Southern Abitibi Greenstone Belt Research Articles

Geology and Controls on Gold Enrichment at the Horne 5 Deposit and Implications for the Architecture of the Gold-Rich Horne Volcanogenic Massive Sulfide Complex, Abitibi Greenstone Belt, Canada

Abstract The Archean Horne 5 deposit, located in the Rouyn-Noranda district in the southern Abitibi greenstone belt, Canada, contains a total resource of 172.4 t Au (5.6 Moz) from 112.7 Mt of ore grading at 1.53 g/t Au. The deposit is part of the Au-rich Horne volcanogenic massive sulfide (VMS) complex that also includes the past-producing Horne mine (i.e., the Upper and Lower H zones plus small subsidiary lenses) that yielded 325.4 t Au (10.5 Moz Au) from 53.7 Mt of ore grading at 6.06 g/t Au. Combined, the Horne mine and Horne 5 deposit contain ~500 t Au (16 Moz), making them the world’s single largest accumulation of VMS-related Au. The Horne 5 deposit consists of stacked lenses of massive to semimassive sulfides alternating with extensive zones of disseminated and stringer sulfides. The mineralization is hosted within thick accumulations of steeply dipping dacitic to rhyodacitic volcaniclastic units of transitional to calc-alkaline magmatic affinity. Dacitic-rhyodacitic synvolcanic units (lobes, sills, and/or domes) intrude the host succession, which is also crosscut by a series of post-ore mafic and younger intermediate to felsic feldspar ± quartz porphyry dikes. A broad and diffuse halo of distal sericite-chlorite-epidote alteration extends outboard of intensely sericite-altered zones proximal to the sulfide lenses. Gold is interpreted to be synvolcanic on the basis of Au-rich massive sulfide clasts in the volcaniclastic units, the presence of preserved Au-rich primary pyrite, Au zones limited to the sulfide envelope, crosscutting deformed but unaltered and barren dikes, and the absence of typical syndeformation, orogenic-style alteration and mineralization despite overprinting high-strain corridors and faults. Gold is spatially associated with pyrite, sphalerite, and chalcopyrite, and its distribution is largely controlled by the higher porosity and permeability of the volcaniclastic host rocks, which are interpreted to have facilitated hydrothermal fluid circulation in the subseafloor environment. Synvolcanic intrusions and fine-grained tuffs overlying auriferous zones also influenced the distribution of the mineralization by acting as cap rocks to ascending fluids. Evidence suggests that Au enrichment at the Horne 5 deposit is due to efficient transport and precipitation of Au in the subseafloor environment, a favorable geodynamic setting (transitional to calc-alkaline magmatism over thick crust), and possible input of magmatic fluids as suggested by high Te and Cu in the mineralization. Minor and very local remobilization of metals occurred in response to regional deformation and associated greenschist facies metamorphism. The detailed study of the Horne 5 deposit geology and a review of the available information on the Horne mine and recent 3-D modeling indicate that the Horne 5 deposit may have formed higher in the stratigraphy than the Upper and Lower H orebodies of the former Horne mine, which originally formed a single lens. Therefore, the Horne Au-rich VMS complex originally formed as a stacked system in which the Horne 5 deposit was deposited above the Upper and Lower H zones and not in a distal or lateral position as previously proposed, indicating that a robust hydrothermal system was responsible for the formation of the world’s largest Au-rich VMS complex.

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.

Hydrothermal Alteration Mineralogy and Geochemistry of the Archean World-Class Canadian Malartic Disseminated-Stockwork Gold Deposit, Southern Abitibi Greenstone Belt, Quebec, Canada

