Synvolcanic Structures Research Articles

New insights into the petrogenesis of rhyolites and evolution of volcanic‐hosted massive sulphide mineralization from shallow borehole core, Betul Belt, Central India

Shallow borehole drilling has been conducted at the Sarni area of Betul Belt, Central India to understand the subsurface geology and nature of mineralization. The predominant lithounits are ferruginous quartzites, felsic volcanic rocks, and with minor intrusions of amphibolites. The encountered felsic volcanic rocks are rhyolites varying from pink (rich in K‐feldspar) to grey (rich in Na‐feldspar and amphiboles). The rhyolitic rocks have undergone intense hydrothermal alteration. The alteration zones are defined by the presence of biotite‐chlorite‐garnet‐gahnite; biotite‐sericite‐chlorite‐anthophyllite and anthophyllite‐actinolite‐tremolite‐carbonate mineral assemblages. Sulphide pits, highly leached zones with occasional bluish tinges, limonitic patches are present in the felsic volcanic rocks, followed by semi‐massive to disseminated sulphide mineralization indicative of Kuroku‐ type Zn–Cu–Pb volcanic‐hosted Massive Sulphide (VHMS) mineralization. Scanning electron microscope and energy‐dispersive x‐ray spectroscopy studies documented sphalerite, pyrite, chalcophyrite, arsenopyrite, pyrrhotite, and minor Au–Ag phases. The Sarni rhyolites exhibit chalcopyrite disease in sphalerite, replacement, and overgrowth ore‐textures. The high HFSE, low Zr/Y ratio, pronounced negative Eu anomalies characterize the felsic volcanic rocks as FIIIb type with high zircon saturation temperatures of 856–931°C. These rhyolites are formed at a back‐arc rift setting in an ensialic island‐arc tectonic regime. The synvolcanic structures facilitated the magma ascent to the surface. The hydrothermal circulation due to adequate heat flow in the back‐arc rift environment, in an episodic multicyclic process provided sufficient time for accumulation of sulphides from the seafloor, which was later deposited to form a VHMS mineralization. We also report tungsten (W) mineralization in the quartz veins of these felsic volcanic rocks. It is suggested that the felsic magma has interacted with metasedimentary sequences and assimilated W‐ore bearing fluids, and during remobilization in the later stages of tectonic activity enabled the deposition of tungsten in these quartz veins.

PALEOVOLCANIC CONDITIONS OF MASSIVE SULPHIDE MINERALIZATION FORMATION OF AKZHARSKOYE ORE FIELD (SOUTHERN URALS)

The article considers the geological structure and paleovolcanic conditions for the formation of massive sulphide deposits and ore mineralizations of the Akzharskoye ore field located within the Dzhusinsko- Dombarovsky paleovolcanic belt. It is shown that three strata participate in the structure of the ore-bearing volcanogenic complex: lower basaltic, rhyolite-basalt and upper basaltic. Paleovolcanological analysis of the complex made it possible to reconstruct the following volcanic structures: 1) Akzharskoye shield volcano; 2) Akzharskaya caldera; 3) rhyolite-basalt volcanic apparatus; 4) Alimbayskaya depression; 5) Tyulkubayskaya depression. The history of the development of these structures is detailed in the work. It is shown that the structures are well mapped in physical fields - the volcanic accumulation region differs by an elevated gradient of gravity, its negative values characterize areas with a significant thickness of terrigenous deposits. The assignment of massive sulphide objects of the Akzharskoye ore field to the Urals type is proved. Moreover, they possess a number of specific features similar to those of the dombarovsky type. The ores were formed during the activity of acidic volcanism and are located on a single stratigraphic level marked by rhyodacitic bodies. It is established that the paleostructure of volcanic complexes is due to the combination of three-order synvolcanic structures. The largest of them are the structures of the first (shield volcanoes, relics of volcanic ridges) and the second (caldera depressions, separate apparatus of the central type) of orders, correspond respectively to volcanogenic complexes and ore fields. Placing of pyrite deposits and ore bodies is controlled by third-order structural elements - volcanic-tectonic depressions, local basalt-rhyodacite structures and vertex calderas of volcanicdome apparatus.

