Origins of metamorphic lode gold deposits: Implications of stable isotope data from the central Rocky Mountains, Canada

Shaw, Robert P.; Morton, R. D.; Gray, John Edward; Krouse, H. R.

doi:10.1007/bf01166891

Cited by 3 publications

(3 citation statements)

References 24 publications

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“…The results of a number of recent studies of Cordilleran vein deposits suggest that chemically evolved meteoric fluids play a much greater role in mesothermal systems than previously reported (Nesbitt and Muehlenbachs, 1989a;Madu et al, 1990;Shaw et al, 1991). Stable isotope analyses of Klondike vein minerals and inclusion fluids imply that the mineralizing fluid was also meteoric in origin but underwent considerable isotopic evolution prior to mineralization.…”

Section: Discussionmentioning

confidence: 75%

A fluid inclusion and stable isotope study of Au quartz veins in the Klondike District, Yukon Territory, Canada; a section through a mesothermal vein system

Rushton

Nesbitt²,

Muehlenbachs³

et al. 1993

Economic Geology

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Gold-bearing quartz veins in theKlondike district of Yukon Territory are of earliest Cretaceous age and are hosted by Permian, middle to upper greenschist facies, metasedimentary and meta-igneous lithologies belonging to the Yukon-Tanana terrane. The veins occur in brittle, extensional structures, which may have developed during extensional deformation during uplift of the Klondike region in the Late Jurassic to Early to middle Cretaceous, following thrust imbrication in the Early Jurassic. The quartz veins are massive and unbanded, with localized carbonate flooding and/or pyritization of the wall rocks.Fluid inclusion studies have delineated significant, regional P-T-X trends in primary inclusion fluids from the Au quartz veins. The fluids have low salinities (<6 wt % NaC1 equiv), but variable CO2 concentrations, decreasing from approximately 0.13 to 0.18 mole fraction CO2 in the southeastern Klondike, through variable concentrations in the central Klondike (Xco2 < 0.01 to •1.00), to dominantly aqueous fluids (Xco2 < 0.01) in veins from the northwest portion of the region. Estimated trapping pressures decrease from 2,300 +_ 700 bars to between 300 to 900 bars, from southeast to northwest. Estimated fluid temperatures range from 300 ø to 350øC in central and southern lodes, to 200 ø to 250øC in the northwest Klondike. The P-X trends are interpreted as evidence for (1) effervescence of CO• from the mineralizing fluid, most likely in response to regional uplift during the life of the hydrothermal system, and (2) that the Au quartz vein system represents a section through a mesothermal vein system with deeper structural levels exposed in the southeastern Klondike. Convection of the hydrothermal fluid may have been driven by an elevated geothermal gradient resulting from the rapid uplift.The calculated isotopic composition of the Au quartz vein fluids (b•aOauia 8.3 4-_ 1.2%oSMOW, bDauia • -150 +_ 18%oSMOW) indicates that the mineralizing fluid was meteoric in origin but underwent considerable isotopic evolution prior to mineralization. Additionally, the •Da•ia values of extracted inclusion fluids (-166 +_ 11%0) and from hydrothermal muscovites (-174 +_ 3%0) from the Sheba vein system are in good agreement and indicate that the deuterium signature reflects that of the primary mesothermal fluid.

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Section: Discussionmentioning

confidence: 75%

A fluid inclusion and stable isotope study of Au quartz veins in the Klondike District, Yukon Territory, Canada; a section through a mesothermal vein system

Rushton

Nesbitt²,

Muehlenbachs³

et al. 1993

Economic Geology

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“…8a) and OH-bearing minerals (Fig. 7b) low dD values are mainly documented from Mesozoic and Cenozoic deposits, and are commonly interpreted to result from late meteoric water infiltration disconnected with the formation of the deposits (Nesbitt et al 1989;Zhang et al 1989;Madu et al 1990;Goldfarb et al 1991;Shaw et al 1991;Rushton et al 1993;Apodaca 1994;Jia et al 2003). However, similar low dD values are also documented for older deposits.…”

Section: Watermentioning

confidence: 74%

“…Isotopic exchange between fluid and hostrock is not considered. Equations 1 and 2 are modified from Sharp (2017):…”

Section: Watermentioning

confidence: 99%

The Light Stable Isotope (Hydrogen, Boron, Carbon, Nitrogen, Oxygen, Silicon, Sulfur) Composition of Orogenic Gold Deposits

