ALBERT

Description: The Larder Lake – Cadillac deformation zone (LLCDZ) is one of two major, auriferous, deformation zones in the southern Abitibi subprovince of the Archean Superior Province. It hosts the Cheminis and the giant Kerr Addison – Chesterville deposits within a strongly deformed band of Fe-rich tholeiitic basalt and komatiite of the Larder Lake Group (ca. 2705 Ma). The latter is bounded on both sides by younger, less deformed, Timiskaming turbidites (2674–2670 Ma). The earliest deformation features are F 1 folds affecting the Timiskaming rocks, which formed either during D 1 extensional faulting or during early D 2 north–south shortening related to the opening and closure, respectively, of the Timiskaming basin. Continued shortening during D 2 imbricated the older volcanic rocks and turbidites and produced regional F 2 folds with an axial planar S 2 cleavage. D 2 deformation was partitioned into the weaker band of volcanic rocks, producing the strong S 2 foliation, L 2 stretching lineation, and south-side-up shear sense indicators, which characterize the LLCDZ. Gold is present in quartz–carbonate veins in deformed fuchsitic komatiites (carbonate ore) and turbiditic sandstone (sandstone-hosted ore), and in association with disseminated pyrite in altered Fe-rich tholeiitic basalts (flow ore). All host rocks underwent strong mass gains in CO 2 , S, K 2 O, Ba, As, and W, during sericitization, carbonatization, and sulphidation of the host rocks, suggesting that they interacted with the same hydrothermal fluids. Textural relationships between alteration minerals and S 2 cleavage indicate that mineralization is syn-cleavage. Thus, gold was deposited as hydrothermal fluids migrated upward along the LLCDZ during contractional, D 2 south-side-up shearing. The gold zones were subsequently modified during D 3 reactivation of the LLCDZ as a dextral transcurrent fault zone.

Description: New structural and geochronological data are presented for two orogenic events, the Blezardian and Yavapai orogenies, which affected the Paleoproterozoic Southern Province near Sudbury, Ontario, Canada. The Southern Province comprises ca. 2452 Ma metavolcanic rocks and metasedimentary rocks of the Huronian Supergroup, which were deposited along the southern margin of the Archean Superior craton during its evolution from a rifted to passive continental margin. Emplacement of the ca. 2415 Ma Creighton pluton during rifting was followed by its deformation and the development of a penetrative gneissic fabric during the ca. 2415 – ca. 2219 Ma Blezardian Orogeny. New laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) U–Pb zircon ages of 2343 ± 17 and 2344 ± 47 Ma on two granitic dikes that cut this fabric provide a new minimum age of ca. 2.34 Ga for the Blezardian Orogeny. The Sudbury area was then impacted by a large extraterrestrial bolide at ca. 1.85 Ga and deformed during the Penokean Orogeny. The southern part of the Southern Province was later reworked by regional folding and north-directed thrusting during the younger 1.7 Ga Yavapai Orogeny. The 1744 ± 29 Ma Eden Lake Complex was emplaced and deformed during this event, which produced a strong foliation overprinting the complex. The foliation formed at pressures of 2.8–4 kbar (1 kbar = 100 MPa) and temperatures of 540–565 °C and was intruded by a weakly deformed 1704 ± 13 Ma old granitic dike, bracketing the Yavapai event between 1744 ± 29 and 1704 ± 13 Ma in the Sudbury segment of the Southern Province. Crustal thickening associated with the Yavapai event resulted, locally, in minor pressure increases before or during regional metamorphism as revealed by phase equilibria modeling in the Raft Lake area; this evolution may be recorded elsewhere in the Ontario segment of the Southern Province.

