Source of Pb in orogenic lode-gold mineralisation: Pb isotope constraints from deep crustal rocks from the southwestern Archaean Yilgarn Craton, Australia

Southern Cross was one of the earliest gold mining centres in Western Australia. Over 142 tonnes of gold have been produced from the district, and, on a gold per hectare basis, the Southern Cross greenstone belt in the southwestern Yilgarn Craton is the most productive of Western Australia's Archaean greenstone belts. The SW Yilgarn Craton is characterised by high-grade (amphibolite- to granulite-facies) metamorphism, extensive granitoid magmatism and older greenstone volcanism ages, compared to the well-known greenschist-facies metamorphism and younger (2.7 Ga) eruption ages which dominate in the Eastern Goldfields Province. The Pb-isotope compositions of deep-seated granitoids in the SW Archaean Yilgarn Craton, which were emplaced coeval with a craton-wide major orogenic lode-gold mineralization event at about 2.64–2.63 Ga, have been determined for 96 whole-rock and 24 K-feldspar samples. The Pb isotope data of the granitoids are consistent with a crustal origin for their genesis, probably by reworking (partial melting) of older continental crust. The Pb isotope composition of greenstones, which are the main host rocks for gold mineralisation, and pyrites from the komatiite-hosted syngenetic Ni deposits in the amphibolite-facies Forrestania greenstone belt, have also been determined, with initial Pb-isotope ratios higher than that for the Eastern Goldfields Province. The Pb isotopic character of the orogenic lode-gold deposits in the region is intermediate between coeval granitoid and greenstone Pb, indicating that the ore fluids contained metals from both reservoirs. The Pb in the ore fluid of the most deeply formed deposit, Griffin's Find, overlaps the isotopic composition of coeval granitoids, indicating the deep-seated granitoid magmatism was the primary source for Pb in the ore fluids. Received: 8 October 1998 / Accepted 22 December 1998     

Woodcutters goldfield: Gold in an Archaean granite,Kalgoorlie, Western Australia

The 43 t (1.4 Moz) of gold in the Woodcutters goldfield 50 km north of Kalgoorlie has wide geological significance in terms of gold in Archaean granite, as well as its local commercial and exploration significance. Woodcutters is already one of the largest Archaean gold systems in granite, and is unusual in being so far laterally from the nearest greenstone belt. Gold in the Federal zone, one of the deposits making up the Woodcutters goldfield, is hosted in hornblende‐biotite granodiorite,6 km from the mapped contact with greenstone. In Federal open pit, the granodiorite is coarse‐grained in the northern half, and a fine‐grained granodiorite in the south, with both hosting gold. These two types of granodiorite are rather similar in both mineralogy and geochemistry. There is also a subordinate fine‐grained monzodiorite. The Federal gold mineralisation is in a northwest‐striking, northeast‐dipping (315° strike/60°E dip) shear zone in the Scotia granite. Variation in grainsize of the host rocks might have affected the style of deformation with more brittle fabrics in the coarse‐grained phase and more ductile fabrics prominent in the fine‐grained granodiorite. Hydrothermal alteration is extensively developed around the Federal deposit and is a useful vector towards gold mineralisation. Distal epidote alteration surrounds a proximal muscovite‐biotite alteration zone that contains quartz‐sulfide veins. The alteration shares some of the common alteration characteristics of Archaean greenstone‐hosted gold, but differs in that carbonate‐chlorite alteration is only weakly developed. This difference is readily explained in terms of host‐rock composition and lower concentrations of Fe, Mg and Ca in the granite compared with greenstone. Fluid‐inclusion studies demonstrate that the fluids associated with the hydrothermal alteration at Woodcutters shared the common characteristics of fluids in Archaean greenstone gold, namely low‐salinity and dominant H₂O–CO₂. Fluid inclusions with moderate salinity were found in one fresh sample away from mineralisation, and are inferred to represent possible magmatic fluid. There is no evidence of a granite‐derived fluid being responsible for gold mineralisation. The granodiorite host rock had cooled, crystallised and had at least started to undergo deformation prior to gold introduction. The distribution of gold mineralisation in the Woodcutters goldfield has the style, shape and orientation comparable with greenstone‐hosted gold deposits in the same region. The northwest trend, the quartz veining and simple pyrite mineralogy are all features common to other greenstone‐hosted gold deposits near Kalgoorlie such as Mt Pleasant. The alteration fluid appears to have penetrated the granite on the scale of many hundreds of metres, causing large‐scale alteration. Woodcutters gold mineralisation resulted from the same metamorphic fluid processes that led to formation of greenstone gold deposits. In this metamorphic model, granitic rocks are predicted to be less‐favourable gold hosts than mafic rocks for two reasons. Granitic rocks do not generally fracture during regional deformation in such a way as to create large‐scale dilation. Furthermore, with less iron and no carbon, granitic rocks have lower potential to precipitate gold from solution by wall‐rock reaction. The metamorphic model predicts that those granite types with higher Fe should host better gold deposits, all other factors being equal. Accordingly, tonalite‐trondhjemite and hornblende‐bearing granodiorite should provide better environments for major gold deposits compared with monzogranite, and granite sensu stricto, as borne out by Woodcutters, but mafic rocks should be better hosts than any of these felsic to intermediate rocks.     

