short course 2017 vii earth science convention, la...

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Millbrook Minerals Inc.

A Private Canadian Corporation

Focused on Resource Development in Cuba

Millbrook Minerals Inc.

VOLCANOGENIC MASSIVE SULFIDE DEPOSITS: CLASSIFICATION, TECTONIC SETTINGS AND A “BOTTOM TO TOP” REVIEW

Alan Galley

James M. Franklin

VII Earth Science Convention, La Habana, Cuba,Short Course 2017

with contributions from

Harold Gibson, Laurentian University, CanadaJan Peter, Geological Survey of Canada

Mark Hannington, University of Ottawa, CanadaChang-Jo Chung

MacDonald Mines, Hecla, Agnico-Eagle, Xstrata-Glencore,Nuinsco, Anconia, Noront, HudBay and many others

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Millbrook Minerals Inc. COURSE CONTENT

• INTRODUCTION

• Definition

• Distribution in space and time

• TECTONIC SETTING

• STRATIGRAPHIC SETTING AND CLASSIFICATION

• Geochemical signature of VMS host volcanic succession

• VMS class characteristics and examples

• ANATOMY OF A VMS HYDROTHERMAL SYSTEM

• Camp scale to deposit scale evolution

• Fluid chemistry and chemical pathways

• EXPLORTION CRITERIA

• SUMMARY AND CONCLUSIONS

Slide 2. Good morning ladies and gentlemen, I would first like to thank Millbrook minerals and the organizers of this conference for inviting us to introduce you to some of the principal characteristics of volcanogenic massive sulphide deposits. I would also like to express the regrets of my short course partner, Dr. Jim Franklin that he could be here today, but he sends his best regards and hopes to have an opportunity in the future to see you again.

Jim and I have worked on and off together for over 30 years in the research and exploration of volcanogenic massive sulphide deposits, both on land and on the seafloor, and therefore have much in common with respect to out understanding of these fascinating ore systems.

Volcanogenic massive sulfide

deposits (VMS)

3

• Also known as Volcanic-hosted (VHMS) or

volcanic-associated massive sulfide deposits

• Syngenetic polymetallic deposits formed at,

or near the seafloor due to focused upflow

and capture of hydrothermal-magmatic fluids

• Are recognized in rocks as old as 3.2 Ga to

modern seafloor settings (SMS) but focused

in extensional periods of plate subduction

• Range up to over 300 Mt (avg 3-5 Mt) and

commonly form in clusters, or districts

Slide 3. The slide shows the distribution of VMS deposits by age both in terms of tons of ore on the left and tons of metal on the right. The distribution through time is controlled very much by times of major supercontinental breakup. The first of these appears to be in the early Proterozoic with the breakup of an Archean supercontinent and formation of a wide range of VMS deposit types in both the trans-Hudson origin and Svecokarelian Shield. Major breakups of Rodina, Pangea and Gondwana led to the formation of younger ocean floor distinct back arc basin formation and in areas of back arc extension, the formation of VMS deposits. Although the breakup events mark the start of major arc formations, these tend to be protracted events; for example, the major breakup of Gondwana in the Cretaceous was accompanied by major VMS districts that formed in the mid Cretaceous in many areas marginal to continents (S and North America, Pontides in western Asia etc.), but in South America VMS formation continued into the Eocene. Slide 4. Volcanogenic massive sulfide deposits

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• World-wide contain about $1.3 trillion in contained metal value

• Sustain much of the world’s supply of zinc and silver, important

sources of Cu, and are a major source of “high tech” metals (Ge, In)

• Occur in strata of all ages

Major Volcanogenic Massive Sulfide Districts of the World

[VMS] occur throughout the world and in strata of virtually all ages from early Precambrian until the present. Although by comparison with giant porphyry copper and Sedex deposits the individual deposits tend to be small, they almost always occur in districts or "camps" which are comprised of anywhere from 3 to more than 10 individual deposits. VMS deposits are one of the world's most significant sources of zinc and lead, a significant source of silver and gold, and are also the sources of many of the "high tech" metals that are in increasing demand. I will explain the five subtypes of VMS deposits in subsequent slides. I will explain the five subtypes of VMS deposits in subsequent slides.

VMS Distribution by Age

5

Largest are associated with periods of continental breakup

Siliciclastic-dominated (Bathurst, Iberia) contain most metal

Early Proterozoic (Trans Hudson-Svecokarelian) is richer than Archean

Cretaceous dominates the Mesozoic

Slide 5. The slide shows the distribution of VMS deposits by age both in terms of tons of ore on the left and tons of metal on the right. The distribution through time is controlled very much by times of major supercontinental breakup. The first of these appears to be in the early Proterozoic with the breakup of an Archean supercontinent and formation of a wide range of VMS deposit types in both the trans-Hudson origin and Svecokarelian Shield. Major breakups of Rodina, Pangea and Gondwana led to the formation of younger ocean floor distinct back arc basin formation and in areas of back arc extension, the formation of VMS deposits. Although the breakup events mark the start of major arc formations, these tend to be protracted events; for example, the major breakup of Gondwana in the Cretaceous was accompanied by major VMS districts that formed in the mid Cretaceous in many areas marginal to continents (S and North America, Pontides in western Asia etc.), but in South America VMS formation continued into the Eocene.

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Here’s Where VMS Deposits are

Forming Today

At Spreading Ridges

At Oceanic SubductionZones

Slide 6. And of course what makes research on VMS deposits so exciting is that they are forming today in modern seafloor environments. Even though we have only investigated <1% of the seafloor for hydrothermal activity, seafloor massive sulfide deposits have been discovered along not only major oceanic spreading centres, such as the Mid-Atlantic Ridge, but also in modern oceanic island arc environments, Such as the Tonga and Kermadec arc and back arc environments.

7

Endeavour Ridge- Example of Mound Growth

Godzilla

Vent,

Endeavour

Ridge

Kelley, D. S et al, 2001

Slide 7. The study of modern day seafloor massive sulfide environments allows us to better understand the composition of the hydrothermal, and sometimes magmatic fluids responsible for metal deposition, and how and where sulfide mounds grow and how they are preserved in the rock record.

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Action At Plate Margins: Making and

Consuming Ocean Floor

Slide 8. It also allows us to better understand the tectonic environments in which ancient VMS deposits form, and how we can recognize these environments in the ancient rock record through a good understanding of volcanic architecture and its geochemical signature. The first modern seafloor massive sulfide systems were discovered along oceanic spreading centres, where new basaltic crust is generated, adding fresh buoyant crust to the migrating slab. For this reason the first comparisons we made were between modern ocean spreading systems that host VMS and ophiolites, which are parts of ancient ocean crust that has been obducted on to adjacent land masses.

Shanks and Thurston, 2010

VMS-hosting tectonic environments

Nascent arc-

fore arcRifted arc

Slide 9. We know from the ancient rock record that most of the VMS deposits that have been discovered and developed since the early 20

th Century were in fact hosted

not at ocean spreading centres, but in oceanic, and sometimes continental island arc regimes. And so as researchers and explorationists we must be aware of the wide variety of oceanic environments where we can search for ancient deposits. This includes fore-arc, arc, rifted arc and back arc environments, as well as continental back arc terranes that develop into peripheral basins. So what do all of these different oceanic terranes have in common that allows them to host VMS deposits?

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Action At Plate Margins: Making and

Consuming Ocean Floor

Slide 10. The common tectonic process for the development of VMS terranes is crustal extension.` Where the ocean crust extends it thins, and as it thins it allows magma to approach closer to the seafloor, which in turn triggers fluid convection, the generation of metal-rich fluids, and the discharge of these fluids to form VMS deposits. We can certainly understand how this extension works at oceanic spreading centres because they are spreading, but why do most VMS deposits actually form in island arc regimes over a subducting platrgin.

•Must have slab roll-back to provide for extension

•Need dense, dehydrated, cool oceanic crust to be effective

•Should be distal to spreading ridge

Compression vs. extension

Slide 11. Intuition would tell us that at converging plate margins responsible for the formation of magmatic arcs the overlying plate is compressed and pushed upwards such as we see along the Cordilleran margin of South America. In reality, whether we have compression or extension depends on the age and thickness of the subducting ocean plate. Young, thin, bouyant ocean plates tend to subduct at a shallow angle, which results in compression. Older, colder and thicker ocean crust tends to subduct at a steeper angle. This causes the subducting plate to pull, or roll back, resulting in periods of extension in the overlying arc environment

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Slide 12. These diagrams illustrate the relationship between lithotectonic setting and the 5 VMS deposit types. On the left the two diagrams represent ocean-ocean subduction; as a cold oceanic plate was subducted beneath another oceanic plate. The arc is formed above the subduction zone and back arc spreading occurs behind it, cased by extension related to the roll-back of the cold subducting plate.. These "primitive" settings and gender basalt dominated copper – rich deposits as shown. Diagrams on the right illustrate subduction near a continental landmass. Shallow buoyant plate subduction results in extension the adjacent continental landmass and VMS deposits typically do not form. Steep subduction, formed by older, colder subducting plates, cause extension at the margins of the continent, where older continental material not only partially melts to form volcanic rocks, but is buoyant relative to the basalt dominated types, engendering a shallow water depositional environment.

The VMS Family

Galley et al., 2007

Tectonic setting

Depositional setting

Deposit type

Slide 13. The variations in extensional tectonic settings in which VMS deposit form can be recognized in the ancient rock record by the physical attributes and geochemistry of the VMS host stratigraphy, which tells us a lot about the their depositional settings. As we will see over the next few slides, each of these depositional settings can be related to a specific member of the VMS family

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Five VMS Lithotectonic Settings

• 1: Bimodal mafic-dominated volcanic

– Ocean-ocean suprasubduction arc; flow-dominant

• 2: Mafic backarc: mafic volcanic dominated, ophiolite-associated

– Oceanic backarc and mid-ocean rifts; flow dominant

– Some plume-related (including alkaline) volcanics

• 3: Pelitic Mafic backarc: sediment, mafic flow/sill dominated

– Oceanic backarc-rift; pelagic sediments

• 4: Bimodal felsic-dominated volcanic

– Ocean-continent suprasubduction arc; pyroclastic dominant, old crust melting

• 5: Siliciclastic- Felsic

– Ocean-continent backarc; pyroclastic dominant, continental-derived sediments

Slide 14. After reviewing the geological setting of approximately 1000 deposits contained in preserved terrains, plus the modern active systems on today seafloor, five subtypes have been defined, as outlined here. The principal element of control on these is the relationship of the back arc system and accompanying subduction zone to continental landmasses. One, two and three form in primitive ocean – ocean subduction environments, distal to continental landmasses. Types four and five form in back arcs typically adjacent to continents.

Petrochemical Assemblages

From Piercey (2007)

Slide 15. The variety of extensional environments in which VMS deposits are generate are characterized by different host rock assemblages. The formation of these assemblages and their physical and geochemical characteristics are controlled by how evolved the magmas were that generated the VMS-associated effusive and volcaniclastic strata and high level intrusions. In seafloor spreading centres, nascent arc and back arc environments the magmas are mantle derived and primitive, leading the the presence of boninite, low-Ti basalt, NMORB to primitive arc tholeiite. These environments are dominated by mafic effusive flows and very little volcaniclastic rock or sediment. The few rhyolitic flows that are generated are not necessarily spatially associated with the VMS deposits, as they are formed late in the volcanic history from the fractionation of gabbroic subvolcanic intrusions. This VMS environment is known as Mafic-dominated, and is most commonly found in the ancient rock record in ophiolitic suites. Because of the very dynamic effusive flow environment the associated VMS deposits tend to be quite small, in the 3-5 Mt range, and more copper rich.

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Ophiolite-hosted

•Usually formed in mature

oceanic backarcs

•May be obducted oceanic

spreading ridges

•Productive areas have

trondhjemite in the

subvolcanic sequence

•biggest deposits low in

stratigraphic section

•Au-rich caps generally

overlooked by early miners

Slide 16. This illustrates the classic Type 2 stratigraphic setting, based on the Cyprus ophiolite complex. Most of the deposits formed in the lower part of the stratigraphic assemblage, some within the uppermost part of the intrusive complex [sheeted dikes], illustrating that intrusive and volcanic activity continued through the period of formation of the deposits. Many are capped by a thin sequence of sulfide – or oxide – rich pelite, and the deposits may have formed immediately under it.

Mafic Dominated (ophiolite)

• Setting: Nascent arc-fore arc, mature ocean-ocean backarc, oceanic spreading ridges

– Ore: Massive sulfides bulbous pyritic, low As, Sb, Sn; prominent Cu-stringer zones.

– Pipe alteration: pronounced, zoned Fe -Mg chlorite core, sericite+/- paragonite rim ; silicic veins

– Semiconformable: Epidote-albite-actinolite, local silicification; regular metal depletion

– Examples: Cyprus, Oman, Lokken (Norway)

Lydon & Galley, 1986

Slide 17. The deposits are pyrite-rich, compact massive sulfide bodies that commonly have a gossanous cap due to seafloor weathering that in some case is Au-enriched. Although mafic-dominated VMS deposits are thought of as Cu orebodies. This is because many were mined before the time when Zn could be recovered. In reality, they are Cu-Zn orebodies, with a Zn-enriched rim and a Cu-enriched margin and a silicic pyrite-rich core that leads down to a quartz-pyrite stockwork vein system enclosed in a chlorite-rich alteration pipe. These deposits are also commonly associated with thin overlying layers of jasper-rich iron formation.

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From Piercey (2007)

Slide 18. The variety of extensional environments in which VMS deposits are generated are characterized by different host rock assemblages. The formation of these assemblages and their physical and geochemical characteristics are controlled by how evolved the magmas were that generated the VMS-associated effusive and volcaniclastic strata and high level intrusions. Next are extensional environments which are dominated by the generation of large volumes of mafic siliciclastic rocks. This most commonly occurs in the forearc wedge to oceanic island arcs or in mature oceanic back arc basins where erosion and sedimentation dominate over volcanism. Where magmatism occurs it is in the form of mafic sills and rare effusive pillowed flows. These Mafic Siliciclastic VMS environments host what are commonly referred to as Besshi-type deposits. They can be quite large, up to 100’s of millions of tons, and are usually quite Co and Ni rich.

VMS-hosting mafic siliciclastic

– Ore: Massive sulfides tabular, pyrite & pyrrhotite, low

As, Sb, Sn; high Co (+/- Ni?), minor Cu-stringer zones.

Some sulfidic sediments, chert, IF, tourmalinites.

– Pipe alteration: zoned Fe -Mg chlorite core, sericite;

– Semiconformable: not well established

– Examples: Besshi, Windy Craggy

(Peter & Scott, 1999)

Slide 19. This cross-section, on the left, illustrates the typical setting of a mafic – siliciclastic deposit. On the right side is a cross-section of the Windy Craggy deposit, one of the best examples of this VMS subtype.

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From Piercey (2007)

Slide 20. The variety of extensional environments in which VMS deposits are generated are characterized by different host rock assemblages. The formation of these assemblages and their physical and geochemical characteristics are controlled by how evolved the magmas were that generated the VMS-associated effusive and volcaniclastic strata and high level intrusions. As an oceanic arc matures we begin to see the generation of volcanic depressions, such as calderas and more linear rift structures that are characterized by a bimodal tholeiitic basalt-rhyolite assemblage. The volcanic architecture is still dominated by effusive flows, but we observe periods of lower magmatic activity where rhyolite dome flow complexes form, associated with hydrothermal activity that first forms thin chert-sulphide horizons or oxide facies iron formation, followed by the formation of massive sulfide deposits in direct spatial association with the rhyolites, which can form up to 10% of the rock volume. These are known as Bimodal Mafic type VMS systems, with Noranda being the most famous camp of this type. The deposits, as will see, are Cu-Zn rich with appreciable amounts of Au and Ag, and can be up to several 100’s of millions of tons, such as the Horne and Kidd Creek deposits.

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Bimodal mafic-dominated (flow)• Setting: Ocean-ocean arc, nascent (early) arc rifting, deep

water

– 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, <25% Na-felsic flows, flow breccia, endogenous domes, minor pyroclastic; sediment minor (tuffite). Felsic (some mafic) subvolcanic intrusions prominent (3x20km)

– Ore: Massive sulfides bulbous to tabular, pyritic, As, Sbminor, Sn variable; prominent Cu-stringer zones.

– Pipe alteration: pronounced, zoned Fe -Mg chlorite core, sericite rim; quartz-sulfide veins

– Semiconformable: Epidote-albite-actinolite, overlain by silicification; irregular metal depletion

– Examples: Noranda, Mattagami, Snow Lake (Anderson-sequence), Tambo Grande, San Nicolas, Norwegian Caledonides

Slide 21. Type I deposits illustrate the diversity within this class, with two quite distinct subgroups. Type IA is the "classic" group of deposits, which occur in bimodal, basalt and flow – dominated settings. These deposits tend to be copper rich, and many have formed on the seafloor as bulbous mounds. These deposits have well-defined alteration pipes, typified by a core of intense chloritization and chalcopyrite – rich stringers, surrounded by a broad zone of sericitization.

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VMS-hosting mafic bimodal setting

• Nascent to rifted oceanic island arc setting commonly defined

by cauldron environments

• Cauldron underplated by subvolcanic intrusive complex (quartz

diorite-tonalite-trondhjemite)

• VMS horizons defined by sulfidic exhalite units

• Noranda, Matagami, Kidd Creek, Flin Flon, Snow Lake,

Ladysmith-Rhinelander

Slide 22. The distribution of deposits in a cross-section of the famous Noranda camp [left diagram]. Note that the deposits in this camp occur at several stratigraphic horizons. Generally speaking, they range from the most copper rich near the base of the section to more zinc – rich at the top of the section. The diagram on the right illustrates the typical position of these deposits in a subduction -related setting. Note that the deposits are associated with a distinct ocean – ocean back arc setting.

