Carbon-isotope systematics of Archean Au–Ag vein deposits in the Superior Province

1990 ◽  
Vol 27 (1) ◽  
pp. 40-56 ◽  
Author(s):  
R. Kerrich
Keyword(s):  
Hydrothermal Fluids ◽  
Superior Province ◽  
Ambient Temperatures ◽  
Rayleigh Fractionation ◽  
Clastic Sediments ◽  
Isotopic Equilibration ◽  
Vein Deposits ◽  
Carbonaceous Rocks ◽  
Isotope Systematics ◽  
Main Population

Abundant carbonate is a characteristic feature of most Archean mesothermal Au–Ag vein deposits, but the source of the C is controversial. For Superior Province deposits collectively, the maximum variation of average δ13C values is from −9.0 ± 0.7‰ (1σ, n = 19; Darius) to −0.6 ± 1.6‰ (1σ, n = 7; Cochenour–Willians), and limiting δ13C values are−13.6 and + 1.3‰. At the deposit scale, Fe dolomites in nongraphitic lithologies are for the most part isotopically uniform, where δ13C = −3.4 ± 0.4 (1σ) (Hollinger), −3.2 ± 0.3 (McIntyre), −4.7 ± 1.7 (Dome), −2.8 ± 0.6 (Buffalo Ankerite), −3.6 ± 0.5 (Macassa), −3.2 ± 0.3 (Bousquet), −5.4 ± 0.9 (Lamaque), and −5.3 ± 0.5‰ (Hasaga): the restricted individual ranges of δ13C values imply a corresponding uniformity to the ambient temperature and δ13CΣC of the ore-forming fluids.Within individual deposits, small systematic variations of δ13C carbonate arise from (i) interaction of hydrothermal fluids with carbonaceous rocks, (ii) immiscible separation of CO2 + CH4, or (iii) Rayleigh fractionation effects. Positive shifts in δ13C result from buffering of the fluid to lower Eh by reaction with reduced C, whereas negative shifts reflect partial isotopic equilibration between 13C-depleted C (δ13C ≈ −26‰) and aqueous hydrothermal C species. Transient immiscibility of CO2 + CH4 acts to precipitate carbonates enriched relative to the main population of Fe dolomites. The δ13C values of carbonates in unmineralized alteration halos (−2.2 ± 1.1‰, n = 42) at the McIntyre deposit are enriched in 13C relative to the main gold-bearing vein systems (δ13C = −3.2 ± 0.3‰): the enrichment is attributed to a Rayleigh fractionation accompanying progressive consumption of CO2 as hydrothermal fluids infiltrate laterally from veins into wall rocks. Fe dolomite and calcite are variably enriched in 18O with respect to equilibrium quartz-carbonate fractionations for ambient temperatures of 270–340 °C. Carbonate δ18O values diminish in an irregular manner with depth, converging on values of ~11‰ (Fe dolomite, 6800 ft (2073 m), McIntyre). Variable degrees of oxygen-isotope disequilibrium represent overprinting of carbonates by post-Archean brines in the Canadian Shield.Synvolcanic vesicle calcite in three groups of metabasalts (δ13C = −4.3 ± 2.1; −2.8 ± 1.5; −2.7 ± 1.3‰) and calcite in two groups of clastic sediments (−6.4 ± 1.8; −4.6 ± 2.5‰) remote from deposits are systematically depleted of 13C relative to average Precambrian limestones (~0 ± 1‰), owing to the involvement of CO2 derived from 13C-depleted organic matter. Consequently, calcite in greenstone belt supracrustal rocks is not restricted to approximately 0‰. The total spread of average δ13CFe dol values (−9.0 ± 0.7 to −0.6 ± 0.6‰) in the Au deposits, which goes in hand with a geographical provinciality in O-, Sr-, and Pb-isotope compositions of the ore-forming fluids, is too large to be accounted for by mantle CO2 (−6 ± 2‰) or magmatic CO2 (−6 ± 2‰) alone but rather is interpreted as reflecting generation of hydrothermal fluids in crustal or subcreted rocks heterogeneous in terms of the distribution of 13C-enriched (carbonate) and 13C-depleted (reduced C) lithologies.

The status of the Makrotantalon Unit (Andros, Greece) within the structural framework of the Attic-Cycladic Crystalline Belt

2013 ◽  
Vol 151 (3) ◽  
pp. 430-446 ◽  
Author(s):  
MAGDALENA H. HUYSKENS ◽  
MICHAEL BRÖCKER
Keyword(s):  
Regional Differences ◽  
Regional Scale ◽  
Structural Evolution ◽  
Rock Types ◽  
Structural Framework ◽  
Facies Metamorphism ◽  
The Status ◽  
Isotopic Equilibration ◽  
Tectonic Contact ◽  
Isotope Systematics

