Auriferous Quartz Veins Research Articles

Wallrock alteration associated with gold mineralisation in Cohen’s Reef, Walhalla, Victoria

Cohen?s Reef in Walhalla is a shear zone-associated, laminated-massive auriferous quartz vein system, and is associated both spatially and temporally with the Woods Point Dyke Swarm. Geochemical analyses of the auriferous Cohen?s Reef and host rocks, have led to a greater understanding of the mechanisms and extent of the alteration and mineralisation events. Geochemical traverses proximal and distal to Cohen?s Reef were based on samples taken from recent exploration drilling. Current research is concentrated on selected gold deposits surrounding Walhalla in response to the latest GeoScience Victoria mapping initiative and current exploration by Goldstar Resources NL. This preliminary geochemical analysis builds on handsample and thinsection studies, and whole rock analyses of the altered and unaltered dyke, and host sediments from drilhole traverses proximal and distal to Cohen?s Reef. Loss on Ignition (LOI) values are a favourable guide to the extent of hydrothermal alteration in dyke and host sediments. Based on the selected samples, broad hydrothermal alteration zones (~5% wt) at least 15 metres from auriferous dyke margins and quartz veins, are characterised by arsenopyrite and pyrite visible in thinsection. More importantly, LOI peaks greater than 12% wt seem to be indicative of greater Au content with narrow alteration zones of less than five metres, and characterised by visible sulphides.

Geology and geochemical aspects of lode gold mineralization at Dimako–Mboscorro, SE Cameroon

The Dimako–Mboscorro lode gold deposit has two styles of mineralization: quartz veins (4.5–54 ppm Au) and altered wallrock (up to 2.6 ppm Au). The veins occur in steeply dipping brittle–ductile shear zones in a foliated biotite metagranite. The alteration envelop around the gold-bearing quartz veins shows lateral zoning comprising a silicified/ ferruginized and mylonitic zone (SFZ) at the centre, through a transitional sericite zone (SZ), to an outer, slightly sheared, but mineralogically less altered, though bleached, metagranite zone (BZ) at the shear zone boundaries. Hydrothermal alteration in these zones is characterized by silicification (SFZ, SZ), hematite formation (SFZ), alkali metasomatism with muscovite/sericite formation (SFZ, SZ, to a much lesser extent BZ), and pyritization (SFZ, SZ). The auriferous quartz veins are concordant to the host structure but discordant to the foliation in the metagranite. The main ore minerals in the veins and in the SFZ and SZ zones are gold, hydrothermal hematite, sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, chalcocite, covellite and colloidal supergene hematite. Gold occurs as inclusions in sphalerite (15.7±0.4 wt% Ag; 83.6±0.2 wt% Au), as coarse grains associated with euhedral hematite pseudomorphs (18.5±1.0 wt% Ag; 82.0±1.1 wt% Au), as stringers within fractures in quartz (13.3±0.1 wt% Ag; 87.1±0.5 wt% Au), as well as fine to coarse grains related to limonite, goethite and colloidal hematite (8.1±1.8 wt% Ag; 92.1±2.2 wt% Au). A hydrothermal metal association of Ag, Au, As, Ba, Pb, V, W, and Zn is identified in the wallrock and quartz veins. These metals, together with K-enrichment, are potential pathfinder elements for wider lithogeochemical exploration in this region, especially when combined with the lateral wallrock zonation pattern. Preliminary fluid inclusion data from the auriferous quartz vein samples indicate that gold was probably deposited from low salinity (<1 to 8 wt% eq. NaCl, with mean at 5.52 wt% eq. NaCl) H 2 O–CO 2 fluids that have total homogenization temperatures of 263–335°C.

The role of decarbonization and structure in the Callie gold deposit, Tanami Region of northern Australia

