Deposits In Ireland Research Articles

A Pleistocene deposit preserved in deep karst at Coolough, County Galway, western Ireland

Buried karst features result from a complex interplay between carbonate bedrock and structural geology. The enhancement of carbonate aquifers through this interplay creates hydrogeologically complex subsurface drainage networks and highly variable surface topographies. This article reports on the sediment fill and fossil pollen record from a large enclosed negative karst landform at Coolough, near Galway City in the west of Ireland. The landform was discovered during the preliminary site investigation for the proposed N6 Galway City Ring Road. Major joint sets developed in the Carboniferous (Mississippian) Limestone bedrock govern the local hydrogeology and structural evidence suggests that a local fault has controlled the east–west propagation of the deep enclosed depression. Fluvial and lacustrine palaeoenvironments are interpreted from the sediments infilling the enclosed depression, within which three discrete diamicts are identified, potentially representing distinct phases of glaciation. Pollen assemblages, which reflect palaeoenvironments of open woods and grasslands, have been correlated with other deposits in Ireland and Europe and suggest a minimum date of the Early Pleistocene for the oldest of the diamicts, and thus an upper age limit on the development of the karst depression. Fluvial disturbance to the deposit is evident and is likely to be responsible for the chronological discrepancy between the Early Pleistocene and Middle Pleistocene sediments both recorded from the deposit infill. The findings of this multidisciplinary study contribute to an understanding of Quaternary landscape evolution in western Ireland.

A global assessment of Zn isotope fractionation in secondary Zn minerals from sulfide and non-sulfide ore deposits and model for fractionation control

We investigated extent and direction of Zn isotope fractionation in secondary zinc minerals formed during low temperature hydrothermal and/or supergene oxidation of primary sulfide deposits. Zinc isotope data have been obtained from non-sulfide zinc mineral separates (willemite - Zn2SiO4, smithsonite - ZnCO3, hemimorphite - Zn4(Si2O7)(OH)2·H2O, hydrozincite - Zn5(CO3)2(OH)6, and sauconite - Na0.3Zn3(Si,Al)4O10(OH)2·4H2O) collected from several Zn deposits in Ireland, Belgium, Poland, Namibia, Peru, Yemen and Zambia. The data are compared with Zn isotope compositions measured on Zn sulfides collected in the same areas and/or derived from the existing literature, to establish the controls of direction and likely extent of any fractionations. We find that willemite has the greatest compositional variability, with measured δ66ZnJCM-Lyon values ranging from −0.42 to 1.39‰, spanning the entire range of terrestrial variation in Zn isotopes recorded to date. Overall, significant fractionations in positive and negative directions are recorded relative to the precursor phase (primary sphalerite or an earlier secondary phase), with primary sphalerite falling in a relatively narrow range of isotopic values (approximately −0.1 to +0.4‰). Most of the data observed on willemite, hemimorphite and hydrozincite can be explained with a model of isotopic fractionation, in which partial dissolution of primary sphalerite is followed by precipitation of an initial secondary phase that preferentially incorporates heavy Zn isotopes. Smithsonite, instead, preferentially incorporates light Zn isotopes. This reflects the variation in the Zn-x bond strengths of these secondary phases with respect to the original sulfides. We also observed that isotope compositions do not depend only on the difference between the fractionation factors of the involved phases but also on the amount of the secondary mineral precipitated after dissolution of primary sulfide, and that the greatest fractionations occur when only small amounts of secondary mineral are precipitated. Progressive precipitation from migrating fluids that form phases enriched in heavy zinc isotopes would lead to a gradual decrease in the δ66Zn values of such phases, and the fluids involved, in time and space. Strong negative isotopic shifts are almost only observed for late crystallizing phases, such as those in vugs. These are interpreted to reflect precipitation from residual, isotopically-light fluids that are the inevitable highly-fractionated product of the above-described process. Where a more complete replacement of primary sulfide has occurred, such as in the high-grade core of non-sulfide zinc orebodies, there is limited net isotopic fractionation because dissolved primary zinc is nearly quantitatively reprecipitated locally. In addition, in only one case (Yanque, Peru) we observed that the fringes of non-sulfide zinc deposit were characterized by isotopically fractionated compositions, with highly negative values implying extensive precipitation (earlier, or elsewhere) of isotopically heavy secondary phases. The higher-grade ore zones, where complete breakdown of primary sulfides and quantitative reprecipitation of zinc have occurred, show instead less fractionated compositions.

