ALKALIC INTRUSION-ASSOCIATED Au-Ag
H08
by Tom G. Schroeter and Robert Cameron
British Columbia Geological Survey

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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 Hűy, 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 (I05) 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 (LO3).

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 prodín (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- B34.

March 25, 1996

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