Quesnel

Scheffer

Beaudoin

2023

Mineral Resource Reviews

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Orogenic gold deposits formed in various terranes of most ages since the Paleoarchean and generally consist of quartz veins hosted in shear zones formed at the ductile brittle transition under greenschist to lower amphibolite metamorphic conditions. Vein mineralogy is dominated by quartz with various amounts of silicates, carbonates, phyllosilicates, borates, tungstates, sulfides, and oxides. The isotopic composition of these minerals and fluid inclusions has been investigated since the 1960s to constrain the characteristics of orogenic fluid systems involved in the formation of gold deposits worldwide. This review is based on 8580 stable isotope analyses, including δ18O, δD, δ13C, δ34S δ15N, δ11B, and δ30Si values, from 5478 samples from 558 orogenic gold deposits reported in the literature from 1960 to 2010. This contribution describes the variability of the light stable isotopic systems as function of the minerals, the age of the deposits, their regional setting, and their country rocks. The temperature of isotopic equilibrium of orogenic gold veins is estimated from mineral pairs for oxygen and sulfur isotopes. Based on these temperatures, and on fractionation between mineral and fluid components (H2O, CO2 and H2S), the isotopic composition of fluids is estimated to better constrain the main parameters shared by most of auriferous orogenic fluid systems. Orogenic gold deposits display similar isotopic features through time, suggesting that fluid conditions and sources leading to the formation of orogenic gold deposits did not change significantly from the Archean to the Cenozoic. No consistent secular variations of mineral isotope composition for oxygen (−8.1‰ ≤ δ18O ≤ 33‰, n = 4011), hydrogen (−187‰ ≤ δD ≤ −4‰, n = 246), carbon (−26.7‰ ≤ δ13C ≤ 12.3‰, n = 1179), boron (−21.6‰ ≤ δ11B ≤ 9‰, n = 119), and silicon (−0.5‰ ≤ δ30Si ≤ 0.8‰, n = 33) are documented. Only nitrogen (1.6‰ ≤ δ15N ≤ 23.7‰, n = 258) and sulfide sulfur from deposits hosted in sedimentary rocks (−27.2‰ ≤ δ34S ≤ 25‰, n = 717) display secular variations. For nitrogen, the change in composition is interpreted to record the variation of δ15N values of sediments devolatilized during metamorphism. For sulfur, secular variations reflect incorporation of local sedimentary sulfur of ultimate seawater origin. No significant variation of temperature of vein formation is documented for orogenic gold deposits of different ages. Quartz-silicate, quartz-carbonate and sulfide-sulfide mineral pairs display consistent temperatures of 360 ± 76 °C (1σ; n = 332), in agreement with the more common greenschist facies hostrocks and fluid inclusion microthermometry. Fluid sources for orogenic gold deposits are complex but the isotopic systems (hydrogen, boron, carbon, nitrogen, oxygen, sulfur) are most consistent with contributions from metamorphic fluids released by devolatilization of igneous, volcano-sedimentary and/or sedimentary rocks. The contribution of magmatic water exsolved from magma during crystallization is not a necessary component, even if permissible in specific cases. Isotopic data arrays can be interpreted as the result of fluid mixing between a high T (~550 °C)—high δ18O (~10‰)—low δD (~−60‰) deep-seated (metamorphic) fluid reservoir and a low T (~200 °C)—low δ18O (~2‰)—high δD (~0‰) upper crustal fluid reservoir in a number of orogenic gold deposits. The origin of the upper crustal fluid is most likely sea- or meteoric water filling the host rock porosity, with a long history of water–rock isotope exchange. Mixing of deep-seated and upper crustal fluids also explains the large variation of tourmaline δ11B values from orogenic gold veins. Regional spatial variations of oxygen and hydrogen isotope compositions of deep-seated fluid reservoirs are documented between orogenic gold districts. This is the case for the Val-d’Or (Abitibi), Coolgardie and Kalgoorlie (Yilgarn) where the oxygen isotope composition of the deep-seated fluid end-member is 4‰ lower compared to that from the Timmins, Larder Lake, and Kirkland Lake districts (Abitibi). However, both mixing trends converge towards a common, low δ18O upper crustal fluid end-member. Such variations cannot be related to fluid buffering at the site of deposition and suggest provinciality of the fluid source. The contribution of meteoric water is mainly recorded by fluid inclusions from Mesozoic and Cenozoic age deposits, but micas are not systematically in isotopic equilibrium with fluid inclusions trapped in quartz from the same vein. This suggests late involvement of meteoric water unrelated to deposit formation. Yet, a number of deposits with low δD mica may record infiltration of meteoric water in orogenic gold deposits. Isotope exchange between mineralizing fluid and country rocks is documented for oxygen, carbon, sulfur and silicon isotopes. Large variations (> 10‰) of sulfide δ34S values at the deposit scale are likely related to evolving redox conditions of the mineralizing fluid during reaction with country rocks. Deposits hosted in sedimentary rocks show a shift to higher δ18O values as a result of fluid/rock oxygen exchange with the regional sedimentary country rocks.

