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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.
IDENTIFICATION
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).
GEOLOGICAL
CHARACTERISTICS
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.
ECONOMIC FACTORS
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.
REFERENCES
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 |