Depositional Setting and Structural Evolution of the Archean Perseverance Volcanogenic Massive Sulfide Deposit, Matagami Mining District, Quebec, Canada

The Perseverance volcanogenic massive sulfide deposit (5.1 Mt at @ 15.8% Zn, 1.24% Cu, 29.4 g/t Ag, 0.4 g/t Au), Matagami district, Abitibi greenstone belt, consists of four pipelike orebodies discordant to local bedding in a shallow-dipping portion of the South Flank volcanic succession. Most of the ore is hosted by rhyolitic lavas of the Watson unit (2725.9 ± 0.8 Ma) and is overlain by a thinly laminated tuffaceous unit known as the Key Tuffite, as well as by rhyodacitic lavas of the Dumagami unit (2725.4 ± 0.7 Ma). The presence of sulfide zones predominantly hosted in the footwall rhyolite, relicts of Key Tuffite within the ore, and intricate sulfide replacement of the laminated tuff are all features consistent with the formation of most of the deposit via subseafloor replacement. The permeability of steep synvolcanic structures likely controlled the migration of metal-bearing fluids and simultaneously allowed the downward infiltration of seawater, causing mixing, cooling, and sulfide deposition in the subseafloor environment. Stratigraphic relationships suggest that massive sulfide formation was active at Perseverance while tuffaceous sedimentation was ongoing on the seafloor. Progressive growth of the massive sulfides along subvertical structures was accompanied by the development of pipelike sericite-chlorite (± talc) alteration halos. Both the hanging wall and the footwall units are characterized by significant MgO mass gains, K 2 O-Na 2 O mass losses, and high alteration index values (e.g., chlorite-carbonate-pyrite index, alteration index). However, these geochemical changes are less intense and widespread in the hanging wall than in the footwall. The record of a decreasing alteration above the orebodies was likely caused by the emplacement of the rhyodacite while massive sulfide deposition and hydrothermal activity were still ongoing. Primary relationships between mineralization and the host lithofacies were overprinted by intense deformation during the main event of compression induced by regional north-south shortening. Deformation is controlled by contrasting rheologies within the sulfide assemblage and between the orebodies and their host rocks. Structural modifications include the transposition of sulfide ores subparallel to the main foliation, the formation of piercement veins at the interface with the host rocks, and the generation of a series of secondary textures within the orebodies, including a vertical mineralogical banding. The distribution of Cu in the deposit is interpreted to be affected by the mechanical remobilization of chalcopyrite during deformation, whereas the zonation of Zn is likely primary. Hydrothermal alteration halos adjacent to the orebodies have accommodated significant strain, as indicated by the presence of tight folds in the Key Tuffite unit and a marked schistosity. Strain localization in the vicinity of the deposit highlights the relative ductility of the ore assemblage with respect to the host volcanic succession and resulted in a spatial association between hydrothermal alteration and deformation, despite the lack of a genetic relationship.

Yakabindie Revisited—Volcanological and Structural Controls on the Komatiite-Hosted Six Mile Well and Goliath North Deposits and Implications for the Architecture of the 2.7 Ga Rift Event in the Agnew-Wiluna Belt, Yilgarn Craton, Western Australia