Single zircon ages for felsic to intermediate rocks from the Pietersburg and Giyani greenstone belts and bordering granitoid orthogneisses, northern Kaapvaal Craton, South Africa

Previous models for the temporal evolution of greenstone belts and surrounding granitoid gneisses in the northern Kaapvaal Craton can be revised on the basis of new single zircon ages, obtained by conventional U---Pb dating and Pb---Pb evaporation. In the Pietersburg greenstone belt, zircons from a metaquartz porphyry of the Ysterberg Formation yielded an age of 2949.7±0.2 Ma, while a granite intruding the greenstones, and deformed together with them, has an age of 2853 + 19/−18 Ma. These data show felsic volcanism in this belt to have been coeval with felsic volcanism in the Murchison belt farther east, and the date of 2853 Ma provides an older age limit for deformation in the region. In contrast, a meta-andesite of the Giyani greenstone belt has a zircon age of 3203.3±0.2 Ma, while a younger and cross-cutting feldspar porphyry has an emplacement age of 2874.1±0.2 Ma. The meta-andesite is intercalated with various mafic and ultramafic rocks and, therefore, the age of 3.2 Ga appears plausible for the bulk of the Giyani greenstones.Granitoid gneisses surrounding the Pietersburg and Giyani belts vary in composition from tonalite to granite and texturally from well-layered to homogeneous but strongly foliated. These rocks yielded zircon ages between 2811 and 3283 Ma. The pre-3.2 Ga gneisses are polydeformed and may have constituted a basement to the Giyani greenstone sequence, while the younger gneisses are intrusive into the older gneiss assemblage and/or into the greenstones. The Giyani and Pietersburg belts probably define two separate crustal entities that were originally close together but were later displaced by strike-slip movement.     

Constraints on Archaean crustal evolution of the Zimbabwe craton: a U-Pb zircon,Sm-Nd and Pb-Pb whole-rock isotope study

 The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd whole-rock isotope data, constrain the timespan of development of the Harare-Shamva granite-greenstone terrain and establish the relative involvement of juvenile mantle-derived and reworked crustal material. Basement-cover field relationships and isotope and geochemical data demonstrate that the greenstones were deposited onto 3.2–2.8 Ga basement gneisses, in ensialic, continental basins. Geodynamic models for the generation of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretations are rift-related: (1) intracontinental rifting related to mantle plume activity or; (2) rifting in a back-arc environment related to a marginal volcanic arc. The data, in conjunction with field evidence, do not indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the greenstone belt, as was recently postulated on the basis of SHRIMP zircon ages. Zircon ages for basal felsic volcanics (2715±15 Ma) and a subvolcanic porphyry (2672±12 Ma) constrain the initiation and termination of deposition of the greenstone sequence. The timespan of deposition of the Upper Bulawayan part of the greenstone sequence corresponds well with radiometric ages for Upper Bulawayan greenstones in the central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667±4 Ma) and a late syn-tectonic intrusive granodiorite (2664±15 Ma) pinpoint the age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659±13 Ma) associated with the latter intrusive phase. The intimate timing relation of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the surrounding greenstone belts, and the structural and strain patterns in the greenstone sequence around and adjacent to the batholiths, imply that the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic activity is indicated by the zircon ages of small, post-tectonic plutons intrusive into the greenstone belt, with a mineralized granodiorite dated at 2649±6 Ma and an unmineralized tonalite at 2618± 6 Ma. The 2601±14 Ma crystallization age of an “external” Chilimanzi-type granite agrees well with existing radiometric ages for similar granites within the southern part of the craton, demonstrating a craton-wide event and heralding cratonization. The similarity between U-Pb zircon ages and T_DM model ages (2.65–2.62 Ga) and the positive ɛNd_T values (+3 to +2) for the late syn-tectonic and post-tectonic intrusive plutons within the greenstone belt indicate magmatism was derived directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model μ₁ values (8.2–8.6) are unlikely to indicate the involvement of significant amounts of older crust and may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older T_DM ages for the felsic volcanics (3.0–2.8 Ga) and the porphyries (2.8–2.7 Ga) suggest that these felsic magmas were derived by partial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle-derived melts and older crustal material. Received: 15 August 1995 / Accepted: 12 January 1996     

