Abstract Two general types of Archean gold deposits are known in the Quebec segment of the Abitibi belt (Robert 1990) (Table 1). Firstly, there are gold-quartz vein deposits where gold is commonly associated with the development of quartz-carbonate vein systems. These deposits are typified by the deposits of such as the Pierre Beauchemin, Silidor and Donalda and Sigma deposits (Robert and Brown 1986). Secondly there are pyritic gold deposits where quartz-carbonate veins are less important and where sulphide generally form a major part of the ore. Deposits of the Bousquet area are good examples of this second type. This classification, although satisfactory to distinguish Archean gold deposits in general sense, needs be further refined in consideration of the broad characteristics of many individual deposits (Tourigny et al. 1989; Tourigny, this volume). The Francoeur #3 deposit presents distinct geological characteristics which make it difficult to classify in either the Au-quartz vein or pyritic gold deposit type. During this mine visit, we will show that there exists a distinct type of gold-only deposit which present significant common geological differences resulting from possible different ore forming processes. The study of such deposits is thus important considering that proposed models might not fully explain their differences nor adequately predict their occurrences.

Abstract The Neoarchean Abitibi greenstone belt in the southern Superior Province has been one of the world’s major gold-producing regions for almost a century with >6,100 metric tons (t) Au produced and a total endowment, including production, reserves, and resources (measured and indicated), of >9,375 t Au. The Abitibi belt records continuous mafic to felsic submarine volcanism and plutonism from ca. 2740 to 2660 Ma. A significant part of that gold is synvolcanic and/or synmagmatic and was formed during the volcanic construction of the belt between ca. 2740 and 2695 Ma. However, >60% of the gold is hosted in late, orogenic quartz-carbonate vein-style deposits that formed between ca. 2660 and 2640 ± 10 Ma, predominantly along the Larder Lake-Cadillac and Destor-Porcupine fault zones. This ore-forming period coincides with the D 3 deformation, a broad north-south main phase of regional shortening that followed a period of extension and associated crustal thinning, alkaline to subalkaline magmatism, and development of orogenic fluvial-alluvial sedimentary basins (ca. <2679–<2669 Ma). These sedimentary rocks are referred to, in the southern Abitibi, as Timiskaming-type. The tectonic inversion from extension to compression is <2669 Ma, the maximum age of the D 3 -deformed youngest Timiskaming rocks. In addition to the quartz-carbonate vein-style, stockwork-disseminated-replacement-style mineralization is hosted in and/or is associated with ca. 2683 to 2670 Ma, early-to syn-Timiskaming alkaline to subalkaline intrusions along major deformation corridors, especially in southern Abitibi. The bulk of such deposits formed late-to post-alkaline to subalkaline magmatism and the largest deposits are early- to syn-D 3 (ca. 2670–2660 Ma), whereas the bulk of the quartz-carbonate vein systems formed syn- to late-D 3 and metamorphism. At belt scale, these illustrate a gradual transition, as shortening increases, in ore styles in orogenic deposits throughout the duration of the D 3 deformation event along the length of the Larder Lake-Cadillac and Destor-Porcupine faults. The sequence of events, although similar in all camps, was probably not perfectly synchronous at belt scale, but varied/migrated with time and crustal levels along the main deformation corridors and from north to south. The presence of high-level alkaline/shoshonitic intrusions, which are spatially associated with Timiskaming conglomerate and sandstone, large-scale hydrothermal alteration, and numerous gold deposits along the Larder Lake-Cadillac and Destor-Porcupine faults indicates that these structures were deeply rooted and tapped auriferous metamorphic-hydrothermal fluids and melts from the upper mantle and/or lower crust, late in the evolution of the belt. The metamorphic-hydrothermal fluids, rich in H 2 O, CO 2 , and H 2 S were capable of leaching and transporting gold to the upper crust along the major faults and their splays. Although most magmatic activity along the faults predates gold, magmas may have contributed fluids and/or metals to the hydrothermal systems in some cases. This great vertical reach explains why the Larder Lake-Cadillac and Destor-Porcupine fault zones are very fertile structures. The major endowment of the southern part of the Abitibi belt (>8,100 t Au) along the corridor defined by the Larder Lake-Cadillac and Destor-Porcupine faults may also suggest that these faults have tapped particularly fertile upper mantle-lower crust gold reservoirs. The concentration of large synvolcanic and synmagmatic gold deposits along that corridor supports the idea of gold-rich source(s) that may have contributed gold to the ore-forming systems at different times during the evolution of the belt.