The Callie deposit is the largest (6.0 Moz Au) of several gold deposits in the Dead Bullock Soak goldfield of the Northern Territory’s Tanami Region, 550 km northwest of Alice Springs. The Callie ore lies within corridors, up to 180 m wide, of sheeted en echelon quartz veins where they intersect the 500-m-wide hinge of an ESE-plunging F1 anticlinorium. The host rocks are the Blake beds, of the Paleoproterozoic Dead Bullock Formation, which consist of a > 350-m-thick sequence of lower greenschist facies graphitic turbidites and mudstones overlying in excess of 100 m of thickly bedded siltstones and fine sandstones. The rocks are Fe-rich and dominated by assemblages of chlorite and biotite, both of which are of hydrothermal and metamorphic origin. A fundamental characteristic of the hydrothermal alteration is the removal of graphite, a process which is associated with bleaching and the development of bedding-parallel bands of coarse biotite augen. Gold is found only in quartz veins and only where they cut decarbonized chloritic rock with abundant biotite augen and no sulfide minerals. Auriferous quartz veins differ from barren quartz veins by the presence of ilmenite, apatite, xenotime, and gold and the absence of sulfide minerals. The assemblage of gold–ilmenite–apatite–xenotime indicates a linked genesis and mobility of Ti, P, and Y in the mineralizing fluids. Geochemical analysis of samples throughout the deposit shows that gold only occurs in sedimentary rocks with high FeO/(FeO+Fe2O3) and low C/(C+CO2) ratios (> 0.8 and < 0.2, respectively). This association can be explained by reactions that convert C from reduced graphitic host rocks into CO2 and reduce ferric iron in the host rocks to ferrous iron in biotite and chlorite. These reactions would increase the CO2 content of the fluid, facilitating the transport of Ti, P, and Y from the host rocks into the veins. Both CO2 and CH4 produced by reaction of H2O with graphite, effervesced under the lower confining pressures in the veins. This would have partitioned H2S into the vapor phase, destabilizing Au–bisulfide complexes; the loss of CO2 and H2S from the aqueous phase caused precipitation of gold, ilmenite, apatite, and xenotime. It is proposed that this process was the main control on gold precipitation. Oxidization of iron in the very reduced wall rocks, resulting in reduction of the fluid, provided a second mechanism of gold precipitation in previously decarbonized rocks, contributing to the high grades in some samples. Although sulfide minerals, especially arsenopyrite, did form during the hydrothermal event, host rock sulfidation reactions did not play a role in gold precipitation because gold is absent near rocks or veins containing sulfide minerals. Sulfide minerals likely formed by different mechanisms from those associated with gold deposition. Both the fold architecture and subsequent spatially coincident sinistral semibrittle shearing ensured that the ore fluids were strongly focused into the hinges of the anticlines. Within the anticlines, a reactive cap of fine-grained, graphitic, reduced Fe-rich turbidites above more permeable siltstones and fine sandstones impeded fluid flow ensuring efficient removal of graphite, and the associated effervescence of CO2 from the fluid caused the precipitation of gold. Exploration for similar deposits should focus on the intersection of east–west shear zones with folds and Fe-rich graphitic host rocks.

Structural and lithological controls on the high-grade Hangingwall Reef quartz – gold veins, Stawell, Victoria

The Hangingwall reefs are high-grade auriferous quartz lodes that occur in the upper levels of the Magdala mine at Stawell, and thus represent an important deposit style in western Victoria. Structural relationships indicate that the Hangingwall Reefs formed coincident with the same metallogenic event that produced the adjacent Magdala deposit. However, the host-rocks to the quartz – gold veins that make up the Hangingwall Reefs display markedly different structural and stratigraphic relationships. Hangingwall Reef mineralisation occurred during east – west shortening in a muscovite-altered turbidite sequence that had little prior iron alteration (cf. Magdala deposit). Lithological and structural data show that the distribution of the quartz – gold veins is related to the geometry of the Stawell Fault and its associated fault splays, with the quartz veining being localised where the faults are discordant with the pre-existing structural fabrics. This discordant relationship produced the dilational sites and jogs in which the auriferous quartz veins formed. Results from 3D numerical modelling show that dilation along the Stawell Fault provided the conduit for fluid flow.

Geology and Mineralogy of the Baolun Hydrothermal Gold Deposit in the Hainan Island, South China

Abstract. The Baolun quartz vein type Au deposit, is located at the southwestern Hainan Island. It occurs next to a Triassic ilmenite‐series/S‐type syenogranite complex. A 40Ar/39Ar plateau age for muscovite from the master orebody is dated to be 219.4±0.6 Ma, suggesting that the gold deposit genetically related to the granite pluton. In the Baolun mining area, orebodies of auriferous quartz vein and wall rock alteration occur in NNW‐striking fracture zones hosted by weakly metamorphosed turbidite of the Lower Silurian age. Eight fracture zones, 400∼1300 m long and 10∼30 m wide, have been identified. The five fracture zones of them form orebodies. The ores are mainly of quartz vein type. More than 20 orebodies in shapes of vein and lens of 195∼751 m in length and 0.20∼7.49 m in thickness are known, and 14 of them occur in the No. 1 vein belt. Silicification, sericitization and pyritization are closely related to the Au mineralization. The ores exhibit commonly 1.54∼29.48 g/t in Au grade, partially &gt;98 g/t. The master orebody is 720 m long and 2.62 m thick in average, with 9.53∼29.27 g/t Au. Gold reserves of more than 70 t have been proven by geological exploration. More than 20 metallic minerals including native gold and sulfides such as pyrite, pyrrhotite, chalcopyrite, and others are identified to be formed in four ore‐forming stages: (1) Au‐coarse grain quartz stage, (2) Au‐fine grained quartz stage, (3) Au‐bismuthide‐bismuth sulfosalt‐sulfide‐quartz stage, and (4) calcite‐sulfide‐quartz stage. The Au mineralization in this deposit occurred mainly in the first three stages. A variety of Bi‐ and Te‐bearing minerals is closely associated with native gold suggesting that the mineralization may take place in a relatively high temperature.

