Alteration zoning and primary geochemical dispersion at the Bronzewing lode-gold deposit, Western Australia

The Late Archaean Bronzewing lode-gold deposit is in the Yandal greenstone belt, Western Australia. It is located in a 500-m-wide, N–S trending, structural corridor consisting of an anastomosing set of brittle–ductile shear zones and is chiefly hosted by tholeiitic basalts, which are metamorphosed at mid- to upper-greenschist facies. Syn-peak metamorphic alteration surround all ore bodies, and alteration extends laterally for ≤80 m from individual mineralised structures. Individual alteration haloes partially overlap and form a >1.5-km-long and ≤300-m-wide domain. The alteration sequence, studied here at 140 m below the present undisturbed surface, comprises distal calcite–chlorite–albite–quartz, intermediate calcite–dolomite–chlorite–muscovite–albite–quartz and proximal ankerite–dolomite–muscovite–albite–quartz–pyrite zones. Mass transfer calculations indicate that chemical changes during alteration include enrichment of Ag, Au, Ba, Bi, CO₂, K, Rb, S, Sb, Te and W, and depletion of Na, Sr and Y. The elements Al, Ca, Cr, Cu, Fe, Mg, Mn, Ni, P, Ti, V, Zn and Zr are immobile. The degree of chemical change increases with proximity to gold ore zones. In addition, abundant quartz veins indicate substantial silica mobility during the hydrothermal event, although there is no large relative silica loss or gain in the host rock. The broadest anomaly surrounding the Bronzewing gold deposit is defined by tellurium (>10 ppb) which, if it is a hydrothermal anomaly, extends beyond the 400 × 600 m study area. Anomalous values for CO₂, K, Rb and Sb also define wider zones than does anomalous gold (>4 ppb), although even the lithogeochemical gold anomaly extends across strike for as much as 80 m away from ore and >600 m along the N–S strike of the shear zone corridor. Also carbonation and sericitisation indices outline large exploration targets at the Bronzewing deposit. Sericitisation indices define anomalies that extend for tens of metres beyond visible potassic alteration, whereas the anomalies defined by the carbonation indices do not extend beyond visible carbonation. None of the individual alteration indices or pathfinder elements are able to define consistent gradients towards ore. However, the respective dimensions of individual geochemical anomalies can be used as an extensive, although stepwise, vector towards ore. This sequence is, from species with broadest dispersion first, as follows: Te > CO₂/Ca ≥ Sb, 3K/Al, Rb/Ti ≥ Au, W > Y/Ti (depletion) > Ag ≥ Bronzewing ore. Received: 25 October 1999 / Accepted: 11 May 2000     

The Syama and Tabakoroni goldfields,Mali

Gold deposits in the Syama and Tabakoroni goldfields in southern Mali occur along a north-northeast trending mineralised litho-structural corridor that trends for approximately 40 km. The deposits are interpreted to have formed during a craton-wide metallogenic event during the Eburnean orogeny. In the Syama goldfield, gold mineralisation in 9 deposits is hosted in the hanging-wall of the Syama-Bananso Shear Zone in basalt, greywacke, argillite, lamprophyre, and black shale. Gold is currently mined primarily from the oxidised-weathered zone of the ore bodies. In the Syama deposit, mineralisation hosted in altered basalt is associated with an intense ankerite–quartz–pyrite stockwork vein systems, whereas disseminated style mineralisation is also present in greywackes. In contrast, the Tellem deposit is hosted in quartz–porphyry rocks.In the Tabakoroni goldfield, gold mineralisation is hosted in quartz veins in tertiary splay shears of the Syama-Bananso Shear Zone. The Tabakoroni orebody is associated with quartz, carbonate and graphite (stylolite) veins, with pyrite and lesser amounts of arsenopyrite. There are four main styles of gold mineralisation including silica-sulphide lodes in carbonaceous fault zones, stylolitic quartz reefs in fault zones, quartz–Fe–carbonate–sulphide lodes in mafic volcanics, and quartz–sulphide stockwork veins in silicified sediments and porphyry dykes. The several deposit styles in the goldfield thus present a number of potential exploration targets spatially associated with the regional Syama-Bananso Shear Zone and generally classified as orogenic shear-hosted gold deposits.     

