GREENSTONE-HOSTED QUARTZ-CARBONATE VEIN DEPOSITS

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GREENSTONE-HOSTED QUARTZ-CARBONATE VEIN DEPOSITS

  1. 1. GREENSTONE-HOSTED QUARTZ-CARBONATE VEIN DEPOSITS BENOÎT DUBÉ AND PATRICE GOSSELIN Geological Survey of Canada, 490 de la Couronne, Quebec, Quebec G1K 9A9 Corresponding author’s email: bdube@nrcan-rncan.gc.ca Abstract Greenstone-hosted quartz-carbonate vein deposits typically occur in deformed greenstone belts of all ages, espe- cially those with variolitic tholeiitic basalts and ultramafic komatiitic flows intruded by intermediate to felsic porphyry intrusions, and sometimes with swarms of albitite or lamprophyre dyke. They are distributed along major compressional to transtensional crustal-scale fault zones in deformed greenstone terranes commonly marking the convergent margins between major lithological boundaries, such as volcano-plutonic and sedimentary domains. The large greenstone- hosted quartz-carbonate vein deposits are commonly spatially associated with fluvio-alluvial conglomerate (e.g. Timiskaming conglomerate) distributed along major crustal fault zones (e.g. Destor Porcupine Fault). This association suggests an empirical time and space relationship between large-scale deposits and regional unconformities. These types of deposits are most abundant and significant, in terms of total gold content, in Archean terranes. However, a significant number of world-class deposits are also found in Proterozoic and Paleozoic terranes. In Canada, they represent the main source of gold and are mainly located in the Archean greenstone belts of the Superior and Slave provinces. They also occur in the Paleozoic greenstone terranes of the Appalachian orogen and in the oceanic terranes of the Cordillera. The greenstone-hosted quartz-carbonate vein deposits correspond to structurally controlled complex epigenetic deposits characterized by simple to complex networks of gold-bearing, laminated quartz-carbonate fault-fill veins. These veins are hosted by moderately to steeply dipping, compressional brittle-ductile shear zones and faults with locally associated shallow-dipping extensional veins and hydrothermal breccias. The deposits are hosted by greenschist to locally amphibolite-facies metamorphic rocks of dominantly mafic composition and formed at intermediate depth (5- 10 km). The mineralization is syn- to late-deformation and typically post-peak greenschist -facies or syn-peak amphi- bolite-facies metamorphism. They are typically associated with iron-carbonate alteration. Gold is largely confined to the quartz-carbonate vein network but may also be present in significant amounts within iron-rich sulphidized wall-rock selvages or within silicified and arsenopyrite-rich replacement zones. There is a general consensus that the greenstone-hosted quartz-carbonate vein deposits are related to metamorphic fluids from accretionary processes and generated by prograde metamorphism and thermal re-equilibration of subducted volcano-sedimentary terranes. The deep-seated, Au-transporting metamorphic fluid has been channelled to higher crustal levels through major crustal faults or deformation zones. Along its pathway, the fluid has dissolved various com- ponents - notably gold - from the volcano-sedimentary packages, including a potential gold-rich precursor. The fluid then precipitated as vein material or wall-rock replacement in second and third order structures at higher crustal levels through fluid-pressure cycling processes and temperature, pH and other physico-chemical variations. Résumé Les gîtes de filoniens à veines de quartz-carbonates dans des roches vertes reposent généralement au sein de cein- tures de roches vertes de tout âge, mais tout particulièrement dans celles qui présentent des basaltes tholéiitiques à tex- ture variolaire et des coulées ultramafiques komatiitiques dans lesquels se sont mis en place des intrusions porphyriques de composition intermédiaire à felsique et, parfois, des essaims de dykes d’albitite ou de lamprophyre. Ces gîtes sont répartis le long d’importantes zones de failles d’échelle crustale formées dans un régime allant de la compression à la transtension, au sein de terrains de roches vertes déformés, où elles coïncident habituellement avec d’importantes lim- ites lithologiques qui témoignent d’une marge convergence, comme celles qui séparent des domaines sédimentaires de domaines volcano-plutoniques. Les plus gros gisements du genre sont souvent associés, sur le plan spatial, à des con- glomérats fluvio-alluvionnaires (p. ex. le conglomérat de Timiskaming) répartis le long d’importantes zones de failles d’échelle crustale (p. ex. la faille de Destor-Porcupine). Cette association suppose un lien empirique aussi bien temporel que spatial entre les gros gisements et les discordances régionales. Les gîtes de ce type sont plus abondants et importants, quant au contenu total en or, dans les terrains archéens. Cependant, de nombreux gisements de calibre mondial reposent aussi dans des terrains protérozoïques et paléozoïques. Au Canada, ils constituent la principale source d’or et sont concentrés dans les ceintures de roches vertes archéennes des provinces du lac Supérieur et des Esclaves, mais on en a aussi découvert dans le terrains de roches vertes paléo- zoïque de l’orogène des Appalaches et dans les terrains océaniques de la Cordillère. Ces gîtes constituent des minéralisations épigénétiques à contrôle structural complexe caractérisées par des réseaux simples à complexes de filons de quartz carbonates laminés porteurs d’or produits par le remplissage de failles. Ces filons sont logés dans des failles et des zones de cisaillement à comportement fragile-ductile formées en régime com- pressif, qui présentent un pendage moyen à fort, auxquels sont associés, par endroits, des brèches hydrothermales et des veines d’extension à faible pendage. Les gîtes, qui se sont formés à des profondeurs intermédiaires (de 5 à 10 km), sont encaissés dans des roches métamorphiques, de composition principalement mafique, du faciès des schistes verts et, par endroits, du faciès des amphibolites. La mise en place de la minéralisation est contemporaine des phases intermédiaires et tardives de la déformation et s’est déroulée après l’atteinte des conditions maximales du métamorphisme au faciès des schistes verts ou lors de l’atteinte des conditions maximales du métamorphisme au faciès des amphibolites. La minéralisation est généralement associée à une altération à carbonates de fer. L’or est en grande partie piégé dans un réseau de filons de quartz-carbonates, mais il est aussi présent en quantités importantes dans les épontes de roches encaissantes sulfurées riches en fer ou de zones des remplacement silicifiées et riches en arsénopyrite. Dubé, B., and Gosselin, P., 2007, Greenstone-hosted quartz-carbonate vein deposits, in Goodfellow, W.D., ed., Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division, Special Publication No. 5, p. 49-73.
