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Lefebure, D.V.and Alldrick, D.J. (1996):
Sediment-hosted Cu+/-Ag+/-Co, 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 13-16.
IDENTIFICATION
SYNONYMS: Sediment-hosted stratiform
copper, shale-hosted copper, Kupferschiefer-type, redbed Cu, Cu-shale,
sandstone Cu.
COMMODITIES (BYPRODUCTS): Cu,
Ag (Co, Pb, Zn, rarely PGE, Au, U, Va).
EXAMPLES (British Columbia (MINFILE #) -
Canada/International): Roo (082GSW020), Commerce (082GSE065),
Chal 4 (092K068); Redstone (Northwest Territories, Canada), Kennicott
(Alaska, USA), Spar Lake (Troy), Rock Creek and Montanore (Montana, USA),
White Pine (Michigan, USA), Creta (Oklahoma, USA), Corocoro (Bolivia),
Mansfield-Sangerhausen and Spremberg, Kupferschiefer district (Germany),
Konrad and Lubin (Poland), Dzherkazgan (Kazakhstan), Copper Claim (Australia),
Kamoto and Shaba, Zambia-Zaire copperbelt (Zaire).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Stratabound
disseminations of native copper, chalcocite, bornite and chalcopyrite in a
variety of continental sedimentary rocks including black shale, sandstone
and limestone. These sequences are typically underlain by, or interbedded
with, redbed sandstones with evaporite sequences. Sulphides are typically
hosted by grey, green or white strata.
TECTONIC SETTINGS: Predominantly
rift environments located in both intracontinental and continental-margin
settings; they can also occur in continental-arc and back- arc settings.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: The characteristic presence of redbed and evaporite sequences
points to deposition of sediments in a hot, arid to semi-arid paleoclimate
near the paleoequator. The host rocks are produced in a variety of local
anoxic depositional environments, including deltaic sediments, Sabkha-type
lagoonal carbonate basins or high intertidal mudflats, and shallow “coal
basins”.
AGE OF MINERALIZATION: Proterozoic
or younger; Middle Proterozoic, Permian and early Mesozoic most favourable
ages.
HOST/ASSOCIATED ROCK TYPES: Most
deposits are hosted by pale gray to black shale, but some are found in
sandstone, siltstone, limestone, silty dolomite, laminated carbonate units
(sabkha origin) and quartzites. Favourable horizons contain reactive
organic matter or sulphur. Algal mats, mudcracks and scour-and-fill
structures indicative of shallow-water deposition are common. Local
channel- conglomerate beds sometimes contain wood fragments. The
associated sequence includes redbed sediments, evaporites and sometimes
volcanics. In many cases the rift-related layered rocks rest unconformably
on older basement rocks.
DEPOSIT FORM: Orebodies are
generally conformable with the bedding, although in detail ore may
transgress bedding at low angles and is typically more transgressive near
the margins of the deposit. Mineralized horizons are from tens of
centimetres to several metres thick (rarely more than 5 m); they are often
contained within broader zones of anomalous copper values. Tabular ore
zones extend laterally for kilometres to tens of kilometres. Less commonly
the deposits are elongate lobes. Some deposits have a C-shaped, “roll
front” configuration in cross-section. Common lateral and/or vertical
zoning is from hematite (barren) > chalcocite > bornite > chalcopyrite >
pyrite, or from a chalcociteñbornite core grading to chalcopyrite with
peripheral galena and sphalerite.
TEXTURE/STRUCTURE: Sulphides are
fine grained and occur as disseminations, concentrated along bedding,
particularly the coarser grained fractions, or as intergranular cement.
Sharp-walled cracks or veinlets (< 1 cm thick, < than a metre in length)
of chalcopyrite, bornite, chalcocite, galena, sphalerite or barite with
calcite occur in some deposits, but are not an important component of the
ore. Pyrite can be framboidal or colloform. Cu minerals often replace
pyrite grains, framboids and nodules; less commonly they form pseudomorphs
of sulphate nodules or blade-shaped gypsum/anhydrite grains. They also
cluster around carbonaceous clots or fragments.
ORE MINERALOGY (Principal and
subordinate): Chalcocite, bornite and chalcopyrite; native copper
in some deposits. Pyrite is abundant in rocks outside the ore zones.
Enargite, digenite, djurleite, sphalerite, galena, tennantite, native
silver with minor Co-pyrite and Ge minerals. In many deposits
carrollite (CuCo2S4) is a rare mineral, however, it is common in the
Central African Copperbelt.
