Characterization of the chemistry of fluid(s) responsible for gold deposition at the South Barnat zone, Canadian Malartic Deposit, Québec

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1 Kayla Helt 03/19/10 MSc Research Proposal Characterization of the chemistry of fluid(s) responsible for gold deposition at the South Barnat zone, Canadian Malartic Deposit, Québec Introduction Uncertainties in Deposit Models Genetic models for Archean gold deposits can rarely be determined with confidence due to overprinting by metamorphism and deformation, and a general lack of preservation for primary evidence of ore-forming processes. In many cases, exploration models are adopted based on preliminary field observations to drive a project forward, and realistic genetic models may emerge only after years or even decades of debate. For example, at Osisko Mining Corporation s Canadian Malartic property, an area with a mining legacy dating back to the 1930s, porphyry, orogenic and intrusion-related gold models have all been proposed for mineralization in the area. At present, Osisko has adopted the porphyry-au type exploration model for their open-pit operation based on the close association between voluminous, low-grade, disseminated gold mineralization and an intermediate porphyry intrusion, as well as the presence of widespread potassic alteration. This study will test the porphyry-au model by evaluating the fluid(s) responsible for ore deposition. For instance, if fluid inclusions are of high salinity, and are vapour rich, this might indicate a porphyry type environment of deposition, but by contrast, if they are carbonic and of moderate salinity, they would have the signature more typical of a mesothermal or orogenic gold deposit. This study will examine the nature of ore-forming fluids in the South Barnat zone. This satellite zone (42.5 Mt at 1.64 g/t Au; (Osisko, 2009) is an integral component of the Canadian Malartic mine project.as it has a significantly higher gold grade than the main Canadian Malartic deposit.

2 Background The Superior Province forms the central part of the Canadian Shield and represents the mining heartland of Canada. This geological province is world-renowned for its numerous high-grade precious metal and polymetallic deposits, but perhaps most of all for the Abitibi Subprovince where most of Canada s world-class gold deposits are located. The Abitibi Subprovince is the largest, and among the richest Archean greenstone belts in the world, approximately 650 km long, 150 km wide, and extending east-northeast from Wawa in central Ontario to Chibougamau in north-central Quebec (Doucet et al., 2009). Gold production from this area comprises approximately 5,000 tons of the total 8,000 tons extracted from the entire Superior Province (Robert, 1996 in Wyman et al., 1999), and is derived from several different types of deposits (Percival, 2007; Figure 1). Osisko is presently developing the Canadian Malartic deposit (283 Mt of 1.12 g/t Au; Osisko, 2009) as a very large tonnage, low grade, disseminated gold deposit hosted by potassically-altered, silicified greywackes and porphyritic intrusives, and schistose, carbonatized and biotitized ultramafics. This open-pit project follows a forty-one year mining legacy ( ) in the area whch included three independent underground operations (Canadian Malartic, Sladen/Barnat, and East Malartic) that accounted for 4.2 million ounces of previous gold production (Belzile, 2009). Excluding past production, the deposits within the Canadian Malartic area currently represent one of the largest gold resources in Canada, at 11.2 million ounces gold at an average grade of 1.1 g/t Au (Osisko, 2009). The enormity of this resource is due to Osisko s vigorous exploration campaign, mainly successful infill drilling between the Canadian Malartic deposit and the South Barnat zone. The South Barnat zone represents the eastern extension of the 3000 m long east-west striking Canadian Malartic mineralized system, along the southern edge of the Cadillac Larder Lake tectonic zone (Figure 2). The average gold grade at the South Barnat is higher than the Canadian Malartic deposit and adding the South Barnat deposit to the Canadian Malartic project decreases operating costs, increases the global measured and indicated resource by 43 percent and will extend the mine life by 25 percent (Osisko, 2009; Figure 2).

3 Figure 1. Simplified geological map of the Abitibi greenstone belt showing the distribution of major fault zones and gold deposits. Modified from Dubé and Gosselin (2006) and Poulsen et al. (2000) in Percival (2007). SOUTH BARNAT Figure 2. Whittle pit projection outlining the Canadian Malartic and South Barnat deposits (Osisko, 2009).

