Basement Structural Controls on Carboniferous-Hosted Base Metal Mineral Deposits in Ireland

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Basement structural controls on Carboniferous-hosted base metal mineral deposits in Ireland

J. D. JOHNSTON l*, D. COLLER 2, G. MILLAR 2 & M. F. CRITCHLEY 2

l Geology Department, Trinity College, Dublin 2, Ireland 2 ERA-Maptec, 5 South Leinster Street, Dublin 2, Ireland

Abstract: Explorationists have long recognized the importance of structural control of Irish base metal deposits, but the kinematics and timing of the structures relative to the mineralization are subject to wildly differing interpretations. This paper presents a combination of structural, magnetic and gravity data that show that the fundamental structural controls are easterly to northeasterly-trending basement (Caledonian) structures which have been reactivated in dextral transtension by northeasterly to north-northeasterly extension during the Dinantian. In general, in the north of the country the ore-controlling faults dip south, while in the south of the country they dip north. The structures have evolved through a temporal kinematic history of early normal faulting, later oblique slip, and some show evidence of later reverse movements. Part of this evolution may reflect burial history, although it also reflects the transition from Dinantian transtension to Variscan (Hercynian) compression. The bulk of the mineralization appears to post-date the normal faulting, and pre-date Variscan compression. Mineralization is thought to be post- compactional and could have occurred during the dextral transtension, although some of the sulphides could post-date most of the transtensional movement.

The Lower Carboniferous of Ireland is host to controlling basement structures. The local a large number of base metal deposits (Fig. 1). faults, although generally clockwise of the These range in size from small occurrences up trend of the zones, show a larger variation in to the Navan deposit of more than 70 million orientation than the zones themselves. tonnes (Mt) of 12% combined Zn-Pb (Table Although the empirical relationship is well 1). Vein-hosted copper ores in red beds are known, only a few studies of the structural abundant in the south of the country, while settings of the deposits have been published further north Zn predominates in carbonate- (Moore 1975; Coller 1984; Reilly 1986; Shearley hosted deposits. Metal ratios vary regionally, et al. 1992, this volume). with Cu and Pb decreasing and Zn increasing Russell (1968, 1972) and Russell & Haszel- northwards (Johnston 1996). This suggests that dine (1992) invoke N-S geofractures to explain all the mineralization may be generated by the the mineralization in the Irish Midlands. same regional phenomenon. All of the major However, as pointed out by Leeder (1988), deposits occur in the hanging walls of normal there is almost no field or geophysical evidence faults. Copper deposits at Aherlow, Mallow for the existence of such structures. Rather, and Ballinalack are in compressional structures, both field and geophysical evidence suggest that and no faulting has been documented at reactivated Caledonian (E to NE-trending) Courtbrown or Carrickittle. These latter cases basement structures were the key in focusing may reflect limited exploration, rather than mineralizing fluids. There is extensive hydro- absence of faults. Generally, individual faults thermal alteration around these faults where trend between ESE (Silvermines) and NE they cut Devonian and Carboniferous rocks. (Keel). Local faults with lengths of hundreds Alteration is less extensive in the basement, but of metres, controlling ore lenses (ESE at may be observed at Navan. In this study, both Silvermines and E at Lisheen), are made up structural and petrographical observations and of en echelon segments defining ENE to NE- magnetic and gravity data are used to locate trending fault zones many kilometres long, basement structures. On the basis of fault and which are inferred to be expressions of mineralized vein geometries, most of the deposits are seen to lie in dextral transtensional settings (oblique extension with a *Dave Johnston lost his life whilst on fieldwork in Mayo during October 1995. His co-authors would like component of clockwise rotation), which are to pay tribute to Dave's great contribution to struct- the result of northeasterly extension of east to ural geology around the world over the last two northeast-trending basement structures. Each decades. mineral deposit has been further localized by

From STROGEN, P., SOMERVILLE, I. D. & JONES, G. LL. (eds), 1996, Recent Advances in Lower Carboniferous Geology, Geological Society Special Publication No. 107, pp. 1-21. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

2 J. D. JOHNSTON ET AL.

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~ lJ ~ Granard ~Oldcasde l J ~..," i~I - "ll. , t /- .+ flY., ,'~ Oldcasfle @ l~.eel . ,• Newt~w~ .~ll IABalhnalack Skerr/es C.ashel • Iklullawornia Sion Hill --~.l • / ~1 Loughshinny ' Aghav&~'den M°Y~A°re ,... • • • I • I • I • ~ll~O~/~ou]hly • Killam .[VIII ! Province ii ip irdl~i i m ii i i iiI i IW I Glentown • LHa~on/Allenwood r.~ rn _ i Boston Hill '~-~ •

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Cmokl~ve~ ~ PRE LOWER CARBONIFEROUS MizenHead • COPPER DEPOSIT / South Munster Basin • PRE-WAULSORTIANHOSTED Zn-Pb • SUB- AND WA~RTIAN HOSTED Zn-Pb-Ba ["1 DISCORDANT Zn - Pb AND Ba DEPOSITS EXTENDING ABOVE WAULSORTIAN Fig. 1. Location map showing main mineral deposits and stratigraphical provinces. National Grid is in kilometres.

specific secondary structures: termination while those in the southern half lie in the hanging zones, bends in faults, or intersections of walls of north-dipping faults. Generally, the second or third order shears with the primary mineralized structures are en echelon and left- basement structures. stepping, trending clockwise of the underlying There is abundant evidence (feeder veins, basement structure. The displacement prior to alteration zones, changing metal ratios, fluid the mineralization is predominantly dip-slip, inclusions) that hot brines came up these fault indicating that the ore-controlling structures zones and deposited metals in the hanging-wall are extensional, and it is postulated here that carbonates. Most of the deposits in the northern these are rooted in dextral transtensional shears half of the Midlands lie in south-dipping faults, in the basement. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND

Table 1. Tonnages and grades of the main Irish sediment-hosted base metal deposits

Deposit Host Tonnage Zn+Pb Zn Pb Zn/Pb Ag Cu BaSO4 Discovery (Mt) (Wt%) (Wt%) (Wt%) (gm/t) (Wt%) (Wt%)

Navan N >70 12.7 10.1 2.6 3.9 3.5 -- -- 1970 Aherlow N 6.0 .... 33.8 0.9 -- 1965 Mallow N 4.2 8.7 7.7 1.0 7.7 27.5 0.7 -- 1973 Gortdrum N 3.8 ------3.5 23.0 1.2 -- 1964 Tatestown N 3.6 6.9 5.3 1.5 3.5 37.0 -- -- 1975 Oldcastle N 3.0 4.9 4.3 0.6 7.2 ------1976 Keel N 1.9 8.7 7.7 1.1 7.0 39.6 -- -- 1964? Abbeytown N 1.1 5.3 3.8 1.5 2.5 40.0 -- -- 1785 Clougherboy N 0.34 7.0 5.8 1.2 5.0 ------1977 Ballyvergin N 0.15 .... 17.1 1.2 -- 1960 Moyvoughly N 0.13 8.0 6.5 1.0 6.5 ------1968 N'town-Cashel N ? 6.3 3.1 3.1 1.0 ------1972 Lisheen W >20.0 13.5 12.0 1.5 8.0 32.0 -- -- 1990 Silvermines W 17.7 8.9 6.4 2.5 2.6 23.0 -- -- 1963 Tynagh W 9.9 8.1 5.8 0.4 0.8* 78.0 -- 8.3 1961 Galmoy W 6.7 11.9 10.9 1.0 10.9 ------1986 Magcobar W 5.0 ...... 85.0 1959 Ballinalack W 3.5 7.0 5.9 1.1 5.4 ------1969 Rickardstown W 3.5 3.3 2.2 1.1 2.0 ------? Courtbrown W 1.0 5.5 3.5 2.0 1.8 14.0 -- -- 1980 Garrycam W 1.4 2.9 2.7 0.2 13.5 -- -- 36.1 1975 Carrickittle W <0.1 7.5 6.0 1.5 4.0 ------1965 Harberton S 3.9 9.3 8.1 1.2 6.7 ------1975 Boston Hill S 0.8 3.8 2.7 1.1 2.6 ------1975 Allenwood S 9 2.0 1.6 0.4 4.0 ------?