  2. 2. B. Dubé and P. Gosselin On croit que l’existence des gîtes de filons de quartz-carbonates dans des roches vertes est liée à celle de fluides métamorphiques issus de processus d’accrétion, et qu’ils sont le produit d’un métamorphisme prograde et d’une remise en équilibre thermique de terrains volcano-sédimentaires subductés. Les fluides métamorphiques de grande profondeur qui ont transporté l’or se sont élevés dans la croûte en empruntant d’importantes failles ou zones de déformation d’échelle crustale. Le long de leur parcours, ils ont dissous divers éléments, dont l’or, dans les assemblages volcano- sédimentaires, qui pouvaient comprendre un précurseur riche en or. Les fluides ont ensuite précipité sous forme de veines ou ont remplacé les roches encaissantes dans des structures de deuxième et de troisième ordres, à des niveaux crustaux supérieurs, selon une succession de cycles liés à des variations de la pression hydrostatique, de la température, du pH et d’autres paramètres physico-chimiques. Definition EPITHERMAL CLAN Simplified Definition ADVANCED ARGILLIC HOTSPRING Greenstone-hosted quartz-car- km HIGH-SULPHIDATION sea level bonate vein deposits occur as 0 LOW SULFIDATION Rhyolite dome PALEOPLACER quartz and quartz-carbonate veins, AU-RICH MASSIVE SULPHIDE ARGILLIC with valuable amounts of gold and (mainly from Hannington et al., 1999) silver, in faults and shear zones 1 STOCKWORK- SERICITE BRECCIA-PIPE AU DISSEMINATED located within deformed terranes AU Carbonate of ancient to recent greenstone Permeable rocks belts commonly metamorphosed Unit CARLIN TYPE at greenschist facies. GREENSTONE VEIN PORPHYRY AND SLATE BELT CLANS AU AU MANTO Scientific Definition 5 Dyke AU SKARN TURBIDITE-HOSTED Stock Vein Greenstone-hosted quartz-car- VEIN bonate vein deposits are a subtype BIF-HOSTED VEIN Wacke-shale INTRUSION-RELATED CLAN (mainly from Sillitoe and Bonham, 1990) of lode gold deposits (Poulsen et Volcanic al., 2000) (Fig. 1). They are also 10 GREENSTONE-HOSTED known as mesothermal, orogenic QUARTZ-CARBONATE VEIN DEPOSITS (mesozonal and hypozonal - the near surface orogenic epizonal Iron formation Shear zone Au-Sb-Hg deposits described by Granitoid Groves et al. (1998) are not FIGURE 1. Inferred crustal levels of gold deposition showing the different types of gold deposits and the included in this synthesis), lode inferred deposit clan (from Dubé et al., 2001; modified from Poulsen et al., 2000). gold, shear-zone-related quartz- carbonate or gold-only deposits (Hodgson and MacGeehan, ically post-peak greenschist-facies or syn-peak amphibolite- 1982; Roberts, 1987; Colvine, 1989; Kerrich and Wyman, facies metamorphism. They are formed from low salinity, 1990; Robert, 1990; Kerrich and Feng, 1992; Hodgson, H2O-CO2-rich hydrothermal fluids with typically anomalous 1993; Kerrich and Cassidy, 1994; Robert, 1995; Groves et concentrations of CH4, N2, K, and S. Gold is mainly con- al., 1998; Hagemann and Cassidy, 2000; Kerrich et al., 2000; fined to the quartz-carbonate vein networks but may also be Poulsen et al., 2000; Goldfarb et al., 2001; Robert and present in significant amounts within iron-rich sulphidized Poulsen, 2001; Groves et al., 2003; Goldfarb et al., 2005; wall rock. Greenstone-hosted quartz-carbonate vein deposits Robert et al., 2005). The focus of the following text is mainly are distributed along major compressional to transpressional on Canadian examples and particularly those deposits found crustal-scale fault zones in deformed greenstone terranes of in the Abitibi Archean greenstone belt. For a complete global all ages, but are more abundant and significant, in terms of perspective, readers are referred to the above list of selected total gold content, in Archean terranes. However, a signifi- key references. cant number of world-class deposits (>100 t Au) are also Greenstone-hosted quartz-carbonate vein deposits are found in Proterozoic and Paleozoic terranes. International structurally controlled, complex epigenetic deposits that are examples of this subtype of gold deposits include Mt. hosted in deformed and metamorphosed terranes. They con- Charlotte, Norseman, and Victory (Australia), Bulyanhulu sist of simple to complex networks of gold-bearing, lami- (Tanzania), and Kolar (India) (Fig. 2). Canadian examples nated quartz-carbonate fault-fill veins in moderately to include Sigma-Lamaque (Québec), Dome and Pamour steeply dipping, compressional brittle-ductile shear zones (Ontario), Giant and Con (Northwest Territories), San and faults, with locally associated extensional veins and Antonio (Manitoba), Hammer Down (Newfoundland), and hydrothermal breccias. They are dominantly hosted by mafic Bralorne-Pioneer (British Columbia). Detailed characteris- metamorphic rocks of greenschist to locally lower amphibo- tics and references are found in the text below. The reader lite facies and formed at intermediate depths (5-10 km). may refer to Appendix 1 for a list of geographical, geologi- Greenstone-hosted quartz-carbonate vein deposits are typi- cal, and economical characteristics of Canadian gold cally associated with iron-carbonate alteration. The relative deposits with more than 250 000 oz Au in combined produc- timing of mineralization is syn- to late-deformation and typ- tion and reserves (data from Gosselin and Dubé, 2005b). 50
  3. 3. Greenstone-Hosted Quartz-Carbonate Vein Deposits New Brittannia Berezovskoe Discovery Kochkar Stepnyak Darasun Yellowknife Svetlinskoe Karalveem Olimpiada Duolanasayi Alaska-Juneau Casa Berardi Aksu Daugyztau Treadwell Chibougamau Bralorne-Pioneer Val d'Or Zun-Holba San Antonio Timmins Qiyiqiu No. 1 Alleghany District Grass Valley District Kirkland Lake ? Mother Lode System Akbakay Paishanlou Larder Lake Baguamiao Renabie Shanggong La Herradura Amesmessa Woxi Wenyu Hetai Zarmitan Erjia El Callao Kolar Hutti Yatela Gross Rosebel Morila Omai Lega Dembi Syama Bulyanhulu Poura Bibiani Shamva Obuasi Mazoe Fazenda Brasileiro Plutonic Cam & Motor Dalny Bronzewing Morro do Ouro Golden Valley Lancefield Morro Velho Lonely Meekatharra Passagem de Mariana Navachab Day Dawn Gympie Blanket Fairview Morning Star / Evening Star Granny Smith Globe and Phoenix New Consort Sheba Sons of Gwalia Golden Mile Mount Charlotte Sunrise Dam - Cleo Royal New Celebration Norseman Victory-Defiance Legend Cenozoic Paleozoic Archean Precambrian Greenstone-hosted quartz-carbonate Mesozoic Proterozoic Phanerozoic Proterozoic-Phanerozoic vein deposit FIGURE 2. World distribution of greenstone-hosted quartz-carbonate vein deposits containing at least 30 tonnes of Au. Economic Characteristics of Greenstone-Hosted The temporal and geographic distribution of greenstone- Quartz-Carbonate Vein Deposits hosted quartz-carbonate vein deposits is shown on Figure 2. Summary of Economic Characteristics Greenstone-hosted quartz-carbonate vein deposits occur in greenstone terranes of all ages. Although they are present in The total world production and reserves of gold, including Paleozoic to Tertiary terranes, they are mainly concentrated the Witwatersrand paleoplacer deposits, stands at more than in Precambrian terranes, and particularly in those of late 126 420 metric tonnes Au (Gosselin and Dubé, 2005a). World Archean age. In Canada, all the world-class deposits but one production and reserves for the greenstone-hosted quartz-car- (Bralorne-Pioneer) are of late Archean age. Their concentra- bonate vein deposit subtype is 15 920 metric tonnes Au tion in the Archean is thought to be related to 1) continental (Gosselin and Dubé, 2005a), which is equivalent to 13% of growth and the related higher number of large-scale