Constraints on the age of the Warrawoona Group, eastern Pilbara Block, Western Australia

Zircons from mafic and felsic volcanic rocks in the type area of the Warrawoona Group, the basal Archaean greenstone succession of the eastern Pilbara Block, have been dated precisely using the ion-microprobe SHRIMP. The results allow two alternative time-frames for the duration of the Warrawoona Group, dependent on how the dated zircons are considered to relate to the volcanic rocks. Our favoured interpretation requires a hiatus of 135±5 Ma between the Duffer Formation at 3.46 Ga and the overlying felsic volcanic rocks of the Wyman Formation, and a hydrothermal or later magmatic origin for zircons of age 3.33 Ga within one Duffer Formation sample and the underlying metabasalts. The alternative time-frame requires a short time for deposition of the entire Group, less than 15 Ma at 3.33 Ga, and a xenocrystic origin for the 3.46 Ga zircons of the Duffer Formation. Outside the type area of the Warrawoona Group, the age of an intrusive granodiorite requires that greenstones be older than 3.43 Ga and the Group formed over an interval of > 120 Ma.Visibly different zircons within one of the Duffer Formation samples were found to be Palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock at low temperature. Similar zircons were found within samples from other rock units but with a spread of Proterozoic ages.     

Nature and source of the ore-forming fluids associated with orogenic gold deposits in the Dharwar Craton

Neoarchean orogenic gold deposits, associated with the greenstone-granite milieus in the Dharwar Craton include(1) the famous Kolar mine and the world class Hutti deposit;(2) small mines at HiraBuddini, Uti, Ajjanahalli, and Guddadarangavanahalli;(3) prospects at Jonnagiri; and(4) old mining camps in the Gadag and Ramagiri-Penakacherla belts. The existing diametric views on the source of ore fluid for formation of these deposits include fluids exsolved from granitic melts and extracted by metamorphic devolatilization of the greenstone sequences. Lode gold mineralization occurs in structurally controlled higher order splays in variety of host rocks such as mafic/felsic greenstones, banded iron formations, volcaniclastic rocks and granitoids. Estimated metamorphic conditions of the greenstones vary from lower greenschist facies to mid-amphibolite facies and mineralizations in all the camps are associated with distinct hydrothermal alterations. Fluid inclusion microthermometric and Raman spectroscopic studies document low salinity aqueous-gaseous(H_2O + CO_2 ± CH_4 + NaCl) ore fluids,which precipitated gold and altered the host rocks in a narrow P-T window of 0.7-2.5 kbar and 215-320℃. While the calculated fluid O-and C-isotopic values are ambiguous, S-isotopic compositions of pyrite-precipitating fluid show distinct craton-scale uniformity in terms of its reduced nature and a suggested crustal sulfur source.Available ages on greenstone metamorphism, granitoid plutonism and mineralization in the Hutti Belt are tantamount, making a geochronology-based resolution of the existing debate on the metamorphic vs.magmatic fluid source impossible. In contrast, tourmaline geochemistry suggests involvement of single fluid in formation of gold mineralization, primarily derived by metamorphic devolatilization of mafic greenstones and interlayered sedimentary rocks, with minor magmatic contributions. Similarly, compositions of scheelite, pyrite and arsenopyrite point toward operation of fault-valves that caused pressure fluctuation-induced fluid phase separation, which acted as the dominant process of gold precipitation,apart from fluid-rock sulfidation reactions. Therefore, results from geochemistry of hydrothermal minerals and those from fluid inclusion microthermometry corroborate in constraining source of ore fluid,nature of gold transport(by Au-bisulfide complex) and mechanism of gold ore formation in the Dharwar Craton.     

An overview of the relationship between granitoid intrusions and gold mineralisation in the Archaean Murchison Province,Western Australia

