Abstract Ancient volcanogenic massive sulfide (VMS) deposits formed in rifted arc, back-arc, and other extensional geodynamic environments and were deformed during later convergent collisional and/or accretionary events. Primary features of deposits influenced the development of tectonic structures. Except for pyrite, common sulfides in VMS deposits are much weaker than their volcanic host rocks. During deformation, strain is taken by the weak sericitic and chloritic alteration envelope surrounding the deposits and by the sulfide bodies themselves, which act as shear zones, undergo hinge thickening and limb attenuation during regional folding, and are deformed into elongate bodies parallel to regional fold hinges and stretching lineations. A tectonic foliation forms as a sulfide banding in the interior of VMS lenses due to shearing and flattening of primary textural and compositional heterogeneities and as a banded silicate-sulfide tectonic foliation along the margins of the VMS lenses due to transposition and shearing of primary silicate (exhalites)-sulfide layers. Other characteristic structures, such as cusps, piercement cusps, piercement veins, and durchbewegung structures (sulfide breccias), formed as a result of the strong competency contrast between the massive sulfide deposits and their host volcanic rocks. Some features of VMS deposits may have both primary and tectonic components, requiring careful mapping of volcanic lithofacies and primary and tectonic structures to assess the nature of these features. One example is the vertical stacking of VMS lenses. The stacking may be primary, due to the rapid burial of lenses by volcanic or sedimentary deposits as the upward flow of hydrothermal fluids continued and precipitated new lenses above the earlier formed lenses. Or it may be tectonic, due to thrusting or isoclinal folding and transposition of the VMS lenses. Metal zoning (Cu/Cu + Zn), produced by zone refining at the seafloor or subseafloor, is refractory to deformation and metamorphism and can be used to delineate hydrothermal fluid upflow zones and, together with stratigraphic mapping, determine if the stacking is primary, tectonic, or both. Similarly, the elongation of VMS lenses may have a primary component due to the deposition and coalescence of sulfide lenses along linear synvolcanic faults or fissures, as well as a tectonic component due to mechanical remobilization of sulfides parallel to linear structural features in the host volcanic rocks. Structural mapping of VMS deposits is hampered by low-temperature recrystallization of sulfides, which masks the effects of deformation, by discontinuous and abrupt lithofacies changes in the volcanic host rocks, and by the weak development of tectonic fabrics and strong strain partitioning in volcanic rocks. To mitigate these issues, mapping of volcanic lithofacies should be done concurrently with structural mapping to delineate repeated stratigraphic panels across reactivated faults and to identify, in the absence of well-developed fabrics, regional folds characterized by abrupt changes in strata orientation from limbs to hinge. Where well-layered sedimentary rocks are intercalated with volcanic rocks, structures should be mapped in the sedimentary rocks and then correlated with those in volcanic rocks to alleviate difficulties in mapping structures in volcanic rocks and defining the sequence of deformation events that affected the volcanic rocks and their VMS deposits.

Abstract The Abitibi greenstone belt, which straddles the border between Ontario and Quebec in eastern Canada, represents one of the largest and best-preserved Neoarchean greenstone belts in the world. The belt consists of E-trending successions of folded volcanic and sedimentary rocks and intervening domes of intrusive rocks. Submarine volcanism occurred between 2795 and 2695 Ma. Six volcanic assemblages have been defined, recording submarine volcanism during specific periods of time. Komatiite successions within some of these volcanic assemblages are host to magmatic sulfide deposits. However, economically more important are volcanogenic massive sulfide (VMS) deposits, which contain a total of ~775 million tonnes (t) of polymetallic massive sulfides. Approximately half of the endowment is hosted by volcanic rocks of the 2704 to 2695 Ma Blake River assemblage. VMS deposits of this assemblage also account for most of the synvolcanic gold in the Abitibi greenstone belt, totaling over 1,100 t (~35 Moz). Submarine volcanism was followed by the deposition of large amounts of sedimentary material derived from a shallow marine or subaerial hinterland, created as a result of crustal thickening during an early phase of mountain building at ≤2690 to ≤2685 Ma. Submarine volcanic rocks and the overlying flysch-like sedimentary rocks of the Porcupine assemblage were affected by large-scale folding and thrusting during at least one deformational event prior to 2679 Ma. At this time, a terrestrial unconformity surface developed between the older and already deformed rocks of the Abitibi greenstone belt and molasse-like sedimentary rocks of the Timiskaming assemblage, which were deposited between ≤2679 and ≤2669 Ma. Deposition of the Timiskaming sedimentary rocks occurred in extensional basins and was locally accompanied by predominantly alkaline volcanism and related intrusive activity. Crustal shortening and thick-skinned deformation resulted in the structural burial of the molasse-like sedimentary rocks of the Timiskaming assemblage after 2669 Ma. Panels of Timiskaming deposits were preserved in the footwall of these thrusts, which are today represented by major fault zones cutting across the supracrustal rocks of the Abitibi greenstone belt. The structural history of these fault zones is complicated by late-stage strike-slip deformation. The Porcupine-Destor and Larder Lake-Cadillac fault zones of the southern Abitibi greenstone belt as well as second- and third-order splays off these fault zones are host to a number of major orogenic gold deposits. The gold endowment of these deposits exceeds 6,200 t (~200 Moz), making the Abitibi greenstone belt one of the economically most important metamorphic terranes in the world.

