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Schroeter, Tom and Poulsen, Howard
(1996): Carbonate-hosted Disseminated Au-Ag, 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 9-12.
IDENTIFICATION
SYNONYMS: Carlin-type gold,
sediment-hosted micron gold, siliceous limestone replacement gold,
invisible ("no-seeum") gold.
COMMODITIES (BYPRODUCTS): Au
(Ag). In rare cases Ag dominates over Au.
EXAMPLES (British Columbia (MINFILE
#) - Canada/International): Golden Bear? (104K079); parts of
Brewery Creek (Yukon, Canada), Carlin, Getchell, Cortez, Gold Acres,
Jerrett Canyon, Post and Gold Quarry (Nevada, USA), Mercur (Utah, USA),
Mesel? (Indonesia),Guizhou (China).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: Very fine
grained, micron-sized gold and sulphides disseminated in zones of
decarbonated calcareous rocks and associated jasperoids. Gold occurs
evenly distributed throughout hostrocks in stratabound concordant zones
and in discordant breccias.
TECTONIC SETTINGS: Passive
continental margins with subsequent deformation and intrusive activity,
and possibly island arc terranes.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: Host rocks to the Nevadan deposits were deposited in
shelf-basin transitional (somewhat anoxic) environments, formed mainly as
carbonate turbidites (up to 150 m thick), characterized by slow
sedimentation. These rocks are presently allochthonous in thrust fault
slices and have been overprinted by Miocene basin and range extension.
There are Mesozic to Tertiary felsic plutons near many deposits.
AGE OF MINERALIZATION: Mainly
Tertiary, but can be any age.
HOST/ASSOCIATED ROCK TYPES:
Hostrocks are most commonly thin-bedded silty or argillaceous carbonaceous
limestone or dolomite, commonly with carbonaceous shale. Although less
productive, non-carbonate siliciclastic and rare metavolcanic rocks are
local hosts. Felsic plutons and dikes are also mineralized at some
deposits.
DEPOSIT FORM: Generally tabular,
stratabound bodies localized at contacts between contrasting lithologies.
Bodies are irregular in shape, but commonly straddle lithological contacts
which, in some cases, are thrust faults. Some ore zones (often higher
grade) are discordant and consist of breccias developed in steep fault
zones. Sulphides (mainly pyrite) and gold are disseminated in both cases.
TEXTURE/STRUCTURE: Silica
replacement of carbonate is accompanied by volume loss so that brecciation
of hostrocks is common. Tectonic brecciation adjacent to steep normal
faults is also common. Generally less than 1% fine-grained sulphides are
disseminated throughout the hostrock.
ORE MINERALOGY (Principal and
subordinate): Native gold (micron-sized), pyrite with arsenian
rims, arsenopyrite, stibnite, realgar, orpiment, cinnabar, fluorite,
barite, rare thallium minerals.
GANGUE MINERALOGY (Principal and
subordinate): Fine-grained quartz, barite, clay minerals,
carbonaceous matter (late-stage calcite veins).
ALTERATION MINERALOGY: Strongly
controlled by local stratigraphic and structural features. Central core of
strong silicification close to mineralization with silica veins and
jasperoid; peripheral argillic alteration and decarbonation (“sanding”) of
carbonate rocks common in ore. Carbonaceous material is present in some
deposits.
WEATHERING: Nevada deposits have
undergone deep supergene alteration due to Miocene weathering. Supergene
alunite and kaolinite are widely developed and sulphides converted to
hematite. Such weathering has made many deposits amenable to heap- leach
processing.
GENETIC MODELS:
- a) Epithermal model: Once widely
accepted but now discounted for most deposits. Mineralization was
thought to result from shallow Miocene magmatism related to basin and
range extension. New discoveries of deep orebodies, overprinting basin
and range deformation, and recognition of a supergene origin of alunite
have cast doubt on this model. b) Distal skarn model: Currently very
popular because many deposits occur near intrusions, skarns and
calcsilicate rocks. Carbonate-hosted disseminated gold is thought to be
related to collapse of intrusion-centred porphyry-type hydrothermal
systems. Although compelling for many deposits, this model fails to
explain several districts (e.g., Jerritt Canyon; Guizhou, China) where
no related magmatism has been observed.
c) Deep crustal fluid model: Recently proposed to account for inferred
deep mixing of different fluids from different reservoirs as demanded by
light stable isotopic and fluid inclusion data. Variants of this model
imply only indirect links to magmatism, suggest a single Paleogene age
for the Nevadan deposits and relate them to a unique period of pre-basin
and range crustal extension and associated faults that are controlled by
pre-existing Paleozoic and Mesozoic structures.
