The Orfée prospect: a Neoarchean orogenic gold
occurrence along the contact between the La
Grande and Opinaca subprovinces (Eeyou Istchee
James Bay, Québec)
Mémoire
Adina Bogatu
Maîtrise en sciences de la Terre
Maître ès sciences (M.Sc.)
Québec, Canada
The Orfée prospect: a Neoarchean orogenic gold
occurrence along the contact between the La
Grande and Opinaca subprovinces (Eeyou Istchee
James Bay, Québec)
Mémoire
Adina Bogatu
Sous la direction de :
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Résumé
Les indices aurifères Orfée, Orfée Est et Le Moyne-extension sont encaissés par des formations de fer archéennes, à 300 m du contact entre la subprovince de La Grande et d’Opinaca, dans la région d’Eeyou Istchee Baie James, Québec, Canada. La zone Orfée (~0,2 Mt à 14,5 g/t Au) a une épaisseur apparente de 40 m et une extension latérale sur 100 m; des valeurs en or atteignent 93,7 g/t Au sur 1 m. Vers le nord, les formations de fer sont en contact faillé avec des amphibolites localement minéralisées (max. 4 g/t Au). Des wackes juste faiblement minéralisées bordent les formations de fer au sud. L’ensemble lithologique est fortement déformé et métamorphisé au faciès des amphibolites syn D2/M2.
L’or natif est associé avec la pyrrhotite semi-massive à massive, concentrée dans des pièges structuraux syn D2 et D3. Des inclusions d’or natif dans la pyrrhotite et dans des silicates métamorphiques,
des reliques de pyrite dans la pyrrhotite et la löllingite aurifère suggèrent l’introduction de l’or pre à syn métamorphisme. La mise en place de la minéralisation a été restreinte entre 2703 ±7 Ma, âge d’un dyke dioritique pre à syn minéralisation, et 2613 ±0,4 Ma, âge d’une intrusion de granite pegmatitique post minéralisation. Deux âges modèles moyens pondérés, obtenues par datation Re-Os sur l’arsénopyrite/löllingite aurifère, sont beaucoup plus jeunes que la mise en place de la minéralisation. Ces âges, 2582 ±13 Ma et 2557 ±12 Ma, représentent la rétroversion de la löllingite en arsenopyrite lors du métamorphisme rétrograde M3. L’or natif associée avec des altérations rétrogrades (e.g. chlorite, épidote,
séricite et prehnite) au long des fractures suggèrent la remobilisation de la minéralisation. L’or à Orfée est interprété comme étant orogénique (2703 Ma à 2613 Ma), pre à syn métamorphique (M2), remobilisé lors
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Abstract
The Orfée, Orfée Est and Le Moyne-extension showings, hosted by Archean iron formations, are located 300 m north of the contact between the La Grande and the Opinaca subprovinces, in the Eeyou Istchee Baie James area, Quebec, Canada. The Orfée showing (~0.2 Mt at 14.5 g/t Au) has an apparent thickness of 40 m and a continuous lateral extension of 100 m; gold values reach up to 93.7 g/t Au. Towards the north, locally mineralized amphibolites (4 g/t Au) are in faulted contact with the iron formations. A wacke unit, bordering the BIFs to the south, is only very weakly mineralized. Syn D2/M2, the lithological assemblage is
highly deformed and metamorphosed up to amphibolite facies.
The native gold is associated with semi-massive to massive pyrrhotite, concentrated in D2 and D3
structural traps. Native gold inclusions in pyrrhotite and in metamorphic silicates, relicts of pyrite in pyrrhotite and gold bearing löllingite suggest gold mineralization was introduced pre to syn metamorphism. The gold mineralization emplacement was constraint between 2703 ±7 Ma, age of a dioritic dyke intruded pre to syn mineralization, and 2613 ±0.4 Ma, age of a pegmatitic granite injected post mineralization. Two weighted average model ages of 2582 ±13 and 2557 ±12 Ma for gold-bearing arsenopyrite were obtained by Re-Os dating. These ages most likely represent the retroversion of löllingite to arsenopyrite during a M3
retrograde metamorphic event. Native gold associated with retrograde alterations (e.g. chlorite, epidote, sericite and prehnite) along fractures suggest gold remobilization. In summary, the gold from Orfée is interpreted to be orogenic (2703 Ma to 2613 Ma), pre to syn M2 metamorphism, remobilized during a
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Table of Contents
Résumé ... iii
Abstract ... iv
Table of Contents ... v
List of Figures ... vii
List of Tables ... viii
Abbreviations ... ix
Foreword ... x
Chapter 1 . Introduction ... 1
1.1 Context ... 2
1.2 Problematics and Objectives ... 2
1.3 Regional geology ... 3
1.4 Presentation of the article ... 4
Chapter 2 . The Orfée prospect: a Neoarchean orogenic gold occurrence along the
contact between the La Grande and Opinaca subprovinces (Eeyou Istchee
Baie-James, Québec, Canada) ... 6
2.1 Abstract ... 6
2.2 Introduction ... 8
2.3 Regional Geology ... 11
2.4 Geology of the Orfée prospects ... 14
2.4.1 Amphibolites ... 14
2.4.2 Graphitic mudrocks ... 16
2.4.3 Banded Iron Formations (BIFs) ... 19
2.4.4 Wackes ... 20
2.4.5 Plagioclase-phyric dioritic dykes... 21
2.4.6 Pegmatitic granites ... 23
2.4.7 Structural analysis ... 23
2.4.8 Mineralization styles ... 29
2.5 Analytical methods ... 32
2.5.1 Electronic Probe Micro-Analysis (EPMA) ... 32
2.5.2 Laser Ablation Inductively Coupled Plasma Mass Spectometry (LA-ICP-MS) ... 32
2.5.3 Lithogeochemistry ... 33
2.5.4 U-Pb geochronology ... 33
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2.6 Analytical Results ... 35
2.6.1 Mineral chemistry: EPMA and LA-ICP-MS ... 35
2.6.2 Thermometry ... 49
2.6.3 Lithogeochemistry ... 52
2.6.3 Hydrothermal alteration geochemistry ... 59
2.6.4 Geochronology ... 62
2.7 Discussion ... 67
2.7.1 Reginal correlation of lithological units ... 67
2.7.2 Paragensis ... 69
2.8 Conclusion ... 75
Chapter 3 . Concluding remarks ... 76
Bibliography ... 78
Appendix 1. Samples characteristics ... 88
Appendix 2. EPMA and LA-ICP-MS analyses data ... 98
Appendix 3. EPMA analyses: classification diagrams ... 137
Appendix 4. EPMA and LA-ICP-MS analyses elementary maps ... 142
Appendix 5. Whole rock geochemistry data ... 151
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List of Figures
Figure 2.1. Regional geological map of the Eeyou Istchee Baie-James region, Québec ... 9
Figure 2.2. Regional geological map of showings and geochronology samples in the Lake Guyer
area... 10
Figure 2.3. Detailed map of the Orfée, Orfée Est, PLE-98-05 and Le Moyne-extension showings
... 11
Figure 2.4. Detailed geological and drill holes map for the Orfée and Orfée Est showings ... 12
Figure 2.5. Detailed geological maps of the Orfée prospect and the Le Moyne-extension showing
... 13
Figure 2.6. Field photographs and photomicrographs of textures and alteration mineral
assemblages in the amphibolites ... 15
Figure 2.7. Graphitic mudrocks field photographs and photomicrographs ... 17
Figure 2.8. Interpreted geology of section E002800 of the Orfée showing ... 18
Figure 2.9. Banded iron formations field photographs and photomicrographs ... 19
Figure 2.10. Wackes field photographs and photomicrographs ... 21
Figure 2.11. Plagioclase-phyric dioritic dykes and pegmatitic granite field photographs and
photomicrographs ... 22
Figure 2.12. D0
/D
1deformation event characteristics ... 24
Figure 2.13. D2
deformation event characteristics ... 25
Figure 2.14. Photomicrographs of metamorphism related textures ... 26
Figure 2.15. D3
deformation event characteristics ... 27
Figure 2.16. Mineral paragenesis for the Orfée gold occurrence ... 30
Figure 2.17. Discrimination diagram for calcic amphiboles ... 35
Figure 2.18. BSE images and photomicrograph for arsenopyrite ... 38
Figure 2.19. EPMA data Fe, S, Ni and As elements diagrams for arsenopyrite... 40
Figure 2.20. Pyrite generations photomicrographs and their respective principal component
analysis (PCA) ... 45
Figure 2.21. LA-ICP-MS data for pyrite at Orfée ... 46
Figure 2.22. Ti (apfu) content in biotite geothermomether ... 51
Figure 2.23. Amphibolites geochemical discrimination diagrams ... 52
Figure 2.24. REE profiles for the lithological assemblage from Orfée ... 54
Figure 2.25. Discrimination diagram for sedimentary units at Orfée ... 56
Figure 2.26. Total alkali vs. silica discrimination diagram ... 57
Figure 2.27. Principal component analysis of major and trace elements for the plagioclase-phyric
dioritic dykes at Orfée ... 57
Figure 2.28. Isocontour maps of alteration minerals on the main trenches at Orfée... 58
Figure 2.29. Isocon diagrams after Grant’s method (1986) for major oxides, trace oxides, S, As,
and Au ... 61
Figure 2.30. Mineralization distribution on the main trenches at Orfée ... Error! Bookmark not
defined.
