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

2021 ◽  
Author(s):  
D. Yergeau ◽  
P. Mercier-Langevin ◽  
B. Dubé ◽  
M. Malo ◽  
A. Savoie
Keyword(s):  
Fractional Crystallization ◽  
Greenstone Belt ◽  
Massive Sulfide ◽  
Hydrothermal Systems ◽  
Lava Flows ◽  
Intrusive Complex ◽  
Calc Alkaline ◽  
Abitibi Greenstone Belt ◽  
Intrusive Rocks ◽  
Ore Zones

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.

The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part II. Hydrothermal Alteration, Mineralization, and Geologic Model

2021 ◽  
Author(s):  
D. Yergeau ◽  
P. Mercier-Langevin ◽  
B. Dubé ◽  
V. McNicoll ◽  
S. E. Jackson ◽  
...  
Keyword(s):  
Hydrothermal System ◽  
Greenstone Belt ◽  
Massive Sulfide ◽  
Alteration Zone ◽  
Intrusive Complex ◽  
Calc Alkaline ◽  
Abitibi Greenstone Belt ◽  
The North ◽  
Vms Deposits ◽  
Ore Zones

Abstract The Westwood deposit, located in the Archean Doyon-Bousquet-LaRonde mining camp in the southern Archean Abitibi greenstone belt, contains 4.5 Moz (140 metric t) of gold. The deposit is hosted in the 2699–2695 Ma submarine, tholeiitic to calc-alkaline volcanic, volcaniclastic, and intrusive rocks of the Bousquet Formation. The deposit is located near the synvolcanic (ca. 2699–2696 Ma) Mooshla Intrusive Complex that hosts the Doyon epizonal intrusion-related Au ± Cu deposit, whereas several Au-rich volcanogenic massive sulfide (VMS) deposits are present east of the Westwood deposit. The Westwood deposit consists of stratigraphically stacked, contrasting, and overprinting mineralization styles that share analogies with both the intrusion-related and VMS deposits of the camp. The ore zones form three distinct, slightly discordant to stratabound corridors that are, from north (base) to south (top), the Zone 2 Extension, the North Corridor, and the Westwood Corridor. Syn- to late-main regional deformation and upper greenschist to lower amphibolite facies regional metamorphism affect the ore zones, alteration assemblages, and host rocks. The Zone 2 Extension consists of Au ± Cu sulfide (pyrite-chalcopyrite)-quartz veins and zones of disseminated to semimassive sulfides. The ore zones are spatially associated with a series of calc-alkaline felsic sills and dikes that crosscut the mafic to intermediate, tholeiitic to transitional, lower Bousquet Formation volcanic rocks. The metamorphosed proximal alteration consists of muscovite-quartz-pyrite ± gypsum-andalusite-kyanite-pyrophyllite argillic to advanced argillic-style tabular envelope that is up to a few tens of meters thick. The North Corridor consists of auriferous semimassive to massive sulfide veins, zones of sulfide stringers, and disseminated sulfides that are hosted in intermediate volcaniclastic rocks at the base of the upper Bousquet Formation. The Westwood Corridor consists of semimassive to massive sulfide lenses, veins, zones of sulfide stringers, and disseminated sulfides that are located higher in the stratigraphic sequence, at or near the contact between calc-alkaline dacite domes and overlying calc-alkaline rhyodacite of the upper Bousquet Formation. A large, semiconformable distal alteration zone that encompasses the North Corridor is present in the footwall and vicinity of the Westwood Corridor. This metamorphosed alteration zone consists of an assemblage of biotite-Mn garnet-chlorite-carbonate ± muscovite-albite. A proximal muscovite-quartz-chlorite-pyrite argillic-style alteration assemblage is associated with both corridors. The Zone 2 Extension ore zones and associated alteration are considered synvolcanic based on crosscutting relationships and U-Pb geochronology and are interpreted as being the distal expression of an epizonal magmatic-hydrothermal system that is centered on the upper part of the synvolcanic Mooshla Intrusive Complex. The North and Westwood corridors consist of bimodal-felsic Au-rich VMS-type mineralization and alteration produced by the convective circulation of modified seawater that included a magmatic contribution from the coeval epizonal Zone 2 Extension magmatic-hydrothermal system. The Westwood Au deposit represents one of the very few documented examples of an Archean magmatic-hydrothermal system—or at least of such systems formed in a subaqueous environment. The study of the Westwood deposit resulted in a better understanding of the critical role of magmatic fluid input toward the formation of Archean epizonal intrusion-related Au ± Cu and seafloor/subseafloor Au-rich VMS-type mineralization.

