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.     

Fluid inclusions in quartz-pebbles of the gold-bearing Tarkwaian conglomerates of Ghana as guides to their provenance area

Quartz-pebbles of the early Proterozoic Au-bearing Tarkwaian conglomerates in Ghana reveal several original (inherited) pre-sedimentary fluid inclusions. These inclusions are CO₂-N₂ rich and display a distinct high density (up to 1.15 g/cm³). The unusual high density and composition compare well with CO₂-N₂-rich inclusions in quartz-vein type gold deposits of the Birimian Supergroup in Ghana and Burkina Faso. This type of fluid inclusions has not been reported from any other lode-gold deposit of greenstone affiliation and is thus a specific characteristic for Birimian-hosted gold deposits. Therefore, it can be used as an unequivocal pathfinder for epigenetic as well as for syn-sedimentary gold mineralization of the early Proterozoic of West Africa. The inherited fluid inclusions with the unique physicochemical characteristics suggest that the Tarkwaian quartz-pebbles and possibly some gold were derived from Au-quartz vein deposits comparable in mineralogy, petrography and genesis to those along the NW-margin of the Ashanti belt (e.g. Ashanti Mine, Prestea Mine).     

Gold mineralisation in the Essakane goldfield in Burkina Faso,West African craton

The West African craton is known for its structurally hosted Au deposits in Ghana, Burkina Faso, Côte d'Ivoire, Mali and Niger. The Essakane goldfield in northeast Burkina Faso has produced 1 606,000 oz of gold since 2010 from the Essakane Main Zone. The Essakane goldfield is made up of several exploration and artisanal sites that include; Essakane Main Zone, Gossey, Falagountou, Sokadie, Tin Zoubratan, Essakane North and South, Korizéna, Bom Kodjélé, Tin Taradat, Tassiri, Gaigou, and Takabangou. Gold mineralisation in sheeted and stockworks quartz–carbonate and tourmaline veins occurs with pyrite, arsenopyrite, and traces of pyrrhotite, galena and hematite. It is hosted in sheared, folded and contact metamorphosed volcanic, volcanoclastic and sedimentary Birimian Supergroup sequences. The maximum age of gold mineralisation in the Essakane goldfield is syn-deformational and formed during the Eburnean Orogeny (D2) at 2130–1980 Ma.     

The metallogenic relationship between Birimian and Tarkwaian gold deposits in Ghana

The traditional concept of the Early Proterozoic gold deposits in Ghana — i.e. gold-bearing shear zones overlain by Tarkwaian paleoplacers containing reworked gold derived from the shear-zones — needs to be reconsidered in the light of recent research in Ghana, the Ivory Coast and French Guiana. This research has revealed a consistent pattern of geostructural and metallogenic evolution in which both the Birimian and the Tarkwaian rocks were deformed by a major Eburnean compression (D2). It has shown that the NE-SW faults controlling the Gold Coast Range shear-zone mineralization (Ashanti-Prestea) were formed during the Eburnean D2 episode of thrusting that was followed by hydrothermal activity with the emplacement of auriferous arsenopyrite and then by the development of quartz veinlets and native gold; thus the shear-zone mineralization could only have appeared during the D2 late-orogenic stage. It has also shown evidence of post-depositional D2 deformation in the gold sites examined in the Tarkwa gold-bearing conglomerate, although the effects are limited and primary lithological controls have been preserved that reveal these deposits to be modified paleo-placers. Thus, the Tarkwaian gold could not be derived from the gold-bearing shear-zones.     

Gold metallogeny in the Birimian craton of Burkina Faso (West Africa)

Primary gold deposits in Burkina Faso occur in Paleoproterozoic Birimian belt formations (2.0 Ga). Mineralization was synchronous with regional metamorphism and deformation, and is either hosted within, or is adjacent to, quartz-bearing veins. These are classical characteristics of epigenetic gold deposits in Precambrian metamorphic terranes and permit to classify the mineralized sites from Burkina Faso as orogenic-type gold deposits. A review of data collected over the past decade by our team permits to recognize two main styles of gold mineralization: (1) Quartz-vein hosted; this style occurs in all lithologies, the veins are deformed and gold is principally concentrated within the veins, associated with either sulfides or tourmaline. (2) Disseminated; this style occurs exclusively in albitites (and to a lesser extent listvenites) with gold occurring mainly within alteration halos of generally undeformed quartz-albite-carbonate vein. Quartz-vein and disseminated styles of mineralization can be associated within the same deposit. Albitites and listvenites are alteration products of mainly calc-alkaline igneous rocks of felsic to ultramafic composition, respectively. The predominant alteration assemblage consists of chlorite, albite, carbonate, and pyrite. Sulfides occur as fine masses commonly in the alteration halos close to vein margins and consist mainly of pyrite and arsenopyrite, depending on host-rock composition. Gold occurs as free native metal and, locally, in form of tellurides, in fissures or as inclusions within pyrite and arsenopyrite. Two main populations of fluid inclusions are associated with the gold deposits, independently of the mineralization style: (1) carbonic inclusions consisting of up to 90 mol% CO₂ (plus N₂ and CH₄) and (2) aqueous-carbonic fluid inclusions with moderate salinities. Interestingly, the disseminated gold style deposits of Burkina Faso, which have the highest economic potential, show strong similarities with the world-class Ashanti deposit, in neighboring Ghana.     