AbstractThe Canadian Malartic stockwork-disseminated gold deposit is an Archean world-class deposit located in the southern Abitibi greenstone belt. It contains over 332.8 tonnes (t; 10.7 Moz) of Au at a grade of 0.97 ppm, in addition to 160 t (5.14 Moz) of past production (1935–1981). Although the deposit is partly situated within the Larder Lake-Cadillac fault zone, most of the ore occurs up to ~1.5 km to the south of the fault zone. The main hosts of the mineralized zones are greenschist facies turbiditic graywacke and mudstone of the Pontiac Group (~2685–2682 Ma) and predominantly subalkaline ~2678 Ma porphyritic quartz monzodiorite and granodiorite. These intrusions were emplaced during an episode of clastic sedimentation and alkaline to subalkaline magmatism known as the Timiskaming assemblage (<2680–2670 Ma in the southern Abitibi). The orebodies define two main mineralized trends, which are oriented subparallel to the NW-striking S2 cleavage and the E-striking, S-dipping Sladen fault zone. This syn- to post-D2 ductile-brittle to brittle Sladen fault zone is mineralized for more than 3 km along strike.The ore mainly consists of disseminated pyrite in stockworks and replacement zones, with subordinate auriferous quartz veins and breccia. Gold is associated with pyrite and traces of tellurides defining an Au-Te-W ± Ag-Bi-Mo-Pb signature. The orebodies are zoned outward, and most of the higher-grade (>1 ppm Au) ore was deposited as a result of iron sulfidation from silicates and oxides and Na-K metasomatism in carbonatized rocks. The alteration footprint comprises a proximal alteration envelope (K- or Na-feldspar-dolomite-calcite-pyrite ± phlogopite). This proximal alteration zone transitions to an outer shell of altered rocks (biotite-calcite-phengitic white mica), which hosts sub-ppm gold grades and reflects decreasing carbonatization, sulfidation, and aNa+/aH+ or aK+/aH+ of the ore fluid.Gold mineralization, with an inferred age of ~2664 Ma (Re-Os molybdenite), was contemporaneous with syn- to late-D2 peak metamorphism in the Pontiac Group; it postdates sedimentation of the Timiskaming assemblage along the Larder Lake-Cadillac fault zone (~2680–2669 Ma) and crystallization of the quartz monzodiorite. These chronological relationships agree with a model of CO2-rich auriferous fluid generation in amphibolite facies rocks of the Pontiac Group and gold deposition in syn- to late-D2 structures in the upper greenschist to amphibolite facies. The variable geometry, rheology, and composition of the various intrusive and sedimentary rocks have provided strain heterogeneities and chemical gradients for the formation of structural and chemical traps that host the gold. The Canadian Malartic deposit corresponds to a mesozonal stockwork-disseminated replacement-type deposit formed within an orogenic setting. The predominance of disseminated replacement ore over fault-fill and extensional quartz-carbonate vein systems suggests that the mineralized fracture networks remained relatively permeable and that fluids circulated at a near-constant hydraulic gradient during the main phase of auriferous hydrothermal alteration.

Early gold-bearing quartz veins within the Rivière-Héva fault zone, Abitibi subprovince, Quebec, Canada

The Malartic Lakeshore showing is a gold-bearing quartz vein system located within the major Rivière-Héva fault zone (RHFZ) of the southern Abitibi greenstone belt. This fault separates the 2702–2700 Ma felsic Héva Formation from the 2708 Ma mafic-ultramafic Dubuisson Formation. A swarm of thin diorite dykes with lamprophyric facies and gold-bearing quartz veins are present only on the Dubuisson side of the fault. The 30–70 cm thick gold quartz veins are boudinaged and folded. Veins are banded and associated with pyrite, chalcopyrite, galena, barite, and gold. The study area is characterized by a high degree of ductile deformation associated with the RHFZ and manifested by the southeast-trending “principal schistosity” (Sp). Stretching lineations plunge moderately to shallowly toward the southeast as a result of shortening followed by late directional shearing during a transpressive deformation. A sample from the Héva Formation yielded a zircon U–Pb age of 2698.2 ± 0.8 Ma, and a diorite dyke produced an age of 2694.3 ± 2.5 Ma. Quartz veins are crosscut by dykes, and both are affected by the Sp fabric, indicating an early emplacement with respect to the deformation. This situation contrasts with the orogenic gold veins found in association with major fault zones. A near-synvolcanic magmatic hydrothermal origin is proposed for this gold vein system. Because all subvertical units in the area are south facing, the presence of the older Dubuisson Formation over the younger Héva Formation is attributed to the RHFZ acting as a significant reverse or thrust fault.