colli- the total world production and puts them in second place for sions between continental fragments (and/or arc complex), world productivity behind paleoplacers. Total Canadian pro- and 2) the associated development of major faults and large- duction and reserves, at 9 280 metric tonnes Au, represent 7% scale hydrothermal fluid flow during the supercontinent cycle of the world total. However, Canadian production and and mantle plume activity (cf. Barley and Groves, 1992; reserves for the greenstone-hosted quartz-carbonate vein sub- Condie, 1998; Kerrich et al., 2000; Goldfarb et al., 2001). type are 5 510 metric tonnes, which constitutes 35% of the world production for this deposit subtype, and 59% of the Grade and Tonnage Characteristics total Canadian production and reserves of gold. The Superior Greenstone-hosted quartz-carbonate vein deposits are sec- province contains 86% (4 760 metric tonnes) of Canadian ond on total tonnage of gold only to the Witwatersrand paleo- gold production and reserves for greenstone-hosted quartz- placers of South Africa. The largest greenstone-hosted quartz- carbonate vein deposits (Gosselin and Dubé, 2005a,b). The carbonate vein deposit in terms of total gold content is the Abitibi sub-province is the main source and represents 81% Golden Mile complex in Kalgoorlie, Australia, with more than (4 470 metric tonnes) of the total Canadian gold. 1800 tonnes Au (Gosselin and Dubé, 2005a). The Hollinger- There are 103 known greenstone-hosted quartz-carbonate McIntyre deposit in Timmins, Ontario, is the second largest vein deposits world-wide containing at least 30 tonnes (~1 M deposit of such type ever found with 987 tonnes Au (Gosselin oz) Au (production and reserves), including 31 Canadian and Dubé, 2005a). In contrast to the Golden Mile complex, deposits, whereas 33 other deposits in Canada, and several open pit mining of the Hollinger-McIntyre deposit is now hundred worldwide, contain more than 7.5 tonnes (~250 000 impossible due to housing, which leaves a significant part of oz) but less than 30 tonnes (Gosselin and Dubé, 2005b). A the total gold content of the deposit inaccessible. select group of 41 world-class deposits contains more than The average grade of greenstone-hosted quartz-carbonate 100 tonnes Au, including 11 giant deposits with more than deposits is fairly consistent, ranging from 5 to 15 g/t Au, 250 tonnes. In this group of world-class deposits, six are from whereas the tonnage is highly variable and ranges from a few the Abitibi greenstone belt of the Canadian Archean Superior thousand tonnes to over 100 million tonnes of ore, although Province (Fig. 3). The Superior Province is the largest and more typically these deposits contain only a few million best preserved Archean craton in terms of greenstone-hosted tonnes of ore (Fig. 4). gold endowment, followed by the Yilgarn craton of Australia. 51
  4. 4. B. Dubé and P. Gosselin Casa Berardi Hollinger - McIntyre PDF Pamour Dome Kirkland Kerr Horne Lake? Addison LLCF Doyon Bousquet-LaRonde Malartic Sigma-Lamaque 100 km Granitoid rock Proterozoic cover World-class greenstone-hosted Other gold deposits quartz-carbonate vein deposits of various types Mafic intrusion Sedimentary rock World-class gold-rich LLCF Larder Lake - Cadillac volcanogenic massive-sulfides Fault Zone Volcanic rock Major fault Other smaller gold-rich VMS PDF Pocupine - Destor Fault Zone FIGURE 3. Simplified geological map of the Abitibi greenstone belt showing the distribution of major fault zones and gold deposits. Modified from Poulsen et al. (2000). See Appendix 1 for deposit details. Comparison of Grade and Tonnage Characteristics with the Global Range 35 In Canada, this type of gold deposit is widely distributed 30 from the Paleozoic greenstone terranes of the Appalachian Number of deposits Orogen on the east coast (e.g. Hammer Down and Deer Cove 25 Newfoundland, Dubé et al., 1993; Gaboury et al., 1996), 20 through the Archean greenstone belts of the Superior (Dome 15 and Sigma-Lamaque) and Slave provinces (Con and Giant) 10 in central Canada, to the oceanic terranes of the Cordillera (Bralorne-Pioneer). 5 The average gold grade of world-class Canadian deposits 0 915 25 35 45 55 65 75 85 105 125 135 145 155 165 15 95 0-5 115 is 10 g/t, which is slightly higher than the average for this type of deposit worldwide (7.6 g/t, Fig. 5). World-class Ore tonnage (Mt) deposits in Canada have on average lower tonnage (20.91 Mt 45 of ore) than those worldwide (39.91 Mt). Perhaps this is in 40 part because mining in Canada has traditionally taken place Number of deposits 35 underground, whereas in other countries open pits have also 30 been developed. 25 20 Geological Characteristics of Greenstone-Hosted 15 Quartz-Carbonate Vein Deposits 10 Physical Properties 5 0 Mineralogy 0-5 10 15 20 25 30 35 40 The main gangue minerals in greenstone-hosted quartz- Ore grade (g/t) carbonate vein deposits are quartz and carbonate (calcite, FIGURE 4. Tonnage and grade repartition for gold deposits in the world con- dolomite, ankerite, and siderite), with variable amounts of taining at least 30 tonnes of Au in combined production and reserves. 52
  5. 5. Greenstone-Hosted Quartz-Carbonate Vein Deposits 100 Grass Valley Kirkland Lake Kolar Bulyanhulu 10 Hollinger-McIntyre KerrAddison Grade (g/t) Sigma-Lamaque Dome GoldenMile 10 00 Alaska-Juneau 0 tA 1 u 10 10 10 00 1 0 tA tA tA tA u u u u 0.1 0 0 0.1 1 10 100 1000 10000 Tonnage (Mt) World 30t (70) 7 Canada (128) FIGURE 5. Tonnage versus grade relationship of Canadian and world Au deposits containing at least 30 tonnes of Au in combined production and reserves. white micas, chlorite, tourmaline, and sometimes scheelite. The sulphide minerals typically constitute less than 5 to 10% of the volume of the orebodies. The main ore minerals are native gold with, in decreasing amounts, pyrite, pyrrhotite, and chalcopyrite and occur without any significant vertical mineral zoning. Arsenopyrite commonly represents the main sulphide in amphibolite-facies rocks (e.g. Con and Giant) and in deposits hosted by clastic sediments. Trace amounts of molybdenite and tellurides are also present in some deposits, such as those hosted by syenite in Kirkland Lake (Thompson et al., 1950; Fig. 6A, B). FIGURE 6. (A) Quartz-breccia vein, Main Break, Kirkland Lake. (B) High- grade quartz veinlets, hosted by syenite with visible gold, disseminated Textures pyrite, and traces of tellurides, Main Break, Kirkland Lake. This type of gold deposit is characterized by moderately Hodgson, 1993; Robert et al., 1994; Robert and Poulsen, to steeply dipping, laminated fault-fill quartz-carbonate 2001). The laminated quartz-carbonate veins typically infill veins (Fig. 7A, B, C) in brittle-ductile shear zones and faults, the central part of, and are subparallel to slightly oblique to, with or without fringing shallow-dipping extensional veins the host structures (Hodgson, 1989; Robert et al., 1994; and breccias (Fig. 7D, E). Quartz vein textures vary accord- Robert and Poulsen, 2001) (Fig. 8). The shallow-dipping ing to the nature of the host structure (extensional vs. com- extensional veins are either confined within shear zones, in pressional). Extensional veins typically display quartz and which case they are relatively small and sigmoidal in shape, carbonate fibres at a high angle to the vein walls and with or they extend outside the shear zone and are planar and lat- multiple stages of mineral growth (Fig. 7E), whereas the erally much more extensive (Robert et al., 1994). laminated veins are composed of massive, fine-grained quartz. When present in laminated veins, fibres are