Geology and geochemical patterns of the Birimian gold deposits, Ghana, West Africa

A major gold province of the world exists in the Proterozoic Birimian and Tarkwaian supracrustal rocks of West Africa. The bulk of the gold comes from the primary lode occurrences of the Birimian rocks of Ghana (formerly The Gold Coast). Birimian lithofacies is characterised by subaqueous fine-grained sediments with bimodal volcanic material. Metasedimentary rocks include phyllites and metawackes. Metavolcanic rocks are predominantly tholeiitic basalts. Komatiites and banded iron formations (BIF) are absent.Gold is in 5 parallel, evenly spaced, more than 300 km long, northeast-trending volcanic belts separated by basins containing pyroclastic and meta-sedimentary units. The most prominent is the Ashanti volcanic “greenstone” belt, which hosts the Ashanti Goldfields Corporation mines at Obuasi (more than 800,000 kg Au since 1896), the Billiton Bogosu Gold mine at Bogosu, and the State Gold Mining Corporation mines at Prestea, Bibiani and Konongo.Gold, ranging from 2 to 30 ppm, is in quartz veins of laterally extensive major orebodies which deeply penetrate fissures and shear zones at contacts between metasedimentary and metavolcanic rocks. The veins consists mainly of quartz with carbonate minerals, green sericite, carbonaceous partings and metallic sulfides and arsenides of Fe, As, Zn, Au, Cu, Sb, and Pb. Gold occurs in carbonate fillings in fractured quartz veins. Country rocks, which contain rutile, anatase and granular masses of leucoxene, along ore channels, have been hydrothermally altered to carbonates, sericite, silica and sulfide minerals. Fluid inclusion evidences suggest that mineral deposition took place at about 350°C and 140 bar from dilute aqueous solutions. Timing deduced from ore textures, however, show complex multi-stage mineralization events, with higher temperature minerals commonly having formed later than lower temperature ones. Geochemical studies of materials produced by tropical processes, especially soils, are essential in prospecting poorly exposed terranes of west Africa. Trace and major element distributions at mines and mineral occurrences can indicate mineralization otherwise difficult to detect.This paper highlights the features of the Ghanaian gold deposits that may aid the current search for new deposits along the gold belts. Exploration based on geochemistry is highly important, but should be integrated with data from accompanying geological, lithologic, mineralogical, and structural studies.     

Manganese deposits of the Oudalan province in the northeast of Burkina Faso,and Ansongo Inlier in eastern Mali

At least 15 manganese deposits crop out in the West African craton in Mali, Burkina Faso, Ghana and Cote d'Ivoire. There average grade estimates range from 17 to 52% total Mn oxide and average volume of 5–19 Mt of ore; they consequently provide an attractive target for exploration and eventual exploitation. The Tambão and Béliata Mn oxide deposits in the Oudalan province (northeast of Burkina Faso), and Takavasita, Agualla and Tondibi Mn oxide deposits in the Ansongo Inlier (eastern Mali) generally crop out as a saprolitic residuum that formed during the Miocene to Eocene and are enriched in manganese to 52% total Mn oxide. The saprolitic residuum is overlain by blocky to botryoidal manganese-rich duricrusts that form part of the resource portfolio. The residuum overlies deeply weathered manganese-bearing carbonates, shales and volcaniclastic siltstones of the Birimian Supergroup (2.3–2.1 Ga) that form a low-grade resource and likely formed in a heat-driven seawater convection system that provided hydrothermal solutions from submarine volcanic centers. They are typically intercalated with mafic-intermediate volcanic rocks.     

