The Eastmain Belt:
the emergence of a new gold camp
By Daniel Bandyayera and Patrick Houle
Discovery of the Roberto deposit
|In 2003, Virginia Gold Mines Inc. discovered the Roberto gold deposit on the Éléonore property, northeast of Opinaca reservoir (Baie James territory), triggering a major staking rush in the Eastmain Belt. The Roberto deposit has now been traced by drilling over a strike length of 1.9 km and a depth of 1300 metres, and remains open in all directions. With estimated resources of three to five million ounces of gold, the Roberto deposit is one of the most significant discoveries made over the past 15 years in North America. In addition, land can still be acquired for exploration in the Opinaca geological subprovince, since a large part of the area remains available for staking.
Gold exploration is booming
The Éléonore property’s gold mineralization, associated with metamorphosed and metasomatized sediments, confirms the potential for high-grade gold mineralization of the Eastmain Belt, including the Eau Claire deposit held by Eastmain Resources Inc.
Exploration in this area has been so successful that it is currently one of the most promising targets for mineral exploration in Québec. For example, Everton Resources Inc. and joint-venture partner Azimut Exploration Inc. have discovered a 1.7-km gold zone, which includes the Inex Zone (3.03 g/t Au over 1.5 m), northeast of the Roberto deposit and a northeast-oriented anomalous gold corridor, 12 km long, with, at each end, the Claude target (1.0 g/t Au over 21.5 m) and the Manuel showing (12.01 g/t Au over 4.6 m on surface). The Eastmain Resources Inc., Goldcorp Inc. and Azimut Exploration Inc. joint venture recently reported an intersection grading 1.49 g/t Au over 16.0 metres on surface, from a new showing in a 10-km-long anomalous gold corridor on the Éléonore South property. The showing, known as the JT Target, is in mineralized sediments that have many points in common with the Roberto deposit. Several other gold showings were reported in 2006 by various companies in the area, including Beaufield Resources Inc., Arianne Resources Inc., Sirios Resources Inc. and Golden Valley Mines Ltd.
Regional exploration guides
As shown in the Roberto mineralized system, the P1 fold hinges and transversal or longitudinal deformation zones that affect the rock are favourable sites for remobilization, hydrothermal replacement and local enrichment of the gold mineralization. Consequently, most of the gold mineralization targets occur:
- along the contact between the sediments and the volcanics,
- in the contact zone between the La Grande and Opinaca subprovinces, and
- in the eastern part of the LaGuiche Basin (Beaumier, 2006).
Geology of the Opinaca sector
The Eastmain Belt is about 300 km long and 10 to 70 km wide. It is composed of assemblages of volcanic and sedimentary rock belonging to the La Grande Subprovince, and of paragneiss belonging to the Opinaca Subprovince . The latter forms a turbiditic basin similar to the Quetico basin in Ontario. The Opinaca sector borders the Lower Eastmain and Middle Eastmain segments.
||In summer 2006, Géologie Québec began a three-year mapping program of the sector. Preliminary results from mapping of NTS sheets 33 C/09 and 33 C/16 show that the geology of the Opinaca sector is centred on the Kasipaskatch synvolcanic plutonic dome, a multiphase synvolcanic intrusive body, 15 km in diameter, consisting of diorite, monzodiorite and porphyritic tonalite, located in the middle of Opinaca reservoir. All the volcanosedimentary units of the La Grande Subprovince are draped around the plutonic dome. They are composed, from base to top, of pillowed or brecciated basalts, intermediate to locally felsic volcaniclastics, and clastic sediments with well-preserved primary structures. Although the sedimentary and volcanic sequences are generally in fault contact, conglomerates or conglomeratic sandstones can locally be observed unconformably overlying the volcanics.
Metallogenic setting of the Opinaca sector
The transition zone between the Opinaca and La Grande subprovinces was influenced by a significant hydrothermal and porphyritic system, centred on the Kasipaskatch multiphase intrusive dome (middle of Opinaca reservoir). The system gave rise to several associations of auriferous mineralization, characterized by significant networks of metasomatic veinlets and by tourmaline and aluminosilicate alteration. The main mineralizations are:
- Roberto-type mineralization, associated with corridors of metasomatic stockworks
|This mineralization is observed in sediments, conglomerates and felsic tuffs. The first generation of metasomatic stockworks is folded and dismembered. Regionally speaking, it is possible that the hydrothermal fluids that formed the Roberto deposit were generated at a deeper level than the Ell Lake Intrusion, probably at the level of the Kasipaskatch Pluton.
Roberto-type metasomatic veins in an andalusite- and sulphide-bearing wacke located 17 km west of Roberto, northwest of the Kasipaskatch plutonic dome.
