GEOLOGICAL SETTING, STRUCTURAL GEOLOGY AND TIMING OF DEFORMATION AT THE GIBRALTAR CU-MO PORPHYRY DEPOSIT, CARIBOO REGION, BRITISH COLUMBIA

The Gibraltar copper-molybdenum calc-alkalic porphyry, near Williams Lake in south-central British Columbia, is hosted in the Late Triassic Granite Mountain batholith. Mineralization occurred at 217 ± 12 Ma (Bysouth et al., 1995). The main ore zone, hosted within the Mine Series Phase tonalite, has been structurally dismembered. The relationship between mineralization and deformation and the timing of deformation is poorly understood.

The mineralized Mine Phase tonalite is variably deformed. There is an early sub-horizontal to shallowly SSE dipping magmatic foliation (S₀) defined by aligned hornblende and chlorite. The main tectonic foliation (S₁) is defined by aligned chlorite and sericte and elongate quartz. S₁ is typically oriented subparallel to early mineralization-stage sheeted chlorite-chalcopyrite veins that dip shallowly SW. Subhorizontal high strain zones associated with boudinaged quartz veins deform S₁. The S₀, S₁ foliations and sheeted chlorite-chalcopyrite veins are locally folded into upright to overturned folds (verging S-SW) that are spatially associated with thrust faults. The thrust faults verge towards NE-E and dip ~25° SW, and contain sulphide-rich quartz-molybdenite veins. A locally prevalent S₂ crenulation cleavage may be related to progressive deformation associated with thrusting. Large N-S trending high-angle high-strain, dextral strike slip zones cross cut and deform S₀/S₁. A well-developed intersection lineation (intersection of S₁ and the steep foliation within the dextral shear fabric) plunges 20° towards SE, which is parallel to fold axes observed within the fault zones. Large N-NE trending high-angle normal faults represent the youngest deformation.

Preliminary results from Ar-Ar cooling ages (illite) show a diverse range of Eocene ages for deformation. Illite was extracted from fabrics developed within thrust faults, dextral high angle faults, low angle high strain zones, normal faults, and S₁ fabrics: and all document Eocene ages. We will further explore the significance of these results with regard to regional deformation and deformation within the deposit.