February 2008    Print this article

Discovery of new mineral potential
northwest of Schefferville

Martin Simard, Charles Gosselin, Isabelle Lafrance
Bureau de l'exploration géologique du Québec (BEGQ)

During the summer of 2007, a new mapping project led to the discovery of three new volcano-sedimentary belts northwest of Schefferville (Figure 1). Moreover, during fieldwork, a gold showing grading 2.1 g/t Au was uncovered in one of the belts, indicating the presence of a potential gold-bearing trend.

As a result of mapping at 1/250,000 scale, three new volcano-sedimentary belts were uncovered in the northern part of the La Grande Subprovince, more specifically in the Rivière Sérigny area (NTS 23N and 24C). In one of these belts, a gold showing (2.1 g/t Au) was discovered in altered sulphide-rich metagabbro (Figure 2). The economic potential of Archean greenstone belts located in the north of this subprovince is well established since the discovery of many mineral occurrences in the Lac Gayot area, located directly west of the Rivière Sérigny area. (RG 99-06).

Following these initial discoveries, work by Virginia Mines confirmed the mineral potential of these belts, with the discovery of several significant Ni-Cu-PGE showings associated with ultramafic lavas (Gayot project) as well as numerous volcanogenic massive sulphide occurrences (Zn-Cu-Pb-Ag) associated with felsic volcanic rocks (Coulon project) (Figure 1).

New volcano-sedimentary belts

The discovery of three new volcano-sedimentary belts opens brand new territories to mineral exploration (Figure 2). Within these belts, mapping by the BEGQ identified many rusty and sulphide-rich zones from 1 to 10 metres in size, as well as silicate and oxide-facies iron formations. Sulphides, mainly disseminated pyrite and pyrrhotite, occur in felsic to intermediate rocks as well as mafic to ultramafic rocks. Several mineralized zones were sampled and analyzed for their base and precious metal content. Based on partial results received so far, a new gold showing in the Piscau belt has already been identified. Two samples from a rusty sulphide-rich metagabbro, 5 metres wide and traced over 50 metres strike length, yielded grades of 2.1 g/t Au and 0.5 g/t Au. The metagabbro is altered to silica, albite, sericite, chlorite, and garnet.

The newly mapped belts are essentially composed of amphibolite, but they also contain various volcanic, effusive and intrusive lithologies ranging in composition from ultramafic to felsic, as well as paragneisses and iron formations. These rocks are metamorphosed to the amphibolite to granulite facies. They are also affected in many locations by late alteration, primarily chlorite, sericite, epidote, and silica alteration. Lithological assemblages within these three new belts are assigned to the Gayot Complex, an early unit of volcano-sedimentary rocks (2880 - 2870 Ma) occurring in the northern part of the La Grande Subprovince. The Angilbert belt is 13.5 km long by 4 km wide, the Piscau belt is 8 km by 6 km, and the Cania belt measures 8 km by 2 km.

The Rivière du Sable fault… A gold-bearing structure?

The Rivière du Sable fault is a new regional fault identified during the summer of 2007 in the south part of the Rivière Sérigny area (Figure 2), which appears to extend eastward (Figure 1) based on aeromagnetic data. This regional structure may have played a role in the emplacement of major gold occurrences located on either side of its path. The new gold showing identified in the Piscau belt is located a few kilometres south of the structure, and many gold showings (MM 95-01) occur to the north of the inferred eastern extension of the Rivière du Sable fault (Figure 1). The latter showings are associated with iron formations and show gold grades ranging from 1 to 40 g/t. This potential relationship between all these gold occurrences and the Rivière du Sable fault remains hypothetical for the moment. In the study area, this major fault, outlined by the presence of mylonite zones and alteration corridors, corresponds to the boundary between the La Grande and Ashuanipi subprovinces. It marks a metamorphic contrast between amphibolite-grade rocks to the north and granulite-grade rocks to the south.

Three geological subprovinces meet

The Rivière Sérigny area lies at the junction between three geological subprovinces of the Superior Province (Figure 1). During the summer 2007 mapping survey, the nature and boundaries of these major lithotectonic assemblages were clarified. Our work namely led to the recognition of an inlier of La Grande rocks within the Ashuanipi Subprovince, just west of the Labrador Trough (Figure 1). The presence of tonalitic and dioritic gneisses, typical of the northern part of the La Grande Subprovince, served as evidence to link this part of the area to the La Grande Subprovince.

The La Grande Subprovince is characterized by the presence of early volcano-sedimentary rocks (2880 - 2870 Ma) and tonalitic and dioritic gneisses (2830 – 2810 Ma). Within the study area, the Minto Subprovince contains abundant tonalitic intrusions (2760 – 2740 Ma) as well as younger granitic and granodioritic intrusions (2735 – 2685 Ma). These intrusions also cut earlier units within the La Grande Subprovince. Finally, the younger Ashuanipi Subprovince (2700 – 2630 Ma) is primarily composed of diatexite units, granitoids, and migmatitic paragneiss. The presence of orthopyroxene in these units is an indication of granulite-grade metamorphism, as opposed to amphibolite-grade conditions observed in the other two subprovinces.

Work planned for the summer of 2008

Fieldwork carried out in the summer of 2007 outlined new areas of interest for mineral exploration. As a result, more than 250 new claims were staked following the presentation of these results during Québec Exploration 2007.

Mapping will continue in 2008 in an area further south, covering map sheet 23K and the southern quarter of map sheet 23N (Figure 1). This survey will complete the geological mapping coverage at 1/250,000 scale of the Ashuanipi Subprovince.

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