The Daduhe gold field at the eastern margin of the Tibetan Plateau: He, Ar, S, O, and H isotopic data and their metallogenic implications

The Daduhe gold field comprises several shear-zone-controlled Tertiary lode gold deposits distributed at the eastern margin of the Tibetan Plateau. The deposits are hosted in a Precambrian granite–greenstone terrane within the Yangtze Craton. The gold mineralization occurs mainly as auriferous quartz veins with minor sulphide minerals. Fluid inclusions in pyrite have 3He/ 4He ratios of 0.16 to 0.86 R a, whereas their 40Ar/ 36Ar ratios range from 298 to 3288, indicating a mixing of fluids of mantle and crust origins. The δ 34S values of pyrite are of 0.7–4.2‰ ( n = 12), suggesting a mantle source or leaching from the mafic country rocks. δ 18O values calculated from hydrothermal quartz are between − 1.5‰ and + 6.0‰ and δD values of the fluids in the fluid inclusions in quartz are − 39‰ and − 108‰. These ranges demonstrate a mixing of magmatic/metamorphic and meteoric fluids. The noble gas isotopic data, along with the stable isotopic data suggest that the ore-forming fluids have a dominantly crustal source with a significant mantle component.

Controls of gold-quartz vein formation during regional folding in amphibolite-facies, marble-dominated metasediments of the Navachab Gold Mine in the Pan-African Damara Belt, Namibia

The Navachab gold mine in central Namibia is situated in amphibolite-facies, marble-dominated metasediments of the Neoproterozoic Damara Sequence in the Pan-African Damara Belt. Gold mineralization is hosted by two main, and at least three minor, sets of auriferous quartz-veins that are developed on the subvertical northwestern limb of the regional-scale, shallow doubly-plunging anticline of the Karibib dome. The orientation, relative timing and progressive deformation of quartz veins indicate that veining occurred during folding and fold amplification of the Karibib dome. This deformation forms part of the main phase of northwest-directed collisional tectonics in the Damara belt at ~540 to 550 Ma. The two main vein sets include (1) bedding-parallel, shallowly-plunging ore lenses and shoots situated at the base of a prominent marble unit, and (2) a laterally extensive swarm of shallowly-dipping quartz veins that truncate the host rocks at high angles. The bulk of the bedding-parallel lenses represent dilational jogs that opened during flexural flow along bedding-parallel slip planes during the amplification of the Karibib dome. Flexural flow and associated bedding-parallel fluid infiltration were concentrated close to the contact between marbles and the underlying siliciclastic formations. This lithological contact, in particular, represented a pronounced rheological contrast resulting in increased slip rates during fold amplification. The highly discordant sets of shallowly-dipping, sheeted quartz veins were emplaced during the fold lock-up stage of the Karibib dome. Quartz veining occurred when the northwestern limb of the dome was rotated to subvertical attitudes, so that bedding-parallel flexural slip ceased to be active. Extensional fracturing and veining was facilitated by the presence of transiently supralithostatic fluid pressures. The high-amplitude fluid pressure cycling associated with extensional fracturing is likely to have triggered gold precipitation in the shallowly-dipping veins. Although bedding-parallel veins and discordant veins are broadly contemporaneous, related to the regional-scale D2 phase of fold-and-thrust tectonics, cross-cutting relationships between the two main vein systems indicate that the two vein sets have represented separate entities and succeeded each other rather than forming an interconnected fracture mesh.

