Blake River Research Articles

The structural setting and evolution of the Upper Beaver gold–copper deposit, Ontario, Canada

The ca. 2680 Ma Upper Beaver deposit is an Archean intrusion-related gold–copper deposit located in the Abitibi greenstone belt, Ontario. The deposit is associated with the Upper Beaver Intrusive Complex, which was emplaced in the hanging wall of an extensional listric fault rooted in metavolcanic rocks of the ca. 2704–2695 Ma Blake River assemblage. The fault formed during the development of an overlying basin of fluvial metasedimentary and alkalic metavolcanic rocks of the ca. 2679–2669 Ma Timiskaming assemblage. The Upper Beaver deposit was subsequently rotated to its current position during the formation of a post-Timiskaming fold, the Spectacle Lakes Anticline, associated with the development of the Larder Lake–Cadillac deformation zone, located 7 km south of the deposit. The Upper Beaver deposit is overprinted by the axial planar cleavage of the anticline but is less deformed than other gold deposits located along the deformation zone. Alteration and other pre-existing planar anisotropies enhanced strain partitioning and the development of folds and fabrics in strained mineralized zones of the deposit. Steeply dipping, sericite-altered mineralized zones developed a continuous foliation surrounding boudinaged and recrystallized quartz–calcite–anhydrite veins, whereas strong, shallowly dipping, stratiform, and skarnoid mineralized zones developed a wavy disjunctive cleavage and deformed mainly by folding. The mineralized zones acted as pre-existing anisotropies during deformation and their orientation and composition were key factors in the development of structures at the Upper Beaver deposit, which can be used as a guide for interpreting the development of structures in similar but more complexly deformed deposits along major deformation zones.

Evaluation of non-destructive tools for preliminary environmental risk assessment during mining exploration

Environmental assessment is generally performed when a mining project is at an advanced development stage, such as the prefeasibility study. It involves several laboratory tests such as detailed characterisation and geochemical mobility prediction tests, which require many samples considered representative of the deposit. Having early characterisation information could be useful to select the most relevant samples for the overall environmental assessment. The present study proposes a selection of non destructive and portable tools for improving the early environmental assessment of a mineral deposit. Three tools (x-ray fluorescence (XRF), mineral spectrometer, carbonate staining) were identified to form a preliminary environmental assessment methodology that can be performed directly on the exploration site. This methodology was applied to two different mining projects: i) a well developed mine, LaRonde zone 5, with available geochemical data in a volcanic-associated massive sulfide (VMS) context, and ii) an exploration project, O’Brien, with ongoing geochemical acquisition in an Au-quartz vein context. Both mine sites are in the south of the Blake River Group within the Abitibi greenstone belt, Quebec, Canada. The first site was used to validate the developed methodology, whereas the second acted as a test site to apply the methodology as a preliminary environmental assessment. The results on the first site confirmed that the selected tools provide geochemical and mineralogical data sets comparable to those of traditional testing methods. The developed methodology then enabled the classification of preliminary geo-environmental domains for the second site. Standard static and kinetic tests were performed on the second site samples to confront the preliminary assessment protocol. The proposed preliminary protocol made it possible to discriminate low-risk samples versus those with a potentially higher risk for the selection of subsequent environmental assessments.

3D geological modeling using Cu grades of the Archean Horne VMS deposit (Blake River Group, Quebec) and beneficiations to the genetical model

3D geological modeling using Cu grades of the Archean Horne VMS deposit (Blake River Group, Quebec) and beneficiations to the genetical model

The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part I. Volcanic Architecture, Deformation, and Metamorphism