Abstract The Noranda camp in the southern Abitibi greenstone belt comprises over 20 volcanogenic massive sulfide deposits hosted by volcanic rocks of the 2704–2695 Ma Blake River Group. Decades of research and exploration have provided a firm understanding of the characteristics of these deposits as well as the geological controls on deposit location. Observations made on the deposits of the Noranda camp significantly contributed to the syngenetic model of massive sulfide formation and shaped the current understanding of ancient and modern sea-floor hydrothermal systems. The Horne and Quemont deposits, which are the largest deposits in the Noranda camp, are hosted by 2702 Ma felsic volcanic successions dominated by volcaniclastic rocks. The massive sulfide ores of these deposits largely formed through processes of subseafloor infiltration and replacement of the highly permeable wall rocks. Laterally extensive hydrothermal alteration halos dominated by chlorite and sericite surround the replacement ores. The Horne deposit formed in an extensional setting in a graben bounded by synvolcanic faults. Rapid extension accompanying deposit formation resulted in the upwelling of mantle-derived mafic melts and the emplacement of a thick package of mafic rocks in the stratigraphic hanging wall of the deposit. Most of the massive sulfide deposits in the Noranda camp are hosted by a 2700–2698 Ma bimodal volcanic succession that formed in a large volcanic subsidence structure to the north. The ~2,000-m-thick lava flow-dominated volcanic package is floored by the large, multiphase, synvolcanic Flavrian pluton. The deposits in this part of the Noranda camp are small (<5 million tonnes) and primarily formed as sulfide mounds on the ancient sea floor. Synvolcanic structures provided cross-stratal permeability for the hydrothermal fluids and controlled the location of volcanic vents. Thin tuffaceous units mark the sea-floor positions hosting the massive sulfide mounds within the flow-dominated volcanic succession. The concordant massive sulfide lenses overlie discordant alteration pipes composed of chlorite- and sericite-altered rocks. Contact metamorphism associated with the emplacement of the ~2690 Ma Lac Dufault pluton converted the hydrothermal alteration pipes into cordierite-anthophyllite assemblages. Recent brownfields exploration successes have demonstrated that massive sulfide discoveries are still possible in one of Canada’s most mature mining camp through three-dimensional geological modeling performed at the camp scale. Geologic target generation through computer modeling has reversed the general trend of progressively deeper exploration with time in the Noranda camp. Deep exploration currently focuses on the reevaluation of a previously uneconomic low-grade ore zone at the Horne deposit.

Abstract The Larder Lake-Cadillac Break is a gold metallotect, which extends for more than 250 km from Matachewan in Ontario to Val-d’Or in Quebec. For much of its length it juxtaposes older komatiitic rocks against younger sedimentary units. Among the adjacent sedimentary rocks are distinctive intervals of polymict conglomerate and crossbedded sandstone, which make up part of the Timiskaming Group that unconformably overlies previously folded volcanic strata. Rocks in the vicinity of the break are commonly strongly carbonatized, with the type and abundance of carbonate minerals being controlled largely by protolith composition. Shoshonitic to alkalic igneous rocks occur along the break as volcanic units within the Timiskaming, as plutonic rocks in syn-Timiskaming stocks and plugs, and as local arrays of albitite dikes of intermediate composition. High-strain dislocative deformation is variably developed along the break but its intensity is in part a reflection of metasomatic phyllosilicates in the affected rocks. Gold deposits tend to form clusters along the break and their relationship to it is two-fold: a subset of geologically similar deposits are localized in direct proximity to the break but the majority of gold in the region is found in diverse settings away from it with no clear genetic connection.