Summary:
The property straddles the Piché Group volcanic rocks that separate Pontiac Group metasedimentary rocks to the south from Cadillac Group metasedimentary rocks to the north. In the property area, all lithologies strike east-west and dip steeply south at approximately 85°.
Cadillac Group
The The Cadillac–Larder Lake Fault Zone (CLLFZ) is a major regional crustal break that consists mainly of chlorite-talccarbonate ultramafic schist, and ranges in thickness from 100 to 300 ft (30 to 100 m) in the mine area, and narrows significantly to about 40 ft (12 m) wide to the east of Zone 36 East. Across the property, the fault is subparallel and close to the Piché Group-Cadillac Group contact, but is generally enveloped by Cadillac Group sedimentary rocks (argillites, greywackes and, to a lesser extent, chert).
Piché Group
The veins of the O’Brien Project were mostly injected into the volcanic and sedimentary rocks of the Piché Group. From south to north, the Piché Group stratigraphy is divided into the following units:
• Southern volcanics: tuff and basaltic schists;
• Southern porphyritic andesite;
• Central volcanics: tuff and basaltic schists;
• Sporadically pebbly greywacke and argillite (“Mine Conglomerate”);
• Northern porphyritic andesite;
• Northern volcanics: tuff and mafic schists (with small quantities of argillite, greywacke, chert and massive to variably porphyritic basalt flows).
All the above lithologies generally strike east-west with more pronounced flexures locally. The rock varies from slightly to highly schistose and foliation increases progressively towards the CLLFZ.
Pontiac Group
The Pontiac Group metasedimentary rocks consist mainly of greywacke and some argillite, which is sometimes graphitic. In general, the sediments are well stratified. Some zones display weak biotitic alteration or chloritization. Small-scale folding is observed in places. Some greyish to smokey quartz veins and veinlets, similar to goldbearing veins, appear locally, and some of these host gold (OB-95-48, 52, 53, 54 and 56A).
Mineralization
O’Brien mine Gold production at the O’Brien mine came from a few quartz veins running almost parallel to the formations. The mine’s productive sector was generally limited to a narrow strip that included the O’Brien conglomerate and the northern porphyritic andesite. Approximately 95% of the O’Brien ore came from four veins (No. 1, No. 4, No. 9 or “F”, and No. 14) in the eastern part of the mine. The veins contained highgrade shoots that occasionally yielded considerable amounts of visible gold. The main veins generally strike from 083° to 098°, and dip steeply to the south (-84° to -90°). The stopes averaged 0.75 to 0.90m (2.5 ft to 3 ft) wide. Gold mineralization extends vertically down to at least the 3450' level.
Deposit Types
Greenstone-hosted quartz-carbonate vein deposits occur as quartz and quartz-carbonate veins, with valuable amounts of gold and silver, in faults and shear zones located within deformed terranes of ancient to recent greenstone belts commonly metamorphosed at greenschist facies (Dubé and Gosselin, 2007). Greenstone-hosted quartz-carbonate vein deposits are a subtype of lode gold deposits (Poulsen et al., 2000). They are also known as mesothermal, orogenic. They consist of simple to complex networks of gold-bearing, laminated quartz-carbonate fault-fill veins in moderately to steeply dipping, compressional brittle-ductile shear zones and faults, with locally associated extensional veins and hydrothermal breccias. They can coexist regionally with iron formation-hosted vein and disseminated deposits, as well as with turbidite-hosted quartz-carbonate vein deposits. They are typically distributed along reverse-oblique crustal-scale major fault zones, commonly marking the convergent margins between major lithological boundaries such as volcanoplutonic and sedimentary domains. These major structures are characterized by different increments of strain, and consequently several generations of steeply dipping foliations and folds resulting in a fairly complex geological collisional setting.
Typically, the alteration haloes are zoned and characterized, at greenschist facies, by iron-carbonatization and sericitization with sulphidation of the immediate vein selvages (mainly pyrite, less commonly arsenopyrite).
The main gangue minerals are quartz and carbonate with variable amounts of white micas, chlorite, scheelite and tourmaline. The sulphide minerals typically constitute less than 10% of the ore. The main ore minerals are native gold with pyrite, pyrrhotite and chalcopyrite without significant vertical zoning. (Dubé and Gosselin, 2007).