Classification and Key Characteristics of VMS Deposits

Contributors (Alphabetical)• D. Ames, Geological Survey of Canada• T. Barrie, T Barrie and Associates, Ottawa• CODES Team (R. Large, B. Gemmel et al)• A. Galley, Geological Survey of Canada• H. Gibson, Laurentian University (MERC)• W Goodfellow, Geological Survey of Canada• M. Hannington, University of Ottawa• I. Jonasson, Geological Survey of Canada• J. Peter, Geological Survey of Canada• R Sherlock, Miramar Mining Ltd

Major Volcanogenic Massive Sulfide Districts of the World• World-wide contain about $500 billion in contained metal value• Sustain much of the world’s supply of zinc and silver, importantsources of Cu, and are a major source of “high tech” metals (Ge, In)• Occur in strata of all ages

Sources of Information• Studies of Ancient Deposits– Precambrian Shield– Kuroko (Japan)– Paleozoic in Canada and Australia– Lower Cretaceous in Peru and Mexico– Mesozoic-Cenozoic in Cyprus, Oman• Modern Hydrothermal Systems on theSeafloor– Spreading ridges– Back-Arcs and Arcs

The Challenge: Some Belts are Rich• Exceptional valuein specific tectonicareas of highpotential shouldlead to explorationinvestment inthese areas• Why are somebelts so wellendowed (Abitibi,Trans-Hudson,Slave) and othersnot (Wabigoon,Grenville) ??and Others Aren’t… Why?

VMS Model• What are the key geological processes thatenable the formation of productive districts?• Are any of these evident in the geological record?• Are these key criteria evident on geological maps,or must we conduct specialized studies toestablish there presence?• What can we learn from other districts?In summary….• What’s the VMS “model” about? What can welearn from it?

Bottom to Top- Some ImportantGuidesWhat are the mostimportant guides?Regional Guides•Subvolcanic Intrusions•Volcanological Style•High Temperaturereaction zones•Exhalite zonesLocal Guides•Alteration pipes•Ore zoning patterns

HydrothermalEscapePrecipitationSiteDischargeZoneReservoirCapReaction ZoneHeat Source

Evolution of theHydrothermal System1: Subvolcanic intrusionemplaced;2: Heat from intrusion raisestrapped seawater temperaturecausing dissolution of Si;3: Cold seawater descends,meets rising HT water, Mg +Sippt forming “cap” to reactionzone4: Reservoir zone becomesisolated from seawater influx,becomes isothermal ~380 o C5: High-T water : rock reactionliberates metals, sets pH ~3.0,forms qtz-epidote-actinolitealbiteassemblage

Five VMS Lithotectonic Settings•1: Bimodal mafic-dominated volcanic– Ocean-ocean suprasubduction arc• 2: Mafic backarc: mafic volcanic dominated,ophiolite-associated– Oceanic backarc and mid-ocean rifts– Some plume-related (including alkaline) volcanics• 3: Pelitic Mafic backarc: sediment, mafic flow/silldominated– Oceanic backarc-rift; pelagic sediments• 4: Bimodal felsic-dominated volcanic– Ocean-continent suprasubduction arc• 5: Siliciclastic- Felsic– Ocean-continent backarc; continental-derived sediments

1: VMS Deposits: Copper-Rich Deposits• Ocean-Ocean Arc -Backarc Terrains– Initial Arc Splitting• 50% mature sedimentary rocks (pelite) in Besshi-type, orophiolite (Cyprus-type)• No (or >1%) felsic rocks• Laterally extensive, Co-enriched Cu ± Zn ±Au• Examples– Windy Craggy (Canada), Besshi, Oman, Cyprus

2: VMS Deposits: Zinc-Lead-Rich Deposits• Ocean-Continent Arc -Backarc Terrains– Steep Subduction– Initial Arc Splitting• >50% felsic volcanic strata• Pyroclastic dominated, at caldera margins• Zn-Pb-Cu Deposits• Examples:– Kuroko (Japan); Skellefte, Bergslagen (Sweden)– Mature Arc Splitting• >50% mature sedimentary rocks (greywacke)• Felsic pyroclastic strata, minor basalt• Low grade Zn-Pb +/- Cu• Examples– Bathurst district (Canada), Iberian Pyrite belt

SubductionDynamics•Must have slab rollback•Need dense,dehydrated, cooloceanic crust to beeffective•Should be distal tospreading ridge

Excellent Oceanic BackarcPotential for Noranda,Besshi, ophiolite typeTonga KermadecArc•Northern End (Lau Basin)•Classic Mature Ocean-Ocean Backarc•Good potential forNoranda-style depositsOcean-ContinentbackarcPotential forKuroko, Iberiatypes•Southern End (Havre Trough)•Underlain by continentalcrust•Good potential formKuroko-style deposits

