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MacIntyre, Don (1995): Sedimentary
Exhalative Zn-Pb-Ag, in Selected British Columbia Mineral Deposit Profiles,
Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors,
British Columbia Ministry of Energy of Employment and Investment, Open
File 1995-20, pages 37-39.
IDENTIFICATION
SYNONYMS: Shale-hosted Zn-Pb-Ag;
sediment-hosted massive sulphide Zn-Pb-Ag; Sedex Zn- Pb.
COMMODITIES (BYPRODUCTS): Zn,
Pb, Ag, (minor Cu, barite).
EXAMPLES (British Columbia - Canada/International):
Cirque, Sullivan, Driftpile; Faro, Grum, Dy, Vangorda, Swim, Tom and
Jason (Yukon, Canada), Red Dog (Alaska, USA), McArthur River and Mt. Isa (Australia);
Megen and Rammelsberg (Germany).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Beds and
laminations of sphalerite, galena, pyrite, pyrrhotite and rare
chalcopyrite, with or without barite, in euxinic clastic marine
sedimentary strata.. Deposits are typically tabular to lensoidal in shape
and range from centimetres to tens of metres thick. Multiple horizons may
occur over stratigraphic intervals of 1000 m or more.
TECTONIC SETTING: Intracratonic or
continental margin environments in fault-controlled basins and troughs.
Troughs are typically half grabens developed by extension along
continental margins or within back-arc basins.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: Restricted second and third order basins within linear,
fault-controlled marine, epicratonic troughs and basins. There is often
evidence of penecontemporaneous movement on faults bounding sites of
sulphide deposition. The depositional environment varies from deep,
starved marine to ? shallow water restricted shelf.
AGE OF MINERALIZATION: The major
metallogenic events are Middle Proterozoic, Early Cambrian, Early Silurian
and Middle to Late Devonian to Mississippian. The Middle Proterozoic and
Devonian-Mississippian events are recognized worldwide. In the Canadian
Cordillera, minor metallogenic events occur in the Middle Ordovician and
Early Devonian.
HOST/ASSOCIATED ROCK TYPES: The most
common hostrocks are those found in euxinic, starved basin environments,
namely, carbonaceous black shale, siltstone, cherty argillite and chert.
Thin interbeds of turbiditic sandstone, granule to pebble conglomerate,
pelagic limestone and dolostone, although volumetrically minor, are common.
Evaporites, calcareous siltstone and mudstone are common in shelf settings.
Small volumes of volcanic rocks, typically tuff and submarine mafic flows,
may be present within the host succession. Slump breccia, fan
conglomerates and similar deposits occur near synsedimentary growth faults.
Rapid facies and thickness changes are found near the margins of second
and third order basins. In some basins high-level mafic sills with minor
dikes are important.
DEPOSIT FORM: These deposits are
stratabound, tabular to lens shaped and are typically comprised of many
beds of laminae of sulphide and/or barite. Frequently the lenses are
stacked and more than one horizon is economic. Ore lenses and mineralized
beds often are part of a sedimentary succession up to hundreds of metres
thick. Horizontal extent is usually much greater than vertical extent.
Individual laminae or beds may persist over tens of kilometres within the
depositional basin.
TEXTURE/STRUCTURE: Sulphide and
barite laminae are usually very finely crystalline where deformation is
minor. In intensely folded deposits, coarser grained, recrystallized zones
are common. Sulphide laminae are typically monomineralic.
ORE MINERALOGY (Principal and
subordinate): The principal sulphide minerals are pyrite,
pyrrhotite, sphalerite and galena. Some deposits contain significant
amounts of chalcopyrite, but most do not. Barite may or may not be
a major component of the ore zone. Trace amounts of marcasite,
arsenopyrite, bismuthinite, molybdenite, enargite, millerite, freibergite,
cobaltite, cassiterite, valleriite and melnikovite have been reported
from these deposits. These minerals are usually present in very minor
amounts.
ALTERATION MINERALOGY: Alteration
varies from well developed to nonexistent. In some deposits a stockwork
and disseminated feeder zone lies beneath, or adjacent to, the stratiform
mineralization. Alteration minerals, if present, include silica,
tourmaline, carbonate, albite, chlorite and dolomite. They formed in a
relatively low temperature environment. Celsian, Ba-muscovite and ammonium
clay minerals have also been reported but are probably not common.
ORE CONTROLS: Favourable sedimentary
sequences, major structural breaks, basins.
GENETIC MODEL: The deposits
accumulate in restricted second and third order basins or half grabens
bounded by synsedimentary growth faults. Exhalative centres occur along
these faults and the exhaled brines accumulate in adjacent seafloor
depressions. Biogenic reduction of seawater sulphate within an anoxic
brine pool is believed to control sulphide precipitation.