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     

Geological setting and SHRIMP U–Pb geochronological evidence for ca. 2680–2660 Ma lode-gold mineralization at Jundee–Nimary in the Yilgarn Craton, Western Australia

The 7 million oz. Jundee–Nimary lode-gold deposit occurs in the northern portion of the Yandal greenstone belt in the northeastern part of the Archean Yilgarn Craton of Western Australia. Gold mineralization at Jundee–Nimary is similar in structural style, mineralogy, geochemistry and relative timing with respect to deformation and metamorphism, to other Western Australian Archean greenstone-hosted gold deposits, but is unusual in the fact that mineralized structures are crosscut by structurally late intermediate to felsic dykes. Within the Deakin South open cut, gold mineralization is hosted in brittle–ductile shear zones primarily developed within the dacitic Mitchell Porphyry. The Moore Porphyry, a broad dyke of porphyritic granodiorite, intrudes the Mitchell Porphyry, crosscutting and post-dating gold mineralization. Analytically indistinguishable SHRIMP U–Pb zircon ages of 2678 ± 5 Ma for the Mitchell Porphyry and 2669 ± 7 Ma for the Moore Porphyry require that gold mineralization at Jundee–Nimary occurred at ca. 2680–2660 Ma, approximately 40 million years earlier than the majority of published robust ages for gold mineralization in the Yilgarn Craton, which mostly overlap at ca. 2640–2630 Ma. The close spatial and temporal relationship between gold mineralization and felsic to intermediate magmatism at Jundee–Nimary also raises the possibility of a genetic link between hydrothermal and igneous activity. However, additional work is required to establish a firm connection. Current research on lode-gold mineralization in Archean, Paleozoic and Phanerozoic terranes suggests a model which postulates that these deposits formed during transpressional to compressional deformation in accretionary and collisional orogens and that their formation is intimately related to orogenic processes. Consequently, mineralization and regional metamorphism are expected to be diachronous, as terranes are accreted and the front of orogenesis migrates. Consideration of the new data presented in this paper in conjunction with previously published dates supports the hypothesis that gold mineralization, along with regional metamorphism, was generally diachronous from northeast to southwest across the Yilgarn Craton, over a period of approximately 40 million years from ca. 2680–2660 Ma to ca. 2640–2630 Ma. This is directly analogous to the accepted model for the timing of orogenic lode-gold mineralization in other provinces and therefore provides further support for a unified model for this style of deposit through geological time. Received: 17 March 2000 / Accepted: 8 September 2000     

Synchronous deformation and hydrothermal activity in the shear zone hosted high-sulphidation Au-Cu deposit at Peak Hill, NSW, Australia

Altered and mineralised rocks at Peak Hill, are confined to a 300–500 m wide, north-south striking, steeply dipping, shear zone that is flanked by the Mingelo Volcanics along its western side, and Cotton Formation siltstones along its eastern side. This shear zone is defined by extensive zones of cataclasite and strongly foliated micaceous schists in marked contrast to the largely undeformed nature of the adjacent rocks. Advanced argillic assemblages (quartz-kaolinite-pyrite ± alunite ± illite) occur throughout the core of the Peak Hill deposit. Propylitic assemblages, including albite, quartz, interlayered chlorite-smectite, illite and ankerite, and a narrow discontinuous zone of argillic (quartz-illite-pyrite) alteration are developed in the Mingelo Volcanics along the western side of the deposit. Propylitic, argillic and advanced argillic assemblages are overprinted by an internally zoned phase of phyllosilicate alteration that grades inwards from a peripheral sericite-clay-chlorite assemblage, through phyllic assemblages (muscovite/illite-pyrite ± paragonite) to a pyrophyllite-pyrite ± diaspore ± andalusite altered core. Au-Cu mineralisation is hosted by barite-pyrite veins that cut the advanced argillic assemblage, but pre-date the phyllosilicate-dominated alteration. Native Au (lacking Ag), calaverite, Te-rich tennantite-tetrahedrite (goldfieldite), chalcopyrite, covellite and chalcocite occur in the barite-pyrite veins. No ore-bearing minerals were detected in any of the alteration assemblages. The total gold content of the Peak Hill deposit is currently 720 K ounces and this includes 100 K ounces of unmined reserves. Within the shear zone phyllosilicate minerals are developed in strain shadows and partly define the stretching lineation associated with dip-slip movement. The zonation within the phyllosilicate assemblages mimics the geometry of bends in the shear zone and minor internal structures. These textures indicate that the phyllosilicate alteration developed synchronous with movement on the shear zone. Earlier advanced argillic alteration and mineralisation are developed in rocks derived from both sides of the shear zone. Hydrothermal activity associated with the earlier advanced argillic alteration was therefore either synchronous with juxtaposition of these distinct rock units, or occurred during a later phase of movement on the shear zone. Cross-cutting fibrous textures in the auriferous barite-pyrite veins indicate that repeated fracturing of the advanced argillic altered rocks accompanied development of successive generations of auriferous veins. Concentrations of auriferous veins are localised in steeply plunging shoots that are oriented parallel to the stretching lineation in the shear zone. These features all indicate movement on the host shear zone accompanied each phase of hydrothermal activity in the Peak Hill deposit. The location, alteration zonation and distribution of mineralised veins within the deposit are intimately controlled by deformation on the host shear zone synchronous with hydrothermal activity. The development of high-sulphidation hydrothermal systems synchronous with deformation along brittle-ductile shear zones is a predictable consequence of intrusive activity during deformation in areas characterised by a high geothermal gradient. The close relationship between tectonism and hydrothermal activity indicates that these deposits are likely to be located in the vicinity of regional-scale shear zones. Deposits are likely to be aligned parallel to the regional-scale structural “grain” and restricted to areas of conspicuous deformation as is the case at Peak Hill (and Temora, NSW). Aluminous alteration zones concentrated in the vicinity of regional-scale structures in the Carolina Slate Belt may be a further example of this style of hydrothermal activity. Received: 30 September 1996 / Accepted: 28 August 1997     