The Yakabindie area hosts significant deposits of disseminated Fe-Ni-Cu sulfide in the Six Mile Well (~2.0 Mt contained Ni) and Goliath North (~0.6 Mt contained Ni) dunite lenses. These dunite lenses form part of the same komatiitic ultramafic unit that hosts the giant Mount Keith MKD5 deposit (>2.6 Mt contained Ni) to the north, but the morphology and internal stratigraphy of the host unit differs significantly. Three-dimensional modeling of the Yakabindie region reveals that each dunite lens is located at the intersection between steeply dipping NNW- and NNE-trending synvolcanic faults, which appear to have acted as conduits from which komatiitic lava vented onto the seafloor. The igneous contacts dip more shallowly to the west and describe a W-facing wedge of olivine-cumulate rock, probably less than 1 km in plunge extent and cored by adcumulate rock. The mineralization at both Six Mile Well and Goliath North consists of multiple stacked lenses of disseminated Fe-Ni-Cu sulfide associated with the olivine adcumulate core. This core is interpreted as the pathway position that focused the flow of komatiite lava away from the vent. Rather than being linked by sheeted bodies of orthocumulate rocks, as is typically the case along strike to the north, the dunite lenses at Yakabindie appear to be capped by a strike-continuous sequence of thin komatiite flow units. There is no evidence of the fractionated sequences of pyroxenite and dolerite, which occur in pathway positions within the Mount Keith ultramafic unit to the north and which reflect late-stage ponding and in situ fractionation of komatiite lava. Three sets of synvolcanic structures are recognized in the broader Agnew-Wiluna belt and a model of oblique extension at ca 2.7 Ga is proposed. A NNW-trending set of structures marks the overall rift orientation at ca 2.7 Ga but is probably inherited from an earlier ca 2.81 Ga phase of rifting. A NW-trending set is thought to represent ca 2.7 Ga transfer faults, but may also have much older roots. These structures produce marked sinistral offsets in both the greenstone stratigraphy and the gravity response and can be used to divide the Agnew-Wiluna belt into rift segments. The third set of early structures trends NNE, subparallel with the volcanic grain of the greenstones as inferred from the orientation of komatiite lava pathways. These are thought to have originated as extension-orthogonal normal faults at ca 2.7 Ga. At the regional scale there is a strong spatial association between the larger Fe-Ni-Cu sulfide deposits of the Agnew-Wiluna belt and the locus of postulated NW-trending transfer zones, whereas at the deposit scale, intersecting NW- and NNE-trending synvolcanic faults exert a control on the location and best development (in terms of thickness and/or grade) of Fe-Ni-Cu sulfide mineralization in some deposits. These spatial associations have major implications for exploration targeting at both regional and deposit scale.

Reconstruction and evolution of Archean intracaldera facies: the Rouyn–Pelletier Caldera Complex of the Blake River Group, Abitibi greenstone belt, Canada

Subvertically- to vertically-dipping Archean strata provide an excellent opportunity to study synvolcanic structures and internal organization of subaqueous volcanic complexes. The Abitibi greenstone belt in Quebec, Canada, hosts a number of these volcanic complexes and specifically the unique Blake River Megacaldera Complex. The Blake River Megacaldera Complex is composed of (i) an initial shield phase known as the Misema Caldera and (ii) two graben-type calderas known as the New Senator and Noranda calderas. The southern portion of the New Senator Caldera, the focus of this study, is of particular interest as the structure hosts the 54 Mt Horne Au-rich volcanogenic massive sulfide deposit. Detailed facies mapping, coupled with geochemical and geochronological analysis at multiple outcrop localities throughout the city of Rouyn-Noranda, Quebec, show that the stratigraphy of the New Senator Caldera is dominated by subaqueous effusive mafic volcanic facies with local felsic effusive and intrusive deposits. With the incorporation of structural data, we have used synvolcanic faults and dyke complexes to divide these facies into specific blocks within this region, here renamed the Rouyn–Pelletier sector. This study focuses on the facies and characteristics of the Pelletier, Senator, and Glenwood blocks and identifies the Évain, Stadacona, and Chadbourne blocks. Geochronological analysis reveals that facies of the Glenwood Block are approximately the same age as those of the felsic flows of the previously identified Horne Block. Together, all extrusive and intrusive volcanic facies from the Stadacona unit north to the Horne Creek fault compose a large caldera complex known as the Rouyn–Pelletier Caldera Complex.

In situ hydroclastic fragmentation of subaqueous ponded lavas; New Senator caldera, Abitibi greenstone belt, Quebec, Canada

