G - Marine Volcanic Association

Gross, G.A. (1996): Algoma-type Iron-fromation, 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 25-28.

IDENTIFICATION

SYNONYMS: Taconite, itabirite, banded iron-formation.

COMMODITIES (BYPRODUCTS): Fe (Mn).

EXAMPLES (British Columbia (MINFILE #) - Canada/International): Falcon (093O 016), Lady A (092B 029); McLeod (Helen), Sherman, Adams, Griffith (Ontario, Canada), Woodstock, Austin Brook (New Brunswick, Canada), Kudremuk (India), Cerro Bolivar (Venezuela), Carajas (Brazil), part of Krivoy Rog (Russia).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: Iron ore deposits in Algoma-type iron-formations consist mainly of oxide and carbonate lithofacies that contain 20 to 40 % Fe as alternating layers and beds of micro- to macro-banded chert or quartz, magnetite, hematite, pyrite, pyrrhotite, iron carbonates, iron silicates and manganese oxide and carbonate minerals. The deposits are interbedded with volcanic rocks, greywacke, turbidite and pelitic sediments; the sequences are commonly metamorphosed.

TECTONIC SETTINGS: Algoma-type iron-formations are deposited in volcanic arcs and at spreading ridges.

AGE OF MINERALIZATION: They range in age from 3.2 Ga to modern protolithic facies on the seafloor and are most widely distributed and achieve the greatest thickness in Archean terranes (2.9 to 2.5 Ga).

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: They formed both near and distal from extrusive centres along volcanic belts, deep fault systems and rift zones and may be present at any stage in a volcanic succession. The proportions of volcanic and clastic sedimentary rocks vary and are rarely mutually exclusive.

HOST/ASSOCIATED ROCKS: Rocks associated with Algoma-type iron-formations vary greatly in composition, even within local basins, and range from felsic to mafic and ultramafic volcanic rocks, and from greywacke, black shale, argillite, and chert interlayered with pyroclastic and other volcaniclastic beds or their metamorphic equivalents. Algoma-type iron-formations and associated stratafer sediments commonly show a prolific development of different facies types within a single stratigraphic sequence. Oxide lithofacies are usually the thickest and most widely distributed units of iron-formation in a region and serve as excellent metallogenetic markers.

DEPOSIT FORM: Iron ore deposits are sedimentary sequences commonly from 30 to 100 m thick, and several kilometres in strike length. In most economic deposits, isoclinal folding or thrust faulting have produced thickened sequences of iron-formation.

STRUCTURE/TEXTURE: Micro-banding, bedding and penecontemporaneous deformation features of the hydroplastic sediment, such as slump folds and faults, are common, and can be recognized in many cases in strongly metamorphosed oxide lithofacies. Ore mineral distribution closely reflects primary sedimentary facies. The quality of oxide facies crude ore is greatly enhanced by metamorphism which leads to the development of coarse granular textures and discrete grain enlargement.

ORE MINERALOGY: Oxide lithofacies are composed of magnetite and hematite. Some deposits consist of siderite interbedded with pyrite and pyrrhotite.

GANGUE MINERALOGY (Principal and subordinate): Quartz, siderite or ferruginous ankerite and dolomite, manganoan siderite and silicate minerals. Silicate lithofacies are characterized by iron silicate minerals including grunerite, minnesotaite, hypersthene, reibeckite and stilpnomelane, associated with chlorite, sericite, amphibole, and garnet.

WEATHERING: Minor oxidation of metal oxide minerals and leaching of silica, silicate and carbonate gangue. Algoma-type iron-formations are protore for high-grade, direct shipping types of residual-enriched iron ore deposits.

GENETIC MODEL: Algoma-type iron deposits were formed by the deposition of iron and silica in colloidal size particles by chemical and biogenic precipitation processes. Their main constituents evidently came from hydrothermal-effusive sources and were deposited in euxinic to oxidizing basin environments, in association with clastic and pelagic sediment, tuff, volcanic rocks and a variety of clay minerals. The variety of metal constituents consistently present as minor or trace elements evidently were derived from the hydrothermal plumes and basin water and adsorbed by amorphous iron and manganese oxides and smectite clay components in the protolithic sediment. Their development and distribution along volcanic belts and deep-seated faults and rift systems was controlled mainly by tectonic rather than by biogenic or atmospheric factors. Sulphide facies were deposited close to the higher temperature effusive centres; iron oxide and silicate facies were intermediate, and manganese-iron facies were deposited from cooler hydrothermal vents and in areas distal from active hydrothermal discharge. Overlapping and lateral transitions of one kind of lithofacies to another appear to be common and are to be expected.