In the Archaean Murchison Province of Western Australia, granitoid batholiths and plutons that intruded into the ca. 2.7–2.8 Ga and ca. 3.0 Ga greenstone belts can be divided into three major suites. Suite I is a ca. 2.69 Ga monzogranite-granodiorite suite, which was derived from anatexis of old continental crust and occurs as syn-tectonic composite batholiths over the entire province. Suite II is a trondhjemite-tonalite suite (termed I-type) derived from partial melting of subducted basaltic crust, which intruded as syn- to late-tectonic plutons into the greenstone belts in the northeastern part of the province where most of the major gold deposits are situated. One of the Suite II trondhjemite plutons has a Pb−Pb isochron age of 2641±36 Ma, and one of the structurally youngest tonalite plutons has a minimum Pb−Pb isochron age of 2630.1±4.3 Ma. Suite III is a ca. 2.65–2.62 Ga A-type monzogranite-syenogranite suite which is most abundant in the largely unmineralised southwestern part of the province. Gold deposits in the province are mostly hosted in brittle-ductile shear zones, and were formed at a late stage in the history of metamorphism, deformation and granitoid emplacement. At one locality, mineralisation has been dated at 2636.8±4.2 Ma through a pyritetitanite Pb−Pb isochron. Lead and Sr isotope studies of granitoids and gold deposits indicate that, although most gold deposits have initial Pb isotope compositions most closely similar to those of Suite II intrusions, both Suite I and Suite II intrusions or their source regions could have contributed solutes to the ore fluids. These preliminary data suggest that gold mineralisation in the Murchison Province was temporally and spatially associated with Suite II I-type granitoids in the northeastern part of the province. This association is consistent with the concept that Archaean gold mineralisation was related to convergent-style tectonic settings, as generation of both Suite II I-type granitoids and hydrothermal ore fluids could have been linked to the dehydration and partial fusion of subducted oceanic crust, and old sialic crust or its anatectic products may also contribute solutes to the ore fluids. Integration of data from this study with other geological and radiogenic isotope constraints in the Yilgarn Block argue against direct derivation of gold ore fluids from specific I-type granitoid plutons, but favour a broad association with convergent tectonics and granitoid magmatism in the late Archaean.     

The source of lead in Archaean lode gold deposits of the Menzies-Kalgoorlie-Kambalda region,Yilgarn Block,Western Australia

The Archaean greenstone terrane between Menzies and Kambalda exhibits a coherent, although deformed, stratigraphic sequence intruded by granitoids and bounded by major NNW-trending shear and/or fault zones. The greenstone terrane hosts a large number of lode gold prospects and deposits, including the giant Kalgoorlie deposits. The initial Pb isotope compositions of lode gold deposits, as determined from ore related galena and pyrite, vary systematically in a linear trend on a²⁰⁷Pb/²⁰⁴Pb versus²⁰⁶Pb/²⁰⁴Pb diagram which reflects crustal heterogeneity at the time of mineralisation. Deposits hosted within a 90 km section of the Menzies-Boorara Shear Zone have a uniform, radiogenic initial Pb isotope composition irrespective of temperature of mineralisation and proximity to granitoid-gneiss in plan view. The Pb in these deposits is considered to be derived largely from older felsic crust underlying the greenstone belt and was accessed via this major shear-zone system. Deposits in a transect unrelated to a major shear zone show a systematic correlation between initial Pb isotope compositions and proximity to granitoid-gneiss and/or to mineralisation temperature. These compositions are less radiogenic than those within the Menzies-Boorara Shear Zone, but trend on a²⁰⁷Pb/²⁰⁴Pb versus⁶⁰⁶Pb/²⁰⁴Pb diagram between this isotope signature and the uniform Pb isotope signature which characterises the >100 km greenstone transect from the Mt Pleasant area through Kalgoorlie to Kambalda. These data are interpreted to reflect Pb derivation from discrete crustal segments within and below the greenstones, and require that mineralisation was related to crustal-scale hydrothermal systems that accessed both sialic mid- to lower-crust and the greenstone succession.     

Geochronology of an archaean tonalitic gneiss dome in Northern Finland and its relation with an unusual overlying volcanic conglomerate and komatiitic greenstone

Archaean gneiss-greenstone relationships are still unresolved in many ancient cratonic terrains although there is growing evidence that most of the late Archaean greenstone assemblages were deposited on older tonalitic crust.We report here well defined basement-cover relationships from a late Archaean greenstone belt in Lapland, north of the Polar Circle. The basal greenstone sequence contains quartzite, schist, komatiitic volcanics and an unusual volcanic conglomerate with well preserved granite pebbles of an older basement. These rocks surround a gneiss dome composed of foliated tonalite which shows a polyphase deformation pattern not seen in the neighbouring greenstones.Zircon fractions of the gneisses plot on two discordia lines and give upper intercept ages with concordia at 3,069±16 Ma and 3,110±17 Ma respectively. One fraction contains metamict zircons with components at least 3,135 Ma old. These are the oldest reliable ages yet reported from the Archaean of the Baltic Shield. Rb-Sr whole-rock dating of the tonalitic gneiss yielded an isochron age of 2,729±122 Ma and an I_Sr of 0.703±0.001. This is interpreted to reflect a resetting event during which the gneisses may have acquired their present tectonic fabric.Rb-Sr model age calculations yield mantle values for I_Sr at about 2,950±115 Ma and suggest that the tonalite was intruded into the crust as juvenile material at about 3.1 Ga ago as reflected by the zircon ages. It was subsequently deformed and isotopically reset at about 2.7 Ga ago, prior to greenstone deposition.Comparison with tonalitic gneisses of eastern Karelia displays significant differences and suggests that the Archaean of Finland may contain several generations of pre-greenstone granitoid rocks.     