Description: The Trout Lake deposit (1878 ± 1 Ma) is a rhyolite-hosted, bimodal-mafic, volcanogenic massive sulfide (VMS) deposit. It is one of the largest VMS deposits in the Flin Flon mining district, containing 21.8 Mt of Cu-Zn rich ore. The deposit underwent greenschist facies metamorphism and polyphase synvolcanic to postvolcanic hydrothermal alteration. Footwall and hanging-wall lithostratigraphic units of the Trout Lake deposit occur as an upright, steeply NE dipping, homoclinal succession of thrust-repeated volcanic and siliciclastic sedimentary units. The deposit consists of several massive sulfide ore lenses hosted within a felsic volcanic unit that represents a localized felsic eruptive center composed of flows, synvolcanic sills, and volcaniclastic lithofacies with an FIIIb-type tholeiitic to transitional rhyolite composition (Zr = 213–385 ppm; Yb cn = 47–100; La/Yb cn = 0.97–2.33). The felsic unit occurs along strike of an andesitic unit comprising an intercalation of high-MgO andesite (MgO 〉 5.0 wt %) and Nb-enriched andesite (Nb = 10–13 ppm) flows that are underlain by a footwall basaltic unit composed of flow lithofacies with a low-Ti tholeiitic basaltic composition (TiO 2 〈 0.48 wt %; Zr = 15–17 ppm; Al 2 O 3 /TiO 2 = 37–45). Postvolcanic gabbros with back-arc basin basalt composition intrude the volcanic units, which are unconformably covered by a younger argillite unit deposited at ca. 〈1843 ± 9 Ma. The felsic volcanic and argillite units were repeated and structurally interleaved during west-directed thrusting. The presence of low-Ti tholeiitic basalts indicates high-temperature melting (1,200°–1,500°C) of strongly depleted refractory mantle sources (Nb/Yb 〈1.45). In contrast, high-MgO and Nb-enriched andesites suggest partial melting of enriched mantle sources (Nb/Yb 〉1.45) variably affected by slab-derived hydrous melts. These petrologic characteristics suggest a geochemically heterogeneous mantle wedge that underwent metasomatism by slab-derived components. In addition, zircon saturation thermometry indicates that FIIIb-type rhyolites formed at temperatures 〉900°C during partial melting of mafic lower crust. Collectively, the occurrence of high-temperature melts indicates that the Trout Lake VMS deposit formed in a hot, intraoceanic, extensional-arc geodynamic setting. Intra-arc rifting, asthenospheric upwelling, and subduction of young and hot oceanic crust are mechanisms that explain the high-temperature magmatism and are features consistent with the formation of the massive sulfide ore lenses. The back-arc basalt geochemical signatures of the postvolcanic gabbros suggest that the tectonic environment of the Trout Lake area evolved from an intra-arc rift, during the deposition of the ore lenses, to an incipient back-arc basin during the intrusion of the gabbros.

Description: The Flin Flon mining district is part of a greenstone belt, the Flin Flon-Glennie Complex, in the Paleoproterozoic Trans-Hudson orogen. Its tectonic history began prior to 1872 Ma with the development of regional folds—the faulted F 1 Burley Lake syncline and F 2 Hidden Lake syncline—during D 1 and D 2 intraoceanic accretion of the 1888 Ma Flin Flon arc to other volcanic terranes. Amalgamation of the Flin Flon to Glennie terrane, possibly during D 3 , produced a W-propagating thrust-fold belt and basins in which fluvial sedimentary rocks were deposited between 1847 Ma and 1842 Ma. As the fold-thrust belt migrated westward, these rocks were incorporated into a stack of E-dipping thrust sheets bounded by NNW-striking thrust faults (1920 fault) and internally folded by W-verging folds (Pipeline, Mud Lake, and Grant Lake synclines). Subsequent D 4 collision of the Flin Flon-Glennie Complex with the Archean Sask microcontinent was broadly coeval with but outlasted the emplacement of 1840 Ma Phantom Lake dikes. D 4 produced a second truncating fold-thrust system characterized by N-directed thrust faults (Club Lake and Railway faults) and E-trending folds (Flin Flon Creek syncline). These folds were overprinted by two regional cleavages, and the thrust faults were reactivated as oblique-slip shear zones, either late during the same collisional event (D 5 ) or during terminal collision (D 6 ) of the Sask craton and Flin Flon-Glennie Complex with the Superior craton at 1.83 to 1.79 Ga. The Flin Flon volcanogenic massive sulfide ore system was thrust-imbricated during D 3 and D 4 , and ore lenses were stretched parallel to a regional, SE-plunging, stretching lineation that formed during D 4 and was later modified during D 5 .