Structural evolution of a gold-bearing transtensional zone within the Archean Porcupine-Destor deformation zone, southern Abitibi greenstone belt, eastern Ontario, Canada

The Garrison camp comprises four structurally distinct orogenic gold deposits that formed in different host lithologies during progressive deformation. Detailed field mapping, drill core logging, and geochronological constraints suggest that the 2678 ± 2 Ma Garrison granitic stock played a fundamental rheological role in the location of the four deposits. Initial local shear movement occurred along the southwestern margin of the stock leading to the development of the NW-trending sinistral NE-side-up Buffonta shear zone, which hosts the Buffonta deposit. Subsequently, a transtensional zone formed between the NE-trending sinistral Porcupine-Destor and Munro deformation zones, which host the 903 and Jonpol deposits, respectively. Finally, a local change in shortening orientation from NE to NNW caused westerly directed extension resulting in the formation of the younger gold-bearing veins composing the Garrcon deposit. In situ U-Pb laser ablation-inductively coupled plasma-mass spectrometry performed on monazite grains formed within the shear fabric of the Munro deformation zone indicates that transtension occurred at 2657 ± 15 Ma. Therefore, at least three of the four deposits formed subsequent to crystallization of the Garrison granitic stock. The reported U-Pb dates represent the first direct age constraints on the movement along a gold-bearing deformation zone in the southern Abitibi greenstone belt.

Origin of the Piché Structural Complex and implications for the early evolution of the Archean crustal-scale Cadillac – Larder Lake Fault Zone, Canada

The Piché Structural Complex (PSC) extends over 150 km within the Cadillac – Larder Lake Fault Zone (CLLFZ), a gold-endowed, east-trending, and high-strain corridor located along the southern edge of the Archean Abitibi Subprovince. The PSC consists of discontinuous units of volcanic rocks (<1 km thick) that host multiple gold deposits. It is spatially associated with molasse-type Timiskaming sedimentary basins. This study describes and interprets the origin of structures and lithologies within the poorly understood PSC to unravel the tectonic evolution of the CLLFZ. Field mapping, chemical analyses, as well as interpretations of cross-sections from drill-hole data, were used to interpret the geometry and structure of the PSC. The PSC is subdivided into six homogeneous fault-bounded segments or slivers. These slivers consist mostly of ultramafic to intermediate volcanic rocks and include some felsic volcanic flows and intrusions. Volcanic facies, chemical compositions, and isotopic ages confirm that these slivers are derived from the early volcanic units of the southern Abitibi greenstone belt, which are located north of the CLLFZ. Cross-cutting relationships between volcanic rocks of the PSC and the Timiskaming-aged intrusions suggest that the slivers were inserted into the CLLFZ during the early stages of the accretion-related deformation (<2686 Ma) and prior to Timiskaming sedimentation and ductile deformation (>2676 Ma). The abundant ultramafic rocks located within the CLLFZ may have focused strain, thereby facilitating the nucleation of the fault as well as the displacements along this crustal-scale structure.

Structural setting of the Young-Davidson syenite-hosted gold deposit in the Western Cadillac-Larder Lake Deformation Zone, Abitibi Greenstone Belt, Superior Province, Ontario

Within the southern Abitibi greenstone belt of the Superior Province, intrusion-related, in particular syenite-hosted gold deposits represent a significant type that is poorly understood. Whether the gold mineralization is genetically linked to the syenite intrusions (as in a magmatic model) or structurally associated with the development of subsequent shear zones remains very controversial. Our new structural data show that the multiply deformed Young-Davidson gold deposit and its surrounding Matachewan area occur along the western extension of the Cadillac – Larder Lake Deformation Zone (CLLDZ), a tectonically and economically important crustal-scale deformation zone in the southern Abitibi greenstone belt. Four generations (D1–D4) of deformation are recognized. The first part of the D1 deformation (D1a) records a NE-SW-oriented shortening and resulted in regional upright folding without penetrative foliation, whereas the subsequent D1b generates regional penetrative foliations and top-to-NE reverse faults. D2 occurs during a NW-SE-oriented shortening and produces asymmetric folds, oblique-slip thrusts and dextral strike-slip shear zones. D3 semi-brittle deformation refolds the D1 and D2 fabrics, whereas the latest D4 sinistral fault records a Paleoproterozoic brittle deformation.The Young-Davidson deposit is a lode-gold deposit that at least in part was structurally controlled. It is associated with the development of the CLLDZ and hosted in a syenite. Four main generations of veins are identified in the syenite. V1 veins are characterized by folded and boudinaged quartz–ankerite veins, V2 veins are represented by folded or en echelon quartz–pyrite veinlets, V3 veins are comprised of en echelon quartz–carbonate veins with sulfide minerals, and V4 veins consist of planar carbonate-quartz veins and minor hematite. Petrological studies reveal that the major phase of gold mineralization is associated with the V2 veins and partially with the V3 veins. Gold mineralization and emplacement of the associated veins appear to have occurred during regional D1b NE-SW-oriented shortening and top-to-NE shearing and the syenite acted as a mechanical trap due to competency contrast to the wall rock

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.