Abstract The Westwood deposit (4.5 Moz Au) is hosted in the 2699–2695 Ma Bousquet Formation volcanic and intrusive rocks, in the eastern part of the Blake River Group, southern Abitibi greenstone belt. The Bousquet Formation is divided in two geochemically distinct members: a mafic to intermediate, tholeiitic to transitional lower member and an intermediate to felsic, transitional to calc-alkaline upper member. The Bousquet Formation is cut by the synvolcanic (2699–2696 Ma) polyphase Mooshla Intrusive Complex, which is cogenetic with the Bousquet Formation. The deposit contains three strongly deformed (D2 flattening and stretching), steeply S-dipping mineralized corridors that are stacked from north to south: Zone 2 Extension, North Corridor, and Westwood Corridor. The North and Westwood corridors are composed of Au-rich polymetallic sulfide veins and stratabound to stratiform disseminated to massive sulfide ore zones that are spatially and genetically associated with the calcalkaline, intermediate to felsic volcanic rocks of the upper Bousquet Formation. The formation of the disseminated to semimassive ore zones is interpreted as strongly controlled by the replacement of porous volcaniclastic rocks at the contact with more impermeable massive cap rocks that helped confine the upflow of mineralizing fluids. The massive sulfide lenses are spatially associated with dacitic to rhyolitic domes and are interpreted as being formed, at least in part, on the paleoseafloor. The epizonal, sulfide-quartz vein-type ore zones of the Zone 2 Extension are associated with the injection of subvolcanic, calc-alkaline felsic sills and dikes within the lower Bousquet Formation. These subvolcanic intrusive rocks, previously interpreted as lava flows, are cogenetic and coeval with the intermediate to felsic lava flows and domes of the upper Bousquet Formation. The change from fractional crystallization to assimilation- and fractional crystallization-dominated processes and transitional to calc-alkaline magmatism is interpreted to be responsible for the development of the auriferous ore-forming system. The Westwood deposit is similar to some Phanerozoic Au ± base metal-rich magmatic-hydrothermal systems, both in terms of local volcano-plutonic architecture and inferred petrogenetic context. The complex volcanic evolution of the host sequence at Westwood, combined with its proximity to a polyphase synvolcanic intrusive complex, led to the development of one of the few known large Archean subaqueous Au-rich magmatic-hydrothermal systems.

Recognizing subsurface breccias in Archean terranes: Implications for district scale metallogeny

Breccias are common in ancient and modern volcanic terranes, where they form at and below surface through volcanic, hydrovolcanic, magmatic, or tectonic mechanisms. They are critical for volcanic reconstruction as stratigraphic markers and indicators of geodynamic change and since they can be associated with mineralization their genesis is also important from an exploration perspective. Their origin can be difficult to ascertain in ancient terranes that have undergone polyphase deformation and associated metamorphism. The Joliet Breccia is a subeconomic Cu-Ag prospect within the Neoarchean Rouyn-Noranda mining district, in the Abitbi greenstone belt, Canada. It was previously interpreted as a phreatic breccia formed on the seafloor. This study presents new data indicating that the Joliet Breccia is a subsurface magmatic-hydrothermal breccia. Specifically, the recognition of gradational contacts with host rocks and between internal breccia domains, lack of sedimentary features and the spatial and temporal association with a tonalite intrusion strongly supports this interpretation. The angular, poorly sorted, lithic clasts derived from the immediate host rocks, hydrothermal cement, complete absence of a rock flour matrix, and presence of a radial and concentric fracture pattern extending into the host rocks, further support a subsurface magmatic-hydrothermal emplacement mechanism.A new TIMS U-Pb zircon age of 2698.0 ± 0.9 Ma (2σ) for a tonalite block within the breccia constrains the maximum age of brecciation. Four LA-ICP-MS U-Pb dates on hydrothermal monazite (2693.7 ± 1.0/8.9 Ma; 2696.9 ± 0.45/8.9 Ma; 2698.7 ± 1.4/8.9 Ma; 2701.2 ± 0.33/8.9 Ma; 2 s/2ssys) found in the cement of the Breccia indicate brecciation and mineralization occurred shortly after the emplacement of the tonalite. Similarly, the ca. 2697 Ma St. Jude intrusive breccia indicates a localized ca. 2699–2695 Ma magmatic-hydrothermal event superimposed on ca. 2704–2701 Ma strata. These breccias are temporally correlative with the youngest units of the Blake River Group strata and associated ca. 2698–2695 Ma volcanogenic massive sulfide (VMS) deposits, which suggests an indirect genetic link between these two hydrothermal systems. The VMS deposits may represent the near surface, distal manifestations of a deeper magmatic-hydrothermal system akin to the porphyry-intermediate sulfidation epithermal continuum in modern subaerial volcanic arcs.

Chemical alteration and preservation of sedimentary/organic nitrogen isotope signatures in a 2.7 Ga seafloor volcanic sequence

AbstractMassive to lobate volcanic flows and brecciated hyaloclastite units in the Abitibi greenstone belt allow investigation of Late Archæan seafloor alteration and associated incorporation into these rocks of nitrogen (N) biogeochemical signatures. In this suite (the Blake River Group), hyaloclastite units containing putative microbial ichnofossils are particularly enriched in large-ion lithophile elements (K, Rb, Ba, Cs), B, and Li, consistent with their having experienced the greatest fluid–rock interaction during subseafloor hydrothermal alteration. Similarly, silicate-δ18O and δ15N values for samples from the hyaloclastites show the greatest shifts from plausible magmatic values. The chemical and isotopic patterns in these tholeiitic igneous rocks greatly resemble those in modern altered seafloor basalts, consistent with the preservation of an Archæan seafloor alteration signature. The N enrichments and shifts in δ15N appear to reflect stabilization of illite and interaction with fluids carrying sedimentary/organic signatures. Enrichments of N (and the δ15N of this N) in altered glass volcanic rocks on Earth's modern and ancient seafloor point to the potential utility of N for tracing past and present biogeochemical processes in similar rocks at/near the Mars surface.