AbstractThis study focuses on the status of the Makrotantalon Unit (Andros, Greece) within the framework of the Cycladic nappe stack. We document unambiguous evidence that this unit has experienced blueschist-facies metamorphism and identify previously unknown lawsonite ± pumpellyite assemblages in glaucophane-free metasediments. The position of the presumed tectonic contact at the base of this unit is vague, but roughly outlined by serpentinites. Only a single outcrop displays a weak angular unconformity with cohesive cataclasites in the footwall. Rb–Sr geochronology was carried out on 11 samples representing various rock types collected within or close to inferred or visible fault zones. Owing to a lack of initial isotopic equilibration and/or subsequent disturbance of the Rb–Sr isotope systematics, isochron relationships are poorly developed or non-existing. In NW Andros, direct dating of distinct displacement events has not been possible, but a lower age limit of ~ 40 Ma for final thrusting is constrained by the new data. Sporadically preserved Cretaceous ages either indicate regional differences in the P–T–d history or a different duration of metamorphic overprinting, which failed to completely eliminate inherited ages. The detachment on the NE coast records a later stage of the structural evolution and accommodates extension-related deformation. Apparent ages of ~ 29–25 Ma for samples from this location are interpreted to constrain the time of a significant deformation increment. On a regional scale, the Makrotantalon Unit can be correlated with the South Evia Blueschist Belt, but assignment to a specific subunit is as yet unconfirmed.

scholarly journals A New Insight Into Seawater-Basalt Exchange Reactions Based on Combined δ18O—Δ′17O—87Sr/86Sr Values of Hydrothermal Fluids From the Axial Seamount Volcano, Pacific Ocean

2021 ◽  
Vol 9 ◽  
Author(s):  
D. O. Zakharov ◽  
R. Tanaka ◽  
D. A. Butterfield ◽  
E. Nakamura
Keyword(s):  
Phase Separation ◽  
Pacific Ocean ◽  
Hydrothermal Fluids ◽  
Exchange Reactions ◽  
Isotope Shifts ◽  
Individual Site ◽  
Dual Porosity Model ◽  
O Isotope ◽  
Axial Seamount ◽  
Isotope Systematics

The δ18O values of submarine vent fluids are controlled by seawater-basalt exchange reactions, temperature of exchange, and to a lesser extent, by phase separation. These variations are translated into the δ18O values of submarine hydrothermal fluids between ca. 0 and + 4‰, a range defined by pristine seawater and equilibrium with basalt. Triple oxygen isotope systematics of submarine fluids remains underexplored. Knowing how δ17O and δ18O change simultaneously during seawater-basalt reaction has a potential to improve i) our understanding of sub-seafloor processes and ii) the rock-based reconstructions of ancient seawater. In this paper, we introduce the first combined δ17O-δ18O-87Sr/86Sr dataset measured in fluids collected from several high-temperature smoker- and anhydrite-type vent sites at the Axial Seamount volcano in the eastern Pacific Ocean. This dataset is supplemented by measurements of major, trace element concentrations and pH indicating that the fluids have reacted extensively with basalt. The salinities of these fluids range between 30 and 110% of seawater indicating that phase separation is an important process, potentially affecting their δ18O. The 87Sr/86Sr endmember values range between 0.7033 and 0.7039. The zero-Mg endmember δ18O values span from -0.9 to + 0.8‰, accompanied by the Δ′17O0.528 values ranging from around 0 to −0.04‰. However, the trajectory at individual site varies. The endmember values of fluids from focused vents exhibit moderate isotope shifts in δ′18O up to +0.8‰, and the shifts in Δ′17O are small, about −0.01‰. The diffuse anhydrite-type vent sites produce fluids that are significantly more scattered in δ′18O—Δ′17O space and cannot be explained by simple isothermal seawater-basalt reactions. To explain the observed variations and to provide constraints on more evolved fluids, we compute triple O isotope compositions of fluids using equilibrium calculations of seawater-basalt reaction, including a non-isothermal reaction that exemplifies complex alteration of oceanic crust. Using a Monte-Carlo simulation of the dual-porosity model, we show a range of possible simultaneous triple O and Sr isotope shifts experienced by seawater upon reaction with basalt. We show the possible variability of fluid values, and the causal effects that would normally be undetected with conventional δ18O measurements.

scholarly journals Thermodynamic and kinetic constraints on molecular hydrogen abundance and isotope systematics in hydrothermal fluids

2021 ◽  
Author(s):  
Andrea Ricci ◽  
Jens Fiebig ◽  
Barbara Kleine ◽  
Jóhann Gunnarsson-Robin ◽  
Kennedy Kamunya ◽  
...  
Keyword(s):  
Molecular Hydrogen ◽  
Hydrothermal Fluids ◽  
Isotope Systematics

The Archean gneiss complex of northern Labrador. 2. Mineral ages, secondary isochrons, and diffusion of strontium during polymetamorphism of the Uivak gneisses

1983 ◽  
Vol 20 (5) ◽  
pp. 707-718 ◽  
Author(s):  
Kenneth D. Collerson
Keyword(s):  
Strontium Isotope ◽  
Gneiss Complex ◽  
History Of ◽  
Mineral Data ◽  
Isotopic Equilibration ◽  
Diffusion Mechanisms ◽  
Place Exchange ◽  
The Times ◽  
Isotope Systematics ◽  
And Diffusion

