Geological Survey of Ireland

IIIlIlIIVHlUlliHIIIlIIIIIIIII

DF000083 Geol Surv.

Ire. Bull.

Vol. 4 (Part 1)1987, pp.

1-10

A review of recent isotope research on mineralization in

Ireland P. S. Kennan, P. McArdle, V. Gallagher, J. H. Morris, P. G. O'Connor, W. G. O'Keeffe, N. A. Reynolds and G. M. Steed

Abstract.

This is a brief review of the conclusions that can be drawn from recent isotope studies on a wide spectrum of mineral deposits hosted in Caledonide rocks. The work was supported by the EEC Commission. A variety of interacting controls on the mineralizations of different areas is indicated. Deuteric solutions derived from granite magma were significant in tungsten mineralization in Leinster. A hydrothermal system involving magmatic fluids carrying lead with a mantle-like isotopic signature operating in an island arc setting is favoured for copper mineralization at Charlestown, Co. Mayo. In the case of stratabound base-metal and tungsten mineralization in the Dalradian of western Ireland, the metal carrying fluids were essentially of magmatic or metamorphic origin initially, with meteoric water significant only in the later stages of an extended history of metal deposition. Diverse fluid sources are also indicated for Caledonian age antimony-arsenic-gold and spatially associated, although younger, mineralization in the Longford-Down Inlier: a magmatic source is strongly indicated for the former, whereas the latter was derived from different fluids, possibly involving seawater. Whatever the geological setting or the apparent nature of the metal- transporting fluids, a regional unity for all the deposits is provided by lead isotope data which reflects a pattern inherited from the Caledonide basement.

INTRODUCTION Isotope techniques have been applied to the study of mineral deposits for many years and with significant success. In Ireland a number of important contributions have been made, largely in the study of carbonate-hosted base-metal deposits (e.g. Greig etal. 1971, Coomer and Robinson 1976, Boast etal. 1981), the Avoca volcanogenic copper deposit (Williams et al. 1986) and granite-hosted lead-zinc veins of the Leinster Granite (Williams and Kennan 1983). This review is concerned with isotopic research carried out as part of the EEC Research and Development Programme in Primary Raw Materials (1982-1985). The research was broadly based and examined mineralization in several geological environments, including tungsten, lithium, base metal and gold deposits in several parts of Ireland (Fig. 1). The research was directed towards establishing what contribution isotopic studies might make in understanding the genesis of, and controls on, metallic mineralization and to determining how such an enhanced understanding might be applied in developing exploration strategy. The pre-Upper Palaeozoic rocks of Ireland, deformed by the Caledonian orogeny, are overlain by relatively undeformed cover rocks of Upper Palaeozoic and younger age (Fig. I). The latter contain a series of important carbonate-hosted base metal deposits, including the Navan deposit, where pre-production ore reserves amounted to 69.9 million tonnes (Ashton etal. 1986). Emerging evidence has suggested that the source of metals for these deposits lies in the Caledonian basement rocks (Russell 1986, Mills et. al. 1987). The research described here has concentrated on these older rocks. Furthermore they have an inherent metallogenic and resource potential since they host a wide variety of metallic deposits, including base metals, gold, tungsten, lithium, molybdenum, beryllium and chromium (Williams and McArdle 1978).

SE IRELAND In SE Ireland the Caledonian rocks (Fig. 2) comprise a volcanosedimentary sequence of Precambrian to Silurian age which was intruded by a variety of igneous rocks including the Leinster

Granite. The eastern margin of this granite is the setting for

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CALEDONIAN AND SOME OLDER ROCITS

Figure 1. Outline geological features of Ireland, showing the districts examined in this study, including Charlestown (CR) and Clontibet (CL). The boxed areas outline the districts containing Dalradianhosted mineralization in western Ireland and lithium and tungsten deposits in SE Ireland.