Evidence for contrasting compositional spectra in comagmatic intrusive and extrusive rocks of the late Archean Blake River Group, Abitibi, Quebec

1988 ◽  
Vol 25 (1) ◽  
pp. 134-144 ◽  
Author(s):  
Suzanne Paradis ◽  
John Ludden ◽  
Léopold Gélinas
Keyword(s):  
Magma Chamber ◽  
Fractional Crystallization ◽  
Open System ◽  
Closed System ◽  
Greenstone Belt ◽  
Magma Mixing ◽  
Volcanic Rocks ◽  
Abitibi Greenstone Belt ◽  
Intrusive Rocks

The Flavrian pluton is a sill-shaped intrusion in the Blake River Group (BRG) volcanic rocks in the Noranda region of the Abitibi greenstone belt. The pluton is dominated by trondhjemites and tonalites, with minor peripheral quartz gabbro and hybrid phases. The BRG volcanic rocks consist of a bimodal suite of basalt–andesite and rhyolite. The Flavrian trondhjemites are geochemically identical to the rhyolitic lavas of the BRG (SiO2 ≥ 72%, La/Sm = 3.4, La/Yb = 3.6, Zr/Y = 3.9, Y/Nb = 3.1), and the Flavrian gabbroic and dioritic rocks are identical to the BRG basalts and andesites (SiO2 < 58%, La/Sm = 3.0, La/Yb = 5.5, Zr/Y = 4.2, Y/Nb = 3.3). However, the tonalitic rocks of the Flavrian pluton have no extrusive equivalents in the BRG. The different compositional spectra of the extrusive and intrusive rocks are interpreted as being a result of a transition in magma-chamber evolution from a zoned open system that was active during the evolution of the volcanic rocks to closed-system plutonic crystallization. The latter destroyed the compositional bimodality of the magma chamber and resulted in the evolution of intermediate compositions (tonalites) generated by both fractional crystallization and magma mixing.

Archean mafic metavolcanics from the Rouyn–Noranda district, Abitibi Greenstone Belt, Quebec. 1. Mobility of the major elements

1982 ◽  
Vol 19 (12) ◽  
pp. 2258-2275 ◽  
Author(s):  
Léopold Gélinas ◽  
Michel Mellinger ◽  
Pierre Trudel
Keyword(s):  
Greenstone Belt ◽  
Great Depth ◽  
Major Elements ◽  
Chemical Degradation ◽  
Type I ◽  
Calc Alkaline ◽  
Calcium Leaching ◽  
Abitibi Greenstone Belt ◽  
Metavolcanic Rocks ◽  
Metamorphic Facies

In a suite of Archean mafic pillows from the Rouyn–Noranda region of Quebec's Abitibi Greenstoné Belt, including both tholeiitic and calc-alkaline varieties spanning the prehnite–pumpellyite to upper greenschist metamorphic facies, three types of alteration can be defined: (I) chlorite–epidote–actinolite; (II) chlorite–epidote; and (III) chlorite ± sericite; the number of mineral phases decreases as a result of progressive hydration from type I to type III alteration. Albitization, resulting from substitution of [Formula: see text], in calcic plagioclase, is highly variable in type I alteration, but in types II and III the plagioclase is totally albitized and in some cases silicified. Chloritization is closely linked to increasing hydration and Ca leaching with MgO and FeO substituting for CaO in ferromagnesian minerals.Calcium was mobilized and carried by solutions, as evidenced by the variable concentration of epidote at the margins of pillows. This calcium leaching generated an excess of Al2O3 with respect to the combined molecular proportions of Na2O, K2O, and CaO, and is shown by the presence of corundum in CIPW norm calculations. In some pillows showing substitution of [Formula: see text], the fo2 of the invading fluid appears to have remained constant, being buffered by the pillow composition; this would be favored by a low water/rock mass ratio. As a result, the initial pillow Fe2O3/FeO ratio remained constant. In other pillows, the fo2 appears to have been imposed by the invading fluid rather than by the mineral assemblage: the FeO/MgO ratios are thus no longer representative of the magmatic composition whereas the ΣFeO/MgO is still representative of the pristine magmatic value.Two types of substitution of CaO by FeO and (or) MgO have been observed: (1) preferential substitution restricted to type I alteration, of FeO over MgO, similar to low-temperature substitution in modern-day sea-floor alteration; and (2) the more common substitution in type II and III alterations in which MgO predominates over FeO, similar to the high-temperature substitution taking place at great depth on the ocean floor.Although the samples were collected to test mineral heterogeneities caused by chemical degradation, more than 40% of the pillows sampled retained their pristine ΣFeO/MgO ratios. The various alteration patterns are independent of the initial tholeiitic or calc-alkaline lineage; this was confirmed using rare earth elements (REE) and inert trace elements such as Zr, Y, and Ti. The chemical changes in the mafic metavolcanic rocks do not obliterate their tholeiitic or calc-alkaline chemical affinities.