Figure 2.31. U-Pb geochronology ... 63
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Figure 2.33. Paragenetic sequence resuming the different deformation, metamorphic, alteration
and mineralization events at Orfée ... 74
List of Tables
Table 2.1. Synthesis table for D0
to D
3deformation events affecting the Orfée, Orfée Est and Le
Moyne-extension gold-rich zones. ... 28
Table 2.2. Chemical reactions table. ... 39
Table 2.3. Mean EPMA data for arsenopyrite of resulted from the retrogression of löllingite .... 41
Table 2.4. Mean EPMA data for late disseminated arsenopyrite and arsenopyrite rims around
individual löllingite crystals ... 41
Table 2.5. Mean EPMA data for löllingite ... 42
Table 2.6. Mean EPMA data for pyrrhotite resulted from pyrite breakdown (PO2
) ... 43
Table 2.7. Mean EPMA data for syn metamorphic pyrrhotite... 44
Table 2.8. Mean LA-ICP-MS data for pyrite from Orfée ... 47
Table 2.9. LA-ICP-MS data for pyrite from Orfée (continued) ... 48
Table 2.10. EPMA data for bismuth tellurides. ... 49
Table 2.11. Electron microprobe data for feldspars, amphiboles and garnets. ... 49
Table 2.12. Electron microprobe data for prograde and retrograde biotite ... 50
Table 2.13. U-Pb chronology data for zircons from a plagioclase-phyric dioritic dyke from Orfée
... 65
Table 2.14. U-Pb geochronology data for monazites from the pegmatitic granite ... 66
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Abbreviations
Minerals* Textures*
AB Albite FO Foliated
AC Actinolite GM Medium grained (1 to 5 mm) AS Arsenopyrite PO Porphyritic
Au Native gold VN Veined
Bi2Te3 Bismuth telluride RL Replacement
BO Biotite GF Fine grained (<1 mm)
CC Calcite GG Coarse grained (5 mm to 3 cm)
CB Carbonates PG Pegmatitic CP Chalcopyrite BO Boudinaged CX Clinopyroxene SS Stringer GN Grunerite BR Brecciated GP Graphite MA Massive GR Garnet HB Hornblende LO Löllingite MG Magnetite OL Oligoclase PG Plagioclase PN Prehnite PO Pyrrhotite PY Pyrite QZ Quartz SF Sulfide SR Sericite TL Tourmaline IL Ilmenite SN Titanite PD Pentlandite AP Apatite MS Marcasite MO Molybdenite SP Sphalerite MV Muscovite ML Microcline
*Minerals and textures abbreviations from MERN
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Foreword
Amongst the many people who supported me throughout this project, I am particularly grateful to my research supervisor François Huot for all his patience, availability and cheerfulness. Working with such a passionate and polyvalent person made me realise that accepting our fears and weaknesses in front of challenge is the path to reaching our full potential. Special thanks to Jean Goutier, of the Ministère de l’Énergie et des Ressources Naturelles du Québec (MERN), for believing in this project, for introducing me to the geological mysteries of the Eeyou Istchee Baie-James area and for passing on his passion. I would also like to thank my co-supervisor, Carl Guilmette, for his constructive comments, encouraging and inspiring words. Georges Beaudoin is also greatly thanked for his constructive comments and scientific input.
This project was entirely supported by MERN, who provided financial and logistical accommodations during two summers of field work, as well as during analytical procedures. Pénélope Burniaux and Joséphine Gigon from the MERN are especially thanked for their technical and logistical support during the summer’s field work campaigns. Special thanks to the Vital Pearson from Osisko Exploration James Bay for technical and scientific support. Many thanks to Marc Choquette, responsible for the Université’s Laval Microprobe Laboratory, to Dany Savard and Marco Prasek from UQAC’s Earth Material Laboratory, and to Jonathan Gagnon from the IRMS Laboratory at ENS de Lyon (Écoles normales supérieures de Lyon) operated by Dr. Jean-Éric Tremblay. Thank you to Don Davis, of the University of Toronto’s Geochronology Lab, and to Robert Creaser from the University of Alberts’a Canadian Centre for Isotopic Micronalysis.
Special thanks to the MERN for granting me financial support, and to DIVEX (Diversification de l’Exploration Minérale au Québec), to the department of Géologie et Génie Géologique and AQUEST for granting me scholarships. I would also like to thank the Geological Association of Canada – Mineralogical Association of Canada for granting me a field trip scholarship.
Thank you to Benoît Dubé for his interest, positive criticism and encouragement. Also, special thanks to Arnaud Fontaine, Donald Grezla, Roman Hanes, Marjorie Sciuba, Nathan Cleven, and William Oswald for constructive comments, scientific input and interest in my project. Many thanks to Pierre Therrien and Martin Plante form the Géologie et Génie Géologique department for their technical support. Thank you to Guylaine Gaumond, Fritz Neuweiler, Marcel Langlois, Julia Lebreux and other Géologie et Génie Géologique department staff for their availability, kindness and understanding. Finally, many thanks to my family, friends and to my other-half, Vincent, for their moral support and understanding troughout the whole duration of this project.
The author of this dissertation wrote the entire article presented as the Chapter 2. The Orfée prospect: a Neoarchean orogenic gold occurrence along the contact between the La Grande and Opinaca subprovinces (Eeyou Istchee Baie-James, Quebec, Canada). The co-authors are François Huot, Robert Creaser, Carl
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Guilmette, Jean Goutier, Georges Beaudoin and Donc Davis. François Huot is my research supervisor, Robert Creaser conducted the Re-Os dating on arsenopyrites, Carl Guilmette and Georges Beaudoin are my cosupervisors, Jean Goutier is an Emeritus geologist for the MERN and Don Davis conducted the U-Pb dating on zircons and monazites.
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To my grandmothers
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Chapter 1 . Introduction
Exploration in the La Grande Subprovince began in the early 1900s, while in the mid-90s the MERN emplaced a program aiming to help exploration in the mid-north of Québec. A new exploration wave was dictated by the discovery of the Roberto gold deposit (Ravenelle et al., 2009, 2010, 2013; Fontaine et al., 2015) along the La Grande and Opinaca subprovinces contact. Both La Grande and Opinaca subprovinces are located in the eastern Superior Province, the largest Archean craton in the world (e.g. Lucas and St-Onge, 1998; Goutier et al., 2001). La Grande Subprovince is composed of a tonalitic basement overlain by sedimentary units and felsic to ultramafic volcanic rocks (Stamatelopoulou-Seymour and Francis, 1980a and 1980b; Goutier et al., 1999b; Bandyayera et al., 2014). Opinaca Subprovince has been interpreted as a metamorphosed sedimentary basin (Card et Ciesielski, 1986). A subprovince is a subdivision of a geological province, and is characterized by similar lithological, structural and metamorphic features (Lucas and St-Onge, 1998). The contact between the La Grande and Opinaca subprovinces was already known as a metallotect for gold exploration in the Eeyou Istchee Baie-James area, with over 150 gold occurrences discovered on the northern contact only. Amongst mineralization styles discovered along this major first-order structure are BIF hosted epigenetic gold (Larocque et al., 1999; Richer-Laflèche et al., 2000; Goutier et al., 2002), shear-zone hosted gold (Aucoin et al., 2012), volcanogenic massive sulphide potential (Richer-Laflèche et al., 2000), carbonate veins hosted gold (Beauchamp et al., 2015), stockwork of quartz-dravite veinlets with phlogopite-sulfides, replacement zones associated with microcline-phlogopite-dravite-sulfides, quartz-diopside-schorl-arsenopyrite veins, quartz-feldspathic veinlets, high-grade quartz veins, high-high-grade paragneiss (Fontaine et al., 2015) and auriferous pegmatitic dykes (Ravenelle et al., 2010; Fontaine et al., 2015). Recent studies (e.g. Côté-Roberge et al., 2016; Gigon and Goutier, 2017) established that the La Grande-Opinaca contact is a boundary between two distinct lithological and structural domains. The contact between the La Grande and the Opinaca subprovinces could be a key player in the concentration of gold-rich fluids. The present thesis is a metallogeny study, part of a broader research group aiming to understand the tectono-metamorphic conditions and their implication for gold mineralization in the La Grande and the Opinaca subprovinces, with particular focus set on the La Grande-Opinaca subprovinces contact.