Structural and Stratigraphic Controls on Magmatic, Volcanogenic, and Shear Zone-Hosted Mineralization in the Chapais-Chibougamau Mining Camp, Northeastern Abitibi, Canada(,)

The structure and stratigraphy of the Chapais-Chibougamau mining camp in the Archean Abitibi greenstone belt of the Superior Craton, Quebec, Canada, is reevaluated from new aeromagnetic data, geochronology, and field studies to provide a framework for syngenetic and epigenetic mineralization. Mineralization in the Chapais-Chibougamau area is synvolcanic (volcanogenic massive sulfide (VMS)), synmagmatic (Fe-Ti-V, Ni-Cu, Au-Ag-Cu-Zn-Pb veins, Chibougamau-type Cu-Au veins), and shear zone hosted (including Opemiska Cu-Au veins subtype). VMS deposits occur within felsic volcanic rocks of tholeiitic affinity and mafic to felsic volcanic rocks of transitional to calc-alkaline affinity at the top of three volcanic cycles of the Roy Group. VMS deposits and early polymetallic veins (Au-Ag-Cu-Zn-Pb) are associated with NNW- to NNE-striking synvolcanic faults. Fe-Ti-V deposits occur within the Layered zone of the Lac Dore Complex, especially where it thickens in areas of interpreted synmagmatic faults. Subeconomic magmatic Ni-Cu deposits occur at the contacts of mafic-ultramafic or tonalitic intrusions. “Chibougamau-type” Cu-Au veins formed through magmatic-hydrothermal processes. They are cut by dikes that predate regional D2 deformation but are located within, and are deformed by, NW- and NE-trending D2 shear zones. Shear zone-hosted Au deposits are developed preferentially within regional E-W–trending deformation corridors and along NNE-striking sinistral shear zones while “Opemiska-type” Cu-Au veins occur within regional overturned anticlines in mafic sills of the Cummings Complex. U-Pb dating of detrital zircons provides maximum depositional ages for metasedimentary rocks of the Blondeau Formation of the Roy Group and the Stella Formation of the Opemisca Group (which unconformably overlies the Roy Group). The Blondeau Formation is <2721 ± 3 Ma, which is coeval to the age of the Windfall Member in the Urban-Barry Belt. Both show similar exploration potential as they host early polymetallic (Au-Ag-Cu-Zn-Pb) veins. The Stella Formation is <2704 ± 2 Ma and is unaffected by first generation folds, thus providing a time constraint for the earliest folding event (D1). This new age also indicates that sedimentation in the Chapais-Chibougamau area occurred earlier than for the Timiskaming-type assemblages in the southern Abitibi greenstone belt, but the structural setting and presence of alkaline intrusions suggests similar potential for syenite-associated Au deposits in the Stella Formation.

An Archean megacaldera complex: The Blake River Group, Abitibi greenstone belt

The Archean Blake River Group of the southern Abitibi greenstone belt is defined as a subaqueous megacaldera. Compelling evidence include: (1) a radial and concentric organization of synvolcanic mafic to intermediate dykes, (2) an overall dome geometry defined by the volcanic strata, (3) a peripheral distribution of subaqueous volcaniclastic units, (4) a zonal distribution of carbonate alteration, and (5) a distinct annular synvolcanic inner and outer ring fault pattern. Three caldera-forming events have been identified: (1) the early Misema caldera, (2) the New Senator caldera, and (3) the well-known Noranda caldera. The multi-vent BRG mafic volcanic complex, developed on a monotonous sequence of tholeiitic basalts, forming a submarine plain, and experienced a first major collapse that created the giant Misema Caldera (80 km in diameter). An endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed. Major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults. This fault system was used as a conduit for CO 2-rich hydrothermal activity. Renewed volcanic activity was associated with a resurgent central dome inside the Misema Caldera. This second collapse event created the 35 km by 14 km NW-trending New Senator Caldera. This caldera was produced by multi-step sagging after the underlying magmatic chamber migrated to the SE and formed the Flavrian-Powel Plutons. The final collapse resulted in the formation of the Noranda Caldera that generated a well-developed 070°-trending fracture pattern associated with several VMS deposits. The multiple-caldera setting provides an effective model to explain the presence of VMS mineralization along the synvolcanic fractures associated with the three collapse episodes. However, the importance of the New Senator Caldera fracture pattern is emphasized because of its role in the positioning of the giant Horne Mine deposit.