- Mineralization associated with tourmalinized volcanosedimentary sequences
Several phases of tourmaline-bearing white pegmatite were injected into the volcanosedimentary sequence. Locally, a dense network of these veins led to the pervasive tourmalinization of the host rock. The pegmatitic bodies can contain up to 20% centimetric tourmaline.
- Mineralization associated with hydrothermal breccias
Hydrothermal breccia in a wacke: the fragments are cemented by the biotite-hornblende-garnet-sulphide assemblage. |Mineralized hydrothermal breccias were developed in the basalts and wackes. The network of fractures gradually filled with biotite, garnet, black chlorite and local tourmaline, while amplifying the brecciation of the host rock. In the final stage, a mineralized brecciated rock resembling a monogenic conglomerate was formed.
- Mineralization associated with basalts
This type of mineralization is often observed on pillow borders, or locally in brecciated basalt. It occurs in the form of massive to semi-massive pyrite in the interstices between pillows. Near the mineralized zone, gradual garnet, biotite and tourmaline enrichment of the host rock has been observed.
- Mineralization associated with the contact between basalts and QFP dykes
|Major mineralized zones, decametric in thickness, have been observed along quartz feldspar porphyry dykes cutting across the basalts. The contact is often deformed or sheared and the basalt is transformed into garnet amphibolite.
Highly disseminated mineralization in the contact zone of a QFP dyke cutting across basalt.
- Mineralization associated with mudstones
Horizons of mudstone, metric in thickness, often occur within the basalts or tuffs, or at the contact between basalts and clastic sediments. They are often rusty, pyritized and silicified.
- Mineralization associated with conglomerates
All the types of conglomerate mapped contain either fragments of sulphides, or fragments of mineralized rock or of rock containing metasomatic and epidotized veins. This suggests the existence of some mineralization prior to the formation of the sedimentary sequence. Mineralization associated with the hydrothermal alteration of conglomerates or with zones of deformation crosscutting them has also been observed.
- Mineralization associated with regional deformation zones
Massive sulphides (pyrite-pyrrhotite-arsenopyrite) in a regional deformation zone at the contact between the basalt and the conglomerate. |This type of mineralization has been observed in the conglomerate south of the Roberto deposit and in the sheared contact between the basalt and the conglomerate to the east and west of the Kasipaskatch Pluton. Disseminated sulphides are preferentially concentrated in sandy conglomeratic horizons. These are mylonitized and rusty zones containing disseminated to semi-massive sulphides (pyrite+pyrrhotite+arsrnopyrite).
Metallogenic setting of the Éléonore sector
The Éléonore sector hosts two main systems of gold mineralization:
- Porphyry-type mineralization
|It consists of Au-Cu-Ag mineralization in diorite. Epidote-bearing stockworks gradually grade into tourmaline-quartz-epidote stockworks and finally into a system of tourmaline-chalcopyrite-pyrrhotite-pyrite-garnet stockworks near the mineralized zone. The mineralization observed consists of pyrite, pyrrhotite and chalcopyrite, either massive or in veinlets, grading up to 3% Cu and 19 g/t Ag.
Lake Ell diorite (Éléonore sector): proximal alteration and porphyry-type mineralization: tourmaline-quartz-garnet-sulphide (chalcopyrite-pyrrhotite-pyrite-arsenopyrite) stockworks
- Hydrothermal-replacement mineralization
The Roberto deposit is part of this type of Au-As-B-Sb-rich mineralization, consisting of pyrrhotite, pyrite and arsenopyrite. It is genetically associated with stockworks of tourmaline-biotite-garnet-arsenopyrite-pyrrhotite metasomatic veins. The hanging wall of the Roberto deposit is characterized by the presence of wacke with stockworks of metasomatic veins composed of hornblende-tourmaline-epidote-garnet, frequently containing aluminosilicates in positive relief. The footwall of the mineralized zones consists of turbiditic units that have not been affected by any hydrothermal alteration.
Two types of alteration can easily be recognized in the Éléonore sector:
Proximal alteration at the Roberto deposit: stockworks of folded veins of tourmaline-quartz-sulphide in the mineralized layers.
- a distal alteration formed by the andalusite-muscovite±cordierite assemblage. This aluminosilicate alteration is traceable for hundreds of metres around the mineralized zones;
- an alteration proximal to the mineralized zones, formed by the tourmaline±epidote±quartz±sulphide assemblage.
Beaumier, M., 2006 - L’or dans les sédiments de lacs dans la partie orientale du Bassin de LaGuiche, Québec Exploration 2006, résumés des conférences et des photoprésentations, page 48.