Abstract Epigenetic gold deposits in metamorphic terranes include those of the Precambrian shields (approx 23,000-25,000 t Au), particularly the Late Archean greenstone belts and Paleoproterozoic fold belts, and of the late Neoproterozoic and younger Cordilleran-style orogens (approx 22,000 t lode and 15,500 t placer Au), mainly along the margins of Gondwana, Laurentia, and the more recent circum-Pacific. Ore formation was concentrated during the time intervals of 2.8 to 2.55 Ga, 2.1 to 1.8 Ga, and 600 to 50 Ma. Prior to the last 25 years, ores were defined by grades of 5 to 10 g/t Au in underground mines; present-day economics, open-pit mining, and improved mineral processing procedures allow recovery of ores of < 1 g/t Au, which has commonly led to the recent reworking of lower gradEzones in many historic orebodies. Most of these deposits formed synchronously with late stages of orogeny and are best classified as orogenic gold deposits, which may be subdivided into epizonal, mesozonal, and hypozonal subtypes based on pressure-temperature conditions of ore formation. A second type of deposit, termed intrusion-related gold deposits, developed landward of Phanerozoic accreted terranes in the Paleozoic of eastern Australia and the Mesozoic of the northern North American Cordillera. These have an overall global distribution that is still equivocal and are characterized by an intimate genetic association with relatively reduced granitoids. The majority of gold deposits in metamorphic terranes are located adjacent to first-order, deep-crustal fault zones, which show complex structural histories and may extend along strike for hundreds of kilometers with widths of as much as a few thousand meters. Fluid migration along such zones was driven by episodes of major pressure fluctuations during seismic events. Ores formed as vein fill of second-and third-order shears and faults, particularly at jogs or changes in strike along the crustal fault zones. Mineralization styles vary from stockworks and breccias in shallow, brittle regimes, through laminated crack-seal veins and sigmoidal vein arrays in brittle-ductile crustal regions, to replacement- and disseminated-type orebodies in deeper, ductile environments (i.e., a continuum model). Most orogenic gold deposits occur in greenschist facies rocks, but significant orebodies can be present in lower and higher grade rocks. Deposits typically formed on retrograde portions of pressure-temperature-time paths and thus are discordant to metamorphic features within host rocks. Spatial association between gold ores and granitoids of all compositions reflects a locally favorable structural trap, except in the case of the intrusion-related gold deposits where there is a clearer genetic association. World-class orebodies are generally 2 to 10 km long, about 1 km wide, and are mined downdip to depths of 2 to 3 km. Most orogenic gold deposits contain 2 to 5 percent sulfide minerals and have gold/silver ratios from 5 to 10 and gold fineness >900. Arsenopyrite and pyrite are the dominant sulfide minerals, whereas pyrrhotite is more important in higher temperature ores and base metals are not highly anomalous. Tungsten-, Bi-, and Te-bearing mineral phases can be common and are dominant in the relatively sulfide poor intrusion-related gold deposits. Alteration intensity, width, and assemblage vary with the host rock, but carbonates, sulfides, muscovite, chlorite, K-feldspar, biotite, tourmaline, and albite are generally present, except in high-temperature systems where alteration halos are dominated by skarnlike assemblages. The vein-forming fluids for gold deposits in metamorphic environments are uniquely CO 2 and 18 O rich, with low to moderate salinities. Phanerozoic and Paleoproterozic ores show a mode of formation temperatures at 250° to 350°C, whereas Late Archean deposits cluster at about 325°to 400°C. However, there are also many important lower and higher temperature deposits deposited throughout the continuum of depths that range between 2 and 20 km. Ore fluids were, in most cases, near-neutral pH, slightly reduced, and dominated by sulfide complexes. Globally consistent ore-fluid δ 18 O values of 6 to 13 per mil and δD values of –80 to –20 per mil generally rule out a significant meteoric water component in the gold-bearing hydrothermal systems. Sulfur isotope measurements on ore-related sulfide minerals are concentrated between 0 and 10 per mil, but with many higher and much lower exceptions, indicating variable sulfur sources and an unlikely dominant role for mantle sulfur. Drastic pressure fluctuations with associated fluid unmixing and/or desulfidation during water/rock interaction are the two most commonly called-upon ore precipitation mechanisms. The specific model(s) for gold ore genesis remains controversial. Although the direct syngenetic models of the 1970s are no longer applicable, the gold itself may be initially added into the volcanic and sedimentary crustal rock sequences, probably within marine pyrite, during sea-floor hydrothermal events. Gold transport and concentration are most commonly suggested to be associated with metamorphic processes, as indicated by the volatile composition of the hydrothermal fluids, the progressive decrease in concentration of elements enriched in the gold deposits with increasing metamorphic grade of the country rocks, and the common association of ores with medium-grade metamorphic environments. Gold deposits of typically relatively low grade, which formed directly from fluid exsolution during granitoid emplacement within metamorphic rocks, are now also clearly recognized (i.e., intrusion-related gold deposits), but there are limited definitive data to implicate such an exsolved fluid source for most gold deposits within orogenic provinces. The fact that orogenic gold deposits are associated with all types of igneous rocks is a problem to a pure magmatic model. Hybrid models, where slab-derived fluids may generate rising melts that drive devolatilization reactions in the lower crust, are also feasible. Although involvement of a direct mantle fluid presents geochemical difficulties, the presence of lamprophyres and deep-crustal faults in many districts suggests potential mantle influence in the overall, large scale tectonic event controlling the hydrothermal flow system.