Extensive reworking of Archaean crust within the Birimian terrane in Ghana as revealed by combined zircon U-Pb and Lu-Hf isotopes

Located in the southeastern parts of the BaouléMossi domain of the West African Craton,the Birimian terrane in Ghana is known to consist of predominantly juvenile crust emplaced during the Eburnean orogeny.Here we present novel coupled zircon U-Pb and Lu-Hf isotope data from nine Birimian granitoids intruding different domains from northwestern,western and southeastern Ghana.Sub-chondriticε_(Hf)-values indicating reworking of Archaean crust are recorded in zircon from both northwestern and southeastern Ghana.As a conservative estimate 71%of all analysed zircon grains spanning in age from2220 Ma to 2130 Ma require contribution from a reworked Archaean source,contradicting the common belief that the Birimian terrane consist of predominantly juvenile crust.A minimumε_(Hf)-value of-10.5 at2139 Ma suggests a Palaeoarchaean to late Mseoarchaean component as the contributing ancient source.Combined with previously reported heavy zirconδ~(18)O signatures from Birimian river zircon in Ghana,our new data suggests reworking of Archaean aged sediments during subduction initiated crustal growth in the vicinity of one,or possibly in between two Archaean aged cratons.     

Compositional characteristics of fluid inclusions as exploration tool for Au-mineralization at Larafella, Burkina Faso

The Larafella Au-prospect (Burkina Faso) lies within dacitic rocks of the Palaeoproterozoic Birimian greenstone belts. Gold mineralization is intimately associated with zones of cataclastic deformation. Whilst the lode-vein mineralization is closely associated with CO₂-rich fluid inclusions, the barren quartz veins are characterized by H₂O ± salt-bearing inclusions. Geochemical studies on the immediate wall-rock of the quartz veins have shown an increase of As in zones of gold enrichment, while alteration overprints such as carbonatization and chloritization cannot be correlated unequivocally with Au-mineralization. Consequently, fluid inclusion studies of quartz veins and As-anomalies constitute important exploration tools for mesothermal gold mineralization, since Au-rich zones can be distinguished from Au-depleted zones.     

Mineralogical siting and distribution of gold in quartz veins and sulfide ores of the Ashanti mine and other deposits in the Ashanti belt of Ghana: genetic implications

The Ashanti belt of Ghana constitutes a gold province which has produced a total of about 1500 t of gold historically. Gold mineralization is found in steep, NNE-SSW to NE-SW trending shear zones predominantly transecting metasediments of the Palaeoproterozoic Birimian Supergroup (2.2–2.1 Ga), disseminated in ca. 2.1 Ga granitoids, in paleo-conglomerates of the Tarkwaian Group (< 2135 Ma), and in recent placers. The distribution of gold, its chemistry, paragenesis and mineralogical siting in the mesothermal ores of the major mines in the Ashanti belt, namely Konongo, Ashanti, Bogosu and Prestea mine, are the subject of this study. At the localities studied, gold is present in two main types of ores: 1. Quartz veins with free-milling gold. The gold is relatively silver-rich (true fineness values from 730 to 954) and is accompanied by a distinct suite of Cu, Pb, Sb sulfides. 2. Sulfide ores, consisting of arsenopyrite, pyrite and rarer pyrrhotite and marcasite, with refractory gold. The ores have apparent fineness values larger than 910. Arsenopyrite and locally (at Bogosu) pyrite were identified as the hosts of submicroscopic gold. Mean concentrations of gold in arsenopyrite in various samples from the different mines, obtained by secondary ion mass spectrometry (SIMS), range from 67 to 314 ppm Au. Gold concentration mapping in individual arsenopyrite crystals from the different deposits revealed similar patterns of gold distribution: the grains possess a gold-poor core, and elevated gold contents are present along distinct crystal growth zones towards their rims. The outermost crystal layer is usually gold-poor. The well-preserved distribution patterns also indicate that remobilization of gold from the sulfides played an insignificant role in the ores of the Ashanti belt. Multiple quartz veining and growth zoning of the sulfides are interpreted as manifestations of multiple episodes of fluid infiltration, fluid flow and mineral deposition. The bimodal occurrence of gold in spatially closely associated quartz vein and sulfide ores indicates a genetic link between these ore types. A model implying a grossly coeval formation of the ores from mesothermal fluids is proposed. Received: 28 September 1995 / Accepted: 11 June 1996     

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.     