The Performance of Road Embankments on Glacial Deposits in Ireland

Glacial deposits are very common as foundation soils supporting road embankments in Ireland and are typically present in almost all road projects. Despite this there is limited research data linking the settlement performance of embankments to the compressibility and consolidation properties of such soils. This is largely due to the difficulties in obtaining representative, high quality undisturbed samples and laboratory testing of glacial deposits, which exhibit a large range of gradation particle sizes including gravels, cobbles and boulders.The paper presents the results of several case histories of foundations and monitored road embankments supported on glacial deposits located throughout Ireland. In situ field compressibility and consolidation parameters are back calculated from field performance and correlated to standard index properties such as liquid limit, plasticity index, particle size gradations and SPT N values. Some general conclusions plus recommended ways to predict embankment performance from commonly obtained geotechnical test data are suggested. Identification of difficult glacial soil types requiring special investigation and treatment or improvement are suggested.

Targeting Stratiform Zn-Pb-Ag Massive Sulfide Deposits in Ireland through Numerical Modeling of Coupled Deformation, Thermal Transport, and Fluid Flow

We explore three- and two-dimensional, regional- and local-scale numerical models for the development of Irish Zn-Pb-Ag mineralization. The calculations examined one of the proposed genetic models for the deposits in which mineralization was associated with mixing of two fluids of different salinities in the hanging walls of faults undergoing active deformation during mineralization. The numerical models involve coupling between volume changes arising from elastic-plastic deformation, single-phase fluid flow, and mixing of fluids with different salinities together with thermal transport both by conduction and by advection. We evaluated critical parameters that could optimize the rate of mineralization in three dimensions at the regional scale and in two dimensions at the local scale. Topographic relief is proposed as one driver of fluid flow. The models indicate a competing process, through convection of high-salinity fluids, mainly within faults and originating as partially evaporated seawater, which mix with lower salinity fluids driven by topographic relief. At the regional scale, although there was substantial fluid flow through the basinal sediments, considerable flow of basin-derived fluids also occurred through the basement, with subsequent focusing through faults. The effect was enhanced if the basement permeability was increased by fracturing. Basinal fluids (fluid 1, initially in the basin or introduced from meteoric sources in the adjacent highlands) were extensively exposed to the basement and became focused into both northward- and southward-dipping faults. A second, more saline fluid (fluid 2) was driven down into the rock column by convection; it had most exposure to the basinal sediments and, in some situations, to the basement before mixing with fluid 1. The positions of the fluid mixing sites involved a balance between topographically driven flow and convection in three dimensions at a regional scale. Changes in porosity and permeability induced by plastic deformation influenced the rates of mineralization at the local scale and local Darcy flow rates at mineralizing sites. Mineralization at the local scale resulted from the interplay of fault dip direction, displacement, deformation-induced dilation, seal thickness, and integrity. The modeling results are compatible with empirical data from the Irish ore field, suggesting that north-facing faults, dipping away from the inferred position of topographic highs, preferentially host economic deposits. There is nothing intrinsic in the modeling that constrains the timing of the mineralization relative to compressional or extensional events, although a body of research favors mineralization overlapping with extension, as at the Navan, Silvermines, and Tynagh deposits. In contrast, at the Lisheen deposit, mineralization postdates the main extensional fault offsets. We suggest that a switch to shortening is a possible driver for this mineralizing system.

Blundell, D.J., Neubauer, F. and Von Quadt, A. (Eds). The Timing and Location of Major Ore Deposits in an Evolving Orogen. Geological Society, London, Special Publications, 204. 2002. 358 pp. Price £85.00. ISBN 1-86239-122-X.