MINERALIZAÇÕES DE OURO DO TIPO OROGÊNICO EM ARCO MAGMÁTICO PALEOPROTEROZÓICO, BORDA OESTE DO CRATON SÃO FRANCISCO, REGIÕES DE SÃO DOMINGOS (GO) E CORRENTINA (BA)

The metavolcanosedimentary sequences and associated plutonic rocks from the Sao Domingos area, State of Goias, and Correntina area, State of Bahia, located at the border of the Sao Francisco Craton, were emplaced during the Paleoproterozoic in an oceanic, island-arc tectonic environment. The metassedimentary rocks from the sequences display different phases of deformation related to the Transamazonico Orogeny, reactivated during the Brasiliano Orogeny only in the Correntina region. The sequences studied and the oceanic island-arc type Almas-Dianopolis Terrane suggest a paleoproterozoic magmatic arc at the western border of the Sao Francisco Craton. The gold mineralizations are hosted by quartz veins which are fillings of Transamazonic fractures. In the Sao Domingos area, the quartz veins occur in phyllite and tonalite and are bordered by hydrothermal alteration zone, mainly kaolinization, while in the Correntina area, the veins crosscut tonalite and basic-ultrabasic rocks and are bordered by hydrothermal alteration zone enriched in biotite and carbonate. Gold generally ocurrs as millimetric grains and pyrite is the only sulphide present in the auriferous quartz veins. Fluid inclusion studies in vein quartz from Sao Domingos revealed the presence of aqueous and carbonic fluids, minimum fluid trapping temperature and pressure of about 300oC and 2 kb, respectively, and low salinity, wich suggests a metamorphic origin for the fluid. Trace element composition of the hydrothermal alteration zones indicates that granitoids and pyrite-rich carbonaceous phyllite in the Sao Domingos area, and monzogranite and basic-ultrabasic rocks in the Correntina area, were the main sources of gold to the hydrothermal metamorphic fluids. The gold mineralizations of the Sao Domingos and Correntina areas and also those of the paleoproterozoic Almas-Dianopolis Terrane, indicate a very good potencial of the paleoproterozoic magmatic arc in the western border of the Sao Francisco Craton for orogenic lode-gold deposits.

Invisible Gold from the Hishikari Epithermal Gold Deposit, Japan: Implication for Gold Distribution and Deposition

Abstract. The presence of invisible gold was confirmed in arsenian pyrite from the Hishikari epithermal gold deposit, Kagoshima, Japan, by means of EPMA and SIMS analyses. The relative concentration of invisible gold may be positively correlated with As contents (0.01 to 10.37 wt%) of fine-grained arsenian pyrite which commonly occurs in the auriferous quartz veins. Although arsenian pyrite occurs widely in any mineralization stage with electrum and other sulfide, sulfosalts, selenide or telluride minerals, arsenian pyrites having higher As contents accompanied by invisible gold occur dominantly in the middle stage of fine-adularia-quartz and in the late stage of coarse-quartz rather than in the early stage of columnar-adularia.

MINERALIZAÇÕES DE OURO DO TIPO OROGÊNICO EM ARCO MAGMÁTICO PALEOPROTEROZÓICO, BORDA OESTE DO CRATON SÃO FRANCISCO, REGIÕES DE SÃO DOMINGOS (GO) E CORRENTINA (BA)

The metavolcanosedimentary sequences and associated plutonic rocks from the São Domingos area, State of Goiás, and Correntina area, State of Bahia, located at the border of the São Francisco Craton, were emplaced during the Paleoproterozoic in an oceanic, island-arc tectonic environment. The metassedimentary rocks from the sequences display different phases of deformation related to the Transamazônico Orogeny, reactivated during the Brasiliano Orogeny only in the Correntina region. The sequences studied and the oceanic island-arc type Almas-Dianópolis Terrane suggest a paleoproterozoic magmatic arc at the western border of the São Francisco Craton. The gold mineralizations are hosted by quartz veins which are fillings of Transamazonic fractures. In the São Domingos area, the quartz veins occur in phyllite and tonalite and are bordered by hydrothermal alteration zone, mainly kaolinization, while in the Correntina area, the veins crosscut tonalite and basic-ultrabasic rocks and are bordered by hydrothermal alteration zone enriched in biotite and carbonate. Gold generally ocurrs as millimetric grains and pyrite is the only sulphide present in the auriferous quartz veins. Fluid inclusion studies in vein quartz from São Domingos revealed the presence of aqueous and carbonic fluids, minimum fluid trapping temperature and pressure of about 300ºC and 2 kb, respectively, and low salinity, wich suggests a metamorphic origin for the fluid. Trace element composition of the hydrothermal alteration zones indicates that granitoids and pyrite-rich carbonaceous phyllite in the São Domingos area, and monzogranite and basic-ultrabasic rocks in the Correntina area, were the main sources of gold to the hydrothermal metamorphic fluids. The gold mineralizations of the São Domingos and Correntina areas and also those of the paleoproterozoic Almas-Dianópolis Terrane, indicate a very good potencial of the paleoproterozoic magmatic arc in the western border of the São Francisco Craton for orogenic lode-gold deposits.