significant tungsten and lithium mineralization (McArdle and Kennan, in press). The Ballinglen (near Tinahely) tungsten deposit occurs in the form of scheelite-bearing quartz-fluorite veins cutting altered microgranite. The microgranite, of similar age to the Leinster Granite, forms a sheet complex conformable with the main Caledonian NE-trending cleavage of the enclosing rocks. The enclosing rocks are Cambro-Ordovician sediments (Ribband Group). These are mainly distal turbidites and contain rare, but

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P.S. KENNAN ETAL.

distinctive, coticule lithologies (McArdle and Kennan in press). The lithium mineralization of the Shillelagh-Borris district occurs in pegmatites spatially associated with the Kilcarry Volcanic Formation of the Ribband Group. The lithium-rich pegmatites occur within the metamorphic aureole of the Leinster Granite in minor granite sheets lying along the prevailing Caledonian cleavage; some occur within the margin of the Leinster Granite itself. Both the tungsten and lithium deposits are present in rocks traversed by the East Carlow Deformation Zone (McArdle and Kennedy 1985). Earlier descriptions of the genesis of these deposits emphasised the important role of granite-related fluids (Steiger and von Knorring 1974, Steìger and Bowden 1982). More recently the possibility that the volcanosedimentary aureole rocks might be the source for the metals in these deposits has been discussed (McArdle eta!, in press). It is a strong possibility that metal mobility might have been structurally promoted, since both types of mineralization lie along the East Carlow Deformation Zone.

Figure 2. Geological setting of tungsten and lithium mineralization in the Caledonide inlier of SE Ireland. Modified after McArdle and

Kennan (in press). Location shown on Figure 1.

Gallagher(1987) reports new isotopic data for these deposits. At the Ballinglen tungsten deposit (Fig. 2), relatively fresh microgranite has a mean del'80 value of + 12.2 per mu and a del D value of -69 per mit. Despite a low strontium isotope initial ratio (0.7045), the 0 and D values suggest a sedimentary protolith, i.e. that the granites are of S-type. Hydrothermally altered mineralized microgranite is isotopically indistinguishable

from unaltered microgranite. The temperature of mineralization, based on quartz-muscovite oxygen isotope fractionation, was about 425°C and varied little duriig deposition: mineral separates display a narrow range of del 0 and del D values. Deposition of metals occurred over a narrow temperature and time interval from a fluid of single source. At 425°C, the hydrothermal fluid would have had a del 0 value of +9.0 to +9.4 per mu and del D value of -42 per mit, close

to that expected for a deuteric fluid derived from the microgranite magma. The walirocks show similar ranges of del' 0 and del D values to those of the microgranites and thus cannot be excluded as sources of the mineralizing fluid on the basis of stable isotopes alone. Oihe assumption that the fluid was magmatic in origin, the del S value of the magma was approximately + 9 per mil, similar to that determined for the Leinster Granite by Williams and Kennan (1983). Isotope data also indicate that a minor degree of meteoric water interaction occurred at some contacts of the microgranite sheets. Isotopic results thus point to an essentially magmatic origin for the microgranite-hosted tungsten mineralization. Stable isotope data do not Indicate any significant interaction between the microgranite and its wallrocks. However the inferred nature of the microgranite source rocks and the location of the mineralized bodies within coticule-bearing Ribband Group sediments may point to a fundamental stratigraphic control. Most lithium pegmatites are spatially associated with the occurrence of tourmaline-bearing rocks (Kennan et al. 1986, Gallagher 1987). Variations in isotope systematics suggest that these rocks do not form a homogeneous group. Tourmaline from quartz-tourmaline rock associated with lithium pegmatite at Coolasnaghta (Fig. 2) has del'80 and del D values of 8.8 to 9.0 per mil and -48 to -57 per mil respectively, typical of pegmatitic tourmaline (Taylor and Slack 1984). Tourmaline in adjacent schist has significantly higher del'80 values (about 10.5 per mil) and this probably reflects derivation of the oxygen in this tourmaline from isotopically heavier wallrock schist during metasomatism. The isotopic results are consistent with the conclusions drawn from chemical studies of these tourmalines (Gallagher 1987). Lithium pegmatite does not occur at Kilcarry Bridge (Fig. 2), where stratiform tourmaline-bearing rock occurs in a sequence containing minor stratabound sulphide mineralization (arsenopyrite, pyrite, chalcopyrite). The tourmaline has del'80 and del D values (9.4 to 10.1 and -40 to -50 per mil respectively) which do not clearly fall within either the pegmatitic or massive sulphide fields of Taylor and Slack's (1984) initial attempt at classification. The isotopic results of this preliminary study indicate that the quartz-tourmaline rock associated with the lithium pegmatite can be distinguished isotopically from the stratiform quartz-tourmaline rock. The lithium and tungsten deposits in SE Ireland are hosted in a succession with a notable content of tourmaline-bearing rocks and coticules. The latter formed widely on the lapetus ocean floor (Kennan and Kennedy 1983). This regional association may be expected because mobilization of manganese, lithium and boron can be a result of the hydrothermal alteration of ocean floor basalt (e.g. Humphris and Thompson 1978, Seyfried et al. 1984). Enhanced levels of other elements, including tungsten, may also be present in sedimentary sequences containing basic igneous rocks not necessarily of ocean floor affinity.