Abstract Mineral deposits exhibit heterogeneous distributions, with each major deposit type showing distinctive, commonly unique, temporal patterns. These reflect a complex interplay between formational and preservational forces that, in turn, largely reflect changes in tectonic processes and environmental conditions in an evolving Earth. The major drivers were the supercontinent cycle and evolution from plume-dominated to modern-style plate tectonics in a cooling Earth. Consequent decrease in the growth rate of continental crust, and change from thick, buoyant sub-continental lithospheric mantle (SCLM) in the Precambrian to thinner, negatively buoyant SCLM in the Phanerozoic, led to progressive decoupling of formational and preservational processes through time. This affected the temporal patterns of deposit types including orogenic gold, porphyry and epithermal deposits, volcanic hosted massive sulphide (VHMS), palaeoplacer Au, iron oxide, copper gold (IOCG), platinum group elements (PGE), diamond and probably massive sulphide SEDEX deposits. Sedimentary mineral deposits mined for redox-sensitive metals show highly anomalous temporal patterns in which specific deposit types are restricted to particular times in Earth history. In particular, palaeoplacer uranium, banded iron formation (BIF) and BIF-associated manganese carbonates that formed in the early Precambrian do not reappear in younger basins. The most obvious driver is progressive oxidation of the atmosphere, with consequent long-term changes in the hydrosphere and biosphere, the latter influencing the temporal distribution and peak development of deposits such as Mississippi Valley types (MVT), hosted in biogenic sedimentary rocks.

The Tombstone, Mayo and Tungsten plutonic suites of granitic intrusions, collectively termed the Tombstone-Tungsten Belt, form three geographically, mineralogically, geochemically and metallogenically distinct plutonic suites. The granites (sensu lato) intruded the ancient North American continental margin of the northern Canadian Cordillera as part of a single magmatic episode in the mid-Cretaceous (96-90 Ma). The Tombstone Suite is alkalic, variably fractionated, slightly oxidised, contains magnetite and titanite, and has primary, but no xenocrystic, zircon. The Mayo Suite is sub-alkalic, metaluminous to weakly peraluminous, fractionated, but with early felsic and late mafic phases, moderately reduced with titanite dominant, and has xenocrystic zircon. The Tungsten Suite is peraluminous, entirely felsic, more highly fractionated, reduced with ilmenite dominant, and has abundant xenocrystic zircon. Each suite has a distinctive petrogenesis. The Tombstone Suite was derived from an enriched, previously depleted lithospheric mantle, the Tungsten Suite is from the continental crust including, but not dominated by, carbonaceous pelitic rocks, and the Mayo Suite is from a similar sedimentary crustal source, but is mixed with a distinct mafic component from an enriched mantle source. Each suite has a distinctive metallogeny that is related to the source and redox characteristics of the magma. The Tombstone Suite has a Au-Cu-Bi association that is characteristic of most oxidised and alkalic magmas, but also has associated, and enigmatic, U-Th-F mineralisation. The reduced Tungsten Suite intrusions are characterised by world-class tungsten skarn deposits with less significant Cu, Zn, Sn and Mo anomalies. The Mayo Suite intrusions are characteristically gold-enriched, with associated As, Bi, Te and W associations. All suites also have associated, but distal and lower temperature Ag-Pb- and Sb-rich mineral occurrences. Although processes such as fractionation, volatile enrichment and phase separation are ultimately required to produce economic concentrations of ore elements from crystallising magmas, the nature of the source materials and their redox state play an important role in determining which elements are effectively concentrated by magmatic processes.