Subaqueous ponded lavas have been recognized within the structure of the New Senator caldera in the Blake River Group of the Abitibi greenstone belt, Quebec, Canada. Sub-vertical to near-vertical dips of extrusive volcanic facies within the southern sector of the caldera permit the identification of the internal architecture of an Archean subaqueous volcanic complex. Detailed facies analyses of the dominantly mafic sequences have shown two localities with atypical primary volcanic structures and facies inconsistent with those observed at modern subaqueous mafic seamounts. Combined with the use of facies-specific geochemical analyses, a coherent model for the two localities accounting for all observed volcanic structures and features has been developed.Mapping completed at a scale of 1:100 at Localities 1 and 2 reveals a series of sub-parallel to parallel hyaloclastite-rich horizons separating aphanitic to medium grained ponded units. Several primary volcanic structures are observed within the hyaloclastite horizons and are identified as pillowed forms, cigar forms and v-shaped structures. Ponded units range in thickness from 2.5 to 5m, with the exception of one unit which is approximately 30m thick. In central portions, this thick unit is medium-grained and has a sub-ophitic texture. Units at Locality 1 strike approximately NW, while those at Locality 2 strike ENE. Several families of mafic dykes cross-cut Locality 1 and trend E-W and NNE. Both volcaniclastic and effusive facies have similar compositions, affinities and trace and rare earth element signatures. All facies have a tholeiitic affinity, range in composition from basalt to andesite and have MORB-type trace and rare earth element signatures. Locality 1 appears slightly more evolved than Locality 2, with later dykes and a late sill being the most evolved facies.Volcanic facies at Localities 1 and 2 are consistent with subaqueous ponded lavas. Bounding synvolcanic structures and volcanic facies validate this interpretation and indicate these ponded lavas are hosted within a large synvolcanic depression at the summit of a mafic shield complex. Primary volcanic structures within hyaloclastite horizons formed via in situ fragmentation when water entered the system as ponded lavas began to cool. Water entered the system through synvolcanic fractures and encountered semi-molten pockets of lava, leading to hydroclastic fragmentation of the lavas. V-shaped and cigar structures formed during more energetic events and changed laterally into pillowed forms as energy dissipated from the system.

Volcanic reconstruction of Paleoproterozoic arc volcanoes: the Hidden and Louis formations, Flin Flon, Manitoba, CanadaThis is a companion paper to DeWolfe, Y.M., Gibson, H.L., and Piercey, S.J. 2009. Petrogenesis of the 1.9 Ga mafic hanging wall sequence to the Flin Flon, Callinan, and Triple 7 massive sulphide deposits, Flin Flon, Manitoba, Canada. Canadian Journal of Earth Sciences, 46: this issue.

The hanging wall to the Flin Flon, Callinan, and Triple 7 volcanogenic massive sulphide deposits of the Flin Flon district is composed of the Hidden and Louis formations. The contact between these formations is marked by mafic tuff that represents a hiatus in effusive volcanism. The formations form a composite volcanic edifice that was erupted and grew within a large, volcanic–tectonic subsidence structure (hosting the deposits) that developed within a rifted-arc environment. The formations are evidence of resurgent effusive volcanism and subsidence following a hiatus in volcanism marked by ore formation since they consist of dominantly basaltic flows, sills, and volcaniclastic rocks with subordinate basaltic andesite and rhyodacitic flows and volcaniclastic rocks. The Hidden formation is interpreted to represent a small shield volcano and the Louis formation a separate shield volcano that developed on its flank. Both the Hidden and Louis volcanic edifices were constructed by continuous, low-volume eruptions of pillow lava. A gradual change from a dominantly extensional environment during the formation of the footwall Flin Flon formation to a progressively more dominant convergent environment during the emplacement of the hanging wall suggests that the Hidden and Louis formations are unlikely to host significant volcanogenic massive sulphide-type mineralization. However, synvolcanic structures in the formations define structural corridors that project downwards into the footwall where they encompass massive sulphide mineralization, indicating their control on ore formation, longevity,and reactivation as magma and fluid pathways during the growth of the Hidden and Louis volcanoes.

Structural Controls on Massive Sulfide Deposition and Hydrothermal Alteration in the South Sturgeon Lake Caldera, Northwestern Ontario