The Orfée, Orfée Est and Le Moyne-extension gold occurrences are located in the Archean Superior Craton, and are hosted by the La Grande Subprovince, roughly 300 m north of the contact with the Opinaca Subprovince, in the Eeyou Istchee Baie-James region (Québec) (Fig. 2.1). In the Orfée area, multiple showings have been discovered, but for this study, emphasis was placed on Orfée and Orfée Est mineralized zones, and on C and C98-13 trenches of the Le Moyne-extension showing (Fig. 2.2). In the Poste Le Moyne region (Fig. 2.2), the geology is characterized by foliated and gneissic tonalites of the Langelier Complex (3452 Ma to 2788 Ma; Goutier et al., 2002; Davis et al., 2014) and the Poste Le Moyne Pluton (2881 ±2 Ma;
2
Goutier et al., 2002) interpreted as an Archean basement on top of which sedimentary units and felsic to ultramafic volcanic rocks were deposited. Multiple volcano-sedimentary belts have been identified in the northern part of the La Grande Subprovince (Stamatelopoulou-Seymour and Francis, 1980a and 1980b; Goutier et al., 1999a and 1999b; Bandyayera et al., 2014). Amongst these, the Guyer Group greenstone belt was dated from 2820 to 2806 Ma (David et al., 2012). The basaltic amphibolites of this greenstone belt have been previously described in the Orfée showing area (Goutier et al., 2001). Mineralization at Orfée is associated with oxide and silicate facies iron formations interbedded with sulphide-rich graphitic mudrocks, bordered by amphibolites to the north and by wackes to the south. The characteristics and styles of mineralization and alteration suggest the Orfée showing is an orogenic gold deposit, emplaced in a dynamic environment, metamorphosed at lower to upper amphibolite facies. Many other gold occurrences have been discovered along the La Grande-Opinaca contact, and could represent an important and complex metallotect for gold exploration. Additionally, the abundant surface and drilling data available in the study area offers a good opportunity to understanding the gold mineralizing events along this contact.
1.1 Context
Orfée, Orfée Est and Le Moyne-extension showings (Fig. 2.3) are located ~160 km southeast of Radisson, and 450 km north-northeast of Matagami, on the territory of the Eeyou Istchee Baie-James municipality, Québec. In 1998, Mines d’Or Virginia exposed the Orfée mineralized zone in the northeast of the 33G06 NTS map (Costa, 2000; Cayer, 2003). Several drilling campaigns followed from 2002 to 2007, which led to the discovery of the Orfée Est mineralized zone, as well as to a resource estimation of 203,483 tons at 14.5 g/t Au, for a total of 95,000 oz of gold at Orfée (Cayer, 2003). The Orfée mineralized zone is accessible by helicopter or by ATV following a 3.5 km trail linked to the Transtaïga road near km 176. The C and C98-13 trenches are also accessible along the same path. The Orfée Est mineralized zone, which has only been intercepted by drill core, is covered by Quaternary sediments and soil with an average thickness of 30-35 metres.
1.2 Problematics and Objectives
As previously discussed, many gold occurrences were discovered along the La Grande-Opinaca subprovinces contact in the past 20 years. Because of its proximity to the contact (roughly 300 m), the Orfée showing represents a rare window into the tectono-metamorphic conditions along this major lithological and metamorphic interface. Moreover, the banded iron formations hosting most of the mineralization at Orfée are ductile rocks that have recorded the quasi-complete complex tectonic history of the area. In such, they offer a good glimpse of the tectonic and metamorphic conditions into which the gold mineralization was emplaced. Furthermore, the lithological assemblage at Orfée is much diversified compared to other showing in the vicinity, and offers the possibility of understanding how alteration, mineralization and deformation are
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controlled by the chemical and physical characteristics of each lithological unit. Rocks are well exposed on the trenches at Orfée and Le Moyne-extension prospects, while abundant drill core data is available from Orfée, Orfée Est, and Le Moyne-extension mineralized zones. Such well exposed rocks and complete drill core sections make of Orfée, Orfée Est and Le Moyne-extension ideal study areas in order to understand spatial distribution of mineralization in this dynamic, high-grade metamorphic environment. The work done as part of this current project aims at:
- Characterizing the magmatic affinity of the lithological assemblages hosting the mineralization, and to integrate them within the already-defined regional lithologies;
- Understanding the spatial distribution of the gold mineralization and its association with different mineralogical assemblages in an orogenic context;
- Characterizing the alterations and defining their temporal relationships to the mineralization; - Defining the paragenetic sequence of the mineralization emplacement, and link it to the hosting
lithologies;
- Constrain the mineralization through U-Pb and Re-Os dating; - Conceive a metallogenic model for the Orfée gold prospect.
1.3 Regional geology
The La Grande and Opinaca subprovinces (Québec) are part of the Superior Province which comprises rocks dated from 3.8 to 2.57 Ga, deformed by multiple phases of the Kenoran Orogeny (Thurston, 1991; Hocq, 1994). The La Grande Subprovince is a volcano-plutonic assemblage composed of an Archean tonalitic basement, volcano-sedimentary sequences and a variety of ultramafic to felsic intrusions (Fig. 2.2). The tonalitic basement belongs to the Langelier Complex and to the poste Le Moyne Pluton. Metamorphic grade increases from west to east in the La Grande Subprovince (Goutier et al., 2002; Gauthier et al., 2007). The Opinaca Subprovince is a metamorphosed sedimentary basin, and comprises metasedimentary and plutonic rocks (Card et Ciesielski, 1986). In general, the metamorphic grade increases from the edges towards the center of the Opinaca Subprovince, with greenschist metamorphosed rocks at the borders and granulite facies towards the centre (Goutier et al., 2002; Gauthier et al., 2007). According to Goutier et al. (2002), these subprovinces might have similar counterparts in Ontario; the La Grande Subprovince resembles the Sachigo-Uchi-Wabigoo Subprovince, while the Opinaca Subprovince shares similarities with the English River and Quetico subprovinces. At its northern contact with the La Grande Subprovince, in the Guyer Lake area, the Opinaca Subprovince comprises biotite paragneiss of the Laguiche Complex (Fig. 2.2) injected by multiple tonalitic to granitic intrusions. Located in the Guyer Lake area, the Orfée prospect - the subject of this current study – comprises Orfée, Orfée Est and Le Moyne-extension mineralized sites. These gold occurrences are located 300 m north of the inferred contact between these two subprovinces (33G06 NTS map).
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According to Stamatelopoulou-Seymour and Francis (1980a and 1980b), Stamatelopoulou-Seymour et al. (1983) and Goutier et al. (2002), in the Guyer Lake area, three major domains can be distinguished based on lithology, metamorphic facies and structure. The northern and central domains are located in the La Grande Subprovince whereas the southern domain is located in the Opinaca Subprovince. The northern domain comprises plutonic and gneissic rocks, the central domain contains a volcano-sedimentary sequence, and the southern domain includes metasedimentary and plutonic rocks. Rocks of the volcano-sedimentary belt in the central domain were attributed to the Guyer Group (Fig. 2.2) greenstone belt by Goutier et al. (2002). This volcano-sedimentary group (2820 to 2806 Ma; David et al., 2012) comprises mostly metabasalts, with local metavolcanites of intermediate composition, a felsic tuff unit, layered iron formations and wackes interstratified with volcanics. Locally, magnesian basalts, komatiites and ultramafic wackes are present (Stamatelopoulou-Seymour et Francis, 1980a and 1980b; Stamatelopoulou-Seymour et al., 1983; Goutier et al., 2002). The age of this group contrasts with the Yasinski Group which is composed of younger rocks (2751 to 2725 Ma; Richer-Laflèche et al., 2000; Goutier et al., 2003). Moreover, the Guyer Group contains ultramafic volcanic rocks which have not yet been identified in the Yasinski Group. The basement and the volcano-sedimentary belt were injected by the Duncan tonalitic to dioritic intrusions (Fig. 2.2). The youngest Archean intrusions in the Guyer Lake area, which cut across both subprovinces, belong to the Bezier Suite (Fig. 2.2) and to the Vieux-Comptoir Granite (Fig. 2.2).