VMS-hosting bimodal mafic

Ore: Massive sulfides bulbous to tabular, pyritic, As, Sb minor, Sn variable; prominent Cu-stringer zones. Commonly stacked lenses

– Pipe alteration: pronounced, zoned Fe -Mg chlorite core, sericite rim; quartz-sulfide veins

– Semiconformable: Epidote-albite-actinolite, overlain by silicification; irregular metal depletion

Slide 23. This diagram illustrates the classic section at Noranda, [left diagram] and topology of a classic system, illustrating that in some districts, the deposits are stacked. Typically, the stacking occurs within a single felsic – effusive section, and at the classic Millenbach deposit in the Noranda camp the deposits are stacked within a single flow -dome complex. Note that the typical composition has significant zinc and copper contents, modest silver contents, and relatively low gold contents.

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From Piercey (2007)

Slide 24. The variety of extensional environments in which VMS deposits are generate are characterized by different host rock assemblages. The formation of these assemblages and their physical and geochemical characteristics are controlled by how evolved the magmas were that generated the VMS-associated effusive and volcaniclastic strata and high level intrusions. As a volcanic arc tectonic environment evolves and thickens further we observe that the rock compositions begin to evolve away from tholeiite towards more calc-alkalic compositions. This is because of the changing influence of the mantle environment, in which the subducting oceanic slab is dehydrated under increasing temperature and pressure, and the resultant fluids rise up into the melt fields to add a higher degree of low field strength elements. At the same time, the crust is thickened to the extent that previously hydrated parts of the oceanic crust melt, adding a more hydrated magmatic assemblage. The thickened crust also allows the formation of mid-crustal intrusions that then differentiate and may be melt-contaminated to form more complex volcanic assemblages such as basaltic andesite, andesite, rhyodacite and dacite. The hydrated compositions result in more explosive volcanism, more rapid evacuation of shallow magma chambers, and the formation of calderas. These environments are dominated by more felsic volcaniclastic assemblages and lesser felsic dome complexes, overlain by thick sequences of effusive basaltic andesite, and are known as Bimodal Felsic VMS environments. The most well-known of these is the Hokuroko district in Japan. The deposits are Zn-Cu-Pb-Ag-Au-rich and are usually in the range of 2-10 Mt in size.

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Bimodal felsic-dominated

• Setting: Ocean-continent arc, nascent arc rifting– Av. Size and Composition: 3.4 m.t., 1.0 Cu, 4.4 Zn, 1.1

Pb, 1.1 g/t Au, 56 g/t Ag

– Geology: <65% Na+/- K felsic pyroclastic strata, endogenous domes; sediment minor (tuffite). >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 sericite dominant, zoned Fe -Mg chlorite core, sericite rim

– Semiconformable: Lower albitic, upper K-spar rich, Na-depleted, uppermost zoned (Fe) carbonate

– Examples: Kuroko, Skellefte, Greens Creek, Buchans, Buttle Lake

Slide 25. The next two VMS types formed in extensional back arcs proximal to a continent. Consequently the volcanic systems associated with them interacted with continental material above the subducting slab. Both of these are significantly more zinc and silver rich, with moderate to low copper contents. Type 4, formally known as “Kuroko-type”, formed in pyroclastic – dominated settings, but with significant amounts of felsic flow material, and formed either as displacement of that pyroclastic material beneath a sedimentary Rock, or as a mound on the paleo seafloor. The alteration pipes tend to be laterally extensive, with intense silicification and sericitization, and significantly less chloritization. Carbonate alteration is commonly present in the footwall. Some formed in exceptionally shallow water environments, are zinc-lead rich and have virtually no copper stringer material; these are commonly exceptionally rich in silver and gold.

Bimodal felsic

Mercier-Langevin et al., 2007

• Part of successor arc oceanic to ocean-continent arc terranes

which develop over thickened arc crust under which lithospheric

mantle melting and crustal partial melting occurs.

• Melting occurs over zones of slab breakoff, which in turn opens

zones of extension in overlying arc terranes

• Result is zones of crustal melting mixing, assimilation and

fractionation, more volatile magma generation and abundant

explosive volcanism

Slide 26. The bimodal felsic – dominated setting generally forms in a back arc that is more proximal to older continental material. Partial melting of this older continent, combined with partial melting of basil hydrated mafic rocks generates an excessive amount of felsic melt. These areas tend to be more buoyant than those associated with ocean – ocean subduction zones, and consequently formed more shallow – water VMS environments. In some cases, these terrains may be transitional to a more mafic – dominated ocean – ocean subduction environment, but recent isotopic studies illustrate that in virtually all cases partial melting of older continental material is a significant source of the felsic melts. Many of these environments form Calc alkaline systems, and some of these may engender a contribution of magmatic fluid. Recent evidence from the Bousquet District proves that a magmatic contribution of gold is possible; this contribution is typically superimposed on slightly earlier more conventional convection – driven hydrothermal products.

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• Ore: Massive sulfides bulbous to tabular, pyritic, As, Sb, Snvariable; some Au-rich, local (late?) high-S zones; limited Cu-stringer zones. Baritie (Au) cap in Phanerozoic deposits

• Pipe alteration: pronounced, silicification and sericite dominant, zoned Fe -Mg chlorite core, sericite rim

• Semiconformable: Lower albitic, upper K-spar rich, Na-depleted, uppermost zoned (Fe) carbonate

Examples: Hokuroko, Skellefte, Sturgeon Lake, Mount Read, Buchans, Buttle Lake

Slide 27. Bimodal felsic deposits are most commonly spatially associated with felsic flow domes and dome complexes. They tend to be massive in composition and have a high thickness to width ratio. Like bimodal mafic VMS, they are strongly compositionally zones, with a siliceous, Cu-rich core and more Zn-Pb-rich margins, with Pb being a major addition from the bimodal mafic variety. In Phanerozoic deposits, there is commonly a barite mantle, are a barite-rich periphery to this deposit type, with the barite sometimes being quite Au-enriched. Although generally missing in the Precambrian, these barite caps are also present in Archean terranes >3 billion year old, which is a story for another time! The proximal footwall alteration has a siliceous core, with a sericite-dominant discordant alteration pipe that include Fe-Mg chlorit enearer the centre. The broader, semi-conformable alteration zones in bimodal felsic VMS districts have a lower albite-amphibole-enriched zone followed upwards by Na-depleted rocks enriched in K-feldspar and carbonate. As we will discuss later the carbonate tends to become Fe-enriched in proximity to VMS mineralization. Examples of bimodal felsic VMS camps include the Archean Sturgeon Lake camp in Canada, the Paleoproterozoic Skellefte camp in Sweden, the Ordovician Mount Read district in Tasmania and the famous Miocene Hokuroko district in Japan.

From Galley et al. (2007)

•Felsic and mafic rocks

with sediments

•Characteristics of

epithermal and VMS

•Aluminous alteration

(pyrophyllite, etc.)

•Hg-Bi-Sb-As-Au-Ag-

S-rich (epithermal)

•Zn-Pb-rich (VMS)

Examples:

•Mt. Lyell, Tasmania,

•Eskay Creek, Boliden,

LaRonde

•Neves Corvo (Sn)?

Hybrid bimodal felsic: Magmatic contribution?

Slide 28. These diagrams illustrate the setting of some of the districts that contain both conventional VMS products and possibly superimposed gold – rich mineralization originating from a magmatic – fluid source. These settings are commonly in mid – to relatively shallow water environments [1500 – 200m].

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From Piercey (2007)

Slide 29. Finally, we have what are known as continental back arc extensional environments which form from the extension of more sialic continental crust where ocean crust subducts along the margin of a continental mass. These environments are dominated by felsic fragmental and reworked fragmental rocks and the presence of areally extensive iron or Fe-Mn formations. Effusive flows are rare, and any associated large subvolcanic intrusions are located in the underlying continental crust. These Felsic Siliciclastic environments are not necessarily thick, they can be at times in the range of hundreds of metres of less, but they are host to some truly impressive VMS deposits in terms of tonnage. Examples include the Bathurst deposit in Canada and the Iberian Pyrite Belt deposits.

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Felsic siliciclastic hosted

• Setting: Mature ocean-continent backarc – Av. Size and Composition: 7.1 m.t., 0.6 Cu, 2.7 Zn, 1.1

Pb, 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. Some with IF, chert

– Pipe alteration: variable size, usually minor, sericitic, silicic, Fe-chlorite

– Semiconformable: not well established (K-rich upper zone, Na-rich (+epidote, actinolite) lower

– Examples: Bathurst, Iberia, Kazakhstan, Bergslagen (?)

Slide 30. Type 5 settings are typified by the Bathurst N.B. and Iberian districts. Their footwall sequences are comprised almost totally of continental – derived wacke, and they occur usually in the lower part of the volcanic strata, many with only a few hundred meters of felsic volcanic strata beneath them. The volcanic rocks are typically pyroclastic, and mafic rocks almost always are above the deposits. Deposits in most of these districts are capped by ferruginous shale or iron formation, and the deposits are tabular and appear to have formed beneath these caps. Stringer zones are usually poorly developed, pyritic, associated with silicification and sericitization. Where present, the stringer zones tend to have stratiform disposition, and in Spain, are amenable to open pit mining. These deposits are typically twice as large as the other types, and zinc rich, but distinctly pyritic. Their precious metal contents are not particularly high.

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VMS-hosting felsic siliciclastic

O2

Syn-riftclastics

SubductionZone andIsland Arc

OceanicCrust

Back-Arc

Rift

Shelf Land

Sea level

Oceanic Crust

Asthenosphere

ContinentalBasementMagma

Syn-rift

clastics

W. Goodfellow , C Van Stal

(Peter & Goodfellow, 1996)

• VMS environment underlain by rifted

continental crust

• Continent-derived sediment-filled basin,

thin veneer of felsic volcaniclastic strata

accompanied by Fe-formation

• Hydrothermal systems driven by coeval

intrusion in underlying clastic sediment.

• Commonly overlain by ocean floor

MORB or alkalic basalts

Slide 31. While the preceding VMS types are generated within oceanic suprsubduction settings the felsic siliciclastic deposits form in continental suprasubduction settings where extension behind the magmatic arc generates a back arc basin. Because it is flanked by continental crust the basin is filled with eroded siliciclastic material accompanies by intermittent felsic magmatic activity and the intrusion of more intermediate to mafic sills.


Iberian Pyrite Belt

• Intracontinental rift or transtensional basin (not supra-subduction)

• Felsic volcanism dominates (boninitic affinity), as at Finlayson Lake, Yukon

• Host rocks dominantly pelitic and psammitic sediments

• Ore horizons at change from volcanism to sedimentation (some wholly in

sediments)

(Carvalho et al. 1999)

Slide 32. Convergent plate boundaries are major sites of mineral deposit formation. Interestingly, the felsic volcanic activity with which this type of VMS deposit is associated is usually represented in the literature as being quite thick, in the range of kilometres. In reality, this is a function of thrust fault repetition that is common in these terranes, and the actual thickness of the VMS-hosting volcanic pile is measured in hundreds of metres at the most.

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From Galley et al. (2007)

• Ore: Massive sulfides huge tabular, pyrite & pyrrhotite, variable As, Sb, Sn; minor Cu-stringer zones.

• Some with multi facies IF, Mn chert

• Pipe alteration:variablesize, usually minor, sericitic, silicic, Fe-chlorite

• Semiconformable: not well established (K-rich upper zone, Na-rich (+epidote, actinolite) lower

• Red Sea model of heavy

brine accumulation?


Examples:

•Iberian Pyrite Belt

•Brunswick No. 6 &12, Bathurst, Wolverine,

•Finlayson Lake area

Slide 33. The felsic siliciclastic deposits are commonly tabular in form, with a metal zonation that includes a pyrrhotite-chalcopyrite -rich footwall stringer zone up into a dominantly Zn-rich pyrite-sphalerite massive sulfide lens that increases in Pb abundance towards the top. As we will discuss in more detail later in the talk, this deposit type is also characterized by a spatial and temporal association with extensive units of iron formation. The change in the metal content and facies from oxide through silicate to carbonate makes these iron formations effective vectoring tools for the exploration of felsic siliciclastic VMS deposits. Examples of this deposit type include the Paleoproterozoic Bergslagen district of Sweden, the Ordovician Bathurst camp of Canada, Devonian Finlayson district of the Yukon and Spanish and Portuguese Iberian Pyrite Belt of Carboniferous age.

Snow Creek

NoneMain subdivisions of the Snow Lake arc

assemblage

Slide 34. Now what must be kept in mind is that ancient island arc assemblages represent an evolution through time of a subducting margin environment. This means that the different stages of this tectonic environment can be stacked upon one another giving us several different VMS environments within a single VMS district. An example of this is the Paleoproterozoic Snow Lake camp we have Cu-Zn rich mafic bimodal deposits in a lower tholeiitic primitive arc assemblage, followed by Zn-Cu-Au-Ag rich VMS in the overlying low-K calc-alkalic bimodal regime representing a maturing island arc succession. This is in turn overlain by a rifted arc succession characterized by NMORB basalts that does not host any known VMS deposits.

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35

Median Metal Content (Millions of Tonnes/Kg)

2016

AVERAGE 1:Bimodal

Mafic

2:Mafic

Dominated

3:Pelitic-

Mafic

4:Bimodal

Felsic

5:Siliciclasic

Felsic

Cu % 1.33 1.7 1.35 1.1 0.6

Pb % 0.01 0.01 0.01 0.8 0.90

Zn % 2.5 1.05 1.58 5.0 3.04

Au g/t 0.37 0.01 0.2 0.4 0.2

Ag g/t 22 11 20 64 40

TOTAL METAL

(TONNES) 121,276 58,529 144,970 198,461 307,490

TOTAL SULFIDE (MILLION

TONNES) 2.84 2.26 4.7 3.34 5.7

N 291 76 95 243 107

Slide 35. This table shows a comparison of compositions for the five types of VMS environments. Note that those that formed in epi – continental environments have significantly higher lead contents relative to the others, and somewhat less copper. Note also that those that form siliciclastic - dominated areas [type 5] are much larger on average than the others.

36

Summary• Cu-rich deposits formed in primitive, basalt-dominant oceanic

arc-backarc systems: deep water (>1500m)

• Zn-rich deposits formed in epicontinental, felsic and sediment-dominant oceanic backarc systems: water depth<1500m

• Subvolcanic intrusions are commonly composite

• High-T reaction (ep-ab-act-qtz) zones are metal depleted with silicified caps (or carbonate in shallow-water systems): These provide good indictors of potentially productive regions

• Alteration pipes occupy synvolcanic faults: vertically extensive, for Cu-rich, laterally extensive Zn-rich deposits

• 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), and in oxidized distal exhalite (possibly with barite)

• Laterally extensive “exhalite” (chert or pelite) contains anomalous metals and conserved elements, useful as vectors to ore

Slide 36. To summarize: deposits formed in mafic, flow – dominated back arc systems that typify primitive ocean – ocean subduction zones are usually copper – rich with very low lead and precious metal contents. However, the type 1 deposits are themselves in either deep-water, basaltic, flow dominated or mid – ocean depths pyroclastic dominated sequences. The latter are typically more zinc and silver rich. In both cases felsic strata regionally comprise less than 10% of the volcanic component; locally, near deposits, this may increase to 25% Most of the more zinc-rich, copper – poor deposits formed in ocean – continent subduction environments. Pyroclastic rocks predominate, and these deposits formed primarily sub-seafloor by displacement of volcaniclastic and sedimentary strata. Each type has a distinctive alteration pattern, with those in types 1A, 2 and 3 having well-defined vertically extensive copper – enriched stringer zones. Types 1B, 3 and 4 have broad alteration zones typified by intense silicification in the core and broad zones of sericite and aluminous mineral formation adjacent. Both types may have caps of laterally extensive exhalite, which is typically silicified volcaniclastic and sedimentary material, but may also be pelite or iron formation.

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37

Birth and evolution of a seafloor

hydrothermal system

Slide 37.

38

3: Major Components of the VMS System

How do they Relate to Size and Composition?• Subvolcanic Intrusions

– Heat and possible metal sources

– May control location and number of discharge sites

• Volcanological and Petrochemical Constraints

– Volcanic products reflect environment (pyroclastics < 2000m = boiling?)

– Flow-dominated regimes = higher T?

– High-T felsics, synvolcanic ultramafic rocks and melt contamination (andesite) indicate more heat, larger deposits

• High-Temperature Reaction Zones and Alteration Systems

– Control metal contents through buffering and metal contents

– Create a “cap” that sealed the system

• Synvolcanic (extensional) Faults

– Create discharge conduits, identified by stratigraphic discontinuity and alteration

• Precipitation Sites

– Subseafloor interaction (alteration pipes) and subseafloor “replacement” deposition (semi-massive sulfide)

– Metal zoning, gold problem, magmatic vs. convective system contributions

• “Distal” products:

– Hydrothermal “escape” products provide vectors to VMS sites

Slide 38. The remainder of this presentation will examine the component parts of the VMS – forming system, and this slide will be repeated for each of the six components that comprise a classic VMS – district. First we will examine subvolcanic intrusions, which primarily provide the heat to drive a convective “reaction zone” system, where seawater at elevated temperatures remove metals from the volcanic rocks, forming the metalliferous hydrothermal fluid.

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VMS deposit clusters/districts

Definition of volcanic depression and

extent of the camp-scale hydrothermal

footprint!

Slide 39. In 1972 Don Sangster published a seminal paper describing the setting and character of VMS deposits. In this paper he observed that this deposit type almost always forms in clusters that has a “footprint” diameter of between 15 to 25 km. We know now that this feature is a function of the volcanic architecture hosting the VMS ore system and the size and nature of an underlying subvolcanic intrusive complex that supplies heat, and in a few districts, metals to an evolving VMS hydrothermal system controls the ultimate size of the district, and its individual deposits.

Extensional environments the key

Slide 40. The volcanic architecture in most VMS camps defines the presence of a volcanic depression that is the focal point for related magmatic activity. This “thermal corridor” of magmatism is related to the previously described need for an extensional crustal environment for VMS formation. When the crust is extended and thinned this causes decompression and melting in the underlying mantle. These mantle melts, in the case of spreading ridge environments, rise to within 2000 m of the ocean floor to initiate a VMS hydrothermal system. Where the ocean crust is thicker, these mantle melts pond at the lithospheric interface and cause melting in the overlying crust. The resulting two sets of magmas then rise up to high crustal levels where their magma chamber evacuate during volcanism, resulting in the collapse of the overlying volcanic environment to form calderas etc.