Synvolcanic structures played a fundamental role in the genesis, morphology, and siting of volcanogenic massive sulfide ores and associated hydrothermal alteration in the Archean South Sturgeon Lake caldera complex. The most voluminous and persistent hydrothermal venting and massive sulfide deposition occurred along synvolcanic rifts and grabens associated with faults and tectonic fissures that created permeable fracture zones deep enough to access the underlying hydrothermal reservoir. The type of fracturing is highly variable and changes with the composition, competency, degree of consolidation, and alteration of host rocks. Synvolcanic structures and fracture styles also vary according to the amount and type of tectonic movement, including extension-related collapse, shearing and faulting perpendicular to the principal direction of extension, and orthogonal faulting and shearing. Permeable conduits were created by tension fracturing along fault zones, brittle deformation at the intersections of orthogonal faults, and by extensional fractures in stockworks. In texturally uniform footwall rocks, the distribution of alteration zones was controlled by the morphology of the structural conduit. In rocks with vertical and/or lateral facies, permeability, and competency changes (e.g., Lyon Lake graben), there was an additional stratigraphic control over fluid migration. Some crosscutting synvolcanic structures, alteration zones, and intrusions appear as stratiform units at the present erosion surface due to regional deformation and the present attitude of the volcanic stratigraphy. Hydrothermal mineral assemblages (e.g., quartz, carbonates, chlorite, pyrite, chalcopyrite) infilling structurally induced fractures provide good evidence of fluid migration pathways. However, mineralogy can vary significantly according to the fluid characteristics, host rock geochemistry, and subsequent metamorphic history of the area. Clearly, one of the best methods for locating volcanogenic massive sulfide deposits is to delineate the attitudes of synvolcanic structures, and explore those that show evidence of associated high-temperature hydrothermal mineral assemblages. Excellent exploration targets occur where synvolcanic structures with hydrothermal alteration intersect paleo-seafloor horizons.

Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (>23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by synvolcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2–15 m thick, which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief, which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate, but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows, proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.

The Spectrum of Ore Deposit Types, Volcanic Environments, Alteration Halos, and Related Exploration Vectors in Submarine Volcanic Successions: Some Examples from Australia

Variations in shape, metal content, alteration mineralogy, and volcanic host rocks of the ore deposits in the two major volcanic-hosted massive sulfide (VHMS) districts of eastern Australia, the Cambrian Mount Read Volcanics and the Cambro-Ordovician Mount Windsor subprovince, strongly reflect their volcanic environment, conditions of ore formation, and hydrothermal alteration processes. Lens and sheet-style polymetallic zinc-rich deposits such as Rosebery, Hellyer, Que River, and Thalanga are considered to have formed in moderate to relatively deep water environments (500-1,000+m). These deposits probably formed either on the sea floor (e.g., Hellyer, Que River) or by replacement of porous volcaniclastic units directly below the sea floor (e.g., Rosebery). The footwall alteration associated with these polymetallic VHMS deposits was controlled by host-rock permeability and porosity, which are in turn related to volcanic facies type, degree of fracturing, and synvolcanic structural architecture. Focusing of hydrothermal fluids along synvolcanic structures has resulted in well-zoned chlorite-sericite footwall alteration pipes within footwall lavas at Hellyer. On the other hand, diffuse fluid flow through very thick pumice breccia at Rosebery and Hercules has resulted in strata-bound, sericite-dominated footwall alteration zones parallel to the paleosea floor and the ore lenses. Massive and disseminated, pyritic Cu-Au deposits, such as those in the Mount Lyell field and at Highway-Reward, formed by subsea-floor replacement and are associated with only minor zinc-lead massive sulfide ore. These deposits formed from higher temperature fluids (>300 degrees C), in which copper transport is enhanced, and are commonly located in felsic volcanic centers dominated by shallow porphyritic intrusions (e.g., Highway-Reward). The Cu-Au ore lenses may be strata-bound (e.g., Mount Lyell) or crosscutting pipes (e.g., Highway-Reward) depending on the structure and permeability characteristics of the felsic volcanic host rocks. The presence of high-sulfidation alteration minerals (e.g., pyrophyllite, zunyite) in some of the Cu-Au deposits (e.g., Mount Lyell field) indicates that fluids were relatively acidic and suggests the possibility of magmatic fluid input into the hydrothermal system. Alteration zonation associated with the Cu-Au VHMS deposits is more symmetrical than that of the Zn-rich deposits, with sericite-rich alteration extending into the hanging wall, in keeping with the subsurface replacement origin of these deposits. Synvolcanic gold-rich deposits, with high gold/base metal ratios are less common than the Cu-Au and Zn-rich VHMS ore types. The gold-rich ores (e.g., Henty, South Hercules) are strata bound in nature, have low sulfide contents, and are associated with central zones of intense silicification, surrounded by envelopes of sericite-pyrite and carbonate alteration. Volcanological and geochemical studies at Henty indicate the gold-rich ore formed by the replacement of particular volcanic units deposited in a relatively shallow water environment dominated by volcaniclastic facies, lavas, and limestones. This spectrum of Cu-Au, Zn-rich, and Au-only deposits in the Mount Read Volcanics and the Mount Windsor subprovince is interpreted to represent a continuum from classic sea-floor VHMS ores toward those with features more akin to porphyry Cu-Au and epithermal Au-Ag deposits. This spectrum relates to the interplay between factors in the submarine volcanic environment and the character of the hydrothermal fluid as follows: (1) proportions of volcaniclastic, lava, and subvolcanic intrusive facies; (2) depth of seawater; (3) permeability and porosity of volcanic host rocks; (4) balance between magmatic components and seawater components in the ore fluid; and (5) temperature and acidity of the ore fluid. Mineralogical, lithogeochemical, and isotopic studies have revealed a range of alteration vectors useful in exploration for both the Zn-rich and Cu-Au VHMS deposits. Carbonate and white mica compositional variations are highlighted as important mineralogical vectors; thallium and antimony halos may be useful trace element vectors; and oxygen and sulfur provide important isotope vectors toward the center of the hydrothermal system.