Archaean precious-metal hydrothermal systems, Dome Mine, Abitibi Greenstone Belt. I. Patterns of alteration and metal distribution

1979 ◽  
Vol 16 (3) ◽  
pp. 421-439 ◽  
Author(s):  
B. J. Fryer ◽  
R. Kerrich ◽  
R. W. Hutchinson ◽  
M. G. Peirce ◽  
D. S. Rogers
Keyword(s):  
Greenstone Belt ◽  
Precious Metals ◽  
Hydrothermal Systems ◽  
Tectonic Deformation ◽  
Hydrothermal Quartz ◽  
Abitibi Greenstone Belt ◽  
Two Stages ◽  
Host Rocks ◽  
Chemical Sediments ◽  
Gold Bearing

The Porcupine District, Abitibi Greenstone Belt is one of the most extensive areas of Archaean auriferous mineralization. At least two stages of lode gold emplacement may be recognized. The first involves gold-bearing ferroan dolomite layers with subordinate chert, sulphides, and graphite deposited as laterally extensive chemical sediments at interflow horizons within the mafic volcanic sequence. The second stage is represented by major gold-bearing hydrothermal quartz–albite–dravite veins which transect diverse host rocks including the carbonate chemical sediments. Differences between gold-bearing chemical sediment and auriferous hydrothermal veins, in terms of texture, mineralogy, and nature of inclusions, together with considerations of chemistry are not compatible with local derivation of veins from enveloping chemical sediments or adjacent host rocks. The chemical sediments display slump structures and predate all tectonic deformation. In general, auriferous hydrothermal quartz veins transect bedding and/or schistosity, and are at a low state of internal strain. They appear to have been emplaced late during the second regional fold episode.Au, Ag, and Pd average 10, 2, and 0.1 ppm respectively in ore types at the Dome mine; representing concentration factors of 10 000,40, and 10 times background values in unmineralized metabasalt, and primary igneous rocks worldwide. Au and Ag are inhomogeneously distributed.Mineralized metabasic rocks adjacent to vein stockworks have Ti/Zr and Ti/Al2O3 ratios comparable to tholeiitic basalts, but display variable enrichment or depletion of silica, systematic depletion in Na2O, and where intensely altered significant extraction of calcium. The low Ni and Cr contents of the carbonate layers, together with low Ti/Zr ratios (43–78) of the carbonates and their enveloping mafic schists, are not consistent with the hypothesis that these auriferous rocks are carbonated ultramafics. Massive banded quartz–fuchsite–dravite veins have Cr and Ni abundances averaging 350 ppm, implying hydrothermal transport of these elements. Ti/Zr ratios of 120, together with high Mg, Cr, and Ni abundances in magnesite–dolomite–quartz– chlorite schists which host the banded veins are compatible with a primary komatiitic composition. Mineralized metabasic rocks are reduced (Fe2+/ΣFe = 0.9) relative to rocks with primary background abundances of precious metals (Fe2+/ΣFe = 0.7). This change of oxidation state implies that large volumes of reducing hydrothermal solutions were involved in vein mineralization.

Geochemical evolution in the Blake River Group, Abitibi Greenstone Belt, Superior Province

1980 ◽  
Vol 17 (9) ◽  
pp. 1292-1299 ◽  
Author(s):  
I. E. M. Smith
Keyword(s):  
Rare Earth ◽  
Volcanic Rock ◽  
Rare Earth Element ◽  
Greenstone Belt ◽  
Earth Element ◽  
Superior Province ◽  
Calc Alkaline ◽  
Abitibi Greenstone Belt ◽  
Rock Types ◽  
Element Fractionation

In well exposed, well developed greenstone belts of the Superior Province there is a clear progression from stratigraphically lower, geochemically primitive volcanic rock types (komatiites, tholeiites) to overlying geochemically evolved calc-alkaline volcanic rock types. In the western Blake River Group of the Abitibi Greenstone Belt the change from tholeiitic to calc-alkaline volcanics represents a geochemical discontinuity defined by an increase in incompatible elements and light/heavy rare-earth element fractionation in the overlying rocks. Quantitative modelling of the parameters of the discontinuity indicates that it can be explained by a change to very small amounts of melting of unmodified mantle lherzolite, although this is not a unique solution. In calc-alkaline suites showing high degrees of rare-earth element fractionation the calculated melt fraction required of unmodified mantle becomes unrealistically low and models involving a geochemically evolved source may have to be considered.