Based on data from Andrew et al. (1986), Bowden et al. (1992). W, Waulsortian; N, Navan Group & equivalents; S, late Chadian or younger; Mt, millions of tonnes. * Primary ore.

It is possible that Variscan inversion of south- The South Munster Basin (Naylor et al. 1989) is dipping normal faults has resulted in uplift and filled with a carbonate-poor, clastic-dominated subsequent erosion of other mineral deposits. succession. Fluviatile red beds are overlain by marine sandstones and shales. Clastic (shale) deposition continued throughout the Courceyan, Regional stratigraphical setting with starvation of the basin later in the Early Carboniferous. Irish Zn-Pb deposits are hosted primarily in Lithological units are widespread and laterally Lower Carboniferous rocks; most are in rocks of persistent in the carbonate-dominated Limerick Courceyan to Chadian age, but in several Province (Philcox 1984; Somerville et al. 1992; see deposits hydrothermal alteration and sulphides also Fig. 1). Transgression occurred during the extend up into Arundian rocks. Some of the early Courceyan. The base of the sequence copper deposits in the south of the country are comprises non-marine red beds and marginal- hosted in Devonian elastic sediments. Strati- marine sandstones and shales. Overlying these are graphic and palaeogeographical settings of the calcareous shales (Ringmoylan Shale Forma- Lower Carboniferous sequences are described in tion), the principal hosts to small copper- detail by Phillips & Sevastopulo (1986) and dominated deposits. The shales pass up into Philcox (1984). The Irish Courceyan has been muddy bioclastic limestones (Ballymartin and subdivided into four provinces based on differ- Ballysteen Limestone Formations), which are in ences in the local succession: (1) the South turn overlain by Waulsortian carbonate mud Munster Basin; (2) the Limerick Province; (3) banks (Lees 1964). These units host the Silver- the Midlands Province; and (4) the mainly post- mines, Lisheen, Galmoy, Courtbrown and Courceyan North West Province (Fig. 1; Phillips Tynagh deposits. Overlying the Waulsortian are & Sevastopulo 1986). either shelf, or in some areas basinal, carbonates. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

4 J.D. JOHNSTON ET AL.

In the Midlands Province, transgression took developed, with north-dipping faults dominant place later, during the mid-Courceyan (Sevas- in the south and south-dipping faults in the topulo 1981; Phillips & Sevastopulo 1986). A north. Typically, sediments adjacent to these formal stratigraphy for the region was erected faults display rapid facies and thickness changes by Strogen et al. (1990). The region is from the late Chadian onwards (e.g. the Tynagh characterized by lateral facies variation, and Fault; Philcox 1984). These faulting events were has been divided into sub-provinces by Philcox sometimes accompanied by chaotic breccias in (1984) and Strogen & Somerville (1984). A the Midlands Province (Nolan 1986, 1989; widespread basal red bed lithology is overlain Crowe 1986; Philcox 1989; Pickard et al. 1992), by a series of shallow-water micritic and oolitic and in the NW Province (Philcox et al. 1989). limestones, collectively referred to as the Navan During the Variscan Orogeny these basins Group (temporally equivalent to the Ballymar- were inverted. Details of the structure of the tin Limestone of the Limerick Province). At Variscan deformation are given by Dolan Navan Mine these are informally called the Pale (1983), Sanderson (1984), Coller (1984), Beds (host to the Navan and Tatestown Cooper et al. (1984, 1986), Ford (1987), deposits). The Pale Beds in turn are overlain Rothery (1988a, b) and deBrit (1989), and only by the argillaceous bioclastic limestone of the the salient points are summarized here. In the Moathill and Slane Castle Formations (litholo- south of the country the rocks have undergone gically equivalent to the Ballysteen Limestone). extensive shortening (50-60%) with upright The majority of sulphides at Ballinalack and folding. The deform-ation has been interpreted Garrycam are hosted by the Waulsortian as being either thin-skinned and thrust-related micrites of the Feltrim Formation, which over- (Cooper et al. 1984, 1986), or thick-skinned and lies the Slane Castle Formation. Overlying the related to dextral transpression (Sanderson Waulsortian is a sequence of basinal graded 1984). While thrusting definitely occurs (e.g. calcarenites and calcisiltites of the Lucan Ford 1987), there is abundant field evidence for Formation, widely referred to as the Upper dextral transpression in the western part of the Dark Limestones. The late Chadian to Arundian South Munster Basin. Folds are transected was a period of tectonic activity, with syn- anticlockwise (i.e. the cleavage related to the depositional tilting apparent at some places (e.g. development of the folds appears axial planar in Navan; Ashton et al. 1986, 1992) and exten- cross-section, but in plan trends anticlockwise of sional deformation. The youngest host rocks for the fold hinge). En echelon fold trains (arrays of mineral deposits in the Midlands Province are right-stepping folds) occur, and pressure fringes Chadian and Arundian platform carbonates, (fibrous mineral fibres, interpreted to indicate which contain the Harberton Bridge and Allen- the stretching direction) growing on pyrite wood deposits. flattened in the cleavage indicates sub-horizon- In the North West Province, much of the tal extension. There is an abrupt drop in strain lower part of the section is made up of very north of a line from Dingle to Dungarvan (Gill shallow water sediments, lithologically equiva- 1962; Cooper et al. 1984). Rocks in this region lent to the Pale Beds, with a slightly later have undergone gentle folding with shortening Courceyan to Chadian transgression. of less than 20%. However, evidence for dextral transpression continues northwards (Johnston 1993; Fitzgerald et al. 1994). Although there are Regional structural setting stable blocks of almost undeformed limestones, approximately 50 kilometres across, these are The structural history of the Carboniferous in bounded by corridors of deformation (Coller Ireland may be divided into two main events: 1984). These are large scale ENE-trending Dinantian extension, and late Carboniferous to dextral transpression zones, up to several kilo- early Permian (Variscan-Hercynian) compres- metres wide, which are characterized by dipping sion. Beginning in the Courceyan and continuing beds, en echelon vein arrays, and weak stylolitic through the Dinantian, 6000 m of shallow-water cleavages that transect gentle folds. Both the sediments accumulated in the South Munster folds and the cleavages generally trend antic- Basin, (Matthews et al. 1983; Price & Todd lockwise of the high strain zones (Coller 1984). 1988; Naylor et al. 1989). South of a line drawn Conjugate NNE-trending sinistral zones are from Dingle to Dungarvan (Fig. 1), major consistent with a NNW-trending compression south-dipping faults controlled sedimentation. direction which is to be expected from regional In the Midlands, a slightly different structural tectonic reconstructions of the Hercynian style existed. A series of interconnected basins (Ziegler 1988). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 5

Geophysical framework geometry of the Carboniferous in the Irish Midlands (Andrew, 1992). However, the Lower The deep structure of the Irish Midlands can be Palaeozoic inliers that expose the basement discerned through use of regional aeromagnetic rocks do not always match the interpretations and gravity data. The Geological Survey of of Williams & Brown (1986) in detail. Never- Ireland holds an aeromagnetic data set which theless, their work identified the existence of may be processed under licence. The magnetic NE-trending, steeply dipping lithological con- map and its interpretation were summarized by tacts in and beneath the basin. The magnetic Max et al. (1983) and Morris & Max (1995). data also show a similar Caledonian grain Murphy (1952, 1960, 1974, 1981) collected and (Murphy 1981; Max et al. 1983). Filtering of compiled gravity data over most of the country the gravity data for the south of the country by with an approximate 1 km spacing. With fairly Ford et al. (1991) has placed some constraints simple processing, these data sets reveal a great on the deep geology of the Munster Basin. deal about the deep (sub-Carboniferous) struc- In this study, the same regional aeromagnetic ture. Using these data, Williams & Brown (1986) data sets and the Bouguer gravity data have and Brown & Williams (1985) identified pro- been studied. The magnetic data are illus- nounced NE-trending gravity anomalies, which trated in Figs 2 and 3. The gravity data are they interpreted in terms of horsts of Ordovician illustrated in Fig. 2 of Readman et al. (1996), volcanic rocks (highs) beneath the Carbonifer- and a summary of the gravity data for the Irish ous, separated by intervening grabens filled with Midlands is shown in Fig. 4. Comparison of the Carboniferous sediments (lows). Exploration magnetic first derivative maps and the gravity drilling has confirmed the ridge and trough map of Readman et al. (1996) and Murphy

100000 150000 200000 250000 300000

100000 150000 200000 250000 300000 Fig. 2. Residual magnetic anomaly map of the Irish Midlands in pseudo-relief, illuminated from NW. The NNW-trending grain is an artefact of the flight lines, and the bright ENE lines (e.g. between Tonduff and Rickardstown) are artefacts of splicing of different data sets. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

6 J.D. JOHNSTON ET AL.

100000 150000 200000 250000 300000 lO0000

!50000

!00000

150000

100~00

100000 150000 200000 250000 300000 Fig. 3. Magnetic first derivative anomaly map of the [fish Midlands. Artefacts as in Fig. 2.

