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-McIntyre Au quartz-carbonate-pyrite vein system, the lar gest known in the Superior province (approximately 995 t Au), is local ized within a 450- to 600-m-wide, northeast-southwest-trending, ductil e-brittle shear zone, the Hollinger shear zone. This shear zone is cha racterized 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 ve ins and fractures, asymmetrical strain shadows, lineations, and asymme tries of foliation trajectories indicate complex movement; however, a reverse, oblique (dextral) component of displacement appears to have b een dominant during and after the main period of Au quartz vein minera lization. Successive stages of vein formation were largely synchronous with shear zone development since dilation occurred in secondary shea r fractures that broadly reflect the shear zone stress system. At the scale of a stope or individual vein structure, vein morphologies commo nly reflect local development of arrays of tension gash veins followed by vein formation by hydraulic fracturing along Reidel (R) shear frac tures, and then principal displacement fractures (D). This process res ulted in the characteristic combination of irregular quartz veins with superimposed banks of parallel veins and stringers slightly oblique t o foliation. Veins parallel to foliation are thought to have resulted from a combination of reorientation of early veins and hydraulic fract uring along shear zone foliation planes. On a broader scale the positi on 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 horiz ons, and competency contrasts, combined with large-scale anisotropies, in this case, the position and attitude of the quartz-feldspar porphy ry intrusions within the shear zone, particularly the Pearl Lake porph yry. The Hollinger-Mclntyre vein system is best interpreted as having developed under lithostatic pressure; fluid migration was localized wi thin a more extensive shear zone by the presence of earlier felsic int rusions and permeable lithologies. Vein orientations and morphologies were then controlled by dilation of a variety of secondary shear and t ension fractures which formed in response to late reverse dextral move ment. The Hollinger shear zone is superimposed on an earlier fabric wh ich also affects Timiskaming sediments; mineralization is thus post-Ti miskaming in age and similar in relative timing and controls to other examples of Au quartz vein mineralization in the southern Abitibi gree nstone belt such as the Kirkland Lake, Larder Lake, and Val d'Or camps . The 2673(+6/-2) Ma age for an albitite dike which immediately predat es Au quartz vein mineralization in the Mclntyre mine corresponds to t he late Timiskaming phase. The Hollinger shear zone is one of three al ignments, two of which are known to be ductile-brittle shear zones, wh ich relate most of the major known Au quartz vein systems in the Timmi ns area (e.g., Dome mine).