Preliminary U-Pb dating of detrital zircons from the Opinaca Subprovince suggested these rocks are younger than the La Grande Suprovince lithological assemblages (Goutier et al., 2002). However, new U-Pb dating allows new time constraints on the deposition age of these sediments. The crystallization of the Frégate Pluton has recently been dated at 2710 Ma (Augland et al., 2016), giving a minimum age for the paragneiss unit into which it is hosted. On the other hand, the 2697 Ma age obtained from detrital zircon crystals in the Lac Bonfait conglomerate, also part of the Opinaca basin, gives a maximum age of deposition for this sedimentary unit (David, 2016, unpublished).
1.4 Presentation of the article
The second chapter of this memoir is composed of the article “The Orfée prospect: a Neoarchean gold occurrence along the contact between the La Grande and Opinaca Subprovinces (Eeyou Istchee Baie-James, Québec, Canada)”. The article will be submitted for publication to Mineralium Deposita scientific journal. The co-authors are François Huot, Don Davis, Robert Creaser, Carl Guilmette, Jean Goutier and George Beaudoin. François Huot is this project’s supervisor, Don Davis conducted the U-Pb dating on zircons and monazites, Robert Creaser conducted the Re-Os dating on arsenopyrite and Carl Guilmette, Jean Goutier and Georges Beaudoin are the co-supervisors of this project. The article was entirely written by this thesis author. Geochemical data from Bandyayera et al. (2014) and Richer-Laflèche et al. (2000) were added for comparison on discrimination diagrams and REE profiles.
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The article begins by a brief introduction to the Eeyou Istchee Baie-James area geology, with emphasis on the contact between the La Grande and Opinaca subprovinces. In order to better expose the tectono-metamorphic environment, the volcano-sedimentary sequence hosting the Orfée prospect is described in detail. A section on analytical methods is followed by the description of the analytical results. The analytical results comprise electron microprobe data, laser ablation inductively coupled plasma mass spectrometry, lithogeochemistry of hosting lithologies, and isotope dating of intermediate and felsic injections by the U-Pb method and of arsenopyrite by the Re-Os method. In the discussion, the Orfée’s hosting lithologies are correlated with regional observed lithologies, the genesis of the sulfidic phases associated with mineralization is interpreted, and the complete paragenetic sequence for Orfée’s mineralization emplacement is constructed.
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Chapter 2 . The Orfée prospect: a Neoarchean orogenic gold
occurrence along the contact between the La Grande and Opinaca
subprovinces (Eeyou Istchee Baie-James, Québec, Canada)
2.1 Abstract
The contact between the La Grande and the Opinaca subprovinces has been a known gold metallotect since the mid-1900s, but only since the “Moyen-Nord” mineral exploration program in 1995-1998 exploration and research along this major interface has been given more attention. Over 150 gold occurrences were exposed along the northern contact between the La Grande and Opinaca subprovinces. Part of these discoveries, the Orfée showing, is located in the La Grande Subprovince, 300 m north of the contact with the Opinaca Subprovince. The mineralized zones comprise the Orfée orebody (~0.2 Mt at 14.5 g/t Au), as well as Orfée Est and Le Moyne-extension gold zones. Gold-rich zones are hosted by highly metamorphosed and deformed Archean oxide and silicate banded iron formations (BIFs) and sulfide-rich graphitic mudrocks, enclosed by amphibolites to the north and by wackes to the south.
Moderately to steeply north dipping mineralized zones follow the geometry of the oxide and silicate facies iron formations and proximal bordering lithologies. At Orfée, the gold mineralized envelope (0.5 to 93.7 g/t Au) is up to 460 m deep, ~100 m long and reaches a maximal thickness of 40 m. Mineralization consists of disseminated to massive pyrrhotite, 5% arsenopyrite and löllingite, 1% chalcopyrite, 1% native gold and trace amounts of pyrite. Disseminated to massive pyrrhotite are concentrated in D2 structural traps,
such as saddle reef and leg reef veins and quartz-pyrrhotite extension veins in necks. WNW-ESE gold-rich shear zones contain disseminated pyrrhotite, gold-bearing arsenopyrite and löllingite, and chalcopyrite. Such shear zones most likely acted as conduits for mineralized fluids, focussing them in reducing rocks of the BIFs and graphitic mudrocks. Calc-silicate and quartz-tourmaline veins host pyrrhotite, arsenopyrite-löllingite, and trace amounts of pyrite, chalcopyrite and gold. These veins, injected as far as ~200 m from the amphibolite-BIFs contact, are folded, sheared and overprinted by M2 metamorphic peak conditions (600 to
650°C for 5.3 to 5.5 kbars).
Native gold inclusions in pyrrhotite and metamorphic silicates, pyrite relicts in pyrrhotite (<500°C) and gold bearing löllingite suggest gold mineralization was emplacement pre to syn metamorphism. The mineralization emplacement was restraint between 2703 ±7 Ma, U-Pb zircon age of a plagioclase-phyric dioritic dyke crystallized pre to syn mineralization, and 2613 ±0.4 Ma, U-Pb monazite age of a post mineralization pegmatitic granite dyke. Two weighted average model ages of 2582 ±13 Ma and 2557 ±12 Ma for gold-bearing composite arsenopyrite/löllingite were obtained by Re-Os dating. These ages represent the retrograde inversion of löllingite to arsenopyrite during a late M3 event. Native gold associated with
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chlorite, epidote, sericite and prehnite veinlets along fractures also suggest gold remobilization during retrograde metamorphism.
Mineralization styles at Orfée are varied, comprising orogenic gold metamorphosed calc-silicate veins, sulfide accumulations in dilatant regions, shear-zone hosted mineralization and native gold remobilized in fractures. Gold from the Orfée showing is orogenic (2703 Ma to 2613 Ma), pre to syn M2
metamorphism, and remobilized (2582 Ma to 2557 Ma) during late M3 retrograde conditions.
Keywords: Orogenic Gold • Archean • Superior Province • Banded Iron formations • Amphibolite • Plagioclase-phyric-dioritic dykes • La Grande • Opinaca • LA-ICP-MS • U-Pb • Re-Os
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2.2 Introduction
Orogenic gold deposits are commonly spatially associated with large-scale tectonic boundaries between two different lithological and/or metamorphic domains. Worldwide, well documented first-order structures with clustered orogenic gold deposits include the Larder Lake, Cadillac and Porcupine-Destor faults (Superior Province, Canada), Boulder-Lefroy shear zone (Kalgoorlie gold field, Yilgarn craton), Champion Reef system (Kolar gold field, Dharwar craton), Obuaski-Ashanti shear zone (Ashanti gold belt, West African craton), Campbell-Giant shear zone (Yellowknife gold district, Canadian Shield), Tamdytau-Sangruntau fault system (Central Asian orogeny, Uzbekistan), Melones fault zone (Mother Lode belt, central California), Tan-Lu fault system (Jiaodong Peninsular, eastern North China craton), and Fanshaw-Sumdum fault system (Juneau gold belt, southern Alaska) (Hodgson, 1989; Robert and Poulsen, 2001; Groves et al., 2003; Goldfarb et al., 2005). More than 50% of the Precambrian gold is hosted by subgreenschist to granulite metamorphosed orogenic gold deposits (Groves et al., 2003) of late Archean greenstone belts of the Yilgarn craton and the Superior Province (Goldfarb et al., 2005). In the Superior Province (Fig. 2.1), many of these orogenic gold deposits formed in the vicinity of major faults and shear zones, such as the Porcupine-Destor, Larder Lake-Cadillac breaks, in the Abitibi gold belt, Québec and Ontario (e.g. Card and Ciesielski, 1986; Card and Poulsen, 1998).