subparal- Stockworks and hydrothermal breccias may represent the lel to the vein walls (Robert et al., 1994; Robert and Poulsen, main mineralization styles when developed in competent 2001). units such as the granophyric facies of differentiated gab- broic sills (e.g. San Antonio deposit, Robert et al., 1994; Dimensions Robert and Poulsen, 2001), especially when developed at shallower crustal levels. Ore-grade mineralization also Individual vein thickness varies from a few centimetres occurs as disseminated sulphides in altered (carbonatized) up to 5 metres, and their length varies from 10 up to 1000 m. rocks along vein selvages. Due to the complexity of the geo- The vertical extent of the orebodies is commonly greater logical and structural setting and the influence of strength than 1 km and reaches 2.5 km in a few cases (e.g. the anisotropy and competency contrasts, the geometry of vein Kirkland Lake deposit, Charlewood, 1964). networks varies from simple (e.g. Silidor deposit), to fairly Morphology complex with multiple orientations of anastomosing and/or conjugate sets of veins, breccias, stockworks, and associated The gold-bearing shear zones and faults associated with structures (Dubé et al., 1989; Hodgson, 1989, Belkabir et al., this deposit type are mainly compressional and they com- 1993; Robert et al., 1994; Robert and Poulsen, 2001). Layer monly display a complex geometry with anastomosing anisotropy induced by stiff differentiated gabbroic sills and/or conjugate arrays (Daigneault and Archambault, 1990; 53
  6. 6. B. Dubé and P. Gosselin FIGURE 7. (A) Laminated fault-fill veins, Pamour mine, Timmins. (B) Close-up of photo A showing a laminated fault-fill vein with iron-carbonatized wall- rock clasts. (C) Boudinaged fault-fill vein, section view, Dome mine. (D) Arrays of extensional quartz veins, Pamour mine. (E) Extensional quartz-tourma- line “flat vein” showing multiple stages of mineral growth perpendicular to the vein walls, Sigma mine (from Poulsen et al., 2000). (F) Tourmaline-quartz vein, Clearwater deposit, James Bay area. within a matrix of softer rocks, or, alternatively, by the pres- layer and its orientation may induce an internal strain differ- ence of soft mafic dykes within a highly competent felsic ent from the regional one and may strongly influence the intrusive host, could control the orientation and slip direc- success of predicting the geometry of the gold-bearing vein tions in shear zones developed within the sills; consequently, network being targeted in an exploration program (Dubé et it may have a major impact on the distribution and geometry al., 1989; Robert et al., 1994). of the associated quartz-carbonate vein network (Dubé et al., 1989; Belkabir et al., 1993). As a consequence, the geometry Host Rocks of the veins in settings with large competence contrasts will The veins in greenstone-hosted quartz-carbonate vein be strongly controlled by the orientation of the hosting bod- deposits are hosted by a wide variety of host rock types; ies and less by external stress. The anisotropy of the stiff mafic and ultramafic volcanic rocks and competent iron-rich 54
  7. 7. Greenstone-Hosted Quartz-Carbonate Vein Deposits differentiated tholeiitic gabbroic sills and granitoid intru- X sions are common hosts. However, there are commonly dis- trict-specific lithological associations acting as chemical and/or structural traps for the mineralizing fluids as illus- trated by tholeiitic basalts and flow contacts within the SLIP PLANE FOLIATION Tisdale Assemblage in Timmins (cf. Hodgson and MacGeehan, 1982; Brisbin, 1997). A large number of deposits in the Archean Yilgarn craton are hosted by gab- broic (“dolerite”) sills and dykes (Solomon et al., 2000) as STAGE II FILLING illustrated by the Golden Mile dolerite sill in Kalgoorlie (Bartram and McGall, 1971; Travis et al., 1971; Groves et al., 1984), whereas in the Superior Province, many deposits are associated with porphyry stocks and dykes (Hodgson and Z McGeehan, 1982). Some deposits are also hosted by and/or Y EXTENSIONAL VEIN along the margins of intrusive complexes (e.g. Perron- (B-AXIS) Beaufort/North Pascalis deposit hosted by the Bourlamaque batholith in Val d’Or (Belkabir et al., 1993; Robert, 1994)). STAGE I FILLING Other deposits are hosted by clastic sedimentary rocks (e.g. FAULT-FILL VEIN Pamour, Timmins). FIGURE 8. Schematic diagram illustrating the geometric relationships Chemical Properties between the structural element of veins and shear zones and the deposit- scale strain axes (from Robert, 1990). Ore Chemistry The metallic geochemical signature of greenstone-hosted Au/Ag ratio typically varies from 5 to 10. Contrary to quartz-carbonate vein orebodies is Au, Ag, As, W, B, Sb, Te, epithermal deposits, there is no vertical metal zoning. and Mo, typically with background or only slightly anom- Palladium is locally present as illustrated by the syndefor- alous concentrations of base metals (Cu, Pb, and Zn). The FIGURE 9. (A) Large boudinaged iron-carbonate vein, Red Lake district. (B) Iron carbonate pervasive replacement of an iron-rich gabbroic sill, Tadd prospect, Chibougamau. (C) Green carbonate rock showing fuchsite-rich replacement and iron-carbonate veining in a highly deformed ultramafic rock, Larder Lake. (D) Green carbonate alteration showing abundant green micas replacing chromite-rich ultramafics, Baie Verte, Newfoundland. 55
  8. 8. B. Dubé and P. Gosselin Typically, the proximal alteration haloes are zoned and char- acterized – in rocks at greenschist facies – by iron-carbona- tization and sericitization, with sulphidation of the immedi- ate vein selvages (mainly pyrite, less commonly arsenopy- rite). Altered rocks show enrichments in CO2, K2O, and S, and leaching of Na2O. Further away from the vein, the alter- ation is characterized by various amounts of chlorite and cal- cite, and locally magnetite (Phillips and Groves, 1984; Dubé et al., 1987; Roberts, 1987). The dimensions of the alteration haloes vary with the composition of the host rocks and may envelope entire deposits hosted by mafic and ultramafic rocks. Pervasive chromium- or vanadium-rich green micas (fuchsite and roscoelite) and ankerite with zones of quartz- carbonate stockworks are common in sheared ultramafic rocks (Fig. 9C, D). Common hydrothermal alteration assem- blages that are associated with gold mineralization in amphi- bolite-facies rocks include biotite, amphibole, pyrite, pyrrhotite, and arsenopyrite, and, at higher grades, biotite/phlogopite, diopside, garnet, pyrrhotite and/or arsenopyrite (cf. Mueller and Groves, 1991; Witt, 1991; Hagemann and Cassidy, 2000; Ridley et al., 2000, and refer- ences therein), with variable proportions of feldspar, calcite, and clinozoisite (Fig. 10). The variations in alteration styles have been interpreted as a direct reflection of the depth of formation of the deposits (Colvine, 1989; Groves, 1993). The alteration mineralogy of the deposits hosted by amphi- bolite-facies rocks, in particular the presence of diopside, biotite, K-feldspar, garnet, staurolite, andalusite, and actino- lite, suggests that they share analogies with gold skarns, especially when they (1) are hosted by sedimentary or mafic volcanic rocks, (2) contain a calc-silicate alteration assem- blage related to gold mineralization with an Au-As-Bi-Te metallic signature, and (3) are associated with granodiorite- diorite intrusions (cf. Meinert, 1998; Ray, 1998). Canadian examples of deposits hosted in amphibolite-facies rocks include the replacement-style Madsen deposit in Red Lake (Dubé et al., 2000) and the quartz-tourmaline vein (Fig. 7F) and