Tectonic and Metallogenic Implications of Regional Seismic Profiles in the Timmins Mining Camp

Four regional seismic reflection profiles totalling 153 line km were acquired near Timmins, Ontario, within the southern Abitibi greenstone belt. Five additional high-resolution lines targeted specific metallogenically important features such as the Porcupine-Destor deformation zone. Interpretation of these profiles individually and in a composite north-south transect reveals a number of prominent bands of reflectors within the upper 15 km of the crust that define a series of folds or antiformal stacks of thrust nappes. Structures and stratigraphy mapped at the surface confirm structural culminations in these locations. At depths greater than 10 km the reflectors have generally shallower dips implying broad folding. Major ore deposits such as those at the Kidd Creek, Hollinger, McIntyre, and Dome mines are located on the northern, steeply dipping limbs of these antiformal stacks, implying that the fold structures focused post tectonic mineralizing fluids within the upper crust into the near surface. The Porcupine-Destor deformation zone is proximal to the large gold deposits near Timmins and is revealed by the new seismic data to be a composite of early fold structures and late transpressive fault arrays.

Depositional Gaps in Abitibi Greenstone Belt Stratigraphy: A Key to Exploration for Syngenetic Mineralization

Models of greenstone belt development are crucial for exploration. Allochthonous models predict belts to be a collage of unrelated fragments, whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals. Superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary “Timiskaming-style” assemblages, and field and geochronological data indicate that the Abitibi greenstone belt developed autochthonously. We describe major revisions to stratigraphy of the Abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization. The Abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous U-Pb zircon ages of pre-2750, 2750 to 2735, 2734 to 2724, 2723 to 2720, 2719 to 2711, 2710 to 2704, and 2704 to 2695 Ma. We present revised lithotectonic and/or stratigraphic nomenclature using these time intervals, including (1) isotopic inheritance in younger episodes which indicates that the older episodes (2750–2735 and 2734–2724 Ma) had greater extent than is presently seen, (2) dikes feeding younger volcanic episodes (2706 Ma) cutting older volcanic units (2734–2724 Ma), and (3) 2710 to 2704 Ma mafic to ultramafic sills intruding the 2719 to 2711 Ma episode. Changes to the nomenclature include the identification of pre-2750 Ma volcanic episode (supracrustal fragments) in the northern and southern Abitibi greenstone belt and subdivision of the 2719 to 2711 Ma, 2710 to 2704 Ma, and 2704 to 2695 Ma episodes into lower and upper parts. We present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and/or lithotectonic map of the Abitibi greenstone belt. Many of the volcanic episodes are intercalated with and capped by a relatively thin “sedimentary interface zone” dominated by chemical sedimentary rocks. Stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m.y. between volcanic episodes. The zones consist of up to 200 m of iron formation, chert breccia, heterolithic debris flows of volcanic provenance, sandstone and/or argillite and conglomerate. Modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years, whereas the time interval between basalt-rhyolite cycles is 106 years. The sedimentary interface zones are therefore interpreted as condensed sections, zones with very low rates of sedimentation in a basinal setting, or zones with negligible rates of sedimentation marked by silicification of existing rock types. The sedimentary interface zones are therefore considered submarine correlative conformities, disconformities, or unconformities separating the equivalent of group level stratigraphic and lithotectonic units. The unconformity-bounded stratigraphic model provides a new regional to deposit-scale interpretive model for use in exploration for syngenetic mineralization.

Geologic and Structural Setting of Gold Mineralization in the Kirkland Lake-Larder Lake Gold Belt, Ontario

