Abstract Hemlo combines several rare to unique features in the spectrum of Archean greenstone gold deposits. It is an isolated, approximately 800-metric ton (t) gold system in a region of otherwise limited known gold endowment. The geology of Hemlo is dominated by deformed and metamorphosed sedimentary, felsic volcanic, and volcaniclastic units, a premineral coherent felsic porphyry, and a swarm of mainly postmineral, intermediate, feldspar-phyric dikes. Ore is dominantly in the form of gold-bearing lenses of pyritic, feldspathic schist derived from deformation of both the clastic rocks and the felsic porphyry. The deposit and its host rocks were metamorphosed at moderate pressures to assemblages diagnostic of the mid-amphibolite facies, followed by progressive retrogression to those of the greenschist facies. The result is a wide range of silicate mineral species in ambiguous textural relationships. The gold system itself is known for ore and related alteration minerals with significant concentrations of Mo-As-Sb-Hg-Tl-V-Ba-K-Na. The inferences derived from lithologic mapping, structural chronology, U-Pb geochronology, and mineral paragenesis favors an interpretation of Hemlo as a deformed and metamorphosed gold system formed from oxidized hydrothermal fluids in an upper crustal setting. Uncertainty remains as to the exact nature and geometry of that ore-forming hydrothermal system, however, and the role subsequent metamorphism and deformation have played in the local remobilization of ore constituents into their present paragenetically late structural sites.

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 The Timmins-Porcupine gold camp, with a total production of more than 2,125 tonnes (75 Moz) Au to date, represents the largest Archean orogenic greenstone-hosted gold camp worldwide in terms of total gold production. The gold deposits of the camp are distributed over 50 km of strike length along the Destor-Porcupine fault zone, including the giant Hollinger-McIntyre and Dome deposits. These two deposits are archetype examples of large Archean orogenic gold systems. The Dome mine, where the ore is centered on a folded unconformity between Tisdale volcanic rocks and Timiskaming sedimentary deposits, also illustrates the spatial relationship between large gold deposits and a regional unconformity. Gold-associated hydrothermal activity in the camp spanned a long period of time, as illustrated by early stage, barren to low-grade ankerite veins formed between ca. 2690 and 2674 Ma, i.e., prior to or very early in the development of the regional unconformity and sedimentation of the Timiskaming assemblage. Such early carbonatization may represent a key hydrothermal event in the formation of large orogenic gold deposits and illustrates the protracted nature of the large-scale CO 2 -rich metasomatism occurring before and during gold deposition. The bulk of the gold is, however, younger than the Three Nations Formation in the upper part of the Timiskaming assemblage (i.e., ≤2669 ± 1 Ma) and consists mainly of syn-main regional shortening deformation (D 3 ) networks of steeply to moderately dipping fault-fill quartz-carbonate ± tourmaline ± pyrite veins and associated extensional, shallow to moderately dipping arrays of sheeted and sigmoidal veins hosted in highly carbonatized and sericitized rocks. Formation of the gold deposits of the Timmins-Porcupine camp can be related to several key factors. The Destor-Porcupine fault zone represents a first-order control on the location of the camp as this major fault zone allowed large scale CO 2 -rich hydrothermal fluid upflow. The fault zone also controlled the location of the Timiskaming clastic basin, which is thought to have been developed as a result of early-stage synorogenic extensional faulting. Several of the orogenic gold deposits of the camp are spatially associated with the regional unconformity separating folded submarine volcanic rocks of the Tisdale assemblage form the syn-orogenic sedimentary deposits of the Timiskaming assemblage. The current level of erosion is deep enough to expose the unconformity and to maximize the chance of discovery of the orogenic deposits or their footprint, but allowed for preservation of at least part of the gold deposits that are mainly hosted in the highly reactive Fe-rich Tisdale basalt. Additional key factors include the presence of komatiitic and/or basaltic komatiite flows, of competent intrusions that predate the main phase of shortening of the belt and the occurrence of bends in the trace of the Destor-Porcupine fault zone that may have facilitated focus to ore-forming fluid upflow. Furthermore, the camp is characterized by complex structural and rheological discontinuities, competency contrasts, and early stage folds with associated fracture and fault networks that provided highly favorable ground preparation conditions. The exceptional gold enrichment of the camp requires that the hydrothermal fluids originated from favorable source rocks, lending support to the concept of provinciality, which may best explain the exceptional gold fertility of the southern Abitibi greenstone belt.