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The Mitchell Au-Cu-Ag-Mo porphyry deposit, hosted by Early Jurassic volcanosedimentary and intrusive rocks in the Stikine terrane of northwestern British Columbia, is considered the largest undeveloped gold resource in Canada. As of 2015 it held a resource of 1777 Mt at 0.61 g/t Au, 0.17% Cu, 3.1 g/t Ag, and 58 ppm Mo (0.5 g/t Au eqiv. cut-off; meas+ind). The calc-alkalic deposit is genetically related to multiple diorite intrusions (Sulphurets suite) that cut volcanosedimentary strata of the Stuhini Group (Upper Triassic) and Jack Formation (basal Hazelton Group, Lower Jurassic). Phase 1 plutons (U/Pb, zircon; 196 ±2.9 Ma and 192.2±2.8 Ma) host Stage 1 potassic and propylitic alteration, veins and copper-gold mineralization. A Phase 2 plug (189.9±2.8 Ma; U/Pb zircon) is central and temporally related to a molybdenum halo (190.3±0.8 Ma; Re-Os, Mo) that is accompanied by phyllic alteration (Stage 2). Phase 3 plutonism is temporally related to diatreme breccia, intrusion breccia dikes and Stage 3 massive pyrite veins and advanced argillic alteration. High-level, gold-rich veins comprise Stage 4.Three phases of progressive deformation related to the mid-Cretaceous Skeena fold and thrust belt structurally modify the Mitchell deposit. Deformation Phase 1 is characterized by a steep, easterly striking pervasive pressure solution cleavage (S₁) and steeply west-plunging buckle folds in veins (F₁); fold geometry and flattening degree are a function of alteration type. In rheologically weak alteration types a pressure solution cleavage is associated with loss of silica, mechanical remobilization of chalcopyrite-molybdenite, and passive enrichment of chalcopyrite-molybdenite-pyrite along the cleavage planes. Strain intensity (i.e., S₁ development) is heterogeneous and this greatly affects the shape of the orebody. In Deformation Phase 2, steeply north-plunging F₂ vein folds overprint S₁ and F₁. The Mitchell thrust fault (Deformation Phase 3) offsets the Snowfield deposit ~ 1600 m to the east-southeast and the Mitchell Basal shear zone displaces the Mitchell deposit from its core zone, located ~1-2 km to the west at a depth of ~ 1 km. It is speculated the Mitchell deposit was emplaced into a structurally influenced, north-trending Jurassic basin and subsidiary east-west structures controlled the intrusion, vein geometry, alteration and metal pattern trends.
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
The role of dolomite on the strength and evolution of calcite-dolomite fold and thrust belts is largely unknown. Field investigations indicate that, under upper- to mid-crustal conditions, strain in natural systems is localized in calcite, resulting in a ductile response, while dolomite deforms in a brittle manner. The effect of dolomite on limestone rheology, and the potential for strain localization in composites have not yet been fully quantified. I conducted 11 constant displacement rate (3x10⁻⁴ and 10⁻⁴ s⁻¹), high confining pressure (300 MPa), and high temperature (750°C and 800°C) torsion experiments to address the role of dolomite on the strength of calcite-dolomite composites. Starting materials were formed by hot isostatic pressing mixtures of dolomite and calcite powders (given as Dm%: Dm25, Dm35, Dm51, and Dm75) and were deformed up to a maximum shear strain of ~5.Mechanical data show a considerable increase in yield strength with increasing dolomite content. Microstructural analysis shows that dolomite grains ~50 μm are characterized by well-defined grain boundaries and cleavage-controlled fracture. Electron backscatter diffraction (EBSD) shows no crystallographic preferred orientation (CPO) development in dolomite, but optical microscopy confirms brittle deformation of dolomite grains by Mode I cracks, shear fractures, and subsequent grain size reduction.Calcite grains are internally strain-free, equiaxed to tabular in shape, and characterized by triple-junction grain boundaries. EBSD confirms a distinct CPO of calcite c-axes perpendicular to the direction of maximum stretching. The microstructure of calcite aggregates suggests grain boundary sliding, accommodated by diffusion and dislocation glide, which accommodates high shear strains without significant change in grains shape and size. Dolomite is essentially undeformed in run products with less than 35% dolomite; calcite accommodates most of the displacement in these experiments. In contrast, for dolomite contents greater than 51%, dolomite accommodates displacement by brittle processes. My experiments provide insights into the processes controlling rheology within bimodal calcite-dolomite systems, suggesting that a minimum dolomite-content exists (between 35% and 51%) above which dolomite significantly influences composite strength.
Carbonates and shales are common in fold and thrust belts worldwide: carbonates typically comprise the hanging wall of fault zones and the shale forms the footwall. Generally, a cataclasite is developed in both the carbonate and shale materials, demonstrating that strain is accommodated in both rock types. Despite the wide occurrence of carbonate and shale cataclasites, little is known about the rheological behavior of these composites. The results of two suites of triaxial frictional sliding experiments designed to analyze the effects of composition, temperature, pore fluid pressure and forcing block composition on gouge strength and stability are presented. Experiments were conducted at 70 MPa effective confining pressure and displacement rates varied between 1 to 100 μm s⁻¹. Gouge material was created from quartz-bearing phyllosilicate-rich shale combined in various volumetric proportions with reagent grade calcite powder with an average grain size of ~5 μm. Experiments were performed on each endmember composition as well as 75%, 50% and 25% mixtures of shale and calcite. At room temperature (T), saturated conditions strain localization in the composite gouges causes significant weakening relative to the strong carbonate endmember. At 150°C and 15 MPa pore fluid pressure (Pf), the carbonate gouge undergoes significant strain weakening followed by the evolution to stick-slip sliding. Microstructure analysis indicates that deformation in the shale endmember gouge is distributed across the gouge zone. In the composite gouges, the fine grained carbonate facilitates phyllosilicate rotation and strain localization. In the carbonate gouge, strain localizes in R₁ and shear zone boundary parallel Y shears. Results show that in the absence of elevated T and Pf, the carbonate hanging wall cataclasite is strong relative to the underlying shale footwall cataclasite. At these conditions strain is most likely to localize in the shale or shale-rich composites. Elevated T and Pf promote strain localization and seismic faulting in the carbonate cataclasite. The coefficient of friction values determined for shale carbonate composites are less than the μ = .85 value predicted by Byerlee for rocks deformed at less than 200 MPa normal stress and should replace Byerlee’s value in numerical thrust sheet models.