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41

Subvolcanic Intrusions Determine Heat and Fluid Flow

Constraints Barrie et al. Modeling Experiments

• More plumes of hydrothermal upwelling develop above a

shallow heat source in permeable “overburden”.

– More, but smaller deposits in volcaniclastic strata?

• The maximum spacing of hydrothermal upwelling above a

horizontal intrusion is ~3.8 times the distance between the

hydrothermal cap and the top of the sill;.

• Narrower reaction zone = smaller, but more abundant, closer spaced

deposits?

• Higher T intrusions = more extensive reaction, larger

deposits and districts:

• maximum volume of water heated to 350 C is equal to:

– the volume of a felsic intrusion at 7000C;

– two times the volume of a mafic intrusion at 10500C; and

– four times the volume of an ultramafic intrusion at 1650°C

Slide 41. This diagram is drawn to represent the surface of an extensional fault, and illustrates points in the previous slide. Heat from the subvolcanic intrusion [red] induced the overlying convective flow of progressively heated seawater, which formed beneath the hydrothermal cap [green]. The composition of the subvolcanic intrusion controls the amount of heat released to the convective system. Clearly mafic or ultramafic systems provide significantly more heat, which leads to much larger VMS districts and deposit sizes. The metalliferous hydrothermal fluid discharges along the fault at regular intervals, as noted in the previous slide. This explains the regular distribution of massive sulfide deposits along a synvolcanic fault system. In well-developed camps, these distances are typically 2.5 to 6 km, reflecting differences in thicknesses of the hydrothermal reaction zone beneath the and above the intrusion. The areas between the discharge points are draw-down zones, which may act to recharge the reaction zone. The reaction zones are essentially isothermal, as documented by ODP drilling at Middle Valley.

Evolution of a VMS

hydrothermal System

A: Depression-wide magma

chamber begins to

conductively cool, setting up

immature, shallow convection

cells

B: Continued magma

emplacement generates a

more mature seawater

convection system and higher

temperature W/R interaction

C: Fluid T increase and pH

decrease generates isotherm

controlled alteration fronts and

sealed high T reaction zone

Overpressure periodically

fractures high T reservoir with

metal-rich fluid escape

Slide 42. We will look at an example of a bimodal mafic, or Noranda-type VMS environment to describe the evolution of a seafloor hydrothermal system. As mentioned in the previous slide this starts with the conductive cooling of an underlying magma chamber which causes extensive but relatively cool (~200oC) heating of shallowly circulating seawater.

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Fe formations and back arcs

• Widespread conductive/convective heat trigger diffuse venting

and shallow seawater convection = break down of glassy

material and low (<200oC) cation exchange with seawater

• Heated seawater causes precipitation of carbonate close to

vents

Slide 43. This shallow circulation and progressive heating of seawater causes the breakdown of volcanic glass in the exposed flows and the release of dominantly Si and Fe. When this fluid return to the seafloor it mixes with colder seawater and precipitates its contained Si and Fe to form cherty sediments. In the case of a relatively oxidized seawater environment, and in some areas, abundant organic activity even in reduced environments, the result is various facies of iron formation. In the case of more anoxic basin conditions it forms a cherty sulfide precipitate known as an “exhalite”.

Bathurst VHMS & Iron Formation Model

After Peter et al. (2004) and Peter (2004)

Silde 42. Various models have been suggested to account for the presence of these VMS related iron formations and exhalite. Rather than an areally diffuse discharge described in the last slide there are those how advocate that these cherty sediments are the result of the focused discharge that forms the actual VMS deposits.

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Main subdivisions of the Snow Lake arc

assemblage

Foot-MuddHorizon = same strike length as underlying subvolcanicintrusive complex

Slide 45. There are several reasons to believe that an early, pre-VMS diffuse hydrothermal discharge is more commonly the process. On reason is that when we look at the distribution of the iron formation or exhalite we commonly observe that its strike extent equals that of the underlying subvolcanic intrusion, as illustrated here for the Snow Lake VMS camp and the exhalite Foot-Mudd horizon.

Slide 46. Secondly, these exhalite horizons commonly directly overlie several tens of metres of strongly bleached and altered volcanic rocks. This bleaching is the result of 200-250

oC reaction of

seawater with the cooling flows, which destroys the glassy rocks and releases Fe, Mg, Ca, and Si into the circulating seawater, plus in some places low concentrations of Zn and Cu as sulfides.

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Exhalite replacement by high T sulfide mound growth

Lasail mine, Oman

Ansil Mine, QC

Vermillion Mine, ON Ansil Mine, QC

Slide 47. Other evidence that the formation of these exhalite deposits happens in the early stages of seafloor hydrothermal activity is the fact that we see ample evidence that they are systematically replaced during the following stages of hotter, more evolved and metal-rich hydrothermal activity associated with massive sulfide deposition. In some cases, the remnants of these earlier formed sediments are found as partially replaced “fragments” within the massive sulfide ore.

Low temperature basin-wide hydrothermal activity

Austin Brook: Bathurst

Magnetite>Chert IF Hematitic Shale Siderite>Chert>Chlorite IF

Examples• Golden Mile, WA (Archean)• Manitouwadge ON (Archean)• Bergslagen, SE (Paleoproterozoic)• Bathurst, NB (Ordovician)• Iberian Pyrite Belt (Carboniferous)

Slide 48. Whereas the preceding slide gave examples of sulfidic exhalite units associated with VMS ore systems this slide gives examples of for really extensive, back arc basin-wide deposits of iron formation temporally and spatially related to the formation of VMS deposits.

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Bacterial Discharge and shimmering water

•The bacteria accumulate and may be trapped in the sediments.•Sediments also trap some low-T

discharge products (Pb, Ag, Mn, Ba, Eu)•Bacterial reeducation fixes metals, and may produce a Ni anomaly

Slide 49. Modern seafloor studies have demonstrated that these fields of shimmering hot water discharge are the focal point for rapid and voluminous bacterial growth that is discharged unto the seafloor by these diffuse, heated waters. The ability of these bacteria to sequester metals and related elements is in places responsible for the hydrothermal chemical signature displayed by these exhalative products

50

Genetic Models & Modern Analogues

For Exhalite

• Plume Fallout:– Precipitation from high-temperature (350-400°C)

hydrothermal fluids in a plume (focused venting)

• Low T Discharge– Precipitation from low-temperature (10-100°C)

hydrothermal fluids (diffuse venting)

• Brine Pool:– Precipitation from hyper-saline, high-temperature

(350-400°C) hydrothermal fluids in a brine pool (focused venting)• Lines of evidence: distribution, textures, mineralogy, bulk

geochemistry, rare earth elements, oxygen isotopes, fluid inclusions

• Rare; Red Sea only?

Slide 50. To summarize the processes attendant on forming exhalite: 1: plume fallout is probably not of major significance except within a few hundred meters of a major venting system. 2: low temperature discharge is by far the most significant process contributing to the formation of exhalite [note discussion of silica and solubility, to follow]. This contributes a significant amount of metal and conserved element addition to the sediments at distances that range from tens to thousands of meters away from any deposit. 3: although brine pools have been proposed for laterally extensive massive sulfide and exhalite systems, in reality these can only form from hyper-saline brines which require interaction of downwelling seawater with evaporate. The Red Sea is an excellent example of this but there are no other documented occurrences of this process. From both modern system studies and examination of preserved exhalative strata, it is clear that the hanging wall exhalite systems primarily formed from low temperature discharge.

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Evolution of a VMS

hydrothermal System


chamber begins to



cells

B: Continued magma












Slide 51. The next stage is the development of a higher temperature convective system, where the metals responsible for forming VMS deposits are leached from the volcanic strata, typically 50 to 4000 m below the paleo seafloor. At the onset of development of this high temperature reaction, downwelling cold seawater interacts with the rising high temperature fluid to form a cap which seals the lower reaction zone. Heat builds up in the lower reaction zone to a critical temperature where the metals are quantitatively leached from the volcanic strata, forming the ore-forming a metalliferous fluid. The To the reaction zone is typically only a few meters thick, and is comprised of a silicified, epidotized replacement of volcanic strata.

Mid-level W/R reactions and metal

stripping

(Greenschist)• Continued shallow

magmatism (higher level?)

• Emplacement of dike

swarms along extensional

faults

• Generation of Mg mixed

clays to Mg-chlorite (pH=4)

• Ca plag to

oligolclase/albite (spilites)

• Destruction of primary

mafics releases Fe, metals

• Oxidation of magmatic

sulfides releases S, Cu, Zn

Cordierite-anthophyllite (metamorphosed Mg-chlorites), Winston Lake

Slide 52. We now see further magmatic activity in the underlying subvolcanic intrusion which results in it’s advance towards the seafloor surface. In the many studies that Jim and I have made of subvolcanic intrusive systems, it appears that the thickness of these composite intrusions is commonly less than 2000 m and their strike length between 20-25 km. It’s thickness cannot exceed that of the overlying volcanic package, or rupture occurs, followed by magma evacuation to surface. So as this composite intrusion nears this 2000 m below seafloor mark it begins to interact directly with seawater trapped in the overlying volcanic rocks, which in some cases have porosities exceeding 50%. As this connate sea water heats it begins to interact with its hosts rocks. At the same time it begins to advect towards the seafloor, which draws in the colder seawater to form the beginning of a deeply convecting hydrothermal system. As these waters reach temperatures over 250

oC we see the destruction of the primary

mineralogy to form Mg-rich clays, zeolites and albite. The formation of the Mg-rich clays radically changes the pH of the circulating, heated seawater from 6-7 to 4. In other words it becomes acidic. The key assemblage, as shown from experimental work, is albite-actinolite-epidote quartz. This forms at above 380

oC and below 402

oC.

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EVOLOVING FLUID COMPOSITON

53

co

olin

g

pa

th

Seawater

(high fO2, low T, neutral pH)

Seawater-rock interaction at

increasing T and evolving pH - fO2

Hydrothermal fluid

(low fO2, high T, acid pH)

Reduced = stable HCL, Oxidized = stable H2S

Slide 53. These heated, acidic, reduced fluids are then capable of sequestering the metals released during the degeneration of the primary silicates and magmatic sulfides as metal chloride complexes. The stability of HCl is critical to the beginning of this process of metal stripping and retention. We will see later that if we have a magmatic component added to this hydrothermal system is may result in the stability of H2S in a more oxidized state which will allow the more stable transport of Au. AuHS- has inverse solubility, and thus is conserved regardless of temperature. It is, as we will see, very sensitive to the presence of oxygen.

54

Compositional Controls• Fundamental controls are pH and temperature

• pH is set by the buffering reaction in the “reservoir zone” (i.e. high-T reaction zone)

• pH may be modified by boiling (e.g. gold problem)

• Temperature is controlled by

– Heat input

– Adiabatic cooling

• Metal content of source rocks is the least important, but may have minor role

• Depositional environment controls zoning, metal redistribution

• Possible magmatic input: Unusual enrichments (Sn, Au, Se?)

Slide 54. Compositional controls that determine the metal contents of VMS deposits are in large part set first by the buffering capacity in the high temperature reaction zone, where the metals are transferred into solution, and secondly by the physical – chemical characteristics of the precipitation site. The first is largely controlled by the composition of the strata within the reaction zone, and secondarily by the heat input to that zone from the subvolcanic intrusive system. The second is largely controlled by adiabatic cooling. As previously noted pH is controlled primarily by the reaction zone, and in the largest number of districts, the reaction zone strata are basaltic, and the key reactions occur at temperatures greater than 380°C. However, in a few districts, and in particular the Type 5 bimodal – siliciclastic dominated systems, the reaction zone may have formed in sedimentary (or felsic volcaniclastic) strata, and the key reactions occurred at temperatures as low as 250°C. Thus the primary metal contents of hydrothermal fluids do not vary significantly, with the exception of those formed in the siliciclastic– dominated environments. Boiling associated with adiabatic cooling has a significant effect on the precipitated assemblage, and the generation of volatiles. In addition, it is clear that some districts have a specific addition of elements from a direct magmatic fluid source, this includes, for example, tin and gold. Although perhaps rare, recent work has demonstrated that significant additions of these elements may occur synchronously with the formation of a massive sulfide deposits, with fluids having been generated within the subvolcanic magmatic intrusive system that also provided the heat for convective reaction zone formation.

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Evolution of a VMS

hydrothermal System


chamber begins to



cells

B: Continued magma












Slide 55. With the maturing and deepening of the convecting seawater hydrothermal system we begin to reach temperatures of over 380

oC in which the sub-

horizontal isotherms above the cooling intrusion control the formation of a number of semi-conformable alteration zones the resultant reduction in rock permeability

High T W/R reactions and reservoir creation

(Amphibolite >350-420oC))

Ep-qz-amph alt’d basalt Silicified andesiteAltered basalt bx

High W/R

Low W/R

Slide 56. Besides the formation of lower temperature zones of clay-zeolite to albite-chlorite we observe the formation of higher temperature reaction zones. These include semi-conformable zones of silica enrichment, amphibolite-grade rocks enriched in base and precious metals and epidosite.

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Silicification-Three Ways

Silicification is a key part of

almost all hydrothermal systems: It

occurs as:

1. Quartz-filled veins

2. Silicification in the immediate

footwall to VMS deposits

3. Silica dumping in the caps of

hydrothermal systems

4. Silica dumping at contacts

with subvolcanic intrusions

Silicification occurs through

three processes:

Rapid pressure drop causing

catastrophic silica dumping;

caused by rapid extension,

resulting in instantaneous

fracturing and vein formation: e.g.

orogenic gold

Slow cooling through

conductive cooling &

mixing: Footwall

silicification, chert/

exhalite

Instantaneous superheating

through introduction of an

intrusion into silica-saturated

hydrothermal fluid: contact

silica dumping & reservoir

cap

57

Slide 57. A feature of several parts of the VMS system; most of these have already been mentioned but this slide illustrates three ways in which silicification can occur in a geological system. Silicification occurs as courts filled veins, which are a less common feature of VMS environments, it occurs also as silicification in the immediate footwall of those deposits that are most associated with volcaniclastic strata and are probably formed in relatively shallow – water systems. Silicification also occurs due to silica dumping at the top of hydrothermal reactions zones, forming the reaction zone. It is also common at the immediate contact with the volcanic intrusions. Each of these is related to a different process and all can be explained using the classic silica solubility diagram. 1: Rapid pressure decrease, causes significant loss of silica solubility, and consequently in seismically -related rapid opening, such as that associated with seismic-related rupture, and quartz is catastrophically precipitated. 2: If renewed magmatic activity causes regeneration of a subvolcanic intrusive system, the contact of hot magma with silica saturated hydrothermal fluid in a reaction zone will cause catastrophic dumping of silica, provided the pressure is relatively low. This is typical of most subvolcanic dykes, for example. Identification of intense silicification adjacent to a probable subvolcanic intrusion is a clear indication that a hydrothermal fluid had formed, and that the area has distinct prospectivity. 3:The third process is slow cooling of silica saturated hydrothermal fluid. This process is inhibited by relatively slow nucleation of quartz at low temperatures, but is prevalent in the footwall zones to deposits that have formed in volcaniclastic strata, where the hydrothermal fluids are distributed laterally through porous media, and cooled conductively. It's clearly important to distinguish the type of silicification observed in the field prior to drawing any conclusions about where within the hydrothermal system any observed silicification has occurred.

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High T W/R reactions and reservoir creation

(Amphibolite >350-420oC))

Ep-qz-amph alt’d basalt Silicified andesiteAltered basalt bx

High W/R

Low W/R

Slide 58. So the rapid precipitation of silica forms an impermeable seal under which the hydrated volcanic strata interact vigorously with the now hot, reduced and acidic evolved seawater. Because this is taking place at amphibolite-grade metasomatic temperatures the results can be spectacular and characteristic of a VMS-related high temperature reaction zone composed of amphibole (typically actinolite), sodic plagioclase, clinozoisite, garnet and sulfide minerals. We will talk about the generation of the epidosite shortly.

59

High Temperature Reaction Zones• As shown in discussion of subvolcanic sills, these

are where the bulk of the mineralizing fluid was made

• Controlled by the buffering capacity of the strata: – Mafic strata require 380-405oC

• Albite-epidote-actinolite-quartz assemblage generates pH<3.5, metals dissolve into solution

– Felsic strata buffer to acid pH at lower temperature(~250oC)

• Difficult to recognize in the field– Quartz-epidote patches , intense silicification along

synvolcanic dyke are good signs

– Exceptional metal depletion; ~90% of Cu, 60% of Zn, much Ba, K lost

Slide 59. Most of this has been covered earlier during the discussion of subvolcanic intrusions and associated high temperature reaction zones. To review, the buffering capacity of the strata within the reaction zone control the loss of metals to the hydrothermal fluid, but the reaction is typified by enrichment in albite – epidotized – actinolite – quartz. As previously noted these are exceptionally metal depleted and therefore geochemical he distinctive but difficult to visually recognize in the field.

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60

Role of Subvolcanic Intrusions:

Heat and Fluid Flow Constraints

Barrie et al. Modeling Experiments

• In the absence of faults, more hydrothermal plumes or sites of

hydrothermal upwelling will develop above a shallow sill heat

source than a deep sill, and there will be more plumes in

permeable “overburden”.

• The maximum spacing of hydrothermal upwelling in ocean

floor rocks above a heated, horizontal plane (magmatic sill) is

~3.8 times the distance between the top of the reaction zone

(base of the hydrothermal cap) and the top of the sill;

hydrothermal plumes may coalesce with time.

• Sill top irregularities on a scale of tens of meters may not effect

hydrothermal fluid flow significantly, whereas irregularities on a

scale of hundreds of meters or greater may strongly effect the

disposition of upwelling fluid flow.