COPPER MINERALIZATION IN THE CHARLESTOWN INLIER The Charlestown Inlier consists of a folded sequence of intrusions, pyroctastic rocks and minor intercalated sediments of Arenig age (O'Connor and Poustie 1986). The igneous rocks are tholeiitic to calcalkaline in character. Basal basalts with associated cherts and siltstones are overlain by an andesitic sequence of reworked tuffs, tuff turbidites and lapitli to blocky tuffs. The tuffs become more silicic higher in the succession, culminating in dacitic and rhyodacitic compositions. The succession is considerably thickened by acidic sill-like ìntrusions, largely contemporaneous. The sequence was folded, ad

ISOTOPE RESEARCH possibly thrusted, before being unconformably overlain by Silurian sandstones. Mineralization is mainly confined to a composite porphyritic intrusion close to the top of the Arenig succession (O'Connor and Poustie 1986). The porphyries are dacitic to rhyodacitic in composition and form a composite intrusive sill which was emplaced before the sequence was lithified. A set of alteration zones, confined to the sill, are concentrically arranged around the mineralization. Much of the mineralization, consisting of pyrite, chalcopyrite, sphalerite and galena, occurs in breccias in the inner silicic alteration zone. Sulphides are also disseminated in the host rocks. O'Connor (1987) concluded on the basis of a study of whole rock H and 0 isotope values that magmatic waters were involved possibly to the exclusion of meoric water or seawater in the metal deposition process. Del S values are low and positive and suggest an igneous rather than a biogenic or sedimentary source. Lead isotope data indicate a relatively unradiogenic lead source (see below). The age of the deposit by reference to the growth curve of Stacey and Kramers (1975) approximates to the stratigraphic age of the host rocks. A hydrothermal system involving mainly magmatic water in an island arc setting is very likely.

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Figure

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Plot

Unaltered Inlrusaes

Siliciled Zone

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Sericitic Zone

Chloritic Zone

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Clay rich Zone

Haematitic Zone

of

del

D

against del18O values

for

the Charlestown

deposit and its vicinity.

BASE METAL AND TUNGSTEN MINERALIZATION IN CONNEMARA The Dairadian inlier of Connemara is an area of high grade metamorphic rocks of late Precambrian and Cambrian age which can be correlated with similar sequences elsewhere in Ireland and Scotland (Harris and Pitcher 1975). Most of the Connemara succession is assigned to the middle Dalradian Argyll Group (Tanner and Shackleton 1979), deposited in an unstable tectonic environment of fault controlled sub-basins as the rate of rifting and volcanicity in the Dalradian basin

