ALBERT

Description: The gold endowment of the western Churchill Province is conventionally attributed to reworking and attendant metamorphism during the Trans-Hudson Orogeny (1.9–1.8 Ga). The Meliadine gold district is regarded as a type example of this inferred Paleoproterozoic gold metallotect and also represents one of Canada’s largest emerging orogenic greenstone- and banded iron formation (BIF)-hosted gold districts (2.8 Moz contained Au in reserves and total resource of 5.8 Moz Au). The largest gold deposits are cospatial with the Pyke fault and associated quartz (± ankerite) veining, which cuts Neoarchean (ca. 2.66 Ga) plutonic and supracrustal rocks comprising the Rankin Inlet greenstone belt. Meliadine gold occurs as inclusions within idioblastic arsenopyrite crystals, at sulfide crystal boundaries, and/or as sulfide fracture fills in hydrothermally altered, sulfidized, and veined BIF. Clusters of gold, Bi-Mo telluride, chalcopyrite, and galena inclusions characterize high-grade ore zones and are spatially associated with sieve-textured arsenopyrite domains. These microtextural relationships suggest that incipient sulfide recrystallization liberated gold, which, along with other precious and base metals, was redistributed during late, fluid-assisted and deformation/metamorphic-driven remobilization. Late precious and base metal enrichment is also demonstrated by in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) arsenopyrite mapping. Hydrothermal xenotime crystals occur with gold in low-strain microtextural sites and yield U-Pb ages at ca. 1.86 Ga, suggesting that remobilization was concomitant with Trans Hudson orogenesis. New Re-Os arsenopyrite model ages range from 2.3 to 1.8 Ga and document a hitherto unrecognized, pre-1.86 Ga hydrothermal activity. The range of Re-Os model ages tends to support partial open-system behavior and/or mixing of disparate arsenopyrite generations that are evident from microtextural observations and in situ LA-ICP-MS element mapping. Replicate analyses of the two most rhenium rich arsenopyrite samples yield reproducible Re-Os model ages at ca. 2.27 and 1.90 Ga. These rhenium-rich samples are also gold poor and likely yield ages that predate gold remobilization and subsequent enrichment along arsenopyrite crystal boundaries and fractures. We suggest that pre-1.86 Ga hydrothermal activity was likely critical to the gold endowment of the district, but is obscured by reworking during the later stages of the Trans-Hudson Orogeny at 1.86 Ga. If correct, these new ages imply that the Paleoproterozoic gold metallotect, which is recorded across the western Churchill Province, in fact comprises multiple, temporally distinct gold events. Differentiating between disparate Paleoproterozoic gold events represents a critical step for effective mineral exploration in the western Churchill Province and at other reworked Archean terranes.

Description: 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 poly-metallic 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-Hébert 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.

Description: The oxygen isotope composition of 169 whole-rock samples from the world-class LaRonde Penna Au-rich volcanogenic massive sulfide deposit range from 6.5 to 22.0. Fractional crystallization from basalt to rhyolite accounts for a variation of approximately 2 in primary whole-rock 18 O values. The remaining variance in whole-rock 18 O values found at LaRonde Penna must therefore be ascribed to water-rock exchange with hydrothermal fluids. The metamorphosed, hydrothermally altered host rocks have been subdivided according to mineralogical assemblages, which display weak covariance with 18 O values and whole-rock geochemical alteration indices. Water-rock equilibrium exchange models indicate that the high 18 O values that characterize the LaRonde Penna deposit are compatible with low-temperature (~150ºC) hydrothermal alteration at high water-rock oxygen atomic ratios (1–50). Zones of lower 18 O values stratigraphically beneath the principal ore lens (20 North) indicate local zones of higher temperature hydrothermal alteration. The oxygen isotope composition of the hydrothermal fluid is estimated to have been ~5, as a consequence of mixing between seawater ( 18 O ~0) and a component of magmatic water degassed from the volcanic and intrusive rocks associated with the LaRonde Penna deposit.

Description: 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 SiO 2 , H 2 O, MgO, Al 2 O 3 , 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.

Description: 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 Na 2 O depletion. Gains in MnO, Fe 2 O 3(total) , MgO, and CaO are recorded in the quartz-muscovite ± Mn-garnet ± biotite ± chlorite assemblage, whereas gains in K 2 O and losses in CaO occur in the quartz muscovite ± pyrite assemblage. In the quartz-muscovite-andalusite-pyrophyllite-pyrite and the proximal massive quartz-pyrite assemblages all oxides, except SiO 2 , Fe 2 O 3(total) , and TiO 2 , 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.

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: 〈span〉〈div〉Abstract〈/div〉The 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 S〈sub〉2〈/sub〉 cleavage and the E-striking, S-dipping Sladen fault zone. This syn- to post-D〈sub〉2〈/sub〉 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 〈span〉a〈/span〉Na〈sup〉+〈/sup〉/〈span〉a〈/span〉H〈sup〉+〈/sup〉 or 〈span〉a〈/span〉K〈sup〉+〈/sup〉/〈span〉a〈/span〉H〈sup〉+〈/sup〉 of the ore fluid.Gold mineralization, with an inferred age of ~2664 Ma (Re-Os molybdenite), was contemporaneous with syn- to late-D〈sub〉2〈/sub〉 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 CO〈sub〉2〈/sub〉-rich auriferous fluid generation in amphibolite facies rocks of the Pontiac Group and gold deposition in syn- to late-D〈sub〉2〈/sub〉 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.〈/span〉