replacement-style Eau Claire deposit in the James Bay area (Cadieux, 2000; Tremblay, 2006). Geological Properties Continental Scale Greenstone-hosted quartz-carbonate-vein deposits are typically distributed along crustal-scale fault zones (cf. Kerrich et al., 2000, and references therein) characterized by FIGURE 10. (A) Diopside vein in biotite-actinolite-microcline-rich gold- several increments of strain (e.g. Cadillac-Larder Lake fault) bearing alteration, Madsen mine, Red Lake. (B) Auriferous metasomatic (Figs. 3, 11A, B, 12A, B), and, consequently multiple gener- hydrothermal layering with actinolite-rich and biotite-microcline-rich ations of steeply dipping foliations and folds resulting in a bands, Madsen mine, Red Lake. (C) Gold-rich No. 8 vein with visible gold in a quartz carbonate-actinolite-diopside-rich vein, Madsen mine, Red Lake. complex deformational history. These crustal-scale fault zones are the main hydrothermal pathways towards higher mation auriferous quartz or hematite-quartz veins hosted by crustal levels. However, the deposits are spatially and genet- Proterozoic iron formation in Brazil (Olivo et al., 1995). ically associated with second- and third-order compressional reverse-oblique to oblique brittle-ductile high-angle shear Alteration Mineralogy and Chemistry and high-strain zones (Fig. 12C), which are commonly At a district scale, greenstone-hosted quartz-carbonate located within 5 km of the first order fault and are best devel- vein deposits are associated with large-scale carbonate alter- oped in its hanging wall (Robert, 1990). Brittle faults may ation (Fig. 9A, B) commonly distributed along major fault also be the main host to gold mineralization as illustrated by zones and associated subsidiary structures. At a deposit the Kirkland Lake Main Break, a brittle structure hosting the scale, the nature, distribution, and intensity of the wall-rock giant Kirkland Lake deposit exploited by seven mines that alteration is controlled mainly by the composition and com- have collectively produced more than 760 metric tonnes of petence of the host rocks and their metamorphic grade. gold (Fig. 13) (Thomson, 1950; Kerrich and Watson, 1984; 56
  9. 9. Greenstone-Hosted Quartz-Carbonate Vein Deposits FIGURE 11. (A) Mylonitic foliation, Cadillac-Larder Lake Break, Val d’Or. (B) Close-up showing mylonitic foliation within Cadillac-Larder Lake Break, Val d’Or. Ayer et al., 2005; Ispolatov et al., 2005 and references therein). Greenstone-hosted quartz-carbonate vein deposits typically formed late in the tectonic-metamorphic history (Groves et al., 2000; Robert et al., 2005) and the mineraliza- tion is syn- to late-deformation and typically post-peak greenschist-facies and syn-peak amphibolite-facies meta- morphism (cf. Kerrich and Cassidy, 1994; Hagemann and Cassidy, 2000). Most world-class greenstone-hosted quartz- carbonate vein deposits are hosted by greenschist-facies rocks. Important exceptions include Kolar (India), which formed at amphibolite facies. Greenstone-hosted quartz-carbonate vein deposits are also commonly spatially associated with Timiskaming-like FIGURE 12. (A) Vertical section of shear bands indicating a reverse-oblique regional unconformities (Fig. 14A, B, C). Several deposits sense of motion recorded by the gold-bearing Cape Ray fault zone, Newfoundland (from Dubé et al., 1996). (B) Section view showing reverse- are hosted by, or located next to, such unconformities (e.g. oblique mylonite, Cape Ray fault zone, Newfoundland. (C) Section view the Pamour and Dome deposits), suggesting an empirical showing auriferous quartz vein hosted by a second-order reverse shear temporal and spatial relationship between large gold deposits zone, Cooke mine, Chapais, Quebec (from Dubé and Guha, 1992). and regional unconformities (Poulsen et al., 1992; Hodgson, 1993; Robert, 2000; Dubé et al., 2003; Robert et al., 2005). (2000), Groves et al. (2003), and Robert et al. (2005), among others, for more information. District Scale Large gold camps are commonly associated with curva- In this section, some of the key geological characteristics tures, flexures, and dilational jogs along major compres- of prolific gold districts are presented with a special empha- sional fault zones, such as the Porcupine-Destor fault in sis on Archean deposits. Only a brief overview is presented Timmins or the Larder Lake-Cadillac fault in Kirkland Lake here, and the reader is referred to key papers by Hodgson and (Fig. 3), which have created dilational zones that allowed MacGeehan (1982), Hodgson (1993), Robert and Poulsen migration of hydrothermal fluids (cf. Colvine et al., 1988; (1997), Hagemann and Cassidy (2000), Poulsen et al. Sibson, 1990; Phillips et al., 1996; McCuaig and Kerrich, 57
  10. 10. B. Dubé and P. Gosselin FIGURE 13. (A) Section view showing the 25 M oz Kirkland Lake Main Break. (B) Close-up of photo (A) showing the Kirkland Lake Main Break in section view; note the brittle nature of the structure with gouges. 1998; Hagemann and Cassidy, 2000; Kerrich et al., 2000; ultramafic komatiitic flows that are intruded by intermediate Groves et al., 2003; Goldfarb et al., 2005; Ispolatov et al., to felsic porphyries, and locally swarms of albitite and/or 2005; Robert et al., 2005). In terms of geological setting, lamprophyre dykes (cf. Hodgson and MacGeehan, 1982). large gold districts, such as Timmins, are mainly underlain Irrelevant to their age, Timiskaming-like regional unconfor- by tholeiitic basalts (commonly variolitic) (Fig. 14D) and mities, distributed along major faults or stratigraphical dis- FIGURE 14. (A) Timiskaming conglomerate, Kirkland Lake. (B) Mineralized quartz veins hosted by a carbonatized Timiskaming conglomerate, Pamour mine, Timmins. (C) Mineralized quartz vein hosted in a discrete brittle-ductile high-strained zone hosted by weakly deformed Timiskaming conglomerate, Kirkland Lake. (D) Variolitic basalt, Vipond Formation, Tisdale Assemblage, Timmins. 58
  11. 11. Greenstone-Hosted Quartz-Carbonate Vein Deposits continuities, are also typical of large gold camps. In terms of et al., 1994; Robert and Poulsen, 2001). As outlined by hydrothermal alteration, the main characteristic at the district Poulsen and Robert (1989), geometric ore shoots are con- scale is the presence of large-scale iron-carbonate alteration, trolled by the intersection of a given structure (i.e., a fault, a the width of which gives some indication as to the size of the shear zone, or a vein) with a favourable lithological unit, hydrothermal system(s) (e.g. Timmins). Protracted mag- such as a competent gabbroic sill, a dyke, an iron formation, matic activity with synvolcanic and syn- to late tectonic or a particularly reactive rock. The geometric ore shoot will intrusions emplaced along structural discontinuities (e.g. be parallel to the line of intersection. The kinematic ore Destor-Porcupine Fault) may also be highly significant. In shoots are syndeformation and syn-formation of the veins, many cases, U-Pb dating of intrusive rocks indicates that and are defined by the intersection between different sets of they are older than gold mineralization, in which case these veins or contemporaneous structures. The plunge of kine- rocks may have provided a competent structural trap or matic ore shoots is commonly at a high angle to the slip induced anisotropy in the layered stratigraphy that influ- direction. enced and partitioned the strain. In other cases, the intrusive Structural traps, such as fold hinges or dilational jogs rocks are post-mineralization. However, the possibility along faults or shear zones, are also key elements in locating