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Riassunti dei lavori presentati: Il feto come paziente: Ruolo della RM

1994

Rivista di Neuroradiologia

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Un'affermazione ricorrente spesso in ambiente neonatologico ricorda che il feto e un paziente che ha gia una storia clinica di 9 mesi. Un paziente, il feto, con una realta biologica del tutto particolare. Tutti i possibili meccanismi eziopatogenetici si realizzano con una fisiopatologia in larga misura sconosciuta, non rapportabili a quanto avviene nell'organismo gia formato.Ogni noxa patogena infettiva, vascolare, tossica o di altro genere agisce su strutture in formazione e il risultato che essa produce e un disturbo dell'organogenesi e quanto resta da osservare sui neonato, e in generale, un quadro malformativo o gli esiti di un processo distruttivo.Allo stato attuale delle conoscenze sulla patologia fetale e con i mezzi diagnostici oggi a disposizione sarebbe azzardata l'ipotesi di poter gia intervenire e prevenire tali situazioni patologiche.Siamo agli albori della medicina fetale ma questa e gia una disciplina esistente per alcune procedure intervenzionistiche prenatali e la diagnostica per immagini deve stare al passo e intervenire in questo processo. I1 maggiore impulso, nell'ultimo ventennio, e venuto dall'ecografia prenatale di recente potenziato dall'uso delle sonde transvaginali 4 • La metodica ad ultrasuoni e rimasta ancorata ai ginecologi che, da sempre, sono riusciti a soddisfare le diverse esigenze diagnostiche.Questo e stato possibile per la semplicita di esecuzione dell'indagine e per l'insita scarsa risoluzione di contrasto tissutale con bassa specificita di metodica; di conseguenza non si e mai resa necessaria una specifica competenza di settore neurora-diologico. Nessuna difficolta per il ginecologo esperto di ecografia a riconoscere i grandi quadri malformativi encefalici come le agenesie totali del corpo calloso, le oloprosencefalie lobari, la malformazione classica di Dandy Walker, l'idrocefalo grave ed evolutivo e via discorrendo. Esiste tuttavia una larga parte di patologia encefalica meno appariscente svelata al ginecologo da piccoli segni quali una moderata dilatazione delle o di una camera ventricolare, una generica condizione di microcefalia, alcune aree parenchimali ipo o iperecogene, sicura spia di patologia che rimane oscura all'interpretazione ecografica.Da circa 10 anni si e cercato di colmare questa lacuna ricorrendo alia RM condotta sull'utero gravidico per lo studio dell'encefalo fetale 5 • 9 • La RM fetale tuttavia, ha subito numerosi arresti, prima frenata da problemi etici in larga misura superati dagli studi tranquillizzanti sugli effetti biologici della RM, ha incontrato poi grande e giustificata diffidenza a causa delle manovre invasive di curarizzazione fetale per ottenere la necessaria immobilita fetale 6 • La nostra casistica comprende 40 esaminati e nella nostra esperienza abbiamo percorse le varie fasi di ottimizzazione della metodica.Circa 20 pazienti sono state sottoposte a curarizzazione fetale nell'arco di 18 mesi. Nell'ultimo semestre il ricorso alia RM fetale e divenuto sempre piu frequente con altre 20 pazienti grazie alia messa a punto di sequenze RM (appar...

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Origins of metamorphic lode gold deposits: Implications of stable isotope data from the central Rocky Mountains, Canada

Cited by 3 publications

References 24 publications

A fluid inclusion and stable isotope study of Au quartz veins in the Klondike District, Yukon Territory, Canada; a section through a mesothermal vein system

A fluid inclusion and stable isotope study of Au quartz veins in the Klondike District, Yukon Territory, Canada; a section through a mesothermal vein system

The Light Stable Isotope (Hydrogen, Boron, Carbon, Nitrogen, Oxygen, Silicon, Sulfur) Composition of Orogenic Gold Deposits

Riassunti dei lavori presentati: Il feto come paziente: Ruolo della RM

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