This excellent compilation of papers is based on a symposium organized by the European Science Foundation scientific programme GEODE in conjunction with the Society for Geology Applied to Mineral Deposits (SGA). GEODE was a five-year scientific programme from 1998–2002 to investigate geodynamics and ore deposit evolution based on five metallogenic provinces in Europe, namely the Alpine, Balkan, Carpathian, Dinaride (ABCD) Belt, Iberian Pyrite Belt, Urals, Fennoscandinavian Precambrian, together with the sediment-hosted deposits of Ireland and Czech Republic. Metallogenic studies in tectonically active regions of S. America and SE Asia supplemented the European work and added context. Blundell sets the scene with an introductory chapter covering the overall geodynamic context in which major ore deposits form in an evolving orogen. An interesting analogy is drawn between the hydrocarbon formation sequence of source, migration pathway …

Timing of mineralization at the Navan Zn-Pb deposit: A post-Arundian age for Irish mineralization

Navan is the largest of the Irish Zn-Pb deposits and contains some of the most important evidence for the inferred early timing of mineralization in Ireland (clasts of ore above a middle Carboniferous erosion surface). We have examined diagenetic fabrics within the main ore (pale beds) and the overlying erosion surface (Boulder Conglomerate) in order to determine the timing of mineralization. This diagenetic analysis (and particularly the presence of premineralization dolomite in the Boulder Conglomerate) has revealed that all of the mineralization postdates the erosion surface and therefore must be post-Arundian (345 Ma) in age. Furthermore, the Navan mineralization must be entirely epigenetic in origin (i.e., similar to Mississippi Valley–type deposits) and is likely to be Holkerian (343 Ma) in age or younger. This conclusion disagrees with the previous model of at least partially syngenetic mineralization, and has important implications for the age of other deposits in Ireland and the nature of “Irish-type” deposits in general.

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

A genetic model for mineralization of lower Windsor (Visean) carbonate rocks of Nova Scotia, Canada

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

Fluid flow connections to basement rocks below sedimentary basins: Evidence from the base metal deposits in Ireland

Lead isotope systematics in Irish base metal deposits and basement rocks indicate that lead and other metals were derived from local basement rocks with little or no addition from basinal sediments hosting the deposits. Thus, a fluid circulation model involving simple basin brine expulsion and an impermeable boundary at the basin‐basement interface cannot be invoked to explain the origin of the mineralization. Production of ore deposits as a result of transport out of the basement would involve a relatively trivial integrated heat addition as compared to the total heat transported within the basin on the time scale of deposit formation.

U-Pb age for some base-metal sulfide deposits in Ireland: Genetic implications for Mississippi Valley-type mineralization

Evidence is presented that links the timing of vein-type (Cu-Ag(U)) to stratiform Mississippi Valley-type (MVT, Pb-Zn) ore events in Ireland. The rare occurrence of pitchblende, coffinite(.), and brannerite mineralization, which is regarded as a precursor component to the sulfide mineralization in the Gortdrum deposit (Ireland), provides the first direct radiometric dating tool for these carbonate-hosted deposits. The U-Pb (340 +25/-20 Ma) and Pb-Pb (359 +/- 26 Ma) whole-rock ages constrain the uranium and base-metal mineralizing events to the Early Carboniferous. The data support a model according to which MVT and earlier uranium mineralization stages of some major ore bodies resulted from fracturing coincident with large basin-dewatering events. The Pb-Pb and concordia data are consistent with an Early Carboniferous age for the mineralization at Gortdrum and agree closely with a previously published Rb-Sr age of 359 +/- 22 Ma, obtained for Missouri glauconites. Furthermore, other comparative geologic data from Ireland and from North American MVT mineral provinces support a model of Pb-Zn-Cu(U) mobilization on a regional scale that implicates the later closing stages of the proto-Atlantic. 40 references, 3 figures, 1 table.

K–Ar ages of clay concentrates from Irish orebodies and their bearing on the timing of mineralisation

ABSTRACTThe K–Ar ages of 117 clay concentrates from samples associated with mineral deposits in Ireland indicate that most, if not all of the major ore deposits were formed during Carboniferous times or earlier. The Avoca orebodies are pre-400 Ma in age. Many of the vein deposits traversing Palaeozoic rocks were either formed or rejuvenated during the Hercynian orogeny c. 300 Ma ago. The SW Ireland Cu–Ba ores are 290 Ma old, and the major, frequently stratiform, base metal deposits stratabound in the Lower Carboniferous were, at least largely, formed during the Carboniferous period. In the case of the Navan orebody, an early Carboniferous age is indicated. Previous models invoking major mineralisations during Mesozoic or Tertiary times are rendered invalid. However, in some areas there appears to have been hydrothermal activity during the Triassic and possibly the Permian. The Triassic event is thought to be genetically related to coeval hydrothermal avtivity found throughout the N Atlantic regions. The data suggest that probably <10 km of cover has been removed from the majority of Ireland since Lower Carboniferous times. K–Ar dating of clays is shown to be a powerful method for constraining the ages of oredeposits.

