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.

Archaean precious-metal hydrothermal systems, Dome Mine, Abitibi Greenstone Belt. II. REE and oxygen isotope relations

1979 ◽  
Vol 16 (3) ◽  
pp. 440-458 ◽  
Author(s):  
R. Kerrich ◽  
B. J. Fryer
Keyword(s):  
Oxygen Isotope ◽  
Greenstone Belt ◽  
Precious Metal ◽  
Igneous Rocks ◽  
Lode Gold ◽  
Quartz Veins ◽  
Hydrothermal Quartz ◽  
Abitibi Greenstone Belt ◽  
Chemical Sediments ◽  
Gold Bearing

The Porcupine District, Abitibi Greenstone Belt, is one of the most extensive areas of Archaean auriferous mineralisation. At least two stages of lode-gold emplacement are recognised: the first stage involves gold-bearing carbonate–chert chemical sediments within the lower mafic volcanic sequence; the second stage is represented by auriferous hydrothermal quartz veins which postdate deformation of the greenstone assemblage and transect diverse host rocks.Rare-earth element (REE) concentrations in the stratiform carbonates are typical of the distinctive patterns recorded for Archaean chemical sediments. Chert in these rocks has a δ18O value averaging 17.1‰, implying exchange from heavier 18/16 ratios during diagenesis and metamorphism. Metabasic volcanic rocks and quartz–feldspar porphyry stocks with background gold abundances have mean whole-rock δ18O values of 9.1‰ and 10.7‰ respectively. This enrichment in 18O relative to primary igneous rocks is attributed to oxygen isotope exchange with seawater at low temperatures during fluid transport through the oceanic crust.Quartz in all of the five hydrothermal vein systems present has a δ18O of 14‰ to 15‰, and quartz-muscovite fractionations are 3.4‰ to 3.8‰. Ambient temperatures of mineralisation are estimated to have been 400 °C to 450 °C, from oxygen isotope thermometers, fluid inclusion filling temperatures, and metamorphic mineral assemblages. The calculated δ18O of the mineralising solutions is~10‰, implying fluids of metamorphic origin. REE patterns in hydrothermal quartz veins suggest that they have been derived from high-temperature solutions in equilibrium with source rocks having relatively flat (chondrite normalised) REE distributions, such as tholeiitic and komatiitic volcanics. Adjacent to hydrothermal veins, quartz in igneous rocks approaches isotopic equilibrium with vein quartz, at 15‰, and whole-rock δ18O values for metabasalts shift to ~11‰, implying extensive water-rock interaction. Strong depletions in heavy REE of metabasic schists adjacent to veins provides further evidence for pervasive hydrothermal alteration. The Eu enrichment of all lode gold deposits analysed at Dome Mine is consistent with the reduced state of the solutions involved in their deposition, as recorded by the predominance of Fe2+. The gold-bearing veins are believed to have formed by focussed flow of fluids outgassed at the greenschist–amphibolite transition. Source volumes for Au in the Porcupine District exceed 600 km3, the carrier fluid volume for mineralisation was 60–90 km3, the Au solute concentration in the low nanogram mL−1 range, and transport distances were of the order of 10 km. Such veins may be the precursors of precious-metal-bearing chemical sediments if fluids debouche into the hydrosphere.

scholarly journals Intrusion-Associated Gold Systems and Multistage Metallogenic Processes in the Neoarchean Abitibi Greenstone Belt

Minerals ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 261
Author(s):  
Lucie Mathieu
Keyword(s):  
Greenstone Belt ◽  
Gold Deposits ◽  
Hydrothermal Systems ◽  
Magmatic Fluid ◽  
Gold Deposition ◽  
Alkaline Magmatism ◽  
Abitibi Greenstone Belt ◽  
Metamorphic Fluids ◽  
Greenstone Belts ◽  
Gold Bearing