ASSOCIATED DEPOSIT TYPES: Associated
deposit types include carbonate-hosted sedimentary exhalative, such as the
Kootenay Arc and Irish deposits, bedded bariteand iron formation.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: The deposits
are typically zoned with Pb found closest to the vent grading outward and
upward into more Zn-rich facies. Cu is usually found either within the
feeder zone of close to the exhalative vent. Barite, exhalative chert and
hematite-chert iron formation, if present, are usually found as a distal
facies. Sediments such as pelagic limestone interbedded with the ore zone
may be enriched in Mn. NH3 anomalies have been documented at some deposits,
as have Zn, Pb and Mn haloes. The host stratigraphic succession may also
be enriched in Ba on a basin-wide scale.
GEOPHYSICAL SIGNATURE: Airborne and
ground geophysical surveys, such as electromagnetics or magnetics should
detect deposits that have massive sulphide zones, especially if these are
steeply dipping. However, the presence of graphite-rich zones in the host
sediments can complicate the interpretation of EM conductors. Also, if the
deposits are flat lying and comprised of fine laminae distributed over a
significant stratigraphic interval, the geophysical response is usually
too weak to be definitive. Induced polarization can detect flat-lying
deposits, especially if disseminated feeder zones are present.
OTHER EXPLORATION GUIDES: The
principal exploration guidelines are appropriate sedimentary environment
and stratigraphic age. Restricted marine sedimentary sequences deposited
in an epicratonic extensional tectonic setting during the Middle
Proterozoic, Early Cambrian, Early Silurian or Devono-Mississippian ages
are the most favourable.
ECONOMIC FACTORS
GRADE AND TONNAGE: The median
tonnage for this type of deposit worldwide is 15 Mt, with 10 % of deposits
in excess of 130 Mt (Briskey, 1986). The median grades worldwide are Zn -
5.6%, Pb - 2.8% and Ag - 30 g/t. The Sullivan deposit, one of the largest
deposits of this type ever discovered, has a total size of more than 155
Mt grading 5.7% Zn, 6.6% Pb and 7 g/t Ag. Reserves at the Cirque are 32.2
Mt grading 7.9% Zn, 2.1% Pb and 48 g/t Ag.
ECONOMIC LIMITATIONS: The large,
near-surface deposits are amenable to high volume, open pit mining
operations. Underground mining is used for some deposits.
IMPORTANCE: Sedimentary exhalative
deposits currently produce a significant proportion of the world’s Zn and
Pb. Their large tonnage potential and associated Ag values make them an
attractive exploration target.
REFERENCES
Briskey, J.A. (1986): Descriptive
Model of Sedimentary Exhalative Zn-Pb; in Mineral Deposit Models, Cox,
D.P. and Singer, D.A., Editors, U.S. Geological Survey, Bulletin
1693, 379 pages.
Carne, R.C. and Cathro, R.J. (1982):
Sedimentary-exhalative (Sedex) Zn-Pb-Ag Deposits, Northern Canadian
Cordillera; Canadian Institute of Mining and Metallurgy, Bulletin,
Volume 75, pages 66-78.
Gustafson, L.B. and Williams, N. (1981):
Sediment-hosted Stratiform Deposits of Copper, Lead and Zinc; in Economic
Geology Seventy-fifth Anniversary Volume, 1905-1980, Skinner, B.J.,
Editor, Economic Geology Publishing Co., pages 139-178.
Large, D.E. (1981): Sediment-hosted
Submarine Exhalative Sulphide Deposits - a Review of their Geological
Characteristics and Genesis; in Handbook of Stratabound and Stratiform Ore
Deposits, Wolfe, K.E., Editor, Geological Association of Canada,
Volume 9, pages 459-507.
Large, D.E. (1983): Sediment-hosted
Massive Sulphide Lead-Zinc Deposits; in Short Course in Sedimentary
Stratiform Lead-Zinc Deposits, Sangster, D.F., Editor, Mineralogical
Association of Canada, pages 1-29.
MacIntyre, D.G. (1991): Sedex -
Sedimentary-exhalative Deposits, in Ore Deposits, Tectonics and
Metallogeny in the Canadian Cordillera, McMillan, W.J., Coordinator, B.
C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4,
pages 25- 69.
Sangster, D.F. (1986):
Classifications, Distribution and Grade-Tonnage Summaries of Canadian
Lead-Zinc Deposits; Geological Survey of Canada, Economic Geology
Report 37, 68 pages. |