Also located in the Superior province (4.3 to 2.6 Ga; Percival et al., 2012), roughly 500 to 600 km north of the Abitibi Subprovince, the contact between the La Grande and Opinaca subprovinces is a major boundary separating two distinct lithological and structural domains (Gauthier, 2000; Goutier et al., 2002; Robertson, 2005; Gauthier et al., 2007). Many gold prospects discovered in the La Grande and Opinaca subprovinces occur as clusters along the contact between the two domains, and could have a close temporal and genetic association with this lithological and metamorphic boundary (Gauthier et al., 2007; Ravenelle et al., 2010; Aucoin et al., 2012; Fontaine et al., 2015). These gold occurrences (Fig. 2.1) include the Marco zone (Aucoin et al., 2012), Ilto and David showings (Cayer, 2011), Zone 32 (Mercier-Langevin et al., 2012), the La Pointe prospect (Fleury et al., unpublished), the Roberto deposit (Ravenelle et al., 2009, 2010; Fontaine et al., 2015), the Cheechoo zone (Fontaine et al., 2017a and 2017b) and the Mustang prospect (Beauchamp et al., 2015). Compared to the large, greenschist facies volcano-sedimentary sequences of the Abitibi Subprovince, those included in the La Grande Subprovince have been seen less exploration activity in the past due to their remoteness, smaller extensions and amphibolite to granulite facies metamorphic overprint. Since the “Moyen-Nord” mineral exploration program in 1995-1998 (e.g. Beaumier et al., 1994; Chartrand and Gauthier, 1995) and the discovery of the Roberto gold deposit (Éléonore Mine) in 2004 (Ravenelle et al., 2009, 2010; Fontaine et al., 2015, 2017a and 2017b), exploration and research along this major structure have received far more attention.
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On-going studies (this study; Hanes et al., 2016; Fleury et al., 2016) aim at understating the role of the major La Grande – Opinaca metamorphic and lithological boundary in gold introduction, distribution and
Figure 2.1. Regional geological map of the Eeyou Istchee Baie-James region, Québec. Modified from Houlé et al. (2015).
1– Marco zone; 2 – Ilto; 3 – Orfée, Orfée Est and Le Moyne-extension; 4 – David; 5 – Zone 32; 6 – La Pointe prospect (Zones 25 and 26); 7 – Roberto deposit; 8- Cheechoo zone; 9 – Wabamisk-Anatacau; 10 – Eastmain.
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concentration in the Eeyou Istchee Baie-James region. The focus of this paper concerns the metallogeny, metamorphic and tectonic study of the Orfée gold occurrences, exposed in the Guyer Lake area. The Orfée gold occurrences include the Orfée prospect, the nearby Orfée Est and Le Moyne-extension showings (Figs. 2.2 and 2.3). Data collected from the Orfée prospect during several field campaigns (1998 to 2007) by Virginia Mines, was used in a resources estimates of 203,483 tons at 14.5 g /t Au, for a total of 95,000 oz
(Cayer, 2003). Because of their proximity to the contact (roughly 300 m), Orfée, Orfée Est and Le Moyne-extension showings represent a rare window into the tectono-metamorphic conditions along the contact between the La Grande and Opinaca subprovinces. Outcrop exposure, trenches and abundant drill core data, make these showings a favorable environment to study and understand the spatial distribution of mineralization in a high-grade metamorphic environment.
Figure 2.2. Regional geological map of showings and geochronology samples in the Lake Guyer area (33G05, 33G06
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2.3 Regional Geology
The lithological assemblage of the La Grande Subprovince comprises an ancient tonalitic basement, volcano-sedimentary sequences and multiple felsic to ultramafic intrusions. The tonalitic basement has been interpreted as belonging to the Langelier Complex (3452 Ma to 2788 Ma; Goutier et al., 2002; Davis et al., 2014) and the poste Le Moyne Pluton (2881 ±2 Ma; Goutier et al., 2002), onto which were deposited sedimentary units and ultramafic to felsic volcanic rocks (Fig. 2.2). At least four different greenstone belts have been identified in the northern La Grande Subprovince (Fig. 2.1). The volcanic rocks in the Orfée showing area have been interpreted as belonging to the Guyer Group (Goutier et al., 2002; Bandyayera et al., 2014). A recent study highlighted geochemical differences between the amphibolites from Orfée and those of the Guyer Group, suggesting the volcanic rocks from Orfée might belong to a different unit (Bogatu et al., 2016). The Guyer Group (Fig. 2.1), dated from 2820 to 2806 Ma (David et al., 2012), consists of ultramafic to intermediate volcanic rocks, felsic tuffs, iron formations and wackes. Ultramafic to felsic volcanic rocks and ultramafic wackes are interstratified with the wacke unit (Goutier et al., 2002). The basement and
volcano-sedimentary units of the La Grande Subprovince have been injected by the tonalitic to dioritic Duncan Intrusions. The youngest intrusions in the region belong to the Bezier (2674 ±12 Ma and 2708.5 +3.6/-2.9 Ma; Goutier et al., 2002; David, unpublished, 2015) and Vieux Comptoir granitic Suite (pegmatitic phase, 2618 to 2013; Goutier et al., 2002; this article); both of them intruded the La Grande and Opinaca subprovinces. The lithological assemblage of the La Grande Subprovince is comparable to those in the Sachigo-Uchi-Wabigoo subprovinces in the northwestern Ontario.
Figure 2.3. Detailed map of the Orfée, Orfée Est, PLE-98-05 and Le Moyne-extension showings in the 33G06
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The northern portion of the Opinaca Subprovince comprises the Laguiche Complex biotite paragneiss and tonalitic to granitic injections. U-Pb dating on detrital zircons from the Opinaca Subprovince indicates younger ages than the La Grande Subprovince lithological assemblage, with a minimum age of 2710 Ma (David, unpublished, 2014). The Opinaca Subprovince is older than 2637 Ma (Morfin et al., 2013), the age of zircons crystallized in leucogranites cutting across the metamorphosed sediments of the Opinaca Subprovince. This latter age is most likely associated with the last migmatization event. The Opinaca Subprovince is composed of a strongly metamorphosed metasedimentary and plutonic assemblage comparable to those in the English River and Quetico subprovinces in Ontario (Goutier et al., 2002; Stott et al., 2010). Both La Grande and the Opinaca subprovinces have been metamorphosed during the Kenorean
orogeny (2720 to 2660 Ma; Thurston, 1991; Hocq, 1994). Along the northern La Grande-Opinaca contact (Fig. 2.1), the metamorphic grade increases from west to east, reaching upper amphibolite facies in the Orfée prospect region. Roughly 100 km west of Orfée, host rocks of the Zone 32 gold occurrence, located in the Yasinski Group, reached greenschist metamorphic conditions (Goutier et al., 1998a, 2000, 2003; Gauthier et
Figure 2.4. Detailed geological and drill holes map for the Orfée and Orfée Est showings. True projections
of the drill holes are represented. Only drillholes considered in this study are projected in this figure. The coordinates are in UTM NAD83, zone 18, easting, northing.
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Figure 2.5. Detailed geological maps of the Orfée prospect (A and B) and the Le Moyne-extension
showing (C and D). A) TR-01-01, TR-01-02 and TR-01-03 trenches; B) B trench ; C) C trench; D) C98-13 trench. Modified from Tremblay (2004). Coordinates are in UTM NAD83, zone 18, easting, northing. See Figure 2.3 for the localisation of trenches. Mineral and textures abbreviation are given at the beginning of this document in the Abbreviations table.
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al., 2007; Mercier-Langevin et al., 2012). Metamorphic grade increases from the outward portions of the Opinaca Subprovince towards its centre, ranging from lower amphibolites to granulite facies (Côté-Roberge, 2016; Gigon and Goutier, 2017, and references in there).
2.4 Geology of the Orfée prospects
The Orfée gold prospects (Orfée, Orfée Est and Le Moyne-extension) are defined by scattered zones of disseminated to massive sulfides, confined at the contact between BIFs and amphibolites (Fig. 2.5). These discontinuous gold-mineralized zones have a lateral distribution of roughly 2 km (Orfée to Le Moyne-extension distance; Fig. 2.3). Most of the gold is concentrated within the BIFs and graphitic mudrocks, located on either side of the BIFs, but a mineralization halo spreads within the proximal amphibolites and wackes. At Orfée, the mineralized halo (0.5 to 1 g/t Au) reaches a maximum width of 30 m, is 350 m deep and continuous on 100 m, as calculated from drill core data.
2.4.1 Amphibolites
The amphibolites are in faulted contact with the structurally over-lying BIFs (Fig. 2.5). They are fine to medium-grained (< 3 mm) and are mostly composed of hornblende and plagioclase, with traces of ilmenite, magnetite, sulfides, titanite and löllingite. Sulfides comprise mostly pyrrhotite, with trace amounts of chalcopyrite, pyrite and arsenopyrite. Massive and pillowed facies compose this unit, with local occurrences of interbedded glomeroporphyritic facies. Flattened pillows and metamorphic hornblende and plagioclase alignment suggest they were affected by major deformation. The amphibolites are affected by various hydrothermal alteration assemblages, including garnet, hornblende and sulfides haloes (Fig. 2.6A), calc-silicates and tourmaline replacement zones (Fig. 2.6B and C), biotite-oligoclase ±albite haloes (Fig. 2.6D), oligoclase-albite-rich shear zones (Fig. 2.6E) and late sericite, epidote, chlorite and prehnite. Gold mineralized quartz-calc-silicate (Fig. 2.6F) and quartz-tourmaline veins, gold-rich shear zones and disseminated sulfides extend into the amphibolites up to 100 m from the contact with the BIFs.