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.

Exogenous and endogenous construction of the subaqueous Glenwood felsic lava flow complex; Abitibi greenstone belt, Québec, Canada

Both intrusive and extrusive volcanic facies of a felsic lava flow complex are exposed as the outcrops of the “Glenwood Rhyolite” within the Blake River Group of the Abitibi greenstone belt, Québec, Canada. Sub-vertically-dipping volcanic facies allows for the identification of the internal architecture of an Archean subaqueous felsic flow complex. Detailed facies analyses show volcanic facies and volcanic textures and structures atypical of subaqueous pyroclastic deposits or deposits of dome complexes. Additionally, a complex network of mafic dykes cross-cut the felsic facies. When integrated with facies-specific geochemical analyses, a coherent model for all volcanic and intrusive facies and supplemental information regarding the host volcanic complex has been developed.Detailed mapping reveals extrusive or “exogenous” aphanitic felsic volcanic facies and intrusive or “endogenous” quartz- and feldspar-phyric felsic volcanic facies. Exogenous facies are comprised of massive lobate, in situ brecciated, flow breccia and flow-front breccia facies, whereas the only visible endogenous facies is a quartz- and feldspar-phyric unit that intrudes the brecciated facies on the eastern side of the complex. All felsic facies demonstrate a west to east flow direction. Both exogenous and endogenous facies have similar chemical compositions ranging from dacite to rhyodacite with a transitional affinity. Mafic to intermediate dykes predominantly strike north–south and east–west with occasional east–northeast-trending units. Wispy terminations at some dyke margins and dismembered dykes indicate they were intruded into a still cooling viscoelastic medium. Geochemical analyses further defines that most east–west-trending dykes have a tholeiitic affinity, whereas north–south-trending dykes are dominantly calc-alkaline and east–northeast-trending dykes are transitional.The subaqueous Glenwood felsic flow complex was emplaced in a fault-bounded depression and consists of stacked exogenous dacite/rhyodacite lobes and intruded by a late quartz- and feldspar-phyric endogenous lobe. The intrusion of mafic dykes occurred relatively soon after the cessation of felsic activity, as evidenced by primary textures consistent with intrusion into a viscoelastic medium. Combined, the extrusive and intrusive facies of the complex attests to the complexity of the host volcanic environment.

Setting and Evolution of the Archean Synvolcanic Mooshla Intrusive Complex, Doyon-Bousquet-LaRonde Mining Camp, Abitibi Greenstone Belt: Emplacement History, Petrogenesis, and Implications for Au Metallogenesis

