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 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.

Abstract The gold-rich Superior, Canada, and Yilgarn, Australia, cratons have similar geologic histories dating back to the Mesoarchean and showing strong parallels in the Neoarchean. Orogenesis in each craton is marked by a shift from dominant volcanism to dominant clastic sedimentation above unconformities, followed by granitic plutonism, progressive deformation, and dynamothermal metamorphism. The terminal stages of orogenesis correspond to the intervals of 2660 to 2650 Ma in the Superior craton and 2660 to 2630 Ma in the Yilgarn cra ton. The Yilgarn and Superior cratons contain an estimated 9,200 and 8,500 t Au, respectively. Most of the sig nificant gold deposits (>100 t Au) are concentrated in a few narrow, highly endowed gold belts along which the deposits cluster into camps, commonly spaced every 30 to 50 km. Gold deposits of both cratons show similar tonnages and grades, and their size distributions define a Pareto, rather than log-normal distribution. Large deposits are rare but account for most of the gold endowment of each craton. Three recurring host-rock associations account for a majority of large deposits: iron-rich mafic igneous rocks, iron-rich sedimentary rocks, and felsic to intermediate porphyry stocks and dikes. Most deposits, particularly large ones, occur in greenschist-grade rocks and are associated with shear zones, faults, or folds. Gold miner alization styles include quartz-carbonate veins, sulfidic replacements in banded iron formation (BIF), crusti form carbonate-quartz veins and associated sulfidic replacement lodes, disseminated-stockwork zones, sulfide rich veins and veinlet zones, and massive sulfide lenses. Wall-rock alteration assemblages vary with mineralization style and metamorphic grade. Most deposits consist of a single style of mineralization, but many of the large ones combine two or more of these, and some large deposits are unique in their metal associations. The diversity of styles of mineralization, wall-rock alteration assemblages, and overprinting relationships re quire more than one episode of gold mineralization and more than one ore-forming process. Geologic parage neses, coupled with isotopic age constraints, show that, although the Archean histories of both cratons span >300]m.y., the majority of gold deposits formed during the final 30 to 50 m.y. of that time span, corresponding to the orogenic phase. The majority of gold deposits can thus be regarded as orogenic in timing, but with the available constraints clearly pointing to the existence of more than one mineralizing event and involving dif ferent mineralization types and processes. The best-endowed gold camps (Timmins and Red Lake in the Superior craton; Kalgoorlie, Granny-Wallaby, and Sunrise Dam in the Yilgarn craton) commonly possess an anticlinorial structure, komatiitic and basaltic rocks in the core giving way to stratigraphically higher volcanic and clastic sedimentary rock units. Such camps are further marked by coarse clastic rocks deposited above the metavolcanic rock sequences, by concentrations of shallow-level porphyritic intrusions, by extensive carbonate alteration, by multiple styles and ages of gold mineralization and, in most, by through-going regional faults. However, these characteristics are also shared by a number of less-endowed gold camps. The best-endowed gold belts are distinguished by substantial volumes of komatiite, by a high degree of preservation of supracrustal rocks, by structural highs that juxtapose the lower and uppermost parts of the stratigraphic column, by multiple styles and ages of gold mineralization, and by world-class deposits of other metals. The gold belts commonly are aligned along crustal-scale faults that rep resent long-lived structures, which acted as crustal-scale magma and fluid conduits and also influenced coarse clastic sedimentation. Abundant komatiites may reflect the first tangible connection to the deep crust and mantle, but the nature of the subvolcanic crust, ensimatic in the Timmins-Val d’Or, Superior craton, and ensialic in the Wiluna-Norseman belts, Yilgarn cration, seems unimportant in determining gold prospectivity. Significant uncertainty remains concerning the timing of formation of the deposits, the models that best explain their characteristics, and the fundamental causes of the high concentration of gold in a few areas. Various models have been proposed, invoking volcanic, magmatic, and orogenic (metamorphism and/or deformation) processes. The synorogenic model best accounts for the Au-only quartz-carbonate veins and temporally related mineralization styles. However, synvolcanic and magmatic hydrothermal models are also required to explain the presence of Au base metal deposits and those deposits overprinted by significant deformation and meta morphism. The specific histories of the gold belts and the known constraints on timing of deposits suggest that all of these processes have contributed to the gold endowment, although it is difficult to separate orogenic from magmatic processes because they closely overlap in time and space. Despite the presence of synorogenic quartz-carbonate veins throughout the greenstone belts of both cratons, large deposits of this style are mainly restricted to the gold belts, where they also coexist with large deposits of other styles of gold mineralization. The presence of multiple ages and styles of gold mineralization in the best-endowed gold belts indicate a unique locus of successive formation of gold deposits from various processes operating at different stages of the orogenic phase of the evolution of these belts. This would explain the common overprinting of the early deposit types, potentially of synvolcanic or synplutonic origin, by syn orogenic ones. The concentration of multiple types and ages of significant gold deposits in well-defined gold belts is not a unique feature of the Superior and Yilgarn cratons but is shared by Tertiary gold belts of Nevada, such as the Walker Lane, the Battle Mountain-Eureka trend, and the Carlin trend. This must be a reflection of fundamental crustal structure, and perhaps composition of subcrustal mantle, as much as local ore-forming hydrothermal processes.