Structure of the Upper Devonian Boyd Volcanic Complex, south coast New South Wales: Implications for the Devonian‐Carboniferous evolution of the Lachlan Fold Belt

Upper Devonian continental and subaqueous sedimentary rocks and bimodal volcanic rocks of the Boyd Volcanic Complex of the south coast of New South Wales were deposited in a rapidly subsiding, 330°‐trending, transtensional basin. Structural analysis of synvolcanic and synsedimentary deformational structures indicate that basin formation is related to a 330°‐orientated subhorizontal σ1 and a 060°‐orientated subhorizontal σ3, which account for the development of the observed intrusion and fracture orientations. Rhyolitic, basaltic and associated clastic dykes are preferentially intruded along extensional 330°‐trending fractures, subordinately along sinistral, transtensional 010°‐trending fractures and along 290°‐trending fractures. One of the implications of such a palaeotectonic reconstruction is that the so called north‐trending Eden‐Comerong‐Yalwal Late Devonian rift does not represent a simple, single palaeobasin entity, but is presently a north‐trending alignment of exposures of sedimentary and volcanic rocks probably emplaced in different basins or sub‐basins, mildly folded during the Carboniferous Kanimblan compression (which also formed the north‐trending Budawang synclinorium) and then extended to the east by the Tasman Sea opening during the Jurassic. The development of scattered, rapidly subsiding, basins characterised by bimodal volcanism during the Late Devonian throughout the Lachlan Fold Belt, can be interpreted in terms of extensional collapse of a forming mountain belt contemporaneous with a sharp decrease of compressional stress after the Middle Devonian Tabberabberan orogenic event. This would promote a reorientation of σ3 and transition from a compressional to a transtensional tectonic environment, which could also favour block rotation and formation of release basins.

The Geology of the Volcanic-associated Polymetallic (Zn, Cu, Ag and Au) Selbaie Deposits, Abitibi, Quebec, Canada

