Summary:
The Bachelor-Desmaraisville Property is situated near the western limit of the Chibougamau-Chapais Greenstone Belt (CCGB). The following description is taken from Darling and Lafontaine (2011).
The mafic to felsic volcanic and volcanoclastic rocks of the Bachelor Lake area are part of the basal mafic dominated sequence referred to as the Volcanic Cycle I (Mueller et al., 1989). The Volcanic Cycle I formed between 2,730 Ma and 2,720 Ma (Mortensen, 1993), and is composed of massive, pillowed, and brecciated, tholeiitic basalt flows with local felsic and sedimentary units. The NVZ of the Abitibi Subprovince is interpreted as a diffuse arc passing laterally into a back-arc environment with numerous felsic and mafic-felsic edifices (Chown et al., 1992) and intra-arc sedimentary basins (Mueller et al., 1996).
The Bachelor-Desmaraisville Property is underlain by Archean volcanic rocks of the Obatogamau Formation in a poorly known and relatively poorly explored area of the AGB. Based on the absence of marker horizons and the paucity of outcrops, it is difficult to establish a well-defined rock sequence in the Coniagas-Bachelor Lake area (Doucet et al., 1998). The Obatogamau Formation includes mafic, intermediate, and felsic flows, and synvolcanic intrusive equivalents which are the host for the volcanogenic massive sulphide occurrences (e.g., Coniagas). The stratigraphic sequence includes the 280 m thick Coniagas Mine sequence composed of a mafic dominated volcanoclastic sequence.
Porphyritic lava flows, prominent in the immediate area of the Coniagas Zn-Pb-Ag deposit (1.5 km west of the Bachelor deposit), cover the volcanoclastic unit. A significant 500 m to 700 m thick, lenticular and dome shaped felsic unit composed of massive to brecciated, rhyolitic to rhyodacitic lava flows occurs upsection. This felsic-dominated unit corresponds to the Bachelor deposit host rocks. Mafic volcanic and volcanoclastic rocks make up the upper part of the sequence. While the Auger Lake and Bachelor Lake sedimentary rocks remain enigmatic, it is probable that they mark the top of the sequence. The late emplacement of several plutons (e.g., O'Brien granodiorititic pluton located east of the Bachelor deposit), adds to the complexity of the region. Gold mineralization at Bachelor has been interpreted to pertain to the rocks of the O’Brien pluton including granitic porphyry and biotite-hornblende granodiorite. Posttectonic lamprophyre dykes are also common at Bachelor and kimberlitic dykes were documented in the Desmaraisville area. This later intrusive phase (N030° and N110° lamprophyre and kimberlitic dykes) has recently been investigated for diamond potential in the Desmaraisville area.
Bachelor is located along an east-southeast trending, southwest-dipping, silicified shear zone with hematitic alteration (Buro, 1984). Bachelor transects northeast trending, steeply dipping volcanic rocks and the O’Brien granitic to granodioritic pluton. Major west-southwest and north-northeast trending faults have affected the ore zone and the emplacement of the granite intrusions. Two types of gold bearing zones have been identified at Bachelor: silica-flooding, and hematite-altered ± stockwork zones, both of which are illustrated by the Main Zone (prolific, representing 90% ore mined to date at the Bachelor Mine) and the B Zone. In both instances, gold is spatially associated with pyrite and gold content correlates well with pyrite content.
Gold mineralization at Bachelor Lake occurs predominately within the pyrite (>70%), as grains attached to the pyrite (-18%) or as free gold enclosed in the gangue (-10%).
The gold is fine grained with an average diameter between 6 to 8 mm (Table 8.1) and visible gold (VG) is more characteristic of the B Zone. Pyrite is usually finely disseminated (2% to 10%) hosted in strongly altered rocks, often brecciaed and occasionally injected by quartz/carbonate veins and veinlets.
At surface, traces of gold, chalcopyrite and ilmenite occurrences have been observed. Gold has been introduced late in the paragenetic sequence as were fluorite and some of the carbonates (Lauzière, 1989).
Much like Bachelor deposit, the Moroy deposit is a series of fractures filled by mineralisation. The Moroy deposit mirrors the Bachelor deposit by dipping north-east. In the current 3D model used for estimation, 37 small veins have been interpreted. Those 37 veins can be divided in 3 major sets. The center set is probably a set of fractures that split the top and bottom sets sub-horizontally with a dip of approximately 25 degrees toward the east. In the Moroy deposits, like in the Bachelor zones, veins are mainly silicified shear zones with hematitic alteration. Both deposits are accessible underground by the Bachelor mine shaft.
Two types of gold-bearing zones have been identified at Bachelor Lake and can be also found in the Moroy part of the mine: silica-flooding and hematite-altered ± stock work zones. In both cases, gold is spatially associated with pyrite and the gold content correlates well with the pyrite content.