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Pb-Zn em carbonatos. Irish Type

by Trygve Hõy
B.C. Geological Survey


Ref: Irlanda, carbonato, Zn-Pb, brecha, plataforma, esfalerita, galena

Hõy, Trygve (1996): Irish-type Carbonate-hosted Zn-Pb, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T, Editors, British Columbia Ministry of Employment and Investment, Open File 1996-13, pages 21-24.


SYNONYMS: Kootenay Arc Pb-Zn, Remac type.

COMMODITIES (BYPRODUCTS): Zn, Pb, Ag; (Cu, barite, Cd).

EXAMPLES (British Columbia (MINFILE #) - Canada/International): Reeves MacDonald (082FSW026), HB (082FSW004), Aspen (082FSW001), Jack Pot (082SW255), Jersey (082SW009), Duncan (082KSE020) , Wigwam (082KNW068); Navan, Lisheen, Tynagh, Silvermines, Galmoy, Ballinalack, Allenwood West (Ireland); Troya (Spain).


CAPSULE DESCRIPTION: Irish-type carbonate-hosted deposits are stratabound, massive sphalerite, galena, iron sulphide and barite lenses with associated calcite, dolomite and quartz gangue in dolomitized platformal limestones. Deposits are structurally controlled, commonly wedge shaped adjacent to normal faults. Deformed deposits are irregular in outline and commonly elongate parallel to the regional structural grain.

TECTONIC SETTING: Platformal sequences on continental margins which commonly overlie deformed and metamorphosed continental crustal rocks.

DEPOSITIONAL ENVIRONMENT/GEOLOGICAL SETTING: Adjacent to normal growth faults in transgressive, shallow marine platformal carbonates; also commonly localized near basin margins.

AGE OF MINERALIZATION: Known deposits are believed to be Paleozoic in age and younger than their host rocks; Irish deposits are hosted by Lower Carboniferous rocks; Kootenay Arc deposits are in the Lower Cambrian.

HOST/ASSOCIATED ROCK TYPES: Hosted by thick, non-argillaceous carbonate rocks; these are commonly the lowest pure carbonates in the stratigraphic succession. They comprise micritic and oolitic beds, and fine-grained calcarenites in a calcareous shale, sandstone, calcarenite succession. Underlying rocks include sandstones or argillaceous calcarenites and shales. Iron formations, comprising interlayered hematite, chert and limestone, may occur as distal facies to some deposits. Deformed Kootenay Arc deposits are enveloped by fine-grained grey, siliceous dolomite that is generally massive or only poorly banded and locally brecciated.

DEPOSIT FORM: Deposits are typically wedge shaped, ranging from over 30 m thick adjacent to, or along growth faults, to 1-2 cm bands of massive sulphides at the periphery of lenses. Economic mineralization rarely extends more than 200 m from the faults. Large deposits comprise individual or stacked sulphide lenses that are roughly concordant with bedding. In detail, however, most lenses cut host stratigraphy at low angles. Contacts are sharp to gradational. Deformed deposits are typically elongate within and parallel to the hinges of tight folds. The Reeves MacDonald deposit forms a syncline with a plunge length of approximately 1500 m and widths up to 25 m. Others (HB) are elongate parallel to a strong mineral lineation. Individual sulphide lenses are irregular, but typically parallel to each other and host layering, and may interfinger or merge along plunge.

TEXTURE/STRUCTURE: Sulphide lenses are massive to occassionally well layered. Typically massive sulphides adjacent to faults grade outward into veinlet- controlled or disseminated sulphides. Colloform sphalerite and pyrite textures occur locally. Breccias are common with sulphides forming the matrix to carbonate (or as clasts?). Sphalerite-galena veins, locally brecciated, commonly cut massive sulphides. Rarely (Navan), thin laminated, graded and crossbedded sulphides, with framboidal pyrite, occur above more massive sulphide lenses. Strongly deformed sulphide lenses comprise interlaminated sulphides and carbonates which, in some cases (Fyles and Hewlett, 1959), has been termed shear banding.

ORE MINERALOGY (Prinicipal and subordinate): Sphalerite, galena; barite, chalcopyrite, pyrrhotite, tennantite, sulfosalts, tetrahedrite, chalcopyrite.

GANGUE MINERALOGY (Prinicipal and subordinate): Dolomite, calcite, quartz, pyrite, marcasite; siderite, barite, hematite, magnetite; at higher metamorphic grades, olivine, diopside, tremolite, wollastonite, garnet.

ALTERATION MINERALOGY: Extensive early dolomitization forms an envelope around most deposits which extends tens of metres beyond the sulphides. Dolomitization associated with mineralization is generally fine grained, commonly iron-rich, and locally brecciated and less well banded than limestone. Mn halos occur around some deposits; silicification is local and uncommon. Fe in iron formations is distal.

WEATHERING: Gossan minerals include limonite, cerussite, anglesite, smithsonite, hemimorphite, pyromorphite.

ORE CONTROLS: Deposits are restricted to relatively pure, shallow-marine carbonates. Regional basement structures and, locally, growth faults are important. Orebodies may be more common at fault intersections. Proximity to carbonate bank margins may be a regional control in some districts.

GENETIC MODEL: Two models are commonly proposed: (1) syngenetic seafloor deposition: evidence inludes stratiform geometry of some deposits, occurrence together of bedded and clastic sulphides, sedimentary textures in sulphides, and, where determined, similar ages for mineralization and host rocks. (2) diagenetic to epigenetic replacement: replacement and open-space filling textures, lack of laminated sulphides in most deposits, alteration and mineralization above sulphide lenses, and lack of seafloor oxidation.