4 Regional Geology The Superior Province and the Abitibi Greenstone Belt The Superior Province is the largest Archean province in the world and forms the central part of the Canadian Shield. It is surrounded by provinces of Paleoproterozoic age to the west, north and east, and of Mesoproterozoic age to the southeast (Percival, 2007). The Superior Province evolved from 2.72 Ga through 2.68 Ga through accretion of Mesoarchean continental fragments and Neoarchean oceanic plates via plate tectonic processes and consequent post-orogenic effects (Percival, 2007). This aggregation of plates generated the linear subprovinces and subparallel boundary faults characteristic of the Superior Province (Figure 1). One prominent zone of boundary faults is the richly gold-mineralized Cadillac- Larder Lake tectonic zone, which forms part of the southern boundary of the Abitibi greenstone belt, dividing the Abitibi and Pontiac subprovinces. This tectonic zone is believed to represent a south-verging thrust, carrying rocks of the Abitibi subprovince over the Pontiac subprovince (Benn et al., 1994; Dimroth et al., 1984; Feng and Kerrich, 1991, 1992; Calvert et al., 1995; Calvert and Ludden, 1999; Ludden and Hynes, 2000; Poulsen et al., 2000; Davis, 2002). Historically, the Abitibi subprovince has been characterized as having northern, central and southern regions with overlapping tectonostraigraphic histories (Percival, 2007). The northern Abitibi subprovince consists of volcanic rocks associated with layered intrustions, the central Abitibi subprovince of plutonic rocks, and the southern Abitibi subprovince of sedimentary and volcanic rocks. The Pontiac subprovince is composed mainly of metasedimentary schist and paragneiss derived from turbiditic greywacke and minor conglomerate (Percival, 2007). Local Geology The South Barnat Deposit The study area lies adjacent to the Cadillac-Larder Lake tectonic zone. The deposit is hosted by potassically-altered porphyry dykes and schistose, carbonatized and biotitic mafic-ultramafic rocks of the Piché Group north of the tectonic zone, and potassically-altered, silicified greywackes of the Pontiac Group to the south. Although gold mineralization does occur north of the tectonic zone, most occurs within the sedimentary domain, where gold is spatially associated with small porphyritic monzodiorite bodies and younger diorite intrusions (Poulsen et al., 2000). At the Canadian Malartic deposit, gold mineralization occurs as fine-grained native gold and subordinate tellurides, and is localized in a nearly continuous shell of 1-5% disseminated pyrite accompanied by trace amounts of chalcopyrite, galena and a suite of Ag-Bi-Pb-bearing telluride minerals. By contrast, at the South Barnat zone, the gold (native gold and minor petzite, Ag 3 AuTe 2 ) occurs in veinlets of quartz-pyrite-carbonate surrounded by alteration haloes of biotite +/- Kfeldspar (Figure 3a). Gold is found both within the veinlets and in the altered rock at vein margins, especially at pyrite-pyrite grain contacts, pyrite-silicate grain contacts, and as inclusions in pyrite and amphibole (Figure 3; J. Clark, pers. comm., 2009). Although gold at the Canadian Malartic deposit and at the South Barnat zone occurs as native grains and subordinate tellurides associated with pyrite, the proximity of the South Barnat zone to the Cadillac-Larder lake tectonic zone may contribute to the higher gold grade, as later stage tectonic zone fluids might have remobilized/enriched the zone.

5 a 1mm Figure 3. a) quartz-pyrite veinlet with biotite alteration. b) enlargement of (a) showing the close association of pyrite with biotite. c) same as in (b) in reflected light showing gold in pyrite. d) quartz-pyrite-carbonate veinlet. e) same as in (d) in reflected light showing gold grain within pyrite. f) same vein as (d) showing gold grain within galena. g) fluid inclusions in quartz from same vein as (d). b c Au 0.5 mm 0.5 mm d e Au 0.5 mm 0.5 mm f g L-V Au V-rich? petzite 0.1 mm 0.1 mm