(1974) highlights regions of steep contacts, Gortdrum juxtaposing rocks of differing densities and magnetic susceptibilities. In many instances In the south of the Irish Midlands, a group of such contacts are tectonic. There are many copper deposits occur within the carbonate examples where this has been confirmed in bearing sequence. Gortdrum (Fig. 5) is the boreholes and Lower Palaeozoic inliers such as only one of these deposits to have been mined. in the Loughshinny-Skerries region (Fig. 1). The sulphides occur in Courceyan shales and bioclastic limestones in the hanging wall of the ENE-trending, steeply NNW-dipping Gortdrum Fault (Steed 1986). The footwall is composed of Geology, geophysics and structure Old Red Sandstone. The hanging wall is of the deposits intruded by E-trending, hydrothermally altered mafic dykes and plugs. The deposit post-dates A number of reviews of the deposits have been the host rocks, including the volcanic rocks, but published in recent years (Hitzman & Large pre-dates Variscan deformation. 1986; McArdle 1990; Andrew 1993; Johnston Mineralization is associated with both the 1996) summarizing the main geological features main faults and an en echelon array of more of the deposits. Detailed descriptions of most of E-W striking extensional faults (Fig. 5a). Both the deposits can be found in Andrew et al. (1986) normal and thrust faulting is present. There is a and Bowden et al. (1992). Here, just the compressional overlap of the main faults to the structural settings and those petrographic rela- south of the deposit consistent with WNW tionships relevant to constraining the timing of compression and complementary to the dilation mineralization are described. area (Fig. 5a) of E-trending en echelon normal Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 7

300

290

280

-10

210

200 0 -10

190

180 0 -10

170

181

150

150 170 190 210 230 250 270 290 Fig. 4. A summary residual gravity map for part of the Irish Midlands, based on Brown & Williams (1985). Contours in gravity units. Hachured regions are lows. faults, which define a NW-trending zone with the main, N-dipping fault. First derivative consistent with NNE extension. This pattern of processing of the magnetic data yields an faulting is typical of a dextral shear zone fault anomaly that is slightly oblique to the fault complex. In section (Fig. 5b) it has the geometry trace (Figs 3, 5c). of a negative flower (oblique extensional) Major gravity linear features (enhanced by structure. This geometry is thought to be the first derivatives) are NE-trending and reflect the product of Dinantian dextral transtension. The distribution of lithologies in the folds adjacent fault zone is also characterized by a series of en to Gortdrum. The gravity features are oblique echelon folds that show a strong clockwise to both the magnetic fabric and the main fault rotation in strike into the fault zone at the (Fig. 5c), and they intersect the fault at the deposit, indicating a large dextral shear displace- deposit. This suggests that a sedimentary basin ment in the mineralized area. The folding, which developed oblique to the basement structure. is compressional and deformed consolidated The folds are likely to have formed in the cover rocks, is interpreted to be Variscan. during the Hercynian deformation, along and This rotation into the footwall is also shown oblique to the basement structure. by the magnetic and gravity data. The principal geophysical elements are shown in Fig. 5c. The deposit lies on the southern margin of a NE- Navan trending magnetic high in a region where the The geology of this deposit (over 70Mt) is strike of the magnetic (basement) signature summarized by Ashton et al. (1986, 1992). The changes from NE- to E-trending (Figs 2, 3). A ore body is subdivided into five stratigraphi- steep S-dipping magnetic gradient is coincident cally stacked lenses hosted in the Pale Beds. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

8 J. D. JOHNSTON ET AL.

B, T and A faults, (Fig. 6a). The B and T faults l~----J "----~ I~ PaloLim~ton~ are spoon-shaped, have a normal component of displacement and die out up-section. They [:('.('.:'.:':':1_ f ...... ~ Old-Red Sandst.°ne... N trend oblique to, and occur between, the A and C faults. Minor folding occurs above the T fault in the Upper Dark Limestone. The B and T faults trend on average about ENE and dip ' ' to the SSE. Adjacent to the T fault and beneath the ore lenses there is a concentration of [~./' ~ Lithological contact ~y ~, ""-. ~ormdFau. sulphide veins that trend ENE. Zone 2 lies in the footwall of the T fault, and the mineralization is less well developed in its hanging wall block. Significant rotation of bedding in the [ ~ ~- Ore body Pale Beds occurs in the hanging wall of the B and T faults, suggesting that they are rotational b SW NE extensional normal faults. The Boulder Con- glomerate represents the chaotic deposit pro- ~Pale Limestone ~:~ Ballyvergin Shale duced by synsedimentary rotational slumping. ~Dark limestone ~=.[.~Transition beds Both the T and the B faults are truncated by ~.-I: :]Old Red Sandstone the A-C fault complex (Fig. 6a). Folding in the ~ ['d'ff~Basalt [ ~[a..2.2]Breccia Upper Dark Limestone that trends anticlock- ~'"-J 0.5% Cu cutoff wise of the A and C faults intensifies in the 100m ~ Fault vicinity of the A fault, which has a large I 7-'- Lithological contact consistent reverse component of displacement. As all of the faults offset the stratigraphic .•Ho!lyford markers that define the ore lenses, they appear to o ,O~m/ II, :" offset the lenses. However, a case can be made for the ore lenses being selectively replacive, and post-dating movement on the B and T faults. While some mineralization terminates at the faults, at least in some cases, within the footwall e ° f"~rortdrum Pi~ block, grade increases towards the faults. Close .,2y~ .,....~ _4__ Antic,n0 examination of mineralization in the footwall of ~ .. ~_ Syncline the T fault reveals that, at several levels, ~,,~,~. r f ""='~V" Reverse Fault o...... '~ Axis of magnetic high mineralization terminates several tens of centi- .." .....'~'"" Magnetic gradient linear metres short of the fault, and is not strictly •" ...... Gravity linear • • Fault with downthmwside truncated by it. The thickest ore in Zone 2 is in the hanging wall block of the B fault. Although Fig. 5. (a) Structural map of the Gortdrum region. displacement on the B fault is well constrained (b) Cross-section of Gortdrum, after Steed (1986). by stratigraphic displacements, ore lenses cannot (e) Interpretation of main geophysical features be identified in the footwall, suggesting that the of Gortdrum. distribution of mineralization is influenced by the B fault. Where sulphides abut the B fault, Structural control on the mineralization is the ore terminates against stylolitic surfaces apparent as the ore lenses overlie steeply defining the fault zone margin. In general dipping and ENE-trending veins of sulphides, terms, where an argillaceous unit lies in the inferred to be feeder veins. The ore lenses are hanging wall of a fault, sulphides terminate at cut by the so-called 'Navan unconformity' (a the fault. Where the succession is shale poor, the major listric slide surface of Chadian age), ore extends across the faults. It is therefore which is overlain by the Boulder Conglomerate possible that the mineralization occurred after and Upper Dark Limestone. The ore body is much of the displacement had occurred on the B also subdivided laterally into three zones by the and T faults.

