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Silvermines - Mogul, Magcobar |
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Ireland |
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Zn Pb Ag Ba
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Super Porphyry Cu and Au
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The Silvermines Pb-Zn-Ag-Ba deposit is located ~6 km west of the village of Silvermines in County Tipperary, in central-southern Ireland and occurs as a series of bodies (#Location; 52° 47' 40"N, 8° 16' 5"W).
The earliest record of mining in the Silvermines district was 1289, but was short-lived. Mining resumed in the 17th century and continued intermittently until 1874. It restarted in 1949, and shortly after a large deposit of baryte was found and opencast mining of this began in 1963 by Magcobar (Ireland) Ltd. Soon after, a large tonnage lead and zinc was also discovered, which was worked underground from 1968 to 1982 by Mogul of Ireland Ltd. The Magcobar mine was closed in September 1992 and the shafts flooded. The site has since been rehabilitated and occupied by a steel fabricator.
The base metal sulphide mineralisation of the Irish Midlands are predominantly hosted by a succession of transgressive Mississippian carbonates and are interpreted to be spatially associated with a series of late Devonian-Early Carboniferous extensional basins and their bounding fault systems (Hitzman and Beaty, 1996; Johnston et al., 1996; Kyne et al., 2019). Onset of the Variscan compressive event at 314 ±1 Ma (Quinn et al., 2005) reactivated both crustal-scale Caledonian faults within basement rocks along the Iapetus Suture zone but also extensional fault systems in the overlying Devonian-Carboniferous sedimentary succession. Sulphide deposits in the Irish Midlands are interpreted to be spatially associated with these extensional fault systems acting as hydrothermal fluid conduits (Johnston et al., 1996; Hitzman, 1999). Fluids infiltrating the shallow water, carbonate mudbanks that constitute the widespread Carboniferous Waulsortian Limestone Formation, interpreted to have resulted in the formation of black matrix breccia (Hitzman et al., 1992), previously known as 'dolomite breccia' at the Silvermines deposit (Andrew, 1986). This black matrix breccia is composed of clasts of undolomitized and dolomitized Waulsortian Limestone Formation, set in a very fine grained dolomitic matrix with trace pyrite, quartz, barite and apatite that was preferentially replaced by sulphide minerals (after Vafeas et al., 2023).
The Silvermines mineralisation comprises several zones of massive sulphides, predominantly pyrite, sphalerite and galena, as well as an adjacent deposit that are primarily composed of barite, within the Magcobar zone (Andrew, 1986). These zones, which are found at the base of the Waulsortian Limestone Formation, beneath and within black matrix breccia, dip gently to the north within the Kilmastulla syncline. However, the black matrix breccia is more widespread than the well-mineralised zones, and is spatially distributed along and within the hanging walls of a set of West to NW trending extensional faults forming a linked normal fault array (Kyne et al., 2019; after Vafeas et al., 2023).
Hitzman et al. (1995) demonstrated that the mineralisation and alteration at Silvermines were initiated by the development of replacive silica-hematite following the base of the Waulsortian Limestone Formation, as well as within the upper portion of the underlying Ballysteen Formation near the Magcobar barite deposit. The Ballysteen Formation comprises light-grey and dark-grey, fine-grained argillaceous bioclastic limestone alternating with calcareous mudstone. This replacive silica-hematite was apparently followed by the formation of the black matrix breccia at the base of the Waulsortian Limestone Formation throughout the deposit area. The Magcobar barite orebody occurs as a stratabound sheet at the base of the Waulsortian Limestone Formation, beneath the breccia. There are apparently no obvious crosscutting relationships between the barite mass and the black matrix breccia. The barite does, however, cross-cut and brecciate earlier-formed silica-hematite altered zones. The barite mineralisation is cut by pyrite ±marcasite veins, some of which contain trace sphalerite ±galena. The Silvermines base metal deposits occur as largely stratabound sheets at the base of the Waulsortian Limestone Formation, at the same stratigraphic level as the Magcobar barite deposit (Andrew, 1986; after Vafeas et al., 2023).