Slide 60. Barrie et al. [1999] examined the theoretical aspects of heat flow from a subvolcanic intrusive sill by developing a set of fluid flow models. They showed that on emplacement, rapidly heated seawater trapped in fractures in the volcanic strata began to react with and rise rapidly [due to increased buoyancy] towards the seafloor. This displaced water induced increased downwelling of cold seawater, and where they met they formed a physical barrier (cap), comprised of silicification, epidotized strata, and Mg- smectite (and anhydrite in modern systems). This, illustrated in light green on the next slide formed an impermeable barrier, trapping progressively heated water beneath it. Seawater within this trapped portion of the system [typically 300 to 1000 m thick] continued to be heated from the sill, and on reaching a critical temperature, established by Seyfried et al.(1999) to be about 380°C, the water: rock reaction caused loss of virtually all of the metals, silica, potassium, barium, and manganese (and more) from the rock into the fluid. The convective cells are quite regular in size, with a width that is approximately 3.8 times the distance between the base of the reaction zone cap and the top of the cell. The convection cell spacing controls the distribution of VMS deposits.

Crystallizing magma interaction with evolved high T seawater

Differential cooling Granophyre/mairolitesRapid cooling

Slide 61. This high temperature reservoir zone is in direct contact with the underlying cooling subvolcanic intrusion. The result is a dynamic fluid rock interaction that causes more rapid cooling of the the intrusive rocks that is characterizes by extensive fracturing, formation of columnar joints and granophyric textures. These fractures then fill with the highly reactive hydrothermal fluids, which then react with the rapidly cooling intrusion to give some very characteristic features of a high level intrusion responsible for the generation of a seafloor hydrothermal system

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Evidence for generation of hydrothermal-magmatic

fluids: Dehydration assimilation of xenoliths

Magmatic contribution? • H2O, CO2, S, metals?

Slide 62. How involved are these cooling subvolcanic intrusions in supplying magmatic-hydrothermal fluids to the overlying convective system? A common feature of high level subvolcanic intrusions is that they enter into, and fracture the host volcanic rocks, which become xenoliths along the outer margin of the intrusive complex. Keeping in mind that these volcanic rocks are water rich, these xenoliths can be very reactive when in contact with the cooling magma. The result is the release of volatiles during xenolith dehydration, and the resultant formation of miarolitic cavities and pegmatitic rims. In some cases elongate fluid channel ways are evidence that these fluids are escaping into the overlying hydrothermal reservoir. Let’s keep this in mind when we later talk about gold content and possible magmatic contributions to VMS ore systems.

63

Fluid Discharge Controlled by Master Fault•Fluid generated in reaction

zone; near and below

subvolcanic intrusion

•Discharge spacing typically

4-6 km for intrusion depth of

1-2 km

•Discharge is up master fault

(lithostatic load)

•In near-seafloor zone

(~200m), subvertical

extension fractures

predominate

•Fluid transfers to fractures,

rises vertically to discharge

100-300m from master fault.

How to Identify Fault:

Diachronous talus breccia; formed a wedge,

“thickens out”

Polymictic “conglomerate” or breccia

Slide 63. A typical extensional rift is bounded by faults on both

sides. Typically one side has a listric normal fault that is deep penetrating to the base of the volcanic succession, whereas the other side has a more planar fault that is less vertically extensive. Only the listric fault will conduct hydrothermal fluids from the high temperature reaction zone, and consequently it's typical that massive sulfide deposits form only on one side of a rift. The geometry of caldera – related faults is much less well understood. Structural studies of normal faults show that they are associated with local, upper crustal near – vertical fracturing. Fluids moving up the faults are primarily under lithostatic load, and remain confined in the fault zone. However within a few hundred meters of the seafloor, the significantly lower lithostatic load enables the fluids to be controlled by the hydrostat. Hydrostatically controlled fluids depart from the listric fault, and move up the vertical fractures and catastrophically cool near and at the seafloor, forming VMS deposits and attendant alteration pipes. Thus if prospecting in well mapped areas, the deposits may not occur at the contact with the master – fault, but rather form a few hundred meters inboard of it. This has been documented in a few camps where such mapping has been undertaken. These faults typically have vertical displacements of several hundred meters, and are composite in nature. On the modern seafloor they are typically covered by thick diachronous wedges of talus and volcaniclastic material. These talus wedges are commonly covered with a thin pelite cover. The talus is comprised of unsorted angular fragments with significant enter fragment open space, which in hydrothermal discharge areas is almost always infilled by massive sulfide. In addition to the talus, episodic seismic activity along these faults causes volcaniclastic and sedimentary material to be easily displaced, forming debris flows which may also host massive sulfide deposits. In many camps, debris flows form a significant footwall succession to the VMS – hosting strata. Clearly, massive sulfide deposits commonly form in relatively high energy seismically active environments, which induce catastrophic failure of the wall rocks, formation of localized direction units, and high permeability zones within which massive sulfides precipitate. In a few cases, continued seismic activity and fault motion cause of the deposits to also be displaced and recreated. In a few cases sulfide talus extends well away from the deposits, and in some, [e.g. Buchans Nfld.] the entire deposit is composed of talus-like material.

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Supralithostatic to hydrostatic = High T fluid pressure release

Epidosite Chl-ser rich pipeOverpressure veins

High W/R

Low W/R

Slide 64. Evidence for cyclic over- pressuring of the hydrothermal reaction zone can be evidenced in the field. This includes the presence of zones of extensive Ca-Al-Si-Mg-Fe alteration in the form of epidosite. These represent the base of the upflow zones during focused hydrothermal fluid discharge up synvolcanic faults. We can also see evidence further up of fracturing and over-pressuring with Fe-rich vein systems cutting across lower zones of silicification, which in turn lead up to what we commonly understand as the more classic chlorite-sericite-sulfide rich alteration pipes that underlay accumulations of seafloor massive sulfide.

65

Semi-Conformable Alteration

Characteristics - Near and FarThree parts to the system

• Lower high temperature reaction zone

– high-temperature convective reactions, metal

source, Na ± Ca ± Si sink

• Cap of high temperature reaction zone

– moderate temperature, low pressure precipitation:

Na loss, Si ± Ca sink

• Immediate sub-seafloor high flux zone: The alteration

pipe

– high seawater flux, progressive downward heating

– Na loss, CO2, Mg sink

Slide 65. Semi-conformable alteration systems consist of three separate entities. I have already discussed the lower high temperature reaction zones, which form above the subvolcanic intrusion, and below the hydrothermal to that zone. The high temperature reaction zones are typified by virtually complete metal depletion [a useful field geochemical guide], some silicification, distinctive epidotization, and slight gain in Na, Ca, and Si. Their contact with the synvolcanic dykes intruding through them is typically highly silicified. The caps to the high temperature systems are typically only a few meters to tens of meters thick, and are rarely observed except in the most well exposed and thoroughly documented VMS districts. They consist of silicified, epidotized and Mg – chlorite enriched strata. Immediately below the paleo VMS horizon, seawater reflux was prominent causing distinctive loss of Na, and possible gain of Mg and CO2. The sizes and litho- chemical attributes of the zones are highly dependent on the paleo – permeability of the strata. This, in turn, may be a function of paleo water depth.

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66

Footwall Mineralogical Indicators: Pipe to

Regional Transition

Identifier

• Mg Chlorite

• Fe Chlorite

• Sericite

• Aluminous Minerals

• Zeolites

• Carbonate Species

Scale

Margins of pipe (?)

Centre of pipe

Throughout pipe, 1.5 X width

In pipe; also kilometers in semiconformable zones

Outside pipe; 100s-1000s

of meters

Outside Pipe; Siderite (core)

Ankerite Fe-dolomite

Calcite (margin)

Slide 66. This summarizes the key mineralogical attributes of alteration pipes, and attempts to summarize the scale of their distributions. Chlorite species in relatively simple, [and unfortunately small] systems are well zoned, with Fe-chlorite in the centre, Mg-chlorite in the margins. However all of these systems have extensive telescoping caused by changes in the locus of the discharge with each seismic event. Thus chlorite zonation is rarely observed. Sericite is are ubiquitous feature in virtually all VMS deposits due to the interaction of potassium in the hydrothermal fluid with access alumina generated by loss of sodium from albite. Sericite also interacts with the hydrothermal fluid to gain barium and thallium. Aluminous minerals (kaolin, andalucite) formed where the drawdown of cold seawater is inhibited due to silicification, and carbonate minerals form more regionally.

Lower Semiconformable Alteration

Assemblages: Volcanic-dominated Settings

VMS Type Mineral Assemblages Chemical Change

Mafic Bimodal Upper zone: feldspar destroyed,

K-mica +/- paragonite

Lower zone: albite-actinolite-

quartz-epidote

Upper zone:-Na, +K,

+Mg (near ore)

Lower zone; +Na, +Si

(local), -Cu, -Zn

Felsic Bimodal Upper zone: K-spar, carbonate

added (Fe-carbonate increases

towards ore). Albite destruction

widespread, aluminosilicate

abundant (andalucite)

Lower zone: albite, epidote added

Upper zone: +CO2, -

Na, +K

Lower zone: +Na ?

Mafic-Backarc Same as Type 1 Same as Type 1

Slide 67. The next two slides provide information on the characteristics of the lower semiconformable reaction zone alteration systems for each of the five VMS subtypes. Note that these are all quite similar, regardless of subtype. In a few, potassium feldspar may occur, but generally these are albitic, but not necessarily albitized.

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Lower Semiconformable Alteration

Assemblages: Sediment-dominated Settings

VMS

Type

Mineral Assemblages Chemical Change

Pelitic -

Mafic

Upper zone; carbonate, K-

feldspar added; widespread

barite, increasing near ore

Lower zone; Complete loss of K-

spar, carbonate, K-mica

Upper zone: +K, +Ba,

+CO2

Lower Zone:-K, -Rb, -Ba, -

Cu, -Zn, +Ca, -CO2

Felsic

Silici-

clastic

Upper zone; K-spar, mixed layer

clays (smectite-chlorite), zeolite,

carbonate

Lower zone: albite-chlorite-

actinolite-epidote

Upper zone: +K, +Mg

Lower zone; +Na, +Ca

Slide 68. Lower semi-conformable alteration associated with the epi-continental districts are less well established. A few of these may have K feldspar. All are metal depleted, including those that formed within sedimentary strata. Many of the systems that formed in intermediate to shallow water depths have significant lateral zones carbonate alteration that formed above the high temperature reaction zone cap. This will be discussed more extensively later.

VMS DEPOSIT

ENVIRONMENT

69

Slide 69. We have looked in a general sense of how we believe VMS hydrothermal systems are created through the emplacement of a high level intrusive complex and the subsequent heating of circulating seawater to interact with volcanic rocks in the subsea floor environment, strip them of their metals and then form a high temperature reservoir zone just above the cooling intrusion. Periodically this reservoir zone over-pressures and ruptures the overlying zones of impermeability to allow the metal rich fluids to escape towards the seafloor.

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70

Subvolcanic Intrusions Cooling and Evacuation Determine

Heat and Fluid Flow Constraints Barrie et al. Modeling Experiments

• Cooling intrusion sets up zones of high strain in overlying rocks

– These zones focus at the intrusion margins and centre

– Also focused on areas of most active magma replenishment

– Results in fracturing of the overlying rocks and upward injection of

magma and allowing reservoir fluid escape!

Slide 70. The injection of a high level magma chamber sets up a local stress regime in the overlying rocks. This stress zones are typically developed along the margins of the intrusive complex and at its center. These zones of weakness then become conduits by which magma leaves the subvolcanic intrusion as dikes along what are essentially synvolcanic faults.

Subvolcanic

intrusions and

upflow zones

71

• Final intrusive form includes

post VMS phases

• Early phases commonly

form stocks and dikes along

synvolcanic faults

• These early phases supply

magma to rhyolite flows

• Same faults and feeder dike

swarms act as hydrothermal

fluid conduits

Slide 71. In many instances the early intrusive phases form stocks and dike swarms that supply magma to rhyolite flow complexes. As the subsea-floor hydrothermal system matures these same faults and magma pathways act as focal points for the upward migrating metal-enriched hydrothermal fluids

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Slide 72. The fact that these synvolcanic faults act as conduits for both magmas and hydrothermal fluids is why we see a close spatial and temporal relationship between rhyolite extrusive complexes and VMS deposits. Furthermore, with continued reactivation of these faults with cyclic discharge of both magmas and hydrothermal fluid can result in the stacking of VMS deposits over time, all connected by the same discordant alteration and sulfide stringer system.

Fluid upflow and discharge

(Ore deposit environment)

73

Slide 73. Let’s now concentrate on the part of the system we consider to be the near deposit environment, which includes the near seafloor are where sulfides are precipitated from within a fluid regime whose interaction with the surrounding rocks results in distinctive alteration mineral assemblages that indicate a proximal deposit environment.

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Sericite Altn

Chloritic Altn with cpy

24% FeO, 19% MgO, 0.08% Na2O, 0.08% K2O

0.59% FeO, 0.95% MgO, 0.18% Na2O, 3.71% K2O

Slide courtesy of Nicole Tardif

Slide 74. As the upflow of hot, acidic, metal-rich hydrothermal fluids begin to come within 1000m of the seafloor they begin to mix with cooler advecting seawater along the margins of the upflow zone. This is responsible for two of the features of the upflow zones: Firstly they are zoned from chlorite-rich cores to sericite rich margins as a function of the degree of fluid mixing. Secondly the entrainment of seawater changes the physical and chemical properties of the hydrothermal fluid resulting in sulfide deposition.

co

olin

g

path Cooling & mixing with seawater are

the main mechanisms of sulfide

precipitation

Fluid Mixing and Cooling

Hannington et al., 1999

Slide 75. Cooling and mixing of seawater with upwelling hydrothermal fluid is one of the major mechanisms for sulfide deposition. The other two are boiling, which we will discuss in a minute, and rapid pressure release due to sudden fracturing. The hydrothermal fluid is rising from a reservoir in which fluid temperatures of over 350oC and low pH allow the fluid to retain base metals, and to a lower degree precious metals, in solution as chloride complexes. As you mix in colder seawater you not only drop the temperature of the metal-bearing solution, but you also rapidly change its pH. Both these processes destabilize the chloride complexes, allowing the metals to complex with sulfur and precipitate.

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Silicification-Three Ways

Silicification is a key part of

almost all hydrothermal systems: It

occurs as:

1. Quartz-filled veins

2. Silicification in the immediate

footwall to VMS deposits

3. Silica dumping in the caps of

hydrothermal systems

4. Silica dumping at contacts

with subvolcanic intrusions

Silicification occurs through

three processes:

Rapid pressure drop causing

catastrophic silica dumping;

caused by rapid extension,

resulting in instantaneous

fracturing and vein formation: e.g.

orogenic gold

Slow cooling through

conductive cooling &

mixing: Footwall

silicification, chert/

exhalite

Instantaneous superheating

through introduction of an

intrusion into silica-saturated

hydrothermal fluid: contact

silica dumping & reservoir

cap

76

Slide 76. This drop in temperature also causes silica to drop out of solution along with the sulfides, which is why VMS stringer zones are chiefly composed of silica and sulfides, and why we see abundant evidence for siliceous alteration directly below Cu-rich sulfide mounds.

Water Depth is Critical

• Water Depth affects:– Volcanological style:

• >2000m: flow dominated.

• <2000m: increasing pyroclastic vs shallowing

– Deposit style:

• Deep water: seafloor mounds or near seafloor deposits:

• Shallow water: subseafloor displacement/replacement formation

– Alteration:

• Deepwater:

– narrow (2x deposit footprint) and vertically extensive (100’s+meters) alteration

– Chlorite-sericite-chalcopyrite-pyrrhotite

• Shallow water

– Broad (5-50+ x deposit footprint

– Sericite-silica-aluminous-pyrite dominated

– Ore composition

• Deepwater: Cu-Zn; usually low Au (~1 g/t), Ag (<20g/t)

• Shallow water; Zn-Pb-Ag (~80-200g/t) - +/-Au (1-20g/t)77

Slide 77. Another physical aspect controlling the deposition of footwall sulfides and silica is boiling due to shallow water seafloor conditions during hydrothermal fluid discharge. Shallow seafloor conditions, and when we say shallow we are referring generally of depths to seafloor of less than 1500m is also responsible in many cases for a change in the morphology of the volcanic deposits. At shallower depths the volatiles in the rising magma exsolve to form abundant tiny bubbles. This causes the rising magma to rapidly lose cohesion, resulting in explosive volcanism and the development of deep piles of volcanic fragmental rocks.

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Deposit morphology and composition

Hannington et al., 1999

Au-Ag epithermalSea level

Dep

th (

mb

sl)

Boiling curve forseawater

Hydrothermal fluid discharge temperature

Temperature (oC)

Precipitation of Cu-Au caused by boiling

Zn-rich

Cu-rich

Cu-Au sulfide precipitationthrough cooled through seawater interaction

Modified from Yargeau, 2014

Boiling zone and formation of a Cu-Au stockwork with little or no surface expression

Slide 78. Both of these factors affect both the composition and morphology of the sulfide ore body. As the hydrothermal fluids rise towards the seafloor and begin to cool they lose their ability to maintain metals in solution according to their saturation index. Copper sulfides will form first during the temperature drop, followed by Zn and Pb sulfides. In deeper water conditions these saturation conditions are telescoped such that Cu and Zn, and attendant Au and Ag are precipitated in a very short interval resulting in a compact VMS deposit. As the ascending fluid crosses the adiabatic boiling curve we see the formation of extended vein stockwork systems that are Cu-rich ending in a Zn-Pb-Au-Ag rich massive sulfide lens. As we approach pressure conditions quite similar to that in which epithermal deposits form the chances are that most of the base metals are precipitated near the base and margin of an extended stockwork vein system, with the top being precious metal rich.