Early Tertiary Pollen Deposits in Ireland

VERY little work has been published on pollen-analysis of Tertiary fresh-water deposits in the British Isles, although there is a considerable literature dealing with Continental work. Recently I attempted to date three Tertiary sites in Ireland by their fossil pollen content. They are: (1) a small deposit of lignitic clay at Ballymacadam, County Tipperary1; (2) the Lough Neagh Clays, an extensive deposit of clays, sands and lignites, about 400 metres thick in the borehole described by Wright2; (3) interbasaltic lignites of the Antrim basalt series3. The Lough Neagh Clays and underlying Antrim basalts occupy a very large area in north-east Ireland. None of the three deposits has been dated satisfactorily. Wright2 tentatively suggested an early Tertiary date for the Lough Neagh Clays; Simpson4 proposed a late Miocene or early Pliocene date for the interbasaltic lignites and an early Tertiary, perhaps late Eocene, age has been tentatively put forward for Ballymacadam1. I examined samples from Ballymacadam, from the interbasaltic beds at several sites, but especially from Craigahulliar quarry near Portrush, and from the Lough Neagh Clays from exposures at Thistleborough near Glenavy, County Antrim, and from boreholes at Washing Bay and Mire House, County Tyrone, which penetrated to the base of the clays. The Lough Neagh Clay samples, especially from Thistleborough, and the Ballymacadam samples contained abundant well-preserved pollen. The interbasaltic pollen was in poor condition.

III.—On the Occurrence of Megaceros Hibernicus, Owen, in the Ancient Lacustrine Deposits of Ireland; with Remarks on the Probable Age of these Beds

The old lacustrine deposits of Ireland do not seem to have attracted so much attention from geologists as they deserve. By the old lacustrine deposits, we understand the dried-up lakes at present occupied by peat-bogs, with their underlying beds of marl and clays. During the past century it was known that in these bogs, or in the marl-beds beneath them, were found antlers and bones of a gigantic extinct deer, which Professor Owen named Megaceros Hibernicus, and now popularly known as the “Irish Elk,”1 not that it was exclusively confined to Ireland, its remains having been found in England and Scotland, also in France and Germany, and it is even reported to occur in caves as far east as the Altai Mountains in Asia.

IV.—On Changes of Climate during the Glacial Epoch

In my last communication to the Magazine I made an attempt to correlate the Scottish Glacial deposits with the equivalent accumulations in Switzerland, Northern Europe, and North America, my purpose being to show that the same order of succession holds good in all those regions where the “Drifts” have been examined, and that in each case there is no proof whatever of any warm period having intervened since the deposition of the clays with Arctic shells and the decrease and disappearance of local glaciers. In the present and a subsequent paper I propose to treat of the superficial deposits of Ireland and England, more especially those of the latter country.

V.—Further Remarks on the Sequence of the Glacial Beds

The paper of Mr. Hull, “On the General Relations of the Drift Deposits of Ireland to those of Great Britain,” in the July number of the Geological Magazine, induces me to recur to one by myself on the Sequence of the Glacial Beds, Vol. VII p. 17 of the same Magazine.

II.—Observations on the General Relations of the Drift Deposits of Ireland to those of Great Britain

The valuable paper by Professor Harkness on “The Middle Pleistocene Deposits of Britain,” published in the pages of this periodical, was a completely successful attempt to synchronize the members of the Drift series of Ireland with those of England and Scotland. He shows that there exist in the East of Ireland detached portions of an Upper Boulder-clay resting on the shelly sands and gravels of the “ Middle Drift,” and that in other localities these sands and gravels may be observed resting on the Lower Boulder-clay, which is generally supported by rock with a glaciated surface. The order of succession of the members of the Drift series is, therefore, precisely similar to that of the North-western counties of England, where we have at the base (a) Lower Till or Boulder-clay; (b) Marine Sands and Gravels; (c) Upper Till or Boulder-clay; —an order of succession which I have for some years felt satisfied would be found ultimately to hold good over the whole of England.