Summary:
The Schaft Creek deposit has been described by many workers as a calc-alkaline Cu-Mo-Au porphyry deposit (Fox et al., 1995; Spilsbury, 1995; Scott et al., 2009; Morrison and Karrei, 2012). Other workers have considered it a shear-hosted, low-sulphidation Cu-Mo-Au-Ag vein deposit (Le Boutillier, 2013). Early mapping assigned the intrusive host rocks of the Schaft Creek deposit to the Early Jurassic (e.g., Logan and Drobe, 1993), but subsequent geochronology work constrained the age of the host rocks to the Late Triassic (Logan et al., 2000; Scott et al., 2009; unpublished U-Pb dating by Richard Friedman, University of British Columbia; unpublished U-Pb dating by Jim Crowley, Boise State University). Interpretation of the deposit is complicated by a lack of outcrop, complex hydrothermal alteration, post-mineral faults, and sparsity of drilling near the fringes of the hydrothermal system.
The deposit has historically been subdivided into two or three distinct mineralized zones, although the boundaries of this subdivision have changed during the history of the Project. These three mineralized zones are named the Liard, Paramount, and West Breccia Zones. Historically, the West Breccia and Liard Zones have been grouped by some workers into a larger domain called the Main Zone. Other workers have grouped the Paramount and West Breccia Zones into a single domain called the Breccia Zone.
Liard Zone
The Liard Zone comprises narrow, porphyritic quartz monzonite to quartz monzodiorite dikes that have been emplaced into andesitic volcanic and volcaniclastic host rocks. The dikes are typically 5 m to 50 m thick, strike north-northwest to north-northeast, and dip steeply to the east. Numerous narrow dikes occur within the eastern part of the Liard Zone, and in this area it can be difficult to trace individual dikes with confidence between drill holes or outcrops. In contrast, a single, thicker “Central Porphyry” dike occurs within the central portion of the Liard Zone.
The porphyritic dikes in the Liard Zone are spatially associated with potassic alteration, increased density of quartz-sulphide veins and vein stockworks, and a zone of elevated Cu-Au grade. The most intense alteration and highest copper grades commonly occur in the host rock immediate adjacent to the porphyry dikes, rather than within the dikes themselves. In some areas, chalcopyrite, bornite, and pyrite all occur disseminated within the host rocks and porphyry dikes, suggesting multiple mineralization episodes that have juxtaposed bornite and pyrite into the same area. Several types of vein-hosted mineralization are recognized in the Liard Zone. These include (1) Cu-Au-Mo mineralization resulting from quartz-biotite-bornite-chalcopyrite ± hematite veins with associated K-feldspar ± green mica selvages; (2) overprinting Cu-Au mineralization resulting from quartz-chlorite-pyrite-chalcopyrite ± calcite ± epidote ± hematite with associated sericite and chlorite-epidote selvages; and (3) late, Mo mineralization resulting from veins of massive to semi-massive molybdenite with no apparent selvage. No preferred structural trend has been identified for this vein-hosted mineralization in the Liard Zone.
Paramount Zone
The Paramount Zone comprises an elongate, multiphase igneous-hydrothermal breccia body that has been emplaced into quartz monzonite and andesitic volcanic host rocks. The breccia body strikes north-northwest, dips steeply east, is 100 m to 300 m thick, has a strike length of approximately 1,200 m, and extends at least 600 m below surface. High-grade mineralization occurs within the breccia body and also extends 100 m to 200 m into the quartz monzonite hanging wall and, to a lesser extent, into the footwall andesitic volcanic rocks. Mineralization outside of the breccia body is associated with stockwork zones containing quartz-sulphide veins.
Three styles of mineralization occur within the Paramount breccia body, each of which is associated with different breccia cement minerals. These mineralization styles include (1) Cu-Mo mineralization associated with K-feldspar-biotite-quartz-chalcopyrite-molybdenite ± bornite veins and breccias with associated potassic alteration (Unit cHBX2), (2) Cu-Au-Mo mineralization associated with anhydrite-bornite-chalcopyrite ± molybdenite veins with associated albitic alteration (Unit cHBX5), and (3) Cu-Au-Mo mineralization associated with tourmaline-quartz-carbonate-chalcopyrite ± bornite ± molybdenite veins and breccias with associated silicic alteration (Unit cHBX3). All three of these breccia styles include sulphide cement, and the assay grade of sample intervals typically correlate with the amount of sulphide cement present (typically 0.5% to 3%). Locally, there appears to be an association between high-grade mineralization and a dark-colored intrusive breccia phase. The mineralogy of this intrusive breccia appears similar to the syn-mineral sPOR dikes in the Liard Zone, but work is required to confirm the link between these two rock types.
A mineral zonation pattern is apparent around the main breccia body in the Paramount Zone. Potassic alteration intensity, vein density, and vein thickness all increase towards the breccia zone. A clear sulphide zonation (from chalcopyrite > pyrite, to chalcopyrite > bornite, to bornite > chalcopyrite) is apparent outside of the breccia body and extends inwards. No pyrite was observed in bornite-bearing areas, which is in contrast to late pyrite that overprints areas of the Liard Zone.
West Breccia Zone
The West Breccia Zone comprises an elongated, hydrothermal breccia body that has been emplaced into andesitic volcanic and volcaniclastic rocks. The breccia body strikes north-northwest, dips steeply east, is 80 m to 160 m thick, has a strike length of approximately 500 m, and extends at least 200 m below surface. Mineralization is limited to the breccia body and seldom extends far into the adjacent footwall or hanging wall. There are a few narrow monzodiorite dikes in the vicinity of the breccia that appear similar to the monzodiorite dikes seen in the Liard Zone.
The West Breccia Zone is similar to the Paramount Zone breccia and comprises different styles of mineralization associated with certain breccia cement minerals. However, the West Breccia Zone is dominated by low- to medium-temperature breccia mineralogy and lacks the higher temperature assemblages that are observed within the Paramount Zone. The three prominent styles of breccia mineralogy at the West Breccia Zone include (1) Cu-Mo-Au mineralization associated with tourmaline-carbonate- chalcopyrite-pyrite ± molybdenite veins and breccias with associated silicic alteration, (2) Cu-Mo mineralization associated with chlorite-calcite-pyrite veins and breccias with associated propylitic alteration (Unit cHBX4), and (3) high-grade Cu-Mo-Au mineralization associated with chlorite- actinolite-calcite-tourmaline breccia cement. Interestingly, although the West Breccia Zone lacks bornite, assay grades in this area are sometimes very high because of the abundance of sulphide cement (typically 2% to 10%).
The boundaries of the West Breccia Zone are generally poorly constrained. Historical drilling in this area is sparse and shallow. The breccia body remains open to the north and south, and the recent drilling has only identified the breccia to a depth of 160 m to 200 m. There is no evidence that the breccia body pinches out in any of these directions; however, the grade in historical drill holes is inconsistent and the structural controls on the breccia body are poorly understood. To the east, the Silica Fault is interpreted to offset the West Breccia Zone with a sinistral sense of movement, but more work is required to confirm this. To the west, the Breccia Footwall Fault is interpreted to truncate the West Breccia Zone, but more work is required for confirmation.