The synvolcanic Mooshla Intrusive Complex intrudes coeval ~2699 to 2696 Ma volcanic rocks of the Blake River Group within the southern margin of the Archean Abitibi greenstone belt. The upper Blake River Group is host to the Doyon-Bousquet-LaRonde mining camp that contains Au-rich volcanogenic massive sulfide (VMS) deposits, possible subsea-floor epithermal-style deposits, and orogenic Au deposits. In total, the camp contains to date in excess of 28 million ounces (Moz) Au, making it a world-class example of Au-rich paleosea-floor environments. The Mooshla Intrusive Complex is spatially, temporally, and most probably genetically associated with all of the above types of mineralization. It is host to parts of the Doyon (5.5 Moz Au), Mouska (0.8 Moz Au), and Mic Mac (0.11 Moz Au) Au deposits and host to the smaller Mooshla A and B Au occurrences. Host volcanic units to the Mooshla Intrusive Complex are intensely deformed, metamorphosed, altered, and mineralized, as is the intrusion itself. The Mooshla Intrusive Complex was formed by nine distinctive phases of subvolcanic dikes, sills, and stocks. These were emplaced in two stages to form a shallow, multiphase synvolcanic intrusion along the contact between the Hebecourt and Bousquet volcanic formations. The Mouska stage is represented by a preliminary swarm of thin diabase sills, intruded by a well-layered gabbroic sill, a more crudely layered quartz diorite, and tonalite. A period of devolatilization accompanied crystallization of the xenolith-rich top of the tonalite magma chamber, as evidenced by the presence of an aplite dike swarm and associated extensive alteration zones and miarolitic cavities. The younger Doyon stage comprises a series of fine-grained aphyric to porphyritic, tonalite and trondhjemite dikes and sills, which also contain evidence of in situ devolatilization. The geochemical signatures of the Mooshla Intrusive Complex indicate emplacement during formation of an evolved, extensional oceanic island arc-style succession. Primitive mantle-normalized spider plots suggest a common origin for this island-arc intrusive suite that is similar to that of the volcanic succession of the upper member of the Bousquet Formation. Various element ratio plots used to further define magma origin and emplacement history suggest that whereas the Mouska-stage magmatic phases have a relatively straightforward, coexisting fractionation history, the Doyon-stage tonalite-trondhjemite has a more complex interplay of assimilation-fractionation-contamination, suggesting midcrustal partitioning and interaction with both earlier formed, partially hydrated ~2720 Ma oceanic crust and upper Blake River host strata (~2699-2696 Ma). The protracted and mulitphased magmatic evolution of the Mooshla Intrusive Complex led to the generation of volatile-rich phases that contributed to the development of a submarine magmatic-hydrothermal system that is thought to be responsible for the formation of the Doyon Au-Cu deposit. Geologic and timing relationships suggest that this magmatic-hydrothermal system might also have contributed to the generation of Au-rich VMS deposits higher in the host volcanic succession as part of a large Archean magmatic and hydrothermal center.

Three-Dimensional Visualization of the Archean Horne and Quemont Au-Bearing Volcanogenic Massive Sulfide Hydrothermal Systems, Blake River Group, Quebec,

The hydrothermal system architecture related to the formation of the contemporaneous Au-bearing Horne and Quemont volcanogenic massive sulfide (VMS) deposits was visualized by employing kriging methods to map whole-rock oxygen isotope compositions, zones of silica addition and loss, and water contents in two- and three-dimension. Zones of alteration were mapped in three-dimensions in the vicinity of the steeply dipping Horne deposit, to depths of as much as 2 km. In all, nearly 300 samples were analyzed for oxygen isotopes and supplemented by previously published whole-rock analyses. Contents of SiO2, H2O, MgO, Al2O3, and S from chemical analyses of nearly 5,000 samples within the two- and three-dimensional study regions were used separately, and in combination with the oxygen isotope data, for modeling and hydrothermal mapping purposes. The Horne and Quemont deposits formed within a similar time frame, but in different magmatic-hydrothermal systems, distinguished by their mapped hydrothermal architecture. The Quemont deposit appears to be centered on the Powell pluton, which intruded late into an apparent volcanic-filled, rift-graben structure. Although structural complexities are apparent, we infer mineralizing high-temperature upflow in the footwall of the Quemont deposit to have emanated from a reaction zone above the Powell pluton (and its precursors), beneath a zone of extensive silicification. Faulting on the Andesite fault and Horne Creek faults, plus erosion, has removed evidence of the upflow zone in the hydrothermal system of the Horne deposit. Areas of silicification correspond, in general, with isotopic evidence of lower temperature alteration. Such alteration east of the Quemont deposit signaled the waning of hydrothermal activity. The suggested cooling, for the most part, promoted the precipitation of silica. In the case of the Horne deposit, mixing of metalliferous hydrothermal fluid with cold seawater in the permeable footwall rocks, in an apparently relatively stratigraphically stable and long-lived hydrothermal system, evidently led to marked footwall silicification. The silicified footwall may have contributed to an increased efficiency of sulfide precipitation in the Horne deposit. Continued intrusion and some post-VMS hydrothermal activity is recorded in the hanging-wall section to the Horne deposit. Our data suggest that deposition of the 10 million ounces (Moz) of Au within the Horne deposit was syngenetic, and not the product of subsequent hydrothermal activity.