Fig. 6. (a) Geological map of Navan, modified after Ashton et al. (1986,1992). (b) Cross-section of Navan redrawn from Ashton et al. (1986). (e) Modified cross-section of Navan in which ore lenses are correlated by absolute position, rather than stratigraphical position (redrawn from Johnston (1996)). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 9

TATESTOWN N t

D Mineralized zone " - " Dark Limestone ulsortian and ABL Red, Pale Beds & Shaley Pales " _'~~"~~~.~.~ ~'_ " _" -'NAVAN ~- " " -" -" - 1"7"l Late Caledonian "Syenite" Lower Palaeozoic Lithological contact '- ii! Fault River J Line of section

SE NW

..... Tertiary Dolerite Dyke ~ Upper Dark Limestone [~ Ore lens number Boulder Conglomerate ~ Conglomerate Group Ore Navan Group Marker Beds ~ Upper ore sheet 0 200m Red Beds / Lower ore sheet I I Lower Palaeozoic

SE NW

t I Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

10 J. D. JOHNSTON ET AL.

All of the ore lenses terminate beneath the Keel and Newtown Cashel unconformity, and clasts of mineralized Pale Bed lithologies occur within the Boulder Con- The Keel deposit (Slowey 1986) is hosted in an glomerate. Where Zn-Pb ore lenses abut the ENE-trending, S-dipping normal fault zone unconformity, iron-rich Zn mineralization (Fig. 7). Mineralization is discordant and occurs occurs above the unconformity (the Conglom- in fractures related to the faulting. The host rocks erate Group Ore). The timing of the mineraliza- range from Lower Palaeozoic slates to upper tion with respect to the unconformity is not Navan Group rocks. Fault segments extend into clear-cut. Stratiform pyritic, sphalerite ore the Waulsortian, and the wallrocks are exten- occurs in the matrix above the unconformity. sively dolomitized adjacent to the faults. A series Clasts of massive sulphides occur in the Boulder of N-dipping, NE-trending magnetic gradient Conglomerate at the base of the Conglomerate boundaries occur on the northern margin of a Group Ore. Some carbonate clasts have been major magnetic high which lies 10km to the replaced in situ by sulphides (for example, south. At the Keel Fault, the anomalies parallel many clasts show a concentric zonation of the strike of the adjacent Lower Palaeozoic inlier, pyrite, sphalerite, dolomite from margin to trending ENE; away from the main fault line, they centre). The replacement is highly selective: swing NE. Mineralization at Newtown Cashel some bioclasts are preferentially replaced by was controlled by an en echelon array of E-W sphalerite. In others, however, complex para- faults, forming a dilation zone at the termination genetic sequences are preserved and some layers or overlap area of the main ENE-trending Keel of sulphides appear to have been truncated by Fault (Fig. 7b). The faulting and dilation zone is a erosion (Ashton et al. 1986). This suggests that mirror image of the controlling faults at Silver- some of the mineralization occurred both prior mines (see below). to and during the slumping that generated the The geometry of the fault system at Keel is Boulder Conglomerate. consistent with dextral transtension. The Keel The Navan ore body is located near the hinge deposit occurs where the main fault on a region of a SW-plunging anticline, and overlies a regional scale has a minor strike swing; with linear SW-plunging basement horst that pro- dextral strike-slip motion this position is duces a regional magnetic high (Figs 3, 4). In favourable for dilation and enhanced fractur- this region the main magnetic trends range from ing. The adjacent Newtown Cashel deposit NE-SW to almost E-W. Magnetic gradients (Crowe 1986) lies on the same ENE-trending from the first derivative processing, like the main structure as at Keel. This structure was active axes of anomalies, follow the two main fault throughout the Dinantian, as there are lateral trends, NE and ENE, diverging at the deposit. facies variations along and across the fault and a South of the mine, a gravity high trends NNE, Chadian unconformity is associated with it and swings in strike to ENE in the area of the (Crowe 1986). The fault pattern is reflected by deposit (Fig. 2 & Fig. 2 in Readman et al. 1996), a gravity lineament swing to E-W, and step off similar but oblique to the main trend of the en echelon to the south. E-trending faults at magnetic anomalies. To the SE, a large gravity Newtown Cashel deposit are also reflected by low has been interpreted as a buried granite magnetic lineaments. (Murphy 1952). The structural location of Navan and the generation of the ENE-trending feeder veins Silvermines and Magcobar oblique to the main B and T faults is considered, therefore, to be related mainly to dilation The geology of the Silvermines and Magcobar created during regional extension. They may be deposits (Fig. 8) was summarized by Andrew en echelon rotational slump structures devel- (1986). The local geology is dominated by the oped on a precursor of the NE-trending A-C ENE-trending, steeply north-dipping, oblique- fault complex which controlled the localization slip, dextral-normal Silvermines fault (Fig. 8). of the deposit. Reactivation and offset on the Dolomitic breccias that are spatially associated original structure was the result of dextral shear. with the tabular mineralization form the hang- The Tatestown-Scallenstown and the Clogher- ing wall; these have been interpreted as debris boy deposits are satellite deposits to Navan and flows (Andrew et al. 1986) and also as hydro- are associated with similar strike swings in the thermal alteration products (Hitzman & Large main faults ('basement' structures) as Navan. 1986). Observations by one of the authors They have similar lensoid geometries to the (J.D.J.) suggest that elements of both hypo- Navan ore bodies. theses may be correct: some parts of the breccia Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 11

S SHAFT N ...... • !i i!!i~i~ ~ii !ii ill; i ...... •" i .e .s , ,, :+::::::::::::. ::::::::::::::::::::::::::::::::::::::::::: iijiiill iii iiiiiiiiiiiYiiiijiiiiiiiili! iii ii!iii!iiiiiiiiiiiiii " ' .,., .,., .~ ~. :::::::::::::::: :::::::::::::::::::::::::::::::::::: ,;,;,; ~;,;," ~;iiiiiiiiii~~ ' ii;ii!iiii;ii!ii!ii~~

.," s s s ~ s :}i~i~i}~? i!!:!:}:::::::

t l contact

/ Drilihole

Mineralized zone

Lower Palaeozoics

N 5km

Kee!

Termination zone l

/~FaFault with dip

Newtown Cashel

Fig. 7. (a) Keel cross-section, redrawn from SIowey (1986), showing the intimate relationship between faulting and mineralization. (b) Schematic map of the fault system Keel-Newtown Cashel fault system, showing a dextral transtensional setting with a termination zone at Newtown Cashel. units show clast alignment and sorting, parallel metal mineralization occurs both in immediately to bedding. Some of the clast margins are sub-Waulsortian stratiform lenses and in dis- sutured and stylolitized, but others show round- cordant feeder zones parallel to the fault. The ing, and soft-sediment slump folds have been ore lenses thicken towards the Silvermines fault, observed underground in the limestones adja- as do the stratigraphic units and the Zn/Pb cent to these breccias (Andrew et al. 1986). ratios increase. There is an upper, tabular, Superimposed on both the sedimentary breccias predominantly stratiform ore body of barite, and the Waulsortian is a dolomite-matrix siderite and marcasite (Fig. 8). In the deeper breccia. This comprises angular fragments of parts of the ore bodies the mineralization dolomitized wallrock with a filigree matrix of occurred in veins which lie en echelon along extremely fine-grained dolomite. In parts of this the Silvermines Fault. breccia the clasts can be matched and fitted The structural history has been documented by together, while in other parts clasts have clearly Coller (1984). A number of en echelon WNW- moved relative to each other. This breccia trending faults with predominantly normal appears to be hydrothermal in origin. Base displacement localize the mineralization. The Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

12 J. D. JOHNSTON ET AL.

...... X" ...... Anticlinal Trace ~ Discordant ore ~ '" ~ Upper Dark Limestone Li~o,og,ca, co°ta~, ~ C..... d..... -__ /...._):::: i Wa,,l~o,,,anReef ...... /~" " "- " " " " [ - - _r"~ Argillaceous Bioclaslic Limes one • mlerreo rauu, wlm mp /i~...... ~ : : - - . . _]___~___.~

N ~" - ~-/.: : : : : : : : : : _ : _ : : : - 1 ..... I Old Red Sandstone • ~ Fault, with dip //'~ ." .- _- _- - _- - _- _" _" _- .- .-~[

.~ ...... :.- - ...... , . .--..,~.--L~ ...... o ~m ~::::i:::!:~'~:~5!!!!!!iii:iiiiiiiiiiiii:-":ii:::::i

:iii!!!ii

Fig. 8. Silvermines map showing the en echelon fault system. Inset is a schematic model indicating the dextral transtensional control of mineralization, based on mapping by D. Coller and structural and geophysical interpretation.