ORE CONTROLS: The primary control is favourable iron-rich stratigraphic horizons with little clastic sedimentation, often near volcanic centres. Some Algoma-type iron-formations contain ore deposits due to metamorphic enhancement of grain size or structural thickening of the mineralized horizon.

ASSOCIATED DEPOSITS: Algoma-type iron-formations can be protore for residual-enriched iron ore deposits
(B01?). Transitions from Lake Superior to Algoma-type iron-formations occur in areas where sediments extend from continental shelf to deep-water environments along craton margins as reported in the Krivoy Rog iron ranges. Oxide lithofacies of iron- formation grade laterally and vertically into manganese-rich lithofacies (G02), and iron sulphide, polymetallic volcanic-hosted and sedex massive sulphide (G04, G05, G06, E14).

COMMENTS: Lithofacies selected for iron and manganese ore are part of the complex assemblage of stratiform units formed by volcanogenic-sedimentary processes that are referred to collectively as stratafer sedimentary deposits, and includes iron-formation (more than 15% Fe) and various other metalliferous lithofacies.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE: Elevated values for Fe and Mn; at times elevated values for Ni, Au, Ag, Cu, Zn Pb, Sn, W, REE and other minor elements.

GEOPHYSICAL SIGNATURE: Electromagnetic, magnetic, and electrical conductance and resistivity survey methods are used effectively in tracing and defining the distribution of Algoma- type beds, either in exploring for iron and manganese ore, or for using these beds as metallogenetic markers.

OTHER EXPLORATION GUIDES: Discrete, well defined magnetite and hematite lithofacies of iron- formation are preferred with a minimum of other lithofacies and clastic sediment interbedded in the crude ore. Iron- formations are usually large regional geological features that are relatively easy to define. Detailed stratigraphic information is an essential part of the database required for defining grade, physical and chemical quality, and beneficiation and concentration characteristics of the ore. Basin analysis and sedimentation modeling enable definition of factors that controlled the development, location and distribution of different iron-formation lithofacies.

ECONOMIC FACTORS

GRADE AND TONNAGE: Orebodies range in size from about 1000 to less than 100 Mt with grades ganging from 15 to 45% Fe, averaging 25% Fe. Precambrian deposits usually contain less than 2% Mn, but many Paleozoic iron-formations, such as those near Woodstock, New Brunswick, contain 10 to 40 % Mn and have Fe/Mn ratios of 40:1 to 1:50. The largest B.C. deposit, the Falcon, contains inferred reserves of 5.28 Mt grading 37.8% Fe.

ECONOMIC LIMITATIONS: Usually large-tonnage open pit operations. Granular, medium to coarse- grained textures with well defined, sharp grain boundaries are desirable for the concentration and beneficiation of the crude ore. Strongly metamorphosed iron- formation and magnetite lithofacies are usually preferred. Oxide facies iron-formation normally has a low content of minor elements, especially Na, K, S and As, which have deleterious effects in the processing of the ore and quality of steel produced from it.

IMPORTANCE: In Canada, Algoma-type iron-formations are the second most important source of iron ore after the taconite and enriched deposits in Lake Superior-type iron-formations. Algoma-type iron-formations are widely distributed and may provide a convenient local source of iron ore.

REFERENCES

El Shazly, E.M. (1990): Red Sea Deposits; in Ancient Banded Iron Formations (Regional Presentations); Chauvel, J.J. et al., Editors, Theophrastus Publications S.A., Athens, Greece, pages 157-222.

Gole, M.J. and Klein, C. (1981): Banded Iron-formations Through Much of Precambrian Time; Journal of Geology, Volume 89, pages 169-183.

Goodwin, A.M., Thode, H.G., Chou, C.-L. and Karkhansis, S.N. (1985): Chemostratigraphy and Origin of the Late Archean Siderite-Pyrite-rich Helen Iron-formation, Michipicoten Belt, Canada; Canadian Journal of Earth Sciences, Volume 22, pages 72-84.

Gross, G.A. (1980): A Classification of Iron-formation Based on Depositional Environments; Canadian Mineralogist, Volume 18, pages 215-222.

Gross, G.A. (1983): Tectonic Systems and the Deposition of Iron-formation; Precambrian Research, Volume 20, pages 171-187.

Gross, G.A. (1988): A Comparison of Metalliferous Sediments, Precambrian to Recent; Kristalinikum, Volume 19, pages 59-74.

Gross, G.A. (1991): Genetic Concepts for Iron-formation and Associated Metalliferous Sediments: in Historical Perspectives of Genetic Concepts and Case Histories of Famous Discoveries, Hutchinson, R.W., and Grauch, R. I., Editors, Economic Geology Monograph 8, Economic Geology, pages 51-81.