U-Pb Geochronology of the Blake River Group, Abitibi Greenstone Belt, Quebec, and Implications for Base Metal Exploration,

The 2704 to 2695 Ma Blake River Group in the southern Abitibi greenstone belt comprises a well preserved submarine volcanic sequence that hosts a large number of VMS and important Au-rich VMS deposits, including the world-class Horne and LaRonde-Penna deposits. Establishing precise chronostratigraphic control on the VMS deposits within the Blake River Group is critical because numerous distinct events took place within a period of 9 m.y. Nineteen new high-precision U-Pb ages temporally constrain the host rocks of many polymetallic VMS deposits and associated synvolcanic intrusions, demonstrating that these VMS deposits formed throughout the protracted volcanic evolution of the entire group. Ages on host rocks of the Horne (2702.2 ± 0.9 Ma), Quemont (2702.0 ± 0.8 Ma), and Fabie (2701.9 ± 0.9 Ma) deposits reveal that they are among the oldest VMS deposits in the Blake River Group. The giant Horne Au-rich VMS deposit had already formed when the Cu-Zn deposits of the Noranda mine sequence, including Millenbach and Amulet, were generated at ~2698 Ma and is thus unrelated, consistent with its different volcanological setting and metal content. Large Au-rich VMS deposits of the Bousquet Formation, including LaRonde Penna and Bousquet 2-Dumagami, were formed at 2698 to 2697 Ma and are distinctly younger than the Horne and Quemont deposits. There were, therefore, two major time-stratigraphic intervals within the Blake River Group that were favorable for the formation of Au-rich VMS deposits. Rhyolite hosting the large Bouchard-Hebert VMS deposits yielded an age of 2695.8 ± 0.8 Ma. Important mineralizing events in the Blake River Group occur at ca. 2-m.y. intervals apart and are associated with major magmatic episodes. Recognition of specific time-stratigraphic intervals for different styles of mineralization and geologic settings is essential to improve exploration models within the Blake River Group and for similar volcanic assemblages elsewhere in the Archean.

The Bousquet 2-Dumagami World-Class Archean Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi, Quebec: Metamorphosed Submarine Advanced Argillic Alteration Footprint and Genesis

The Archean Bousquet 2-Dumagami deposit is an Au-rich volcanogenic massive sulfide deposit (VMS) with a total production of 3.87 Moz Au, 2.77 Moz Ag, 80,000 metric tons (t) Cu, and 5,000 t Zn. The deposit is located within the Doyon-Bousquet-LaRonde mining camp in northwestern Quebec and hosted by the 2704 to 2695 Ma Blake River Group, the world’s most productive volcanic assemblage for Au-rich VMS deposits. The Bousquet 2-Dumagami deposit consists of stacked, deformed, and transposed semimassive to massive pyrite-rich lenses, breccia zones, and associated sulfide veins and stringer zones hosted by the upper member of the Bousquet Formation, ~50 to 100 m stratigraphically below the <2687 Ma Cadillac Group sedimentary rocks. The main ore zone is known as the Massive Hangingwall zone at the Bousquet 2 mine and Zone 5 at the Dumagami mine. Another semimassive to disseminated pyrite-rich auriferous zone with coarsely recrystallized massive pyrite is present in the footwall (Massive Footwall zone). The Massive Hangingwall zone is an Au-Ag- Cu-Zn sheet-like, semimassive to massive, pyrite-rich sulfide lens intermixed with vein and breccia zones. The dominant ore type consists of Au-Cu mineralization, but its upper and eastern parts are enriched in Zn. The ore consists of a complex assemblage of sulfides, sulfosalts, and native gold, including abundant pyrite, sphalerite, a few percent of chalcopyrite, bornite, and galena, with some visible gold. The Massive Hangingwall zone was formed by subsea-floor replacement of footwall calc-alkaline dacitic volcaniclastic rocks and hanging-wall blue quartz-phyric rhyolite. Despite significant north-south shortening and metamorphism, which was responsible for transposition, flattening, folding, and recrystallization, mineralogical gradients related to alteration-induced compositional variations can still be identified. A number of different metamorphic mineral assemblages can be mapped over several tens of meters from distal to proximal to the ore: (1) quartz-muscovite ± Mn-garnet ± biotite ± chlorite; (2) quartz-muscovite ± pyrite; (3) quartz-muscovite-andalusite-pyrophyllite-pyrite with topaz and diaspore; and (4) massive quartz-pyrite. A quartz-carbonate-biotite assemblage occurs in the hanging wall of the Zn-rich Massive Hangingwall zone and is hosted by andesitic sills. The thickness of each of these assemblages varies from a few meters to tens of meters. All metamorphosed alteration assemblages are characterized by strong progressive Na2O depletion. Gains in MnO, Fe2O3(total), MgO, and CaO are recorded in the quartz-muscovite ± Mngarnet ± biotite ± chlorite assemblage, whereas gains in K2O and losses in CaO occur in the quartz muscovite ± pyrite assemblage. In the quartz-muscovite-andalusite-pyrophyllite-pyrite and the proximal massive quartzpyrite assemblages all oxides, except SiO2, Fe2O3(total), and TiO2, were strongly to almost entirely leached. The andalusite-kyanite-pyrophyllite–bearing aluminous assemblages are interpreted to represent metamorphosed equivalents of synvolcanic alteration produced by acidic and oxidizing hydrothermal fluids (i.e., metamorphosed advanced argillic-style alteration), whereas the massive quartz-pyrite assemblage is similar to the massive silicic alteration commonly associated with advanced argillic alteration. The timing of Au mineralization is considered to be close to the age of the host rhyolite (2697.8 ± 1 Ma) and the age of the overlying felsic volcanic rocks (2697.5 ± 1.1 Ma). The major Au endowment of the Doyon-Bousquet-LaRonde mining camp may be related to favorable source rock or Au reservoirs specific to the lower crust or upper mantle beneath the eastern Archean Blake River Group. Exploration for additional Au-rich VMS in this environment should focus on distal quartz- and Mn-rich garnet-biotite and proximal aluminous assemblages with anomalously high Au and/or Cu and Zn in intermediate to felsic transitional to calc-alkaline volcanic or volcaniclastic rocks located underneath a younger sedimentary cover.