Silvermines Fault is parallel to, and lies on the Tynagh southern margin of, a regional magnetic high, and north of a smaller magnetic low. Silvermines ore The Tynagh deposit occurs in the hanging wall of bodies lie where the axis of the high swings from the Tynagh fault, with E-trending mineralized ENE to E. West of the deposits, the boundary to sections linked by short barren NE faults the magnetic high is approximately coincident (Clifford et al. 1986). The role of the Tynagh with the Silvermines Fault. However, this Fault in the development of the deposit (Fig. 9) boundary side-steps to the SE at Silvermines, has been discussed by Moore (1975). Early suggesting an en echelon side-step or offset of the faulting with a major normal component main fault (see inset on Fig. 8). Magnetic linears (greater than 600m) is reflected by thickness apparent on the first derivative map (Fig. 3) show changes and slumping in the Courceyan sedi- this same offset. The axis of a NNE-trending mentary record. The throw is variable along the gravity low (Murphy 1974) passes through the fault, with the ore concentrated in the vicinity of deposit. North of the fault this low is coincident the maximum throw. Later reverse movement with a regional syncline in the Carboniferous. was the result of NW horizontal compression. However, in the vicinity of the deposit, this fold This is the same orientation as the regional has been tightened and rotated into the Silver- Variscan shortening direction, producing dextral mines Fault, while the anomaly has not been transpression. Mineralization overgrows stylo- deflected. lites, which lie at a high angle to bedding adjacent The overall geometry suggests an ENE- to the fault, suggesting that significant burial and trending dextral motion, but the component of deformation pre-dated at least some of the motion on individual WNW-trending segments mineralization. Detailed petrography reveals is extensional (i.e. normal). Mineralization was the occurrence of carbonate pressure fringes most intense at points of maximum normal with horizontal and vertical fibres growing on throw. All structures show evidence of post-ore sulphides. This observation suggests that at least dextral reactivation (sub-horizontal slickensides some of the reverse movement, with associated are developed on some ore lenses). Thus, the high pore-fluid pressures, post-dated at least faults appear to have been active during some of the mineralization. Late stage veins and sedimentation and mineralization, and have slickensides indicate that later sinistral strike-slip been subsequently reactivated. motion of unknown magnitude occurred. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 13

,tone

•~..,.~,~i~ ~ Ore t ...... mtones I ..... Barren link fault I 0 150(O Argillaceous Bi~:lastic Limestone ..... Fault l I l Lower Limestone Shales Lithological contact Old Red Sandstone

Upper Dark Limestone """"" " "'.

Supra Reef Breccia "o'#'o'o'o'#" ",o,',o," 0"0" ¢" 0" Watflsortian •0" 0" 0"o" 0" o" "#" o" o',o," 4% Pb + Zn 0 100m Fault

Fig. 9. (a) Map of Tynagh, showing E-trending mineralized faults linked by ENE barren faults in a dextral transtensional zone, redrawn from Clifford et al. (1986). (b) Cross-section of Tynagh redrawn from Clifford et al. (1986), showing fault control of mineralization.

Tynagh is on the northern margin of a ENE parallel to the main mineralized zone, and magnetic high, and the main Tynagh Fault is oblique to the E-W controlling faults. Gravity coincident with a major magnetic boundary that features trend NE, with the deposit located on extends ENE to the Moyvoughly prospect. First the north side of a linear low and to the south of derivative structures diverge from ENE to NE in a large gravity gradient associated with a the vicinity of the deposit. Minor linears trend regional high. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

14 J. D. JOHNSTON ET AL.

The Tynagh deposit, like Silvermines, is a to and just south of the interpreted boundary classic example of mineralization controlled by to the large gravity low associated with the en echelon extensional faults that conform to an Leinster Granite. To the north of the fault at oblique extension with dextral shear on the main the Derrykearn prospect there is a similarly- ENE fault (Fig. 9). Further evidence is provided trending gravity high. The main 'basin margin' by en echelon veins and slickenfibres. The reason linear has minor strike swings at both Lisheen for the dilation at this segment of the fault is and Galmoy and a more significant swing at either related to the main strike change in the the Tonduff-Derrykearn prospects. Lisheen and basement (magnetic) fabric from near E-W to Galmoy are both spatially associated with a NE, or a dilational termination zone of the fault second set of linear gravity features trending strand. 030 °, oblique to the main fault trend. These are part of a regional-scale zone of features arranged in en echelon groups side-stepping Lisheen and Galmoy westwards, that can be traced for 70 km to the southwest. Lisheen (Hitzman et al. 1992; Shearley et al. As in the Tynagh deposit, steep stylolites 1992, this volume) and Galmoy (Doyle et al. adjacent to the fault are mineralized. This 1992 ) are both fault-controlled tabular depos- suggests that much of the normal displacement its at the base of the Waulsortian. Both had occurred across the Killoran Fault prior to deposits lie on the NE-trending Killoran Fault mineralization. Carbonate pressure fringes are Zone (Shearley et al. this volume). Other minor also developed on the sulphides adjacent to the prospects have been discovered further north fault (as at Tynagh), indicating some post-ore along this trend at Derrykearn and Glasha. The fault movement. Both the Galmoy and Lisheen Killoran Fault is a dextral transtensional fault deposits occur in the footwall of a regional with individual E to ENE-trending strands dolomitization front that affects much of the (Shearley et al. this volume). At Lisheen, as at Lower Carboniferous limestones of SE Ireland. Silvermines and Tynagh, sulphides are spatially At Lisheen the sulphides and their associated associated with an ENE-trending feeder fault dolomites crosscut, brecciate and clearly post- zone. Aggregate normal displacement across date this regional dolomite, which affects rocks the Killoran Fault at Lisheen is 210m. Its dip from Courceyan to late Vis6an in age. varies between 40 ° and 70 ° to the north. Three main ore lenses have been identified: the Main, Derryville and North zones. Most of the mineralization occurred within 30m of the Discordant deposits hosted in base of the Waulsortian. Several sulphide the Waulsortian and bodies, containing iron sulphides, sphalerite supra- Waulsortian succession and chalcopyrite, occur within oolitic limestones in the footwall at approximately the A cluster of small pipe-like, breccia-hosted same elevation as the orebodies at the base of deposits have been discovered in County the Waulsortian. Kildare. The largest of these are Harberton A series of parallel magnetic lineaments (060 °- Bridge (Fig. 1; Emo 1986) and Allenwood trending) define changes in gradient in a zone (Andrew 1993). These prospects are hosted in parallel to the main Killoran Fault between pipes that crosscut the Waulsortian and extend Lisheen and Galmoy. A magnetic high to the into the overlying Chadian and Arundian NE of Galmoy cannot be traced in the Lisheen- carbonate sediments. There are no magnetic Galmoy region. All of the gradients dip south signatures associated with the pipe-like deposits and a second derivative linear is coincident with in Kildare, but the gravity trends are oblique to the main mapped fault. In addition, NNW- the main faults. trending cross-lineaments occur at both Lisheen Several of the smaller occurrences are vein- and Galmoy. hosted. At Allenwood (Andrew 1993), E-trend- Two main 060°-trending linear features may ing faults are mineralized, and brecciation be inferred from the Bouguer gravity data. occurs in patches which coincide with bends on They side-step with an en echelon overlap near the faults. Mineralized faults in the prospect Lisheen. Two oblique-striking (050 °) first area step eastwards and northwards in an en derivative linear features may represent link echelon manner, consistent with dextral trans- faults in the overlap zone between the main tension. NW-directed thrusting is assumed to be linear features. The Killoran Fault lies parallel post-mineralization and Hercynian in age. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 15

Discussion of each of the deposits shows that most of the deposits (Keel, Navan) that occur in the north of There is clear structural control of all of the major the basin are located on southerly dipping Irish Carboniferous-hosted sulphide deposits. structures, while those in the south all dip to Mineralization in the Pale Beds sequence either the north (Silvermines, Lisheen, Tynagh; Fig. abuts a fault (Tatestown, Moyvoughly, Keel; 11). In the central part, faults dip in both Fig. 7) or an 'unconformity' (Navan; Fig. 6). All directions. They usually have a normal throw of the Waulsortian-related deposits are in the with a mineralized hanging wall and barren hanging walls of normal faults (Fig. 9). The footwall. All of the faults have some element Harberton Bridge, Allenwood and related depos- of post-mineralization strike-slip reactivation. its are in breccia pipes related to normal faulting. Many faults appear to have had predominantly A fault map of the Midlands of Ireland (Fig. 10) dip-slip motions during an early, synsedimen- reveals that the mineralized faults generally dip tary history. They almost all trend ENE to NE. north in the southeast and to the south in the In every major fault associated with the Irish northwest. A cartoon of the structural geometries deposits where sufficient exposure is available,