Gross, G.A. (1993): Iron-formation Metallogeny and Facies Relationships in Stratafer Sediments; in Proceedings of the Eighth Quadrennial IAGOD Symposium, Maurice, Y.T., Editor, E. Schweizerbart'sche Verlagsbuchhandlung (Nagele u. Obermiller), Stuttgart, pages 541-550.

Gross, G.A. (1993): Industrial and Genetic Models for Iron Ore in Iron-formation; in Mineral Deposit Modeling, Kirkham, R.V., Sinclair, W.D., Thorpe, R.I. and Duke, J.M., Editors, Geological Association of Canada, Special Paper 40, pages 151-170.

Gross, G.A. (in press): Sedimentary Iron Deposits; in Geology of Canadian Mineral Deposit Types, Eckstrand, O.R., Sinclair, W.D. and Thorpe, R.I, (Editors), Geological Survey of Canada, Geology of Canada, Number 8.

James, H.L. (1954): Sedimentary Facies in Iron-formation; Economic Geology, Volume 49, pages 235-293.

Puchelt, H. (1973): Recent Iron Sediment Formation at the Kameni Islands, Santorini (Greece); in Ores in Sediments, Amstutz, G.C. and Bernard, A.J., Editors, Springer-Verlag, Berlin, pages 227-246.

Shegelski, R.J. (1987): The Depositional Environment of Archean Iron Formations, Sturgeon-Savant Greenstone Belt, Ontario, Canada, in Precambrian Iron-Formations, Appel, P.W.U. and LaBerge, G.L., Theophrastus Publications S.A., Athens, Greece, pages 329-344.

Hõy, Trygve (1995): Besshi Massive Sulphide, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Employment and Investment, Open File 1995-20, pages 49-50.

IDENTIFICATION

SYNONYMS: Besshi type, Kieslager.

COMMODITIES (BYPRODUCTS): Cu, Zn, Pb, Ag, (Au, Co, Sn, Mo, Cd).

EXAMPLES (British Columbia - Canada/International): Goldstream (082M 141), Standard (082M 090), Montgomery (082M 085), True Blue (082FNE002), Granduc (104B 021), War Eagle (114P 020); Greens Creek (Alaska, USA), Besshi (Japan).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: Deposits typically comprise thin sheets of massive to well layered pyrrhotite, chalcopyrite, sphalerite, pyrite and minor galena within interlayered, terrigenous clastic rocks and calcalkaline basaltic to andesitic tuffs and flows.

TECTONIC SETTINGS: Oceanic extensional environments, such as back-arc basins, oceanic ridges close to continental margins, or rift basins in the early stages of continental separation.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Terrigenous clastic rocks associated with marine volcanic rocks and sometimes carbonate rocks; these may overlie platformal carbonate or clastic rocks.

AGE OF MINERALIZATION: Any age. In British Columbia, most deposits are Cambrian, Late Triassic and less commonly Mississippian-Permian in age.

HOST/ASSOCIATED ROCK TYPES: Clastic sediments and marine volcanic rocks; basaltic tuffs and flows, shale and siltstone, commonly calcareous; less commonly chert and Fe formations. Possibly ultramafics and metagabbro in sequence.

DEPOSIT FORM: Typically a concordant sheet of massive sulphides up to a few metres thick and up to kilometres in strike length and down dip; can be stacked lenses.

TEXTURE/STRUCTURE: Massive to well-layered, fine to medium-grained sulphides; gneissic sulphide textures common in metamorphosed and deformed deposits; durchbewegung textures; associated stringer ore is uncommon. Crosscutting pyrite, chalcopyrite and/or sphalerite veins with chlorite, quartz and carbonate are common.

ORE MINERALOGY (Principal and subordinate): Pyrite, pyrrhotite, chalcopyrite, sphalerite, cobaltite, magnetite, galena, bornite, tetrahedrite, cubanite, stannite, molybdenite, arsenopyrite, marcasite.

GANGUE MINERALOGY (Principal and subordinate): Quartz, calcite, ankerite, siderite, albite, tourmaline, graphite, biotite.

ALTERATION MINERALOGY: Similar to gangue mineralogy - quartz, chlorite, calcite, siderite, ankerite, pyrite, sericite, graphite.

ORE CONTROLS: Difficult to recognize; early (syndepositional) faults and mafic volcanic centres.

GENETIC MODEL: Seafloor deposition of sulphide mounds in back-arc basins, or several other tectonic settings, contemporaneous with volcanism.

ASSOCIATED DEPOSIT TYPES: Cu, Zn veins.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE: Cu, Zn, Ag, Co/Ni>1; Mn halos, Mg enrichment.

GEOPHYSICAL SIGNATURE: Sulphide lenses usually show either an electromagnetic or induced polarization signature depending on the style of mineralization and presence of conductive sulphides.