“Les Mines Selbaie” is a large, generally low-grade, polymetallic (Zn, Cu, Ag, Au) “giant” volcanic-associated ore deposit in the Harricana-Turgeon belt, which is part of the northern Abitibi Subprovince of Quebec. Economically recoverable mineralization occurred in three zones: the A1 (open pit), A2, and B Zones (underground). The pre-mining resource was 54.9 Mt, including 8 Mt of sub-economic, locally silver-rich massive pyrite. In the Selbaie region, calc-alkaline andesitic volcanics form the “basement.” The Brouillan Volcanic Complex formed within an island-arc setting on this regional basement. Evolution of this complex involved the formation of a felsic magma chamber and the development of a large caldera structure over the Brouillan batholith. Second-order linear and sub-parallel faults and rifts developed as a result of back-arc rifting/extension. The edges of these rifts were feeders for linear felsic volcanism (similar to fissural volcanism). During felsic volcanism, the feeder zones acted as conduits for hydrothermal fluids with associated pervasive potassic alteration. The alteration is characterized both vertically and laterally by well-defined quartz, sericite, K-feldspar and/or, locally, biotite halos, to produce a mushroom-shaped morphology. This K-feldspar alteration is not common in most other volcanic-associated ore deposits. Mafic dikes cut all of the geological units and appear to occupy earlier synvolcanic structures. Regional green-schist-assemblage metamorphism and pervasive deformation affect all units. Thrust faults moved part of the Brouillan batholith and basement units over the Mine Sequence. Although there is no relationship between the earlier mineralizing events and these late thrust faults, the thrust faults probably played an important role in location of ore lenses. Three types of mineralization are present: 1) copper-rich veins and hydraulic breccias; 2) disseminated and stringer, low-grade and high volume zinc-copper mineralization; and 3) finely laminated, massive, locally silver-rich pyrite mineralization within structural basins. At the bottom of these basins and at the contact of welded acid tuff (WAT) units, massive pyrite zones are locally enriched in zinc, and contain the possible vestiges of black smoker-like material. The ore metals were probably derived from leached basement andesitic volcanics in a hydrothermal system powered by one or more heat sources within the Brouillan Volcanic Complex. The Selbaie deposits are mainly characterized by: (1) the presence of small volcano-tectonic subsidence basins, which host chemical sediments and volcaniclastic materials; (2) pervasive potassic alteration; (3) locally high-grade silver mineralization (up to 1-2 kg/t Ag); and (4) the presence of random zones of Pb mineralization. The Selbaie deposits can be closely compared with the copper-rich Murgul deposits, which belong to the eastern Pontides metallogenetic province in Turkey. If conventional classification criteria for the volcanogenic massive sulfide deposits (e.g., Noranda, Mattabi types, and recent subsea floor deposits) are taken into account, the Selbaie deposits still exhibit many differences in terms of alteration assemblages, physical volcanology and some styles of mineralization (low-grade and large volume). Consequently, the “Selbaie” deposits belong to a new sub-type of volcanic-associated sulfide deposit, which may be used in future exploration programs.

The Selbaie Zn-Cu-Ag deposits, Quebec, Canada; an example of evolution from subaqueous to subaerial volcanism and mineralization in an Archean caldera environment

The Selbaie deposits of western Quebec are hosted in calc-alkalic pyroclastic rocks of the Abitibi subprovince. Rocks range upward from subaqueous debris flows through pumiceous tuff to densely welded ignimbrite; they form an intracaldera sequence over 1,200 m thick which records evolution from subaqueous to subaerial deposition. Mineralization occurred within this sequence on the southwest flank of the Brouillan tonalite, a central intrusion and possibly a resurgent dome, and includes regionally extensive and temporally early exhalite, local massive sulfide, and later epithermal veins. These veins were emplaced by open-space filling along synvolcanic structures and make up the bulk of the economic resource.Mineralization and alteration show similarities to both massive sulfide and epithermal deposits. Cu/Zn increases with depth but also increases laterally toward veins and toward the intrusion. Ag/Cu in veins is higher in the upper levels of the system. Whole-rock mass balance calculations indicate enrichment in Si, Fe, Mg, and Mn with proximity to mineralization and depletion in Ca, K, and Na. Na metasomatism, common in host rocks to Archean massive sulfide deposits, is absent in subaerially emplaced tuff at Selbaie and K feldspar is preserved.Selbaie is unusual (1) as an Archean example of a subaerial caldera that evolved from subaqueous to subaerial volcanism, (2) in representing economically important, Archean epithermal mineralization, and (3) in demonstrating the link between subaqueous volcanogenic massive sulfide deposits and subaerial volcanogenic epithermal veins. It is, perhaps, unique in preserving so well all three of these aspects in a single mineralized system. The rarity of similar, mineralized subaerial volcanic settings in the Abitibi subprovince reflects its primitive crustal development, but may also be due to incomplete preservation of subaerial environments.