Stable isotope geochemistry of the Archean Val-d‘Or (Canada) orogenic gold vein field

A systematic study of the auriferous quartz veins of the Val-d’Or vein field, Abitibi, Quebec, Canada, demonstrates that the C, O, S isotope composition of silicate, carbonate, borate, oxide, tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole Superior Province. The oxygen isotope composition of quartz from early quartz–carbonate auriferous veins ranges from 9.4 ‰ to 14.4 ‰ whereas later quartz-tourmaline-carbonate veins have δ18Oquartz values ranging from 9.2 ‰ to 13.8 ‰. Quartz-carbonate veins have carbonate (δ18O: 6.9–12.5 ‰; δ13C: −6.2– −1.9 ‰) and pyrite (δ34S: 1.2 and 1.9 ‰) isotope compositions comparable to those of quartz-tourmaline-carbonate veins (δ18O: 7.9–11.7 ‰; δ13C: −8.0 – −2.4 ‰; δ34S: 0.6–6.0 ‰). δ18Oquartz values in quartz-tourmaline-carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality, irrespective of the type of structure, the texture of the quartz or its position along strike, across strike or down-dip a vein. In contrast, the oxygen isotope composition of quartz in quartz-tourmaline-carbonate veins displays a regional distribution with higher δ18O values in the south-central part of the vein field near the Cadillac Tectonic Zone, and which δ18O values decrease regularly towards the north. Another zone of high δ18O values in the northeast corner of the region and along the trace of the Senneville Fault is separated by a valley of lower δ18O values from the higher values near the Cadillac Tectonic Zone. Oxygen isotope isopleths cut across lithological contacts and tectonic structures. This regional pattern in quartz-tourmaline-carbonate veins is interpreted to be a product of reaction with country rocks and mixing between (1) a deep-seated hydrothermal fluid of metamorphic origin with minimum δ18O=8.5 ‰, δ13C=0.6 ‰ and δ34S=−0.4 ‰, and (2) a supracrustal fluid, most likely Archean seawater with a long history of water-rock exchange and with maximum δ18O=3.9 ‰, δ13 C=−5.6 ‰ and δ34S=5.0 ‰.

Geology, geochronology, and tectonic setting of the Jiapigou gold deposits, southern Jilin Province, China

The Jiapigou area, at the northeastern corner of the North China Craton (NCC), is one of the most important gold-producing areas in China. It was responsible for nearly half of China's gold production in the 1960s and at present has >100 t Au reserves. Although numerous studies, mostly not published in international literature, have been undertaken on these deposits, their genesis, age, and tectonic setting are still controversial, mainly because of the lack of reliable geochronological data. Gold deposits at Jiapigou are hosted by several ductile shear zones within late Archean gneissic tonalite, trondhjemite, and granodiorite (TTG) and amphibolite (2523±6 Ma), which have been retrogressed to greenschist facies. The deposits occur as massive auriferous quartz veins that contain sulfides (<10 vol.%) and less common disseminated- and stockwork-style mineralization. The lodes are characterized by high Au/Ag ratios and relatively higher Cu (up to 1.3%) and Pb (as much as 10.3%) contents than most other gold deposits in the NCC. Although phyllic alteration is most common, potassic and chloritic alterations characterize felsic and mafic host rocks, respectively. Compiled stable isotope and fluid inclusion data show δ 34S, δ 13C, δ 18O and δD values ranging from −0.2‰ to 12.6‰, −4.2‰ to −5.0‰, 5‰ to 12‰, and −70‰ to −94‰, respectively. These data likely indicate consistent sources of sulfur, carbon, and hydrothermal fluids, but do not actually indicate any specific reservoir. Fluid inclusions from the deposits show variable homogenization temperatures between 150 and 350 °C, and are characterized by low salinity (0.7 to 6.5 wt.% NaCl equiv.) and H 2O–CO 2±CH 4, N 2 solutions. The initial lead isotope compositions of the deposits have considerable variations, even within individual deposits, possibly indicating multiple lead sources for the deposits. In the Jiapigou gold orefield, there are no major intrusions, but felsic, alkaline, and mafic dikes are common. These dikes usually occupy the same fault system as the gold lodes, with dikes locally hosting gold mineralization. Zircon SHRIMP U–Pb dates on pre-ore dikes yield ages of ca. 220 Ma, which constrains the maximum age of gold mineralization, whereas 40Ar/ 39Ar dates on hydrothermal sericite give an age of ca. 204 Ma. The timing of major gold mineralization in the Jiapigou area is therefore constrained between 220 and 204 Ma; this suggests a late Triassic gold-forming event. This episode of gold mineralization is coeval with, or slightly later than, the post-orogenic magmatism that followed the final orogenesis of the Inner Mongolian-Daxinganling orogenic belt (IMDOB), suggesting that a post-orogenic tectonic setting is appropriate for the late Triassic gold mineralization. Zircon SHRIMP dating of a granitic batholith adjacent to the Jiapigou region and also hosting gold lodes that have been previously constrained by Ar–Ar geochronology between 144 and 120 Ma yielded an age of 166±2 Ma, indicating that a mid-Jurassic (Yanshanian) gold-forming episode occurred at Jiapigou and in neighboring regions. This Yanshanian gold mineralization was contemporaneous with subduction of the Pacific plate. On the basis of the nature, timing, and tectonic setting, we favor a classification of ‘orogenic gold’ for the Jiapigou deposits.