OTHER EXPLORATION GUIDES: Mafic volcanic rocks (tholeiitic, less commonly alkalic) associated with clastic rocks; Mn-rich garnets in metamorphosed exhalative horizons, possible structures, such as faults; possible association with ultramafic rocks.

ECONOMIC FACTORS

GRADE AND TONNAGE: Highly variable in size. B.C. deposits range in size from less than 1 Mt to more than 113 Mt. For example, Goldstream has a total resource (reserves and production) of 1.8 Mt containing 4.81 % Cu, 3.08 % Zn and 20.6 g/t Ag and Windy Craggy has reserves in excess of 113.0 Mt containing 1.9 % Cu, 3.9 g/t Ag and 0.08% Co. The type-locality Besshi deposits average 0.22 Mt, containing 1.5% Cu, 2-9 g/t Ag, and 0.4-2% Zn (Cox and Singer, 1986).

IMPORTANCE: Significant sources of Cu, Zn and Ag that can be found in sedimentary sequences that have not been thoroughly explored for this type of target.

REFERENCES

Cox, D.P. and Singer, D.A., Editors (1986): Mineral Deposit Models; U.S. Geological Survey, Bulletin 1693, 379 pages. Höy, T. (1991): Volcanogenic Massive Sulphide Deposits in British Columbia; in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera, McMillan, W.J., Coordinator, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991- 4, pages 89-123.

Franklin, J.M., Lydon, J.W. and Sangster, D.M. (1981): Volcanic-associated Massive Sulfide Deposits; Economic Geology, 75th Anniversary Volume, pages 485-627.

Hutchinson, R.W. (1980): Massive Base Metal Sulphide Deposits as Guides to Tectonic Evolution; in The Continental Crust and its Mineral Deposits, Strangway, D.W., Editor, Geological Association of Canada, Special Paper 20, pages 659-684.

Fox, J.S. (1984): Besshi-type Volcanogenic Sulphide Deposits - a Review; Canadian Institute of Mining and Metallurgy, Bulletin, Volume 77, pages 57-68.

Slack, J.F. (in press): Descriptive and Grade-Tonnage Models for Besshi-type Massive Sulphide Deposits; Geological Association of Canada, Special Paper.

Hõy, Trygve (1995): Cyprus Massive Sulphide Cu (Zn), in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Employment and Investment, Open File 1995-20, pages 51-52.

IDENTIFICATION

SYNONYMS: Cyprus massive sulphide, cuprous pyrite.

COMMODITY (BYPRODUCTS): Cu, (Au, Ag, Zn, Co, Cd).

EXAMPLES (British Columbia - Canada/International): Chu Chua (092F 140), Lang Creek (104P 008), Hidden Creek (103P 021), Bonanza (103P 023), Double Ed (103P 025); Cyprus; York Harbour and Betts Cove (Newfoundland, Canada); Turner-Albright (USA); Lokken (Norway).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: Deposits typically comprise one or more lenses of massive pyrite and chalcopyrite hosted by mafic volcanic rocks and underlain by a well developed pipe-shaped stockwork zone.

TECTONIC SETTINGS: Within ophiolitic complexes formed at oceanic or back-arc spreading ridges; possibly within marginal basins above subduction zones or near volcanic islands within an intraplate environment.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Lenses commonly are in tholeiitic or calcalkaline marine basalts, commonly pillowed, near a transition with overlying argillaceous sediments. Many lenses appear to be structurally controlled, aligned near steep normal faults.

AGE OF MINERALIZATION: Any age. Deposits in British Columbia are primarily Mississippian-Permian or Late Triassic.

HOST/ASSOCIATED ROCK TYPES: Tholeiitic or calcalkaline pillow and flow basalts, basaltic tuff, chert, argillite. Overlying “umbers” consist of ochre [Mn-poor, Fe-rich bedded mudstone containing goethite, maghemite (Fe3O4-Fe2O3 mixture) and quartz] or chert.

DEPOSIT FORM: Concordant massive sulphide lens overlying cross-cutting zone of intense alteration and stockwork mineralization and hydrothermally altered wallrock, and overlain by chert.

TEXTURE/STRUCTURE: Massive, fine-grained pyrite and chalcopyrite, sometimes brecciated or banded?; massive magnetite, magnetite-talc and talc with variable sulphide content; associated chert layers, locally brecciated, contain disseminated sulphides; disseminated, vein and stockwork mineralization beneath lenses.

ORE MINERALOGY (Principal and subordinate): Pyrite, chalcopyrite, magnetite, sphalerite, marcasite, galena, pyrrhotite, cubanite, stannite-besterite, hematite. Sometimes goethite alteration of top of sulphide layer.

GANGUE MINERALOGY: Talc, chert, magnetite, chlorite.