Footwall sulfide stockwork

• Multiple phases of

fracturing and

quartz-sulfide

deposition is typical

of VMS footwall

vine stockwork

zones

Slide 79. The third factor in the development and composition of a subsea-floor stockwork vein system is sudden pressure release, which results in a rapid change in the fluids saturation index and precipitation of silica and sulfides. As many of you are already aware, these vein stockworks commonly consist of a complex set of mutually crossing veins, with individual veins consisting of several generations of vein infill. This is the result of the cyclic, catastrophic pressure release as the vein stockwork seals itself through mineral precipitation, allowing hydrostatic pressure to repeatedly build up and then exceed lithostatic pressure multiple times during the lifetime of the hydrothermal discharge zone.

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80

Volcanological Setting Determines

Shape of Sulfide Distribution

Slide 80. The diagram on the right illustrates the typical form and distribution of VMS deposits in flow dominated regimes. Note that in many districts the deposits are stacked, each forming at an hiatus in volcanic activity. Although vertical stacking occurs in some districts [e.g. Noranda], the deposits in others are laterally time – transgressive Where vertically stacked, alteration continues into the hanging wall. Careful mapping to delineate synvolcanic faults may enable prediction of additional mineralization stratigraphically above or below a discovery. In some districts, synvolcanic faults are occupied by narrow synvolcanic dikes, many which feed flow-dome complexes. Identifying these dikes and tracing them stratigraphically upwards is a useful exploration tool, and has resulted in new discoveries in the Noranda camp. The diagram on the left illustrates deposits which formed sub – seafloor in volcaniclastic strata. Here displacement may occur in several stacked lenses, with intervening poorly mineralized pyroclastic units, representing resumption of volcanic activity, separating more intensely mineralized high permeability strata. In many deposits, there may be as many as five separate mineralized "beds". As with the flow dominated systems, deposits may be distributed in a time – transgressive system [Sturgeon Lake].

Upper fluid conduit and interaction

with seawater

From Gemmell and Fulton

(2001)

Peter & Goodfellow, 2003

Slide 81. Seafloor depth, original hydrothermal fluid composition and host rock competency then all play a role in not only the shape and composition of the sulfide stringer zone that typically underlies a massive sulfide deposit, but also the nature and composition of the attendant footwall alteration zone.

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82

Footwall Alteration• Must discriminate between alteration induced by ore fluids

(both convective and magmatic) and local advection

– Ore fluids add base and precious metals, Fe, K, S, Si, Ba

– Advecting fluids remove Na, Ca, add Mg, & may add SO4

(post Precambrian)

• Relationship between water depth and alteration assemblage

is critical

– Fluid discharge rate increases with water depth

– Slower discharge = more “ore fluid” – related alteration

(metals, conserved elements - SiO2,K,Ba,Tl), less

advection

• The physical characteristics (porosity) of the sub-ore zone

determine the alteration distribution and composition?

– Shallow water strata are more permeable, form broad,

poorly defined pipes, prone to silicification, carb ppt.

Slide 82. One key aspect of understanding footwall alteration is to recognize that the velocity of the fluid in the discharging fault zone is a key factor in determining whether the fluids move upwards and out into seawater, or travel laterally through the relatively porous strata that occur at the seafloor interface. This velocity is controlled by the pressure differential between the hydrostatic load at the seafloor and the list of static load in the high temperature reaction zone. Under relatively high hydrostatic pressure [i.e. water depth greater than 2000 m] this pressure differential engenders very high velocity fluid discharge. Consequently, the amount of drawdown of cold seawater is also very large, leading to deep penetrating Na – depletion zones, Mg – chlorite alteration, and vertically extensive stringer zones. It also leads to formation of massive sulfide bulbous deposits on the seafloor. In relatively shallow water systems, the pressure differential engenders much lower velocity fluid rise, and these fluids then tend to disperse laterally into porous media at the seafloor. This engenders a much different alteration system, where conductive cooling of a hydrothermal fluid in the sub-seafloor enables silicification and laterally extensive sulfide precipitation, leading to tabular deposits. Silicification impedes the discharge of the hydrothermal fluid as well, and prevents local drawdown immediately under the deposit. Thus Mg – chlorite is much less common in the systems, and if present is at the periphery of the alteration pipe.

83

Footwall Alteration - Zones and Sizes

• Cu-Zn (Bimodal Mafic, Mafic Backarc & Pelitic-Mafic type)

– Well defined

– Great vertical extent

– Width =< deposit width

– Well-zoned

– Fe chlorite + Cu-Fe sulfide

core

– Sericite+ Mg chlorite margin

– Silicification not prominent

– Aluminous minerals minor

• Zn-Pb-+/-Cu (Bimodal Felsic and Siliciclastic types)– Poorly defined, disseminated

– Vertical extent <= deposit thickness

– Width > deposit width

– Poorly zoned

– Silicic +Fe sulfide core; minor Fe-chlorite; some Zn-rich (ore!)

– Local intense silicification

– Sericite +/-carbonate + aluminosilicate t margin

– Carbonate species zonation

– Some Mn – enriched

– Less Mg chlorite; sealed, less draw-down of seawater

Slide 83. This summarizes the chemical effects observed in alteration pipes. Most of these have been noted already, but it is important to recognize that Ca is also a depleted, but then "replaced" by carbonate in those areas where thermogenic carbonate is forming. Another feature not mentioned previously is that in addition to barium incorporation into sericite, thallium and fluorine may also be similarly incorporated. In boiling systems, the halogen content, marked by significantly increased fluorine, is an excellent marker of footwall alteration.

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Hanging wall fluid flowthrough

• Buried deposit causes internal zonation

• HW permeability allows continued fluid flow-through and sulfide deposition

• Can break through to form stacked deposits in single hydrothermal corridor

Noranda camp

Interpillow sulfides

Buttercup Hill siliceous flow breccia

Slide 84. Continued emplacement of high temperature fluid into a pre-existing sulfide zone causes local zoning within the deposit, as well as movement of some components into the immediate hanging wall, if the deposit is forming sub – seafloor. This is particularly noticeable in the relatively high temperature, deep water deposits such as Ansil. Commonly, chalcopyrite displaces sphalerite and other minerals outwards, forming a copper rich spine in some deposits.

85

Hangingwall Mineralogical Indicators• Barite

– Restricted to post-Archean; seawater must have been oxidizing• Extends for 100’s of meters to 10s of kilometers

• Pyrite– Must have appropriate geochemical signature

• Ag, Pb enrichment (+ base metals?)• Gold -rich?• Low Ni (?)• non-radiogenic Pb isotope signal• S isotopes; increasing organic origin away form ore

• Albite +/- K-spar• Shallow water only (= boiling?)• Epithermal-VMS transition• Can be distal; scale unknown

• Fe-Mn Oxides• Bacteria-related?• 1-25+ km

• non-radiogenic Pb isotopes?• Mn zoning

• Carbonate (thermogenic only)

Slide 85. These are some of the principal minerals that are present in hanging wall exhalite and capping pelite. Base and precious metal enrichments are closely associated with pyrite distribution. Sulfide precipitation occurs as a combination of hydrothermal sulphur input and, with increasing distance away from hydrothermal centres, bacterial reduction of seawater sulfate. Bacteria are prolific in the warm sediments surrounding then sites, contributing to high organic carbon contents, and as scavengers for base and precious metals. Conserved components include iron and to a lesser extent manganese as oxides [if carbonate is present in the footwall then manganese is generally stripped]. Excess sodium introduced into the fluid by stripping from the footwall Na – depleted zones typically precipitates as albite in the capping hanging wall. This creates anomalously high Na values, with coincident positive europium anomalies. Eu is conserved in clays, albite and sulfates. As previously mentioned, the thermal anomaly associated with discharge centres causes carbonate precipitation, which in some cases [Greens Creek] forms an impermeable carbonate rich pelite, and in others causes precipitation of carbonate replacement nodules and infilling of porous structure [e.g. Middle Valley, Kidd Creek, Sturgeon Lake, Hackett River].

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86

Summary-Footwall & Proximal HW Alteration

• Alteration pipes occupy synvolcanic faults

• These are typically displaced from rift (or caldera) boundary master faults by a few hundred meters

• Pipe alteration is formed from– Very local interaction between high-T hydrothermal fluid and wall

rocks (add Fe, Cu, Mn, K)

– Massive refluxing of cold seawater around the pipe, causing loss of Na, Ca, depletion of Eu, “increase” in Al. local addition of Mg

• Hanging wall alteration follows pre-existing fractures, which perpetrate into HW after next flow or eruptive product covers the system– Primarily formed from the interaction of High T fluid with wall rocks

– Also strongly influenced by volatile and conserved components (Hg, Tl, As, Sb, Ba)

Slide 86. In summary, alteration associated with the hydrothermal discharge zones is the most used and practical application of geochemical work and prospecting for VMS deposits. Geochemical anomalies associated with these pipes and the footwall strata surrounding them provide a multitude of vectors that when used as a group, can be very powerful in identifying drill targets. Prior to their use, however it is important to understand the processes of alteration attendant on any system, and also to know which part of the system you're samples come from when undertaking such an analysis.

Black smokers: sulphide-rich

White smokers: sulphate-rich

Slide 87. So if the conditions are right and the hydrothermal fluids manage to maintain some of its metal budget long enough to reach the seafloor we can then start to see the formation of a massive sulfide ore body. Of course, the spectacular videos and pictures we see of black and white smoker smoking hydrothermal vents discharging on the seafloor are in reality losing their metals to be disbursed in the overlying seawater column. What has to happen then in order for these sulfides and attendant silicates to be captured?

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Modern

Chimneys

Slide 88. Of course we know that these chimneys are the main conduit for metal-rich hydrothermal fluids from our ability to not only sample them on the modern seafloor, but also to monitor temperature and chemical and physical changes that take place as these smoker fields evolve to form massive sulfide deposits

Ancient Chimneys

Alexandrinka, Urals

Safyanovka,Urals

Tash Tau, Urals

R. Herrington

Slide 89. From these modern observations we can recognize in ancient massive sulfide deposits textures that tell us that these modern processes of mound formation were the principal mechanisms of mound growth throughout the earths rock record.

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From Lydon (1988)

Mound

Growth

Slide 90. We mentioned a few slides ago that in the generation of the footwall vein stockwork system that the fluid channelways periodically become clogged as minerals precipitate. The same happens to the black smoker systems on the seafloor. Individual sulfide chimneys become clogged with sulfide precipitates. At the same time, anhydrite that has been precipitated along with the sulfides becomes unstable under cooling conditions and dissolves back into the seawater causing the chimney structure to becomes unstable. As the individual chimneys collapse into a rubble pile in which the upper surface becomes relatively impermeable due to sulfide oxidation and silica precipitation a sulfide mound is generated through inflation.

From Large (1992)

Zone Refining & Metal Zonation

Slide 91. As this mound growth continues through inflation the insulation built up from both silica and sulfide growth in the mound and the development of a ore impermeable fluid conduit due to alteration of the footwall rocks results in a steep thermal gradient within the mound. The 350oC Cu-rich hydrothermal fluids then systematically replace the core of the mound, pushing out previously formed lower temperature sulfide minerals such as sphalerite and galena to concentrated them on the outer margins of the sulfide pile. On this outer margin we will also see an accumulation of chert and in some cases barite, a more stable sulphate mineral then anhydrite. This zone refining is also responsible for the concentration of Au and Ag in the Zn-Pb-rich outer margins of the sulfide mound. Under oxidized seafloor conditions the sulfides forming in this outer mound margin can oxidize, a process that can further concentrate precious metals. It was in the oxidized parts of ophiolite-hosted VMS deposits that ancient civilizations in Cyprus, Turkey and Oman mined for gold.

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No reduced sulphur but extra Fe?

Ansil deposit, Galley, 1994

Slide 92. When a VMS system is allowed to grow to maturity we can sometimes find a situation where the rising fluids become deficient in base metals and insulated from their sulfur source. The result can be the late stage replacement of sulfides by magnetite, as observed especially in bimodal mafic type VMS deposits that have very focused hydrothermal upflow zones.

93

Subseafloor Displacement Origin• The majority of VMS deposits formed this way

• Largest deposits include Volcanic-siliciclastic (Type 5) types (and also most Sedex deposits)

• Formed in either sediment-dominated or volcaniclastic - dominant environments

• They typically form in shallow-water (<1000m) environments

• Buried deposits in active systems more difficult to find– Sedimentary “inflation leads to mound formation in

extensional basins

– Low T discharge forms the capping mound

– No seafloor sulfide, but carbonate halo - cap: Mn enriched

Slide 93. The majority of massive sulfide deposits have formed sub – seafloor, by displacement of unconsolidated sedimentary and volcaniclastic material. This is particularly true for deposits formed in mid – to shallow – water depths, where fluid velocities were reduced relative to the deeper water, higher temperature systems. These deposits formed by inflation of the sediments, forming sediment coated mounds by sub-seafloor displacement. The high temperature fluids induce instantaneous consolidation and low – temperature metamorphism of the immediate hanging wall sedimentary strata, and in some cases induce both silicification and carbonatization of these strata. Discharge from these sub-seafloor systems includes both focused and unfocused fluid flow and tends to engender significantly enhanced biological activity. This displacement process has been observed in both the Middle Valley and Guaymas active seafloor hydrothermal systems.

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Subsea-floor sulfide emplacement

Replacing rhyolite carapace, Buttle

Lake

Replacing talus, Eskay Creek Replacing columnar jointed rhyolite,

Ansil

Slide 94. During the process of the development of subsea-floor massive sulfide deposits the coarser clasts within the fragmental host rocks can be preserved, resulting in textures that caused many early researchers of VMS deposits to suggest that they formed as epigenetic rather than syngenetic deposits, or that they formed much later than their hosts rocks, such as orogenic gold deposits do.

95

Summary: Precipitation Sites

• Metal zoning is almost entirety in response to metal solubility

• Deposits grow by mound inflation on the seafloor (flow-dominant regimes) or by displacement of semi-consolidated strata (volcaniclastic-dominated regimes)

• Gold abundance and distribution is controlled by aS, which varies in response to one or more of :– Boiling in deposits formed <1900m

– Metal redistribution due to recirculating fluids through sulfide mounds

– External source of gold (magmatic, orogenic gold ?)

• Understanding the volcanological setting will assist in predicting metal content and size of deposits

Slide 95. To summarize, metal zoning in massive sulfide deposits is almost entirely in response to the solubility of the individual metals. This is largely controlled by temperature, and secondarily controlled by sulfur activity. Consequently the copper to zinc +/- lead zonation is purely a function of the thermal structure of the precipitation environment. Cold seawater is a highly effective heat transfer agent causing precipitation of chalcopyrite in the lower stringer zone and central parts of mounts, and the remaining constituents precipitated due to super-cooling and mixing at the outer margins of the system. Gold is a special case; it is clearly precipitated through zone refining in boiling systems to form significant enrichments, particularly at the top of deposits. However recent studies in the Bousquet camp of Québec have clearly demonstrated a syn-depositional magmatic addition of gold. In this case the gold content correlates very well with molybdenum, tellurium, and bismuth, but not with the base metals or silver. Furthermore it is evident that the magmatic system is calc-alkaline, and developed a porphyry – or high sulphidation epithermal – like fluid phase. Consequently there is no "one size fits all" mechanism to explain gold enrichment.

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96

VMS EXPLORATION INDICATORS

From fertility indicators

to target vectors

Slide 96. Recognizing the characteristics of the

different members of the VMS family and having

some knowledge of where they form and what

processes are involved is all very well, but we must

now be able to translate this information into

practical exploration indicators that will allow us to

recognize the presence of a fertile volcano-

sedimentary terrane in which it would be possible to

recognize a VMS system, and secondly the tools

necessary to recognize the size and extent of the ore

system footprint, and how to vector within this

footprint towards the actual VMS deposit.

Discovery criteriaMineral systems approach to understanding telescoping

scales for the characteristics and dimensions ore systems

Deposit scale

Camp scale

District scale

Thébault et al., 2014

Slide 97. We have very briefly looked at some of the processes through which VMS deposits form under various tectonic and environmental conditions. The various scales at which we make these observations tell us whether the physical and chemical conditions were right for the generation of a VMS hydrothermal system. We will now look at how the geochemical characteristics of the VMS tectonic environment, depositional environment and finally the deposit-scale primary and alteration geochemistry can be used as a telescoping set of exploration guides towards VMS mineralization.

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98

Petrochemistry as an Exploration Guide

• Petrochemical trends most useful as indicators of tectonic regime and paleo heat regime– magma contamination, high-T crustal melting may be

guides

– fractional crystallization a possible guide

– some direct relationship between magma type and ore composition or size, but big and/or hot magma supply = big district (e.g. Iberia, Kidd Creek)

• Subvolcanic intrusions provide a measure of paleo-heat supply– “collapsed” alteration in intrusions may be important

– need to distinguish magmatic –sourced metals from convectively generated metals; may be source of former

– may not be present (?) in felsic siliciclastic districts

Slide 98. Much has been made of the use of petrochemistry as an exploration guide. To summarize, and already pointed out in the previous slides, the more mafic-dominated systems tend to be much hotter, but the size of the magma system as well as its composition are equally important in determining the size of a district or deposit. As noted above, subvolcanic intrusions provide some of the best guides for paleo heat supply. Because of the aforementioned structural issues that have removed the component of the stratigraphy that contains some volcanic intrusions, understanding the composition of volcanic and related volcaniclastic and volcanic – derived sedimentary strata is important.


From Piercey (2007)

Slide 99. We have seen that different types of VMS deposits are hosted by characteristic host volcano-sedimentary assemblages. In almost all cases it is the mafic volcanic rocks that form the majority of the district scale volcanic packages and so it is these rocks that are the primary geochemical targets to determine the VMS potential, or fertility of a particular geologic terrane.

