum Fault, and 62° with the Mother Vein, the latter being close to the angle predicted by the Coulomb criterion for the formation of R shears in dry rock. The calculated σ₁=17° to 174°, is at 45° to that for the Barren Vein–Mother Vein pair.

6. Age of the quartz-breccia veins

There are probably two distinct populations of quartz-breccia veins in the Tyndrum–Dalmally area, the River Vein–proto-Mother Vein conjugate pair and the Barren Vein–Mother Vein conjugate pair, which may be significantly different in age. Field relationships described in the previous section suggest that the Cononish Vein set is possibly older than both of them, but there are no radiometric age data with which to quantify any age differences. The younger Pb–Zn-bearing veins associated with the mineralisation are not considered here, as they are poorly exposed at the surface, and have been little studied. In the absence of minerals suitable for radiometric dating, the only age determination directly linked to the formation of the quartz-breccia veins, is a K–Ar age of 410±14 Ma on K-feldspar from the metasomatised wallrock of the Cononish Vein (Treagus et al. Reference Treagus, Pattrick and Curtis1999).

The age of the lamprophyre suite in the Tyndrum–Dalmally area is of critical importance, as it is of approximately the same age as the major veins and faults. It has not been dated radiometrically but belongs to the Scottish Highlands Silurian Suite (also termed the ‘Newer Granites’), members of which crop out between the Garabal Hill–Glen Fyne Centre, south of Tyndrum, and Rubha Mhor, near Ballachulish (Fig. 16). Previous U–Pb ages on these two intrusions (Rogers & Dunning Reference Rogers and Dunning1991) were revised by Neilson et al. (Reference Neilson, Kokelaar and Crowley2009), to 428±9·8 and 426·5±4·2 Ma, respectively. The latter concluded that the entire suite was emplaced 430–415 Ma ago. In general the lamprophyres were associated with the emplacement of the ‘Newer Granites’, but locally pre-dated the intrusion of individual plutons. A maximum age of 430 Ma may therefore be inferred for a putative igneous intrusion present beneath the Tyndrum–Dalmally area, which could have provided Si-rich fluids to form the major quartz veins (see further discussion below) (Table 2).

Figure 16

Regional setting of the Tyndrum–Dalmally area (boxed) in relation to major transcurrent faults and to the Lochaber Batholith (modified from Neilson et al. Reference Neilson, Kokelaar and Crowley2009). For A and B, see text.

Table 2

Major events in the geological history of the Tyndrum–Dalmally area

New high-precision U–Pb (zircon) and Re–Os (molybdenite) radiometric dating of certain plutons and lavas in the region around Tyndrum places limits on the age(s) of movement(s) on the faults that bound the Dalmally compartment, and hence on the age(s) of the major quartz-breccia veins. All of the plutonic rocks NW of the Tyndrum–Dalmally area as far as Loch Linnhe, currently designated the Argyll and Northern Highland Subsuite of the Scottish Highland Silurian Suite (‘Newer Granites’), together with contemporaneous volcanic rocks, were collectively termed the Lochaber Batholith by Neilson et al. (Reference Neilson, Kokelaar and Crowley2009) (Fig. 16). Part of this batholith, the Cruachan Monzodiorite Pluton of the Etive Centre yielded an ID-TIMS U–Pb (zircon) age of 414·9±0·7 Ma (Neilson et al. Reference Neilson, Kokelaar and Crowley2009), an emplacement age recently confirmed by Porter & Selby (Reference Porter and Selby2010) who obtained a Re–Os date of 414·7±2·1 Ma from the skarn to the Cruachan Monzodiorite. As elements of the Etive Centre cut across both limbs of the Orchy Dome, albeit NW of the Ericht–Laidon Fault, their emplacement at 415 Ma provides a minimum age limit for the formation of the veins that transect the Dome. Similarly, if the microdiorite dykes intruded into the Mother Vein set of faults (Fig. 4) are equated with the Etive dykes, they provide a minimum age for the early, River and proto-Mother conjugate vein set of 418–414 Ma (Neilson et al. Reference Neilson, Kokelaar and Crowley2009).

The age and field relationships of the Lorn Plateau Lava Formation provide another indirect line of evidence for the age of the veins. The formation belongs to the Scottish Lower Old Red Sandstone magnafacies, and has given a U–Pb (zircon) age of 425±0·7 Ma from a sample near the top of the lava pile (Neilson et al. Reference Neilson, Kokelaar and Crowley2009). It consists of sub-horizontal, largely basaltic to andesitic, lavas, sills and ignimbrites, with a minimum known thickness of 800 m (Browne et al. Reference Browne, Smith and Aitken2002) and lies above, and interfingers with, the Kerrera Sandstone Formation.

South-west of the Tyndrum–Dalmally area (Fig. 16) the Lorn Lavas onlap unconformably on to the Dalradian rocks, and Treagus et al. (Reference Treagus, Pattrick and Curtis1999) concluded that the base of the lava pile W of Loch Awe (Fig. 16, location A) had been little, if at all, displaced by the Ericht–Laidon Fault. Examination of Sheet 37W (Furnace) shows that a line drawn SE across Loch Avich from the base of the lava succession between [NM 935 155] and [NM 950 139], E of the Ericht–Laidon Fault, is within 1° plunge of its equivalent position on the opposing hillside, W of the fault (i.e. the lavas are not displaced, unless the movement vector on the fault plane is precisely horizontal).