Description: Three-dimensional geologic modeling by integrated analysis and interpretation of drill core, 2-D and 3-D seismic, mine survey, and geologic map data provides new insights into the structural setting of the 85.5 million tonne (Mt) Flin Flon-Callinan-777 volcanogenic massive sulfide (VMS) ore system hosted in the Paleoproterozoic Flin Flon belt of Manitoba and Saskatchewan, Canada. The resultant camp-scale 3-D geologic model was, in addition to the densely drill intersected VMS-hosting mine horizon, constrained by lithostratigraphic reference drill holes intersecting the footwall and hanging-wall rock successions of the ore system, which were established by relogging 52 drill holes and systematically reconciling the lithofacies classes in their log descriptions with the lithofacies units of the 1:10,000-scale Flin Flon mining district geologic map. The lateral persistence and large stratigraphic range of these drill hole markers supported seismic interpretation and allowed tracing lithostratigraphic horizons and structures in areas with low drill hole density, expanding 3-D subsurface insight from the local to the more regional structural setting of the ore system. The overall 3-D modeled geometry and topological relationships between lithostratigraphic surfaces and multiple generations of thrust faults suggest that the Flin Flon mining district is underlain by an E-dipping stack of W-vergent thrust imbricates that formed during precollisional and collisional stages of the 1.9 to 1.8 Ga Trans-Hudson orogeny. The imbricate stack was subsequently deformed by E-trending ductile thrust faults that internally imbricated the Flin Flon arc assemblage and the molasse cover rocks of the Missi Group and brought up rocks of the former over the latter in a northerly direction. The VMS-hosting Millrock Member has been stacked on at least four structural levels, enhancing the VMS potential in the footwall and hanging wall of the known ore deposits where both thrust systems intersect.

Description: 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 Hébécourt 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.

Description: The Lemoine auriferous volcanogenic massive sulfide deposit (0.76 Mt at 4.6 g/t Au, 4.2 wt % Cu, and 9.5 wt % Zn) is part of the Chibougamau camp located in the northeastern part of the Abitibi greenstone belt. The deposit is hosted by a steeply dipping, S-facing homoclinal volcanic succession (~2729–2726 Ma Waconichi Formation, Lemoine Member) composed of effusive and intrusive tholeiitic rhyolites and andesites cut by comagmatic diorite and gabbro dikes and overlain by transitional to mildly calc-alkaline basalts, andesites, and rhyolites. Seven predominant synvolcanic alteration assemblages, now intensely deformed and metamorphosed to upper greenschist facies, were defined based on their mineralogy and position relative to ore. Albite-quartz, sericite-carbonate, sericite-chlorite, sericite-chlorite-(Zn), and chlorite-sericite-epidote-carbonate assemblages define semiconcordant zones that are stacked from the paleoseafloor to the deep footwall. In contrast, chlorite and chlorite-sericite-chloritoid assemblages overprint the other alteration zones and form subconcordant to now transposed discordant zones in the deposit footwall. Most alteration assemblages are characterized by relative SiO 2 , CaO, and Na 2 O mass losses and relative FeO, MgO, K 2 O, and CO 2 mass gains. Whole-rock oxygen isotopes indicate that the temperatures of alteration ranged from ~100° to 150°C (sericite-carbonate assemblage) to ≥350°C (sericite-chlorite, chlorite-sericite-chloritoid, and chlorite assemblages). The chlorite-sericite-chloritoid assemblage, and to some extent the sericite-chlorite assemblage, are associated with strong to near total depletion of light rare earth elements possibly due to reaction with Cl-bearing, mildly acidic fluids at depth in the ore-forming hydrothermal system. The sulfide lens was formed in part on the seafloor and in part by subseafloor replacement. The massive sulfides are particularly rich in Bi, suggesting a possible magmatic input into the Lemoine ore-forming hydrothermal system. High Bi concentrations in the mineralizing system are likely to have enhanced Au precipitation by scavenging the precious metal from the hydrothermal fluids. The Au-rich nature of the Lemoine auriferous volcanogenic massive sulfide deposit can be explained by a combination of very efficient metal transport, highly effective capture of metals at or near the seafloor, and a possible magmatic contribution to the hydrothermal system.

