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Amphibolite facies gold mineralization: an exemple from the Roberto deposit, Eleonore property, James Bay, Quebec.

Ravenelle, Jean-François (2013). Amphibolite facies gold mineralization: an exemple from the Roberto deposit, Eleonore property, James Bay, Quebec. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de la terre, 325 p.

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The Roberto gold deposit represents one of the most significant gold discoveries of the past 10 years in Canada. The latest resource estimation (publicly released in February 2010) indicated a total of more than 9 million ounces (280 t) of inferred (6.25 Moz, 194 t) and indicated resources (3.15 Moz, 98 t). The geology of the Roberto deposit differs from most Archean gold deposits of the Superior Province. After initial inspection of the deposit prior to this study, it was recognized that it could not clearly be identified as a typical quartz-carbonate gold deposit or as a gold deposit that formed at shallower depth. It was also recognized that the geological context of the deposit is complex, being characterized by stockwork and replacement-style mineralization hosted within highly deformed and metamorphosed turbiditic metagreywacke and paragneiss. The Roberto deposit is a landmark for future gold exploration within the under-explored northern part of the Superior Province. However, being an atypical gold deposit, the key geological parameters involved in its genesis need to be identified in order to understand its formation and define exploration criteria that will help discover similar gold deposits in the James Bay territory and elsewhere in amphibolite-grade terranes of the Superior Province. The purpose of this thesis is to present a geological description of the Roberto gold deposit and provide insights on geological features that played a role in its formation. Using regional and local geological mapping, structural analysis, 3D modelling, core logging, petrography, and geochronology, efforts are focused on acquiring an understanding of the regional geology, the relative ages of the main lithological units, the geochemical characteristics of the deposit, and the relative timing between gold mineralization, deformational events, and magmatic phases. The Roberto deposit is located on the Éléonore property which straddles the contact between the Archean metasedimentary Opinaca Subprovince and the volcano-plutonic La Grande Subprovince. The main deformation responsible for regional metamorphism and the predominant east-west structural fabric is attributed to D2 within both subprovinces. The Opinaca Subprovince is a migmatite terrane with metamorphic grade reaching the granulite facies. The metamorphic grade in the La Grande Subprovince ranges from greenschist to amphibolite facies. The presence of widespread conglomeratic sequences within the La Grande indicates that an important uplift (potentially associated with D1) occurred in the area. Such conglomeratic sequences may mask important deformation zones that may have played a role in the metallogenic history of the region. Further work is required in to determine whether such deformation zones juxtaposed the two terranes or whether the current terrane distribution reflects the original setting of the area. The bulk of the auriferous system primarily occurs in a series of sub-parallel decametre- scale auriferous zones globally confined to a km-scale steeply-plunging polyfolded geometry that has been traced to more than 1500 meters below surface. Two principal auriferous zones, referred to as Roberto and East-Roberto, can be distinguished from one another based on their structural style, mineralogy, gold content, and stratigraphic position. The Roberto auriferous zone (typically averaging 12 g/t Au over 10 meters) represents the main phase of gold mineralization and consists of stockworks of K-bearing veins and veinlets and Ca-bearing quartz veins contained within K-altered thinly-bedded greywacke. The K-bearing veinlets are typically composed of quartz, microcline, dravite (Mg-rich tourmaline), biotite, arsenopyrite/lôllingite, and pyrrhotite. Ca-bearing quartz veins of the Roberto zone are typically composed of quartz, amphibole, diopside, titanite, carbonate, pyrrhotite, arsenopyrite/lôllingite, and locally visible gold. The East-Roberto auriferous zone (up to 8 g/t Au over 6 m) is composed of several mineralogical assemblages that have different texturai characteristics including hydrothermal breccias, veins, and zones of silica replacement. In terms of mineralogy, the Ca-bearing alteration of the Roberto and East-Roberto zones in association with microcline, tourmaline, arsenopyrite, and pyrrhotite shares analogies with the mineralogical assemblage that characterizes the gold-skarn hydrothermal signature of amphibolite-facies gold deposits such as those found in hypozonal orogenic settings. Geochronological work indicates that a wide span of maximum ages occurs within Roberto's sedimentary