ALTERATION MINERALOGY: Chlorite, talc, carbonate, sericite and quartz veins in the core of the stringer zone, sometimes with an envelope of weak albite with illite alteration.

ORE CONTROLS: Prominent structural control with clustering or alignment of sulphide lenses along early normal faults, near transition from mafic pillow basalts; less commonly mafic tuff; to overlying fine pelagic material.

GENETIC MODEL: Seafloor deposition of sulphide mounds contemporaneous with mafic volcanism, such as spreading ridges.

ASSOCIATED DEPOSIT TYPES: Vein and stockwork Cu (-Au) mineralization; Mn and Fe- rich cherts; massive magnetite (-talc) deposits.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE: Cu, Zn; common depletion of Ca and Na; less common, local minor Na enrichment; possible local K enrichment; prominent Fe and Mn enrichment in footwall stringer zone.

GEOPHYSICAL SIGNATURE: Sulphide lenses usually show either an electromagnetic or induced polarization signature depending on the style of mineralization and presence of conductive sulphides.

OTHER EXPLORATION GUIDES: Mafic ophiolitic volcanic rocks; transition to argillite; clustering or alignment of deposits indicative of fault control; ochre and exhalite (chert) horizons; regional pyritic horizons.

ECONOMIC FACTORS

GRADE AND TONNAGE: Published average is 1.6 Mt containing 1.7 % Cu, 0-33 g/t Ag; 0-1.9 g/t Au, 0-2.1 % Zn (Cox and Singer, 1986).  B.C. examples: Chu Chua reserves - 1.043 Mt, 2.97 % Cu, 0.4 % Zn, 8.0 g/t Ag, 1.0 g/t Au; Anyox deposits - 0.2 to 23.7 Mt, approx. 1.5% Cu, 9.9 g/t Ag and 0.17 g/t Au.

IMPORTANCE: Deposits at Anyox produced 335,846 tonnes copper, 215,057 kg silver and 3,859 kg gold. Worldwide these deposits are generally significant more for their higher grades and polymetallic nature, than their size.

REFERENCES

Cox, D.P. and Singer, D.A., Editors (1986): Mineral Deposit Models; U.S. Geological Survey, Bulletin 1693, 379 pages.

Höy, T. (1991): Volcanogenic Massive Sulphide Deposits in British Columbia; in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera, McMillan, W.J., Coordinator, B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 89-123.

Franklin, J.M., Lydon, J.W. and Sangster, D.M., (1981): Volcanic-associated Massive Sulfide Deposits; Economic Geology, 75th Anniversary Volume, pages 485-627.

Lydon, J.W. (1988): Volcanogenic Massive Sulphide Deposits, Part 2: Genetic Models; Geoscience Canada; Volume 15, pages 43-65.

Constantinou, G. and Govett, G.J.S. (1972): Genesis of Sulphide Deposits, Ochre and Umber of Cyprus; Institution of Mining and Metallurgy, Transactions, Volume 8, pages B36-B46.

Spooner, E.T.C. (1980): Cu-pyrite Mineralization and Seawater Convection in Oceanic Crust - The Ophiolite Ore Deposits of Cyprus; in The Continental Crust and its Mineral Deposits, Strangway, D.W., Editor, Geological Association of Canada, Special Paper 20, pages 685-704.

Hõy, Trygve (1995): Noranda/Kuroko Massive Sulphide Cu-Pb-Zn, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Hõy, T., Editors, British Columbia Ministry of Employment and Investment, Open File 1995-20, pages 53-54.

IDENTIFICATION

SYNONYM: Polymetallic volcanogenic massive sulphide.

COMMODITIES (BYPRODUCTS): Cu, Pb, Zn, Ag, Au (Cd, S, Se, Sn, barite, gypsum).

EXAMPLES (British Columbia - Canada/International): Homestake (082M 025), Lara (092B 001), Lynx (092B 129), Myra (092F 072), Price (092F 073), H-W (092F 330), Ecstall (103H 011), Tulsequah Chief (104K 011), Big Bull (104K 008), Kutcho Creek (104J 060), Britannia (092G 003); Kidd Creek (Ontario, Canada), Buchans (Newfoundland, Canada), Bathurst-Newcastle district (New Brunswick, Canada), Horne-Quemont (Québec, Canada), Kuroko district (Japan), Mount Lyell (Australia), Rio Tinto (Spain), Shasta King (California, USA), Lockwood (Washington, USA).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: One or more lenses of massive pyrite, sphalerite, galena and chalcopyrite commonly within felsic volcanic rocks in a calcalkaline bimodal arc succession. The lenses may be zoned, with a Cu-rich base and a Pb-Zn-rich top; low-grade stockwork zones commonly underlie lenses and barite or chert layers may overlie them.