Using Physical Volcanology, Chemical Stratigraphy, and Pyrite Geochemistry for Volcanogenic Massive Sulfide Exploration: An Example from the Blake River Group, Abitibi Greenstone Belt,

An innovative approach to enhance volcanogenic massive sulfide (VMS) exploration in regions outside of mining camps is to first use physical volcanology with litho- and chemostratigraphy to establish the location of effusive centers in the volcanic units and use pyrite geochemistry in sulfide-bearing stratified intervals with whole-rock geochemistry in the underlying volcanic units to identify hydrothermal upflow zones. This methodology is illustrated by this study in the Archean Blake River Group within the Abitibi greenstone belt of Quebec and Ontario. The Blake River Group contains numerous VMS deposits, yet large segments remain underexplored, including the Hebecourt Formation which contains four tholeiitic units that range from basalt to rhyolite in composition. Effusive centers are located for three felsic units and subunits: (1) low Ti (porphyritic) subunit of the main rhyolite, (2) high Ti (aphyric) subunit of the main rhyolite, and (3) the upper rhyolite, and for a basaltic andesite unit. Stringer and disseminated Zn-Cu mineralization occurs within the flank breccia of the low Ti rhyolite dome. An inferred vent area for an overlying basaltic andesitic unit has also been identified in this area, illustrating the coincidence of hydrothermal upflow zones with volcanic vents. LA-ICP-MS analysis of pyrite grains from several sulfide-bearing stratified intervals indicates two broad areas of higher Cu, Zn, Au, and Ag contents. The eastern region corresponds to known volcanic vents and mineralization. The western region also indicates upflow of Cu-bearing hydrothermal fluids and corresponds to a possible effusive center for the high Ti subunit. The western region does not contain known mineralization at lower stratigraphic positions, but it has not been thoroughly explored.

Sampling oxygenated Archean hydrosphere: Implications from fractionations of Th/U and Ce/Ce* in hydrothermally altered volcanic sequences

Hydrothermally altered Archean igneous suites erupted in the submarine environment record variable excursions of Ce/Ce* and Th/U from primary magmatic values of 1 and ~4 respectively. Rhyolites of the 2.96Ga bimodal basalt–rhyolite sequence of the Murchison Domain, Yilgarn Craton, Western Australia, hosting the Golden Grove VMS deposit, are enriched in MnO up to ten fold over primary values. Th/U ratios span 2.6–4.7, Ce/Ce*=2.5–16, and Eu/Eu*=1.3–3. The 2.8Ga Lady Alma ultramafic–mafic subvolcanic complex of the same domain features highly dispersed MREE and LREE due to intense hydrothermal alteration. Th/U ratios span 0.005–0.16 from preferential addition of U, with Ce/Ce*=0.6–2.2, and Eu/Eu*=1–1.4. The eastern Dharwar Craton, India, includes greenstone terranes dominantly 2.7–2.6Ga. Adakites of the Gadwal terrane preserve near primary magmatic Th/U, Ce/Ce*, and Eu/Eu*. In contrast, igneous lithologies of the Hutti greenstone terrane are characterized by total ranges of Th/U=2–5.8, Ce/Ce*=1.01–1.28, and Eu/Eu*=0.82–1.26, and counterparts of the Sandur terrane have Th/U=0.4–6.0, Ce/Ce*=0.9–1.25, and Eu/Eu*=0.8–1.8. Coexistence of Ce and Eu anomalies may reflect a two-stage process: low-temperature hydrothermal alteration at high water–rock ratios by oxidizing fluids, with evolution of the hydrothermal systems to high temperature, low water–rock ratios, under reducing conditions. Uranium is dominantly added to these lithologies over Th in common with Recent altered ocean crust. Iron-rich shales in the Sandur terrane record U-enrichment where Th/U=2–4. Three shales record true negative Ce anomalies and Eu/Eu*=0.8–2.4: true negative Ce anomalies, present in some other Archean iron formations, are interpreted as a signature of precipitates from the ocean water column whereas Eu anomalies are hydrothermal in origin. Volcanic flows of the 2.7Ga Blake River Group, Abitibi greenstone terrane, Canada, preserve Th/U=1.5–8.5, the conjunction of low Th/U values with Ce/Ce*=1.4 in two samples, and Eu/Eu*=0.15–1.3. Mobility of U and Ce in these hydrothermally altered Archean lithologies is in common with their mobility in Phanerozoic counterparts by oxygenated fluids.