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100

REEs and High Field Strength (Immobile) Elements

Mainly as petrogenetic indicators

– Mafic rocks– Higher values, larger -ve Eu*, higher slopes (Lan/Ybn) in

contaminated basalt (andesite) indicates good potential

– Felsic rocks• “High-T” rhyolite/dacite :

– High REE abundance, flat pattern, -ve Eu* anomaly

– High Y and Ybn contents, low Zr/Y and Lan/Nbn contents

– Usually related to Cu-Zn deposits (primitive ocean-ocean backarcs)

• “Intermediate -Low T” dacite/rhyolite– High LREE abundance, steep slope, negative Eu*

anomaly

– Low Y and Ybn contents, low Zr/Y and intermediateLan/Nbn contents

– Usually associated with Zn-Pb-Cu deposits (ocean-continent backarcs)

Slide 100. Given the mobility of all of the major elements except TiO2 and Al2O3 in alteration zones associated with VMS districts,, the significantly less mobile Rare Earth and high field strength elements are most useful in determining primary petrochemical compositions. Elements such as Zr, Y and Nb, along with the rare earths, are particularly useful. Using the composition of felsic volcanic rocks as a surrogate for paleo temperature has been examined by numerous authors. Basically the higher the content of the high field strength elements, and the flatter the rare earth pattern, the higher the temperature of the felsic melt, and therefore the better prospectivity for large VMS deposits.

Snow Creek

NoneMain subdivisions of the Snow Lake arc

assemblage

Slide 101. If we go back to the Paleoproterozoic Snow Lake VMS district in central Canada we have a good example for using basalt geochemistry as a tectonic vector. The lower part of the island arc succession is characterized by very flat to LREE depleted signatures accompanied by a Nb depletion and Th high. This is typical of primitive boninitic to low-Ti tholeiitic basalts formed in a nascent arc regime in which bimodal mafic Cu-Zn VMS systems are the most common. Up section we move into a maturing magmatic arc assemblage typified by more fragmental andesite, rhyodacite and rhyolite of the bimodal felsic type VMS environment. The elevation in the La/Yb ratio and relative depletion in Nb, Ta and Zr indicate more hydrous mantle conditions in which hornblende and garnet stability is present. The more fractionated volcanic assemblage means a thickened crust in which mid-level magma chambers can evolve. Finally, the upper part of the volcanic package indicates that it contains mostly N-MORB to BAAB basalts more typical of mature back arc ocean basins in which there may be the possibility of mafic-dominated VMS systems.

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102

REEs as a guide to VMS Footwall Indicators:

•REE patterns can help

define tectonic setting

and ore potential

•Low La/Yb ratios =

high T melt conditions-

thus high heat flow and

VMS potential

•Strong -ve Eu*

indicates assimilation of

older crust: typical of

excess magma supply,

high heat flow

Slide 102. In footwall felsic strata, typically europium is strongly depleted. This is in part due to its mobility in high water:rock alteration systems, but also in part is caused by its loss from the precursor mafic volcanic rocks prior to their partial melting, and due to halmyrolysis on the seafloor. Exceptionally large negative europium anomalies are good indicators of proximity to a VMS deposit, regardless of composition of the footwall rocks. As previously mentioned, the slope of the rare-earth pattern is also a key guide, not only to the highest temperature felsic volcanic melts [flat pattern], but as a useful stratigraphic guide.

Rhyolite fertility

103

Tholeiitic

Low-K calc-alkalic

Low-high-K calc-alkalic

Alkalic

Slide 103. Besides using normalized REE and spider element plots to determine the type of rhyolites present we can also use a REE ratio plot as shown here. The variation in the normalized La-Yb ratio to normalized Yb allows you to plot rhyolite compositions roughly according to their petrogenetic association. Back in the early days of VMS petrogenesis many of the exploration models were based on tholeiitic bimodal mafic and felsic VMS environments, known as Type 3 rhyolites. As exploration expanded into more evolved submarine arc terranes it was found that VMS deposits of the bimodal felsic and felsic siliciclastic types could form, and more importantly as we will discuss at the end of this short course, are commonly Au-enriched.

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VMS-hosting bimodal mafic

Piercey, 2010

• Dominantly bimodal: IAT

basalt, basaltic andesite with

10-15% rhyolite domes

• Rhyolite FIII type – tholeiitic

and high T with 200-300 ppm

Zr

• Fractionated, porphyritic HW

- Fe-Ti enriched Icelandites

Slide 104. During greenfields, or reconnaissance exploration stages we can then combine the petrogenetic indicators for both mafic and felsic volcanic assemblages to determine the VMS potential, or fertility of a particular belt of volcanic rocks, and what type of VMS systems we can expect to find. In the case of bimodal mafic, or Noranda Cu-Zn type VMS deposits we are looking for tholeiitic island arc basalts as the principal host rock in the 2000 m underlying a potential VMS camp. The normalized spider plots for this basalt type would have a relatively flat signature with the exception of a Nb depletion, which is typical of all arc-type basalts due to the sequestration of Nb in refractory, or resistant minerals that remain in the mantle during melting. The signature it typical of relatively low pressure melting of the lithospheric mantle below relatively thin overlying ocean crust. The rhyolite component of the bimodal VMS sequence should have a Type IIIb signature, which again indicates a low La/Yb ratio typical of plagioclase dominant low pressure melts that are relatively dry and high temperature, with characteristic high Zr contents.

VMS-hosting bimodal felsic

• Calc-alkalic, differentiated footwall

succession with andesite and

rhyodacite abundant

• Spatially associated with FII-FI

rhyolite domes and assoc. breccias

• Overlain by volcaniclastic-mudstone-

mafic volcanic successions –

commonly of MORB composition

Slide 105. In contrast there is the bimodal felsic, or Kuroko-style, polymetallic VMS environment that is characterized by basalts with a more calc-alkalic affinity typified by elevated LREE and LFSE, but still maintaining the characteristic Nb depletion that is matched in calc-alkalic basalts by a dip in Ti, as rutile becomes a restite mineral as the mantle magmas are sourced at higher pressure under more hydrous conditions. For the associated rhyolite and rhyodacite the La/Yb ratio begins to increase due to the retention of HREE in garnet and hornblende at source, giving these felsic rocks an FII signature.

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106

Summary: Petrochemistry• Principal; use is in identifying paleo-heat environment,

also arc-backarc transitions

• Major elements provide guidance only where alteration is absent

• Trace elements are essential; Zr/TiO2, Nb/Y. Lan/Gdn, Eu* all are very useful in discriminating primary lithology

• Most useful in field using a portable XRF is Zr/TiO2

• Must modify any chemically-determined lithology with field knowledge (e.g. don’t mis-classify sediments as volcanics, don’t use chemical plots to classify polylithic breccias etc)

• Most VMS deposits form in association with high-T felsic and mafic rocks, but Ag-Sb-Au rich deposits may form in Low-T environments

• Melt contamination and/or fractionation yields andesite-indication of super-heat flow hydrothermal convection

• Use discriminate plots with care, as exceptions may exceed the “rules”

Slide 106. In summary, the primary use of petrochemistry is to establish paleo heat flow. However key ratios, such as Zr/TiO2 are exceptionally useful to determine primary rock types, particularly in highly altered environments. Understanding the paleo-temperature regime for the formation of a massive sulfide deposits may directly reflect in their composition, with Cu-rich deposits formed in higher temperature [typically deep-water i.e.>2000m] regimes. Zn –Pb and Ag –Sb – Au deposits formed in lower temperature [shallow water, i.e.<1500m] environments. Use of portable XRF units in the field can provide excellent guidance for rock types, as well as alteration attributes.

107

Exhalite Geochemistry

as an Exploration Tool

• Aim: provide a vector towards ore

• Complications:– Understanding mode of primary dispersal

– “destructive interference” of multiple point vent sources

– Selection of “background” values

– Dilution of hydrothermal signal by detrital and hydrogenous material

– Paleo environment (bathymetry, bottom currents, redox, etc.)

– Structural deformation & dismemberment

Slide 107. There are numerous studies of composition of exhalite, but few have examined the spatial distribution of these various indicators from hydrothermal and hydrogenous sources. Nevertheless there are a large number of useful vectors, many of which have been noted in earlier parts of this presentation. Each district must be evaluated on its own merits, as although some characteristics, such as the Ba and Tl contents of sericite are relatively uniform indicators of mineralization, these are primarily sequestered in the footwall to VMS systems. Following slides provide a few examples from the Bathurst Camp.

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Low temperature basin-wide

hydrothermal activityAustin Brook: Bathurst

Magnetite>Chert IF Hematitic Shale Siderite>Chert>Chlorite IF

Examples• Golden Mile, WA (Archean)• Manitouwadge ON (Archean)• Bergslagen, SE (Paleoproterozoic)• Bathurst, NB (Ordovician)• Iberian Pyrite Belt (Carboniferous)

Slide 108. Let’s go back to the Bathurst siliciclastic VMS camp as an example of how the trace element variations in iron formations temporally associated with VMS formation can be used to vector at a regional, or camp scale towrads individual hydrothermal upflow centres.

109

Slide 109. Detailed geochemical studies on the Bathurst camp iron formations by Jan Peter at the Geological Survey of Canada resulted in the recognition of both geochemical and mineralogical changes with distance from the iron formation-VMS deposit interface. This included not only variations in metal contents, but also trace and rare earth elements, which vary with the hydrothermal component of these exhalitive deposits.

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110

Heath Steele

Belt IF Rare

Earth Element

Plots

(Peter et al., 2003)

Proximal HW BIF Proximal HW BIF

Proximal HW BIF Distal HW BIF

Proximal Chl alt FW

Proximal

Alk-depleted alt

FW

Proximal

Alk-depleted alt

FW

Footwall Sediments

Slide 110. This is an example of the use of earth elements in providing a vector towards ore, based on work by Peter et al. in the Bathurst Camp. The diagram on the left illustrates the rare earth content of hydrothermal fluids associated with active VMS systems in the Mid-Atlantic Ridge; note the distinct positive europium anomaly in these fluids. This is reflected in the rare earth content of the hangingwall iron formation at Bathurst [right, top 4 diagrams]. Note the strong positive europium anomaly associated with iron formation proximal to the massive sulfide deposits, but almost no such anomaly exists in the distal iron formation. The lower four plots illustrate the distinct negative europium anomaly associated with the alkali depleted footwall rocks. These trends in the hanging wall and footwall rocks at Bathurst are very similar to those observed in virtually all districts where an exhalite unit has formed. The footwall europium depletion trends are ubiquitous in all VMS systems.

111

Mn Content of Chlorite In Iron Formation, HSB

(Peter et al. , 2003)

Slide 111. This diagram illustrates the distribution of manganese in chlorite at Bathurst, further emphasizing the use of manganese as a factor to ore. Generally, as noted above in the Sturgeon Lake example, the manganese content of footwall rocks is a more reliable vector to ore than that in the hanging wall.

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112

Footwall Geochemical IndicatorsIdentifier

• Na depletion– No.1 indicator

• Ca depletion

• Base metals– Copper

– Zinc

– Lead

• Others– Barium

– Fluorine

– Excess Alumina

– Thallium

– Silica

Scale– Pipe =1.5 x deposit width

– Semiconformable = kilometers

– Pipe-1.5 x deposit width

– Pipe - deposit width, 10s to 1000m

below seafloor

– Marginal to pipe; 1.5 x width

– Marginal to pipe; 2 x width

– In mica; pipe width

– In mica; not known

– Normative corundum; as per Na

– In Sericite

– Quartz: reduces permeability

Slide 112. This summarizes the chemical effects observed in alteration pipes. Most of these have been noted already, but it is important to recognize that Ca is also a depleted, but then "replaced" by carbonate in those areas where thermogenic carbonate is forming. Another feature not mentioned previously is that in addition to barium incorporation into sericite, thallium and fluorine may also be similarly incorporated. In boiling systems, the halogen content, marked by significantly increased fluorine, is an excellent marker of footwall alteration.

Alteration Indices

Lost ElementsGained Elements

Gained ElementsAlt.Index

+=

• Hashimoto Index = 100*(MgO+K2O)/(MgO+K2O+CaO+Na2O) [Ishikawa et

al. (1976)]

• Spitz-Darling Index = Al2O3/Na2O [Spitz + Darling (1978)]

• Chlorite-Carbonate-Pyrite-Index (CCPI) =

100*(MgO+FeO*)/(MgO+FeO*+Na2O+K2O) [Large et al. (2001a)]

• Sericite Index = K2O/(K2O+Na2O) [Saeki and Date (1980)]

• Au Indexes: CO2/Ca, K/Al, 3Na/Al, 1000Y/Tidepletion [Eilu et al. (2001)]

• Barrett and McLean Mass Change Index = (Ti/Zr)

Slide 113. There are numerous variations in alteration indices used by explorationists to determine first of all the presence of VMS-related hydrothermal alteration and secondly whether variations in these indices allow them to vector towards the most rock destructive part of the system where we would expect to find the core of the hydrothermal upflow, attendant sulfide stringer zones and hopefully and overlying massive sulfide component. Many of these alteration indices are based on variations in major element chemistry due primarily to destruction of primary feldspar and creation of chlorite and sericite.

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Distal hydrothermal alteration

Proximal hydrothermal alteration

Large et al., 2001

Slide 114. One of the more popular alteration plots is one developed by Ross Large’s group at CODES in Tasmania, and involves the combination of two older alteration indices and the comparison of the resultant trends to groups of alteration minerals commonly present in different VMS type alteration systems. Based on the large scale hydrothermal model we went through earlier the plot should in theory tell you: a) are there altered rocks present; b) whether the geochemical variations shown by the varying element ratios took place in a distal or proximal footwall environment; and c) whether a trend is evident within the sample suite that would allow the explorationist to vector towards the high temperature upflow zone. Note that this plot, as with many other “formula” approaches to typifying alteration, has limited applicability.

115

Alteration

IndexElement Ratios Alteration Process

Residual

SilicaSiO2 vs. Zr/TiO2 (silicification)

residual to feldspar fractionation

line

Pearce

Element

Ratios

molar Fe/Zr, Mg/Zr, Mn/Zr, K/Zr,CO2/Zraddition of Fe, Mg etc. relative to

conserved Zr

molar (Na + K + 2Ca - Al/Zr) (alkali

depletion)


line

molar (Si/Zr) - 7.5 Al/Zr + 6.25)

(silicification)


line

Other e.g. Zn/Na2Oe.g. sphalerite staining and

sodium depletion

Normative

plotse.g. corundum > 0.5%, feldspar ratio Alkali depletion, other

Mass

Balance plots

All major and trace elements, altered vs.

unaltered

Addition, loss, metasomatism,

volume/mass changes vs.

unaltered sample, immobile

elements

Alteration

Box Plot

Chlorite-carbonate-pyrite (CCPI;Y axis) vs.

Ishikawa Index (X axis)

CCPI=100(MgO+(FeOt)/

(MgO+FeOt+Na2O+K2O)

Addition of ferromagnesian

elements normalized to loss of Na

and K vs.

Slide 115. These are a continuation of the last slide, but provide an outline of some of the more complex systems that have been used primarily to understand the processes of alteration. Such methods as Pearce Element Ratios and normative plots require significant mathematical manipulation of the data, and others, such as mass balance plots are very good starting points for estimating what's been gained in what's been lost in any district. It's recommended that at the onset of study of a district, mass balance plots, using good-quality whole rock data, be undertaken. The last indicator, the "alteration box plot" is an example of a system determined for the Tasmanian massive sulfide district; this has significantly limited application in many other districts and as with the indices provided on the previous slide should be used with caution.

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Noranda Na Depletion

116

• Note well

clustered

anomalies

• Width in the

Amulet area

<1km.

Slide 116. Here is an example of sodium depletion in the Noranda camp, a classic high temperature, deep water, bimodal basalt dominated [Type Ia] system. Note that the Na loss is quite extensive around all of the deposits , Extending for hundreds of meters away from them in their footwall strata.

117

Sturgeon Lake Chemical Data Na2O

MattabiF

Zone

Lyon Lake

Slide 117. The slide illustrates the loss of sodium in volcaniclastic – dominated system at Sturgeon Lake Ontario. Note that sodium depletion extends laterally throughout virtually the entire footwall system here, and thus although it provides a useful first – order regional guide to VMS prospectivity, does not provide very specific targeting for discovery. Na depletion is an excellent first order guide to understanding the stratigraphic limitations to prospectivity, but may on its own not be specific enough to provide drill targets.

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K- Enrichment : Sericite

• K in boiling systems may be enriched as either

sericite (most common) or K-spar (epithermal)

– Test: K correlates with Rb in mica, Sr in feldspar

• Sericite is dominant in all systems

• Sericite may be variably enriched in Ba and Tl near

discharge points; Ba/K and Tl/K are useful guides

Source of K: All hydrothermal? Needs study.

Good Rb-K2O

correlation:

Sericite

dominant

No Sr-K2O

correlation

Slide 118. Analytically distinguishing hydrothermal sericite from other forms of K – mica or K feldspar is easily done by examining the K – Rb relationship. Rb is sequestered only in mica, whereas Sr is sequestered primarily in feldspar. We presume that the source of potassium for sericite associated with alteration systems is entirely derived from the hydrothermal fluid, but this has not been proven. Nevertheless, the first test for the presence of hydrothermal sericite should be the examination of the K versus Rb plot.

Barite- Sericite: Data Manipulations and Use

• Ba is conserved in all VMS-forming

hydrothermal fluids

• Ba is sequestered in K species:

– Sericite in FW alteration

– Best normalized to K2O to remove effects of

variable sericite content.

• Ba as barite: Only in post-Archean (mainly

Phanerozoic).

– Mainly in HW and peripheral to deposits; in

HW may be Au-rich

119

Slide 119. As previously noted, barium is conserved in all hydrothermal systems, and as it does not form a significant sulfide mineral, and is sequestered either in sericite or barite. Formation of barite requires access to cold, sulfate – bearing seawater, which limited to the post Archean, and most likely to the post Paleoproterozoic. Thus the barium content of sericite increases towards the deposit and is an excellent vector to ore. Used in combination with thallium, described below, studies of the distribution of barium in footwall strata provides a useful and relatively easily determined vector. Portable XRF units may be used for this. Because the content of sericite may vary significantly, and may be controlled by the primary porosity of the strata, it's best to normalize the barium [and thallium] contents to K2O. Barite is always a relatively low temperature product, and in many cases is associated with exceptionally high silver and gold contents.

