ETH Library Gold precipitation and hydrothermal alteration during fluid flow through the vein network of the mesothermal gold deposit of Sigma, (Abitibi belt, Canada) Doctoral Thesis Author(s): Garofalo, Paolo Stefano Publication date: 2000 Permanent link: https://doi.org/10.3929/ethz-a-004090477 Rights / license: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information, please consult the Terms of use. DISSETH Nr. 13952 Gold precipitation and hydrothermal alteration during fluid flow through the vein network of the mesothermal gold deposit of Sigma, (Abitibi belt, Canada) A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of Doctor of Natural Science presented by Paolo Stefano Garofalo Laurea in Scienze Geologiche - Université di Firenze - Italy MSc. Ore Geology- Univ. of Wales - College of Cardiff - UK Born the 24th March 1965 Italian Citizen Accepted on the recommendation of Prof. Dr. C. A. Heinrich ETH Zurich Examiner - Dr. S. K. Matthäi - ETH Zurich Co-examiner Dr. J. R. Ridley - Macquarie University, Sydney Co-examiner Prof. Dr. L. Diamond University of Leoben Co-examiner - 2000 DISS ETH Nr. 13952 Gold precipitation and hydrothermal alteration during fluid flow through the vein network of the mesothermal gold deposit of Sigma, (Abitibi belt, Canada) A dissertation submitted to the SWISS FEDERAL INSTITUTE OF TECHNOLOGY ZURICH for the degree of Doctor of Natural Science presented by Paolo Stefano Garofalo Laurea in Scienze Geologiche - Universita di Firenze - Italy MSc. Ore Geology - Univ. of Wales - College of Cardiff - UK Born the 24th March 1965 Italian Citizen Accepted on the recommendation of Prof. Dr. C. A. Heinrich - ETH Zurich Examiner Dr. S. K. Matthäi - ETH Zurich Co-examiner Dr. J. R. Ridley - Macquarie University, Sydney Co-examiner Prof. Dr. L. Diamond - University of Leoben Co-examiner 2000 I Contents CONTENTS I ABSTRACT 1 RIASSUNTO 3 CHAPTER 1 5 GEOLOGICAL SETTING OF THE ABITIBI GREENSTONEBELT AND OF THE VAL D'ORAREA 5 paleogeographic evolution andarchitecture oftheabitibigreestone belt 5 GeologyoftheVald'Orarea 7 Shorthistoricalnotes JO CHAPTER 2 12 THE GEOLOGY OF THE SIGMA DEPOSIT 12 Abstract 12 mainfeatures ofthedeposit 13 GeochronologicaldataontheVald'Orarea andonSigma 18 Characteristics ofthequartz-tourmaline veins 20 The steepveins 27 The subhorizontal veins 29 Mechanismofformationofverticalandhorizontalveins 30 CHAPTER 3 33 ESTIMATION AND TESTING OF STANDARD MOLAR THERMODYNAMIC PROPERTIES OF TOURMALINE END-MEMBERS USING DATA OF NATURAL SAMPLES 33 Abstract 33 Introduction 33 Crystalchemistryoftourmaline anddumortierite 34 Experimentaldatafornaturaltourmalineanddumortierite 36 Estimation ofheatcapacityfunctions fortourmalineend-members 40 Il Estimation ofstandard molarentropies 46 Estimationofstandard molarenthalpies 48 Boron solubility determined byLA-ICP-MS microanalysis ofsinglefluid inclusions 50 Appendix 3.1 57 CHAPTER 4 58 THE PTX PROPERTIES OF THE GOLD-BEARING HYDROTHERMAL FLUID OF THE SIGMA DEPOSITAS RECONSTRUCTED FROM FLUID INCLUSION DATA..58 Abstract 58 Introduction 58 Samplepreparationandfluid inclusionpetrography 60 Undergroundsamples 60 Surface samples (Audet vein) 61 Samplepreparation 61 Relations betweenfluid entrapment, quartz, gold andtourmalinedeposition 62 Thefluidinclusionassemblages 64 Fluidinclusionpetrography 70 Compositionaltypesofthetrappedfluid 73 Analyticalmethods 74 Microthermometry 74 Raman spectroscopy 74 LaserAblation-ICP-MassSpectrometry 75 LaserRamanResults 80 Microthermometricresults 82 Low temperature determinations 82 High temperature determinations 86 LaserAblation-ICP-MassSpectrometry ofsinglefluidinclusions 88 Composition ofthe ore bearingfluids 88 Composition ofthe Shieldbrines 90 Interpretation ofthefluid inclusionpetrography 91 Interpretation ofthemicrothermometricdata 95 Phaseseparationofthe Sigmafluid as a mechanism for goldprecipitation 101 Appendix4.1: Microthermometric characteristicsofthe studiedfluidinclusions Appendix4.2: ResultsoftheLaserAblation-ICP-MS study 122 CHAPTER 5 135 CHARACTERISTICS OF THE HYDROTHERMALALTERATIONIN THE