remains that the thermal energy provided by some intrusions the richest part of an orebody. However, multiple factors are contributed to large-scale and long-lived hydrothermal fluid commonly involved, as mentioned by Groves et al. (2003), circulation (cf. Wall, 1989). and world-class and giant-size deposits commonly exhibit The presence of other deposit types in a district, such as complex geometries. This complexity is mainly due to the volcanogenic massive sulphide (VMS) or Ni-Cu deposits, is longevity of the hydrothermal system and/or multistage, bar- also commonly thought to be a favourable factor (cf. ren and/or gold-bearing hydrothermal, structural, and mag- Hodgson, 1993; Huston, 2000). The provinciality of the high matic events (Dubé et al., 2003; Groves et al., 2003; Ayer et Au content of a district may be related to specific funda- al., 2005). This is especially well illustrated at the Dome mental geological characteristics in terms of favourable mine, where low-grade colloform-crustiform ankerite veins source-rock environments or gold reservoirs (Hodgson, cut the 2690 ± 2 Ma Paymaster porphyry (Corfu et al., 1989) 1993). The local geological “heritage” of the district, in addi- (Fig. 15A). These ankerite veins have been deformed; they tion to ore-forming processes, may thus be a major factor to are typically boudinaged and are cut by extensional, en ech- take into account. elon, auriferous quartz veins (Fig. 15B, C). The <2673.9 ± Knowledge Gaps at District Scale: One of the main 1.8 Ma Timiskaming conglomerate (Ayer et al., 2003, 2005) remaining knowledge gaps at district scale is the structural contains clasts of the ankerite veins in the Dome open pit evolution, and in some cases, the tectonic significance of the (Fig. 15D, E), whereas the Timiskaming conglomerate is large-scale faults that control the distribution of the green- itself carbonatized, cut by auriferous quartz veins and locally stone-hosted quartz-carbonate-vein deposits. The nature and contains spectacular visible gold (Fig. 15F). Argillite and significance of the early stage(s) of deformation (e.g. D0- sandstone above the Timiskaming conglomerate are them- D1) of major fault zones to the circulation of gold-bearing selves folded and cut by auriferous quartz veins (Dubé et al., fluids and the formation of large gold deposits remain 2003). These chronological relationships illustrate the super- obscure. For example, despite decades of work in the imposed hydrothermal and structural events involved in the Timmins’ district, the structural evolution of the Porcupine- formation of the giant deposit with post-magmatic carbonate Destor Fault, a poorly exposed, regionally extensive, steeply veining predating the deposition of the Timiskaming con- dipping, long-lived fault (active between ca. 2680-2600 Ma), glomerate, which in turn precedes formation of the bulk of and its definite relationship to gold mineralization, remain the gold mineralization. controversial (cf. Hurst, 1936; Pyke, 1982; Bleeker, 1995; 1997; Hodgson and Hamilton, 1989; Hodgson et al., 1990; Distribution of Canadian Greenstone-Hosted Brisbin, 1997; Ayer et al., 2005; Bateman et al., 2005, and Quartz-Carbonate Vein Districts references therein). The processes controlling the distribu- The most productive Canadian metallogenic districts for tion of the large gold districts along such crustal-scale struc- greenstone-hosted quartz-carbonate vein deposits occur in tures are poorly understood and therefore remain an avenue (Late) Archean greenstone belts of the Superior, Churchill, for future research (Robert et al., 2005). Key questions and Slave provinces (Table 1). The Abitibi greenstone belt remain, such as the reason(s) why the Timmins district con- contains the majority of the productive districts, including tains a large number of world-class gold deposits, why some the very large Timmins, Kirkland Lake, Larder Lake, large-scale Archean fault zones in greenstone belts are Rouyn-Noranda, and Val d’Or districts. The Kirkland Lake devoid of significant gold deposits, and why the gold grade gold deposit is included here as a greenstone-hosted quartz- in some districts is significantly higher. carbonate deposit, however, the structural timing of gold deposition and its origin is still the subject of debate (Kerrich Deposit Scale and Watson, 1984; Cameron and Hattori, 1987; Robert and The location of higher grade mineralization (ore shoots) Poulsen, 1997; Ayer et al., 2005; Ispolatov et al., 2005) as the within a deposit has been the subject of investigation since deposit shares strong analogies with tellurium-rich syndefor- the early works of Newhouse (1942) and McKinstry (1948). mation gold deposits associated with alkaline magmatism as Ore shoots represent a critical element to take into account defined by Jensen and Barton (2000). Other younger green- when defining and following the richest part of an orebody. stone belts of the Appalachian and Cordilleran orogens are Two broad categories of ore shoots are recognized: 1) geo- also favourable terranes for quartz-carbonate vein-type gold metric, and 2) kinematic (Poulsen and Robert, 1989; Robert deposits (Fig. 16). Districts listed in Table 1 also include 59
  12. 12. B. Dubé and P. Gosselin A D B E C F Figure 15. (A) Boudinaged ankerite vein with late quartz veins cutting the Paymaster porphyry, Dome mine. (B) Boudinaged ankerite veins with syndefor- mation late extensional quartz veins, Dome mine (from Poulsen et al., 2000). (C) Massive ankerite Kurst vein cut by late gold-bearing extensional quartz vein, Dome mine area. (D) Ankerite vein clast within Timiskaming conglomerate, Dome mine (from Dubé et al., 2003). (E) Close-up of photograph (D) (from Dubé et al., 2003). (F) High-grade Timiskaming conglomerate hosting folded carbonate-pyrite veins with spectacular visible gold. The specimen was pre- sented to the Geological Survey of Canada in 1923 by the then Board of Directors of Dome Mines. Weight is 136 lbs (61.8 kg) of which about 20% by weight is gold. It most likely came from the bonanza East Dome area, which was discovered in 1910. It consists of subrounded to subangular altered and nonaltered clasts and folded crosscutting veins of coarse pyrite, ankerite, and minor quartz shattered and invaded by gold. Geological Survey of Canada National Mineral collection Sample No. 1003. Photograph by Igor Bilot, Geological Survey of Canada. deposits hosted by iron formation (BIF-hosted vein or Archean in Canada (Fig. 16). Proterozoic gold deposits Homestake-type; Poulsen et al., 2000). occur in the United States as exemplified by the Homestake The geographical and temporal distribution of greenstone- deposit, a giant iron-formation-hosted vein and disseminated hosted quartz-carbonate vein deposits containing at least 30 Au-Ag deposit, as well as in greenstone belts of Brazil and t Au is included in Figure 2. The greatest concentration of western Africa. However, Canadian deposits of Proterozoic deposits is found in the Archean, particularly in the Late age are rare; they include the New Britannia deposit in the 60