Abstract Veins are common components of greenstone gold deposits. Their analysis is one key aspect in understanding the sequence of events leading to the formation or deformation of gold deposits. This analysis is essential for the determination of controls on mineralization and ore-forming processes, and for the prediction of the geometry and plunges of deposits and orebodies. Many greenstone gold districts have experienced a common structural evolution: D 1 thin skin-style shortening and D 2 thick skin-style shortening are largely responsible for the structural trend and penetrative fabrics in a district, whereas D 3 and D 4 transcurrent deformation are largely focused along preexisting major fault zones. A majority of greenstone gold deposits consists of quartz-carbonate veins in or adjacent to high-angle reverse, and less commonly transcurrent, shear zones, viewed as splays or subsidiaries of major, complex, belt-scale fault zones. In other deposits, veins simply overprint gold mineralization and provide important information about the postore deformation history. Three main types of veins occur in greenstone gold deposits and each records small increments of bulk strain. Laminated fault-fill veins form by slip along the central parts of active shear zones in low-angle di-lational bends, or less commonly by extensional opening of foliation planes. Extensional and oblique-extension veins form within or adjacent to shear zones, at high angles to foliation and elongation lineation. They represent opening and filling of extensional and hybrid extensional-shear fractures, respectively. In more competent host rocks, extensional veins can form arrays of en echelon planar or sigmoidal veins, or of stacked planar veins, and can also combine into multiple sets to form stockwork and breccia bodies. Multiple types and sets of auriferous veins commonly combine to form variably complex vein networks, especially in large deposits. These vein networks record deposit-scale bulk incremental strain, with axes of elongation and shortening that can be compared with those of the main deformation increments in the district as a further way of constraining their timing of formation. The formation of vein networks in many districts is compatible with D 2 , and in a number of others with D 3 , reflecting their formation in contractional or transcurrent deformation regimes, likely involving subhorizontal compressional stress under high fluid pressures. Veins in many districts also systematically display evidence of overprinting deformation, in the form of folds, boudins, striated vein margins, and a number of internal vein textures such as recrystallized quartz and stylolites. Overprinting deformation is a natural consequence of vein formation in active shear zones, but it can also result from overprinting of veins by a younger increment of regional deformation. This can lead to local shear zone reactivation or wholesale folding or boudinage of a deposit. The confident determination of the structural timing of veins in deposits is critical but challenging, and is at the center of divergences of interpretation of the origin of many greenstone gold deposits. A number of guidelines are offered to help distinguish pre-orogenic veins and deposits from those with syn- to postorogenic timing.

Abstract It has been the custom of the Geological Survey of Canada (GSC) to periodically publish a summary account of the geology and economic mineral deposits of Canada. Such ‘Geology and Economic Minerals of Canada’ volumes were published in 1909, 1926, 1946, 1957, and 1970. During the 1980s, in lieu of a sixth edition, the GSC embarked on the production of a series of more comprehensive ‘Geology of Canada’ volumes as part of the Geological Society of America (GSA) Decade of North American Geology (DNAG) series. Much of the material for this review of the geology and economic minerals of Superior Province was prepared as part of the DNAG initiative but, due to revised operational requirements, the integration of the geological and metallogenic aspects arose much later. In this integrated review, we place particular emphasis on advances that have taken place in the 25 years since the last summaries of this kind (Stockwell et al., 1970; Lang et al., 1970; Goodwin, 1972). The Archean Superior Craton forms the core of the North American continent (Fig. 2.1) and is surrounded and truncated on all sides by Proterozoic orogens, the collisional zones along which the elements of the Precambrian Canadian Shield were amalgamated (Hoffman, 1988, 1989). The Superior Province (Fig. 2.2) is the 2 × 10 6 km 2 region of this craton which is free of significant post-Archean cover rocks and deformation. The rocks of Superior Province are of mainly Meso- and Neoarchean age and have been significantly affected by post-Archean deformation only along

Abstract The Superior province, a major Archean craton of the Canadian Shield, was formed during middle and late Archean tectono-magmatic events from rocks of mantle or recycled juvenile crustal origin assembled by accretionary mechanisms in convergent tectonic settings. The Superior province consists of northern and southern high-grade gneiss terranes and a broad central region of alternating lower grade greenstone- and metasediment-rich subprovinces, all intruded by voluminous granitoid plutons. Subprovince boundaries are complex zones of facies, metamorphic, and structural transition, commonly telescoped by crustal-scale faults. Volcanism, plutonism, and sedimentation occurred at ca 3.1 to 2.8 Ga, notably in the north, and again at ca 2.75 to 2.7 Ga throughout the Superior province, in settings analogous to modern oceanic island arc-interarc basin-accretionary wedge systems. Polyphase deformation, metamorphism, and plutonism at ca 2.7 Ga, all products of subduction-driven accretion, resulted in ductile deformation and granulite facies metamorphism at deep crustal levels represented by the high-grade gneiss terranes and in ductile to brittle deformation and lower grade metamorphism at high crustal levels represented by the greenstone-granite subprovinces. Lode gold deposits formed during late ductile-brittle stages of deformation are related to major fault systems and to zones of rock alteration within greenstone-granite subprovinces. Granulite metamorphism at depth, involving dehydration and formation of HO 2 − and CO 2 -rich fluids, was synchronous with brittle deformation, rock alteration, and formation of lode gold deposits at high crustal levels. Although granulitization models probably best account for most genetic aspects of the Superior province lode gold deposits except their confinement to greenstone-granite terranes, degree of greenstone belt preservation is an important factor in their present distribution.