SIGMA DEPOSIT 135 Abstract 135 Introduction 136 Types ofhydrothermalalteration attheoutcrop scale 138 minescalevariations 144 Outcrop-scalepatterns ofalteration haloes 147 Mineralogicalcharacteristics ofthehydrothermalalterationtypes 150 Albite alteration 150 Tourmaline alteration 152 mass balancecalculations 154 Conclusions 169 CHAPTER 6 171 THREE-DIMENSIONAL GEOMETRY AND GOLD GRADE DISTRIBUTION OF A SECTION OF THE STEEP VEIN NETWORK OF SIGMA 171 Abstract 171 Introduction 172 Minedata andmethodusedtobuildtheAudistributiondatabase 174 Reconstruction ofthethreedimensionalgeometry ofsingleveins 179 Networkvolumeand volumetricestimation ofveintourmaline 183 Characteristics ofthethree-dimensionalveinnetwork 186 Discussionand conclusions 196 CHAPTER 7 201 ESTIMATION OF THE PHYSICAL-CHEMICAL PROPERTIES OF THE HYDROTHERMAL FLUID DURING GOLD DEPOSITION 201 Abstract 201 Thermodynamic analysis 201 Appendix7.1 213 CHAPTER 8: SYNOPSIS 216 IV SIGMA —A SHALLOW MESOTHERMAL QUARTZ-TOURMALINE-GOLDVEIN SYSTEM WITH A DEEP FLUID SOURCE? 216 Abstract 216 Large-scalecharacteristics ofthedeposit 217 Compositionoftheorefluid and conditions ofmineralization 218 Quantificationoffluid fluxes 219 Thermodynamicanalysis offluid-mineralreactions 220 Constraints on sourceofauriferousfluids 222 REFERENCE LIST 224 ACKNOWLEDGMENTS 243 1 Abstract The mesothermal vein network of Sigma is one of the largest Archean lode-gold deposits of the Abitibi greenstone belt, and together with the nearby Lamaque vein network represents a world-class deposit in terms of total historical production (i e ca 400 t Au) Sigma is an ideal case study of a vertically extensive gold-bearing vein network, and its three-dimensional underground exposure offers the opportunity to document the possible geochemical evolution of a gold-bearing hydrothermal fluid across a large vertical section (i e ca 2 km) of an Archean greenstone belt The aim of this work is to characterize this evolution using a series of different approaches, namely the determination of the three-dimensional geometry of the vein network and its volumetric characteristics using a compilation of the underground mine database, the determination of the PTX properties of the hydrothermal fluid using a detailed fluid inclusion study, geometrical and mass-balance calculation study of the zones of hydrothermal alteration developed around the gold-bearing veins, and the thermodynamic analysis of fluid-mineral reactions in veins and alteration haloes that uses also thermodynamic properties of tourmaline end-members derived ad hoc This study reports the first quantitative analyses of Au and B concentrations of an Archean mesothermal fluid as determined from Laser Ablation-ICP-Mass Spectrometry of single fluid inclusions These concentrations range between 0 5 and 5 ppm for gold and 78 and 2200 ppm for boron Sigma is made of a vertically extensive network of quartz-tourmaline veins contained by a network of anastomosing, E-W/ENE-WSW trending, south dipping brittle-ductile shear zones developed during a strike-slip deformation regime coupled with a reverse component The vein network is mainly made of vertically extensive steeply dipping veins and mechanically associated subhorizontal extension veins Gold is found mainly within the veins and in fewer amounts also in the proximal alteration zones Fluid inclusion data, vein network characteristics, and geometrical development of carbonatic alteration across the deposit indicate that Sigma-Lamaque formed probably at a relatively shallow level of ca 3 km with near-hydrostatic fluid pressures estimated at ca 300 bars and temperature between 380 and 400 °C Fluid inclusion data and thermodynamic analysis of fluid-mineral reactions are consistent with gold precipitation in the veins as a result of boiling of the hydrothermal fluid In contrast, gold precipitated in the wallrocks as a result of decrease of the total sulfur concentration in the fluid 2 caused by sulfide deposition. During fluid flow within the veins and fluid-rock interaction with the wallrocks, the hydrothermal fluid