Timing and characteristics of the Archean subaqueous Blake River Megacaldera Complex, Abitibi greenstone belt, Canada

The Archean Blake River Group of the Abitibi greenstone belt represents a megacaldera complex which evolved over 8–11M.y. from approximately 2704 to 2696Ma. The early Misema Caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems. These remnants are found in the Jevis-Clericy, Montsabrais-Renault, Clifford-Tannahill and Colnet regions, where summit calderas are delineated by circular ring dyke structures. The secondary 330°-trending New Senator Caldera formed within the envelope of the Misema Caldera and exhibits a box-work graben-type structure. Finally, the felsic-dominated, 070°-striking Noranda Caldera, well known for its VMS endowment, represents the final collapse of the megacaldera complex.Deformation has overprinted the calderas, but structural patterns can be used in order to reconstruct the pre-existing volcanic architecture. Structures such as mafic ring dyke complexes and rhyolitic dome-flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation. Precise U-Pb geochronological analyses were conducted in selected areas with specific emphasis on the Misema and New Senator calderas. Felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution. The Misema Caldera formed between 2704 and 2702Ma via the amalgamation of shield volcanoes that were probably active prior to 2704Ma. The New Senator Caldera was generated between 2702 and 2700Ma during paroxysmal felsic volcanism, followed by the collapse of the Noranda Caldera culminating between 2700–2696Ma. The Misema Caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture, whereas the SE-trending New Senator Caldera is more compatible with a SW-trending principal compression direction related to oblique convergence in the Abitibi belt. The Noranda Caldera is interpreted as a NE rift structure that formed in the final stages of megacaldera evolution.

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.

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas1This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology. Canadian Journal of Earth Sciences, 48: this issue.2MRNF Contribution BEGQ 8439-2010/2011-1. Natural

The Archean Blake River Group (BRG) of Ontario and Quebec is dominated by submarine mafic to intermediate lavas, with more restricted felsic volcanic rocks. Given the good quality of outcrop, and high level of preservation of some BRG rocks, the mafic to intermediate lavas were used in the 1970s and 1980s to better understand the evolution of massive and pillowed submarine flows, and their associated fragmental facies (pillow breccias, hyaloclastite). Potentially, the BRG could also represent a useful volcanic succession for the study of explosive submarine eruption products in the ancient record. Before this is possible, however, a regional inventory of the mafic to intermediate volcaniclastic units is needed to clarify their characteristics and origins. In this paper, we compare and contrast volcaniclastic rocks from three areas within the same formation of the northern BRG in Quebec: the Monsabrais area, the Lac Duparquet area, and the D’Alembert tuff area. Close examination reveals pronounced differences in terms of lateral continuity, thickness, grading, bedding, clast shapes, textures, etc. in the volcaniclastic rocks. These differences are interpreted to reflect vastly different emplacement processes, ranging from hyaloclastite generation as a result of self-fragmentation and lava contact with water (dominant in the Monsabrais and Lac Duparquet areas) to aqueous density currents likely fed directly by explosive submarine eruptions (dominant in the D’Alembert tuff).

Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, geochemistry, and geochronology 1This article is a companion paper to Ross et al. 2011. Basaltic to andesitic volcaniclastic rocks in the Blake River Group, Abitibi Greenstone Belt: 1. Mode of emplacement in three areas. Canadian Journal of Earth Sciences, 48: this issue.2Ministère des Ressources naturelles

In the Archean Blake River Group, mafic to intermediate fragmental units have controversially been proposed to have formed during the collapse of a giant submarine caldera. This paper describes and interprets these rocks, summarizing their physical characteristics, inferred origins, age relationships, and geochemical signatures. The widespread Stadacona member, south of Rouyn-Noranda, consists of several hundred metres of bedded volcaniclastic rocks interpreted to have been mostly deposited from aqueous density currents fed directly by explosive eruptions. The magmas involved in these eruptions were plagioclase-phyric, tholeiitic to transitional basalts. The similarly widespread D’Alembert tuff, in the northern part of the Blake River Group, shares many physical characteristics with the Stadacona member and is thought to have a similar origin. However, the D’Alembert tuff is approximately six million years younger than the Stadacona member. It is composed mostly of transitional to calc-alkaline andesites and basaltic andesites with very distinct trace element profiles. Volcaniclastic rocks from other areas, such as Tannahill Township in Ontario and the Monsabrais area in Quebec, are interpreted to represent mostly in situ to remobilized hyaloclastite, with no explosive eruptions involved in their genesis. Our observations and interpretations are not compatible with models in which the volcaniclastic units are emplaced during a catastrophic event in relation with the collapse of a giant caldera. Instead, the fragmental rocks were produced by various mechanisms at many distinct times during the evolution of the Blake River Group.

Gold Potential of a Hidden Archean Fault Zone: The Case of the Cadillac-Larder Lake Fault

By compiling geological, structural, geophysical, and geochemical information into a 3D geological model, we evaluated the orogenic gold potential in the vicinity of a hidden segment of an important Archean fault zone, the Cadillac–Larder Lake fault (CLLF) in the region of Rouyn-Noranda. The segment of CLLF in the present study is partly covered by Proterozoic sedimentary rocks. Because more than 2000 t Au have been extracted along the CLLF to date, our objective is to evaluate the gold potential at depth along a poorly known segment of this fault. A 3D geological model (50 km × 9 km × 1.5 km) including the covered segment was built through the compilation and homogenization of available geological data and the construction of 23 cross sections. The geology under the Proterozoic cover was evaluated using geophysical inversions, drill holes (42 in total), and surrounding geology. All available assays were filtered and upscaled to a 250 m × 250 m × 250 m regular cell grid to determine and quantify spatial relationships between geological features and mineralized occurrences using the weights of evidence method. Structural features, such as E–W-trending faults and fault intersections, and certain lithologies with a high primary porosity such as volcanoclastic rocks of the Blake River Group and Timiskaming sedimentary rocks, proved to be very prospective, yielding favourable factors with a weight of evidence index W + > 0.24. These salient features were then assigned a combination index for ultimately evaluating the orogenic gold potential under the sedimentary cover. The zones resulting in an optimization of exploration targeting were attributed the highest probability, representing ~1% of the initial volume.

A volcanic habitat for early life preserved in the Abitibi Greenstone belt, Canada

A breccia facies of mafic composition in the subaqueous Misema shield volcano building phase and Misema caldera of the Blake River caldera complex, Abitibi Greenstone belt, Canada preserves putative microbial ichnofossils in formerly glassy breccia fragments. These Neoarchean ichnofossils are comparable in morphology, distribution, and mineral association to similar textures found in other Archean cratons as well as Phanerozoic ophiolite successions and are interpreted to represent an early Earth subaqueous biosphere. However, a detailed volcanological facies analysis of the rocks that host these purported microbial ichnofossils is lacking from previous studies. The present field locality (Hurd property) is a 62 m-thick volcanic succession that displays two main lithological facies: (i) a massive to lobate facies and (ii) breccia facies. The 5–25 m-thick massive to lobate facies contains massive basalts that grade into lobate flows toward the top. The 1–15 m-thick breccia facies displays delicate volcanic textures that are preserved due to extensive silica precipitation, a low temperature hydrothermal process, and the subgreenschist metamorphic grade. Geochemically, the mafic flows are tholeiitic basalts with TiO 2 contents of 1–2% but high SiO 2, up to 71%, related to hydrothermal silica precipitation. Microbial ichnofossils are found in the glassy fragments of the breccia facies. The breccia facies that displays evidence of associated hydrothermal activity, as in modern seafloor and summit caldera smoker settings, provided a favourable environment for ancient biological interactions between glassy rocks and microorganisms during formation of the Abitibi Greenstone belt.