Geological mapping using drilling data and Quaternary sediment geochemistry, Lac Bigniba area (32F04-SE); Phase 1 of the Rivière Octave project, Abitibi, Québec
Pierre Rhéaume (MRNF), Michel Parent (GSC), Charles Maurice (MRNF), Éric Boisvert and Vicki. McNicoll (GSC)

The main objective of the Rivière Octave project is to compensate for the lack of knowledge of geological settings and exploration targets under the thick cover of glaciolacustrine sediments that extends from Normétal to Lebel-sur-Quévillon. The selected method, which consists in systematically sampling this environment in drillhole, will also lead to a characterization of tills and of underlying geological settings. The results of Phase 1 of this project, conducted under the MRNF Copper Plan and the GSC Targeted Geoscience Initiative, are presented here.

The study area in 2006-2007 covers NTS sheet 32F04-101, located southeast of the Sleeping Giant mine. Supracrustal rocks in this area are bordered by the Marest and Bernetz plutons. A sequence of dominantly volcanic rocks cut by E-W to NW-SE-trending deformation zones was described. This sequence mainly consists of tholeiitic to transitional mafic and intermediate lavas, locally interbedded with tuffs and calc-alkaline felsic lavas. These volcanic rocks are locally overlain by a thick sedimentary unit composed of wacke and mudstone with graded bedding. This unit unconformably overlies the volcanic rocks and occupies the core of a regional west-plunging syncline. The youngest zircons recovered from a wacke sample from this unit yielded a TIMS age of 2700 Ma.

Under a variable thickness of glaciolacustrine silt, the 37 diamond drillholes (HQ-3) intersected 162.1 m of till, related glacial sediments or older Quaternary sediments; the average recovery rate was 63% (ranging from 0 to 100%; standard deviation = 22%). Given the proximity of the Harricana Moraine, drillholes frequently intersected, under the silt layer, variable thicknesses of proglacial sediments, mainly pebbly pavement, diamict with a washed-out matrix, and well-sorted sand and gravel. These proglacial facies were underlain by the basal till with a silty-sandy matrix or directly by the bedrock. Under the till, one drillhole intersected oxidized sandy sediments and an earlier till unit.

A total of 102 m of drill core was recovered, from which 130 samples of 400 to 800 g were collected for geochemical analyses (major and trace elements by ICP-AES and INA) and 128 samples weighing 1200 to 2000 g for heavy mineral concentrate studies and microprobe analyses of indicator minerals.