GANGUE MINERALOGY (Principal and
subordinate): Not well documented; in several deposits carbonate,
quartz and feldspar formed synchronously with the ore minerals and exhibit
zonal patterns that are sympathetic with the ore minerals. They infill,
replace or overgrow detrital or earlier authigenic phases.
ALTERATION MINERALOGY: Lateral or
underlying reduced zones of green, white or grey colour in redbed
successions. In the Montana deposits these zones contain chlorite,
magnetite and/or pyrite. Barren, hematite-rich, red zones grade into ore
in the Kupferschiefer. Kupferschiefer ore hosts also show elevated
vitrinite reflectances compared to equivalent stratigraphic units.
WEATHERING: Surface exposures may be
totally leached or have malachite and azurite staining. Near surface
secondary chalcocite enrichment is common.
ORE CONTROLS: Most sediment-hosted
Cu deposits are associated with the sag phase of continental rifts
characterized by deposition of shallow-water sediments represented by
redbed sequences and evaporites. These formed in hot, arid to semi-arid
paleoclimates which normally occur within 20-30øof the paleoequator.
Hostrocks are typically black, grey or green reduced sediments with
disseminated pyrite or organics. The main control on fluid flow from the
source to redoxcline is primary permeability within specific rock units,
commonly coarse-grained sandstones. In some districts deposits are located
within coarser grained sediments on the flanks of basement highs. Growth
faults provide local controls in some deposits (e.g., Spar Lake).
ASSOCIATED DEPOSIT TYPES: Sandstone
U (D05), volcanic redbed Cu , Kipushi Cu-Pb-Zn evaporite halite, sylvite, gypsum and anhydrite; natural gas in Poland.
GENETIC MODELS: Traditionally these
deposits have been regarded as syngenetic, analogous to sedex deposits or
late hydrothermal epigenetic deposits. Currently most researchers
emphasize a two-stage diagenetic model. Carbonaceous shales, sandstones
and limestones deposited in reducing, shallow subaqeuous environments
undergo diagenesis which converts the sulphur in these sediments to
pyrite. At a later stage during diagenesis, saline low-temperature brines
carrying copper from a distant source follow permeable units, such as
oxidized redbed sandstones, until they encounter a reducing unit. At this
point a redoxcline is established with a cuperiferous zone extending
“downstream” until it gradually fades into the unmineralized, often
pyritic, reducing unit. The source of the metals is unresolved, with
possible choices including underlying volcanic rocks, labile sediments,
basement rocks or intrusions.
COMMENTS: Sediment-hosted Cu
includes Sabkha Cu deposits which are hosted by thin- bedded
carbonate-evaporite-redbed ‘sabkha’ sequences.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Elevated
values of Cu, Ag, Pb, Zn and Cd are found in hostrocks, sometimes with
weaker Hg, Mo, V, U, Co and Ge anomalies. Dark streaks and specks in
suitable rocks should be analysed as they may be sulphides, such as
chalcocite.
GEOPHYSICAL SIGNATURE: Weak
radioactivity in some deposits.
OTHER EXPLORATION GUIDES: Deposits
often occur near the transition from redbeds to other units which is
marked by the distinctive change in colour from red or purple to grey,
green or black. The basal reduced unit within the stratigraphy overlying
the redbeds will most often carry the highest grade mineralization.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE: Average
deposit contains 22 Mt grading 2.1 % Cu and 23 g/t Ag (Mosier et al.,
1986). Approximately 20% of these deposits average 0.24 % Co. The Lubin
deposit contains 2600 Mt of >2.0% Cu and ~ 30-80 g/t Ag. Spar Lake
pre-production reserves were 58 Mt grading 0.76% Cu and 54 g/t Ag.
Montanore contains 134.5 Mt grading 0.74% Cu and 60 g/t Ag, while Rock
Creek has reserves of 143.7 Mt containing 0.68 % Cu and 51 g/t Ag.
ECONOMIC LIMITATIONS: These
relatively thin horizons require higher grades because they are typically
mined by underground methods. The polymetallic nature and broad lateral
extent of sediment-hosted Cu deposits make them attractive.
IMPORTANCE: These deposits are the
second most important source of copper world wide after porphyry Cu
deposits. They are an interesting potential exploration target in British
Columbia, although there has been no production from sediment-hosted Cu
deposits in the province. The stratigraphy that hosts the Spar Lake,
Montanore and Rock Creek deposits in Montana extends into British Columbia
where it contains numerous small sediment-hosted Cu-Ag deposits.
REFERENCES
ACKNOWLEDGEMENTS: Nick Massey
contributed to the original draft of the profile.
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Renfro, A.R. (1974): Genesis of
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