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Ba/K2O at Sturgeon Lake

120

• Well defined

anomaly clusters

associated with

mines

• 75% of the highest

values (>1000)are

within 250m of

deposits

• Lower anomalous

values occur along

ore horizon

• Only sample

containing >1%

K2O should be

used for this

determination, to

avoid analytical

uncertainties.

Slide 120. This example illustrates the usefulness of the Ba/K2O ratio is an exploration guide to VMS deposits. Note that values of this ratio in excess of 1500 are generally tightly clustered to massive sulfide deposits. Some care should be taken however to ensure that the plotted samples have enhanced K2O contents, otherwise high ratios may be an artifact of analytical thresholds.

Manganese as a Vector to Ore

• Manganese is conserved in a hydrothermal fluid

• It may partition into sphalerite. But most “passes

through” a hydrothermal system

• It is most effectively sequestered in carbonate

• It may also partition into chlorite (smectite)

• During prograde metamorphism it partitions into

chloritoid (upper greenschist), garnet ( reducing

metamorphic boundary to mid greenschist),

orthoamphibole and staurolite

• Its distribution is most strongly controlled by

primary carbonate distribution: (primary

porosity/fracture density)121

Sldie 121. Manganese is an excellent footwall alteration vector to ore. It is conserved only in carbonate, then during progressive metamorphism transferred to various silicate minerals. One issue is that the content of manganese in a rock will become a function of the amount of carbonate, and that in turn is related to the primary porosity of the host rock. Rocks with large open spaces [vesicles, inter- fragment space] may have a large amount of carbonate and therefore a very high manganese content but those with relatively reduced porosity may have a small amount of carbonate. Thus it is important to normalize the amount of manganese to the amount of carbonate [CO2] in the rock.

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MnO Distribution Sturgeon Lake• Anomalous MnO is

broadly distributed in

the FW to all deposits

• Most abundant near

mines

• Problem: Its primary

distribution is a

function of:

– Proximity to the Mn

source (vent

area/alteration pipe)

– Also related to the

amount of precursor

carbonate, which is a

function of the

porosity of the

primary lithology

• Need to normalize

Mn content to CO2

content to remove

the physical

distribution

irregularities 122

Slide 122. The Sturgeon Lake District Illustrates the usefulness of manganese as a vector to ore. This diagram illustrates the distinct increase in manganese content near the deposits. Manganese anomalies near deposits on the eastern side of the district are separated from them by a late intrusions, shown in orange and mauve. Note the tight distribution of anomalous manganese near the Mattabi and F – Zone deposits.

Normalizing Mn to CO2: Improved Anomaly Definition

123

Most anomalous samples are within 200 meters of

an orebody: Some define discharge faults

Isotopic studies indicate that CO2 is derived from

seawater (and probably Ca)

Carbonate species vary as a function of temperature

and activity of hydrothermal Fe and Mn

Slide 123. Normalization of the manganese content to CO2 significantly improves the definition of the alteration systems. Note that under them at Mattabi deposit, Mn – enriched carbonate is associated with synvolcanic faults that extend for several hundred meters into the footwall.

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Carbonate Usefulness as a Vector to VMS

Carbonate presence…is it useful?

• Its presence may indicate

proximity to an anomalous amount

of heat, i.e. to VMS discharge

zone

• From field studies, calcite forms

early in the VMS –forming process

(pre-ore)

• Its species is controlled by

temperature (increasing Mg at

higher temperatures) and by

interaction with VMS-related fluids

• Thus the regional transition from

carbonate to dolomite may be

significant.

• In a few areas, VMS deposits

formed in early thermogenic

carbonate mounds (Bergslagen,

Sweden)

Interaction of early-formed

carbonate with hydrothermal fluid: a

useful observation!

• Carbonate species vary as a

function of temperature and

activity of Fe and Mn

• Both are present in hydrothermal

fluids, but Mn is sequestered

only in carbonate, or some Fe-

silicates.

• The classic case is Sturgeon

Lake; from 10 km away to ore we

have increasing Fe and Mn:

– Calcite

– Dolomite

– Ferruginous dolomite (100’s of m)

– Ankerite (surrounding the pipe)

– Siderite (only in pipe) 124

Slide 124. We have mentioned previously the role of carbonate as an indicator of VMS potential. Although the precipitation process is relatively poorly understood, mass balance studies show that all of the components of the carbonate are externally [seawater] derived. The important aspect from an exploration viewpoint is the carbonate species zonation. Typically distal to the deposits [several kilometres away] calcite is predominant and as the deposit or alteration pipe system is approached, carbonate gains magnesium, then more proximally, manganese, and finally iron. Siderite forms only immediately under most VMS deposits. Progressive metamorphism causes decarbonatization of the rocks, but the components added during the hydrothermal interaction phase with carbonate are retained in metamorphic minerals such as chloritoid, staurolite and orthoamphibole.

125

Volatile Element Halos

• Usage of elements such as Tl, Sb, Hg, Bi, As etc.

ICP-MS technology made this possible

• Have been used extensively in Australian VMS and SEDEX systems (e.g., Smith and Huston, 1992;Large and McGoldrick, 1998; Large et al., 2001) and in N America (Kidd, Greens Creek).

• Most of the volatile elements are more soluble in hydrothermal fluids than base/precious metals.

• Some (As, Sb, Hg, Tl) are partially transported in the vapour phase

• Hg, Tl and Sb particularly useful in delineating large footwall and hangingwall haloes distal from mineralization (up to several 100’s of meters).

• Tl is sequestered mainly in sericite, but also in low-T sulfides

• Sb (As) , Hg are in low T sulfides; may be “epithermal”-like late addition, or in HW exhalite

Slide 125. In boiling hydrothermal systems, a number of elements are transferred into the volatile phase. These include antimony, arsenic, mercury, thallium and gold. The use of thallium and antimony has been shown by Large et al. [2001] and others to be particularly useful in examining the hanging wall systems associated with massive sulfide deposits. These elements are typically contained in relatively low temperature sulfide assemblages, or those associated with high sulphur activities (aS).

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126

Tl

(pp

m)

Sb (ppm)

0.1

0.1 1 10 100 1000

1

10

100

background

Tl,Sb increasingtowards ore

oreproximal

After Large et al. (2001)

Cambrian Mt. Read Volcanics, TasmaniaCu-Au deposits; Western Tharsis, Highway Reward, Gossan Hill, Rosebery

Slide 126. This simple plot illustrates the use of the combination of Tl and Sb in determining the prospectivity of the hanging wall system of a VMS district. It has been demonstrated by Large et al. [2001] for the Tasmanian deposits, as well as other studies that Kidd Creek and many other districts. It’s important when selecting an analytical method to ensure that the lower cut-off for Tl and Sb is <1ppm.

Large et al., 2001

Hydrothermal leakage

Slide 127. A spatial example of some of these variations in volatile elements, combined with other alteration indices already mentioned is given in this stylized cartoon of a Ordovician VMS system from the Mount Read VMS district in Tasmania. The scales are variable, but we are generally speaking in terms of tens of metres in the proximal deposit environment to kilometres at the camp scale.

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128

Oxygen Isotopes

• Excellent measure of fluid-rock interaction: another element to use for regional lithogeochemical surveys.

• Most seawater-derived hydrothermal fluids have d18O ~ 0‰ (e.g., Huston, 1999). These exchange with

• Wall rocks typically have d18O>0‰ (e.g., Huston, 1999).

• The interaction of lots of low d18O fluids at high temperatures with wall rock leads to altered rocks with d18O depletions relative to parent.

• We search for d18O depletions relative to background to find high T reaction zones

• We search for d18O enrichments relative to background for alteration pipe (high water/rock reactions)

• Technology is now making d18O much more readily available and has been applied successfully in exploration

Slide 128. Studies of oxygen isotope distributions associated with a VMS districts began in the late 1970s [Spooner and others] as summarized in Huston [1999]. In the reaction zone there is an almost complete exchange of oxygen with the high temperature seawater, leading to strong

18O

depletion, but in the upper parts of the system [alteration pipes] there is a consequent enrichment in

18O. The advent of easily obtainable oxygen isotope

data makes this a useful, yet little – used prospecting tool.

Oxygen isotope indicator

129Cathles, 1983; Taylor, 2002

• <6 per mil variation

in δO16/18 indicates

high temperature

seawater-rock

interaction

• > 12 per mil δO16/18

indicates low

temperature

seawater-rock

interaction

commonly above

the VMS-bearing

interval

Slide 129. A classic example of the use of oxygen isotopes was Larry Cathles work in the 1980’s in the Noranda VMS camp. Through the collection of about 200 samples throughout this camp he was able to demonstrate that the presence of a high temperature hydrothermal core above, and including the Flavrian subvolcanic intrusion, with high temperature apophyses going upsection along synvolcanic faults to generate the VMS deposits. The blue areas indicate where lower temperature seawater-rock interaction took place within the hanging wall stratigraphy to the VMS host sequence.

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130

Sturgeon Lake Oxygen Isotope Data

Discharge Zones

Reaction Zone

Slide 130. The Sturgeon Lake area provides an excellent example of the usefulness of oxygen isotope data, with the depleted values [turquoise triangles] defining the high temperature reaction zone and the heavy values [red triangles] indicating the discharge zones. Similar studies have been undertaken in Cyprus, Noranda, and several other districts.

Modified from Gemmell and Fulton, 2001

Hanging wall hydrothermal fingerprints

Pyrite nodules

Slide 131. As noted previously, deposits that form sub – seafloor commonly have a distinctive hanging wall alteration package associated above them. In modern systems, such as Middle Valley in teh Juan de Fuca Ridge, these have been observed to contain talc, anhydrite, and various smectite minerals.The hangignwall becomes "inflated" due to teh gfrowth oif theese minerals. On the "death" of the system, anydrite re-dissolves, leaving talc and smectite, and a collapse breccia in the HW zone. In this example, a deposit in Tasmania has a distinct hanging wall alteration zone. In many others, carbonate replaces the immediate hangignwall sedimentary strata, forming a massive unit of carbonate-rich shale or volcaniclastic material.

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GOLD-RICH VMS SYSTEMS

132

Slide 132. Finally, let’s look at a special type of VMS system that is the prime target for exploraitonists looking for this particular deposit type: gold-rich VMS ore systems.

Robert et al. 2007

VMS-EPITHERMAL CONTINUUM?

Slide 133. Commonly Au-rich VMS systems are thought of in terms of their genetic relationship to other gold-enriched ore systems. In particular, there has been some serious efforts made to link them to some sort of porphyry-epithermal-VMS continuum as there is a change from subaerial to shallow submarine environments. The implication is that the only way to form a Au-rich VMS system is through the addition of a magmatic fluid component as evidenced for epithermal gold systems. This is not true, although some may become enriched through magmatic fluid addition, most form in another way.

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Mercier-Langevin et al. (2010)

Au-rich VMS deposits

Slide 134. There have been many attempts to define what is really a Au-rich, as opposed to a Au-enriched VMS system. Whereas in the past the rule of thumb was that a Au-rich VMS must contain >2 g/t Au, Patrick Mercier-Langevin of the Geological Survey of Canada took a more statistical approach using a global VMS data base. Looking at gold grade vs. tonnage the dividing line for Au-rich VMS deposits is then 3.46 g/t Au. These are divided into deposits containing >31 t Au as world class Au deposits. This category also includes VMS deposits, such as Flin Flon that have average gold grades but over 60 Mt or ore and therefore produced over 30 t Au.

135

Gold Issue• Gold can be an orebody-maker

• Discoveries in the Abitibi Belt (LaRonde) and other Au-rich VMS deposits (e.g. Eskay Creek, Tambo Grande, San Fernando, Romero) illustrate that the Au content is independent of tectonic setting and size

• Seafloor studies indicate that gold is enhanced either:– By “seafloor zone refining” (TAG, Tambo Grande)

– In boiling systems (Axial Seamount, Eskay Creek)

– Magmatic processes (LaRonde; Horne)

– Others, including overprinting by orogenic systems?

• In almost all cases, Au content in the primary fluid is not the major factor: it’s controlled either an external source of gold or precipitation (solubility) constraints

• Cuba/DR (Cretaceous) and Axial Seamount (modern) are illustrative

Slide 135. The gold content of VMS systems varies widely, with most containing less than 1g/t, but a few with 5 to 10 g/t and Eskay Creek with more than 10 times that amount of gold. Many have distinct zoning of gold, with the highest contents in the uppermost parts of the system or even in areas within the immediate hanging wall or adjacent to the principal massive sulfide zones. In some however, gold distribution is not correlated with any of the base metals, and in some its distribution seems to cross-cut the sulfide zones. Several mechanisms have been demonstrated to enable gold enrichment. These include gold enrichment by seafloor zone refining, by speciation differentiation in boiling systems, or in synvolcanic magmatic – fluid additions. Other mechanisms, such as overprinting by orogenic gold may also occur, although these have not been documented. We have learned from seafloor studies that the gold contents of hydrothermal fluids are consistent everywhere, and although there may be exceptions to this, it's generally considered that variations in the primary gold content of the hydrothermal fluid is not a significant factor in explaining the different contents of gold in the deposits.

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• The Au content appears independent of tectonic setting and deposit size

• Au content in the primary fluid is not necessarily the major factor, but rather how it is concentrated

• Seafloor studies indicate that gold is enhanced either: Size of depositSeafloor zone refiningSubcritical phase separation (boiling)Magma devolatilization

Controls on Au enrichment

Slide 136. There may therefore be a number of processes that may influence the degree to which a VMS deposit may be gold enriched. Whereas magma composition as defined by arc setting and evolution may be important, it does not appear to be critical. The Au content of a VMS deposit may be a function of primary fluid composition, but may be related to processes undergone by originally Au undersaturated fluids that resulted in Au enrichment within the depositional environment. Three processes that we can quickly review are seafloor zone refining, subcritical phase separation, or boiling, and magma devolatilization.

137

What Causes the Variation in Gold Content in Seafloor Deposits?

• Gold contents within sites are

fairly consistent, but differ

between sites

– Southern Juan de Fuca and

Endeavour: 0.75 g/t

– Escanaba: 1.4 g/t, highly

variable (up to 10g/t) Bi

association

– Explorer Site: 2 g/t

– Axial Seamount: 8-10g/t

• Variations in water depth may

relate to gold content…. but why?

Middle Valley

(2520m)

Axial

Seamount

(1530m)

Endeavour

Ridge

(2500m)

Southern

Juan de Fuca

(2500m)Escanaba

Trough

(3250m)

Explorer Site

(1900m)

Slide 137. Variations in gold content along the spreading ridges in the Northeast Pacific are illustrative of the issue. Active venting systems have been discovered in numerous areas; many of these are at water depth greater than 2000m, but two sites, the Explorer site on most northerly segment of the Juan de Fuca system [Explorer Ridge] is at 1900 meters and Axial Seamount is at 1530 m. Interestingly the gold content generally varies with water depth, with massive sulfides at the Explorer site containing about 2 g /t, and the Axial Seamount sulfides containing 8 to 10 g/t. What does water depth [i.e. adiabatic cooling] have to do with gold content? This is explained in the next three slides.

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138

Metal-Producing VentTemp:136-323oCpH=3.5

Metals(mol kg-1)Zn 118Cu 10Fe 925Mn 1018Au 100ppt

Majors(mmol kg-1) Cl 515Na 415Ca 37.3K 22.0H2S 8.1Si(OH)4 15.8

Axial Seamount: ASHES Vent

Slide 138. Axial seamount is forming over a magma plume that has existed for several hundred thousand years at least, and the plate has migrated over this plume forming a series of volcanic edifices that extend to the west. Axial Seamount has a well-developed caldera, with several vent sites, including a major one near its southwestern corner, called the Ashes vent. Fluids from the main Ashes sulfide mound have a maximum temperature of 323° C; at this water depth that is close to the maximum temperature allowable. The metal contents shown here are typical of most seafloor hydrothermal vents, Note that the gold content is approximately 100 ppt, as determined by Dave Butterfield at the NOAA PMEL laboratory. Note also that the major element constituents are also very similar to those observed at most seafloor vents regardless of water depth. Remember the metal contents for comparison with the next slide.

139

Axial Seamount: Virgin Vent (25m from ASHES Vent)

Metal-Free Vent

Temp:299oCpH=4.4

Metals(mol kg-1)Zn 2.3Cu 0.7Fe 9Mn 162Au 150ppt

Majors(mmol kg-1) Cl 188Na 159Ca 10.2K 7.5H2S 19.5Si(OH)4 13.8

Slide 139. About 25 meters away from the main sulfide mound, a second vent was observed, emitting clear water and forming an anhydrite mound. Anhydrite is precipitated because of its retrograde solubility, and all of its constituents are obtained from seawater. We determined the metal content of the fluid, and you can see that relative to the Ashes mound, the base metal contents are very low, reduced by 10 to 100 times. However the gold content is at least as high as and perhaps a bit higher than that measured at Ashes. Note also that the major element constituents are reduced by a factor of 3 to 5 except for H2 S which is more than double and Si(OH)4, which is not reduced. This fluid represents the vapour phase which has separated from the residual fluid and is discharging separately. We also noted [not shown] that this fluid contained enhanced antimony, arsenic and mercury contents, all elements preferentially transported in the vapour phase. Clearly hydrothermal phase separation [boiling] has had a profound effect on the distribution of gold. This is explained in the next slide.

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Magmatic fluids or boiling?

(After Allen, 1997)

Au-Ag epithermalSea level

Dep

th

(mb

sl)

Boiling curve forseawater

Hydrothermal fluid discharge temperature

Temperature (oC)

Precipitation of Cu-Au caused by boiling

Zn-rich

Cu-rich

Cu-Au sulfide precipitationthrough cooled through seawater interaction

Modified from Yargeau, 2014

Boiling zone and formation of a Cu-Au stockwork with little or no surface expression

Slide 140. So this is an example of phase separation due to boiling, and we have already seen the boiling curve diagram and how boiling can change both the morphology of a VMS deposits as well as it’s composition, with Cu being precipitated in a lower part of a stringer zone followed by Zn and Pb, and finally the “epithermal”-like suite of Au, Ag, As, Sb and Hg

Magmatic fluids?