Archean high-Mg monzodiorite–syenite,epidote skarn,and biotite–sericite gold lodes in the Granny Smith–Wallaby district,Australia: U–Pb and Re–Os chronometry of two intrusion-related hydrothermal systems

The Granny Smith (37 t Au production) and Wallaby deposits (38 t out of a 180 t Au resource) are located northeast of Kalgoorlie, in 2.7 Ga greenstones of the Eastern Goldfields Province, the youngest orogenic belt of the Yilgarn craton, Western Australia. At Granny Smith, a zoned monzodiorite–granodiorite stock, dated by a concordant titanite–zircon U–Pb age of 2,665 ± 3 Ma, cuts across east-dipping thrust faults. The stock is fractured but not displaced and sets a minimum age for large-scale (1 km) thrust faulting (D2), regional folding (D1), and dynamothermal metamorphism in the mining district. The local gold–pyrite mineralization, controlled by fractured fault zones, is younger than 2,665 ± 3 Ma. In augite–hornblende monzodiorite, alteration progressed from a hematite-stained alkali feldspar–quartz–calcite assemblage and quartz–molybdenite–pyrite veins to a late reduced sericite–dolomite–albite assemblage. Gold-related monazite and xenotime define a U–Pb age of 2,660 ± 5 Ma, and molybdenite from veins a Re–Os isochron age of 2,661 ± 6 Ma, indicating that mineralization took place shortly after the emplacement of the main stock, perhaps coincident with the intrusion of late alkali granite dikes. At Wallaby, a NE-trending swarm of porphyry dikes comprising augite monzonite, monzodiorite, and minor kersantite intrudes folded and thrust-faulted molasse. The conglomerate and the dikes are overprinted by barren (<0.01 g/t Au) anhydrite-bearing epidote–actinolite–calcite skarn, forming a 600-m-wide and >1,600-m-long replacement pipe, which is intruded by a younger ring dike of syenite porphyry pervasively altered to muscovite + calcite + pyrite. Skarn and syenite are cut by pink biotite–calcite veins, containing magnetite + pyrite and subeconomic gold–silver mineralization (Au/Ag = 0.2). The veins are associated with red biotite–sericite–calcite–albite alteration in adjacent monzonite dikes. Structural relations and the concordant titanite U–Pb age of the skarn constrain intrusion-related mineralization to 2,662 ± 3 Ma. The main-stage gold–pyrite ore (Au/Ag >10) forms hematite-stained sericite–dolomite–albite lodes in stacked D2 reverse faults, which offset skarn, syenite, and the biotite–calcite veins by up to 25 m. The molybdenite Re–Os age (2,661 ± 10 Ma) of the ore suggests a genetic link to intrusive activity but is in apparent conflict with a monazite–xenotime U–Pb age (2,651 ± 6 Ma), which differs from that of the skarn at the 95% confidence level. The time relationships at both gold deposits are inconsistent with orogenic models invoking a principal role for metamorphic fluids released during the main phase of compression in the fold belt. Instead, mineralization is related in space and time to late-orogenic, magnetite-series, high-Mg monzodiorite–syenite intrusions of mantle origin, characterized by Mg/(Mg + Fe_TOTAL) = 0.31–0.57, high Cr (34–96 ppm), Ni (22–63 ppm), Ba (1,056–2,321 ppm), Sr (1,268–2,457 ppm), Th (15–36 ppm), and rare earth elements (total REE: 343–523 ppm). At Wallaby, shared Ca–K–CO₂ metasomatism and Th-REE enrichment (in allanite) link Au–Ag mineralization in biotite–calcite veins to the formation of the giant epidote skarn, implicating a Th + REE-rich syenite pluton at depth as the source of the oxidized hydrothermal fluid. At Granny Smith, lead isotope data and the Rb–Th–U signature of early hematite-bearing wall-rock alteration point to fluid released by the source pluton of the differentiated alkali granite dikes.     