Auriferous sediments of the Yilgarn Block, Western Australia

Gold-bearing sediments of mixed sedimentary-igneous composition, and associated largely with various basic volcanic rocks (greenstones), abound in the Archaean of the Yilgarn Block. In the Coolgardie district, where perhaps they are best developed, many important gold mines and several series of minor gold deposits occur in a few such horizons. Practically identical occurrences, even when highly metamorphosed, are also found elsewhere in greenstone belts-e.g. in the Southern Cross-Westonia-Yellowdine metamorphic zone.     

Yandal greenstone belt, Western Australia: 12 million ounces of gold in the 1990s

After a century of virtual neglect, exploration in the Yandal greenstone belt of the Yilgarn Craton of Western Australia has yielded resources of 12 Moz Au during the 1990s. Success has come from a combination of conceptual geological models, surface prospecting, understanding the weathering environment, and systematic drilling. The Archaean Yandal greenstone belt comprises a lowermost banded iron formation, extensive basalt and dolerite sills, ultramafic rocks, intermediate to felsic volcanic rocks, and variable clastic sedimentary rocks. Early shear zones trend NNW and form the greenstone belt margins, or trend N–S within the belt. Later brittle cross-faults are critical in gold localization. Gold resources and past production at major deposits include Bronzewing (4 Moz Au), Jundee (5 Moz) Mt.␣McClure (1 Moz) and Darlot (3␣Moz, some of which was produced before 1990). All major deposits are hosted by Fe-rich mafic rocks, and mineralization displays a combination of different orientations and morphologies. Quartz veins are surrounded by broad carbonate alteration with proximal K-mica and Fe-sulphides. The recognition of a critical role for the late brittle structures in localizing gold implicates mid-crustal processes within the greenstone belt for fluid generation, and with the host rock control, supports the model in which fluid was derived by metamorphic devolatilization. Received: 19 September 1997 / Accepted: 7 January 1998     

Syn-amphibolite facies archaean lode gold mineralisation in the Mt. York District,Pilbara Block,Western Australia

The Archaean lode gold deposits in the Mt. York District, Pilbara Block, Western Australia are hosted in banded iron formation (Main Hill/Breccia Hill prospect) of the ca. 3.33 Ga Gorge Creek Group and in amphibolites (Zakanaka prospect) of the ca. 3.46 Ga Warrawoona Group. Gold mineralisation at the Main Hill/Breccia Hill prospect is associated with breccias comprising quartz clasts in a quartz-pyrrhotite matrix, and quartz-amphibole veins, with löllingite being the major host for gold. Minimum temperatures for gold mineralisation at the prospect are constrained as 455°C to 550°C by arsenopyrite thermometry. Gold mineralisation at the Zakanaka prospect is spatially associated with quartzclinopyroxene-calcite-microcline-calcic-amphibole veins and biotite altered wallrock adjacent to the veins. Temperatures for vein emplacement are estimated as 480°C to 570°C using both plagioclase-amphibole thermometry and mineral equilibria with respect to T and X_CO2. The timing of gold mineralisation relative to the peak of metamorphism is constrained by mineral textures and the relative temperatures of hydrothermal alteration and metamorphism. Gold mineralisation at both deposits was broadly synchronous with the peak of regional amphibolite facies metamorphism, which reached temperatures of 520°C to 640°C based on amphibole-plagioclase and garnet-biotite thermometry. In this respect, the deposits are similar to other well documented syn-amphibolite facies lode gold deposits from the Archaean Southern Cross greenstone belt in the Yilgarn Block, and represent the deeper section of a crustal continuum of lode gold deposits that includes mesothermal deposits such as those at Kalgoorlie at higher crustal levels.     

