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Schroeter, T.G. and Cameron, R. (1996):
Alkalic Intrusion-associated 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 49-51.
IDENTIFICATION
SYNONYMS: Alkalic epithermal,
Au-Ag-Te veins.
COMMODITIES (BYPRODUCTS): Au,
Ag (Zn, Pb).
EXAMPLES (British Columbia - Canada/International):
Flathead (082GSE070), Howell (082GSE037), Howe (082GSE048); Cripple
Creek (Colorado, USA), Zartman, Landusky, Golden Sunlight (Montana, USA),
Golden Reward (South Dakota, USA).
GEOLOGICAL
CHARACTERISTICS
CAPSULE DESCRIPTION: These deposits
include quartz veins with pyrite, sphalerite and galena in structural
zones and stockworks within alkalic intrusions and/or disseminated pyritic
zones in alkalic intrusions, diatremes, coeval volcanics (Cripple Creek)
and surrounding sediments. Argillic alteration, +/- silicification,
carbonatization, and barite and fluorite veins are common.
TECTONIC SETTINGS: Associated with
alkalic intrusive rocks in sedimentary cover rocks above continental crust,
generally associated with extensional faulting. Tertiary examples in the
USA are related to continental rifting; Rio Grande rift for Cripple Creek,
Great Falls tectonic zone for the Montana deposits. Flathead area of
British Columbia is in a continental setting but the extensional component
is not as apparent.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL
SETTING: Diatreme-intrusive complexes, high-level alkalic plugs, and
dikes that intrude Proterozoic to Mesozoic continental clastic and
carbonate rocks. Cripple Creek is within a large maar diatreme complex.
Flathead intrusions are coeval with chemically similar volcanic rocks, the
Crowsnest volcanics, in southern Alberta.
AGE OF MINERALIZATION: Any age;
Flathead intrusions are early Cretaceous (98.5 Ma)
HOST/ASSOCIATED ROCK TYPES: (Flathead
area): Intrusions include alkali feldspar syenite, foid-bearing syenite (nepheline,
leucite, nosean, analcite), mela- syenite and related diatreme breccias
with 10 % to 100 % intrusive component. Textures include coarse
porphyritic sanidine, micro-syenite, tinguaite. Host sedimentary rocks
include clastic rocks, shales and argillites to sandstones, and impure
fine-grained carbonaceoous limestone and massive calcarenitic limestone.
Gold may be present in all rock types.
DEPOSIT FORM: Deposits may be in the
form of sheeted veins in structural zones within intrusions (e.g, Zortman,
Cripple Creek) with dimensions of 50 m to 100 m in width and hundreds of
metres in length to, less commonly, large disseminated, diffuse zones
within diatremes (e.g., Montana Tunnels, Cripple Creek), volcanic rocks
(e.g., Cripple Creek) or stratabound within favourable sedimentary
lithologies.
TEXTURE/STRUCTURE: Ore minerals in
quartz and quartz-adularia veins, vein stockworks, disseminated zones and
minor breccias.
ORE MINERALOGY (Principal and
subordinate): Fine-grained (auriferous, arsenical?) pyrite,
galena, sphalerite, gold tellurides; chalcopyrite, magnetite, gold,
bismuth and tellurium minerals are suspected at Flathead from elevated
geochemical values in samples (to 31 ppm Te, 356 ppm Bi).
GAUNGE MINERALOGY (Principal and
subordinate): Quartz, calcite; adularia, barite, fluorite.
ALTERATION MINERALOGY: Widespread
pyrite and carbonate (calcite) alteration of intrusive rocks, silicic and
argillic (illite, sericite, jarosite, roscoelite) alteration of
wallrocks; also albite and adularia.
WEATHERING: Oxidation with limonite,
jarosite, hydrozincite.
ORE CONTROLS: Mineralization is
controlled by structural zones within or proximal to alkalic intrusions;
also in permeable (e.g., sandstone) or chemically favourable units
(impure carbonates or bedding contacts) in country rocks. Diatreme
breccias are favourable permeable hosts for focused flow of volatiles.
