TURBIDITE-HOSTED Au VEINS
I03
by R.H. McMillan
Consulting Geologist, Victoria, British Columbia
McMillan, R.H. (1996): Turbidite-hosted Au Veins, 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 59-62.
IDENTIFICATION
SYNONYMS : Saddle reefs, Bendigo-type.
COMMODITIES (BYPRODUCTS) : Au (Ag, W, Sb).
EXAMPLES (British Columbia (MINFILE #) - Canada/International) : Frasergold (093A150), Valentine Mountain (092B108), Island Mountain (093H019), Mosquito Creek (093H025), Sheep Creek Deposits - Reno (082FSW036), Queen (082FSW048), Kootenay Belle (082FSW044) and Gold Belt (082FSW040); Ptarmigan, Burwash, Thompson-Ludmar and other Yellowknife district deposits (Northwest Territories, Canada), Meguma district (Nova Scotia, Canada), Bendigo and Ballarat (Victoria, Australia).
GEOLOGICAL CHARACTERISTICS
CAPSULE DESCRIPTION : Gold-quartz veins, segregations, lodes and sheeted zones hosted by fractures, faults, folds and openings in anticlines, synclines and along bedding planes in turbidites and associated poorly sorted clastic sedimentary rocks.
TECTONIC SETTING : Hostrocks were deposited in submarine troughs, periarc basins, foreland basins and remnant ocean basins. The sediments were typically formed on continental margins or back-arc basins. Typically these sequences experienced one or two deformational phases with associated metamorphism.
DEPOSITIONAL ENVIRONMENT/GEOLOGICAL SETTING : Thick sediment sequences that have been deformed and metamorphosed; relatively few igneous rocks.
AGE OF MINERALIZATION : Archean to Tertiary; the Bendigo and Meguma districts are underlain by Early Paleozoic strata. The veins are generally considered to be related to later deformational event.
HOST/ ASSOCIATED ROCK TYPES : The predominant rock types are greywackes, siliceous wackes, shales and carbonaceous shales. Bedded cherts, iron formations, fine- grained impure carbonate rocks; minor polymictic conglomerate, tuffaceous members and minor marine volcanic flows may also be part of the stratigraphic sequence. There are younger granitic intrusions in many belts. Metamorphic grade is generally greenschist, but may reach amphibolite rank.
DEPOSIT FORM : Typically deposits are composed of multiple quartz veins up to a few metres in width that are commonly stratabound (either concordant or discordant), bedding-parallel, or discordant, and parallel to fold axial planes. Veins are variably deformed and occur as single strands, as sheeted arrays or as stockworks. Bedding-parallel veins within anticlines and synclines in the Bendigo-Ballarat and Meguma districts are commonly called saddle reefs or saddle troughs.
TEXTURE/STRUCTURE : Veins are well defined with sharp contacts. Bedding veins can be massive or laminated (ribbon texture) with columnar structures or stylolites, while discordant veins are generally massive. Veins can be associated with a variety of structures. Most common are folded veins and saddle reefs related to anticlinal folds. Sheeted, en echelon sigmoidal veins, ladder veins, tension gashes or stockworks may be related to zones of extension or to Reidel shear structures.
ORE MINERALOGY (Principal and subordinate) : Native gold, pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, galena, molybdenite, bismuth, stibnite, bournonite and other sulphosalt minerals. Low sulphide content (<2.5%).
GANGUE MINERALOGY (Principal and subordinate) : Quartz, carbonates (calcite, dolomite or ankerite), feldspar (albite) and chlorite.
ALTERATION : Generally not prominent, however, disseminated arsenopyrite, pyrite and tourmaline, and more pervasive silica, sericite and carbonate, may develop in wallrocks adjacent to veins.
WEATHERING : In unglaciated terrains deep weathering and alluvial recycling may produce related rich placer deposits, such as the Bendigo region.
ORE CONTROLS : A strong structural control within dilatent areas in fold crests (saddle and trough reefs), discordant veins and tension gashes. This structural control may extend to district scale alignment of deposits. In some districts the veins appear confined to a specific stratigraphic interval, often near a change in lithologies. In the Meguma district, a more subtle stratigraphic control related to the upper (pelitic) portions of individual bouma cycles as well as regionally to the upper portion of the turbidite section. In the Bendigo district there is a relationship between ore and an abundance of graphite in the adjacent wallrocks.
GENETIC MODEL : Genetic theories range from veins formed by magmatic hydrothermal fluids or metamorphogenic fluids to deformed syngenetic mineralization. Most current workers prefer the metamorphogenic-deformational or lateral secretion theories and interpret the laminations as “crack-seal” phenomena formed during episodic re-opening of the veins during their formation. Workers favoring a syngenetic origin interpret the laminations as primary layering. Structural relationships in the Meguma and Bendigo districts indicate that the veins formed contemporaneously with, or prior to the major deformational event and were metamorphically overprinted during the intrusion of Devonian batholithic granitic rocks. Late post-deformational tension veinlets are generally non- auriferous.