陕西省铧厂沟金矿围岩蚀变和元素迁移特征

陕西省铧厂沟金矿床位于勉略缝合带以南,矿体受控于近东西向逆冲断层和韧脆性剪切带。本文以细碧岩矿带为例,系统研究了围岩蚀变分带及蚀变矿物组合,总结了矿床的蚀变分带模式。围岩蚀变以穿切细碧岩透镜体的剪切带为中心向外依次可划分为黄铁绢英岩化带、绢云碳酸盐化带和绿泥赤铁矿化带。蚀变矿物组合分别为黄铁矿＋铁白云石＋铬云母＋绢云母＋钠长石＋石英＋方解石、铁白云石＋绢云母＋钠长石＋石英±黄铁矿、（铁）绿泥石＋钠长石＋铁白云石＋赤铁矿＋钛铁氧化物＋石英±绿帘石。蚀变岩石组分迁移分析表明,在围岩蚀变过程中, SiO2、Na2O、Fe2O3T、MgO与Y等组分发生不同程度的迁出, K2O、CaO、Ba、Rb、Sr、Cr、Cu、Pb和挥发组分等迁入,并以黄铁绢英岩化带最为显著。金在成矿流体中以Au（HS）–2络合物迁移,成矿流体与富铁细碧岩之间的反应是金沉淀重要机制。     

The Hanshan gold deposit in the Caledonian North Qilian orogenic belt, NW China

The recently discovered Hanshan gold deposit in northern Gansu Province, northwestern China, is hosted by a WNW-striking shear zone in Ordovician andesite and basalt. Mineralization consists of surface to near-surface oxidized ore (the yellow sandy gossan type) and three types of primary ore, i.e. early-stage quartz-sericite-pyrite ores in stockworks, early-stage disseminated ore, and the most important late-stage quartz ± calcite-sulfide veins. The ore system is characterized by variable degrees of potassic and silicic alteration. Late-stage gold-related fluid inclusions have homogenization temperatures between 170 to 310 °C, with a peak around 260 °C and low salinities. The ore fluids had high contents of CO₂, CH₄, and N₂. Sulfur isotope measurements of −1.9 to +1.7 per mil for hydrothermal pyrites could be consistent with a hydrothermal fluid source from the mantle, but the oxygen and carbon isotope data from calcite and quartz suggest mixing between mantle and crustal fluid sources. K-Ar ages for hydrothermal sericite from ore zones are 213.9 ± 3.1 and 224.4 ± 3.2 Ma. Due to the arid Cenozoic climate, a yellow gold-bearing gossan developed, which consists of jarosite, gypsum, and relict quartz. It could be a widespread and useful prospecting guide for gold in northwestern China. Received: 1 February 1999 / Accepted: 1 August 1999     

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     

The geology of the Lega Dembi gold deposit, southern Ethiopia: implications for Pan-African gold exploration

The Lega Dembi deposit is the largest gold producer in Ethiopia. It is situated in late-Precambrian metamorphosed sediments of the N-S trending, volcano-sedimentary Megado belt, which forms part of the late-Proterozoic Adola granite-greenstone terrane in southern Ethiopia. The lode-gold mineralization occurs in a N-S trending, steep westerly dipping quartz-vein system that follows the structural contact between underlying feldspathic gneisses and the volcanosedimentary sequence of the Megado belt. This contact also marks the northernmost extension of the regional-scale, sinistral strike-slip Lega Dembi-Aflata shear zone. Mineralization and intense quartz-veining is best developed in graphite-rich sediments within an area not more than 80 m away from this tectonic contact. Hydrothermal wall-rock alteration includes actinolite/tremolite-biotite-calcite-sericite and chlorite-calcite-epidote assemblages. Gold occurs preferentially in the sericite alteration zone, where it is closely associated and intergrown with galena. The variable deformation of the gold-quartz veins suggests a syn-kinematic timing for the gold mineralization during transcurrent shearing in a dilational segment of the shear zone. In addition to the structural control, lithological control on gold deposition is indicated by the almost exclusive occurrence of the gold mineralization in graphite-rich metasediments. This close relationship suggests that gold precipitation was the result of chemical reduction of regional ore-bearing fluids. Temperature conditions of mineralization are constrained by the actinolite-biotite alteration assemblage and by arsenopyrite chemistry, which indicate that ore deposition occurred at or close to peak metamorphic conditions at upper-greenschist to lower-amphibolite metamorphic grades. Rb-Sr dating of sericite indicates an age of about 545 Ma. for hydrothermal alteration and, thus, for gold mineralization. The style of gold mineralization, structural pattern and lithological assemblages at Lega Dembi are very similar to lode-gold deposits most commonly reported from Archaean granite-greenstone terranes. These similarities may open new perspectives for the exploration of lode-gold deposits, which has previously primarily focused on Archaean greenstone belts rather than Proterozoic or even Phanerozoic meta-volcanosedimentary belts. Received: 26 July 1996 / Accepted: 8 January 1997     