100000 200000 300000 300000

,.-\ I A,

HOMASTOWN

i I 100000 200000 300000

STRUCTURE LITHOLOGIES

FAULTS (UNCLASSIFIED)

FAULTS WITH A NORTHERN OOWNTHROW BASAL CLASTICS

..... FAULTS WITH A SOUTHERN DOWNTHROW LOWER PALAEOZOICS .... FOLDS MINERAL LOCATIONS Fig. 10. Fold and fault map of the Midlands of Ireland. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

16 J. D. JOHNSTON ET AL.

~ jY'jV~W ~ Tatestown Newtown Cashel ~'~f .Ballinalack "- .-~)(r.L __..~ ,v . ~ .... ~ .. //

, Navan ,Z / " ~7

..'.-~ o~.e o ~ ~,,.~ lw - .o Harberton /,~

Tynagh /..c:~," -.. ~.._~ o ~).. ~

.J /

. "-" ~"" ~ Galmoy

Silvermines Lisheen ~ " y "~__~

_ __~ - - Go.drum South dipping fault @ @ ~ ~ ~ inferred .,. North dipping fault s ~" inferred J South dipping fault well constrained North dipping fault Mineral deposit ~ well constrained ~'~Allihies I McCoss construction movement vector with error bars

Fig. 11. Cartoon map of the structural settings of the Irish deposits. The deposits are located in their approximate relative positions, but not drawn to scale. McCoss constructions indicate movement vectors for deformation. All of the deposits are in dextral transtensional settings. This is the result of NE extension of an E to ENE-trending basement template. there is evidence of early dip-slip. The evidence to mineralization occurred in pure extensional support this view is a combination of stratigra- orientations in en echelon systems (see below) of phical (e.g. formations thicken in the hanging wall overall transtension. In localized occurrences, at Tynagh, Silvermines and Lisheen) and textural these structures were subsequently reactivated by (e.g. the orientation of fibres in the fault zones at reverse movements. Gortdrum, Tynagh, and Silvermines, of slick- Millar (1990) demonstrated the same general enlines and shear bands in the case of Navan, and sequence of structural events on a regional basis early compacted veins at Ballinalack). In all cases in the North West Province, and Nolan (1986, the dip-slip motions were followed by strike-slip 1989), de Brit (1988) and Johnston (1993) have motions as indicated by the orientation of fibres, identified the same sequence in the Irish Mid- veins and faulting. In every case the strike-slip lands. To a certain extent the division of post-dates the mineralization. This is because structures into strike-slip and dip-slip is an Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 17 artificial one. Individual, en echelon, pure faults rotate and link at depth to form an ENE extension veins (dip-slip fractures) can link and dextral fault. rotate with depth to form oblique extension veins In an attempt to semi-quantify the motions, (strike-slip fractures) as is apparent at Silver- McCoss constructions (McCoss 1986; Fig. 12a) mines (Fig. 8), where ESE-trending dip-slip for the Irish deposits were evaluated. It was assumed that the magnetic first derivative trend gave the deformation zone boundary. This is justified on the grounds that it is likely to give a ] BulkDisplacement the orientation of the major basement structure, Vector which in turn has controlled later deformation in the Carboniferous. Secondly, the best esti- mate of the orientation of extension fractures at the time of mineralization was used to define the strike of extension veins. Every one of the deposits sits in dextral transtensional zones Pureextension vein Dip slip faults ./~ k'e~'~~0~e'~ (Fig. 11). These were the product of a NE extension of Carboniferous rocks deposited on a pre-existing Caledonian template. As pointed out by Millar (1990), the transition from dip-slip through strike-slip to thrusting Tensile Compressional throughout the Irish Carboniferous may simply Stress '~ reflect increasing burial depths (Fig. 12b). All of the textural evidence presented here suggests ___~_ _ Dip slip - pure extension that the mineralization is related to NE-extension. At deeper structural levels, this may have Miner aliza-fion: I-nsh-l~i~ lands" been contemporaneous with NW-compression. Burial [ Therefore, some of the copper deposits in the ~ ~ ~t.ee Strike slip South Munster Basin (Fig. 1; Reilly 1986) may have formed at the same time as the dextral Time I transtensional deposits in the Irish Midlands Miaeralization,South Munster Basin i ~ ~ Thrusting, __ Fig. 12. (a) Figure illustrating McCoss construction ement shear zone (McCoss 1986). A line is constructed parallel to the margin of a shear zone. A circle is constructed with this line as tangent. A line is drawn parallel to the Basemen~t~tri~j// / strike of pure extensional structures (veins, dip-slip faults) from the tangent point. A perpendicular is constructed. A diameter drawn from the intersection of the circle with the strike line to the intersection with the perpendicular gives the resultant vector of the " , Harberton,~ridge shear zone. (b) Diagram to illustrate the switch from T / Allenwood ~ I t ~ _ i __ /Moyvoughly normal faulting to strike-slip faulting to thrusting with ] / tynagn/~ ](e-el --'~---" burial. The tensile part of the figure is only appropriate / / ~ Newcastle in some cases. During the Dinantian the greater .t~ ..,..\~------Lisheen ~ ,'5"~ Galmoy horizontal stress axis was oriented NE and the smaller ~~~ l]allinalack trended SE. Progressive burial resulted in increasing Silvermines compression and the vertical stress increased linearly and slowly, while the horizontal stresses increased Local mineral controllingfault more rapidly. As a result, initial dip-slip switches to Gravity anomaly strike-slip and eventually thrusting. Miilar (1990) has Magnetic anomaly, suggested that all of these regimes may have been revealing basement structure with inferred dip of structure active simultaneously at different stratigraphic levels in the same place. However, as the Variscan deformation propagated northwards there must have been a general Fig. 13. Idealized structural model for Irish-style switch from transtension to transpression. deposits. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