Mineralisation was found in two main forms, namely:
i). concordant, tabular, shallow dipping ore lenses within the early Carboniferous carbonate mudbanks of the Waulsortian facies - with Pb-Zn mineralisation and accompanying barite, pyrite and siderite, in a range of facies. Sulphides occur as massive stratabound lenses as well as zones of partial replacement of portions of the adjacent black matrix breccia (Hitzman and Beaty, 1996) where it forms the breccia matrix, separating clasts in debris flow breccias but also to a lesser extent in finely laminated carbonates.
ii). transgressive, fault controlled vein mineralisation cutting the footwall carbonates and shales of the Waulsortian, down to and including the underlying Upper Devonian Old Red Sandstone. This mineralisation is associated with replacive, ferroan hydrothermal dolostone within the Ballysteen Formation along the G fault, with adjacent disseminated sulphides in the Shallee area south and west of the G fault (Andrew, 1986; Kyne et al., 2019). Sulphide zones are locally crosscut by late dolomite-sulphide veins and veinlets.
A third form comprised the 'residual ores' consisting mainly of smithsonite and hemimorphite with cerussite and hydrocerussite. Four significant deposits occur in the district all hosted by the Lower (Courceyan) Dolomite horizon. Three are in the hangingwall of the Silvermines Fault, while the fourth is some distance from the fault. In all examples the mineralisation is only poorly consolidated and concentrated in weathered out, steeply sided troughs replacing the primary sulphides and locally the host carbonates. Grades range from 3% Pb and 2 to 3% Zn in the gossan to 8% Pb and 15 to 20% Zn in the smithsonite and hemimorphite deposits. Hemimorphite and geothite are found at surface and smithsonite at depth with supergene siderite.
U-Pb dating of unusually coarse-grained apatite crystals from hydrothermal dolostone breccia in the barite-rich Magcobar zone at the Silvermines deposit indicates an age of 331 ±5.6 Ma for hydrothermal alteration. This age is in agreement with an Re-Os dating of pyrite-sphalerite but differs from previous estimates that were based on palaeo-magnetism and sphalerite Rb-Sr geochronology at Silvermines. This U-Pb age is consistent with the sampled section of the deposit being largely formed epigenetically rather than as a syn-sedimentary deposit (Vafeas et al., 2023).
Total production + reserves after Andrew (1986) were:
17.679 Mt @ 2.53% Pb, 6.43% Zn, 23 g/t Ag; comprising
Concordant mineralisation
• Upper G zone - 7.977 Mt @ 2.10% Pb, 8.18% Zn, 22 g/t Ag;
• B zone - 4.639 Mt @ 3.38% Pb, 4.53% Zn, 30 g/t Ag;
• Magcobar South zone - 0.327 Mt @ 1.7% Pb, 4.72% Zn, 26 g/t Ag.
• Magcobar Barite zone 5 Mt @ 85% BaSO4;
• B zone barite 0.5 Mt @ 75% BaSO4
Transgressive mineralisation
• Lower G zone - 1.965 Mt @ 3.98% Pb, 6.58% Zn, 34 g/t Ag;
• K zone - 1.434 Mt @ 1.38% Pb, 4.40% Zn, 15 g/t Ag;
• P zone - 0.562 Mt @ 1.23% Pb, 4.01% Zn, 17 g/t Ag.
Transgressive mineralisation in basal clastics and Silurian
• C zone - 0.433 Mt @ 1.19% Pb, 6.06% Zn, 32 g/t Ag;
• K zone - 0.342 Mt @ 1.88% Pb, 5.68% Zn, 64 g/t Ag.
Non-sulphide mineralisation is estimated to have totalled 1 Mt @ ~21% Zn.
The Mogul mine produced 10.784 Mt @ 2.7% Pb, 7.36% Zn between 1968 and 1982 (Andrew, 1986).
Remaining resources in 1986 amounted to 6.985 Mt @ 2.26% Pb, 4.98% Zn (Anderew, 1986 after Ennex International, plc).
To 1986, the Magcobar operation had produced 4 Mt @ 85% BaSO4 (Andrew, 1986).
For more detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2023.