The Nassara gold prospect,Gaoua District,southwestern Burkina Faso

The Nassara-Au prospect is located in the Birimian Boromo Greenstone Belt in southwestern Burkina Faso. It is part of a larger mineralized field that includes the Cu–Au porphyry system of Gaoua, to the north. At Nassara, mineralization occurs within the West Batié Shear Zone that follows the contact between volcanic rocks (basalt and andesite) and volcano-sediments (pyroclastics and black shales) at the southern termination of the Boromo Belt. Gold is associated with pyrite and other Fe-bearing minerals that occur disseminated within the sheared volcanic and volcano-sedimentary rocks. In particular, highest grades are distinguished in alteration halos of small quartz–albite–ankerite veins that form networks along the shear zone. Here, pyrites are marked by As-poor and As-rich growth zones, the latter containing gold inclusions. Gold mineralization formed during D2_NA. Subsequent shear fractures related to D3_NA related are devoid of gold. Nassara is a classical orogenic gold occurrence where gold is associated to disseminated pyrite along quartz veins.     

Archaean Au−Ag mineralisation at Racetrack,near Kalgoorlie,Western Australia: a high crustal-level expression of the Archaean composite lode-gold system

The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold, and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO₂ content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate that ore fluid pH ranged between 4.2 and 5.3, f_{O 2} between 10^−38.8 and 10^−39.6 bars, and m_Σs between 10^−3.2 and 10^−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean lode-gold mineralisation.     

Alteration and primary geochemical dispersion associated with the Bulletin lode-gold deposit, Wiluna, Western Australia

The Bulletin lode-gold deposit is within the northernmost part of the Norseman–Wiluna greenstone belt in the Archaean Yilgarn Block, Western Australia. It is located within a brittle–ductile shear zone and hosted by tholeiitic metavolcanic rocks. Syn-metamorphic wallrock alteration envelops the gold mineralisation and is pervasive throughout the entire shear zone and extends up to 150 m into the undeformed wallrocks. Alteration is characterised by the sequence of distal chlorite–calcite, intermediate calcite–dolomite, outer proximal sericite and inner proximal dolomite–sericite zones. The thickness of the alteration envelope, and the occurrence of dolomite in the alteration sequence, can be used as a rough guide to the width, extent and grade of gold mineralisation, because a positive correlation exists between these variables. Mass transfer evaluations indicate that chemical changes related to the wallrock alteration are similar in all host rocks: in general, Ag, As, Au, Ba, CO₂, K, Rb, S, Sb, Te and W are enriched, Na and Y are depleted, and Al, Cr, Cu, Fe, Mg, Mn, Nb, Ni, P, Se, V, Zn and Zr are immobile, while Ca, Si and Sr show only minor or negligible relative changes. The degree of mobility of each component increases with proximity to gold mineralisation. The largest potential exploration targets are possibly defined by regional As (>6 ppm) and Sb (>0.6 ppm) anomalies. These anomalies, if real, extend laterally for >150 m from the mineralised shear zone into areas of apparently unaltered rocks. Anomalies defined by Te (>10 ppb), W (>0.6 ppm), carbonation indices, local enrichment of Sb (>2.0 ppm) and As (>28 ppm), and potassic alteration indices also form significant exploration targets extending beyond the HJB shear zone and the Au anomaly (>6 ppb) and, locally, into apparently unaltered rock. Gold, itself, has a restricted dispersion, with an anomaly extending for 1–35 m from ore, and being restricted to within the shear zone itself. Amongst individual geochemical parameters, only As and Sb define significant, consistent and smooth trends (vectors) when laterally approaching the ore. However, the respective dimensions of individual geochemical anomalies can be used as an extensive, though stepwise, vector towards ore.     