North of Tyndrum and NE of location B (Fig. 16), Hinxman et al. (Reference Hinxman, Carruthers and MacGregor1923) and Jacques & Reavy (Reference Jacques and Reavy1994) inferred that the Rannoch Monzodiorite–granite Pluton (formerly Rannoch Moor Pluton) had been affected by a sinistral strike-slip displacement of up to 7 km on the Ericht–Laidon Fault. However, the offset is difficult to confirm in poorly exposed ground where the upper surface of the granite may be undulating and gently dipping. In order to resolve this problem, Treagus (Reference Treagus1991) suggested that the major sinistral displacement on the Ericht–Laidon Fault occurred before the ∼425 Ma Lorn Lavas were laid down, and that a later transcurrent movement on this fault N of the northern tip of Loch Awe (Fig. 16) was taken up by the Pass of Brander Fault, which acted as a ring fault to the Etive Centre. The 422·5±0·5 Ma age of the Rannoch Pluton, and the 425 Ma age of the Lorn Lavas (Neilson et al. Reference Neilson, Kokelaar and Crowley2009) provide a further upper age limit for movement on the Ericht–Laidon Fault and, by association, for the age of the quartz-breccia veins.

Taking into account the inferred upthrow of ∼2 km W of the Ericht–Laidon Fault (Treagus Reference Treagus1991), the base of the Lorn Lavas in Lower ORS times was probably not more than a few kilometres above the present-day hill summits in the Tyndrum–Dalmally area. The intrusions of the Kilmelford Diorite–granodiorite complex SW of locality A (Fig. 16) probably acted as a feeder for the Lorn Lavas: they abut against, but are not cut by, the Ericht–Laidon Fault. This interpretation is supported by the Re–Os (molybdenite) age on the Kilmelford intrusions of 425·8±1·7 Ma (Conliffe et al. Reference Conliffe, Selby, Porter and Feely2010).

The constraints provided by these recent age data, indicate that the Ericht–Laidon–Tyndrum shear couple was operative for a short period between 428 Ma and 425 Ma. The age bracket is coincident with the inferred age of 428±1·9 Ma for the commencement of left-lateral movement on the related Great Glen Fault (Stewart et al. Reference Stewart, Strachan, Martin and Holdsworth2001), and is 2 Ma from being within error of the 410±14 Ma age of the Cononish Vein (Treagus et al. Reference Treagus, Pattrick and Curtis1999).

7. Source of the Si-rich fluids

The source(s) of the fluids that gave rise to most lode-gold deposits is not known for certain, and it is difficult to reconcile some of the competing hypotheses. Possible sources that have been proposed, include:

1. 1. Regional-scale circulation of fluids originating from metamorphic dehydration reactions;

2. 2. Fluids resulting from the crystallisation and cooling of igneous rocks, in particular those with a granitoid composition;

3. 3. Meteoric and basinal fluids.

Craw (Reference Craw1990) and Craw & Chamberlain (Reference Craw and Chamberlain1996) proposed that metamorphic fluids were responsible for the formation of the major veins in the Tyndrum–Dalmally area, whereas Pattrick et al. (Reference Pattrick, Boyce and MacIntyre1988), Curtis et al. (Reference Curtis, Pattrick, Jenkin, Fallick, Boyce and Treagus1993) and Treagus et al. (Reference Treagus, Pattrick and Curtis1999) inferred that the fluids were, in part at least, of igneous origin. Thus, gold present in some of the quartz veins could have originated either from fluids released during crystallisation of igneous rocks, especially those linked to granite and lamprophyre intrusions, or have been scavenged from the host rocks by circulating meteoric or basinal fluids. Quantitative assessment of the source of the fluids responsible for forming the quartz veins relies upon the determination of H, O and S isotopic ratios, backed up by fluid inclusion studies, and is outwith the scope of this paper. A brief synopsis of available data and hypotheses is presented below.

7.3. Depth of emplacement

Fluid inclusion studies on quartz from Au-bearing veins at Tyndrum (Halliday's Vein; Pattrick et al. Reference Pattrick, Boyce and MacIntyre1988) and at Cononish (Cononish Vein; Curtis et al. Reference Curtis, Pattrick, Jenkin, Fallick, Boyce and Treagus1993) showed that the veins were emplaced at a shallow depth in the Earth's crust of around 3–4 km. This agrees well with the conclusions of Hamidullah (Reference Hamidullah2007) who calculated first, that lamprophyre-appinite members of the Argyll–Northern Highlands Subsuite correlated with those in the Tyndrum–Dalmally area, crystallised under ∼1 kb fluid pressure and, secondly, that the associated explosion breccia pipes probably reached the (then) surface (as shown in cartoon form on Fig. 17).

Figure 17

A two-part model (A and B) for the origin and development of quartz-breccia veins of Lower Devonian age in the Tyndrum–Dalmally area. The Si-rich fluid that gave rise to these bodies is inferred to be of igneous origin, but the possibility of a meteoric or ‘mixed’ origin is not totally discounted. A–D and box marked X are referred to in the text.

A line drawn from the base of the lava pile east of Loch Avich to the summit of Ben Lui, (the highest peak in the area (1130 m), 4 km SW of Cononish), along which the Dalradian rocks are not affected by any major faults, plunges at 1·6°SW. Thus, the base of the lava pile when it was laid down could have been almost horizontal across the whole of the Tyndrum–Dalmally area, and was probably not far above the present-day hill summits in that area. However, fluid inclusion studies (e.g. Curtis et al. Reference Curtis, Pattrick, Jenkin, Fallick, Boyce and Treagus1993) show that the cover above Cononish was ∼4 km thick. When this is taken into account, the required apparent dip along the above line increases to 7°, as shown in Figure 17.