Pontiac metavolcanic rocks within the Cadillac tectonic zone, McWatters, Abitibi Belt, Quebec

1993 ◽  
Vol 30 (7) ◽  
pp. 1521-1531 ◽  
Author(s):  
David Morin ◽  
Michel Jébrak ◽  
Marc Bardoux ◽  
Normand Goulet
Keyword(s):  
Rare Earth ◽  
Trace Element ◽  
Rare Earth Element ◽  
Greenstone Belt ◽  
Earth Element ◽  
Calc Alkaline ◽  
Tectonic Zone ◽  
Southern Boundary ◽  
Abitibi Greenstone Belt ◽  
Metavolcanic Rocks

The McWatters metavolcanic rocks are structurally bounded lenses within the Cadillac tectonic zone on the southern boundary of the Abitibi greenstone belt. They comprise komatiite, tholeiitic basalt and gabbro, and calc-alkaline andesitic lavas and volcaniclastic rocks cut by calc-alkaline dioritic and lamprophyric dykes. The McWatters basalts are mid-ocean-ridge basalt type tholeiites exhibiting low incompatible trace element contents and [La/Yb]N < 1. They may have formed via relatively high degree partial melting of a rare-earth element depleted mantle source. The andesites exhibit chondrite-normalized trace-element patterns with light-rare-earth and large-ion lithophile element enrichments and negative Nb and Ti anomalies, comparable to those of subduction-related calc-alkaline andesites. McWatters units are distinct from nearby Blake River Group rocks, despite comparable lithological assemblages and some common geochemical characteristics. The McWatters basalts exhibit lower Ti/Y, Zr/Y, and La/Yb than the Blake River tholeiites, whereas the McWatters andesites display lower Ti/Zr and higher Zr/Y than the Blake River calc-alkaline units. The McWatters tholeiites can be correlated with northern Pontiac Group tholeiitic units based on similar trace-element ratios and parallel rare-earth-element patterns. Thus, the McWatters tholeiites represent Pontiac rocks, underthrust beneath the southern Abitibi belt and appearing as isolated and retrograded lenses in the Cadillac tectonic zone. They may represent the remnants of an ocean basin that once separated the southern Abitibi greenstone belt from the Pontiac Subprovince.

Metallogenesis of the Abitibi greenstone belt of Canada: a contribution from the analysis of trace lead in sulfide minerals

1989 ◽  
Vol 26 (12) ◽  
pp. 2529-2540 ◽  
Author(s):  
Etienne Deloule ◽  
Clément Gariépy ◽  
Bernard Dupré
Keyword(s):  
Greenstone Belt ◽  
Igneous Rocks ◽  
Lava Flows ◽  
Ore Formation ◽  
Abitibi Greenstone Belt ◽  
Isotopic Compositions ◽  
Leaching Experiments ◽  
Iron Formations ◽  
Different Sources

Pb-isotopic compositions are reported for 12 ore localities within the late Archean Abitibi greenstone belt. The studied samples carry massive or disseminated sulfides hosted in a variety of materials, including mafic–ultramafic igneous rocks, felsic lavas, porphyries, and sedimentary iron formations. Repeated leaching experiments on these sulfides frequently revealed the presence of a radiogenic Pb component, which is attributed to in situ decay of U and Th. The leaching experiments make it possible, in some cases, to separate the radiogenic Pb from the initial Pb included in the minerals. Six Pb–Pb isochrons formed by the analyses on leachates and residues show little evidence of secondary perturbations and yield ages that are, within error, similar to those determined for the supracrustal assemblage. This implies that the ores were concentrated synchronously with the main phases of magmatic activity, close to 2.7 Ga.The initial isotopic compositions of the sulfide specimens point to the existence of two different sources of metals: (i) juvenile, mantle-derived igneous rocks and (ii) older recycled supracrustal series. Ore formation frequently involves mixing of metals from these two sources in variable proportions. The initial isotopic composition of these two reservoirs is best evaluated by examining the composition of sulfides associated with komatiitic lava flows and with sedimentary iron formations, respectively.

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