Abstract The Serra Pelada Au-PGE deposit is located within the Carajás mineral province of the southeastern Amazon craton, Brazil. Gold-PGE ores are epigenetic and display a strong structural control, being hosted in subgreenschist facies carbonaceous and calcareous metasiltstone, within the hinge zone of a reclined, tight, regional-scale F 2 synform. Although the entire orebody has undergone deep tropical weathering, some evidence of the original hydrothermal alteration is preserved. Gold-PGE mineralization is associated with the formation of magnetite- and hematite-rich hydrothermal breccias, massive zones of hematite metasomatism, intense sericite (white mica)-kaolin metasomatism, siderite veining, and a jasperoid envelope of amorphous silica alteration hosting rare disseminated pyrite. All other Au-PGE orerelated mineral assemblages have undergone intense weathering to hydrated Fe oxides and secondary clay minerals, preventing further description of primary ore and alteration features. The geochemistry of the primary Au-PGE ores at Serra Pelada displays many similarities to that of Fe oxide Cu-Au deposits within the Carajás mineral province, and indeed worldwide, in terms of metal association (e.g., Co, Ni, Cu, U), LREE enrichment, and associated Fe metasomatism. The Au-Pd-Pt association also suggests ore metal transport in acid, oxidizing, chloride-rich fluids, similar to those for Fe oxide Cu-Au deposits. In combination with these similarities and the location of the Serra Pelada Au-Pd-Pt deposit, it is suggested that the latter represents a distal equivalent to the Fe oxide Cu-Au deposits and, as such, a target that may have been overlooked during exploration programs around such terrains globally.

Abstract When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.

Abstract Economically significant Archean Au quartz vein mineralization in the Renabie area, north Ontario, is hosted entirely within a tonalite-trondhjemite terrane which is marginal to a greenstone belt and constitutes a part of an exposed, 120-km-wide (∼26 km estimated true thickness) uplifted Archean crustal section. Mineralization shows marked structural control and is confined to variably oriented, brittle-ductile shear zones showing oblique displacements. Host shear zones are superimposed on a regional foliation and appear to have formed under a bulk stress field which was reoriented with respect to the stress field responsible for earlier regional flattening. Strain features shown by the mineralization, together with evidence for reaction-enhanced ductility associated with hydrothermal fluid-rock interaction within shear zones, indicate that mineralization and hydrothermal alteration were synchronous with deformation. Banded quartz vein geometries are interpreted to be the result of repeated hydraulic fracturing in response to fluid overpressuring, with fracture orientation largely controlled by shear fabric anisotropy. On a microscale, gangue and opaque minerals show variation in style and intensity of deformation. Gold and closely associated tellurides, together with galena and chalcopyrite, appear to be paragenetically late with respect to pyrite and fill brittle dilational sites which developed during shear deformation. The δ 34 S data and vein mineralogy reflect the oxidized nature of the fluid system at Renabie. Carbonate δ 13 C and δ 18 0 data suggest a close relationship between fluids associated with gold mineralization and more pervasive fluids active within tonalite remote from mineralization in the Renabie area. The carbonate δ 13 C data (x = — 4.2 ± 0.2; 1< T, n = 14) are consistent with fluid derivation from a magmatic source and/or a juvenile (i.e., mantle) source. The Renabie mine, though a rather unusual tonalite-trondhjemite-granodiorite-hosted Au deposit by virtue of its size and grade, exemplifies the potential of tonalitic-trondhjemite-granodiorite terrane-hosted Au quartz vein mineralization, given the correct combination of geologic conditions. Renabie-type deposits should therefore be considered viable exploration targets.

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.

Abstract The Superior province of Canada hosts hundreds of gold mines which have recorded production ranging from less than 1 metric ton up to 1,000 metric tons of gold. All of them occur within large-scale, transcurrent and oblique slip-shear deformation zones, which were active during the latest Archean. The deformation zones constitute a conjugate set to a north-northwest-directed compression of the Canadian Shield. Within these deformation zones gold camps are localized in extensional structures; many of these are pull-apart structures which were the loci of fluvial-alluvial sedimentation, a suite of felsic intrusions, and alkaline volcanism. The localization of gold at all scales, from individual veins up to camp scale, is attributed to the dilation zones produced by the shear deformation. Although mineralization and attendant wall-rock alteration are the products of auriferous hydrothermal fluids of similar composition ascending along deformation zones, they are manifested in many styles in different deposits and even within a single deposit. Wall-rock alteration assemblages vary according to metamorphic grade of host rocks and reflect alteration mineral stabilities under elevated ambient pressure and temperature of regional metamorphism; metamorphism does not overprint alteration mineral assemblages. Thus mineralization formed during, not only late Archean regional shearing, but also regional metamorphism; wall-rock metamorphic grade is therefore an indicator of depth of formation of mineralization within deformation zones. Wall rocks range in metamoprhic grade from subgreenschist to amphibolite facies, indicating that gold deposition took place over a substantial range of depths, possibly in excess of 10 km. The depositional model is based on observations of more than 30 deposits in various terrains in the Superior province. Distribution and style of mineralization and alteration are a function of fluid access to wall rocks through permeability generated by shearing; with increasing depth brittle, brittle-ductile, and ductile deformation corresponds to breccia-style, veining, and foliation-parallel mineralization, respectively. Alteration mineralogy also reflects depth of formation. The most prominent aspects of the mineralogy are the predominance of pyrrhotite over pyrite as the characteristic alteration sulfide and the absence of ankerite carbonatization in deposits which formed under higher pressure and temperature conditions. Physical and chemical characteristics of varied lithologies also affect response to shearing and alteration. The model therefore provides a degree of predictability to the distribution and styles of mineralization and alteration expected in any specific lithologic and metamorphic environment. The correspondence of the observations with the model suggests that Archean gold mineralization is one single deposit type. At several widely separated locations, the mineralizing event took place around or after 2,680 Ma, at least 20 Ma after the cessation of greenstone volcanism. The mineralization is somewhat younger than the felsic intrusions, which are spatially associated with many gold deposits, and may be, temporally, more closely related to slightly younger, minor magmatism of a more alkaline or a lamprophyric affinity. In the Canadian Shield, the timing of the mineralization roughly corresponds to a late batholith emplacement and granulitization and accompanying magmatism in the lower crust. Thus, the mineralization, which involves heat and mass transfer, is a manifestation in the upper crust of the widespread cratonization processes in the very late Archean. The latest Archean was the most important metallogenic epoch for this style of mineralization.