18 J.D. JOHNSTON ET AL. during the mid to late Dinantian. However, it is We thank D. J. Sanderson, W. E. A. Phillips, D. generally accepted that the transpressional Romer, G. D. Sevastopulo, J. Ashton, M. W. Hitzman deformation associated with the Hercynian and P. Redmond for stimulating discussions. This work deformation post-dates the late Dinantian. would not have been possible without the gravity data set gathered by T. Murphy and access to the magnetic Marine sedimentation persisted into the Namur- data set under license by the Geological Survey of ian. Nevertheless, it is possible that the deforma- Ireland. Reviews by M. W. Hitzman, M. J. Kennedy tion was diachronous and the Irish Midlands and P. Strogen greatly improved the manuscript. may have been in transtension while the Munster Basin was in transpression. A generalized model for the structural References controls of the deposits is shown in Fig. 13. ANDREW, C. J. 1986. The tectono-stratigraphic Each of the deposits is in a unique setting related controls to mineralization in the Silvermines to the precise basement geometry of its location. area, County Tipperary, Ireland. In: ANDREW, However, they are all located adjacent to C. J., CROWE, R. W. A., FINLAY, S., PENNELL, basement structures, in regions of strike swings W. M. & PYNE, J. F. (eds) Geology and Genesis of along these structures. This is seen in the Mineral Deposits in Ireland. Irish Association for Economic Geology, Dublin, 377 417. geophysical data as a divergence of the gravity --1992. Basin development chronology of the lower- and magnetic linears. This may reflect obliquity most Carboniferous strata in the north-Central of lithological and structural grain, or obliquity Midlands. In: BOWDEN, A., EARLS, G. V., of major and minor structures. Either way, O'CONNOR, P. & PYNE, J. (eds) The Irish Minerals complexity of the trends in the basement greatly Industry 1980 1990. Irish Association for Eco- enhanced the potential for dilatancy in the cover nomic Geology, Dublin, 143-169. during reactivation, and favoured the generation 1993. Mineralization in the Irish Midlands. In: of ore deposits. PATTRICK, R. A. D. & POLYA, D. A. (eds) Mineralization in the British Isles. Chapman & Hall, London, 208-269. --, GROWL, R. W. A., FINLAY, S., PENNELL, W. M. Conclusions & PYNE, J. F. 1986. Geology and Genesis of Mineral Deposits in Ireland. Irish Association for Most of the Irish carbonate-hosted base metal Economic Geology, Dublin. deposits appear to sit in dextral transtensional ASHTON, J. H., BLACK, A., GERAGHTY, J., HOLD- structural traps. The precise geometry of the STOCK, M. & HYLAND, E. 1992. The geological traps varies from one deposit to another and setting and metal distribution patterns of Zn/Pb/Fe the variations are the product of different mineralization in the Navan boulder conglomerate. basement structural geometries. Most of the In: BOWDEN, A., EARLS, G. V., O'CONNOR, P. & PYNE, J. (eds) The Irish Minerals Industry 1980- deposits appear to have formed at intermediate 1990. Irish Association for Economic Geology, burial depths, rather than on the sea floor. Dublin, 171-210. Precise timing is difficult to constrain, but the --, DOWNING, D. H. & FINLAY, S. F. 1986. The best estimates seem to be early Chadian for geology of the Navan Zn-Pb orebody. In: Navan, and late or post-Chadian for Lisheen. ANDREW, C. J., CROWE, R. W. A., FINLAY, S., The fundamental structural controls are E- to PENNELL, W. M. & PYNE, J. F. (eds) Geology and NE-trending basement (Caledonian) structures, Genesis of Mineral Deposits in Ireland. Irish which have been reactivated in dextral transten- Association for Economic Geology, Dublin, sion by NNE- to ENE-extension during the 243 -280. BOWDEN, A. A., EARLS, G., O'CONNOR, P. G. & Dinantian. In general, in the north of the PYNE, J. F. 1992. The Irish Minerals Industry country, the ore-controlling faults dip south, 1980-1990. Irish Association for Economic while in the south of the country they dip Geology, Dublin. north. The structures have evolved through a BROWN, C. & WILLIAMS, B. 1985. A gravity and temporal kinematic history of early normal magnetic interpretation of the structure of the faulting, later oblique slip, and some show Irish Midlands and its relation to ore genesis. evidence of later reverse movements. Part of Journal of the Geological Society, London, 142, this evolution may reflect burial history, but it 1059-1075. also reflects the temporal transition from CLIFFORD, J. A., RYAN, P. & KUCHA, H. 1986. A review of the geological setting of the Tynagh Dinantian transtension to Variscan compres- orebody, Co. Galway. In: ANDREW, C. J., sion, which propagated northwards with time. CROWE, R. W. A., FINLAY, S., PENNELL, W. M. The bulk of mineralization appears to post-date & PYNE, J. F. (eds) Geology and Genesis of Mineral the normal faulting, but pre-date the Variscan Deposits in Ireland. Irish Association for Eco- compression. nomic Geology, Dublin, 419-439. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND 19

COLLER, D. W. 1984. Variscan structures in the Upper HITZMAN, M. W. & LARGE, D. 1986. A review Palaeozoic rocks of west central Ireland. In: and classification of the Irish carbonate-hosted HUTTON, D. H. W. & SANDERSON, D. J. (eds) base metal deposits. In: ANDREW, C. J., CROWE, Variscan Tectonics of the North Atlantic Region. R. W. A., FINLAY, S., PENNELL, W. M. & PYNE, Geological Society, London, Special Publication, J. F. (eds) Geology and Genesis of Mineral 14, 185-194. Deposits in Ireland. Irish Association for Eco- COOPER, M. A., COLLINS, D. A., FORD, M., nomic Geology, Dublin, 217 238. MURPHY, F. X. & TRAYNER, P. M. 1984. --, O'CONNOR, P., SHEARLEY, E., SCHAFFA- Structural style, shortening estimates and the LITZKY, C., BEATY, D. W., ALLAN, J. R. & thrust front of the Irish Variscides. In: HUTTON, THOMPSON, T. 1992. Discovery and geology of D. H. W. & SANDERSON, D. J. (eds) Variscan the Lisheen Zn-Pb-Ag prospect, Rathdowney Tectonics of the North Atlantic Region. Geologi- trend, Ireland. In: BOWDEN, A., EARLS, G. V., cal Society, London, Special Publication, 14, O'CONNOR, P. & PYNE, J. (eds) The Irish 167-175. Minerals Industry 1980-1990. Irish Association .... & O'SULLIVAN, M. 1986. for Economic Geology, Dublin, 227-246. Structural evolution of the Irish Variscides. JOHNSTON, J. n. 1993. Three-dimensional geometries Journal of the Geological Society, London, 143, of veins and their relationship to folds: examples 53-61. from the Carboniferous of eastern Ireland. Irish CROWE, R. W. A. 1986. The stratigraphic and structural Journal of Earth Sciences, 12, 47-63. setting of Zn-Ba-Pb mineralization at Newtown 1996. Regional fluid flow and the genesis of Irish Cashel, Co. Longford. In: ANDREW, C. J., CROWE, Carboniferous base metal deposits. In: RICKARD, R. W. A., FINLAY, S., PENNELL, W. M. & PYNE, J. D. (ed.) Mineral Deposits of Europe. Wylie, F. (eds) Geology and Genesis of Mineral Deposits in London, in press. Ireland. Irish Association for Economic Geology, LEEDER, M. R. 1988. Recent developments in Carbon- Dublin, 331-339. iferous geology: a critical review with implications DEBRIT, T. J. 1989. Timing structural events and for the British Isles and N. W. Europe. Proceedings basement emplacement using extension veins of the Geologists'Association, 99, 73-100. and cements in the Carboniferous of north LEES, A. 1964. The structure and origin of the central Ireland. Irish Journal of Earth Sciences, Waulsortian (Lower Carboniferous) 'reefs' of 10, 13-31. west-central Eire. Philosophical Transactions of DOLAN, J. M. 1983. A structural cross-section through the Royal Society of London, B247, 483-531. the Carboniferous of northwest Kerry. Irish MCARDLE, P. 1990. A review of carbonate hosted Journal of Earth Sciences, 6, 95-108. base metal-barite deposits in the Lower Carboni- DOYLE, E., BOWDEN, A. A., JONES, G. V. & STANLEY, ferous of Ireland. Chronique de la Minidre, G. A. 1992. The geology of the Galmoy deposit. In: Recherche, 500, 3-29. BOWDEN, A., EARLS, G. V., O'CONNOR, P. & McCoss, A. M. 1986. Simple constructions for PYNE, J. (eds) The Irish Minerals Industry 1980- deformation in transpression/transtension zones. 1990. Irish Association for Economic Geology, Journal of Structural Geology, 8, 715-718. Dublin, 211-226. MATTHEWS, S. C., NAYLOR, D. & SEVASTOPULO, EMO, G. T. 1986. Some considerations regarding the G. D. 1983. Palaeozoic sedimentary sequence as a styles of mineralization at Harberton Bridge, Co. reflection of deep structure in southwest Ireland. Kildare. In: ANDREW, C. J., CROWE, R. W. A., Sedimentary Geology, 34, 83-95. FINLAY, S., PENNELL, W. M. & PYNE, J. F. (eds) MAX, M. D., RYAN, P. D. & INAMDAR, D. D. 1983. A Geology and Genesis of Mineral Deposits in Ireland. magnetic deep structural interpretation of Ire- Irish Association for Economic Geology, Dublin, land. Tectonics, 2, 431-451. 461-469. MILLAR, G. 1990. Fracturing in the Northwest FITZGERALD, E., FELLY, M., JOHNSTON, J. D., Carboniferous Basin, Ireland. PhD Thesis, CLAYTON, G., FITZGERALD, L. J. & SEVASTO- Queen's University of Belfast. PULO, G. D. 1994. The Variscan thermal history MOORE, J. MCM. 1975. Fault tectonics at Tynagh of west Clare, Ireland. Geological Magazine, 131, mine, Ireland. Transactions of the Institution for 545-58. Mining and Metallurgy, 84B, 141-145. FORD, M., 1987. Practical application of the sequen- MORRIS, P. & MAX, M. D. 1995. Magnetic crustal tial balancing technique: an example from the character in Central Ireland. Geological Journal, Irish Variscides. Journal of the Geological Society, 30, 49-68. London, 144, 885-891. MURPHY, T. 1952. Measurement of gravity in --, BROWN, C. & READMAN, P. 1991. An analysis Ireland - gravity study of Central Ireland. Dublin and tectonic interpretation of gravity data over Institute of Advanced Studies, Memoirs, 3, 31 pp. the Variscides of south-west Ireland. Journal of 1960. Gravity anomaly map of Ireland. Dublin the Geological Society, London, 148, 137-148. Institute of Advanced Studies, Geophysical GILL, W. D. 1962. The Variscan fold belt in Bulletin, 18. Ireland. In: COL, K. (ed.) Some Aspects of the 1974. Gravity anomaly map of Ireland. Commu- Variscan Fold Belt. Manchester University Press, nications of the Dublin Institute for Advanced 49-64. Studies, Bulletin, 32. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