Description: The thrust-bounded McLeod Road – Birch Lake (MB) sequence occurs within the Paleoproterozoic Snow Lake arc (SLA) assemblage of the Flin Flon belt. Stratigraphic correlation of volcanic strata of the MB sequence with strata of the thrust-bounded Chisel sequence indicates that distinctive, submarine, eruption-fed, pyroclastic flow deposits are more extensive and voluminous than previously recognized (〉10 km 3 ). These voluminous felsic pyroclastic deposits define a distinct magmatic and explosive volcanic event during bimodal volcanism that accompanied rifting of the SLA. The felsic pyroclastic deposits define the remnants of a basin, or of nested basins, that formed during arc rifting and subsidence, and their eruption immediately preceded formation of the Chisel sequence volcanogenic massive sulfide (VMS) deposits. Although the Chisel sequence ore interval is recognized in the MB sequence, the lack of VMS-related alteration indicates that VMS hydrothermal activity was restricted to the Chisel portion of the basin. However, the MB sequence is host to the younger Snow Lake gold mine, a 1.4M oz (43 699 kg) gold producer. The overlying MORB-like Birch Lake basalts, if conformable with the MB sequence, may represent a progression from a rifted-arc to a back-arc setting. However, if they are thrust fault bounded, then they may represent the initial phases of arc-rifting, prior to the voluminous felsic pyroclastic eruptions. Correlation and integrity of stratigraphy between the thrust-bounded MB and SLA sequences indicates that the bounding thrust faults, which developed during accretionary processes, have less regional significance than previously interpreted.

Description: Two coincident high-resolution airborne gravity and magnetic profiles of the Sudbury structure were forward modelled to better understand the geology of the structure at depth. A north–south profile was used to further investigate the deep geological setting of the Sudbury Igneous Complex (SIC) along Lithoprobe seismic transect, while an east–west profile was selected to examine a discontinuity in the magnetic and gravity fields near the centre of the SIC in the North Range. Constraints imposed on the best-fit model were the location of surface magnetic contacts, interpreted seismic and geological sections, and petrophysical data acquired from surface and borehole data. The constrained model computed for the north–south profile, elements of which are consistent with known Lithoprobe seismic reflectors, defines a north-verging fold in the deeper portion of the SIC. It may have developed during the modification of the initial geometry of the SIC by either a post-SIC thick-skinned basement shortening event, or by a compressive event that puts the tholeiitic basalts of the Elliot Lake Group against the SIC during the Penokean orogeny. The interpreted deep-seated basal folding explains the changes in dip of the seismic reflectors of the Archean basement and the SIC at about 4–8 km depth that were not fully accounted for in previous models of the Sudbury structure. This deformational event is interpreted to displace the base of the SIC rocks northwards to the depth of about 5 km, which is now reflected by a linear gravity high within the southern part of the Sudbury Basin. Lithological fence diagrams of the two interpreted sections, across and along a magnetic anomaly located in the northwest portion of the SIC, show that features of the observed anomaly pattern can be explained by a series of closely spaced deep-seated growth faults trending north around the Sandcherry fault, which has been previously interpreted as a reactivated pre-impact fault that affects the thickness and topography of both the SIC and highly magnetic Levack Gneiss Complex in that locality.