Abstract Historically, and in recent years, most of Zimbabwe’s gold production has been derived from lode deposits, which range from pervasively silicified and sulfidized schistose zones and shear zone vein arrays to ribbon-textured and massive quartz veins. Stable isotope and fluid inclusion data are consistent with those for hypothermal lode deposits in other cratons, although complex Au-Te mineralization at the Commoner mine was deposited below 200° C. Precipitation of gold and sulfide minerals was constrained largely by fluid-rock interaction and less commonly by fluid-cooling, phase separation within local areas of dilation and explosive hydraulic fracturing. The lodes formed in predominantly compressional environments, during a major, late Archean, ca. 2.7 to 2.6-Ga, tectonic event, but generally they postdate at least one phase of deformation. A close spatial association and general temporal equivalence is evident with trondhjemite-tonalite-granodiorite stocks which intruded the greenstone belts. Auriferous orebodies in iron-formation are mostly structurally controlled, and the size and tenor of the deposits are defined in a complex manner by the ductile or brittle behavior of the metasediments during deformation and concurrent hydrothermal activity. Sulfidation of the ferruginous units was an important depositional mechanism. Gold enrichment to 50 ppb or more is evident in some sulfide iron-formations and geochemical data suggest a bimodal fumarolic and/or ambient marine origin for some of these sulfidic metasediments. At the Athens gold-copper mine the orebodies have been interpreted as volcanogenic sulfide deposits by Fabiani (1987). Gold deposits in late Archean rhyodacitic volcaniclastic rocks exhibit an important epigenetic component, but a gold-enriched, mixed chemical, clastic sedimentary protolith is indicated for some deposits. Most deposits are relatively small, but the Shamva mine has produced more than 52 metric tons of gold from a complex chemogenic sediment-lode deposit which evolved initially as a porphyry (Au-Mo-As)-linked exhalative system. The lode deposits, and in some cases their exhalative equivalents, developed in response to regional deformation and metamorphism which marked the culmination of komatiitic → tholeiitic → calc-alkaline volcanism and preceded the voluminous granitic magmatism which marked final stabilization of the craton. Near-synchronous deformation, metamorphism, and trondhjemite-tonalite-granodiorite magmatism provided optimum conditions for crustal dewater-ing, focused fluid-fluid, and eventually gold precipitation. By far the greatest proportion of gold produced from lode deposits in the late Archean greenstone belts has come from the volcanic-dominated western succession which was also the locus of the intrusive (trondhjemite-tonalite-granodiorite) and extrusive calc-alkaline magmatism. This is tentatively identified as a zone of rifting, perhaps multiback-arc spreading, in which subsequent crustal foundering (subduction?), anatexis, and compressional tectonic activity facilitated crustal dewatering. With this framework a number of first-order exploration targets can be identified, in particular lode deposits in tholeiitic (not calc-alkaline) sequences, high-strain zones at the margins of some greenstone belts, porphyry-intruded rhyodacitic sequences, and regional zones of rifting which became the focus of subsequent deformation and magmatism.