Metallogeny and lead isotopic composition of base metal sulfide deposits in the Rappen area,northern Sweden

Early Proterozoic volcanic and sedimentary rocks of the Rappen district in northern Sweden were deposited at a destructive plate margin to the south of the Archaean craton of the western Baltic Shield. The volcano-sedimentary suite was intruded by two generations of early Proterozoic granites at ca. 1.89–1.85 Ga and ca.1.82–1.78 Ga, respectively, and metamorphosed at upper amphibolite facies conditions. Small stratabound iron, copper, and zinc deposits occur in felsic to mafic tuffs and arkosic sediments. Small deposits of molybdenum, tungsten, and uranium formed during the emplacement of the younger granites. The lead isotopic compositions of sulfide trace lead from the various deposits are highly heterogeneous. In the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram they fall on mixing arrays between little evolved early Proterozoic lead and highly radiogenic Caledonian lead. The least radiogenic lead isotopic compositions from the various deposits have a wide range of ²⁰⁷Pb/²⁰⁴Pb ratios and thus indicate variable involvement of Archaean crustal lead in the Proterozoic deposits. Deposits hosted by siliciclastic rocks have higher ²⁰⁷Pb/²⁰⁴Pb ratios than deposits hosted in mafic to felsic tuffites. The lead isotopic heterogeneity suggests that the lead in the various deposits was locally derived and, furthermore, that the sedimentary rocks in part originated from the Archaean craton to the north. Lead mixing arrays in the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram demonstrate that in Paleozoic time radiogenic lead was mobilized and transported in the basement. Source ages calculated from the mixing arrays (ca.1.9 Ga and ca.1.8 Ga) correspond to the age of the Early Proterozoic volcanism and metamorphism respectively. One group of deposits includes lead from at least three sources and illustrates that radiogenic lead was multiply mobilized and transported in the Proterozoic basement. It occurs in deposits that occur in zones that became permeable during the reactivations of the basement.     

The relationship between Archaean gold mineralization and spatially associated minor intrusions at the Kambalda and Norseman gold camps,western Australia: Lead isotope evidence

Many Archaean mesothermal gold deposits are spatially associated with felsic to lamprophyric minor intrusions and it has been suggested that magmatic processes related to such intrusions may be important in the genesis of these deposits. A comparison of the Pb-isotopic signature of gold-related galenas from Kambalda and Norseman with that of spatially associated minor intrusions (at the time of mineralization) indicates that the ore-fluid Pb cannot have been derived solely from the intrusions or their source regions. For both study areas, the galena Pb-isotopic compositions are bracketed by those of local volcanic (mafic) and intrusive (largely felsic) rock types. This is consistent with the ore fluid having derived metallic components from the crust (or crustally derived granitic rocks) and the mantle (or mantle-derived rocks of the greenstone succession) via metamorphic dewatering or mantle/crustal degassing. Interaction of granite-derived magmatic fluids with greenstone lithologies could plausibly produce a similar array of Pb-isotopic signatures. The Norseman data, as a whole, are more radiogenic than the Kambalda data for broadly synchronous mineralization, reflecting the greater abundance of older granitic rocks with respect to mafic/ultramafic rocks in the Norseman district. The provinciality exhibited by the Pb-isotopic composition of the ore fluid indicates that the gold-mineralizing process formed galena whose Pb-isotopic composition was very sensitive to local variations in crustal Pb-isotopic composition, either within the source region of the fluid or along fluid conduits.     

SHRIMP U–Pb geochronology of magmatism and thermal events in the Archaean Marymia Inlier, central Western Australia

Zircons from porphyry and granitoid samples collected in and around the Marymia Gold Mine in the Marymia Inlier, Western Australia, record a complex history. The results of U-Pb studies confirm that the Plutonic Well greenstone belt, and the surrounding granitoid envelope (including a 2,721Lj Ma intrusion), represent an Archaean terrain, which was intruded by high-level, felsic to intermediate porphyries at 2,694lj Ma and potentially also at 2,660dž Ma. Zircon xenocrysts (Sca. 3.35, 2.93 and 2.74 Ga) indicate that there was older crust within, or below, the greenstone belt at the time of porphyry emplacement. Zircons from the granitoid envelope and intrusions within the greenstone belt record subsequent metamorphism and/or hydrothermal activity coeval with magmatism in the Late Archaean (ca. 2.66-2.63 Ga), and peak metamorphism, magmatism and gold mineralisation in the Yilgarn Block. A later period of metamorphism and hydrothermal activity at ca. 1.72 Ga is coeval with orogenesis in the southern Capricorn Orogen. Both the Late Archaean and Palaeoproterozoic thermal events have altered zircons, redistributed trace elements and caused zircon recrystallisation, which is distinctive in its isotope chemistry (in particular Th/U ratios >1) and morphology (e.g. homogeneous in transmitted light and back-scattered electron images, but sector-zoned in cathodoluminescence).     