TECTONIC SETTING: Island arc; typically in a local extensional setting or rift environment within, or perhaps behind, an oceanic or continental margin arc.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Marine volcanism; commonly during a period of more felsic volcanism in an andesite (or basalt) dominated succession; locally associated with fine-grained marine sediments; also associated with faults or prominent fractures.

AGE OF MINERALIZATION: Any age. In British Columbia typically Devonian; less commonly Permian-Mississippian, Late Triassic, Early (and Middle) Jurassic, and Cretaceous.

HOST/ASSOCIATED ROCK TYPES: Submarine volcanic arc rocks: rhyolite, dacite associated with andesite or basalt; less commonly, in mafic alkaline arc successions; associated epiclastic deposits and minor shale or sandstone; commonly in close proximity to felsic intrusive rocks. Ore horizon grades laterally and vertically into thin chert or sediment layers called informally “exhalites”.

DEPOSIT FORM: Concordant massive to banded sulphide lens which is typically metres to tens of metres thick and tens to hundreds of metres in horizontal dimension; sometimes there is a peripheral apron of "clastic" massive sulphides; underlying crosscutting “stringer” zone of intense alteration and stockwork veining.

TEXTURE/STRUCTURE: Massive to well layered sulphides, typically zoned vertically and laterally; sulphides with a quartz, chert or barite gangue (more common near top of deposit); disseminated, stockwork and vein sulphides (footwall).

ORE MINERALOGY (Principal and subordinate): Upper massive zone: pyrite, sphalerite, galena, chalcopyrite, pyrrhotite, tetrahedrite-tennantite, bornite, arsenopyrite. Lower massive zone: pyrite, chalcopyrite, sphalerite, pyrrhotite, magnetite.

GANGUE MINERALOGY: Barite, chert, gypsum, anhydrite and carbonate near top of lens, carbonate quartz, chlorite and sericite near the base.

ALTERATION MINERALOGY: Footwall alteration pipes are commonly zoned from the core with quartz, sericite or chlorite to an outer zone of clay minerals, albite and carbonate (siderite or ankerite).

ORE CONTROLS: More felsic component of mafic to intermediate volcanic arc succession; near centre of felsic volcanism (marked by coarse pyroclastic breccias or felsic dome); extensional faults.

ASSOCIATED DEPOSIT TYPES: Stockwork Cu deposits; vein Cu, Pb, Zn, Ag, Au.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE: Zn, Hg and Mg halos, K addition and Na and Ca depletion of footwall rocks; closer proximity to deposit - Cu, Ag, As, Pb; within deposit - Cu, Zn, Pb, Ba, As, Ag, Au, Se, Sn, Bi, As.

GEOPHYSICAL SIGNATURE: Sulphide lenses usually show either an electromagnetic or induced polarization signature depending on the style of mineralization and presence of conductive sulphides. In recent years borehole electromagnetic methods have proven successful.

OTHER EXPLORATION GUIDES: Explosive felsic volcanics, volcanic centres, extensional faults, exhalite (chert) horizons, pyritic horizons.

ECONOMIC FACTORS

GRADE AND TONNAGE: Average deposit size is 1.5 Mt containing 1.3% Cu, 1.9 % Pb, 2.0 % Zn, 0.16 g/t Au and 13 g/T Ag (Cox and Singer, 1986). British Columbia deposits range from less than 1 to 2 Mt to more than 10 Mt. The largest are the H-W (10.1 Mt with 2.0 % Cu, 3.5 % Zn, 0.3 % Pb, 30.4 g/t Ag and 2.1 g/t Au) and Kutcho (combined tonnage of 17 Mt, 1.6 % Cu, 2.3 % Zn, 0.06 % Pb, 29 g/t Ag and 0.3 g/t Au).

IMPORTANCE: Noranda/Kuroko massive sulphide deposits are major producers of Cu, Zn, Ag, Au and Pb in Canada. Their high grade and commonly high precious metal content continue to make them attractive exploration targets.

REFERENCES

Cox, D.P. and Singer, D.A., Editors (1986): Mineral Deposit Models; U.S. Geological Survey, Bulletin 1693, 379 pages.

Höy, T. (1991): Volcanogenic Massive Sulphide Deposits in British Columbia: in Ore Deposits, Tectonics and Metallogeny in the Canadian Cordillera, W.J. McMillan, Coordinator, British Columbia Ministry of Energy, Mines and Petroleum Resources, Paper 1991-4, pages 89-123.

Franklin, J.M., Lydon, J.W. and Sangster, D.M. (1981): Volcanic-associated Massive Sulphide Deposits; Economic Geology, 75th Anniversary Volume, pages 485-627.