ASSOCIATED DEPOSIT TYPES: Distal
base metal mantos are indicated in the Flathead and South Dakota deposit
areas. Possible link with porphyry Mo deposits; polymetallic veins.
COMMENTS: Some authors consider this
deposit type to be a subset of the low- sulphidation epithermal suite of
precious metal deposits. This deposit model relates to continental rift
settings, but related deposit types are present in oceanic arc settings
and include Emperor (Fiji), Porgera and Ladolam (Papua New Guinea)
deposits. Similar British Columbia settings may include the Quesnel and
Stikine Terrane alkalic volcanic belts which host the alkalic porphyry
copper-gold deposits.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE: Au, Ag, As,
Sb, Pb, Zn, F, Ba, V, Te, Bi
GEOPHYSICAL SIGNATURE: High
chargeability (I.P.) will outline pyritic zones; magnetic surveys will
outline magnetite-bearing zones.
ECONOMIC FACTORS:
TYPICAL GRADE AND TONNAGE: Highly
variable, from very low mineable grades (e.g., 0.53 g/t Au at Zortman) to
very high bonanza grades (e.g., 126 g/t Au at the Cresson vug, Cripple
Creek). Recovered gold from the Cripple Creek district totals in excess of
600 tonnes. Grades at Howell Creek include 58 m of 1.3 g/t Au in
silicified limestone, with grab samples containing up to 184 g/t at
Flathead. Tonnages and grades from a number of deposits include: Cresson
deposit, Cripple Creek 70 mt 0.99 g/t Au Cripple Creek, historical prod’n
(1891-1989) 41 mt 17.14 g/t Au Golden Sunlight (Dec., 1994) 42.8 mt 1.9
g/t Zortman (Dec., 1994) 55.7 mt 0.68 g/t Au Montana Tunnels (Dec., 1994)
26.6 mt 0.61 g/T Au
IMPORTANCE: Although these deposits
have not been mined in British Columbia, they remain a viable exploration
target.
REFERENCES
Bonham, H.F. (1988): Models for
Volcanic Hosted Epithermal Precious Metal Deposits; in Bulk Mineable
Precious Metal Deposits of the Western United States, Schafer, R.W.,
Cooper, J.J., and Wikre, P.G., Editors, Geological Society of Nevada,
Symposium Proceedings, pages 259-271.
Cameron, R.S. (1989): Reverse
Circulation Drilling Report for the Howe Claims, Fort Steele Mining
Division, British Columbia; B.C. Ministry of Energy, Mines and
Petroleum Resources, Assessment Report 18629. Mutschler, G.E. and
Mooney, T.C. (1993): Precious-metal
Deposits Related to Alkalic Igneous Rocks: Provisional Classification,
Grade-Tonnage Data and Exploration Frontiers; in Mineral Deposit
Modelling, R.V. Kirkham, W.D. Sinlcair, R.I. Thorpe and J.M. Duke,
Editors, Geological Survey of Canada, Special Paper 40, pages 479-
520.
Richards, J.P. and Kerrick, R., (1993):
The Porgera Gold Mine, Papua New Guinea: Magmatic Hydrothermal to
Epithermal Evolution of an Alkalic-type Precious Metal Deposit;
Economic Geology, Violume 88, pages 1017-1052.
Sillitoe, R.H. (1983): Epithermal
Models: Genetic Types, Geometrical Controls and Shallow Features; in
Mineral Deposit Modelling, R.V. Kirkham, W.D. Sinlcair, R.I. Thorpe and
J.M. Duke, Editors, Geological Survey of Canada, Special Paper 40,
pages 403-417.
Skupinski. A. and Legun, A. (1989):
Geology of Alkalic Rocks at Twentynine Mile Creek, Flathead River Area,
Southeastern British Columbia; in Exploration in British Columbia 1988,
B.C. Ministry of Energy, Mines and Petroleum Resources, pages B29-
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