Structural controls,temperature–pressure conditions and fluid evolution of orogenic gold mineralisation at the Betam mine,south Eastern Desert,Egypt

The Betam gold deposit, located in the southern Eastern Desert of Egypt, is related to a series of milky quartz veins along a NNW-trending shear zone, cutting through pelitic metasedimentary rocks and small masses of pink granite. This shear zone, along with a system of discrete shear and fault zones, was developed late in the deformation history of the area. Although slightly sheared and boudinaged within the shear zone, the auriferous quartz veins are characterised by irregular walls with a steeply plunging ridge-in-groove lineation. Shear geometry of rootless intra-folial folds and asymmetrical strain shadows around the quartz lenses suggests that vein emplacement took place under a brittle–ductile shear regime, clearly post-dating the amphibolite-facies regional metamorphism. Hydrothermal alteration is pervasive in the wallrock metapelites and granite including sericitisation, silicification, sulphidisation and minor carbonatisation. Ore mineralogy includes pyrite, arsenopyrite and subordinate galena, chalcopyrite, pyrrhotite and gold. Gold occurs in the quartz veins and adjacent wallrocks as inclusions in pyrite and arsenopyrite, blebs and globules associated with galena, fracture fillings in deformed arsenopyrite or as thin, wire-like rims within or around rhythmic goethite. Presence of refractory gold in arsenopyrite and pyrite is inferred from microprobe analyses. Clustered and intra-granular trail-bound aqueous–carbonic (L_CO2 + L_aq ± V_CO2) inclusions are common in cores of the less deformed quartz crystals, whereas carbonic (L_CO2 ± V_CO2) and aqueous H₂O–NaCl (L + V) inclusions occur along inter-granular and trans-granular trails. Clathrate melting temperatures indicate low salinities of the fluid (3–8 wt.% NaCl eq.). Homogenisation temperatures of the aqueous–carbonic inclusions range between 297 and 323°C, slightly higher than those of the intra-granular and inter-granular aqueous inclusions (263–304°C), which are likely formed during grain boundary migration. Homogenisation temperatures of the trans-granular H₂O–NaCl inclusions are much lower (130–221°C), implying different fluids late in the shear zone formation. Fluid densities calculated from aqueous–carbonic inclusions along a single trail are between 0.88 and 0.98 g/cm³, and the resulting isochores suggest trapping pressures of 2–2.6 kbar. Based on the arsenopyrite–pyrite–pyrrhotite cotectic, arsenopyrite (30.4–30.7 wt.% As) associated with gold inclusions indicates a temperature range of 325–344°C. This ore paragenesis constrains f _S2 to the range of 10⁻¹⁰ to 10^−8.5 bar. Under such conditions, gold was likely transported mainly as bisulphide complexes by low salinity aqueous–carbonic fluids and precipitated because of variations in pH and f _O2 through pressure fluctuation and CO₂ effervescence as the ore fluids infiltrated the shear zone, along with precipitation of carbonate and sericite. Wallrock sulphidation also likely contributed to destabilising the gold–bisulphide complexes and precipitating gold in the hydrothermal alteration zone adjacent to the mineralised quartz veins.     

Shear-related gold mineralization in Northwest Ghana: The Julie deposit

The Julie deposit is currently the largest gold prospect in NW Ghana. It is hosted in sheared granitoids of TTG composition of the Paleoproterozoic Julie greenstone belt. The main mineralization consists of a corridor of gold-bearing quartz veins forming a network of a few tens of metres in thickness, trending E–W and dipping 30–60° N, contained within the main shear zone that affects these rocks. The core of this vein corridor is altered by sericite, quartz, ankerite, calcite, tourmaline and pyrite, and is surrounded by an outer halo consisting of albite, sericite, calcite, chlorite, pyrite and rutile. A second set of veins, conjugate to the first set, occurs in the area. These veins have alteration halos with a similar mineralogy as the main corridor, however, their extent, as well as the size of the mineralization, is less important. In the main corridor, gold forms micron-sized grains that occur in pyrite as inclusions, on its edges, and in fractures crosscutting it. Silver, tellurium, bismuth, copper and lead commonly accompany the gold. Pyrite occurs disseminated in the veins and in the surrounding rocks. Up to several ppm Au occurs in the structure of pyrite from the main mineralization.     