Strontium isotope systematics of minerals from the ca. 3600 Ma old Uivak gneisses and of compositionally layered Uivak I gneisses, which define secondary isochrons, are the result of Sr isotopic homogenization during thermal events ca. 2800, ca. 2500, and ca. 1800 Ma ago. The strontium isotope data provide information about the thermal history of the Archean gneiss complex in northern Labrador, the scale of Sr isotopic equilibration, and mechanisms of Sr diffusion during polymetamorphism.The mineral data indicate that biotite and amphibole were both partially open to Sr diffusion during the Hudsonian Orogeny ca. 1800 Ma ago. However, cooling rates and closure to diffusion of Sr in these phases varied throughout the terrane. Potassium feldspars were largely closed to diffusive loss of Sr during the 1800 and 2500 Ma events, and they record evidence of a ca. 2800–2900 Ma thermal event.Ages defined by secondary isochrons correlate with variations in the widths of the gneissic layers and correspond with geologically established events in the Nain and Churchill Provinces. The secondary isochrons developed as a result of local isotopic equilibration on different scales during these thermal events. The variable scales of equilibration reflect the effects of different Sr diffusion mechanisms. As a melt phase was not produced in the gneisses at the times indicated by the secondary isochrons, diffusion must have occurred under subsolidus conditions. Temperatures were therefore too low for extensive volume diffusion to have taken place. Exchange of Sr isotopes over distances greater than 5–10 cm must have involved grain boundary diffusion or some form of infiltration metasomatism.

Carbon in Archean rocks of the Abitibi belt (Ontario–Quebec) and its relation to gold distribution

1985 ◽  
Vol 22 (12) ◽  
pp. 1945-1951 ◽  
Author(s):  
Janet S. Springer
Keyword(s):  
Hydrothermal Fluids ◽  
Carbonate Content ◽  
Gold Leaching ◽  
Independent Evidence ◽  
Quartz Veins ◽  
Mass Balancing ◽  
Gold Enrichment ◽  
Carbonaceous Rocks ◽  
Hydrothermal Event

In the Abitibi belt of Ontario and Quebec, sedimentary black argillites with carbon contents of up to 5% C contain up to 0.5 ppm gold. Sheared black argillites, now graphitic schists, reflect the values of the sedimentary precursors, although their appearance is very different. Gold values of 15 ppm and more are found in graphitic schists only where alteration, shown by quartz veins and increased carbonate content, gives independent evidence of a later hydrothermal event. Gold-rich graphitic schists are characterized by veining; raised gold values closely follow vein margins. As mass balancing shows gold leaching and redeposition to be unlikely, gold enrichment is interpreted to reflect introduction of gold and its preferential deposition in carbonaceous rocks by reduction at vein margins. Natural carbons in these rocks may have been surface activated by both shearing and hydrothermal fluids.

The genesis of the Deer Trail Zn–Pb–Ag vein deposits, northeast Washington, U.S.A.: evidence from fluid-inclusion and stable-isotope studies

1987 ◽  
Vol 24 (8) ◽  
pp. 1715-1726 ◽  
Author(s):  
D. W. Fluet ◽  
A. Changkakoti ◽  
R. D. Morton ◽  
J. Gray ◽  
H. R. Krouse
Keyword(s):  
Fluid Inclusion ◽  
Isotope Exchange ◽  
Hydrothermal Fluids ◽  
Geothermal System ◽  
Metasedimentary Rocks ◽  
Isotope Studies ◽  
Three Stages ◽  
Vein Deposits ◽  
Homogenization Temperatures ◽  
Insight Into

The Zn–Pb–Ag-bearing vein deposits of the Deer Trail mine, northeastern Washington, U.S.A., are hosted by Beltian metasediments that have been intruded by the Jurassic–Cretaceous Loon Lake granitoid batholith.Vein infilling took place in three stages: pre-ore, ore, and post-ore. Fluid-inclusion studies revealed homogenization temperatures and salinities of approximately 300 °C and 6.5–8.5 wt. % NaCl equivalent, respectively, for the pre-ore stage; 250–150 °C and 4–7.5 wt. % NaCl equivalent for the ore stage; and less than 150 °C and 3 wt. % NaCl equivlent for the post-ore stage. The calculated δ18O values of the hydrothermal fluids and δD values of fluid inclusion waters were +8 to +11‰ (SMOW) and −128 to −134‰ (SMOW), respectively, for the pre-ore stage; 0 to +10‰ and −89 to −143‰ for the ore stage; and 0 to −5‰ and −139‰ for the post-ore stage. The δ34S(ΣS) of the hydrothermal fluids was approximately +10‰(CDT).The results provide insight into the evolution of a geothermal system dominated by meteoric waters and driven thermally by the cooling batholith. The chemical and isotopic compositions of the veins were influenced by chemical reactions and isotope exchange of the hydrothermal fluids with the Precambrian metasedimentary rocks.

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