Structural geology of the vein system in the San Antonio Gold Mine, Bissett, Manitoba, Canada

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Lau, Meng Hoo Sebastian
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The San Antonio mine occurs in a mafic sill-like unit within the Rice Lake Group known as the San Antonio Mine (SAM) unit. Within the SAM unit there are five major fracture sets, each with distinct characteristics, now filled by dike and vein materials. The cross-cutting relationships between the five sets of fractures are very consistent and reveal age relationships, from oldest to youngest (1) dikes, (2) stockworks, (3) 16-type vein-filled shear zones, (4) NE/SE vein-filled shear zones and (5) NW/SW vein-filled fractures. The dikes and the fractures in which they occur are coeval and were developed as the result of hydraulic tension fracturing of the SAM unit. The stockworks are composed mainly of three components: marginal ladder-veins, central breccia zones and central veins. The ladder-veins were developed by hydraulic tension fracturing, and the central breccia zones developed as a result of either hydraulic or shear brecciation at advanced stages of ladder-vein development. The central veins which occur within the central breccia zones are in later fractures developed by shear or tension. In either case the fracturing was brittle. The 16-type veins occur within 16-type ductile shear zones. The veins were emplaced by hydraulic dilation of the shear zone schistosity. The NE/SE veins occur within pre-vein NE/SE ductile shear zones and were emplaced by process similar to the 16-type veins. The NW/SW veins occur in brittle fractures which may have originated by either shear or tension. The orientations of these sets of fractures, along with kinematics and/or stress orientation interpretations indicate that they were developed independently of each other at different times. In contrast the veins in the fractures have similar compositions indicating that vein forming fluids changed very little over the time span of the development of the four vein-filled fractures sets. The 16-type shear zones are the only structures within the SAN unit which have been related to observed deformation in the surrounding Rice Lake Group. They represent spaced ductile shear zones that have caused segmentation of the SAM unit into elongate prisms. Kinematic analyses of these features have revealed that displacement of the segments is subparallel to their elongation and similar in direction and sense to the interpretation of displacement represented by the schistosity and lineations in the surrounding Rice Lake Group.