Summary:
The O’Brien Project is located along the Cadillac-Larder Lake Fault Zone (CLLFZ), in the southeastern part of the Cadillac Mining Camp (CMC), Québec. The Project straddles the Piché Group volcanic rocks and CLLFZ that separate Pontiac Group metasedimentary rocks to the south from Cadillac Group metasedimentary rocks to the north.
Across the Project, the CLLFZ shows a general east-west strike and dips steeply south at approximately 85°. On the property, the CLLFZ consists mainly of chlorite-talc-carbonate ultramafic schist, and ranges in thickness from 30 to 100 m in the mine area and narrows significantly to about 12 m wide to the east of Zone 36 East. The CLLFZ is in places closely associated to the Piché Group-Cadillac Group contact, but in most places, the fault is hosted by sedimentary rocks of the Cadillac Group (argillites, greywackes and, to a lesser extent, chert).
Most lithological contacts are sub-parallel to the CLLFZ. The main lithologies can be described as following:
Cadillac Group
Found to the north, the Cadillac Group metasedimentary rocks are in the footwall of the CLLFZ and of most of the mineralized zones, and hence the majority of the diamond drill holes did not intersect the Cadillac Group rocks. The limited drilling shows the presence of argillite, greywacke, some pebble conglomerate-like units, and some iron formation.
Piché Group
The Piché Group is a relatively thin band of interlayered mafic volcanic rocks, conglomerates, and porphyric andesitic sills. From north to south, the Piché Group stratigraphy is divided into the following units:
• Northern volcanics: tuff and foliated basalts (with small quantities of argillite, greywacke, chert and massive to variably porphyritic basalt flows).
• Northern porphyric andesitic sills
• Polygenic matrix supported conglomerate (“Mine Conglomerate”)
• Central volcanics: tuff and foliated basalt
• Southern porphyric andesitic sills
• Southern volcanics: massive to well foliated, locally pillowed basalts
All the above lithologies generally strike east-west with more pronounced flexures locally. Schistosity is more developed in the central and northern volcanic units than the southern unit.
Pontiac Group
The metasedimentary rocks of the Pontiac Group 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.
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 volcano-plutonic 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).