sequence suggesting the presence of two sedimentary sequences. The greywacke host rock of the Roberto zone, dated at <2675 Ma (Geochron-ELE-05-08), is significantly younger than the phase of the EII Lake diorite dated at ca. 2705 Ma (David, 2005), which rules out the hypothesis that Roberto is genetically related to the dated phase of the EII Lake intrusion. The Roberto deposit is affected by polyphase folding but is not hosted within a shear zone. Field observations indicate that the main part of the gold mineralization has been overprinted by D2 and D3. Only in one specific location has our structural analysis enabled us to generate arguments supporting syn-D2 ore deposition. The argument consists in the presence of a meter-scale F2Z-fold where Roberto's auriferous stockwork is preferentially developed in the short limb of the fold and is primarily composed axial-planar and bedding-parallel auriferous veins and veinlets, suggesting that its formation was controlled by folding. However, since field observations commonly suggest that auriferous bodies have been overprinted by D2 rather than being generated during active F2 folding, it can only be concluded that the main auriferous event either occurred before of very early during D2. At deposit-scale, auriferous zones are confined to the F2/F3 refolded geometry which suggests that auriferous zones were preserved in fold hinges as opposed to being destroyed and remobilized on highly stretched limbs. F2 and F3 folds control the finite deposit-scale geometry of the main auriferous zones and the attitudes of ore shoots within those zones. The mineralogy and the metamorphic textures of auriferous material suggest that the veins and disseminated ore have been generated before or during the peak of metamorphism, which agrees with the pre- to early-D2 timing established from the structural analysis. A study of sulphide minerals with a scanning electron microscope (SEM) confirms that gold mineralization either occurred before the peak of metamorphism in association with pre-peak arsenopyrite, or precipitated with lôllingite at near peak conditions. Geochronology constrains the absolute age of the main stage of gold mineralization to occur between ca. 2675 Ma and 2603 Ma, the maximum age of Roberto's host rock and the age of a pegmatite dyke that crosscuts the main auriferous zones, respectively. Some of the pegmatite dykes that crosscut the mineralized zones are auriferous, which is interpreted to result from a contamination process caused by emplacement of dykes through mineralized material. Other pegmatite dykes are in petrogenetic continuity with auriferous quartz-feldspar veins, and crosscutting relationships between two aplite dykes and an auriferous quartz vein indicate that such dykes were broadly coeval with a part of the gold mineralization which occurred between ca. 2615 Ma and 2607 Ma. Pegmatite magmatism was therefore contemporaneous with a part of the gold mineralization and/or represents a second stage of gold mineralization. Even though geochronological data from the EII Lake diorite and Roberto's host rocks rules out the hypothesis that Roberto's hydrothermal system is genetically related to the EII Lake intrusion, the potential input from magmatic fluids in generating the main part of the gold mineralization should not be discarded, as indicated by the presence of feldspar porphyry dykes dated at ca. <2680 Ma in the vicinity of the deposit. The contrasts between the various styles of gold mineralization, from early stockwork to late pegmatitic veins, indicate that Roberto was affected by a long-lasting tectono-metamorphic event during which gold mineralization was generated, deformed, metamorphosed, and remobilized. The tectonic setting of the area represents an important clue in understanding the genesis of the Roberto deposit, and even though the origin of the low-P high-T metamorphism is unknown, a "deep-earlier" type model explains the early timing of gold mineralization relative to deformation and metamorphism. Accordingly, the potential input of metamorphic fluids should not be ruled out on the basis that gold mineralization was introduced early relative to tectono-metamorphic events. This could further suggest that Ca-bearing veins and replacement represent metamorphosed equivalents of quartz-carbonate veins and carbonate alteration typical of orogenic deposits, respectively. Targeting regions affected by large-scale folds, especially near the contact between the Opinaca and La Grande Subprovinces, might prove to be a valuable prospecting criterion.

Type de document: Thèse
Directeur de mémoire/thèse: Dubé, Benoît
Co-directeurs de mémoire/thèse: Malo, Michel
Informations complémentaires: Résumé avec symboles
Mots-clés libres: géologie; or; amphibolite; facies; minéralisation; aurifère; gisement Roberto; Baie James
Centre: Centre Eau Terre Environnement
Date de dépôt: 13 janv. 2014 19:08
Dernière modification: 17 mars 2016 18:11
URI: http://espace.inrs.ca/id/eprint/1945

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