Eleonore, Roberto

Quebec, Canada

Main commodities: Au
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The Eleonore district and Roberto gold deposit are located in in the eastern part of the Opinaca reservoir in the James Bay area of central Quebec in Canada, 320 km north of Matagami and 825 km north of Montreal.

The minerlisation of the Eleonore district straddles the contact between the 2700 to 2648 Ma Opinaca metasediments and the 2752 to 2697 Ma La Grande volcano-plutonic sub-province within the Archaean Superior Province of Canada. The La Grande Sub-province comprises 4 volcanic cycles, represented respectively by the Kauputauch, Natel, Anatacau-Pivert and Komo-Kasak Formations, which were progressively erupted between 2752 and 2705 Ma, and are overlain by sedimentary rocks of the Wabamisk, Anaconda, and Clarkie Formations which were deposited between 2703 Ma and 2697 Ma. The Opinaca sub-province comprises a younger sedimentary cycle, represented by the Auclair Formation which is dominated by 2697 and 2674 Ma paragneiss. These supracrustal rocks are intruded by 2747 to 2710 Ma syn-volcanic, 2710 to 2697 Ma syn-tectonic and 2697 to 2618 Ma post- or late-tectonic tonalite-trondhjemite- granodiorite suites.

Three deformational events are interpreted to have affected these rocks. D1 generated a penetrative fabric oriented sub-parallel to bedding that is best developed in paragneiss, locally represented by tight to isoclinal asymmetric folds in S0 and local high strain zones can be attributed to that event. D2 is the most important of the area, having generated a W- to NW-trending penetrative fabric that locally affects alumino-silicate and biotite porphyroblasts, with the S2 fabric in the paragneiss locally present as a crenulation cleavage. F2 folds are open and asymmetric, and are best observed in the mineralised zones. D3 generated folds and fabrics similar to those associated with D2. D1 generated an ~E-trending foliation; D2 generated a NE- to N- trending foliation, and D3 generated a WNW- to NW-trending non-penetrative fabric. Regional metamorphic grades vary from greenschist to amphibolite facies, reaching granulite facies near the center of the Opinaca basin, while contact metamorphism nearby syn- to post-tectonic intrusions is at amphibolite grade.

The Eleonore district is centered on a discrete, ~10 km diameter tonalite-diorite intrusion, to the north of the margin of an extensive batholitic complex, and is within the contact zone between the Opinaca and La Grande sub-provinces. The intrusion occurs as a large crescent shape open to the east, with lamprophyre and aplite dykes cross-cutting the intrusion with 40 to 90° trends. Aluminous metasediments and conglomerates bound this intrusion to the north and south while it is in contact with basaltic units and conglomerates to the west. A thin wedge of sediments separates the intrusion from tonalitic gneisses to the east. The La Grande sequence is represented by volcanic rocks to the south, comprising locally pillowed basalts and minor intermediate to felsic lapilli tuffs. This volcanic sequence is unconformably overlain by a conglomerate unit, containing large size clasts of mostly dioritic composition, which grades grades to the north into a turbidic greywacke sequence which hosts the Roberto Deposit.

The Roberto Deposit is hosted by the sedimentary rocks along the northern edge of the intrusion, which comprise an assemblage of sandstone, wacke and conglomerate. These sedimentary rocks evolve into pegmatite-bearing paragneiss to the north and to the west, implying a steep metamorphic gradient.

Gold mineralisation was ongoing during D2, but could have been initiated as early as D1. Since the Ell Lake intrusion is multiphase and locally affected by S1, it may have been temporally coincident with gold mineralisation.

The Roberto deposit is composed of numerous gold zones, the two main of which are known as the Roberto and East-Roberto lenses. The Roberto packages are located in close proximity to the Ell Lake intrusion and are hosted by polydeformed sedimentary rocks. The main geological characterisitcs if the Roberto lenses are:   i). The Roberto gold bearing system is stratabound, sediment hosted and possibly intrusive related deposit.   ii). Mineralisation is partially controlled by rock type, associated with thinly-bedded sedimentary units (greywacke), part of a turbiditic sequence.   iii).  The mineralized zones comprise stockworks of quartz-tourmaline-actinolite-arsenopyrite-pyrrhotite veins and veinlets contained within microcline (potassic alteration) and brown to black tourmaline replacement zones with variable amounts of disseminated arsenopyrite and pyrrhotite.

The structural hanging wall of the mineralised zones is characterized by greywacke containing centimetre-scale aluminosilicate porphyroblasts, metre-scale garnetiferous metasomatic zones and moderate potassic alteration. The structural footwall is made up of paragneiss units containing a high percentage of biotite.

The sulphide content of the mineralised zones varies from 2 to 5% mostly as arsenopyrite, pyrrhotite and pyrite. The host rock may contain trace to 2% sulphide, and mostly in the structural hanging wall. In places the 'ore' zones occur as stacked sub-parallel stratabound lenses (up to 5 to 6 lenses) separated by 10 to 50 m of low grade to barren 'waste' rock. The 'ore' zones are generally 5 to 6 m in true thicknesses, occasionally varying from 2 to >20 m. At the surface the lithologic units that contain the mineralised zones have a minimum strike length of of 1.9 km and persist for at least 1000 m below the surface.

Resources within the Eleonore District in December 2008 were (Gold Corp. website):
    Measured + indicated resource - 7.05 Mt @ 10.05 g/t Au = 71 t of contained Au
    Inferred resource - 7.39 Mt @ 12.75 g/t Au = 94 t of contained Au

Main source: Simoneau, et al., 2007, available from Gold Corp website http://www.goldcorp.com

The most recent source geological information used to prepare this summary was dated: 2009.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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