The Kirkland Lake-Larder Lake gold belt includes the giant Kirkland Lake and world-class Kerr-Addison-Chesterville gold deposits, along with several smaller deposits and occurrences. It corresponds to an east-trending band of Timiskaming clastic and volcanic rocks that unconformably overlie older volcanic assemblages in the southern Abitibi greenstone belt of the Archean Superior province. The gold belt is bounded to the south by the Larder Lake-Cadillac deformation zone, which roughly follows the contact between the younger Timiskaming rocks and the older volcanic units. Three generations of fabrics formed during post-Timiskaming regional deformation (D2, D3, and D4) of the belt. North-south shortening during D2 produced a penetrative, generally east-striking, steeply dipping, S2 foliation, and an east-plunging, L2 stretching lineation. S2 is most intense within the syn-D2 Larder Lake-Cadillac deformation zone and its northeast-trending splay, the Upper Canada deformation zone. The D2 structures are overprinted by a north-trending crenulation cleavage S3, which formed during east-west D3 shortening across the belt. A northeast-trending regional S4 foliation associated with Z-shaped F4 folds and overprinting both S2 and S3 formed during northwest-southeast D4 shortening. Gold mineralization is localized along the Larder Lake-Cadillac deformation zone (Anoki and McBean deposits), the Upper Canada deformation zone (Upper Canada deposit), and the brittle Kirkland Lake fault and ‘04 Break (Kirkland Lake deposit). The Upper Canada, McBean, and Anoki deposits formed during D2, and, along with Kerr-Addison-Chesterville, Omega, and Cheminis deposits, are probably related to a regionally extensive hydrothermal system associated with the Larder Lake-Cadillac deformation zone. The sulfide-poor gold- and telluride-bearing quartz veins of the Kirkland Lake deposit are interpreted to have been emplaced during D4, synchronous with reverse-dextral movement along the ore-controlling brittle Kirkland Lake fault. The Kirkland Lake mineralization has a distinct metal signature (Te>Au, Mo, Pb, Ag, high Au/Ag, low As) and probably represents a separate hydrothermal system linked to a deep magmatic (alkalic) fluid source and unrelated to mineralization along the syn-D2 deformation zones.

2D-3C high-resolution seismic data from the Abitibi Greenstone Belt, Canada

One high-resolution seismic profile acquired as part of a 3-component (three orthogonal seismometers) regional seismic reflection survey within the southern Abitibi greenstone belt was investigated in more detail using tomography and by processing all three components. Several high-resolution lines had targeted the Porcupine–Destor deformation zone, a zone proximal to the large gold deposits of the Timmins mining camp, and revealed by the new seismic data to be a composite of early fold structures and late transpressive fault arrays that host quartz-vein gold. Processing of the horizontal-component data assumed that seismic waves were converted from P-waves to S-waves between Vibrator source and receivers; S-wave static corrections proved especially important. Radial-component sections showed most features observed previously on vertical-component sections, but some amplitudes became enhanced, others reduced. The transverse-component stack demonstrated that a few structures are 3-D or contain significant seismic anisotropy. Two dimensional, P- and S-wave travel-time tomography analysis using data from the 10-km-long linear receiver array revealed low-resolution (> 200 m) physical property variations, possibly related to mineralization in near-surface structures of the Porcupine–Destor deformation zone; these variations were especially apparent using Poisson's ratio.

Evolution of the southern Abitibi greenstone belt based on U–Pb geochronology: autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation

New mapping and U–Pb zircon geochronological results in the southern Abitibi greenstone belt (SAGB) support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages, rather than the collage of allochthonous terranes proposed in recent publications. Based on lithological and geochronological criteria these supracrustal assemblages also encompass the Swayze, Shining Tree and Montcalm greenstone belts indicating that, despite separation by large granitic intrusions, they are in fact part of one extensive greenstone belt. In conjunction with the mapping, the geochronological results support coherent, upward-facing stratigraphic sections with about 20% of the samples containing inherited zircons with ages similar to those found in underlying assemblages. The seven oldest assemblages represent semi-continuous volcanism from 2750 to 2697 Ma. The volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc–alkaline mafic to felsic lavas. Intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section. Regional fault control on the distribution of the volcanic assemblages provides evidence for early dip–slip movement associated with volcanic extension dating back to at least 2725 Ma. The two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults. The earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 Ma, and the youngest one of subaerial conglomerates, fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 Ma. The sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons, regional folding and reactivated movement on the regional faults probably related to accretion of the Abitibi to the Superior Province craton. Autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic ± komatiitic assemblages and two dominantly calc–alkaline assemblages, as well as the extensive intermingling of these different magma clans in a number of the assemblages, appears to be unique to the Archean. It suggests complex, large-scale and long-lived interaction between mantle plumes and subduction zone magmas. The Nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with ε Nd=2.5±0.5 demonstrating a lack of contamination by ancient enriched components.