Garnet-hornblende ±sulfide haloes
Penetrative alteration haloes affect the amphibolites and are characterized by garnet-hornblende-plagioclase-sulfide ±magnetite mineral assemblage (Fig. 2.6A). In drill core, the alteration haloes were noted as far as a few tens of meters away from the BIFs (~100 m). The irregular character and discordance in respect to the main foliation, suggest these haloes predate deformation. Inclusions in euhedral garnets comprise pyrrhotite, chalcopyrite, magnetite, titanite, quartz, plagioclase, hornblende and rare pyrite. The inclusions very fine grained (<0.2 millimeters) compared to the minerals composing the matrix (~1-2 millimeters). Brittle deformed garnets have thin inclusion-free rims, and prehnite-calcite-sulfide and sulfide
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veinlets precipitate along the fractures. Hornblende in these alteration selvages is euhedral to subhedral, dark-green, while in non-altered samples, euhedral hornblendes have a medium-green tint.
Figure 2.6. Field photographs and photomicrograph of textures and alteration mineral assemblages in the amphibolites.
A) Garnet-hornblende-plagioclase-sulfide ±magnetite irregular alteration haloes. B) Calc-silicate replacement zone. C) Tourmaline-calc-silicate alteration zone. D) Biotite-oligoclase ±albite alteration zones, contemporaneous to calc-silicate alteration E) Oligoclase-albite-rich shear zone affecting the amphibolites and calc-silicate-rich zone. F) Composite löllingite-arsenopyrite in quartz-calc-silicate vein.
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Calc-silicates and tourmaline replacement zones
Calc-silicate and tourmaline replacement zones (Fig. 2.6B and C) are proximal (0 to 10 m) to the mineralized zones. The mineralogical assemblage associated with the calc-silicate-rich zones comprise Ca-amphiboles, clinopyroxene, garnet, plagioclase and sulfide. Tourmaline-rich zones are rare at Orfée, but abundant at Orfée Est. Calc-silicates alteration haloes are proximal (0 to 1 m) to mineralized quartz-calc-silicate veins (Fig. 2.6F). Garnet crystals in the calc-quartz-calc-silicate replacement zones do not have any internal record of deformation, and contain very little inclusions. Calc-silicates (e.g. clinopyroxene, garnet) are commonly part of the proximal alteration mineral assemblage in orogenic gold deposits metamorphosed at amphibolite to granulite facies (Nguyen et al., 1997; Bodon, 1998; Eilu et al., 1999; Oswald et al., 2015).
Biotite-oligoclase ±albite haloes
Alteration haloes comprising biotite ±oligoclase ±albite are intermediate to distal (0 to 5 m) to the mineralized quartz-calc-silicate veins (Fig. 2.6D). Overprinting metamorphism makes it difficult to distinguish cross-cutting relations with the calc-silicate rich zones (Fig. 2.6D). The biotite-rich alteration haloes are concordant to the main foliation and are folded during subsequent deformation. Affected samples contain up to 20 % biotite.
Oligoclase-albite-rich shear zones
Amphibolites affected by gold-rich shear zones have a whitish and leached aspect due to high oligoclase, albite and quartz content. The shear zones have a WNW-ESE orientation and they overprint the calc-silicate replacement zoens (Fig. 2.6E). Gold in the shear zones occurs as free grains or associated with löllingite and arsenopyrite. A 1 m wide albite-rich shear zone affects both graphitic mudrocks and amphibolites at the contact between these two units.
2.4.2 Graphitic mudrocks
Fine grained (<0.5 mm) recrystallized graphite-rich mudrocks (Fig. 2.7), up to 2.5 meters thick, occur mostly on the boundaries of the iron formations. In the main trench at Orfée, this unit is preferentially sheared and sulfidized (Fig. 2.7A) in a WNW-ESE corridor, as it is bordered by more competent banded iron formations and amphibolites (Fig. 2.5A). The graphite-rich mudrocks are composed of fine to very fine-grained quartz, feldspar, biotite, graphite, pyrrhotite, hornblende and <1% arsenopyrite, löllingite, chalcopyrite and pyrite. Trace amounts of garnet, titanite, magnetite and ilmenite are found. Graphite occurs as flakes (<10%, 0.05 to 0.15 mm) and is homogeneously distributed in the matrix. The crystals are aligned along the main WNW-ESE foliation (Fig. 2.7B). Native gold was observed in arsenopyrite (Fig. 2.7C), along with disseminated to massive pyrrhotite, and in boudinaged and deformed quartz-sulfide veins (Fig. 2.7D to F).
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Figure 2.7. Graphitic mudrocks field photographs and photomicrographs. A) Interface between the amphibolites and
BIF, defined by a sheared graphitic mudrocks layer. A leached amphibolite enclave is hosted by the mudrocks. B) Graphite flakes and pyrrhotite define the main foliation. C) Gold-bearing arsenopyrite, adjacent to pyrrhotite. Löllingite remnants were also noted. D) Boudinaged quartz-sulfide vein. E) Folded sulfide-quartz vein. F) Quartz-chalcopyrite-pyrrhotite vein with visible gold
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Figure 2.8. Interpreted geology of section E002800 of the Orfée showing. Section looking west. Modified from Cayer,
2007. Detailed structural features of the Orfée deposit are discussed in section 2.5.7. Structural analysis. A=PLE98-02, B=PLE02-14, C=PLE02-21, D=PLE021-20, E=PLE02-24, F=PLE-02-31, G=PLE02-49, H=PLE06-87, I=PLE03-60, J=PLE06-89; 1=PLE98-02-14.9, 2=PLE98-02-15.5,3=PLE98-02-20.4, 4=PLE98-02-27, 5=PLE02-14-87, 6=PLE02-14-91, 7=PLE02-14-106, 8=PLE02-21-56.8, 9=PLE02-21-72.3, 10=PLE02-21-102.4, 11=PLE02-21-111, 12=PLE02-21-114,8, 13=PLE02-21-124.6, 14=PLE02-21-127, 15=PLE02-21-148.5, 16=PLE-02-20-130, 17=PLE02-20-140, 18=PLE-02-20-154.75, 19=PLE02-20-162.75, 20=PLE02-20-166.75, 21=PLE02-24-182.5, 22=PLE02-24-190.5, 23=PLE02-31-20.5, 24=PLE02-31-22, 25=PLE02-31-141, 26=PLE02-31-241.6, 27=PLE02-49-25.2, 28=PLE02-49-102.4, 29=PLE02-49-162.45, 30=PLE02-49-241.6, 31=PLE02-49-321.38, 32=PLE02-49-332, 33=PLE02-49-337, 34=PLE02-49-344.3, 35=PLE06-87-154, 36=PLE06-87-208.9, 37=PLE06-87-366.8, 38=PLE06-87-370, 39=PLE06-89-400.71.
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2.4.3 Banded Iron Formations (BIFs)
The banded iron formations at Orfée, Orfée Est, and Le Moyne-Extension showings correspond to a discontinuous unit of 2 km, reaching a maximal continuous lateral extension of 250-300 m, with an apparent thickness of 30 m at the Orfée Est zone. At Orfée, the BIF reaches a maximum thickness of 5 to 20 m with a continuous lateral extension of 25-30 m, while the mineralized envelope (0.5 to 93.7 g/t Au) is up to 460 m
Figure 2.9. Banded iron formations field photographs and photomicrographs. A) Layered BIF, with boudinaged iron
oxides layers replaced by stratabound sulfides; quartz flooding and sulfides in boudin necks. B) Calc-silicate alteration in a silica-rich BIF facies; the alteration is concentrated in a fold hinge. C) Ferro-(calc)-silicate facies of the BIF replaced by massive pyrrhotite, with visible gold inclusiosn. D) Saddle reef hosted pyrrhotite in silicate facies BIF. E) Late, remobilized gold in a chlorite veinlet along a fracture in hornblende. F) Grunerite-sulfide replacement of hornblende in the ferro-(calc)-silicate facies of the BIF. The altered layers are subsequently folded.
