Structural controls on formation of the Hollinger-McIntyre Au quartz vein system in the Hollinger shear zone, Timmins, southern Abitibi greenstone belt, Ontario

The Hollinger-MeIntyre Au quartz-carbonate-pyrite vein system, the largest known in the Superior province ( approximately 995 t Au), is localized within a 450- to 600-m-wide, northeast-southwest-trending, ductile-brittle shear zone, the Hollinger shear zone. This shear zone is characterized by a strong east-northeast-striking foliation that dips 80 degrees S and a prominent elongation-stretching lineation plunging 60 degrees to the east. Kinematic and strain indicators such as folded veins and fractures, asymmetrical strain shadows, lineations, and asymmetries of foliation trajectories indicate complex movement; however, a reverse, oblique (dextral) component of displacement appears to have been dominant during and after the main period of Au quartz vein mineralization.Successive stages of vein formation were largely synchronous with shear zone development since dilation occurred in secondary shear fractures that broadly reflect the shear zone stress system. At the scale of a stope or individual vein structure, vein morphologies commonly reflect local development of arrays of tension gash veins followed by vein formation by hydraulic fracturing along Reidel (R) shear fractures, and then principal displacement fractures (D). This process resulted in the characteristic combination of irregular quartz veins with superimposed banks of parallel veins and stringers slightly oblique to foliation. Veins parallel to foliation are thought to have resulted from a combination of reorientation of early veins and hydraulic fracturing along shear zone foliation planes.On a broader scale the position and plunge of the most productive central ore zone is controlled by the occurrence of favorable lithologic units (e.g., permeable Central subgroup breccias), favorable stratigraphic contacts, interflow horizons, and competency contrasts, combined with large-scale anisotropies, in this case, the position and attitude of the quartz-feldspar porphyry intrusions within the shear zone, particularly the Pearl Lake porphyry.The Hollinger-McIntyre vein system is best interpreted as having developed under lithostatic pressure; fluid migration was localized within a more extensive shear zone by the presence of earlier felsic intrusions and permeable lithologies. Vein orientations and morphologies were then controlled by dilation of a variety of secondary shear and tension fractures which formed in response to late reverse dextral movement. The Hollinger shear zone is superimposed on an earlier fabric which also affects Timiskaming sediments; mineralization is thus post-Timiskaming in age and similar in relative timing and controls to other examples of Au quartz vein mineralization in the southern Abitibi greenstone belt such as the Kirkland Lake, Larder Lake, and Val d'Or camps. The 2673 (super +6) (sub -2) Ma age for an albitite dike which immediately predates Au quartz vein mineralization in the McIntyre mine corresponds to the late Timiskaming phase.The Hollinger shear zone is one of three alignments, two of which are known to be ductile-brittle shear zones, which relate most of the major known Au quartz vein systems in the Timmins area (e.g., Dome mine).