Re-Os Isotopic Study of Komatiitic Volcanism and Magmatic Sulphide Formation in the Southern Abitibi Greenstone Belt, Canada

We have investigated the Re–Os isotope geochemistry of 2.7-Ga metakomatiitic flows and associated Ni–Cu sulfide deposits from Alexo, Texmont and Hart in the Abitibi greenstone belt of Ontario in order to refine the thermal erosion model and evaluat e the superimposed effects of metamorphism and hydrothermal alteration on ore environments and non-ore environments. Although the geochemical characteristics of these komatiites have led to the belief that these lavas were uncontaminated, radioge nic Os isotopic compositions (Os = +20.2 and +33.2) obtained from well-preserved komatiites and olivine separates suggest that the Alexo flow has been contaminated by crust-derived material. These data are compatible with the trace-element enrichment observed in melt inclusions trapped within olivine. Redistribution of Os and Re did occur at least at the mineral scale and most likely during the Grenville orogeny. Hydrothermal fluids were channeled along the contact between the komatiites and their basement, and were responsible for the remobilization of Re or Os (or both) within the sulfides at Alexo and Hart. Matrix and disseminate d sulfides from Texmont are located within the pile of cumulates and seem to have escaped this localized alteration. Although the Abitibi sulfides have experienced various degrees of metamorphism (from prehnite–pumpellyite to low amphibolite facies), the initial Re–Os isotopic composition of the flows appears to have been preserved at the whole-rock scale. Re–Os isotopic heterogeneity of the Abitibi sulfides is best explained by variable R-factor of the sulfides. Re and Os concentrations and Os isotopic heterogeneity of the Abitibi sulfides are consistent with the current model of nickel sulfide formation, which implies that the assimilation of sulfidic sedimentary rocks was the trigger for sulfide saturation.

Structural evolution and early accretion of the Archean Malartic Composite Block, southern Abitibi greenstone belt, Quebec, Canada

The Malartic Composite Block of the southern Abitibi belt underwent at least two phases of ductile deformation over a period of 25 Ma during the late Archean. It is divided into seven tectono-stratigraphic domains on the basis of their lithologicai content, structural styles, and geochemical affinities of the volcanic and plutonic rocks. The Malartic Composite Block is bounded to the south by the sedimentary rocks of the Kewagama Group that have been also affected by deformation. The crosscutting relationships between the structural elements, along with precise geochronological data, provide a basis for evaluation of the evolution of the deformation in this part of the southern Abitibi. The D1 tectonic event produced ductile thrust faults and folds in the mafic and ultramafic domains of the Malartic Composite Block. The calc-alkalic rocks of the Val-d'Or Domain (2705 Ma) were erupted on top of these deformed domains, thus establishing the minimum age for D1. The D2 event is divided into three increments that are linked to the same progressive deformation. D2.1 produced northwest-trending folds in the rocks of the Kewagama Group, but did not affect the rocks of the Malartic Composite Block. The age of the youngest detrital zircon in the Kewagama indicates that D2.1 was active after 2687 Ma. D2.2 developed a consistent east–west regional foliation that overprints all the rocks of the area. Radiometric data on related metamorphic minerals indicate an age of 2680 Ma for this event. Locally, S2.2 is modified by Fo2.3 folds of asymmetric Z shape associated with a late dextral transcurrent shearing. A minor D3 event produced small, local conjugate kink folds recording an east–west shortening.

Hf isotopic composition and age constraints on the evolution of the Archean central Uchi Subprovince, Ontario, Canada

The Archean Uchi Subprovince, located in the northwestern Superior Province, comprises greenstone belts and granitoid complexes that formed in the course of a multistage history between about 3000 and 2690 Ma. The earliest volcanic-plutonic complexes were assembled into a microcontinent, which then progressively grew by the addition of magmatic arcs and tectonic slivers. A major compressional orogeny concluded this evolution at around 2700 Ma. LuHf isotopic compositions for well dated zircon populations from volcanic and plutonic rocks of the central part of the subprovince have ϵ Hf values that range from +6.1 to +1.4. These results show that the bulk of the province was derived predominantly from depleted mantle sources but there is a superimposed trend showing involvement of the oldest greenstone and crustal sequences in the generation of the youngest magmatic complexes. The temporal and isotopic relationships of the Uchi Subprovince contrast with the situation observed in the southern Abitibi greenstone belt, which developed within a time span of less than 80 m.y. in an intraoceanic setting, and exhibits quite uniform, depleted-mantle-like isotopic values revealing only very subordinate degrees of recycling of older crust.