• Au-Cu transported as chloride/sulfide complexes in acid/oxidized fluid or high salinity brine

• Acid leaching defined by presence of Qtz-Al silicates-ser-py + bornite

(After Allen, 1997)

Slide 141. The close spatial relationship of many VMS deposit with rhyolite vent complexes and associated underlying hypabyssal intrusive complexes leads to the question of magmatic devolatilization as a possible source for metals. The close association of Cu- and Au in some Au-enriched VMS systems leads to the comparison with porphyry Cu-Au and associated high sulfidation epithermal systems. Although the presence of quartz-sericite-aluminosilicate rich alteration with both the Chisal/Lalor and Laurel Lake seafloor systems may be the result of low pH, acidic fluids, there is not much evidence in the form of the sulphur-poor metal assemblages that suggest a particularly oxidized fluid was involved in the deposition of Au.

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Low vs. high sulfidation VMS?

D. Lentz

Slide 142. In the context of both shallow submarine boiling and a possible magmatic contribution also think of VMS deposits as low and high sulfidation, as is proposed for epithermal systems. On an oxygen fugacity vs. pH plot the presence of both boiling and or magmatic fluid increases the oxidation state of the fluid and decreases the pH. This allows Au to be complexed as a bisulfide rather than a chloride complex, as the system becomes H2S dominant. This highly acidic environment has a strong effect on fluid rock interaction, leaching almost everything except for Si and Al in extreme cases resulting in “argillic” to “advanced argillic” alteration assemblages similar to subaerial epithermal deposits.

Doyon-Bousquet-LaRonde camp

From Mercier-Langevin et al. (2007)

Slide 143. A good example of a Au-rich VMS district is the Archean Doyon-Bousquet-LaRonde camp along the southern margin of the Abitibi Subprovince in Quebec. The host Blake River Group tholeiitic to calc-alkalic volcanic rocks host six known Au-rich VMS systems and a Au-Cu subsea-floor epithermal system at Doyon. It is also host to a number of orogenic gold deposits. The Au-Cu Doyon submarine epithermal system is hosted in part by a composite synvolcanic intrusive complex.

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2 km

Westwood

Bousquet 2-

Dumagami

LaRonde Penna

67 Mt @ 4 g/t

8.6 Moz Au

( + Zn-Cu-Ag)Regard vers le nord

Total camp: 146 Mt et 26 Moz Au

(SMV: LaRonde Penna, Bousquet 2-Dumagami, Bousquet 1, Westwood-

Warrenmac – Environ 105 Mt et 18.4 Moz Au)

Bousquet 1

Tiré de Mercier-Langevin et al. (2009)

Doyon-Bousquet-LaRonde camp

Slide 144. To date the camp contains in excess of 150 Mt of ore with 26 M oz gold plus significant tonnages of Cu, Zn and Ag. What is interesting from the exploration point of view in terranes such as the San Fernando VMS-hosting volcanic belt here in Cuba and the Tireo Group in the Dominican Republic is that we are not looking at a single large deposit, but rather a series of deposits spaced approximately every 2-4 km along a 20 km strike length of the ancient island arc assemblage.

Yargeau, 2014

Slide 145. As with most Au-rich VMS systems the gold is not distributed evenly throughout the entire VMS deposits, but rather the gold is sequestered within certain zones within each deposit. At the Westwood deposit there is a classic Zn-Cu bimodal felsic type VMS lens in the upper part of the system, which was subsequently overprinted by a shallow submarine epithermal-type Au-Cu vein stockwork that can be traced 2 km to the west to join with the Doyon Au-Cu submarine epithermal system. At the other end of the belt the large LaRonde-Penna deposit is similar, except it is the lower massive sulfide lenses that are Zn-Cu rich.

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LaRonde PennaWorld-class Au-rich VMS

67 Mt at 0.5% Cu, 2.8% Zn, 4

g/t Au, 45 g/t Ag

• Hosted in transitional to

calc-alkaline felsic

volcaniclastics, rhyolite

domes/cryptodomes, mafic

sills complex

• Large Qz-Bo-Gt-Sr

alteration

> 2.2 km

-600 m

-2800 m

From Mercier-Langevin et al. (2007)

Slide 146. It was when Agnico-Eagle followed the mineralized system down plunge that the alteration system changed from chlorite-sericite rich to a much more quartz-sericite-aluminosilicate rich assemblage rich in Au and Cu.

20 North lens – Au-Cu zone

1.5 cm

Mercier-Langevin et al., 2011

Slide 147. In epithermal terms this would be called advanced argillic alteration as part of a high sulfidation system. In fact, at the nearby Bousquet deposit a part of the Cu-Au stringer system consists of quartz-bornite, a much more oxidized assemblage.

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Cuba: An Example of Au-rich vs Au-normal Districts

148

• Numerous VMS

districts

• All are in mafic-

dominated bimodal

settings

• San-Fernand & Santa

Rosa have anomalous

Au (3-33 g/t)

• Antonio & Los

Cerros have typical

Au (1-2 g/t)

• All have peripheral

Ba zones

• Only Santa Rosa/ San

Fernando are Au rich

3-33 g/t Au

1 g/t Au

1-2 g/t Au

Slide 148. The Greater Antilles have several near- or past- producing VMS districts. Some are gold-rich, others are not. What causes this difference, in seemingly similar geological setting? What can we learn from study of these, in addition to active VMS-producing systems, that give us better exploration guidance in finding high-value, Au and Ag-rich deposits.

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Final Thoughts and Summary

149

Improvements needed:

Petrochemical data:• Paleo temperature indicators; no well-established constraints

• Alteration data: Some issues – Na depletion and relative Al gain works 99% of the time:

• what about sediment-hosted deposits?

– Carbonate issue: Thermochemical constraints• is it all thermogenic?

– Sericite: Rb-K identifies hydrothermal sericite; Ba constraints?

– Mg addition of limited use; lithology-sensitive

• Volcanological constraints (pyroclastics vs. flows)– Need better paleo-water depth indicators

• Geochronology: better tests for subvolcanic intrusions

• Geophysical models– Demagnetized alteration pipes

– Conductivity issues

Slide 149. This review has attempted to show how virtually all mappable and measurable attributes of a back arc volcanic system can be studied to determine their prospectivity for VMS deposits. Much remains to be learned however and some of these indicators are strictly based on observation without sufficient underpinning research to establish either their validity or scale. Much more information is needed in order to quantify paleo temperature indicators, and we have a relatively poor understanding of the boundary conditions for carbonates. We know that barium in sericite increases towards the deposits but we do not understand well the partitioning of barium between sericite and barite in the upper parts of the system. Volcanological attributes clearly help us to define the overall litho-tectonic setting for a VMS district and enable us to predict which alteration attributes are most applicable. To some extent these will also be useful to establish the likely compositional and size distribution of deposits but much more work is needed on this. As we move forward, use of geochronology to better establish the presence of subvolcanic intrusions, for example, and also to refine stratigraphy within a back arc system continues to add to our exploration efficiency. Geophysics, a long used method for finding conductive orebodies, has been significantly less effective in recent years as we learned that many sulfide bodies are non-– conductive. By integrating the data from a variety of geophysical methods, and integrating that further with geochemical constraints and our knowledge of stratigraphy associated with prospective districts, we will see in future much better multivariate vectoring to ore, particularly at depth. We need to remember that virtually all of the discovered VMS deposits occur within 200 meters of the earth surface, and that probably 75 to 80% of these sub-cropped or outcropped. In steeply dipping deformed terrains extant in most ancient back arc sequences, there is an equal opportunity for discovery of additional resources at depths well below 200 meters, once we can develop more fully integrated exploration models.

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Selected References

150

Barrie, C. T., Cathles, L. M., Erendi, A., Schwaiger, H., and Murray, C., 1999, Heat and fluid flow in volcanic-associated massive sulfide-forming

hydrothermal systems, in Barrie, C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern

and ancient settings., 8. Reviews in Economic Geology: Socorro, NM, United States, Society of Economic Geologists, p. 201-219.

Butterfield, D. A., Massoth, G. J., McDuff, R. E., Lupton, J. E., and Lilley, M. D., 1990, Geochemistry of hydrothermal fluids from Axial Seamount

Hydrothermal Emissions Study Vent Field, Juan de Fuca Ridge: Seafloor boiling and subsequent fluid-rock interaction: Journal of Geophysical Research,

v. 95, p. 12895-12921.

Franklin, J. M., Gibson, H. G., Jonasson, I. R., Galley, A. G., and . 2005, Volcanogenic Massive Sulfide Deposits: Economic Geology, v. 100th

Anniversary Volume, p. 523-559.

Galley, A. G., Hannington, M. D., and Jonasson, I. R., 2007, Volcanogenic massive sulphide deposits, in Goodfellow, W. D., ed., Mineral Deposits of

Canada, Special Publication No. 5: St Johns, Geological Association of Canada, p. 141-162.

Hannington, M. D., deRonde, C., and Petersen, S., 2005, Hydrothermal Ore Deposits in the Submarine Environment: Economic Geology, v. 100th

Anniversary Volume.

Huston, D. L., 1999, Stable isotopes and their significance for understanding the genesis of volcanic-hosted massive sulfide deposits; a review, in Barrie,

C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern and ancient settings., 8. Reviews

in Economic Geology: Socorro, NM, United States, Society of Economic Geologists, p. 157-179.

Kelley, D. S., Delaney, J. R., and Yoerger, D. R., 2001, Geology and venting characteristics of the Mothra hydrothermal field, Endeavour segment, Juan

de Fuca Ridge Geology, v. 29 p. 959-962.

Knuckey, M. J., Comba, C. D. A., and Riverin, G., 1982, Structure, metal zoning and alteration at the Millenbach Deposit, Noranda, Quebec, in

Hutchinson, R. W., Spence, C. D., and Franklin, J. M., eds., Precambrian sulphide deposits., 25. Special Paper - Geological Association of Canada:

Toronto, ON, Canada, Geological Association of Canada, p. 255-295.

Large, R. R., Allen, R. L., Blake, M. D., and Herrman, W., 2001, Hydrothermal Alteration and Volatile Element Halos for the Rosebery K Lens

Volcanic-Hosted Massive Sulfide Deposit, Western Tasmania: Economic Geology, v. 96, p. 1055-1072.

Peter, J. M., Kjarsgaard, I. M., and Goodfellow, W. D., 2003, Hydrothermal Sedimentary Rocks of the Heath Steele Belt, Bathurst Mining Camp, New

Brunswick: Part 1:Mineralogy and Mineral Chemistry, in Goodfellow, W. D., McCutcheon, S. R., and Peter, J. M., eds., Massive sulfide deposits of the

Bathurst Mining Camp, New Brunswick and Maine, Monograph 11, Economic geology, p. 361-390.

Rudnicki, M. D., and Elderfield, H., 1992, Theory appleid to the Mid Atlantic ridge hydrothermal plumes:the finite elemenn approach: Journal of

Volcanology and Geothermal Research, p. 161-172.

Schardt, C., Cooke, D. R., Gemmell, J. B., and Large, R. R., 2001, Geochemical modeling of the zoned footwall alteration pipe, Hellyer volcanic-hosted

massive sulfide deposit, western Tasmania, Australia, in Gemmell, J. B., and Hermann, W., eds., A special issue devoted to alteration associated with

volcanic-hosted massive sulfide deposits, and its exploration significance., Economic Geology Publishing Company. Lancaster, PA, United States. 2001.

Seyfried, W. E., Jr., Ding, K., Berndt, M. E., and Chen, X., 1999, Experimental and theoretical controls on the composition of mid-ocean ridge

hydrothermal fluids, in Barrie, C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern


Spooner, E. T. C., Bechinsdale, R. D., England, P. C., and Senior, A., 1977, Hydration, 18O enrichment and oxidation during ocean floor hydrothermal

metamorphism of ophiolitic metabasic rocks from E. Liguria, Italy: Geochem. et Cosmochim. Acta, v. 41, p. 857-872.

Slide 150. Barrie, C. T., Cathles, L. M., Erendi, A., Schwaiger, H., and Murray, C., 1999, Heat and fluid flow in volcanic-associated massive sulfide-forming hydrothermal systems, in Barrie, C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern and ancient settings., 8. Reviews in Economic Geology: Socorro, NM, United States, Society of Economic Geologists, p. 201-219. Butterfield, D. A., Massoth, G. J., McDuff, R. E., Lupton, J. E., and Lilley, M. D., 1990, Geochemistry of hydrothermal fluids from Axial Seamount Hydrothermal Emissions Study Vent Field, Juan de Fuca Ridge: Seafloor boiling and subsequent fluid-rock interaction: Journal of Geophysical Research, v. 95, p. 12895-12921. Franklin, J. M., Gibson, H. G., Jonasson, I. R., Galley, A. G., and . 2005, Volcanogenic Massive Sulfide Deposits: Economic Geology, v. 100th Anniversary Volume, p. 523-559. Galley, A. G., Hannington, M. D., and Jonasson, I. R., 2007, Volcanogenic massive sulphide deposits, in Goodfellow, W. D., ed., Mineral Deposits of Canada, Special Publication No. 5: St Johns, Geological Association of Canada, p. 141-162. Hannington, M. D., deRonde, C., and Petersen, S., 2005, Hydrothermal Ore Deposits in the Submarine Environment: Economic Geology, v. 100th Anniversary Volume. Huston, D. L., 1999, Stable isotopes and their significance for understanding the genesis of volcanic-hosted massive sulfide deposits; a review, in Barrie, C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern and ancient settings., 8. Reviews in Economic Geology: Socorro, NM, United States, Society of Economic Geologists, p. 157-179.

of 79

Selected References

151

Barrie, C. T., Cathles, L. M., Erendi, A., Schwaiger, H., and Murray, C., 1999, Heat and fluid flow in volcanic-associated massive sulfide-forming

hydrothermal systems, in Barrie, C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern


Butterfield, D. A., Massoth, G. J., McDuff, R. E., Lupton, J. E., and Lilley, M. D., 1990, Geochemistry of hydrothermal fluids from Axial Seamount

Hydrothermal Emissions Study Vent Field, Juan de Fuca Ridge: Seafloor boiling and subsequent fluid-rock interaction: Journal of Geophysical Research,

v. 95, p. 12895-12921.

Franklin, J. M., Gibson, H. G., Jonasson, I. R., Galley, A. G., and . 2005, Volcanogenic Massive Sulfide Deposits: Economic Geology, v. 100th

Anniversary Volume, p. 523-559.

Galley, A. G., Hannington, M. D., and Jonasson, I. R., 2007, Volcanogenic massive sulphide deposits, in Goodfellow, W. D., ed., Mineral Deposits of

Canada, Special Publication No. 5: St Johns, Geological Association of Canada, p. 141-162.

Hannington, M. D., deRonde, C., and Petersen, S., 2005, Hydrothermal Ore Deposits in the Submarine Environment: Economic Geology, v. 100th

Anniversary Volume.

Huston, D. L., 1999, Stable isotopes and their significance for understanding the genesis of volcanic-hosted massive sulfide deposits; a review, in Barrie,

C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern and ancient settings., 8. Reviews

in Economic Geology: Socorro, NM, United States, Society of Economic Geologists, p. 157-179.

Kelley, D. S., Delaney, J. R., and Yoerger, D. R., 2001, Geology and venting characteristics of the Mothra hydrothermal field, Endeavour segment, Juan

de Fuca Ridge Geology, v. 29 p. 959-962.

Knuckey, M. J., Comba, C. D. A., and Riverin, G., 1982, Structure, metal zoning and alteration at the Millenbach Deposit, Noranda, Quebec, in

Hutchinson, R. W., Spence, C. D., and Franklin, J. M., eds., Precambrian sulphide deposits., 25. Special Paper - Geological Association of Canada:

Toronto, ON, Canada, Geological Association of Canada, p. 255-295.

Large, R. R., Allen, R. L., Blake, M. D., and Herrman, W., 2001, Hydrothermal Alteration and Volatile Element Halos for the Rosebery K Lens

Volcanic-Hosted Massive Sulfide Deposit, Western Tasmania: Economic Geology, v. 96, p. 1055-1072.

Peter, J. M., Kjarsgaard, I. M., and Goodfellow, W. D., 2003, Hydrothermal Sedimentary Rocks of the Heath Steele Belt, Bathurst Mining Camp, New

Brunswick: Part 1:Mineralogy and Mineral Chemistry, in Goodfellow, W. D., McCutcheon, S. R., and Peter, J. M., eds., Massive sulfide deposits of the

Bathurst Mining Camp, New Brunswick and Maine, Monograph 11, Economic geology, p. 361-390.

Rudnicki, M. D., and Elderfield, H., 1992, Theory appleid to the Mid Atlantic ridge hydrothermal plumes:the finite elemenn approach: Journal of

Volcanology and Geothermal Research, p. 161-172.

Schardt, C., Cooke, D. R., Gemmell, J. B., and Large, R. R., 2001, Geochemical modeling of the zoned footwall alteration pipe, Hellyer volcanic-hosted

massive sulfide deposit, western Tasmania, Australia, in Gemmell, J. B., and Hermann, W., eds., A special issue devoted to alteration associated with

volcanic-hosted massive sulfide deposits, and its exploration significance., Economic Geology Publishing Company. Lancaster, PA, United States. 2001.

Seyfried, W. E., Jr., Ding, K., Berndt, M. E., and Chen, X., 1999, Experimental and theoretical controls on the composition of mid-ocean ridge

hydrothermal fluids, in Barrie, C. T., and Hannington Mark, D., eds., Volcanic-associated massive sulfide deposits; processes and examples in modern


Spooner, E. T. C., Bechinsdale, R. D., England, P. C., and Senior, A., 1977, Hydration, 18O enrichment and oxidation during ocean floor hydrothermal

metamorphism of ophiolitic metabasic rocks from E. Liguria, Italy: Geochem. et Cosmochim. Acta, v. 41, p. 857-872.

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