Abstract Komatiitic rocks occur mainly in Archean greenstone belts, less commonly in Paleoproterozoic volcano-sedimentary belts, and only rarely in younger volcanic settings. As in most other greenstone belts worldwide, komatiitic rocks are locally abundant in the Abitibi greenstone belt but generally represent only a small proportion of the volcanic rocks in the volcanic succession. Although only locally exposed, glacially sculpted exposures of only weakly metamorphosed and mildly deformed komatiites of mineralized and unmineralized komatiites in the Abitibi greenstone belt are among the best in the world, characterized by excellent textural preservation and, in some cases, excellent mineralogical preservation. Komatiitic rocks in the Abitibi greenstone belt occur predominantly within the Pacaud (2750–2735 Ma), Stoughton-Roquemaure (2723–2720 Ma), Kidd-Munro (2720–2710 Ma), and Tisdale (2710–2704 Ma) assemblages, but have recently also been recognized in lesser abundances within the Deloro (2734–2724 Ma) and Porcupine (≤2690–≤2685 Ma) assemblages. Overall, the komatiitic rocks present in these assemblages are characterized by a wide variety of lithofacies (textural, compositional) and flow facies; however, a regional analysis of komatiite physical volcanology reveals some fundamental differences between each of the komatiite-bearing assemblages. The Kidd-Munro and Tisdale komatiite-bearing assemblages contain the largest volumes of komatiitic rocks, in particular thick, highly magnesian cumulate lava channels and channelized sheet flows. This suggests that the magma discharge rates were higher for these assemblages and/or that they formed more proximal to the eruptive site. However, the recently discovered Grasset Ni-Cu-(PGE) deposit hosted within relatively high MgO cumulate rocks that are interpreted to occur within the Deloro assemblage highlights the possibility of the other komatiite-bearing assemblages to contain similarly prospective volcanic and/or subvolcanic facies. Geochemical data indicate that regardless of age or petrogenetic affinity (Al-undepleted vs. Al-depleted vs. Ti-enriched vs. Fe-rich), almost all of the parental magmas were undersaturated in sulfide prior to emplacement and therefore represent favorable magma sources for Ni-Cu-(PGE) mineralization. Volcanological data indicate that almost all komatiite-associated Ni-Cu-(PGE) deposits in the Abitibi greenstone belt appear to be localized in lava channels or channelized sheet flows, which have the capacity to thermomechanically erode S-bearing country rocks and to efficiently transfer metals from the magma to sulfide xenomelts. Three type localities (Spinifex Ridge in La Motte Township, Pyke Hill in Munro Township, and Alexo in Dundonald Township) illustrate how physical volcanology (lava channelization) and stratigraphic environment (S source) need to operate quasi-simultaneously to allow for the genesis of significant amounts of Ni-Cu-(PGE) sulfides within a komatiitic succession. As not all komatiite magma pathways are mineralized, one of the most important challenges is to be able to distinguish potentially mineralized successions from barren successions.

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 2698 Ma LaRonde Penna deposit, with over 71 Mt of ore at 3.9 g/t Au (280 t Au or ~9 Moz Au), is the second largest Au-rich volcanogenic massive sulfide (VMS) deposit in the world. It is part of the Doyon-Bousquet-LaRonde mining camp in the eastern part of the Blake River Group. The deposits of the Doyon-Bousquet-LaRonde mining camp are hosted by the volcanic rocks of the Hébé-court (base) and Bousquet (top) formations that form a southward-younging homoclinal sequence, with nearly vertical dips due to a north-south compressional event responsible for the development of an E-W–trending, steeply S-dipping, penetrative schistosity under prograde, upper greenschist to lower amphibolite facies meta-morphism. The E-trending, steeply S-dipping schistosity is associated with strong flattening, transposition, and minor folding of the volcanic rocks, alteration zones, and sulfide lenses. The ore lenses at LaRonde Penna, which are stacked in the upper half of the Bousquet Formation, are characterized by semimassive to massive sulfides or narrow intervals of transposed sulfide veins and veinlets. The synvolcanic hydrothermal alteration at LaRonde Penna now corresponds to mappable upper greenschist-lower amphibolites-grade metamorphic assemblages. In the upper part of the deposit, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulfide lens (20N Zn zone). The 20 North lens (20N Au and 20N Zn zones) is underlain by a large, semiconformable alteration zone that comprises a proximal quartz-Mn-garnet-biotite-muscovite alteration assemblage. The 20N Zn zone tapers with depth in the deposit and gives way to the 20N Au zone. At depth in the deposit, the 20N Au zone consists of semimassive sulfides (Au-rich pyrite and chalcopyrite) enclosed by a large aluminous alteration assemblage interpreted to be the metamorphic equivalent of an advanced argillic alteration zone. At LaRonde Penna, the presence of sulfide lenses characterized by Au-rich portions and base metal-rich portions demonstrates that a VMS system can generate mineralization styles that gradually evolve, both in space and time, from neutral (Au-Cu-Zn-Ag-Pb ore), to transitional, to acidic (advanced argillic alteration and Au ± Cu-rich ore) in response to the evolving local geologic setting.