Hutchinson, R.W. (1980): Massive Base Metal Sulphide Deposits as Guides to Tectonic Evolution; in The Continental Crust and its Mineral Deposits, D.W. Strangway, Editor, Geological Association of Canada, Special Paper 20, pages 659-684.

Lydon, J.W. (1984): Volcanogenic Massive Sulphide Deposits, Part 1: A Descriptive Model, Geoscience Canada, Volume 11, No. 4, pages 195-202.

Ohmoto, H. and Skinner, B.J., Editors (1983): The Kuroko and Related Volcanogenic Massive Sulfide Deposits; Economic Geology, Monograph 5, 604 pages.

Scott, S.D. (1985): Seafloor Polymetallic Sulfide Deposits: Modern and Ancient; Marine Geology, Volume 5, pages 191-212.

Sangster, D.F. (1972): Precambrian Volcanogenic Massive Sulphide Deposits in Canada: a Review; Geological Survey of Canada; Paper 72-22, 44 pages.

Alldrick, D.J. (1995): Subaqueous Hot Spring Au-Ag, in Selected British Columbia Mineral Deposit Profiles, Volume 1 - Metallics and Coal, Lefebure, D.V. and Ray, G.E., Editors, British Columbia Ministry of Employment and Investment, Open File 1995-20, pages 55-58.

IDENTIFICATION

SYNONYMS: Epithermal massive sulphide; subaqueous-hydrothermal deposits; Eskay- type deposit; Osorezan-type deposit.

COMMODITIES (BYPRODUCTS): Ag, Au (Cu, Pb, Zn, As, Sb, Hg).

EXAMPLES (British Columbia - Canada/International): Eskay Creek (104B 008), Lulu (104B 376); Osorezan, Vulcano Islands and Jade hydrothermal field (Japan), Mendeleev Volcano (Kurile Islands, Russia), Rabaul (Papua New Guinea), White Island (New Zealand), Bacon-Manito and Surigao del Norte (Phillippines).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: Vein, replacement and synsedimentary bedded sulphides are deposited in volcanic rocks and associated sediments in areas of shallow lacustrine, fluvial or marine waters or in glacial subfloors.

TECTONIC SETTING: Active volcanic arcs (both oceanic island arcs and continental margin arcs) are likely setting.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: 1) Water-filled reservoirs in active continental volcanic areas (crater lakes, playa lakes, stream flood plains, glacier subfloors). 2) Sea-flooded, breached calderas, or unconsolidated shallow marine sediments at the foot of a volcano.

AGE OF MINERALIZATION: Presumably any age, oldest known example is Jurassic.

HOST/ASSOCIATED ROCK TYPES: Mineralization hosted by intermediate to felsic flows and tuffs and minor intercalated sedimentary rocks. Pillow lavas, coarse epiclastic debris flows, and assorted subvolcanic feeder dikes are all part of the local stratigraphic package.

DEPOSIT FORM: Highly variable. Footwall stockwork or stringer-style vein networks. Large, textureless massive sulphide pods, finely laminated stratiform sulphide layers and lenses, reworked clastic sulphide sedimentary beds, and epithermal-style breccia veins with large vugs, coarse sulphides and chalcedonic silica. All types may coexist in a single deposit.

TEXTURE/STRUCTURE: Range from fine clastic sulphides and "framboid"-like chemical precipitates to very coarse grained sulphide aggregates in breccia veins. Structural styles include: vein stockworks, major breccia veins, stratabound and stratiform sulphide lenses and layers.

ORE MINERALOGY (Principal and subordinate): Sphalerite, tetrahedrite, boulangerite, bournonite, native gold, native silver, amalgam, galena, chalcopyrite, enargite, pyrite, stibnite, realgar, arsenopyrite orpiment; metallic arsenic, Hg-wurtzite, cinnabar, aktashite, unnamed Ag-Pb-As-S minerals, jordanite, wurtzite, krennerite, coloradoite, marcasite, magnetite, scorodite, jarosite, limonite, anglesite, native sulphur.

GANGUE MINERALOGY (Principal and subordinate): Magnesian chlorite, muscovite (sericite), chalcedonic silica, amorphous silica, calcite, dolomite, pyrobitumen, gypsum, barite, potassium feldspar, alunite with minor carbon, graphite, halite and cristobalite.

ALTERATION MINERALOGY: Massive chlorite (clinochlore)-illite-quartz-gypsum-barite rock or quartz-muscovite-pyrite rock are associated with the near-footwall stockwork zones. Chlorite and pyrite alteration is associated with the deep-footwall stockwork zones where alteration minerals are restricted to fractures. Stratabound mineralization is accompanied by magnesian chlorite, muscovite, chalcedonic silica, calcite, dolomite and pyrobitumen. At the Osorezan hot spring deposits, pervasive silica and alunite microveinlets are the dominant alteration phases.