Tectonic Controls on VMS Deposits- Summary• Paleo-Tectonic Environment– Best back-arcs form where older, cooler (somewhatdehydrated) more dense oceanic crust was subducting• steeper subduction, continental retreat lead tooffshore, classic oceanic island arc and back arcdevelopment: Lots of VMS– Proximity to paleo-oceanic spreading ridge is important• too close (e.g. EPR to S. America) yields hot,hydrated, buoyant subducted oceanic crust, formingcontinental arcs: No VMS, lots of epithermal &porphyry deposits– Very old, cold, thick oceanic crust, including oceanicplateaus, may not subduct: No oceanic arcs, backarcsor VMS• e.g. Ontong -Java plateau locked against New Irelandarc, subduction shifted

Median Metal Content (Millions of Tonnes/Kg)GEOMETRICMEAN1:BimodalMafic2:MaficBackarc3:Pelitic-Mafic4:BimodalFelsic5:SiliciclasicFelsicCu % 1.24 1.82 1.23 1.04 0.62Pb % 0.3 0.02 0.68 1.14 1.09Zn % 2.32 0.84 1.58 4.36 2.70Au g/t 0.81 1.40 0.75 1.06 0.59Ag g/t 21.14 10.62 19.29 56.35 38.54TOTAL METAL(TONNES) 128,515 63,035 132,968 198,461 324.748TOTAL SULFIDE(TONNES) 3,421,075 2,699,466 4,721,093 3,420,784 7.139,305N 291 76 90 241 1062007

Normalized Metal Contents ofPrincipal VMS Types

Metal Correlation MatrixCu Pb Zn Au AgCu 1Pb -0.921 1Zn -0.756 0.757 1Au 0.695 -0.745 -0.327 1Ag -0.659 0.773 0.953 -0.286 1

Characteristics of the Five Types of VMS• Although types are distinct, significant variationsexist within each type• Compositional characteristics are specific• Physical volcanological characteristics vary,depending on water depth, melt volatile contents• Greatest variations in Bimodal mafic andBimodal felsic types• Bimodal mafic can be divided into twosubgroups:– 1A: flow dominant, Zn-Cu, form on seafloor– 1B: pyroclastic dominant, Zn±Pb, Cu, Ag, form subseafloor

Bimodal mafic: Comparison of SubtypesFlow dominated• Higher Cu, noPb• Lower totalmetal content

Type 1A:Bimodal mafic-dominated (flow)• Setting: Ocean-ocean arc, nascent (early) arc rifting– Median Size and Composition: 2.8m.t., 1.41 Cu, 2.6 Zn,0.1Pb, 0.8 g/t Au, 22 g/t Ag– Geology: >75% pillowed tholeiitic basalt-andesite,

Noranda Cross Section

Noranda Camp: Bimodal Mafic-Dominated setting (1a)HorneAnomalous because:>25% felsicstratamany shallowintrusions, flowdomesNo pyroclasticsformed eitheroutside calderon oron its marginmore sediment totrap precipitates

• Snow Lake- two major VMSepisodes, twosubtypes• late: Zn-Pb-Cu-Au, BimodalFelsicPyroclasticdominated(Chisel-Ghost)• early: Cu-Zn,Bimodal Maficdominated(Anderson-Stall)

Kidd Creek vs. Potter MineContrasting settings in the samegreenstone beltBoth with strong ultramaficvolcanic associationKidd Creekhas a uniquesetting; Type1BPotter isBesshi/Ophiolitestyle: Type 1A

Typical Noranda-type Deposit•Sulfide mound formed on seafloor: Zn-Cu, low (1 g/t) Au•Many with Fs-porphyry dykes in FW, occupy synvolcanic fault•Several in / on felsic cryptodomes•Some detrital (talus) sulfide on margin•Very vertically extensive stringer - alteration zone•Chlorite-quartz core, sericite outer margin

Type 1B:Bimodal mafic-dominated volcaniclastic• Setting: Ocean-ocean arc, nascent (early) arc rifting– Median Size and Composition: 4.5m.t., 1.1 Cu, 3.4 Zn,0.8 Pb,1.1 g/t Au, 37 g/t Ag– Geology: >75% pillowed tholeiitic basalt-andesite,

Sturgeon Lake District: Example of Type 1BCamp Size20.42 mt 8.25% Zn;0.76% Pb; 0.97% Cu109g/t Ag

Sturgeon Lake Geology

Sturgeon LakeBimodal mafic 1BPyroclasticdominatedCaldron sequence:•base of sequencesubmarineandsubaerial basalt•episodic calderacollapse:heterolithicdebris flow•deposits formed atonset of felsicvolcanism•cryptodome formedafter deposits•Andesitic HW

Mattabi deposit: Five stratiform lenses, banded sphalerite-galena pyrite,minor stringer zone, late cpy-asp-tetrahedrite zone

Type 2: Mafic-Backarc (ophiolite)• Setting: Mature ocean-ocean backarc, oceanicspreading ridges– Av. Size and Composition: 2.7 m.t., 1.8 Cu, 0.84 Zn,0.02 Pb, 1.4 g/t Au, 11 g/t Ag– Geology: Tholeiitic: MORB to LILE and LREE-enriched, ocean island basalt– Ore: Massive sulfides bulbous pyritic, low As, Sb, Sn;prominent Cu-stringer zones.– Pipe alteration: pronounced, zoned Fe -Mg chloritecore, sericite+/- paragonite rim ; silicic veins– Semiconformable: Epidote-albite-actinolite, localsilicification; regular metal depletion– Examples: Cyprus, Oman, Lokken (Norway)

