Summary:
The gold mineralization at the West Cache Property is a mesothermal lode gold deposit in an Archean greenstone belt setting. Mesothermal gold deposits in the Abitibi Greenstone Belt are spatially associated with large-scale regional structures such as the PDFZ. These large-scale structures and associated Timiskaming-type sedimentary units are interpreted to be zones of transpressive terrain accretion. The general consensus is that greenstone-hosted mesothermal lode gold deposits formed from metamorphic fluids generated by prograde metamorphism and liberated during accretionary processes and thermal re-equilibration of subducted volcano-sedimentary terranes.
The West Cache Property porphyry-hosted gold deposits resemble those at the Hollinger and McIntyre Mines, located approximately 15 km to the east. The deposits are characterized by chalcopyrite-pyrite stringers and veins, and quartz-tourmaline veins, and hosted by altered and sheared QFP. MacDonald (2010) suggests that the gold mineralization and porphyry intrusions are not genetically related, but occur along common emplacement conduits.
Pyrite, the most important sulphide for visually estimating gold grade, is observed in many different styles. Fine-grained disseminated pyrite is typically associated with lower, but consistent gold grades, whereas coarser-grained euhedral to subhedral pyrite grains (“buckshot pyrite”) are a reliable indicator of higher gold grades. When the coarser-formed pyrite grains occur in the form of stringers/bands, gold grades are elevated. A combination of banded pyrite and other base metal sulphides, such as sphalerite and chalcopyrite, is also a reliable indicator of gold. Chalcopyrite is commonly observed as blebs and filling fractures. Sphalerite typically occurs as “stringer-style” mineralization. In the absence of pyrite, sphalerite mineralization doesn’t return significant gold grades. Conversely, solitary chalcopyrite occurrences have locally returned large spikes in gold grade. Within the Bristol Porphyry Unit, sphalerite is generally the “honey” type (yellowish to rose colour), whereas metasedimentary-hosted sphalerite is of the reddish brown “ruby jack” or “blackjack” types.
Free grains of visible gold are observed in quartz-carbonate and chlorite veins, and as inclusions in pyrite and chalcopyrite, but not sphalerite. Chlorite-calcite-silica-sulphide stringers and wisps (veinlets) overprint the strongly foliated chloritized pyrite bands. The stringers are only weakly deformed compared with the host rock, and therefore likely formed during the later stages of deformation. In addition, the associated chlorite alteration overprints the earlier sericitic alteration. Late quartz-carbonate-chlorite, hematite, and tourmaline veinlet stockworks cross cut the QFP, although these alteration styles have no direct correlation to gold.
Six zones of gold mineralization have been identified on the West Cache Property: East Zone, East Zone Extension, West Zone, West Deep, South Zone and Zone #9). The geology of each of these zones is summarized below.
East Zone
The term “East Zone” refers to the area of gold mineralization hosted in the Bristol Porphyry Unit, which has been explored since its discovery in the mid-1980s during several drilling campaigns. The main rock type, a quartz feldspar porphyry (“QFP”), displays a wide range of textural and alteration styles. Steeply-dipping Proterozoic diabase dykes trend NNW-ESE through the East Zone area. Greywacke and argillite of the Porcupine Assemblage occur as 1 m to 10 m wide intercalations in the East Zone area, accompanied by elevated sulphide mineralization at the contacts.
East Zone Extension
The geology of the East Zone Extension is a direct extension of the East Zone and therefor similar in that it represents an eastward extension of the same rock types, structures and sulphide zones, and dips steeply to the north. Multiple gold zones are present, some of which are on strike from the East Zone, while others represent up-dip, near-surface extensions from deeper historical drilling. Lithology is largely dominated by volcanic and intrusive rocks that characterize the Bristol Stock area, much of which has historically been labelled QFP. There are also minor metasedimentary units composed of interbedded metawackes and meta-argillite host rocks.
West Zone
West Zone lithology consists mainly of mixed (intercalated) QFP and metasedimentary rocks. The QFP is less altered and transitions to a metasedimentary host rock moving westward and southward to the contact with the Porcupine Assemblage. Beginning around section 464,100 E an argillite-rich metasedimentary unit becomes the primary lithology and extends westward to the West Deep area. This meta-argillite unit may be important to locating additional high-grade metasedimentary-hosted gold zones at the Project, as it appears to be associated with both the West Deep and Zone #9 Zones. The Gap Area, the unofficial border between the East Zone and West Zone areas, is a diabase dyke swarm referred to as the “Snowflake” diabase due to its striking porphyritic feldspar texture. The feldspar is pale green to yellow in colour, ranges from euhedral to sub-rounded, and generally decreases in size and abundance downhole (southwards).
Zone #9
Zone #9 is hosted in turbiditic metasedimentary rocks, composed of metasandstone, metasiltstone, and metamudstone of the Porcupine Assemblage. The zone is situated south of the main lithological contact between the Bristol Porphyry Unit and Porcupine metasedimentary rocks. The discovery of Zone #9 highlighted a deposit style that was interpreted previously to exist only at >500 m depth below surface and had not previously been identified near-surface. Zone #9 plunges 60° northwest from the bedrock interface to a depth of 275 m below surface, with an average strike length of 100 m. Its average true thickness is approximately 7.5 m with widths up to 15 m in the central area. The upper parts of the deeper Zone #9 holes are in the transition zone between QFP and metasedimentary lithologies and show fragmental textures.
South Zone
The geology of the South Zone is similar to the Zone #9 and West Deep Zones, with thinly to thickly bedded turbiditic metasandstone, metasiltstone, and metamudstone (argillite) of the Porcupine Assemblage. Graded bedding is common. The sandstone is generally well sorted, fine- to medium- grained, and may contain granule to fine grained conglomerate components. Siltstone is very fine grained to fine grained, grey to beigegrey in colour, and forms narrower and less well-defined beds. The argillite lithology is aphanitic to very fine-grained ranging from grey to dark grey- black in colour. A 150 m wide porphyry was intersected approximately 150 m below surface in deeper South Zone drilling.
West Deep Zone
The West Deep Zone, like Zone #9, is hosted in Porcupine Assemblage (metasandstone, siltstone, and mudstone) and has similar alteration styles. West Deep drill holes collar in the Bristol Porphyry Unit and transect the area between the two main lithologies, which has been historically referred to as the “deformation zone”. There are E-W trending diabase sills that run north of the West Deep (near the drill hole collars) and within the zone.