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 Archean orogenic lode gold deposits are the result of large, complex mineralizing systems that have developed within many Archean terrains. Mineralizing systems are defined to include all geologic factors that control the generation and preservation of mineral deposits and emphasize the processes responsible for deposit formation at a variety of scales. Deposits belonging to Archean orogenic lode gold mineralizing systems comprise epigenetic mineralization that formed as a result of focused fluid flow late during active deformation and metamorphism of volcano-plutonic terranes. They can occur in any lithology and formed at a range of paleocrustal levels through site-specific and local physical and chemical processes. All Archean orogenic lode gold deposits formed through broadly similar geologic processes, with the unique character of individual deposits resulting mainly from variations at the depositional site. The key feature of Archean orogenic lode gold systems is a broadly uniform low-moderate salinity, mixed aqueous-carbonic fluid that is capable of carrying Au but has limited capacity to transport base metals. Models for development of orogenic lode gold mineralizing systems are generally poorly constrained, although geologic and geochemical characteristics are consistent with terrane- or larger-scale processes. Archean terranes containing orogenic lode gold systems include accretionary and collisional settings. Mineralization is generally late in the tectonic evolution of the host terranes; is typically syn- to postpeak metamorphism, becoming increasingly postpeak at higher paleocrustal levels; and is indicative of clockwise metamorphic P-T paths and implicating processes involving “deeper later”-type metamorphism. There are few robust absolute ages on mineralization although, in most terranes, available ages indicate mineralization follows major volcanic, sedimentary, and plutonic episodes. In many terranes, mineralization is coincident with mid-crustal felsic magmatism. Young absolute ages recorded in some deposits probably reflect resetting and/or new mineral growth during post-gold mineralization hydrothermal activity and/or slow cooling of host terranes. The source(s) of fluids and metals in orogenic lode gold systems is poorly understood; however, mineral equilibria and isotope tracers implicate sources deeper than presently exposed greenstones. Isotope tracers and mineral equilibria are also consistent with derivation from and/or equilibration of the ore fluid with felsic rocks during transport of hydrothermal fluids to depositional sites. Stable and radiogenic isotope tracers alone do not distinguish between fluid derivation through metamorphic devolatilization and magmatic fluid evolution. Some deposits formed at high paleocrustal levels, however, and record the influx of surface waters. Transport of large volumes of broadly uniform hydrothermal fluids over relatively long distances is implicated and requires channelized fluid flow with minor modification of major molecular components en route from source to depositional sites. Selected major deformation zones that are truly “crustal scale,” as demonstrated by deep seismic profiling, provide ideal fluid pathways for deeply sourced hydrothermal fluids. The largest gold provinces show spatial proximity of world-class lode gold deposits to “crustal-scale” deformation zones (e.g., Boulder-Lefroy, Destor-Porcupine). Linking of active faults is important for fluid focusing and effective transport of hydrothermal fluids. The hydrothermal fluids transport gold along the pathways as one or more neutral and reduced sulfide species. Chemical modeling demonstrates that the inferred hydrothermal fluids can effectively transport gold over long distances and over a significant crustal-depth range. Provided the fluids remain effectively channelized during transport to higher crustal levels, camp- and deposit-scale structural focusing and associated local gold precipitation mechanisms are required to ensure development of economic gold mineralization at the trap site. Archean orogenic lode gold mineralizing systems have distinctive depositional site characteristics at both the camp and deposit scale. Camp-scale features include geochemical signatures related to regional alteration surrounding major deformation zones, large-scale structural inhomogeneities (e.g., bends in major deformation zones, district-scale granitoid-greenstone contacts), and the presence or absence of overlying rock successions that can act as barriers (e.g., seals, aquicludes) to fluid movement. Deposit-scale variables include the local host rocks, structural traps (fault intersections, contacts between contrasting lithologies) typically in zones of low mean stress, and the P-T conditions in the host sequence. Resulting alteration assemblages primarily reflect interaction of host lithologies with the hydrothermal fluid at a particular pressure and temperature. Alteration assemblages generally show enrichment in K, CO 2 and S in deposits irrespective of paleocrustal level. A metal association of Au, Ag ± As, B, Bi, Sb, Te, and W is displayed by most deposits. Fluid-inclusion-derived data at individual deposits show a range in compositions, although salinity is generally low to moderate, with mixed aqueous-carbonic compositions. Variations in CO 2 , CH 4 , N 2 , salinity, and redox-state may reflect district- to local-scale processes and/or specific host rocks rather than differences in fluid source. Deposition at the trap site is likely to reflect catastrophic effects in response to physical changes (e.g., large pressure fluctuations, seismic events), with resultant chemical changes due to local fluid wall-rock interaction, phase separation, and/or fluid mixing. The extreme diversity of Archean orogenic lode gold deposits reflects the complex interplay of physical and chemical processes at a trap (depositional) site localized at various crustal levels ranging from sub-greenschist to upper-amphibolite facies metamorphic environments, with gold precipitation occurring over a correspondingly wide range of pressures and temperatures. The variability in deposit characteristics largely reflects the P-T conditions, variability in host rock, and local changes in ore fluid composition.

Abstract Certain aspects of the genesis of Archean epigenetic gold deposits remain controversial, in particular the source of the auriferous fluids, which are arguably magmatic, metamorphic, or mantle derived. In an attempt to constrain the fluid source, it is essential to consider Archean gold mineralization in terms of the tectonic, magmatic, and metamorphic history of greenstone terranes. Asymmetries in the distribution of volcanic, sedimentary, and plutonic rock types, the pattern of deformation, and the rapid evolution of the greenstone sequences within the Norseman-Wiluna belt in the eastern Yilgarn block are akin to those of younger orogenic belts at obliquely convergent continental plate boundaries. Archean gold deposits show many similarities to younger, cordilleran-style gold deposits (e.g., the Mother Lode) which occur in a similar tectonic setting, particularly in terms of their strong dependence on structural controls and the composition of the ore fluids. In the eastern Yilgarn block there is a coincidence of lode gold mineralization, calc-alkaline porphyry, and lamprophyre dike swarms and craton-scale oblique-slip faults with their attendent mantle-derived carbonation. With no compelling evidence for direct derivation of ore fluids from felsic magmas, gold mineralization is best viewed as the upper crustal expression of a deep-seated tectono-thermal event with mantle-crustal outgassing, occurring in response to a deep mantle heat source, related to convergent tectonics. In all probability the ore fluid contained magmatic, metamorphic, and mantle components, but it is impossible at this stage to determine with which component the gold was predominantly associated.