ASSOCIATED DEPOSIT TYPES : Placers (C01), iron formation hosted gold deposits (I04) are also mainly hosted in turbidites - some of the Northwest Territories turbidite- hosted deposits are associated with chemical sediments. In several camps, slate horizons carrying finely disseminated, very low grade gold have been reported.
COMMENTS : Although past classification schemes have not recognized this type of deposit in British Columbia, the Valentine Mountain deposit hosted in Leech River schists and Frasergold hosted in Late Triassic clastic Quesnel River Group can be included. Elsewhere, several important vein gold districts in clastic sedimentary (possibly turbiditic) rocks might also be included. For example, the Sheep Creek camp and some of the Barkerville deposits are hosted in siliceous wackes and phyllites.
EXPLORATION GUIDES
GEOCHEMICAL SIGNATURE : Si, Fe, S, As, B, Au and Ag generally show strong enrichment in the deposits, while Cu, Mg, Ca, Zn, Cd, Pb, Sb, W and Mn generally show moderate enrichment, and Hg, In, Li, Bi, Se, Te, Mo, F, Co and Ni may show low levels of enrichment.
GEOPHYSICAL SIGNATURE : The low sulphide content of the majority of quartz veins renders most geophysical techniques ineffective as direct exploration tools. However, airborne and ground electromagnetic and magnetic surveys and induced polarization surveys can be useful where deposits show an association with iron formation, massive sulphides or graphite.
OTHER EXPLORATION GUIDES : Standard prospecting techniques to trace mineralization directly or in float trains in glacial till, talus or other debris derived from the gold mineralization remains the most effective prospecting tool. Areas where there has been past gold production from placers are good candidates for prospecting.
ECONOMIC FACTORS
TYPICAL GRADE AND TONNAGE : Gold production from the Meguma region has come from 60 deposits at grades ranging from 8 to 50 g/t - a total of 35.13 tonnes has been produced from the district. The Bendigo field is much more significant, having produced a minimum of more than 373.3 t (12 M.oz.) of non-alluvial gold from more than 40 Mt of ore since 1851 - grades ranged from a minimum of approximately 5 g/t to more than 30 g/t. The three Barkerville mines produced an aggregate of 2.75 Mt to yield 38.29 t of gold between 1933 and 1987.
ECONOMIC LIMITATIONS : Deposits such as those in the Bendigo and Barkerville districts constitute attractive exploration targets. Although the hand sorting required to recover gold from the Nova Scotia deposits would probably render them uneconomic today, new techniques such as photometric sorting might improve the economics.
IMPORTANCE : Some districts/deposits, such as Bendigo, rank as world class and remain attractive exploration targets. The limited information available about the immense Muruntau deposit suggest that it may be similar to this type.
REFERENCES
ACKNOWLEDGMENTS : Howard Poulsen, Chris Ash, Dani Alldrick and Andre Panteleyev reviewed the profile and provided constructive comments.
Boyle, R.W. (1979) : The Geochemistry of Gold and its Deposits, Geological Survey of Canada, Bulletin 280, 584 pages.
Boyle, R.W. (1986) : Gold Deposits in Turbidite Sequences: Their Geology, Geochemistry and History of the Theories of their Origin, in Keppie, J. Duncan, Boyle, R.W. and Haynes, S.J., Editors, Turbidite-hosted Gold Deposits, Geological Association of Canada, Special Paper 32, pages 1-14.
Graves, M.C. and Zentilli, M. (1982) : A Review of the Geology of Gold in Nova Scotia; in Geology of Canadian Gold Deposits, Hodder, R.W. and Petruk W., Editors, Canadian Institute of Mining and Metallurgy, Special Volume 24, pages 233-242.
Haynes, S.J. (1986) : Geology and Chemistry of Turbidite-hosted Gold Deposits, Greenschist Facies, Eastern Nova Scotia, in Turbidite-hosted Gold Deposits, Keppie, J. D., Boyle, R.W. and Haynes, S.J., Editors, Geological Association of Canada, Special Paper 32, pages 161-178.
Mathews, W.H. (1953) : Geology of the Sheep Creek Camp, B.C. Ministry of Energy, Mines and Petroleum Resources, Bulletin 31, -- pages.
Padgham, W.A.: (1986) : Turbidite-hosted Gold-quartz Veins in the Slave Structural Province, N.W.T, in Turbidite-hosted Gold Deposits, Keppie, J. D., Boyle, R.W. and Haynes, S.J., Editors, Geological Association of Canada, Special Paper 32, pages 119-134.
Panteleyev, A. P. (1991) : Gold in the Canadian Cordillera - a Focus on Epithermal and Deeper Environments, in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 163-212.
Schroeter, T.G. and Lane, R.A. (1991) : A Century of Gold Production and Reserves in British Columbia (1890 to 1990). B.C. Ministry of Energy Mines and Petroleum Resources, Open File 1991-19, 42 pages.
Sharpe, E.N. and MacGeehan, P.J. (1990) : Bendigo Goldfield; in Geology of the Mineral Deposits of Australia and Papua New Guinea, Hughes, F.E., Editor, The Australasian Institute of Mining and Metallurgy, Monograph No. 14, Volume 2, pages 1287-1296.
March 19, 1996