  13. 13. Greenstone-Hosted Quartz-Carbonate Vein Deposits TABLE 1. Most productive Canadian districts for greenstone-hosted Flin Flon district (Manitoba) and other smaller deposits of quartz-carbonate vein deposits. the Churchill Province, as well as gold-bearing quartz-car- Production & Resources bonate veins in the central metasedimentary belt of the District Geological Province Reserves (tonnes Au)* (tonnes Au)* Grenville Province (Carter, 1984; Jourdain et al., 1990; Timmins Superior/Abitibi 2,072.9 78.5 Easton and Fyon, 1992). Mesozoic and Cenozoic deposits Kirkland Lake Superior/Abitibi 794.8 72.6 are less common, but are important within Circum-Pacific Val d'Or Superior/Abitibi 638.9 171.6 collisional orogenic belts (e.g. the Mesozoic Mother Lode Rouyn-Noranda Superior/Abitibi 519.6 66.5 and Alleghany districts, and the Cenozoic Alaska-Juneau and Larder Lake Superior/Abitibi 378.7 14.5 Treadwell deposits, USA). The only world-class Mesozoic Malartic Superior/Abitibi 278.7 23.2 Canadian deposit (Fig. 16) is the Bralorne-Pioneer deposit Red Lake** Superior/Uchi 128.0 17.2 (British Columbia). Other smaller deposits (not represented Joutel Superior/Abitibi 61.4 27.5 in Fig.16) were also formed in the Cordilleran during the Matheson Superior/Abitibi 60.4 9.7 Mesozoic, and in the Appalachians during Paleozoic times. Cadillac Superior/Abitibi 22.1 25.1 Additionally, three important unexploited deposits (as of Pickle Lake Superior/Uchi 90.4 8.1 December 31, 2004) are noted on Figure 16: Rice Lake Superior/Uchi 51.6 25.2 1) Hope Bay (Hope Bay district, Northwest Territories, Beardmore-Geraldton Superior/Wabigoon 123.5 35.1 210 t Au in unmined reserves and resources), Michipicoten Superior/Wawa 41.1 2.8 2) Moss Lake (Shebandowan district, Ontario, 69 t Au, Mishibishu Superior/Wawa 26.7 16.8 Goudreau-Lolshcach Superior/Wawa 8.8 19.6 resources), Flin Flon Churchill 62.2 12.7 3) Box (Athabaska district, Saskatchewan, 29 t Au, Lynn Lake Churchill 19.5 14.6 resources, as of December 1998). La Ronge Churchill 3.4 5.6 The following deposits, which are located inside districts Keewatin Churchill-Hearne 7.2 252.4 represented on Figure 16, also contain important unmined Yellowknife Slave 432.8 16.6 resources (as of December 31, 2004, unless otherwise indi- MacKenzie Slave 38.1 286.6 cated): Cassiar Cordillera 14.9 55.4 1) Tundra (Mackenzie district, Northwest Territories, 262 t Baie Verte Appalachian/Dunnage 10.3 8.9 Au), *as of December 31, 2002 **does not include the Campbell-Red Lake, Cochenour, and MacKenzie 2) Goldex (Val d’Or district, Quebec, 56 t Au), Red Lake deposits as they are not considered typical greenstone- hosted quartz-carbonate deposits Arctic Platform Hope Bay Bear Slave Keewatin- MacKenzie Churchill Cordillera Yellowknife Box Hudson Bay Cassiar Interior Lowlands Lynn Lake Churchill Platform Flin Flon LaRonge Superior Pickle Lake Rouyn-Noranda Beardmore- Bralorne-Pioneer Geraldton Matheson Cadillac Grenville Rice Lake Timmins Moss Lake Abitibi Baie Legend Val d'Or Verte Mishibishu Malartic Cenozoic Phanerozoic Michipicoten Larder Lake Mesozoic Proterozoic-Phanerozoic Goudreau Paleozoic Precambrian Proterozoic Archean Kirkland Lake Greenstone-hosted quartz- (>30 t Au) Central meta- Appalachians sedimentary Belt carbonate vein deposit (<30 t Au) FIGURE 16. Location of Canadian greenstone-hosted quartz-carbonate vein districts. See Appendix 1 for deposit details. 61
  14. 14. B. Dubé and P. Gosselin Consequently, once a deposit is appropriately classified, exploration models are relatively well defined (cf. Hodgson, 1990, 1993; Groves et al., 2000, 2003). Since the early 1980s, several different genetic models have been proposed to explain the formation of greenstone-hosted quartz-carbon- ate vein deposits and this has resulted in significant contro- versy. Some of this controversy is caused by the difficulty in metamorphosed greenstone terranes to classify certain key deposits, such as Hemlo (Lin, 2001; Muir, 2002; Davis and Lin, 2003), due to the poor preservation of primary charac- teristics largely obscured by post-mineralization deforma- tion and metamorphism. Thus, adequate classification of gold deposits is a key to formulating successful exploration models (Poulsen et al., 2000). An excellent review of the various proposed genetic models, and the pros and cons of FIGURE 17. Fine-grained chloritized albitite dyke on the 4175 foot level of each of these, has been presented by Kerrich and Cassidy the McIntyre mine, intruding sericitized Pearl Lake porphyry. Both the (1994). Since then, Hagemann and Cassidy (2000), Kerrich albitite dyke and the altered porphyry are cut by quartz-ankerite-albite et al. (2000), Ridley and Diamond (2000), Groves et al. veins (from Brisbin, 1997; photograph by Nadia Melnik-Proud, caption after Melnik-Proud, 1992; photo obtained by B. Dubé from D. Brisbin). (2003), and Goldfarb et al. (2005), among others, have also revisited the subject. Only a brief summary is presented here. 3) Taurus (Cassiar district, British Columbia, 50 t Au, as of Several genetic models have been proposed during the December 1999), last two decades without attaining a definite consensus. One 4) Lapa-Pandora-Tonawanda (Cadillac district, Quebec, 54 t of the main controversies is related to the source of the flu- Au including 36 t Au as reserves). ids. The ore-forming fluid is typically a 1.5 ± 0.5 kb, 350 ± 50°C, low-salinity H2O-CO2 ± CH4 ± N2 fluid that trans- Associated Mineral Deposit Types ported gold as a reduced sulphur complex (Groves et al., Greenstone-hosted quartz-carbonate vein deposits are 2003). Several authors have emphasized a deep source for thought to represent the main component of the greenstone gold, with fluids related to metamorphic devolatilization, deposit clan (Fig. 1) (Poulsen et al., 2000). However, in and deposition of gold over a continuum of crustal levels (cf. metamorphosed terranes, other types of gold deposits Colvine, 1989; Powell et al., 1991; Groves et al., 1995). formed in different tectonic settings and/or crustal levels, Others have proposed a magmatic source of fluids (cf. such as Au-rich VMS or intrusion-related gold deposits, may Spooner, 1991), a mantle-related model (Rock and Groves, have been juxtaposed against greenstone-hosted quartz-car- 1988), drifting of a crustal plate over a mantle plume bonate vein deposits during the various increments of strain (Kontak and Archibald, 2002), anomalous thermal condi- that characterize Archean greenstone belts (Poulsen et al., tions associated to upwelling asthenosphere (Kerrich et al., 2000). Although these different gold deposits were formed at 2000), or deep convection of meteoric fluids (Nesbitt et al., different times, they now coexist along major faults. 1986). Hutchinson (1993) has proposed a multi-stage, multi- Examples include the Bousquet 2 - Dumagami and LaRonde process genetic model in which gold is recycled from pre- Penna Au-rich VMS deposits that are distributed a few kilo- enriched source rocks and early formed, typically subeco- metres north of the Cadillac-Larder Lake fault east of nomic gold concentrations. Hodgson (1993) also proposed a Noranda (Fig. 3), where the fault zone hosts the former multi-stage model in which the gold was, at least in part, O’Brien and Thompson Cadillac greenstone-hosted quartz- recycled from gold-rich district-scale reservoirs that resulted carbonate vein deposits. Intrusion-related syenite-associated from earlier increments of gold enrichment. disseminated gold deposits, such as the Holt-McDermott and The debate on gold genesis was, at least in part, based Holloway mines in the Abitibi greenstone belt of Ontario, upon interpretations of stable isotope data, and after more occur mainly along major fault zones, in association with than two decades, it is still impossible to unequivocally dis- preserved slivers of Timiskaming-type sediments and conse- tinguish between a fluid of metamorphic, magmatic, or man- quently are spatially associated with greenstone-hosted tle origin (Goldfarb et al., 2005). The significant input of quartz-carbonate vein deposits (Robert, 2001). meteoric waters in the formation of