ORE CONTROLS:
- 1. Selective replacement of carbonaceous
carbonate rocks adjacent to and along high-angle faults, regional thrust
faults or bedding.
2. Presence of small felsic plutons (dikes) that may have caused
geothermal activity and intruded a shallow hydrocarbon reservoir or area
of hydrocarbon-enriched rocks, imposing a convecting geothermal system
on the local groundwater.
3. Deep structural controls are believed responsible for regional trends
and may be related to Precambrian crystalline basement structures and/or
accreted terrane boundaries.
ASSOCIATED DEPOSIT TYPES: Porphyry Au, W or Mo skarns
, polymetallic manto.
COMMENTS: B.C.: 1. Limestone fault
slices (part of accreted Stikine terrane) which have been intruded by
felsic plutons, especially near high-angle fault zones, may host deposits
(e.g., Golden Bear mine area). 2. Interior Plateau region - if carbonate
units present - potential basin and range setting.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Two
geochemical asemblages - Au+As+Hg+W or ? Mo and As+Hg+ Sb+Tl or Fe. NH3
important in some deposits. Au:Ag 10:1 or greater. Anomalous values in
rock: As (100-1000 ppm); Sb (10-50 ppm); Hg (1-30 ppm).
GEOPHYSICAL SIGNATURE: Resistivity
lows for some deposits. Aeromagnetic surveys may highlight spatially
associated intrusions, skarns if present and possibly regional trends.
OTHER EXPLORATION GUIDES: In Nevada
the deposits exhibit regional alignments or trends. Satellite imagery is
useful to identify regional structures.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE: Grades
range from 1 to 35 g/t Au and deposit sizes from 1 to 150 Mt of ore. For
43 significant deposits the median tonnages and grades for low-grade oxide
and higher grade hypogene deposits are 20 Mt grading 1.2 g/t Au and 6 Mt
containing 4.5 g/t Au, respectively. Supergene deposits amenable to heap
leaching typically grade 1-2 g/t Au; whereas, production grades for
deposits with hypogene ore typically grade 5 to 10 g/t or greater.
ECONOMIC LIMITATIONS: Parts of
deposits are amenable to open-pit mining and heap leaching (especially
oxidized zones), but roasting and autoclave extraction is required for
more refractory ores. New discoveries of high-grade hypogene ore have
resulted in increased underground mining.
IMPORTANCE: Between 1965 and 1995,
deposits along the Carlin Trend (70 x 10 km), have yielded approximately
750 t of Au. Deposits that are unquestionably of this type are not
presently known in Canada but may be present.
REFERENCES
Bagby, W.C. and Berger, B.R. (1985):
Geologic Characteristics of Sediment-hosted, Disseminated Precious-metal
Deposits in the Western United States; in Geology and Geochemistry of
Epithermal Systems, Berger, B.R. and Bethke, P.M., Editors, Reviews in
Economic Geology, Volume 2, Society of Economic Geologists, pages
169-202.
Bakken, B.M. and Einaudi, M.T. (1986):
Spatial and Temporal Relations Between Wallrock Alteration and Gold
Mineralization, Main Pit, Carlin Gold Mine, Nevada; in Proceedings of Gold
'86 Symposium, Macdonald, A.J. , Editor, Geological Association of
Canada, pages 388-403.
Berger, B.R. and Bagby, W.C. (1991):
The Geology and Origin of Carlin-type Gold Deposits; in Gold
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Christensen, O.D., Editor (1993):
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C., Huang, Z., Wan, C. and Li, W. (1988): Newly Discovered Sedimentary
Rock-hosted Disseminated Gold Deposits in the People’s Republic of China;
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Geology and Geochemistry of Wall-rock Alteration at the Carlin Gold
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Sawkins, F.J. (1990):
Sediment-hosted Gold Deposits of the Great Basin; in Metal Deposits in
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