experienced physical and compositional changes. Combining the three-dimensional reconstruction of the geometry of a significant section of the vein network with image analysis of quartz-tourmaline veins in the deposit, a mass of total boron of 71500 tons is estimated to have precipitated within the vein network. On the basis of this estimation and of the range of boron concentration in the ore fluid, the total volume of hydrothermal fluid that produced the deposit is estimated to be less than 0.5 km3. This indicates that a small amount of gold-rich fluid was involved in the production of the Sigma vein network. Simple mass balance constraints between rock and fluid amounts and their respective gold concentrations indicate that devolatilization of metamorphic rocks at depth cannot be the only mechanism that produced the gold-rich hydrothermal fluid. Leaching from an inherently Au-rich source present in the area is therefore necessary to produce the ore fluid of Sigma. Such a source can be identified in the igneous bodies present in the Val d'Or area and possibly in the mantle. These rocks emplaced at the time of mineralization, and had therefore a possible role in the formation of the Sigma deposit. 3 Riassunto Le vene aurifère a quarzo e tormalina di Sigma (Quebec) formano uno dei piu grandi giacimenti archeani mesotermali delPAbitibi greenstone belt, ed insieme al contiguo giacimento Lamaque formano un deposito di importanza mondiale in termini di produzione totale d'oro (i.e. ca. 400 t Au). Sigma è formate da un reticolo di vene con un'estensione verticale esplorata di circa due chilometri, ed ha un'eccellente esposizione tridimensionale che permette la documentazione délia possibile evoluzione geochimica a grande scala del fluido idrotermale. Lo scopo di questo lavoro di dottorato è quello di cararatterizzare questa possibile evoluzione attraverso l'uso delle seguenti metodologie: studio tridimensionale délia geometria delle vene aurifère attraverso una opportuna compilazione di dati di sottosuolo, determinazione delle propriété PTX del fluido idrotermale con l'uso di inclusioni fluide, studio geometrico e di bilancio di massa delle zone di alterazione idrotermale, analisi termodinamica di equilibri fluido-roccia nelle vene e nelle zone di alterazione. Questo studio riporta altresl le prime analisi quantitative di singole inclusioni fluide contenenti fluido mesotermale ad oro effettuate attraverso spettrometria di massa ad ablazione laser (Laser Ablation-ICP-MS). In particolare, le concentrazioni di oro e boro nel fluido mineralizzante sono comprese, rispettivamente, tra 0.5 e 5 ppm e 78 e 2200 ppm. Il reticolo di vene aurifère di Sigma è contenuto alPintemo di un reticolo anastomizzante di zone di taglio di tipo fragile-duttile con direzione E-W/ENE-WSW ed immersione di ca. 80°S sviluppato durante una deformazione di tipo strike-slip con componente inversa. Il reticolo è composta prevalentemente da vene subverticali e suborizzontali, le ultime meccanicamente associate alle prime, che si sono sviluppate nelle zone a più alta dilatanza poste all'apice delle vene subverticali. Gran parte dell'oro è estratto da vene verticali, ma le vene orizzontali sono anche un importante corpo minerario, anche se contribuiscono poco al volume totale délia produzione. Le zone di alterazione idotermale contengono pure limitate quantité d'oro. I dati delle inclusioni fluide, le caratteristiche del reticolo di vene e la geometria dell'alterazione carbonatica présente intorno alle vene indicano che il deposito Sigma-Lamaque si è formata ad una profondità di circa 3 km, con una pressione del fluido di circa 300 bar ed una temperatura di 380-400 °C. I dati delle inclusioni fluide e lo studio termodinamico degli equilibri fluido-roccia sono coerenti con un modello di precipitazione dell'oro in segito ad ebollizione del fluido idrotermale. La precipitazione del'oro nelle zone d'alterazione è invece dovuta alla diminuzione del contenuto totale di zolfo nel fluido in seguito alla precipitazione di soifuri. Durante il flusso idrotermale