Abstract The Yilgarn block is a major metallogenic province, currently enjoying its highest ever annual gold production from greenstone-hosted Archean mesothermal gold deposits. Gold mineralization occurs in ca. 2.95 to 2.7-Ga greenstone belts throughout the block, with over 2,000 deposits known, but is best developed in the ca. 2.7-Ga greenstones of the Norseman-Wiluna belt. Most mineralization is sited in brittle-ductile structures, at or below the amphibolite-greenschist transition, commonly in rocks with high Fe/(Fe + Mg) ratios. Sulfidation, K-(± Na-) metasomatism and carbonation are important alteration styles associated with such mineralization in shear zones, quartz veins, and/or breccias. Gold occurs most commonly within Fe sulfides and mineralization has a typical element association of Au-Ag-As-W±Sb±Te±B with low Pb-Zn-Cu contents. Gold was deposited from reduced to slightly oxidized, near-neutral, moderate-density, low-salinity H 2 O-CO 2 fluids at 250° to 350°C and 0.5 to 2 kbars in response to sulfidation and/or oxidation-reduction reactions, changes in pH, and pressure decrease over a limited temperature range. On the regional scale, the distribution of gold deposits is controlled by kilometer-scale, oblique-slip, reverse or normal faults-shears linked to crustal-scale, largely strike-slip, shear zones that also appear to control the distribution of mantle-derived carbonation and the emplacement of I- and A-type granitoids, felsic porphyries, and/or calc-alkaline lamprophyres. These associations, combined with radiogenic and stable isotope data, suggest that gold mineralization was related to fluid flow on a crustal or even lithospheric scale, rather than simply being related to greenstone belt devolatilization or local magmatic intrusions; lamprophyres do, however, represent a potential gold donor to the metamorphic-hydrothermal systems. Despite this gross control, the provinciality of isotope data suggests that ore components were strongly influenced by upper crustal fluid pathways, probably controlled by transient fluid flow into brittle-ductile structures under the influence of fluid-pressure gradients. The preferred model is that gold mineralization was the result of high lithospheric heat and fluid flux during compressional, oblique-slip deformation and mantle to crustal magmatism related to closure of the partly sialic-floored, marine basin now represented by the Norseman-Wiluna belt. As such, it shows similarities to Phanerozoic gold provinces in convergent-margin tectonic settings.

Abstract The Abitibi greenstone belt, the largest and best-preserved Archean granite-greenstone complex in the world, consists of five major lithologic assemblages which formed in four distinctive geotectonic environments. These comprise: (1) a tholeiitic and komatiitic assemblage formed in an oceanic extensional environment; (2) a calc-alkalic volcanic and volcanic-derived sedimentary rock assemblage formed in an island-arc environment; (3) an assemblage of craton-derived quartzose sedimentary rocks with interbedded komatiitic volcanic rocks formed on a passive continental margin; and (4) an assemblage of molasse-type sedimentary rocks, alkalic volcanic rocks, and felsic intrusions. The oceanic, arc, and continental margin assemblages were imbricated and tectonically stacked during the collision of a northward-moving continent and its attached marginal sediments with the arc, as a result of subduction of the intervening ocean crust. The molasse assemblage formed along the collisional suture zone. Interbedded alkalic volcanic rocks, syn- to postdeformational felsic intrusions, and the CO 2 -rich hydrothermal fluids responsible for widespread hydrothermal alteration and gold mineralization, all broadly part of this molasse assemblage, were generated by linked lower crust-mantle processes. The effect of the high density of the oceanic rocks on pressure-temperature conditions at the base of the tectonically thickened crust may have been instrumental in triggering the processes which resulted in auriferous fluids. Anatectic melts and gold ore-forming hydrothermal fluids were generated at depth as an end-stage manifestation of the collisional event. Rocks of the oceanic assemblage were the possible source of the gold. This model provides a rational and internally consistent explanation for the association of gold mineralization with greenstone belts, major faults, molasse-type sedimentary rock assemblages, felsic porphyry intrusions, and widespread hydrothermal alteration. Also, the model is consistent with the late timing of the mineralization in the geologic development of the Abitibi belt in particular, and both Precambrian and Phanerozoic greenstone belts in general.

Abstract The maximum age of gold mineralization and the chronostratigraphy of the Timmins and Kirkland Lake-Matheson areas in the external zone and the Detour Lake mine in the internal zone of the Abitibi subprovince, Ontario, are established through U-Pb zircon geochronology. In the Timmins area, two episodes of felsic volcanism are determined at 2,727 ± 1.5 and 2,698 ± 4 Ma which support existing stratigraphic interpretations. In the Kirkland Lake area, three periods of felsic volcanism are dated at 2,747 ± 2 (J. K. Mortensen, pers. commun., 1988), 2,705 ± 2, and 2,701 +3 −2 Ma. In the area south of the Kirkland Lake-Larder Lake break, the dates do not correspond to the current stratigraphic interpretation, rather, they indicate reversals which possibly resulted from stacking. In the Matheson area, north of the Destor-Porcupine break, an episode of felsic volcanism is determined at 2,714 ± 2 Ma. The dates in this area indicate structural repetition of some units. Therefore, caution is exercised in stratigraphic correlation across structural discontinuities. The youngest lithologies hosting gold mineralization consist of late Archean intrabelt intrusions which cut the youngest, folded and/or tilted volcanic rocks. They are silica-saturated quartz porphyries which in the Timmins mines are 2,691 to 2,688 Ma and silica-poor or undersaturated monzonites and lamprophyres(?) which are 2,678 to 2,673 Ma. In the Detour Lake mine, feldspar porphyry which is the youngest lithology hosting gold-bearing structures dates at 2,722 +3 −2 Ma. An apparent age difference between the late plutonic event of the internal and the external zones of the Abitibi subprovince suggests a different timing of cratonization of the two terranes. Gold mineralization shows a distinct spatial and temporal association with major deformation zones such as the Destor-Porcupine and the Kirkland Lake-Larder Lake breaks, and the silica-poor or undersaturated intrusions, emplaced along these breaks. Gold is temporally separated from volcanism by 25 Ma and from emplacement of quartz porphyries by 13 to 15 Ma. Along the Destor-Porcupine break, gold is synchronous with or later than 2,673 +6 −2 Ma.