Archaean lode gold mineralisation in banded iron formation at the Kalahari Goldridge deposit, Kraaipan Greenstone Belt, South Africa

The Kalahari Goldridge Mine is located within the Archaean Kraaipan Greenstone Belt, about 60 km southwest of Mafikeng in the North West Province, South Africa. The ore body thickness varies from 15 to 45 m along a strike length of about 1.5 km within approximately N–S striking banded iron formation (BIF). The stratabound ore body is hosted primarily by BIF, which consists of alternating chert and magnetite–chlorite–stilpnomelane–sulphide–carbonate bands of millimetre- to centimetre scale. A footwall of sericite–carbonate–chlorite schist underlain by mafic amphibolite occurs to the west and carbonaceous metapelites in the hanging wall to the east. Overlying the hanging wall, carbonaceous metapelites, units of coarse-grained metagreywackes fining upwards, become increasingly conglomeratic up the stratigraphy. Small-scale isoclinal folds, brecciation, extension fractures and boudinage of cherty BIF units reflect brittle-ductile deformation. Fold axial planes have foliation, with subvertical plunges parallel to prominent rodding and mineral lineation in the footwall rocks. Gold mineralisation is associated with two generations of quartz–carbonate veins, dipping approximately 20° to 40° W. The first generation consists of ladder-vein sets (group IIA) preferentially developed in centimetre-scale Fe-rich mesobands, whereas the second generation consists of large quartz–carbonate veins (group IIB), which locally crosscut the entire ore body and extend into the footwall and hanging wall. The ore body is controlled by mesoscale isoclinal folds approximately 67° E, orthogonal to the plane of mineralised, gently dipping veins, defining the principal stretching direction and development of fluid-focussing conduits. The intersections of the mineralised veins and foliation planes of the host rock plunges approximately 08° to the north. Pervasive hydrothermal alteration is characterised by chloritisation, carbonatisation, sulphidation and K-metasomatism. Gold is closely associated with sulphides, mainly pyrite and pyrrhotite, and to a lesser extent, with bismuth tellurides and carbonate minerals. Mass balance transfer calculations indicate that hydrothermal alteration of BIF involved enrichment of Au, Ag, Bi, Te, S and CO₂ (LOI), MgO, Ba, K and Rb, but significant depletion of SiO₂ and, to a lesser extent, Fe₂O₃. Extensive replacement of magnetite and chlorite in BIF and other pelitic sedimentary rocks by sulphide and carbonate minerals, both on mesoscopic and microscopic scales, is evidence of interaction of CO₂- and H₂S-bearing fluids with the Fe-rich host rocks. The fineness of gold grains ranges from 823 to 921, similar to that of other epigenetic Archaean BIF-hosted gold deposits, worldwide.     

Gold mineralisation throughout about 45 Ma of Archaean orogenesis: protracted flux of gold in the Golden Mile, Yilgarn craton, Western Australia

The Golden Mile deposit was discovered in 1893 and represents today the largest Archaean orogenic lode gold system in the world (50 M oz produced gold). The Golden Mile deposit comprises three major styles of gold mineralisation: Fimiston, Oroya and Charlotte styles. Fimiston-style lodes formed at 250 to 350 °C and 100 to 200 MPa and are controlled by brittle–ductile fault zones, their subsidiary fault zone and vein networks including breccias and open-cavity-infill textures and hydrothermally altered wall rock. Fimiston lodes were formed late D₁, prior to D₂ regional upright folding. Hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite, an increase in sericite and in Fe content of carbonates. Lodes contain siderite, pyrite, native gold, 17 different telluride minerals (Au–Ag tellurides contain ~25% of total gold), tourmaline, haematite, sericite and V-rich muscovite. Oroya-style lodes formed at similar P–T conditions as the Fimiston lodes and are controlled by brittle–ductile shear zones, associated dilational jogs that are particularly well developed at the contact between Paringa Basalt and black shale interflow sedimentary rocks and altered wall rock. The orebodies are characterised by micro-breccias and zones of intense shear zone foliation, very high gold grades (up to 100,000 g/t Au) and the common association of tellurides and vanadian mica (green leader). Oroya lodes crosscut Fimiston lodes and are interpreted to have formed slightly later than Fimiston lodes as part of one evolving hydrothermal system spanning D₁ and D₂ deformation (ca. 2,675–2,660 Ma). Charlotte-style lodes, exemplified by the Mt Charlotte deposit, are controlled by a sheeted vein (stockwork) complex of north-dipping quartz veins and hydrothermally altered wall rock. The Mt Charlotte orebody formed at 120 to 440 °C and 150 to 250 MPa during movement along closely spaced D₄ (2,625 Ma) and reactivated D₂ faults with the quartz granophyre in the Golden Mile Dolerite exerting a strong lithological control on gold mineralisation. Veins consist of quartz–carbonate–minor scheelite, and wall-rock alteration comprises chlorite destruction and growth of ferroan carbonate–sericite–pyrite–native gold. Pyrite–pyrrhotite is zoned on the scale of vein haloes and of the entire mine, giving a vertical temperature gradient of 50–100 °C over 1,000 vertical metres. The structural–hydrothermal model proposed consists of four major stages: (1) D₁ thrusting and formation of Fimiston-style lodes, (2) D₂ reverse faulting and formation of Oroya-style lodes, (3) D₃ faulting and dissecting of Fimiston- and Oroya-style lodes, and (4) D₄ faulting and formation of Mt Charlotte-style sheeted quartz vein system. The giant accumulation of gold in the Golden Mile deposit was formed due to protracted gold mineralisation throughout episodes of an Archaean orogeny that spanned about 45 Ma. Fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing, efficiency and duration. In addition to the long-lasting fluid plumbing system, the wide variety of transient structural and geochemical traps, multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world.Editorial handling: B. Lehmann     

