Rivière Octave Project: initial results from a new type of survey for sectors with scarce surface data

Pierre Rhéaume (BEGQ-MRNF)
Michel Parent (NRCAN-CGC)
Charles Maurice (BEGQ-MRNF)

As part of the MRNF's Copper Plan and the Geological Survey of Canada's Targeted Geoscience Initiative (IGC/TGI), we have developed a new type of drill-hole survey that combines the study of Quaternary deposits with sampling of the basement rock in the outcrop-poor areas of the northern part of the Abitibi region. The first phase of the survey led to the discovery of new mineralized settings likely to interest the mineral exploration industry. The most interesting discoveries are the following:

• A new unit of felsic lava with chlorite alteration and traces of disseminated sulphides
• New deformation zones with a context favourable for orogenic gold mineralization

Objectives and methodology

The objective of the Rivière Octave project is to sample and characterize the till and basement rock beneath the significant layer of glaciolacustrine deposits along a corridor about ten kilometres wide, stretching over 170 kilometres from Normétal to the Bell River (PDF Format, 1,4 Mb). The target area, for the first three years, is bordered to the north by the Marest Pluton and to the south by the Bernetz Pluton, east of Highway 109. The first phase of the project began in 2006 on NTS map sheet 32F04-SO (Bigniba Lake), where 37 vertical holes were drilled.

The sampling was done by diamond drilling along the existing road network, with an average spacing of two kilometres between holes. The drill holes were positioned to sample units of the basement rock characterized by distinct signatures on the regional aeromagnetic survey. A few outcrops along accessible logging roads were also described.

For the Quaternary sediments, the primary objective is to establish the provenance of the various till layers present by characterizing their geochemical, mineralogical and lithological composition. For each drill hole, the plan is therefore to characterize the stratigraphy, the geochemistry of the fine fraction (< 63 μm ), the heavy mineralogy of the sand fraction and the assemblage of lithic fragments. Other types of data may also be obtained, depending on the nature of the materials discovered. The results of the work on Quaternary sediments will be the subject of an upcoming publication.

For the basement rock, the plan is to characterize the lithologies of each drill hole using macroscopic and petrographic descriptions, lithogeochemistry of fresh and altered facies, and structural and magnetic susceptibility measurements. This characterization of the basement rock will lead to a new geological map that can be used as a reference to evaluate the mineral potential of the sector and to provide a geological context for exploration targets derived from the geochemistry of glacial sediments.

Preliminary geological map

The preliminary geological results stem from the visual examination of drill core collected during the first phase of the project. They are presented in Table 1 in the form of summaries. The lithological descriptions, considered with the regional magnetic survey (Format PDF, 693 Ko), made it possible to sketch the outline of a geological map for this area (Format PDF, 684 Ko). A lithogeochemical characterization of these rocks is underway. The latter, combined with the integration of data from regional compilations, will make it possible to refine the current geological picture.

Table 1

The 2006 work identified a series of major geological domains characterized by certain lithological and magnetic properties and bordered by deformation zones. The latter were traced on the basis of the degree of rock deformation observed and turned out to correspond to the southern border of two linear magnetic highs generally oriented WNW-ESE. The main lithological domains identified, from south to north, were:

1. the Bernetz Pluton,
2. basalts and amphibolites,
3. mudstones and wackes,
4. intermediate and felsic volcanics, and
5. gabbros (Format PDF, 693 Ko) (Format PDF, 684 Ko).

Although our work did not bring to light any new mineralized showings, the preliminary results make it possible to suggest certain regional exploration targets and to determine settings favourable for the following types of mineralization:

1. volcanogenic massive sulphide deposits,
2. orogenic gold deposits associated with regional deformation corridors,
3. sulphide deposits of unknown affinity.

Volcanogenic massive sulphide deposits

Click on photo to enlarge Felsic lava (dacite?) with well develloped flow banding in drill hole RO-31. The progressive change in attitude of the flow banding is not the result of folding but reflects the internal structure of a lobe.

The potential for this type of deposit is mainly related to newly identified felsic volcanic rocks in the northern part of the study area. The lobate felsic lava and tuffs with chlorite alteration suggest the presence of a felsic centre that could be conducive to the genesis of volcanogenic massive sulphides.

This alteration, which consists of dark chlorite accompanied by silica and traces of carbonates, is disseminated in small clusters within the lava. It has also been noted as fill in angular areas between the fragments of brecciated zones. The overall sulphide content of the rock is less than 1% but the chloritic areas are up to 1 cm wide and contain up to 2% finely disseminated pyrite accompanied by traces of chalcopyrite. Lithogeochemical characterization of the felsic rocks, examination of the drill core from old holes, drilling of new holes or additional geophysical surveys would make it possible to further clarify this promising setting.

Click on photo to enlarge Autobreccic felsic lava (dacite?) with chlorite alteration and traces of disseminated sulfide in drill hole RO-32.

Orogenic gold deposits associated with regional deformation corridors

The work underway has led to the identification of potential for the presence of orogenic gold deposits associated with deformation zones. The zones, with an average dip of 60° north, form a fan-shaped swarm converging in the southeastern part of the map; they vary in direction from E-W to NW-SE. Among these zones, it should be possible to recognize the extensions of the Laflamme and Harricana deformation zones.

Click on photo to enlarge Tectonic breccia with strong sericite-carbonate-silica alteration in drill hole RO-15. This breccia, located within 300 meters of a SE-trending deformation zone, contains about 2% disseminated pyrite with traces of chalcopyrite .

Holes drilled on the periphery of these deformation zones intersected numerous quartz-carbonate-pyrite±chlorite±albite veins. Drill hole RO-15 (Table 1), in particular, cut a sheared and altered tectonic breccia over 2.3 m. This breccia is associated with significant sericite, carbonate and silica alteration, locally accompanied by 2% disseminated pyrite and traces of chalcopyrite. The hole was collared in a zone of low magnetic susceptibility, about 300 m south of a deformation zone.

A gold showing, the Coigny showing, was discovered about four kilometres south of hole RO-15, on another NW-SE structure, in 1997. The Coigny showing consists of gold-bearing NW-SE quartz-carbonate veins in sheared basalt.

Sulphide deposits of unknown affinity

Sulphides, whose metallogenic context could not be identified, are present in the amphibolites adjacent to the Bernetz Pluton (RO-36). These occurrences locally contain up to 5% sulphides and consist of pyrite and chalcopyrite as elongated clusters and very fine disseminations.

Next steps

The project's raw data will be entered into SIGEOM, and a first geological report will be published in the RP series in November 2007. A synthesis report in the RG series will also be prepared at the end of the project (2010) and will integrate the overall results of the work.

The results of the work on Quaternary sediments will be the subject of a separate series of publications.

uring the next phases, the Rivière Octave project will continue toward the east into Fonteneau, Thémines and Comtois townships, into the eastern extensions of the rocks presented here.

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© Gouvernement du Québec, 2007