Abstract The Dome mine, in the Porcupine district of the Abitibi greenstone belt, has produced about 360 metric tons of gold from a variety of ore types in a block of ground 2.5 km long, 1.2 km wide, and 1.4 km deep. The ore zones are hosted in regionally metamorphosed, greenschist facies Archean metavolcanic and metasedimentary rocks. Attention is focused here on two structurally and morphologically different types of gold veins: one concordant with its metavolcanic host rocks, and the other discordant. These veins are exposed together in several parts of the mine, and the latter clearly crosscuts the former. Veins of the first type consist of layers along flow contacts within the metavolcanic sequence; they occasionally extend into facies-equivalent metasedimentary rocks. These corcordant veins can be traced along strike for up to 500 m, downdip for 1,000 m, and are up to 2 m thick. Their mineralogy changes along strike from ankerite + quartz + sulfides to quartz + tourmaline + sulfides ± ankerite. The veins are often well laminated and predate regional metamorphism. Veins of the second type consist of en echelon sets of discordant, lenticular quartz veins. Individual veins are generally less than 5 m in length, have a vertical extent of 2 to 5 m, and are up to 30 cm thick. Their mineralogy consists of approximately 93 percent quartz + 5 percent sulfides + 2 percent ankerite. These discordant lodes were emplaced during, or immediately after, regional metamorphism. Geochemical studies comparing the two types of veins reveal many differences. These results, combined with evidence from field relationships, indicate a multistage and multiprocess origin for these differing veins. The concordant veins formed as auriferous chemical sediments deposited on the sea floor which underwent lithification and burial prior to regional metamorphism. During metamorphism, auriferous metamorphogenic fluids were channeled upward along major structural breaks, depositing gold in a single pulse and filling extension fractures to form unlaminated, quartz-dominated discordant veins.

Abstract Archean mesothermal gold mineralization is commonly located in brittle-ductile structures active during the late deformation events of greenstone belt evolution, and in several cases, reactivation of earlier, mechanically weak structures was a controlling factor. Intensely mineralized greenstone belts in both Western Australia and southern Africa are characterized by an early compressional to oblique-compressional event. Within these belts, zones of low-strain greenstones are bounded by narrow high-strain zones of up to hundreds of kilometers in strike length, which are in turn linked to the brittle-ductile structures hosting gold mineralization. This structural pattern ensured that there was strongly focused fluid flux and probably accounts for the generally high productivity of Archean greenstone belts. In Western Australia, the most mineralized greenstone belt (the Norseman-Wiluna belt in the Yilgarn block) is one in which deformation followed shortly after volcanism and sedimentation and probably resulted from the closure of an extensional basin. Other Yilgarn greenstone belts do, however, host large gold deposits, probably in structures synchronous with and related to those in the Norseman-Wiluna belt. In southern Africa, heterogeneously deformed greenstone belts with high-strain zones of reverse fault-thrust regimes related to externally imposed tectonics are highly prospective. Other important sites are reverse shear zones close to or at granitoid dome margins, which may be due to jostling of these rigid granitoids and differential movement of greenstones during externally imposed deformation.

Abstract Two massive to banded strata-bound magnetite-rich ironstones (Fe 2 O 3total + SiO 2 = 96%, Fe 2 O 3total > 60%) host Au-Cu mineralization in an intracratonic rift setting within the Mount Isa eastern succession. The deposits are of international interest because of the present divergent views on exhalative versus epigenetic genesis; these ores have features which support both origins. Starra is the main orebody cluster, consisting of four geographically separate lodes, totaling 5.3 million metric tons at 5.0 g/metric tons Au, and 1.98 percent Cu. The origin of the Starra ores is complicated by an intense deformational history. The lodes lie on the margin of a major D 1 decollement, which was subsequently reactivated during D 2 and D 4 . Starra ores are deformed by all recognizable stages of deformation, occurring both in folded and unfolded segments. The footwall is extensively altered to albite-magnetite-pyrite-bearing assemblages, whereas the hanging wall shows only sporadic albite alteration overprinted by D 4 calcite gash veining not spatially related to ore. Ores are massive to banded, characterized by Fe, Si, Au, Cu, W, and Sn enrichment and by Pb, Zn, Ag, and Ba depletion. Au shows good correlations with Si, W, and Cu but is inversely correlated to Fe in the only lode studied in detail. Trough Tank, 40 km southeast of Starra, a similar but less highly strained deposit, is characterized by Co, Mo, and P enrichment in addition to the above elements. A syngenetic exhalative origin with transport of Au as Au chloride complexes at 280° to 380°C into a low S oxidizing environment is invoked. This best explains local and regional features such as high background Au levels in banded iron-formation, a lack of replacement textures in massive ore, zoning of geochemistry and mineralogy, high Cu/Au ratios, and location at the boundary between a basic-acid sequence and calcareous metasediments.