6 Chemical characterization of fluids responsible for gold deposition at the South Barnat deposit, Canadian Malartic Property, Québec Research Objectives The main goal of this project is to characterize the fluids responsible for gold deposition at the South Barnat deposit, and provide information for or against the currently adopted Archean porphyry-au type exploration model for the deposit. To achieve this objective it will be necessary to elucidate i) the nature and distribution of alteration, mineralization, and veining; ii) the timing of gold mineralization; iii) the physicochemical conditions of the hydrothermal system; iv) the composition of the mineralizing fluids; and v) the effects of changes in physiochemical conditions which led to to the deposition of gold. Methodology The nature and distribution of alteration, mineralization and veining, as well as mineral phase chemistry and mineral evolution, will be examined using a combination of optical microscopy, scanning electron microscopy (SEM), and electron microprobe analysis (EMPA). Guided by the mineral paragenesis and availability of fluid inclusions suitable for microthermometric analysis, fluid inclusion assemblages will be selected for study. Primary gold mineralization in greywacke and porphyry post-dates porphyry intrusion, and occurs in veinlets of quartz-pyrite-carbonate with alteration haloes of biotite +/- kspar, and is particularly associated with pyrite (Figure 3a). Preliminary fluid inclusion petrography has identified suitable quartz-hosted fluid inclusions in these veinlets (<5 μm liquid-vapour inclusions; Figure 3d), and if pyrite proves to be sufficiently transparent such that inclusions can be observed with infrared (IR) microscopy, they will also be studied. Inclusions of fluid in pyrite would be optimal for studying the ore-forming fluid, given the close association of pyrite and gold, and the likelihood that the ore-forming fluid co-precipitated the pyrite and gold. A complete study of an ore-forming system should, however, contain more than just fluid inclusion analyses from the ore stage to address the question of whether the fluids responsible for mineralization are significantly different from the fluid responsible for precipitation of the alteration assemblage (Stoffell et al., 2008). Therefore, fluid inclusions in feldspar and carbonate will also be incorporated into the study. With explicit samples of ore-forming fluids, the physicochemical conditions of the hydrothermal system can be evaluated. The quartz-biotite geothermometer will be used to estimate the temperature of the system during the mineralizing event and microthermometry will be used to estimate total salinity, and constrain pressure and temperature conditions of fluid entrapment. Quantitative microanalysis of fluid inclusions will describe the composition of the mineralizing fluids, but the nature of the fluid inclusions (% liquid, liquid-vapour, etc) will ultimately control the method of microanalysis (crushing, IC, ICP-AES, ICP-MS, AAS etc.); since the fluid inclusions petrology is not complete, the method has yet to be determined. Once the physiochemical conditions and fluid compositions have been determined, geochemical reaction path computer modeling will be used to evaluate a variety of possible depositional mechanisms and the effects of changes in physiochemical conditions on the solubility of gold. Two computer programs that would enable this

7 modeling are SOLVEQ, which evaluates speciation and concentration of metals in solution, and CHILLER, which evaluates depositional mechanisms (Reed, 1982). Other programs are of course possible alternatives, but the main thought is that computer modeling will integrate fluid inclusion data and simulate ore-forming processes, to give a complete characterization of the chemistry of fluids responsible for gold deposition and subsequently provide constraints on possible geochemical models. Expected contributions By conducting a thorough fluid inclusion study, the compositions of ore-forming fluids will be determined, which can then be used to clarify the origin and evolution of the mineralizing system, and ultimately provide valuable controls for the genetic model for the deposit (e.g., Gleeson, 2003). This project will represent the first fluid inclusion study of the Malartic area deposits, and will thus provide information necessary for understanding the chemistry of the fluid(s) responsible for gold deposition at the South Barnat deposit. The currently adopted Archean porphyry-au type exploration model will be tested, and perhaps a new genetic model will be embraced to help guide Osisko s evaluation of the deposit and the exploration for similar types of mineralization elsewhere.

8 References Belzile, E. Technical Report for South-Barnat Deposit on behalf of Osisko Mining Corporation. Rouyn-Noranda (QC): Belzile Solutions Inc; p. Online Available < Doucet, P., Morehead, J., Lesage, D., and Côté, S. DV : Report on Mineral Exploration Activities in Québec 2008, Chapter 1B, Southern Superior Province (Abitibi and Pontiac Subprovinces) and Westernmost Grennville Province. Resource Naturelles et Faune, Government of Québec. Online Available: < Gleeson, S.A Bulk analysis of electrolytes in fluid inclusions in Fluid Inclusions: Analysis and Interpretation. Eds. Samson, I., Anderson, A., and Marshall, D. Mineralogical Association of Canada, Short Course 32, Gourd, Benoit-Beaudry. "Malartic", in The Canadian Encyclopedia (Edmonton: Hurtig Publishing, 1988), Volume 2, p Online Available: < >. Planning for Profits - Report on Mining Feature: Osisko Exploration. Online Available: < Poulsen, K H, Robert, F, and Dubé, B Geological classification of Canadian gold deposits. Geological Survey of Canada, Bulletin 540, 106 pages. Reed, M.H Calculation of multicomponent equilibria and reaction processes in sstems involving minerals, gases, and aqueous phase. Geochimica et Cosmochimica Acta. 46: Stoffel, B., Appold, M.S., Wilkinson, J.J., McClean, N.A., Jeffries, T.E Geochemistry and evolution of Mississippi-Valley-Type mineralizing brines from the Tri-State and Northern Arkansas districts determined by LA-ICP-MS Microanalysis of fluid inclusions. Economic Geology. 100: System for Electronic Document Analysis and Retrieval (SEDAR). Company profiles: Osisko. News release, December Online Available: < >. Wyman, D.A., Kerrich, R., and Groves, D.I Lode gold deposits and archean mantle plume-island arc interaction, Abitibi Subprovince, Canada. Journal of Geology. 107:

9 Figures 1. Percival, J.A., 2007, Geology and metallogeny of the Superior Province, Canada, in Goodfellow, W.D., ed., Mineral Deposits o 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 Osisko, Outline of new whittle pit December