Ophiolite-hosted•Usually formed in matureoceanic backarcs•May be obducted oceanicspreading ridges•Productive areas haveplagiogranite in thesubvolcanic sequence•biggest deposits low instratigraphic section•Au-rich caps generallyoverlooked by early miners

Type 3: Pelitic - Mafic Backarc (Besshi)• Setting: Mature ocean-ocean backarc– Av. Size and Composition: 4.7 m.t., 1.2 Cu, 1.6 Zn, 0.7Pb, 0.75 g/t Au, 19 g/t Ag– Geology: pelitic to semi-pelitic distal, minor siliciclasticsediments. Tholeiitic basalt, MORB to LILE and LREEenriched, ocean island basalt– Ore: Massive sulfides tabular, pyrite & pyrrhotite, lowAs, Sb, Sn; high Co (+/- Ni?), minor Cu-stringerzones.Some sulfidic sediments, chert, IF.– Pipe alteration: zoned Fe -Mg chlorite core, sericite;– Semiconformable: not well established– Examples: Besshi, Windy Craggy

Besshi-Type (Mafic-Dominated Siliciclastic)

Type 4: Bimodal felsic-dominated• Setting: Ocean-continent arc, nascent arc rifting– Av. Size and Composition: 3.4 m.t., 1.0 Cu, 4.4 Zn, 1.1Pb, 1.1 g/t Au, 56 g/t Ag– Geology: 35%pillowed tholeiitic to calc-alkaline andesite, Felsic subvolc.intrusions, not prominent– Ore: Massive sulfides bulbous to tabular, pyritic, As, Sb,Sn variable; some Au-rich, local (late?) high-S zones;limited Cu-stringer zones.– Pipe alteration: pronounced, silicification and sericitedominant, zoned Fe -Mg chlorite core, sericite rim– Semiconformable: Lower albitic, upper K-spar rich, Nadepleted,uppermost zoned (Fe) carbonate– Examples: Kuroko, Skellefte, Sturgeon Lake, Buchans,Buttle Lake

•These are a “special case” of the bimodal-felsic-dominated class•There are two types•1:Unusual compositions: Boliden, Neves Corvo: possible direct magmatic input•High Hg, Sn, Te; unusual non-stratiform shapes, not high S•Unusually enriched in “high volatility “elements (Hg, Sb, As, S):•probable exceptionally high vapour-phase (boiling) systems•not really “high S” either!

Type 5: Felsic siliciclastic hosted• Setting: Mature ocean-continent backarc– Av. Size and Composition: 7.1 m.t., 0.6 Cu, 2.7 Zn, 1.1Pb, 0.6 g/t Au (?), 38 g/t Ag– Geology: Siliciclastic (volcanic +/- continent derived)sediments, locally prominent felsic pyroclastic flows,minor calc-alkaline mafic flows (typically H.W.)– Ore: Massive sulfides huge tabular, pyrite & pyrrhotite,variable As, Sb, Sn; minor Cu-stringer zones. Somewith IF, chert– Pipe alteration:variable size, usually minor, sericitic,silicic, Fe-chlorite– Semiconformable: not well established (K-rich upperzone, Na-rich (+epidote, actinolite) lower– Examples: Bathurst, Iberia, Kazakhstan, Bergslagen (?)

Bathurst Mining Camp: SedimentedBack-Arc Continental Rift BasinOceanicCrustSubductionZone andIsland ArcBack-ArcRiftShelfLandSea levelOHS 2 2Oceanic CrustAsthenosphereFelsicVolcanicsSyn-riftclasticsMassiveSulfidesBasinalsedimentsMagmaContinentalBasementSubductingOceanicCrustAsthenosphereUpwelling

Geologyof theBrunswickDistrict

Bimodal Siliciclastic Environment

Summary• Cu-rich deposits form in primitive, basalt-dominant oceanic arcbackarcsystems: deep water• Zn-rich deposits form in epicontinental, felsic and sedimentdominantoceanic backarc systems• Subvolcanic intrusions are more common, and are commonlymafic or composite• High-T reaction (ep-ab-act-qtz) zones are metal depleted withsilicified caps (or carbonate in shallow-water systems): Theseprovide good indictors of potentially productive regions• Alteration pipes occupy synvolcanic faults: usually verticallyextensive, Mg-metasomatized, Na ± Ca depleted and Cu-rich• Cu-rich VMS typically form as mounds on the paleo-seafloor;extensive metal redistribution may result in Cu-rich core, Au ±Ba cap (post PC)• Zn-rich VMS form sub-seafloor, some metal redistribution,copper in limited stringer zones, Au-rich (cap), oxidized distalexhalite• Laterally extensive “exhalite” (chert or pelite) containsanomalous metals and conserved elements, useful as vectorsto ore