Mineralizing age of the Rushan lode gold deposit in the Jiaodong Peninsula: SHRIMP U-Pb dating on hydrothermal zircon

Zircons from the auriferous quartz vein in the Rushan gold deposit, Jiaodong Peninsula, have high common Pb contents (206Pb6 = 2.00%–15.88%) and Th/U ratios (0.31–1.35), and trap primary CO2-H2O fluid inclusion similar to ore-forming fluids, indicating that they grew from high Th/U ratio ore-forming fluids responsible for gold mineralization. SHRIMP U-Pb analyses of these zircons gave an age of 117±3 Ma comparable with the ages reported by the former researches in the other areas in the Jiaodong Peninsula, which is interpreted as the age of gold mineralization at Rushan. The age of SHRIMP zircon U-Pb dating with 160±3 Ma from host rock Kunyushan monzogranite is different from that of the gold mineralization, indicating that there is no genetic relationship between gold mineralization and the granitic magmatism. SHRIMP U-Pb dating on hydrothermal zircons from auriferous quartz veins could be used to constrain the timing of lode gold mineralization and the relationship to relevant hydrothermal event.

Fluid Geochemistry and Metallogenesis of the Hatu Gold Deposit in the Junggar Basin, Xinjiang

Abstract The Hatu large gold deposit is located on the western margin of the Junggar basin, Xinjiang. Its mineralization is characterized by auriferous quartz veins and Au‐bearing altered fracturing zones. Studies on mineralogy, inclusions and decrepitation temperature indicate that the gold deposit was formed by overlapping of two kinds of fluid of different origins, instead of gradual evolution of a single fluid. The auriferous quartz veins are related to magmatism‐originated fluid, but the Au‐bearing altered fracturing zones to deep‐derived fluid. Bonanzas in quartz veins were formed and localized at overlapping positions of two types of fluid under intensive compression.

Geophysical characterisation of the Challenger gold deposit, Gawler Craton, South Australia

The Challenger Project is located in Archaean rocks of the Gawler Craton and is the most significant gold-only discovery made in South Australia. The deposit was discovered through geochemical sampling of calcrete. It has a measured and indicated resource of 1.7M t at a grade of 8.5 g/t for 465,000 oz of gold. The resource is open down-plunge and has the potential to be significantly greater. Mineralisation occurs as discrete shallowly (30°) plunging shoots hosted by granulite facies paragneiss, which lacks marker horizons to aid in geological and structural mapping. High-grade moderate to steeply dipping auriferous quartz veins occur within the shoots. No major structures are observable in the area in regional (400 m line spacing) aeromagnetic data. However, the deposit lies on the edge of a regional-scale gravity high, as do the majority of significant Archaean lode gold sites in the region. The gravity high is interpreted to indicate a buried intrusive, or crustal underplating. A high-resolution aeromagnetic survey has mapped a subtle, but easily discernible, magnetic high (5-6 nT) that is spatially coincident with the mineralisation. Modelling, constrained by core-magnetisation studies and known mineralised geometries, shows pyrrhotite within the mineralising system is the probable source of the anomaly. However, interpretation is complicated by lateral interference from lamprophyre dikes/sills. During exploration in the Challenger area, radiometrics, digital elevation models and enhanced Landsat data provided a basis for mapping the regolith. The deposit is coincident with a slight topographic high and a pronounced increase in the depth of weathering. The former is mapped by digital terrain data gathered during airborne geophysical surveys. The latter has been mapped by ground electrical resistivity, electromagnetic and gravity surveys, and drilling. A TEMPEST AEM survey has mapped the conductivity structure of the regolith in good agreement with that derived from deposit-scale ground electrical geophysics. The increase in weathering over the Challenger deposit is mapped as a second- or third-order effect. However, the survey was extremely useful because it identified and delineated hitherto unknown 2 km wide, 200 m deep, north-south striking, palaeochannels. Drilling of one of these has shown they are excellent sources of water for the future mine development. A weak bedrock IP response is coincident with the mineralisation, but is not diagnostic. Extreme difficulties were experienced in collecting high-quality IP data due to the highly resistive nature of the surface calcrete and silcrete, and the highly conductive nature of the underlying clays and saprolite material.