Abstract Middle to upper Proterozoic marine sedimentary successions of the Paterson province host gold-copper mineralization in quartz sulfide reefs at Telfer, the largest single producing gold mine in Australia during 1987. The origin of Telfer is controversial: most previous models have emphasized the very continuous, stratiform-strata-bound nature of the auriferous Middle Vale reef and postulated a syngenetic exhalative origin. However, recent deeper mining and drilling of hypogene ore provides evidence that Telfer is an epigenetic deposit. Reconnaissance data suggest that northwest-trending elongate domes in the Paterson province (including the Main and West domes which host the Telfer gold deposits) formed during a noncoaxial deformation event. Veins associated with mineralization form probable conjugate sets, possibly controlled by later coaxial deformation. Widespread bedding-plane slip and dilation was synchronous with vein formation, resulting in the preferential development of laterally extensive concordant veins and mineralized reefs within less competent siltstone units, either during or after the late stages of doming. The Middle Vale reef hosts most of the gold mineralization, and the highest grade ore is spatially coincident with zones of highest vein density in both its footwall and hanging wall. Economic gold mineralization higher in the stratigraphy is in strata-bound horizons and vein stockworks, hosted by pervasively altered sandstones and siltstones. Field observations suggest that at least part of the Middle Vale reef consists of subconcordant quartz sulfide veins. Textural studies suggest that most of the sulfides within the reef are epigenetic and replace carbonate horizons and calcareous mudstones and siltstones, which locally contain carbonaceous material. Rounded aggregates of fine-grained, well-crystallized pyrite within a thin laminated interval at the top of the reef have anomalous geochemistry and provide the only evidence for probable syngenetic pyrite within the mine sequence. A number of granitoids intruded the middle to upper Proterozoic succession late in its tectono-magmatic history. A Pb-Pb mineral isochron for one of these, the Mount Crofton Granite, gives an age of 690 ± 48 Ma. The Pb isotope compositions of ore-associated sulfides in discordant veins and the Middle Vale reef at Telfer, and skarns at two other prospects, are heterogeneous and dissimilar to that shown by most volcanogenic sulfide ores. The Pb isotope sulfide data are best explained by derivation of most of the Pb from the host rock (or basinal brines), with some contribution from a magmatic source such as the Mount Crofton Granite. Fluid inclusions from quartz in veins and the Middle Vale reef contain very complex saline and CO 2 -rich fluids that homogenize between 225° and 440°C: salinities range between 21 and 54 equiv wt percent NaCl. Daughter minerals include halite, Fe-bearing calcite, sylvite (?), and dawsonite. Fluid inclusion data are indicative of mixing of hot, very high salinity (magmatic) fluids with cooler, lower salinity (basinal or meteoric) waters with liberation of CC 2 during replacement of carbonate host rocks. Telfer is reinterpreted to be an epigenetic vein-hosted replacement deposit in which gold mineralization was controlled by both structure and composition of the host rocks. Hot, saline fluids introduced Cu and Au during late deformation and granitoid emplacement, but the source of the Au is not yet established.

Abstract Gold, bismuth, and copper mineralization at Tennant Creek is hosted by magnetite-hematite replacement bodies in lower Proterozoic sediments of the Warramunga Group. The sediments have been folded about east-west axes, are characterized by a pervasive axial-plane slaty cleavage, and are intruded by pre- and postfolding granites. Marked structural and stratigraphic control yields lines of lode (ironstones) that can be traced for distances of up to 40 km. Ironstone lodes are restricted to the magnetite-rich Black Eye Member of the Carraman Formation and concentrate adjacent to argillaceous banded iron-formations. They are aligned parallel to the regional axial-plane cleavage, commonly lying in the cores of third-order folds, especially in areas of fold hinge plunge reversal. Faulting and shearing parallel to the cleavage may also play a role in the localization of some lodes. Gold, bismuth, and copper mineralization and associated alteration form a late-stage overprint on the magnetite lodes, with gold typically concentrated toward the footwall of the ironstone or at its margins in distinct pods associated with chlorite and muscovite. Copper and bismuth mineralization occurs in overlapping zones around these pods, and this zonation is complemented by gangue mineralogy, and trace element and sulfur isotope zonation patterns. A model for the formation of the ironstone lode involves the movement of hot connate brines into developing fold axes during regional deformation of the Warramunga Group. The fluids in equilibrium with the magnetite in the sedimentary pile reacted with more oxidized horizons (e.g., hematite shales), resulting in the deposition of hematite (or a hydrated precursor) that was subsequently converted to magnetite as equilibrium was restored. Economic mineralization is associated with faulting and fracturing of the ironstone lodes and introduction of hot, saline, relatively reduced and sulfur-bearing solutions. Reaction of these solutions with chlorite in the lodes resulted in its replacement by muscovite with a consequent increase in pH and reduction in f O2 of the fluid. This reaction is likely to have controlled gold, bismuth, and copper deposition. The relative availability of sulfur, metals, and fluid between ironstone lodes is thought to be responsible for the spectrum from unmineralized to copper- and gold-rich ironstone lodes.