UPb isotopic geochemistry of komatiites and pyroxenes from the southern Abitibi greenstone belt, Canada

New U and Pb concentrations and Pb isotopic data are reported for several komatiitic flows and for primary magmatic clinopyroxenes (CPX) from flows and sills of basaltic compositions from the southern Abitibi greenstone belt, for which RbSr, SmNd and ReOs isotope data were previously published. A suite of komatiites from Pyke Hill displays a narrow range of 206Ph/ 204Pb (17.91–19.20) when compared to previously reported values of ∼ 16–30 for komatiites in the same area. Whole-rock samples leached with HCl underwent up to 40% weight loss. The leachates display essentially the same isotopic composition as the bulk samples but the residues have higher 206Pb/ 204Pb (19.9–24.0). The CPX display 206Pb/ 204Pb (13.96–17.98) lower than whole rocks and leaching resulted in either higher or lower Pb ratios for the residues compared to bulk samples. For both whole rocks and mineral separates, leaching induced a significant decrease of Pb concentrations in the residues, which contained < 0.04 ppm compared to 1–2 ppm for the bulk samples. The U concentrations did not vary systematically with the degree of leaching but the residues always had higher 238U/ 204Pb than the bulk samples. Whole rocks and CPX exhibit U loss relative to Pb. All the samples define a ∼ 2.73-Ga PbPb isochron, identical to that previously reported for komatiites in the same area. However, our isochron is displaced toward slightly more radiogenic 207Pb/ 204Pb compared to the previously reported isochrons and this is interpreted as reflecting mantle source heterogeneities for komatiites. These mantle sources are depleted and the most radiogenic one is similar in composition to that of carbonatites and other alkaline complexes in the Superior Province. We also show that the Pb concentration in komatiites from Pyke Hill displays a coherent variation with respect to MgO, FeO, Re and Os, and that Pb behaves like Os in all the samples, suggesting a common bearing phase controlling the distribution of these two elements. The “hydrothermal Pb removing” inferred by B. Dupré and N.T. Arndt to account for large variations of the 206Pb/ 204Pb ratios of some komatiite flows is not universal. If our samples were affected by any hydrothermal remobilisation, it did not mask the primary magmatic distribution of Pb.

The subsea-floor formation of volcanic-hosted massive sulfide; evidence from the Ansil Deposit, Rouyn-Noranda, Canada

The Ansil Cu-Zn volcanic-hosted massive sulfide deposit is located approximately 14 km north-northwest of Rouyn-Noranda, Quebec, within the Blake River Group of the Archean southern Abitibi greenstone belt. The deposit is the second lowest within the Central Mine sequence, a series of bimodal rhyolite-andesite formations that fill in the Noranda cauldron, host to 17 volcanic-hosted massive sulfide deposits. The deposit consists of a single massive sulfide lens situated at the contact between rhyolite of the Northwest formation and overlying andesite of the Rusty Ridge formation. The rhyolite flow forms a ridge up to 500 m high which was inundated by the andesite flows. The orebody is hosted within a unit of quartz porphyritic, finely layered, felsic volcaniclastite (Cranston tuff) that fills in an east-west-trending graben formed on the upper surface of the rhyolite flow. At the east end of the graben the Cranston tuff is interlayered with a thin unit of dacite flows. The deposit lies within an extensive discordant alteration zone characterized by feldspar destruction and Na depletion. The zone extends from 400 m below the deposit at the contact of the synvolcanic Flavrian Intrusive Complex to over 400 m above the deposit. This alteration zone is controlled by a fault system that served as a focus for hydrothermal fluid upflow from the beginning of cauldron formation until the end of the first cauldron cycle. Within the Na depletion zone there are three distinct alteration facies associated with the orebody. Early hydrothermal activity is defined by hydrothermal explosion breccias that formed along the base of the east-west trending graben walls. They were filled with finely banded quartz-albite- sphalerite-pyrite, and the wall rock altered to sericite-quartz, with finely disseminated sphalerite and pyrite- pyrrhotite. The graben was then filled with the Cranston tuff, which was selectively altered and mineralized by the Zn-rich fluid. Subsequent development of northerly trending faults was accompanied by the upflow of Cu-rich fluid that selectively chloritized the footwall rhyolite below the graben floor and formed a ...