GENETIC MODEL: Deposits are formed by "hot spring" (i.e.: epithermal) fluids vented into a shallow water environment. Fluids are magmatic in character, rather than meteoric. This concept contrasts with some characteristics of the process model for volcanogenic massive sulphides. Lateral and vertical zoning has been recognized within a single lens. Lateral zoning shows changes from Sb, As and Hg-rich mineral suites to Zn, Pb and Cu-rich assemblages. Vertical zoning is expressed as a systematic increase in Au, Ag and base metal content up-section. Fluid conduits are fissures generated by seismic shock, aggradation of the volcano over a later expanding magma chamber, or fracturing in response to regional compressional tectonics. A near-surface subvolcanic magma body is an essential source of metals, fluids and heat.

ASSOCIATED DEPOSIT TYPES: Hot spring Hg (H02), hot spring Au-Ag (H03), epithermal veins (H04, H05), volcanogenic exhalative massive sulphides (G06).

COMMENTS: This deposit type is the shallow subaqueous analogue of hot spring Au- Ag, and both of these are subtypes of the "epithermal" class of mineral deposits. Considering the recent discoveries at Osorezan (1987) and Eskay Creek (1988), the brief discussion by Laznicka (1985, p. 907) seems especially prophetic.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE: Ag, Au, Cu, Pb, Zn, As, Sb, Hg.

GEOPHYSICAL SIGNATURE: The pyrite associated with stockwork mineralization and ubiquitous alteration should produce a widespread induced polarization anomaly, but the best targets may be local peaks within this broad anomalous 'plateau'. Airborne magnetometer surveys may help delineate favourable strata and fault offsets.

OTHER EXPLORATION GUIDES: The geological deposit model and its regional setting may be the best exploration tools available. Broad hydrothermal systems marked by widespread sericite-pyrite alteration; evidence of a volcanic crater or caldera setting; accumulations of felsic volcanic strata: 1) in a local subaqueous setting in a regionally subaerial environment, 2) along the near shore zone of a regional subaerial/subaqueous volcanic facies transition (e.g.: the western margin of the Hazelton trough). Focus on the sedimentary intervals within the volcanic pile.

ECONOMIC FACTORS

GRADE AND TONNAGE: These deposits are not well known. The Eskay Creek deposit is attractive because of the polymetallic signature and high precious metal contents. It contains an estimated mining reserve of 1.08 Mt grading 65.5 g/t Au, 2930 g/t Ag, 5.7 % Zn, 0.77 % Cu and 2.89% Pb with geological reserves of 4.3 Mt grading 28.8 g/t Au and 1 027 g/t Ag.

IMPORTANCE: These deposits are attractive because of their bonanza grades and polymetallic nature.

REFERENCES

Aoki, M. (1991): Gold and Base Metal Mineralization in an Evolving Hydrothermal System at Osorezan, Northern Honshu, Japan; Geological Survey of Japan, Report No. 277, pages 67-70.

Aoki, M. (1992a): Magmatic Fluid Discharging at the Surface from the Osorezan Geothermal System, Northern Honshu, Japan; Geological Survey of Japan, Report No. 279, 1992, pages 16-21.

Aoki, M. (1992b): Active Gold Mineralization in the Osorezan Caldera; 29th International Geological Congress Field Trip, Epithermal Gold and Kuroko Mineralizations, in Northeast Honshu, Shikazono, N., Aoki, M., Yamada, R., Singer, D.A., Kouda, R. and Imai, A., Editors, pages 69-75.

Britton, J.M., Blackwell, J.D. and Schroeter, T.G. (1990): 21 Zone Deposits, Eskay Creek, Northwestern British Columbia; in Exploration in British Columbia 1989, B. C. Ministry of Energy, Mines and Petroleum Resources, pages 197-223.

Izawa, E. and Aoki, M. (1991): Geothermal Activity and Epithermal Gold Mineralization in Japan; Episodes, Volume 14, No. 3, pages 269-273.

Laznicka, P. (1985): Subaqueous-hydrothermal Deposits, in Empirical Metallogeny, Elsevier, Amsterdam, 1758 pages.

Macdonald, A.J. (1992): Osorezan, in Japan '92 - A Technical Report, Mineral Deposit Research Unit, University of British Columbia, pages 29-67.

Mitchell, A.H.G. (1992): Andesitic Arcs, Epithermal Gold and Porphyry-type Mineralization in the Western Pacific and Eastern Europe, Institution of Mining and Metallurgy, Transactions, Volume 101, pages B125-B138.

Roth, T. (1982): Eskay Creek 21A Zone: An Update, in Iskut Project Annual Report, Year 2, Mineral Deposit Research Unit, University of British Columbia May 1992.