ASSOCIATED DEPOSIT TYPES: Mississippi Valley type Pb-Zn (E12), sediment-hosted barite (E17), sedimentary exhalative Zn-Pb-Ag , possibly carbonate-hosted disseminated Au-Ag .

COMMENTS: Although deposits such as Tynagh and Silvermines have structures and textures similar to sedex deposits, and are associated with distal iron formations, they are included in the Irish-type classification as recent work (e.g., Hizman, 1995) concludes they formed by replacement of lithified rocks. Deposits that can be demonstrated to have formed on the seafloor are not included in Irish- type deposits. It is possible that the same continental margin carbonates may host sedex , Irish-type and Mississippi Valley-type (E12) deposits.

EXPLORATION GUIDES GEOCHEMICAL SIGNATURE: Elevated base metal, Ag and Mn values in both silt and soil samples; however, high carbonate content, and hence high Ph may reduce effectiveness of stream silts.

GEOPHYSICAL SIGNATURE: Induced polarization surveys are effective and ground electromagnetic methods may work for deposits with iron sulphides. Deposits can show up as resistivity lows and gravity highs.

OTHER EXPLORATION GUIDES: The most important control is stratigraphic. All known deposits are in carbonate rocks, commonly the lowest relatively pure carbonate in a succession. Other guides are proximity to growth faults and intersection of faults, regional and local dolomitization and possibly laterally equivalent iron formations.


TYPICAL GRADE AND TONNAGE: Irish deposits are typically less than 10 Mt with 5-6% Zn, 1-2% Pb and 30g/t Ag. Individual deposits can contain up to 90 g/t Ag. The largest, Navan, produced 36 Mt and has remaining reserves of 41.8 Mt containing 8% Zn and 2% Pb. Mined deposits in the Kootenay Arc averaged between 6 and 7 Mt and contained 3-4 % Zn, 1-2 % Pb, and 3-4 g/t Ag. Duncan has reserves of 2.76 Mt with 3.3% Pb and 3.1% Zn; Wigwam contains 8.48 Mt with 2.14% Pb and 3.54% Zn.

ECONOMIC LIMITATIONS: These deposits are attractive because of their simple mineralogy and polymetallic nature, although significantly smaller than sedex deposits. In British Columbia the Kootenay Arc deposits are generally lower grade with up to only 6 % Pb+Zn. These deposits are also structurally complex making them more complicated to mine. IMPORTANCE: Production from these deposits makes Ireland a major world zinc producer. Recent discovery of concealed deposits (Galmoy in 1986 and Lisheen in 1990) assures continued production. In British Columbia, a number of these deposits were mined intermittently until 1979 when H.B. finally closed. Some still have substantial lead and zinc reserves. However, their current potential for development is based largely on the precious metal content. The high carbonate content of the gangue minimizes acid-rock drainage problems.


Addie, G.G. (1970): The Reeves MacDonald Mine, Nelway, British Columbia; in Pb-Zn Deposits in the Kootenay Arc, N.E. Washington and adjacent British Columbia; Department of Natural Resources, State of Washington, Bulletin 61, pages 79-88.

Fyles, J.T. (1970): Geological Setting of Pb-Zn Deposits in the Kootenay Lake and Salmo Areas of B.C.; in Pb-Zn Deposits in the Kootenay Arc, N.E. Washington and Adjacent British Columbia; Department of Natural Resources, State of Washington, Bulletin 61, pages 41-53.

Fyles, J.T. and Hewlett, C.G. (1959): Stratigraphy and Structure of the Salmo Lead-Zinc Area; B. C. Department of Mines, Bulletin 41, 162 pages.

Hitzman, M.W. (1995): Mineralization in the Irish Zn-Pb-(Ba-Ag) Orefield; in Irish Carbonate-hosted Zn-Pb Deposits, Anderson K., Ashton J., Earls G., Hitzman M., and Sears S., Editors, Society of Economic Geologists, Guidebook Series, Volume 21, pages 25-61.

Hitzman, M.W. (1995): Geological Setting of the Irish Zn-Pb-(Ba-Ag) Orefield; in Irish Carbonate-hosted Zn-Pb Deposits, Anderson, K., Ashton, J., Earls, G., Hitzman, M., and Sears, S., Editors, Society of Economic Geologists, Guidebook Series, Volume 21, pages 3-24.

Höy, T. (1982): Stratigraphic and Structural Setting of Stratabound Lead- Zinc Deposits in Southeastern British Columbia; Canadian Institute of Mining and Metallurgy, Bulletin, Volume 75, pages 114-134.

Nelson, J.L. (1991): Carbonate-hosted Lead-Zinc Deposits of British Columbia; in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 71-88.

Sangster, D.F. (1970): Metallogenesis for some Canadian Lead-zinc Deposits in Carbonate Rocks; Geological Association of Canada, Proceedings, Volume 22, pages 27-36.

Sangster, D.F. (1990): Mississippi Valley-type and Sedex Lead-Zinc Deposits: a Comparative Examination; Transactions of the Institution of Mining and Metallurgy, Section B, Volume 99, pages B21-B42. T. Hoy Draft 3: March 27, 1996

DEPÓSITOS - 26/04/2004 18:22:00

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