The timing and isotopic character of regional hydrothermal alteration and associated epigenetic mineralization in the western sector of the Kaapvaal Craton (South Africa)

The Kaapvaal Craton of South Africa comprises an Archaean core of ≈3.5 Ga lithospheric and crustal rocks surrounded by younger accreted terrains of ≈3.0–2.7 and ≈2.1–1.9 Ga. The craton is covered by relatively undeformed 3.0–2.4 Ga supracrustal rocks, which show the effects of thermal and hydrothermal interaction. Part of this activity is manifested by a large number of epigenetic Pb–Zn (±Ag, Au, Cu, F) deposits in the cover rocks of the Kaapvaal Craton. These include small volcanic and breccia hosted deposits in mafic and felsic volcanic rocks of the 2.7 Ga Ventersdorp Supergroup and the Mississippi Valley-type (MVT) deposits in the carbonates of the Transvaal Supergroup.MVT mineralization at the Pering (and other Zn–Pb deposits) is hosted in fracture-generated N–S breccia bodies in the Paleoproterozoic carbonate succession of the western Kaapvaal Craton. The fluids carrying the metals were focused in vertical bodies within the fracture zones (FZ), the metals and the sulphur being carried together and precipitated in organic-rich sectors of the basin. Two small Pb–Zn deposits within mafic rocks of the Ventersdorp Supergroup, stratigraphically below the basin-hosted MVTs on the southwestern part of the Kaapvaal Craton have secondary chlorite which is extremely Rb-rich, associated with the mineralization. This chlorite and the associated altered basaltic host rocks give a Rb–Sr date of ≈1.98 Ga, and the associated galena Pb isotope data plot on the same array as those of other Pb–Zn deposits, the radiogenic intercept giving a date of ≈2.0 Ga. We interpret these data to indicate a craton-wide epigenetic fluid-infiltration event, which exploited the Maquassie Quartz Porphyry (MQP) as the aquifer and metal source.Sr isotopic results for the ore-zone gangue minerals show highly radiogenic ⁸⁷Sr/⁸⁶Sr ratios (>0.710) which support earlier models that the origin of radiogenic Sr isotopic composition in the calcite cements is the felsic tuffs (MQP) of the Ventersdorp Supergroup occurring at deeper levels within the basin. Relationships between δ¹⁸O and δ¹³C performed on carbonate cements within the aquifers are complex: the range in δ¹³C for some of the cements represents a mixture from two sources and with a progression from heavy carbon in the host to somewhat lighter carbon in the cements. Similarly, the lighter δ¹⁸O values have a narrow range indicative of rapid exchanges between hydrous fluid and rock.     

津巴布韦太古宙花岗-绿岩型金矿类型与资源潜力

津巴布韦太古宙花岗-绿岩型金矿床受构造的控制,金矿可分为褶皱控矿型、剪切带控矿型、层控型、深成花岗岩控矿型和构造-蚀变岩型等5类。津巴布韦花岗-绿岩型金矿多产于花岗质片麻岩与绿岩带的接触带上,金以自然金或金的硫化物形式产出。金矿成矿时代大致分为(2660±50)Ma和(2410±70)Ma(米德兰德绿岩带北部白钨矿Sm-Nd测年)2个时段,分别与新太古代TTG事件和大岩墙侵入相关。津巴布韦花岗-绿岩型金矿较多,但发现的中-大型绿岩型金矿较少,具有较好的金矿找矿前景。     

The Central Bundelkhand Archaean greenstone complex,Bundelkhand craton,central India: geology,composition, and geochronology of supracrustal rocks

Analysis of 3.3 Ga tonalite–trondhjemite–granodiorite (TTG) series granitoids and greenstone belt assemblages from the Bundelkhand craton in central India reveal that it is a typical Archaean craton. At least two greenstone complexes can be recognized in the Bundelkhand craton, namely the (i) Central Bundelkhand (Babina, Mauranipur belts) and (ii) Southern Bundelkhand (Girar, Madaura belts). The Central Bundelkhand greenstone complex contains three tectonostratigraphic assemblages: (1) metamorphosed basic or metabasic, high-Mg rocks; (2) banded iron formations (BIFs); and (3) felsic volcanics. The first two assemblages are regarded as representing an earlier sequence, which is in tectonic contact with the felsic volcanics. However, the contact between the BIFs and mafic volcanics is also evidently tectonic. Metabasic high-Mg rocks are represented by amphibolites and tremolite-actinolite schists in the Babina greenstone belt and are comparable in composition to tholeiitic basalts-basaltic andesites and komatiites. They are very similar to the metabasic high-Mg rocks of the Mauranipur greenstone belt. Felsic volcanics occur as fine-grained schists with phenocrysts of quartz, albite, and microcline. Felsic volcanics are classified as calc-alkaline dacites, less commonly rhyolites. The chondrite-normalized rare earth element distribution pattern is poorly fractionated (La_N/Lu_N = 11–16) with a small negative Eu anomaly (Eu/Eu* = 0.68–0.85), being characteristic of volcanics formed in a subduction setting. On Rb – Y + Nb, Nb – Y, Rb – Ta + Yb and Ta – Yb discrimination diagrams, the compositions of the volcanics are also consistent with those of felsic rocks formed in subduction settings. SHRIMP-dating of zircon from the felsic volcanics of the Babina belt of the Central Bundelkhand greenstone complex, performed for the first time, has shown that they were erupted in Neoarchaean time (2542 ± 17 Ma). The early sequence of the Babina belt is correlatable with the rocks of the Mauranipur belt, whose age is tentatively estimated as Mesoarchaean. The Central Bundelkhand greenstone complex consists of two (Meso- and Neoarchaean) sequences, which were formed in subduction settings.     

