Abstract:

The Croxall breccia pipe is located approximately 30 km northeast of Kirkland Lake, Ontario, within the Abitibi Greenstone Belt’s Blake River Group, an Archean, succession of basaltic through rhyolitic volcanic rocks and associated synvolcanic plutons, which extends 140 km from Cadillac Township in Quebec to Clifford Township in Ontario. <br/>The Croxall breccia is hosted by feldspar porphyritic andesite, is roughly circular in plan, with a diameter of approximately 32 m and has been intersected by diamond drilling at least 150 m below surface. It is clast supported and has a matrix comprised of quartz, calcite and ankerite. Clasts in the core of the breccia are mostly angular, tabular in shape, poorly sorted and are typically larger than clasts in the mineralized margins where they tend to be well- to sub-rounded and are moderately to well-sorted. <br/>Porphyry-style Cu-Mo-Au mineralization is primarily hosted within the peripheral “mill” breccia, which occurs at the contact between the breccia pipe and the host andesite. Mineralization consists of 3 to 5 percent sulphides with localized areas of up to 10 percent, which consist of mostly pyrite and chalcopyrite with lesser molybdenite and gold. The sulphides occur as disseminations, veinlets and semi-massive accumulations in the breccia matrix and in fractures within the adjacent andesite. <br/>Sericite alteration (with lesser chlorite) is pervasive throughout the breccia, whereas silica, sericite, chlorite, epidote and potassic alteration are readily observed in drill core proximal to the breccia pipe. In the core of the breccia, where sericite alteration is most intense, plagioclase phenocrysts are barely distinguishable from the sericite groundmass. Rocks in and around the breccia pipe have all undergone net losses of Na2O and gains of K2O. Losses in Na2O, Sr, Cr and Zn and gains in K2O, MnO, Rb and Cs coincide with mineralized portions of breccia.<br/>The shape, contacts with host rocks, hydrothermal alteration, distribution of textures and mineralization all suggest that the Croxall breccia is the product of collapse and ensuing fluidization and mineralization. Relationships observed suggest there was at least one episode of prolonged fluidization and mineralization along the margins of the breccia, which post-date the initial brecciation that produced the barren, angular breccia observed in the core of the breccia pipe. It is thought that fluids responsible for fluidization and mineralization of the periphery were restricted to the weaker, fractured, margins of the pipe rather than the less permeable core of the breccia.<br/>Collectively, the geological setting, host rocks, metal associations, distribution and characteristics of alteration and mineralization in the Verna Lake area suggest mineralization of the area is analogous to porphyry Cu-Mo-Au mineralization associated with Phanerozoic subduction environments, yet at a much smaller scale.<br/>Future exploration efforts in the area should focus on identifying and defining the extent of features that contain porphyry-style characteristics in an effort to locate oregrade mineralization. Futhermore, late high-level intrusions and associated dykes of calcalkaline composition throughout the Blake River Group should be considered prospective for porphyry-style Cu-Mo-Au mineralization, especially if they are closely linked to hydrothermal alteration (particularly sericite-quartz and/or K-feldspar-biotite) and/or hydrothermal breccias and disseminated or stockwork-like sulphide mineralization (particularly pyrite +/-quartz and/or quartz-pyrite-chalcopyrite-molybdenum +/-gold).