Record last updated: 11/9/2023
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Silvermines - Magcobar
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Selected References:
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Andrew C J 1986 - The tectono-stratigraphic controls to mineralization in the Silvermines area, County Tipperary, Ireland: in Andrew C J, Crowe R W A, Finlay S, Pennell W M, Pyne J F (Ed.s) 1986 Geology and Genesis of Mineral Deposits in Ireland Irish Association for Economic Geology, Dublin pp 377-417
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Boland M B, Clifford J A, Meldrum A H, Poustie A 1992 - Residual base metal and barite mineralization at Silvermines, Co. Tipperary, Ireland: in Bowden A A, Earls G, O Connor P G, Pyne J F 1992 The Irish Minerals Industry 1980-1990 Irish Association for Economic Geology pp 247-260
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Boni M, Large D 2003 - Nonsulfide zinc mineralization in Europe: an overview: in Econ. Geol. v98 pp 715-729
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Boyce A J, Little C T S, Russell M J 2003 - A new fossil vent biota in the Ballynoe Barite deposit, Silvermines, Ireland: evidence for intracratonic sea-floor hydrothermal activity about 352 Ma: in Econ. Geol. v98 pp 649-656
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Hitzman M W 1995 - Mineralisation in the Irish Zn-Pb-(Ba-Ag) Orefield: in Anderson K, Ashton J, Earls G, Hitzman M, Tear S (Eds.), Irish Carbonate Hosted Zn-Pb Deposits SEG Guidebook Series, Littleton, Colorado, USA v21 pp 25-61
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Hitzman M W 1995 - Geological setting of the Irish Zn-Pb-(Ba-Ag) Orefield: in Anderson K, Ashton J, Earls G, Hitzman M, Tear S (Eds.), Irish Carbonate Hosted Zn-Pb Deposits SEG Guidebook Series, Littleton, Colorado, USA v21 pp 3-23
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Kyne, R., Torremans, K., Guven, J.F., Doyle, R. and Walsh, J.J., 2019 - 3-D Modeling of the Lisheen and Silvermines Deposits, County Tipperary, Ireland:Insights into Structural Controls on the Formation of Irish Zn-Pb Deposits: in Econ. Geol. v.114, pp. 93-116.
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Lee M J, Wilkinson J J 2002 - Cementation, hydrothermal alteration and Zn-Pb mineralization of carbonate breccias in the Irish Midlands: textural evidence from the Cooleen zone, near Silvermines, County Tipperary: in Econ. Geol. v97 pp 653-662
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LeHuray A P, Caulfield J B D, Rye D M, Dixon P R 1987 - Basement controls on sediment-hosted Zn-Pb deposits: a Pb isotope study of Carboniferous mineralization in central Ireland: in Econ. Geol. v82 pp 1695-1709
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Phillips W E A, Sevastopulo G D 1986 - The stratigraphy and structural setting of Irish mineral deposits: in Andrew C J, Crowe R W A, Finlay S, Pennell W M, Pyne J F (Eds.), Geology and Genesis of Mineral Deposits in Ireland Irish Association for Economic Geology, Dublin pp 1-30, xvi
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Reed C P,Ê Wallace M W 2004 - Zn-Pb mineralisation in the Silvermines district, Ireland: a product of burial diagenesis: in Mineralium Deposita v39 pp 87-102
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Samson I M, Russell M J 1987 - Genesis of the Silvermines Zinc-Lead-Barite deposit, Ireland: fluid inclusion and stable isotope evidence: in Econ. Geol. v82 pp 371-394
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Taylor S 1984 - Structural and paleotopographic controls of Lead-Zinc mineralization in the Silvermines orebodies, Republic of Ireland: in Econ. Geol. v79 pp 529-548
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Torremans, K., Kyne, R., Doyle, R., Guven, J.F. and Walsh, J.J., 2018 - Controls on Metal Distributions at the Lisheen and Silvermines Deposits: Insights into Fluid Flow Pathways in Irish-Type Zn-Pb Deposits: in Econ. Geol. v.113, pp. 1455-1477
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Vafeas, N.A., Slezak, P., Chew, D., Brodbeck, M., Hitzman, M.W. and Hnatyshin, D., 2023 - U-Pb Dating of apatite from Silvermines deposit, Ireland: a model for hydrothermal ore genesis: in Econ. Geol. v.118, pp. 1521-1527. doi: 10.5382/econgeo.5016.
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Wilkinson J J 2003 - On diagenesis, dolomitisation and mineralisation in the Irish Zn-Pb orefield: in Mineralium Deposita v38 pp 968-983
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Wilkinson JJ, 2010 - A Review of Fluid Inclusion Constraints on Mineralization in the Irish Ore Field and Implications for the Genesis of Sediment-Hosted Zn-Pb Deposits : in Econ. Geol. v105 pp. 417-442
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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