increased (Anderton 1985). This was accompanied by extensive hydrothermal activity during which widespread developments of metallic mineralization, including the Aberfeldy base metalbarite deposit in Scotland (Coats et al. 1980), originated. The Dalradian rock sequence was deformed and metamorphosed in the early Ordovicia Grampian orogeny and intruded by post-tectonic granities (Leake 1978). The Dairadian of Connemara consists of a diverse sequence of sediments and minor volcanic rocks. Widespread Dalradianhosted mineralization reflects three principal controls (McArdle et al. 1986, Reynolds 1987), i.e. stratigraphic position, granite association and structural setting. Most mineralization occurs within the Lakes Marble Formation associated with calcareous and metavolcanic amphibolite lithologies. Mineralization also occurs in graphitic marbles of the Cornamona Formation deposited in a tectonically active basinal setting. Three styles of mineralization can be distinguished (Fig. 4). Stratabound massive to disseminated iron sulphide bodies with low copper grades and local tungsten, molybdenum and gold values occur in E Connemara in the Oughterard Granite area. High temperature skarn-and vein-hosted tungsten, molybdenum and copper mineralization also occurs around all the Connemara granites. In the Glan area of E Connemara skarn, vein and massive sulphide mineralization is structurally controlled by the Maam Valley fault zone. Low temperature vein and replacement base metal-barite mineralization is also largely granite related and often forms a continuum with earlier skarn mineralization. De134S values for stratabound sulphides are typically in the + 4 to + 10 per mil range (Fig. 5) and suggest a significant component of Dalradian sedimentary sulphur. The range is similar to that of disseminated sygenetic mineralization in the Scottish Dalradian, but not that of the significant Aberfeldy deposits (Willan and Coleman 1983). Sulphide in skarn mineralization and some associated massive sulphide mineralization is characterized by lower positive values (0 to + 2 per mu) which are compatible with a predominantly magmatic sulphur so%ce. Carbon isotopes also have a magmatic character; del C values lie in the range -6 to -8 per mu. Low temperature vein and replacement mineralization probably has a bulk sulphur composition similar to that of stratabound sulphides (+5 to + 10 per mil) suggesting a similar sulphur source and/or remobilization of earlier mineralization. In this case, carbon isotopes suggest mixing of sedimentary and igneous carbon sources. Lead isotope data suggest an important role for granitic lead. Oxygen and hydrogen isotope data distinguish relatively '0and D-enriched metamorphic fluids (del'80 values of 10 to 15 per mil and delD values of -30 to -50 per mil) and relatively 18Ø and D-depleted granitic fluids (del'80 values of 5 to 10 per mu and delD values -50 to -80 per niìl). The isotopic composition of silicate assemblages in prograde skarn and stratabound sulphide bodies suggests a predominantly granitic fluid source. Del0 values in stratabound sulphide mineralization were locally buffered to metamorphic values due to interaction with metamorphic rocks at low water/rock ratios. Retrograde skarn stages and widespread retrogressive alteration in granite and metamorphic rocks record the influx of large amounts of 180-depleted (1 to 6 per mil) and D-enriched (-10 to -30 per mil) water. This suggests the development of large scale convective systems around granites and the drawing in of extraneous water, probably of low latitude meteoric origin.

BASE METAL AND GOLD DEPOSITS IN THE

CLONTIBRET DISTRICT The Clontibret district is located in the Lower Palaeozoic inlier of Longford-Down, a southwesterly extension of the Southern Uplands of Scotland. The inlier contains a sequence of

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80 90 100 Figure 4. Geological setting of metallic mineralization in the Dalradian of the Connemara District (from McArdle et al. 1986). The numbered localities are as follows (grid references in brackets): 1 High Island (L 507575). 2. Omey Granite deposits. 3. Cleggan (L 614575). 4. Letterfrack Granite deposits. 5. Derrylea Lough (L 714495). 6. Benbaun (L 785539). 7. Maumean (L 907505). 8. Cur (L 935533). 9. Kilmeelickin (L 933554). 10. Teemakill (L 985505 N 971505). 11. Clements Mine(L 993518). 12. Dereenagusfoor(M 004471). 13. Doorus (M 049520 078505). 14 Glan area (M 035485 090487). 15. Oughterard area (M

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