An indirect lead isotope age determination of gold mineralization at the Corinthia mine,Yilgarn Block,Western Australia

The Corinthia lode‐gold deposit in amphibolite‐facies greenstone belt rocks in the Southern Cross Province of the Archaean Yilgarn Block contains a largely undeformed pegmatite dyke emplaced during the last phases of movement along the Fraser's‐Corinthia shear zone. Gold mineralization and shear zone development were synchronous, and a Pb‐Pb isochron age of 2620 ±6 Ma for pegmatite emplacement either indirectly dates mineralization, or places a minimum age constraint on the timing of mineralization. This age is in accord with a broadly synchronous dominant episode of Archaean lode‐gold mineralization throughout the Yilgarn Block.     

Role of fluid mixing and wallrock sulfidation in gold mineralization at the Semna mine area, central Eastern Desert of Egypt: Evidence from hydrothermal alteration, fluid inclusions and stable isotope data

The Semna gold deposit is one of several vein-type gold occurrences in the central Eastern Desert of Egypt, where gold-bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks. The Semna gold deposit is related to a series of sub-parallel quartz veins along steeply dipping WNW-trending shear zones, which cut through tectonized metagabbro and granodiorite rocks. The orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre-existing fault segments. Textural, structural and mineralogical evidence, including open space structures, quartz stockwork and alteration assemblages, constrain on vein development during an active fault system. The ore mineral assemblage includes pyrite, chalcopyrite, subordinate arsenopyrite, galena, sphalerite and gold. Hydrothermal chlorite, carbonate, pyrite, chalcopyrite and kaolinite are dominant in the altered metaggabro; whereas, quartz, sericite, pyrite, kaolinite and alunite characterize the granodiorite rocks in the alteration zones. Mixtures of alunite, vuggy silica and disseminated sulfides occupy the interstitial open spaces, common at fracture intersections. Partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins.Petrographic and microthermometric studies aided recognition of CO₂-rich, H₂O-rich and mixed H₂O–CO₂ fluid inclusions in the gold-bearing quartz veins. The H₂O–CO₂ inclusions are dominant over the other two types and are characterized by variable vapor: liquid ratios. These inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids. The generally light δ³⁴S of pyrite and chalcopyrite may suggest a magmatic source of sulfur. Spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing. Conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226–267 °C and 350–1100 bar, under conditions transitional between mesothermal and epithermal systems.The Semna gold deposit can be attributed to interplay of protracted volcanic activity (Dokhan Volcanics?), fluid mixing, wallrock sulfidation and a structural setting favoring gold deposition. Gold was transported as Au-bisulfide complexes under weak acid conditions concomitant with quartz–sericite–pyrite alteration, and precipitated through a decrease in gold solubility due to fluid cooling, mixing with meteoric waters and variations in pH and fO₂.     

Deep seismic reflection profiling in the Archaean northeastern Yilgarn Craton, Western Australia: implications for crustal architecture and mineral potential

Deep seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia, have provided information on its crustal architecture and on several of its highly mineralised belts. The seismic reflection data allow interpretation of several prominent crustal scale features, including an eastward thickening of the crust, subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite–greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This geometry forms a suitable fluid focusing wedge in which upward to subhorizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.