20 J.D. JOHNSTON ET AL.

--1981. Geophysical evidence. In: HOLLAND, C. H. & PYNE, J. F. (eds) Geology and Genesis of (ed.) A Geology of Ireland. Scottish Academic Mineral Deposits in Ireland. Irish Association for Press, Edinburgh, 225-229. Economic Geology, Dublin, 513-544. NAYLOR, D., SEVASTOPULO, G. D. & SLEEMAN, ROTHERY, E. 1988a. En-echelon vein array develop- A. G. 1989. Subsidence history of the South ment in extension and shear. Journal of Structural Munster Basin, Ireland. In: ARTHURTON, R. S., Geology, 10, 63-71. GUTTERIDGE, P. & NOLAN, S. C. (eds) The Role 1988b. Transpression in the Variscan foreland: a of Tectonics in Devonian and Carboniferous study in east-central Ireland. Irish Journal of sedimentation in the British Isles. Occasional Earth Sciences, 10, 1-12. Publication, Yorkshire Geological Society, 6, RUSSELL, M. J. 1968. Structural controls of base metal 99-109. mineralization in relation to continental drift. NOLAN, S. C. 1986. The Carboniferous Geology of Transactions of the Institution for Mining and the Dublin Area. PhD Thesis, University of Metallurgy, 77B, 11-28. Dublin. --1972. The Geological Environment of Post Cale- --1989. The style and timing of Dinantian syn- donian Base Metal Mineralization in Ireland. PhD sedimentary tectonics in the eastern part of the Thesis, University of Durham. Dublin Basin, Ireland. In: ARTHURTON, R. S., & HASZELDINE, R. 1992. Accounting for GUTTERIDGE, P. & NOLAN, S. C. (eds) The Role of geofractures. In: BOWDEN, A., EARLS, G. V., Tectonics in Devonian and Carboniferous Sedimen- O'CONNOR, P. & PYNE, J. (eds) The Irish tation in the British Isles. Occasional Publication, Minerals Industry 1980-1990. Irish Association Yorkshire Geological Society, 6, 83-98. for Economic Geology, Dublin, 135-142. PHILCOX, M. E. 1984. Lower Carboniferous litho- SANDERSON, O. J. 1984. Structural variation across stratigraphy of the Irish Midlands. Special Pub- the northern margin of the Variscides in NW lication, Irish Association for Economic Geology, Europe. In: HUTTON, D. H. W. & SANDERSON, Dublin. D. J. (eds) Variscan Tectonics of the North --1989. The mid-Dinantian unconformity at Navan, Atlantic Region. Geological Society, London, Ireland. In: ARTHURTON, R. S., GUTTERIDGE, P. Special Publication, 14, 149-166. & NOLAN, S. C. (eds) The Role of Tectonics in SEVASTOPULO, G. D. S. 1981. Lower Carbonif- Devonian and Carboniferous Sedimentation in the erous. In: HOLLAND, C. H. (ed.) A Geology of British Isles. Occasional Publication, Yorkshire Ireland. Scottish Academic Press, Edinburgh, Geological Society, 6, 67-82. 147-172. , SEVASTOPULO, G. D. & MACDERMOT, C. V. SHEARLEY, E., HITZMAN, M., WALTON, G., 1989. Intra-Dinantian tectonic activity on the REDMOND, P., DAVIS, R., KING, M., DUFFY, Curlew Fault, north-west Ireland. In: ARTHUR- L. & GOODMAN, R. 1992. Structural control' TON, R. S., GUTTERIDGE, P. & NOLAN, S. C. on mineralization and dolomitisation, Lishe~.n (eds) The Role of Tectonics in Devonian and Zn-Pb-Ag deposit, County Tipperary, Ire'.,nd. Carboniferous Sedimentation in the British Isles. Geological Society of America, Abstro .s with Occasional Publication, Yorkshire Geological Programs, 34, A354. Society, 6, 55-66. , REDMOND, P. KING, M. & C jODMAN, R. PHILLIPS, W. E. A. & SEVASTOPULO, G. D. 1986. The 1996. Structural controls on rr',~eralization and stratigraphic and structural setting of Irish mineral dolomitisation, Lisheen Zn-" o--Ag deposit, Co. deposits. In: ANDREW, C. J., GROWL, R. W. A., Tipperary, Ireland. This ," .ume. FINLAY, S., PENNELL, W. M. & PYNE, J. F. (eds) SLOWLY, E. P. 1986. The ~,l-Pb and barite deposits Geology and Genesis of Mineral Deposits in Ireland. at Keel, Co. Lot ,.ord. In: ANDREW, C. J., Irish Association for Economic Geology, Dublin, CROWE, R. W ,., FINLAY, S., PENNELL, W. 1-30. M. & PYNF J. F. (eds) Geology and Genesis PICKARD, N. A. H., JONES, G. LL., REES, J. G., of Minerr" ,3eposits in Ireland. Irish Association SOMERVILLE, I. D. & STROGEN, P. 1992. Lower for Ec,- Jmic Geology, Dublin, 319-330. Carboniferous (Dinantian) stratigraphy and struc- SOMERVi ,.E, I. D., STROGEN, P. & JONES, G. EL. ture of the Walterstown-Kentstown area, Co. 1992. The biostratigraphy of Dinantian lime- Meath, Ireland. Geological Journal, 27, 35-58. stones and associated volcanic rocks in the PRICE, C. A. & TODD, S. P. 1988. A model for the Limerick Syncline, Ireland. Geological Journal, development of the Irish Variscides. Journal of the 27, 201-220. Geological Society, London, 145, 935-939. STEED, G. M. 1986. The geology of the Gortdrum READMAN, P. W., O'REILLY, B. M., EDWARDS, Cu-Ag-Hg orebody. In: ANDREW, C. J., CROWE, J. W. F., SANKEY, M. J. 1996. A gravity map of R. W. A., FINLAY, S., PENNELL, W. M. & PYNE, Ireland and its surrounding waters. In: CROKER, J. F. (eds) Geology and Genesis of Mineral P. F. & SHANNON, P. M. (eds) The Petroleum Deposits in Ireland. Irish Association for Eco- Geology of Ireland's Offshore Basins. Geological nomic Geology, Dublin, 481-499. Society of London, Special Publication, 93, 9-16. STROGEN, P. & SOMERVILLE, I. D. 1984. The REILLY, T. A. 1986. A review of vein mineralization in stratigraphy of Upper Palaeozoic rocks in the SW County Cork, Ireland. In: ANDREW, C. J., Lyons Hill area, Co. Kildare. Irish Journal of CROWE, R. W. A., FINLAY, S., PENNELL, W. M. Earth Sciences, 6, 155-173. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

BASE METAL MINERAL DEPOSITS, IRELAND

, & JONES, G. LL. 1990. Stratigraphy and M. & PYNE, J. F. (eds) Geology and Genesis of sedimentology of Lower Carboniferous (Dinan- Mineral Deposits in Ireland. Irish Association for tian) boreholes from west Co. Meath, Ireland. Economic Geology, Dublin, 579-590. Geological Journal, 25, 103-137. ZIEGLER, P. A. 1988. Evolution of the Arctic-North WILLIAMS, B. & BROWN, C. 1986. A model for the Atlantic and the western Tethys. American genesis of Zn-Pb deposits in Ireland. In: ANDREW, Association of Petroleum Geologists, Memoir, 43, C. J., CROWE, R. W. A., FINLAY, S., PENNELL, W. 1-198.