Abstract The White Devil gold mine is 33 km northwest of Tennant Creek, in the Northern Territory, Australia. Production began in August 1987, and as of 30 June 1988 the measured and indicated resource totaled 343,000 metric tons at 20.6 g/metric ton Au, with a further 73,000 metric tons having been mined to that date. The full extent of the deposit is yet to be defined. Well-bedded siliciclastic sedimentary rocks of the lower Proterozoic Warramunga Group, which host the mineralization, have undergone two main deformations. The early ductile deformation (D 1 ) was a moderate deformation, which produced upright east-west-trending open-close folds, with a regular plunge of 40° to 50° toward 245°. The late semiductile to brittle deformation (D 2 ) was a progressive deformation, which was associated with at least three closely spaced events, that is, the intrusion of a set of quartz-feldspar porphyry dikes, an early east-west shearing associated with the emplacement of hydrothermal magnetite-rich bodies (ironstones), and a progressive shearing associated with the mineralization. The shearing produced a slickenside-growth fiber lineation (L 2 ), that suggests a vertical-oblique displacement, with the south block eastwardly uplifted and the north block westwardly downthrown. The magnetite veins commonly have a brecciated texture, and en echelon arrays of sygmoidal tension microfractures are extensively developed. The geometric forms of these microfractures and the internally crystallizing vein fibers indicate formation during a progressive and incremental deformation. Gold-bismuth-copper mineralization was emplaced during the late stage of this progressive deformation, and the ore minerals are mainly concentrated in the tension fractures, replacing quartz and chlorite fibers. Continued progressive shearing was the latest event in the second deformation, when it displaced the porphyries and caused the brecciation of the ironstones. Gold-bismuth-copper mineralization is mainly confined to the Main Zone and Deeps Zone orebodies, which are associated with magnetite-rich bodies in the hinge region of an anticlinal F 1 fold. The mineralogical composition of the ore is relatively simple and consists of gold, chalcopyrite, bismuthinite, bismuth, pyrite, marcasite, and molybdenite associated with magnetite, chlorite, quartz, and hematite with minor carbonate and talc. Ore grades are quite variable over 1-m lengths of drill core, with Bi up to 15 percent, Cu up to 8 percent, and Au up to 1,000 g/metric ton. The primary zone in the Main Zone orebody averages 0.5 to 0.8 percent Cu and 17.6 g/metric ton Au and the Deeps Zone orebody 0.1 percent Cu and 25.2 g/metric ton Au. Gold is generally fine grained and not visible in hand specimens, except in very rich sections of the orebodies. Textural relationships of the minerals and studies on fluid inclusions in quartz demonstrate that there were two distinct phases of hydrothermal fluid involved in the formation of the deposit. Magnetite was formed from a fluid of relatively high temperature (approximately 350°C) and high salinity (probably CaCl 2 -NaCl), whereas later gold-bismuth-copper mineralization was formed from a fluid of lower temperature (approx. 300°C) and lower salinity. The origin of the solutions is uncertain, but a preliminary sulfur isotope study suggests a magmatic source for the sulfur. The close association of the ore and magnetite suggests that the magnetite was an important factor in controlling ore deposition.

Abstract Bulk mineable gold deposits in the western United States contain reserves of more than 55 million oz (1,710 metric tons) Au and produce more than 3 million oz (93,300 kg) Au/yr. They can be subdivided into five main deposit types: porphyry-related, sediment-hosted (includes deposits of the Carlin type), metamorphic-hosted, volcanic-hosted , and hot spring gold deposits. Each deposit type can be further divided into several deposit subtypes. Examples of porphyry-related gold deposits are Zortman-Landusky in Montana and the Fortitude and McCoy skarn-hosted gold deposits in Nevada. Major sediment-hosted gold deposits in the western United States occur mainly in Nevada and include the world-class gold deposits located within the Carlin and Getchell trends. Metamorphic deposits include Mother Lode and Mesquite types, located principally in California and Arizona. Volcanic-hosted deposits can be subdivided into three main deposit subtypes: high sulfur deposits such as Summitville, Colorado, and Paradise Peak, Nevada; low sulfur deposits such as Round Mountain, Nevada; and alkalic deposits such as Cripple Creek, Colorado. Hot spring gold deposits are typified by the McLaughlin gold deposit in northern California.