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deep, ~100 m long and reaches a maximal thickness of 40 m (Fig. 2.8). The iron formations comprise mostly oxide and silicate facies, with up to 20% replacement by sulfide minerals. The thickness of each individual mineral layer varies from 2 to 5 cm (Fig. 2.9A). Two main groups were distinguished based on mineralogy and geochemistry. One biotite-poor group (BIF-1), composed of oxide and silicate facies, is characterized by alternating ribbons of ~25% quartz, ~25% magnetite- and ~50% ferro-(calc)-silicate-rich layers (Fig. 2.9A). Calc-silicate minerals include hornblende, grunerite, plagioclase, garnet and clinopyroxene; magnetite is the main iron-oxide mineral. Sulfides mean of 10%, but reach locally up to 80% replacement of pre-existing magnetite-rich and iron-rich-silicate layers (Fig. 2.9B and C). In BIF-2 group, the mean pyrrhotite content reaches 5%, but locally can be as high at 40% (Fig. 2.9D). In both BIF-1 and BIF-2, disseminated to massive pyrrhotite can occurs interstitial to metamorphic mineral (e.g. hornblende, clinopyroxene; Fig. 2.9C) or as inclusions in metamorphic minerals (Fig. 2.9E). BIF-2 group represents the silicate facies and its mineralogical assemblage is characterized by quartz, biotite, plagioclase, hornblende, garnet, and magnetite with trace amounts of apatite, clinopyroxene and tourmaline. The biotite content is higher (up to 20%) compared to that in the BIF-1 group (<5%).
Hydrothermal alterations
Grunerite and sulfides (pyrrhotite) replace pre-existing hornblende-rich layers in the BIFs (Fig. 2.9F). Gruneritization of the silicate and oxide facies in BIFs is a common alteration associated with iron formations-hosted orogenic gold deposits (Eilu et al., 1999). Biotite crystallizing in the wall-rock of quartz-pyrrhotite veins, suggests a potassic alteration was contemporaneous to the sulfidation event. Along with the biotite alteration, silicification is observed as quartz flooding filling tension gashes and quartz accumulations in boudin necks.
2.4.4 Wackes
The wacke unit is in conformable contact with the structurally under-lying iron formations (Fig. 2.5), and has been interpreted as belonging to the Laguiche Group (Goutier et al., 2002). At least two facies were identified based on mineralogy and geochemistry (Fig. 2.10A; section 2.6.3 Lithogochemistry section). The first group has a mineral assemblage characterized by quartz, plagioclase, biotite and ~5% actinolite to actinolitic-hornblende. Accessory minerals comprise garnet, muscovite, pyrrhotite with pentlandite exsolution, pyrite, chalcopyrite, arsenopyrite, magnetite and ilmenite. Mafic minerals (biotite and actinolite) constitute a maximum modal proportion of 40%. The second group has a mineral assemblage comprising actinolite (30%), chlorite, biotite, plagioclase, and quartz with minor traces of pyrite, chalcopyrite, pyrrhotite with pentlandite exsolution, arsenopyrite, magnetite and ilmenite. Retrograde chlorite partially replaced the amphibole. Mafic minerals (biotite and actinolite) reach a maximum modal proportion of 90% (Fig. 2.10B). Both populations are fine-grained (<1 mm). Layers of actinolite-poor and actinolite-rich facies vary in
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thickness from 2 to 20 centimeters, and are commonly interstratified. Quartz-calc-silicate veins (Fig. 2.10C), quartz-tourmaline and quartz-plagioclase-sulfide veins (Fig. 2.10D) cut this unit within 5 m from the mineralized zone. Up to 10% sulfides occur in the actinolite-poor wackes while <1% were noted in the actinolite-rich unit.
2.4.5 Plagioclase-phyric dioritic dykes
Plagioclase-phyric intermediate dykes (quartz-diorite), 0.5 to 3 m thick, cut across the main lithological assemblage at Orfée (Figs. 2.5A and 2.11A). The groundmass is homogeneous and fine-grained (<1 mm), and phenocrysts are limited to 0.5 cm. Locally, these dykes contain accessory quartz phenocrysts, but overall, quartz is limited to a maximum of 10% modal proportion. Their mineral assemblage comprises andesine and hornblende, biotite, quartz and albite with trace amounts of sulfides, zircon, ilmenite, titanite and bismuth telluride. Disseminated sulfides are pyrrhotite with pentlandite exsolution, arsenopyrite, pyrite and chalcopyrite. Centimeter scale and fine-grained mafic enclaves in the dykes, have irregular shapes and
are homogeneously distributed (Fig. 2.11A). These enclaves contain löllingite, as well as pyrrhotite with
Figure 2.10. Wackes field photographs and photomicrographs. A) Actinolite wacke bed in an actinolite-poor wacke. B)
Actinolite-rich wacke unit. C) Calc-silicate veins cutting across the wacke. D) Quartz-tourmaline vein, with intermediate calc-silicates and quartz in the center.
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pentlandite exsolution, chalcopyrite, pyrite and arsenopyrite. Garnet pseudomorphs are replaced by hornblende in the enclaves.
Figure 2.11. Plagioclase-phyric dioritic dykes and pegmatitic granite field photographs and photomicrographs. A)
Dioritic dyke cutting across the amphibolites. The dyke also cuts a quartz-calc-silicate vein. Amphibolite enclaves are visible. B) Dioritic dyke cut by a quartz-calc-silicate veins, both affected by a WNW-ESE shear zone. C) Native gold associated with a bismuth telluride and arsenopyrite mineral assemblage. Prehnite alteration affects the plagioclases. D) Pegmatitic granite boudinaged in the wacke unit, at the contact with the BIF. E) Sericite alteration along a fractures in the pegmatitic granite. F) Pyrrhotite ±chalcopyrite precipitate along fractures in garnet in the pegmatitic granite.
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The dykes cut (Fig. 2.11A) and are cut by quartz-calc-silicate veins (Fig. 2.11B) suggesting both are contemporaneous. Both the dykes and the quartz-calc-silicate veins are sheared and boudinaged in WNW-ESE gold-mineralized shear zones (Fig. 2.11B). Quartz-plagioclase stringers cut the dykes and contain a native gold-arsenopyrite, pyrrhotite, chalcopyrite, pyrite, bismuth telluride and molybdenite mineral assemblage (Fig. 2.11C). A sample of plagioclase-phyric dioritic dyke was selected for U-Pb dating (section 2.6.4 Geochronology).
2.4.6 Pegmatitic granites
Pegmatitic dykes and sills of granitic composition (Fig. 2.5) cut across all previously described lithologies (Fig. 2.11D). The thickness of this intrusions reach up to 5 meters in the trenches, and based on drill core data they also occur as irregular intrusions of several tens of meters thick and a few hundred meters long (Fig. 2.8). The matrix crystals reach a granulometry of up to 5 cm, but finer grained portions (<5 mm) are associated with shearing. The most common minerals are quartz, potassic feldspar, plagioclase, biotite and muscovite, while sulfides, garnet, tourmaline and monazite account for accessory minerals. Locally, muscovite can reach up to 65% modal proportion. These felsic injections are granitic in composition as they contain a modal proportion of ≥40% potassic feldspar. This unit is affected by at least one deformation event, and pegmatitic dykes are boudinaged and sheared (Fig. 2.11D). Plagioclase can display brittle deformation. Sulfides observed in this unit comprise pyrrhotite, chalcopyrite and arsenopyrite, often associated with a sericite alteration (Fig. 2.11E). Sulfides crystallized along fractures in garnet (Fig. 2.11F). A sample of pegmatitic granite was selected for radiometric dating (section 2.6.4 Geochronology).
2.4.7 Structural analysis
Interpreting the timing of the gold events in Archean deposits is a complex question as multiple deformation and metamorphic events can obliterate geological features linked to mineralization and alteration processes (e.g. Groves et al., 1998; Goldfarb et al., 2005). This is also the case of the Orfée prospect, with at least two distinct deformation events affecting the mineralized zones, as suggested by field and microscopic petrography observations of structural features (Table 2.1, Figs. 2.12, 2.13 and 2.14). The amphibolites have even a more complex deformation history, and are affected by at least three distinct events (D1, D2 and D3).
For the current study, D1, D2 and D3 were defined in respect to observed structural features on the trenches
and sparse outcrops from Orfée and Le Moyne-extension showings (Fig. 2.5), and do not necessarily correspond to what have been identified as reginal D1, D2 and D3 deformations events (Goutier et al., 2002).
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Because of the magnetic character of BIFs, the structural measures in this unit were taken using a protractor. In the remaining lithologies, the measures were taken with a Brunton compass, and have an error of ±5°.
