UBC Theses and Dissertations
Structural geology and timing of deformation at the Gibraltar copper-molybdenum porphyry deposit, south-central British Columbia Mostaghimi, Nader
The Gibraltar Cu-Mo porphyry deposit, near Williams Lake in south-central British Columbia, is hosted in the Late Triassic Granite Mountain batholith. The main ore zone, hosted within the Mine Phase tonalite, is variably deformed and structurally dismembered. Alteration assemblages are used to map out the geometry of deformation. Quartz-chlorite (QC) alteration is strongly associated with mineralization, and QC and ankerite-quartz (AQ) are associated with ductile shear zones (thrust faults) that typically host or bound the ore. Deformation structures are divided into two deformation events, D₁ and D₂. D₁ contains a variably developed, tectonic foliation (S₁) that is folded into gentle to open folds. S₁ is associated with shallowly to moderately south- to southwest-dipping ductile thrust faults and smaller-scale imbricate ductile thrusts that deform the Gibraltar porphyry system. D₂ resulted in the formation of NW- to NE- (N-S) trending dextral faults ± normal displacement, and variably striking low-angle normal faults (e.g., northeast-striking Fault 10) that offset (~60 to <220 metres of vertical- and/or lateral-slip separation) and rotate (CW) D₁ and mineralization. Shallowly SE-plunging mineral lineations (e.g., intersections (L₁) and fold axes (F₂) are associated with a sub-horizontal crenulation cleavage (S₂) that likely formed during extension. The Mine Phase tonalite yields a U-Pb (zircon) crystallization age of ca. 216.17 ± 0.24 Ma (CA-TIMS). In contrast, Ar-Ar (white mica) minimum cooling ages ranged from 54-36 ± 5 Ma for mica collected from S₁, ductile thrusts faults, and N-S striking, dextral faults ± normal displacement. It is proposed that D₁ and D₂ are associated with movement along the Paleocene-Eocene, dextral strike-slip, Quesnel River and Fraser River fault systems, and therefore deformation significantly post-dates porphyry emplacement. This interpretation is supported by deformation microstructures in quartz and plagioclase that constrain the temperatures during deformation to be less than 450°C; too low to be contemporaneous with pluton emplacement.
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