Fluid Inclusion Characteristics of Auriferous Quartz Veins in Archean and Paleoproterozoic Greenstone Belts of Eastern and Southern Finland

Fluid inclusions in auriferous quartz veins in five deposits in the Archean (2.9–2.7 Ga) and Paleoproterozoic (2.0–1.75 Ga) greenstone belts of eastern and southwestern Finland have been characterized on the basis of their composition, reconstructed isochores, and decrepitation behavior. The dominant fluids associated with gold mineralization are H2O-CO2 ± CH4 ± halite (Archean) and H2O-CO2-CH4 or H2O-CH4 (Paleoproterozoic) in composition; phase ratios were generally uniform H2O/CO2 in the Archean but were variable in the Paleoproterozoic. In the Archean Au ± sulfide quartz veins, the mineralizing fluids contain less CH4 (<30 mol %) and have higher salinities (up to 39 wt % NaCl equiv) than the corresponding fluids in the Paleoproterozoic Au-sulfide-quartz veins (up to 38 mol % CH4 and below 13 wt % NaCl equiv). Additionally, the estimated pressure-temperature conditions for gold mineralization differ markedly between the two domains. Mineralization occurred in a pressure-temperature range of 300° to 500°C at 2 to 3.5 kbars in the Archean domain and 200° to 300°C at 1 to 2 kbars in the Paleoproterozoic domain. Decrepigrams for the Archean and Paleoproterozoic auriferous quartz veins show different characteristics and permit classification of the different fluid inclusion populations on a regional scale.

Fluid Inclusion Characteristics of Auriferous Quartz Veins in Archean and Paleoproterozoic Greenstone Belts of Eastern and Southern Finland

Fluid Inclusion Characteristics of Auriferous Quartz Veins in Archean and Paleoproterozoic Greenstone Belts of Eastern and Southern Finland

Fluid inclusion studies of the Abawso gold prospect, near the Ashanti Belt, Ghana

The major Ghanaian lode gold deposits are preferentially aligned along the western and eastern contacts of the Kumasi Basin with the Ashanti and Sefwi Belts, respectively. The investigated area of the Abawso small-scale concession, covering the workings of the old Ettadom mine, is situated 3 km west of the lithological contact of the Birimian metavolcanic rocks of the Akropong Belt in the east with the Birimian metasedimentary rocks of the Kumasi Basin in the west. The rocks of the Abawso concession represent a steeply NW-dipping limb of a SE-verging anticline with an axis plunging to the SW. Quartz veining occurs predominantly in the form of en echelon dilatational veins along NNE–SSW-striking shear zones of a few metres width and shows evidence of brittle and ductile deformation. Also stockwork-style quartz veining occurs in the vicinity of the main shaft of the old Ettadom mine. Hydrothermal alteration includes sericitisation, sulphidation and locally carbonatisation. The auriferous quartz veins mainly follow the trend of brittle to ductile deformed quartz veins; however, some occur in stockwork. Fluid inclusion studies reveal a large number of H2O inclusions along intragranular trails in auriferous quartz vein samples, as well as an overall dominance of H2O and H2O-CO2 inclusions over CO2 inclusions. Textural observations and physico-chemical fluid inclusion properties indicate post-entrapment modifications for all quartz vein samples due to grain boundary migration recrystallisation. This process is interpreted to be responsible for the generation of the CO2 inclusions from a H2O-CO2 parent fluid. In comparison with mineralisation at the Ashanti and Prestea deposits, which are characterised by CO2±N2 inclusions, the observed inclusion assemblage may be due to a shallower crustal level of mineralisation, or different degrees and styles of recrystallisation, or a less pronounced development of laminated quartz veins due to comparably restricted pressure fluctuations. Furthermore, the microthermometric observations allow the reconstruction of a possible retrograde P-T path, depicting near-isothermal decompression in the P-T range of the brittle/ductile transition.