Figure 2.12. D0/D1 deformation event characteristics. A) Stereonet showing bedding (S0) and S1 foliation; S1 is defined by aligned minerals in the wackes and BIFs. Aligned minerals in the BIFs define mineralogical distinct layers which could correspond to S0. Structural data from Orfée trenches (Fig. 2.5A). A) S1 foliation, folded by D2, refolded by D3. Massive sulfides preferentially replace iron oxides beds. Massive sulfides are accumulated in P2 and P3 fold hinges. C) Field photo of bedding with normal grading, defining a polarity towards south, on Trench C (Fig. 2.5C). D) Accumulation of massive sulfides in the P2 fold hinges. E) Graphite inclusions in hornblende define a foliation at an angle to the main S2 foliation. F) Aligned inclusions in metamorphic garnet defining a S0//S1 foliation, or early S2 foliation.
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D1 deformation
A first S1 foliation is particularly visible in the amphibolites and BIFs, and has a mean WNW-ESE
(~289°/75°) attitude. The S1 foliation (Fig. 2.12A) could be contemporaneous to diagenesis and is parallel to
bedding (S0). Compositions layering defined bedding, while the preferential alignment of minerals define S1.
Bedding and the first S1 foliation have been folded during subsequent D2 and D3, explaining the wide range
of S1 attitude from E to N (070° to 357°; Fig. 2.12A). Bedding, when preserved, displays normal grading
(Fig. 2.12C) suggesting a stratigraphic polarity towards south (Fig. 2.5C). Sulfides concordant to S0-S1 have
been folded by the D2 event (Fig. 2.12D), which suggest that part of the sulfides are either syngenetic or are
associated to a sulfidic event pre to syn D2. At microscale, aligned inclusions in syn D2 metamorphic garnet
and hornblende in the amphibolites could also represent relicts of the S1, or early S2 foliation
(Fig. 2.12E and F). In both cases, the inclusions are at an angle with respect to the main S2 foliation in the
matrix. Another relict of a possible D1 deformation are P1 folds, refolded by P2 isoclinal folds.
Figure 2.13. D2 deformation event charcteristics. A) S2 foliation and L2 and P2 fold hinges stereonet. Stretching lineations L2 are parallel to the P2 fold hinges. Structural data from trenches at Orfée (Fig. 2.5A). B) Stratiform pyrrhotite replacing the iron oxide beds, following the S0-S1. Pyrrhotite accumulation along the axial plane of a second-order z-fold. C) D2 pyrrhotite-quartz vein, feeling a tension gash. Flanking veins follow the bedding in the BIF. D) D2 shear zone and extension quartz-calc-silicate vein.
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A first deformation/metamorphic event (D1/M1) was described regionally by Goutier et al. (1999a
and 2002) in the La Grande, but it is unclear whether this first event affected the rocks from Orfée. This Neoarchean event pre-dates 2709 to 2016 Ma, age of the Duncan intrusions (Fig. 2.2; Goutier et al., 1999a).
Figure 2.14. Photomicrographs of metamorphism related textures. A) Native gold inclusion in clinopyroxene, in a
quartz-calc-silicate vein. B) Pyrite partially replaced by pyrrhotite in a quartz-quartz-calc-silicate vein. C) Calcite remnants in a pre metamorphic quartz-calc-silicate vein. D) Pyrite-chalcopyrite remnants in semi-massive pyrrhotite accumulation in a dilatant region. E) Pyrrhotite-Arsenopyrite-Löllingite composite crystals. The arsenopyrite is retrograde and replaced metamorphic löllingite. F) Individual löllingite, with adjacent retrograde arsenopyrite. Pyrite pseudomorph are spatially associated.
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D2 deformation
The S2 schistosity (Fig. 2.13A) is defined by aligned metamorphic amphibole (Fig. 2.12F) and
feldspar following a WNW-ESE (~279°/74°) orientation. Weakly plunging (05° to 35°) P2 isoclinal folds
(Fig. 2.12B) are steeply inclined (62° to 81°), with asymmetrical second-order z- and s-folds; the steeply dipping P2 axial planes are parallel to the S2 foliation. The second-order asymmetrical P2 z-folds (Fig. 2.13B)
are accentuated by flexural slip as a result of a late D2 or syn D3 dextral apparent movement. The dextral
movement is also responsible for the thickening of the iron formation unit on the main trench (TR-01-01; Fig. 2.5A), the rotation of syntectonic garnets (Fig. 2.12F) and boudinaged layers (Fig. 2.9A) in the iron formations. D2 tension gashes (Fig. 2.13C) are filled with quartz-sulfide.
Plagioclase-phyric dioritic dykes have weakly recorded the main S2 foliation. Pre to syn D2
mineralized quartz-calc-silicate veins cut across the plagioclase-phyric dioritic dykes (Fig. 2.11B and 2.13D) and both are sheared in WNW-ESE corridors; new laminated shear zone-hosted veins form (Fig. 2.11B).
The early D2 quartz-calc-silicate veins (Fig. 2.13D) are overprinted by the M2 metamorphic event,
as suggested by native gold inclusions in metamorphic clinopyroxene (Fig. 2.14A), pyrite remnants partially
Figure 2.15. A) S3 foliation and L3 fold hinges to P3 folds. Structural data from trenches at Orfée (Fig. 2.5A). B) A crenulation cleavage (S3) affects the main S2 foliation. C) Grain size reduction in a sheared pegmatitic granite. D) Boudinaged and stretched pegmatitic granite in the wacke unit.
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replaced by pyrrhotite (Fig. 2.14B) and calcite remnants (Fig. 2.14B). Prograde metamorphic relicts also include pyrite in semi-massive to massive pyrrhotite (Fig. 2.14D and E) and metamorphic löllingite (Fig. 2.14F). Regionally, a M2 metamorphic event affects the La Grande and the Opinaca subprovinces between
2684 to 2640 Ma (Wodicka et al., 2009; David et al. 2009 and 2011a; Morfin et al., 2013; Rhéaum-Ouellet, 2017, unpublished).
Table 2.1. Synthesis table for D0 to D3 deformation events affecting the Orfée, Orfée Est and Le Moyne-extension gold-rich zones.
Event Description Observations Orientation
D0 Bedding (S0)
- bedding defined by mineral layering in the BIF unit;
- normal grading shows a polarity towards south;
Bedding is averagely oriented at ~289°/75° but varies between 134° and 355°; it is
unlikely this represents original S0 as subsequent S2 and S3 affect the lithological
assemblage from Orfée
D1/M1
Primary foliation S1 parallel to bedding; P1
folds
- S1 is parallel to bedding, and is affected by P1, P2 and P3 folds;
- remnants of the S1 foliation are defined by inclusions in metamorphic garnet porphyroblastes (in garnet-hornblende alteration halos);
- graphite and pyrite inclusions in hornblende are also relicts of the S1 foliation; - P1 folds are affected by isoclinal P2 folds;
S1 is averagely oriented at ~289°/75° (parallel to bedding) but varies between
070° and 357°; folded by subsequent S2 and S3
D2/M2
S2 foliation and isoclinal folds; L2 stretching lineations and P2 fold axis; apparent dextral movement and thickening of the iron
formations; late shear zones and “boudin necks”
- foliation is defined by alignment of metamorphic minerals (e.g. hornblende); - Isoclinal P2 folds result from an apparent dextral movement, also responsible for thickening of iron formations and « boudin necks » formation;
- Late-tectonic shear zones form in response to folding;
- Late shear zones cut a generation of plagioclase-phyric dioritic dykes dated at 2703 ±7 Ma;
- The dioritic dykes cut the S2 foliation, but locally seem to have been recorded the apparent dextral movement;
- The D2 deformation event is responsible for the parallelisation of any pre-existing foliation as well at the bedding;
The average orientation of S2 is ~279°/74° (261° to 333°). P2 isoclinal folds are oriented at 21°,099°. P2 fold hinges are parallel to the L2 stretching lineations. Late shear zones are oriented at ~292°/72°
D3/M3
S3 foliation is defined by a crenulation cleavage;
opened P3 folds
- A S3 crenulation cleavage is parallel to the axial planes of P3 opened folds; - The calc-silicate veins are folded during
this event;
- The P3 opened folds affect the isoclinal P2 folds;
- Shearing and boudinage of granite pegmatitic dykes;
The S3 crenulation cleavage is oriented at ~256°/79° (201° to
283°). The S2-S3 planes intersection is defined by a
steeply dipping lineation (~71°,076°). P3 fold hinges
are parallel to the L3 intersection lineations