Gold mineralisation and alteration of Penakacherla schist belt, India, constraints on Archaean subduction and fluid processes

In the Dharwar Craton, southern India, gold deposits are found mostly along the six arcuate shear zones passing through late Archaean greenstone belts (2.7 Ga). One such shear zone complex extends for about 400 km within and along the Ramagiri–Hungund schist belt. The Penakacherla sector of this shear zone is excellently exposed, enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration.Metamorphism and deformation under NE–SW compression associated with Archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists. Continued compression generated a 50–100-m-wide shear zone complex consisting of mafic phyllonites. Advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified, K-metasomatic assemblage mainly consisting of chlorite, amphibole, K-mica, plagioclase, ankerite, quartz, Fe-oxides, pyrite, chalcopyrite and arsenopyrite. Networks of quartz and carbonate veinlets, a few millimeters to a few centimeters thick, formed along the foliation planes giving rise to microscopic alteration envelope, in which individual veinlet systems are merged into one another to form a composite alteration system. Gold is found within these quartz veinlets, mafic phyllonites and at their mutual contacts.Hydrothermal fluids have modified the primary major, minor, trace and LREE compositions of host rocks such that their mutual behaviour became non-systematic. Some HFSE and HREE also show minor mobility but the overall REE pattern generally resembles that of the precursor mafic volcanic rocks. Mass and volume loss/gain by Si and Ca has made significant impact on Al, Ti and Zr abundances, which are generally immobile during hydrothermal alteration. However, element pairs such as Zr–Hf, V–Sc and Nb–Ta maintain primary inter-element ratios, although their absolute abundances are drastically diluted. Similarly, ΣREE in highly silicified and carbonatised samples are reduced, but patterns remain similar to those of relatively least altered mafic phyllonites with (La_N/Yb)_N between 1 and 3. In some samples, LREE enrichment is observed elevating in (La_N/Yb)_N from 3 to 11. Pathfinder elements and base metals such as As, Cd, Cu, Pb, Zn and Sb have been added along with the Au and Ag.δ¹³C of carbon varies from −16‰ to −21‰ suggesting a biogenic origin, whereas coexisting pyrite δ³⁴S ranges from 1‰ to 3‰, pointing towards the involvement of magmatic or average crustal sulphur. Overall concentrations of K, Rb, Sr, Ba, Nb, Ta, Ti, Cs, Cr, Co, V, Y and Sc and many of the ratios such as K/Rb, La/Sc, La/Yb indicate that metamorphism, devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks. Fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity.     

Auriferous Precambrian conglomerates of Witwatersrand

Geological and mineralogical specificity of the world’s largest Witwatersrand gold deposit was defined by geodynamic processes in the Late Archean. The primary crust composed of felsic rocks (“gray gneisses”) was intruded 2.9–3.1 Ga ago by mafic melts that gave rise to greenstone belts. This was followed by the appearance of long-lived granulite mobile zones that promoted the formation of hydrosphere and atmosphere accompanied by an intense acidic leaching (weathering) of rocks in the greenstone belts. Numerous conglomerate interbeds were formed in the Witwatersrand Basin section due to vigorous eolian processes and floodwater washouts that produced a fan channel system mainly filled with quartz pebbles. At present, most researchers support a modified paleoplacer model of the origin of gold mineralization in Witwatersrand, suggesting a hydrothermal-metamorphic redistribution of the primary placer gold. According to various hydrothermal models, gold was introduced into sedimentary rocks in a water medium from outer deep sources or during the filtration of postsedimentary hydrothermal fluids. The present communication suggests that a significant portion of gold contained in reefs could be delivered to the sedimentation basin by the auriferous hydrothermal quartz of pebble or sand dimension that was metamorphosed at approximately 350–400°C. Metamorphism of gold was accompanied by its purification, transfer to matrix, and hydrothermal intrastratal redistribution.