40Ar/39Ar ages of auriferous quartz veins from the Fengyang and Zhangbaling regions and their geological significance

Geotectonically the Fengyang and Zhangbaling regions belong to the North China craton and the Dabie-Sulu orogene, respectively. Neo-Archean gneiss and amphibolite and metamorphosed sea-facies sodic volcanic rocks are the main outcrops in the two regions, respectively. The Zhangbaling terrane strike-skipped along the Tancheng-Lujiang fault zone in Mesozoic and Cenozoic eras and got close to the Fengyang terrane. Mesozoic Yanshanian intrusions occur broadly in the two regions. Gold-bearing quartz veins occur in the metamorphic rocks in the Fengyang region and in the granodiorite and metamorphosed sea-facies sodic volcanic rocks in the Zhangbaling region. Generally, the formation of the auriferous quartz veins involved three stages. At the first stage, gold-poor sulfide quartz veins were formed; at the second stage gold-rich quartz sulfide veins were formed; and at the third stage gold-poor barite and/or carbonate veins were formed. The40Ar/39Ar step-heating plateau ages of the first-stage and the second-stage quartz aggregates from the Zhuding, Maoshan and Shangcheng gold deposits range between 116.1±0.6 Ma and 118.3 ± 0.5 Ma and are pretty close to their least apparent ages and isochronal ages, respectively. All plateau, least apparent and isochronal ages range between 113.4 ± 0.4 Ma and 118.3 ± 0.5 Ma, which are considered as the formation age range of the quartz. It is reasonable and reliable to take the40Ar/39Ar age range of the quartz as the formation age range of gold-bearing quartz veins on the basis of spatial relationship between gold-bearing quartz veins and their country rocks. The gold deposits in the two regions were formed in Aptian, Cretaceous, when the Tancheng-Lujiang fault zone moved as a normal fault with slightly right-lateral strike-skip, was extensional and experienced very strong magmatic process. It is shown that the magmatic hydrothermal fluid is a very important part of the gold ore-forming hydrothermal fluid in the Fengyang and Zhangbaling regions. The formation of the gold ore deposits in the Fengyang and Zhangbaling regions had genetic relations with the extensional movement of the Tancheng-Lujiang fault zone and magmatic activities and took place under the extensional dynamic condition in Late Cretaceous. Therefore, the extensional movement of the Tancheng-Lujiang fault zone presented the energy and space for magmatic and gold ore-forming processes.

Age constraints on Tarkwaian palaeoplacer and lode-gold formation in the Tarkwa-Damang district, SW Ghana

Two major epigenetic gold-forming events are recorded in the world-class gold province of southwest Ghana. A pre-Tarkwaian event was the source of the world-class Tarkwa palaeoplacers whereas post-Birimian and Tarkwaian deformation, which was related to the Eburnean orogeny, gave rise to the world-class (e.g. Prestea) to giant (e.g. Obuasi) orogenic gold deposits which have made the region famous for more than 2,500 years. A maximum age of 2133±4 Ma for Tarkwaian sedimentation is provided by 71 of 111 concordant SHRIMP II U–Pb dates from detrital zircons in Tarkwaian clastic rocks from Damang and Bippo Bin, northeast of Tarkwa. The overall data distribution broadly overlaps the relatively poorly constrained ages of Birimian volcanism and associated Dixcove-type granitoid emplacement, indicating syntectonic development of the Tarkwaian sedimentary basin. These zircon ages argue against derivation of the palaeoplacer gold from an orogenic gold source related to the compressional phase of an orogeny significantly older than the Eburnean orogeny. Instead, they suggest that the gold source was either orogenic gold lodes related to an earlier compressional phase of a diachronous Eburnean orogeny or ca. 2200–2100 Ma intrusion-related gold lode. The CO2-rich fluid inclusions in associated vein-quartz pebbles are permissive of either source. At the Damang deposit, an epigenetic, orogenic lode-gold system clearly overprinted, and sulphidised low-grade palaeoplacer hematite–magnetite gold occurrences in the Banket Series conglomerate within the Tarkwaian sedimentary sequence. Gold mineralisation is demonstrably post-peak metamorphism, as gold-related alteration assemblages overprint metamorphic assemblages in host rocks. In alteration zones surrounding the dominant, subhorizontal auriferous quartz veins, there are rare occurrences of hydrothermal xenotime which give a SHRIMP U–Pb age of 2063±9 Ma for gold mineralisation. The similar structural timing of epigenetic gold mineralisation in Tarkwaian host rocks at Damang to that in mainly Birimian host rocks elsewhere in southwest Ghana, particularly at Obuasi, suggests that 2063±9 Ma is the best available age estimate for widespread orogenic gold mineralisation in the region. Argon–argon ages of 2029±4 and 2034±4 Ma for hydrothermal biotite from auriferous quartz veins appear to represent uplift and cooling of the region below about 300 °C, as estimates of the temperature of gold mineralisation are higher, at around 400 °C. If peak metamorphism, with temperatures of about 550 °C, is assumed to have occurred at about 2100 Ma, the biotite ages, in combination with the xenotime age, suggest a broadly constant uplift rate for the region of about 1 km per 10 million years from about 2100 to 2025 Ma.