quartz-carbonate green- stone-hosted gold deposits is now, however, considered Genetic and Exploration Models unlikely (Goldfarb et al., 2005). The magmatic and mantle- Poulsen et al. (2000) has indicated that one of the main related models mainly based on spatial relationships problems in deformed and metamorphosed terranes, such as between the deposits and intrusive rocks, are challenged by those underlain by greenstone belts, is that many primary crosscutting field relationships combined with precise U-Pb characteristics may have been obscured by overprinting zircon dating. These show that, in most cases, the proposed deformation and metamorphism to the extent that they are magmatic source for the ore-forming fluid is significantly difficult to recognize. This is particularly the case with gold- older than the quartz-carbonate veins. For example, in the rich VMS or intrusion-related deposits. But since green- Timmins area, the quartz-carbonate veins hosting the gold stone-hosted quartz-carbonate vein deposits are syn- to late mineralization at the Hollinger-McIntyre deposit cut an main phase of deformation, their primary features are, in albitite dyke intruding the Pearl Lake porphyry (Fig. 17). most cases, relatively well preserved (Groves et al., 2000). One such albitite dyke was dated at 2673 +6/-2 Ma 62
  15. 15. Greenstone-Hosted Quartz-Carbonate Vein Deposits TURBIDITE-hosted (Marmont and Corfu, 1989) and more recently at 2672.8 ± VEIN 1.1 Ma (Ayer et al., 2005). Thus the albitite dyke is ca.15 Ma WACKE-SHALE GREENSTONE-hosted VEIN younger than the 2689 ± 1 Ma Pearl Lake porphyry and var- ious porphyries in the regions ranging in age from 2691 to 3 2687 Ma (Corfu et al., 1989; Ayer et al., 2003). These HOMESTAKE- BRITTLE - TYPE chronological relationships rule out the possibility that the SULPHIDE BODY DUCTILE ZONE 1 ore fluids could be related to known intrusions. An alterna- tive to the magmatic fluid source model is one in which VOLCANIC intrusions have provided the thermal energy responsible, at least in part, for fluid circulation (cf. Wall, 1989). The man- tle-related model was mainly based on the close spatial rela- IRON FORMATION SHEAR ZONE tionship between lamprophyre dykes and gold deposits GRANITOID (Rock and Groves, 1988). Key arguments against such a FIGURE 18. Schematic diagram illustrating the setting of greenstone-hosted model have been presented by Wyman and Kerrich (1988, quartz-carbonate vein deposits (from Poulsen et al., 2000). 1989). Recently, Dubé et al. (2004) have demonstrated that the lamprophyre dykes spatially associated with gold miner- geometry of mixed lithostratigraphic packages; and 3) evi- alization at the Campbell-Red Lake deposit, although differ- dence for multiple mineralization or remobilization events ent than the typical greenstone-hosted quartz-carbonate vein (Groves et al., 2003). The empirical spatial and potentially deposit, are at least 10 Ma younger than the main stage of genetic (?) relationship between large gold deposits and a gold mineralization. Timiskaming-like regional unconformity represents a key Each of these models has merit, and various aspects of all first-order exploration target irrelevant to the deposit type or or some of them are potentially involved in the formation of the mineralization style, as illustrated by large gold districts quartz-carbonate greenstone-hosted gold deposits in meta- such as Timmins, Kirkland Lake, and Red Lake (Poulsen et morphic terranes. However, the overall geological settings al., 1992; Hodgson, 1993; Robert, 2000; Dubé et al., 2000, and characteristics suggest that the greenstone-hosted 2003, 2004; Robert et al., 2005). quartz-carbonate vein deposits are related to prograde meta- Knowledge Gaps morphism and thermal re-equilibration of subducted vol- cano-sedimentary terranes during accretionary or collisional Several outstanding problems remain for greenstone- tectonics (cf. Kerrich et al., 2000, and references therein). hosted quartz-carbonate vein deposits. As mentioned above, The deep-seated, Au-transporting fluid has been channelled the sources of fluid and gold remain unresolved (Ridley and to higher crustal levels through major crustal faults or defor- Diamond, 2000). Other critical elements are listed in mation zones (Figs. 1, 18). Along its pathway, the fluid has Hagemann and Cassidy (2000) and Groves et al. (2003). In dissolved various components, notably gold, from the vol- practical terms, the three most outstanding knowledge gaps cano-sedimentary packages, which may include a potential to be addressed are 1) better definition of the key geological gold-rich precursor. The fluid will then precipitate sulphides, parameters controlling the formation of giant gold deposits; gold, and gangue minerals as vein material or wall-rock 2) controls on the high-grade content of deposits or parts of replacement in second- and third-order structures at higher deposits; 3) controls on the distribution of large gold districts, crustal levels through fluid-pressure cycling processes such as Timmins or Val d’Or; and 4) the influence of the early (Sibson et al., 1988) and temperature, pH, and other physico- stage structural history of crustal scale faults on their gold chemical variations. endowment. The classification of gold deposit types remains Nevertheless, the source of the ore fluid, and hence of a problem, which is more than an academic exercise as it has gold in greenstone-hosted quartz-carbonate vein deposits, a major impact on exploration strategies (e.g. what type of remains unresolved (Groves et al. 2003). According to deposit to look for, where, and how?) (Poulsen et al., 2000). Ridley and Diamond (2000), a model based on either meta- However, the reasons why geological provinces, such as the morphic devolatilization or granitoid magmatism best fits Superior province and the Yilgarn craton are so richly most of the geological parameters. These authors indicated endowed are now much better understood (Robert et al., that the magmatic model could not be ruled out simply on 2005). It is also believed that integrated research programs, the basis of a lack of exposed granite in proximity of a such as the Geological Survey of Canada EXTECH, Natmap, deposit with a similar age, because the full subsurface archi- or Targeted Geoscience Initiative, where various aspects of tecture of the crust is unknown. Ridley and Diamond (2000) the geology of a gold mining district or camp are addressed, also indicated that the fluid composition should not be remain an excellent approach for developing additional expected to reflect the source. The fluid travels great dis- understanding of these deposits. The most fundamental ele- tances and its measured composition now reflects the fluid- ments to take into account to successfully establish the com- rock interactions along its pathway, or a mixed signature of plex evolution and relationships between mineralizing the source and the wall rocks (Ridley and Diamond, 2000). event(s), geological setting, and deformation/metamorphism phase(s) are 1) basic chronological field relationships, com- In terms of exploration, at the geological province or ter- bined with 2) accurate U-Pb geochronology. rane scale, geological parameters that are common in highly auriferous volcano-sedimentary belts include 1) reactivated Acknowledgements crustal-scale faults that controlled emplacement of por- phyry-lamprophyre dyke swarms; 2) complex regional-scale This synthesis has been made possible by the kind co- operation of numerous company, government, and university 63

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