In gold-endowed greenstone belts, ore bodies generally correspond to orogenic gold systems (OGS) formed during the main deformation stage that led to craton stabilization (syntectonic period). Most OGS deposits postdate and locally overprint magmatic-hydrothermal systems, such as Au-Cu porphyry that mostly formed during the main magmatic stage (synvolcanic period) and polymetallic intrusion-related gold systems (IRGS) of the syntectonic period. Porphyries are associated with tonalite-dominated and sanukitoid plutons, whereas most IRGS are related to alkaline magmatism. As reviewed here, most intrusion-associated mineralization in the Abitibi greenstone belt is the result of complex and local multistage metallogenic processes. A new classification is proposed that includes (1) OGS and OGS-like deposits dominated by metamorphic and magmatic fluids, respectively; (2) porphyry and IRGS that may contain gold remobilized during subsequent deformation episodes; (3) porphyry and IRGS that are overprinted by OGS. Both OGS and OGS-like deposits are associated with crustal-scale faults and display similar gold-deposition mechanisms. The main difference is that magmatic fluid input may increase the oxidation state and CO2 content of the mineralizing fluid for OGS-like deposits, while OGS are characterized by the circulation of reduced metamorphic fluids. For porphyry and IRGS, mineralizing fluids and metals have a magmatic origin. Porphyries are defined as base metal and gold-bearing deposits associated with large-volume intrusions, while IRGS are gold deposits that may display a polymetallic signature and that can be associated with small-volume syntectonic intrusions. Some porphyry, such as the Côté Gold deposit, demonstrate that magmatic systems can generate economically significant gold mineralization. In addition, many deposits display evidence of multistage processes and correspond to gold-bearing or gold-barren magmatic-hydrothermal systems overprinted by OGS or by gold-barren metamorphic fluids. In most cases, the source of gold remains debated. Whether magmatic activity was essential or marginal for fertilizing the upper crust during the Neoarchean remains a major topic for future research, and petrogenetic investigations may be paramount for distinguishing gold-endowed from barren greenstone belts.

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.

Fluid diversity in the gold-endowed Archean orogenic systems of the Abitibi greenstone belt (Canada) I: Constraining the PTX of prolonged hydrothermal systems

2021 ◽  
pp. 104221
Author(s):  
Györgyi Tuba ◽  
Daniel J. Kontak ◽  
Brandon G. Choquette ◽  
Jérémie Pfister ◽  
Evan C.G. Hastie ◽  
...  
Keyword(s):  
Greenstone Belt ◽  
Hydrothermal Systems ◽  
Abitibi Greenstone Belt

scholarly journals Oxygen Fugacity and Volatile Content of Syntectonic Magmatism in the Neoarchean Abitibi Greenstone Belt, Superior Province, Canada

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 966
Author(s):  
Baptiste Madon ◽  
Lucie Mathieu ◽  
Jeffrey H. Marsh
Keyword(s):  
Oxygen Fugacity ◽  
Greenstone Belt ◽  
Inductively Coupled Plasma ◽  
Gold Mineralization ◽  
Gold Deposits ◽  
Hydrothermal Systems ◽  
Chemical Diversity ◽  
Temporal Association ◽  
Volatile Content ◽  
Abitibi Subprovince

Neoarchean syntectonic intrusions from the Chibougamau area, northeastern Abitibi Subprovince (greenstone belt), may be genetically related to intrusion related gold mineralization. These magmatic-hydrothermal systems share common features with orogenic gold deposits, such as spatial and temporal association with syntectonic magmatism. Genetic association with magmatism, however, remains controversial for many greenstone belt hosted Au deposits. To precisely identify the link between syntectonic magmas and gold mineralization in the Abitibi Subprovince, major and trace-element compositions of whole rock, zircon, apatite, and amphibole grains were measured for five intrusions in the Chibougamau area; the Anville, Saussure, Chevrillon, Opémisca, and Lac Line Plutons. The selected intrusions are representative of the chemical diversity of synvolcanic (TTG suite) and syntectonic (e.g., sanukitoid, alkaline intrusion) magmatism. Chemical data enable calculation of oxygen fugacity and volatile content, and these parameters were interpreted using data collected by electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry. The zircon and apatite data and associated oxygen fugacity values in magma indicate that the youngest magmas are the most oxidized. Moreover, similar oxygen fugacity and high volatile content for both the Saussure Pluton and the mineralized Lac Line intrusion may indicate a possible prospective mineralized system associated with the syntectonic Saussure intrusion.

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