Age and tectonic significance of the Louth Volcanics: implications for the evolution of the Tasmanides of eastern Australia

Abstract

Re-evaluation of geochemical and geophysical datasets, and analysis of magmatic and detrital zircons from drill-core samples extracted from the Louth region of the southern Thomson Orogen (STO), augmented by limited field samples, has shown that two temporally and compositionally distinct igneous groups exist. The older Lower Devonian, calc-alkaline group corresponds to complexly folded, high-intensity curvilinear magnetic anomalies in the Louth region (Louth Volcanics) and are probable equivalents to Lower Devonian volcanics in the northern Lachlan Orogen. A younger Permo-Triassic alkaline assemblage forms part of an E–W corridor of diatremes that appears to relate to focussed lithospheric extension associated with the later stages of the Hunter–Bowen Orogeny in the New England Orogen. The alkaline group includes gabbros previously considered as Neoproterozoic, but all magmatic rocks, including alkaline basalts, contain an unusual number of xenocrystic zircons. The age spectra of the xenocrystic zircons mimic detrital zircons from Cobar Basin sedimentary rocks and/or underlying Ordovician turbidites, suggesting incorporation of upper crustal zircons into the alkaline basaltic magmas. A distinct difference of detrital zircon age spectra from central Thomson Orogen metasediments indicates the STO metasediments have greater affinities to the Lachlan Orogen, but both orogens probably began in the Early Ordovician during widespread backarc extension and deposition of turbidites in the Tasmanides. A surprising result is that Ordovician, Devonian and Permo-Triassic basaltic rocks from the STO and elsewhere in the Tasmanides, all yield the same Nd-model ages of ca 960–830 Ma, suggesting that Neoproterozoic subcontinental lithospheric mantle persisted throughout the evolution of the Tasmanide orogenic system.     

The Loulo gold and tourmaline-bearing deposit

The stratabound polymorph Loulo gold deposits and occurrences were discovered in sedimentary rocks of the Early Proterozoic Birimian Formation, in the Kéniéba region of SW Mali. Early work showed reserves in the Loulo-0 and Loulo-3 deposits of 28 t of gold in ore averaging 4.43 g/t Au, to depths of 140 m; recent drilling has found mineralization to at least 300 m depth. Early tourmalinization of the sedimentary host rock is indicated by reworked tourmalinite pebbles in non-tourmalinized hanging-wall rocks. Mineralization, restricted to highly tourmalinized zones, occurs as: (a) pre-D1 (deformation) disseminated pyrite with micron-size inclusions of gold particles; (b) syn-/late-D2 stockworks with minor disseminated sulphides and native gold. Gold in the sulphides is small (10–40 gm), and occurs at pyrite-crystal edges, filling cracks and/or as inclusions. Post-D2 stockwork mineralization derived from the pre-D1 one that was emplaced with tourmaline, as is indicated by the small differences between pre-D1 and post-D2 sulphur isotopes, as well as by the type of quartz vein that indicates an almost closed system. In West Africa, the pre-D1 mineralization of Early Birimian age represents the first major introduction of gold into the Proterozoic. It attributes a major metallogenic role to the tourmalinized sandstone that indicates crustal accretion in West Africa during the Early Proterozoic. Editorial handling: D.D. Klemm     

Petrology,geochemistry, and zircon U-Pb geochronology of the Zambezi Belt in Zimbabwe: Implications for terrane assembly in southern Africa

The Zambezi Belt in southern Africa has been regarded as a part of the 570-530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts.Here,we report new petrological,geochemical,and zircon U-Pb geochronological data of various metamorphic rocks(felsic to mafic orthogneiss,pelitic schist,and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe,and evaluate the timing and P-T conditions of the collisional event as well as protolith formation.Geochemical data of felsic orthogneiss indicate within-plate granite signature,whereas those of mafic orthogneiss suggest MORB,ocean-island,or within-plate affinities.Metamorphic P-Testimates for orthogneisses indicate significant P-T variation within the study area(700-780 C/6.7-7.2 kbar to 800-875 C/10-11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks.Zircon cores from felsic orthogneisses yielded two magmatic ages:2655±21 Ma and 813士5 Ma,which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism.Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic(ca.2700-750 Ma).The Neoarchean(ca.2700-2630 Ma) and Paleoproterozoic(ca.2200-1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe,respectively.The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso-to Neoproterozoic(ca.1500-950 Ma) and early Neoproterozoic(ca.900-750 Ma) detrital zircons,respectively.Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic(744-670 Ma),and subsequently underwent highgrade metamorphism at 557-555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian.In contrast,670-627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports,but consistent with~680-650 Ma metamorphic ages reported from different parts of the Kuunga Orogen,suggesting Cryogenian thermal events before the final collision.     

Terrigenous zircon of archean greenstone belts as a source of information on the early earth’s crust: Azov and Dnieper domains,Ukrainian shield

Zircons from the metasedimentary rocks of the Mesoarchean greenstone belts of the Azov and Middle Dnieper blocks of the Ukrainian shield were studied and dated by U-Pb method on a NORDSIM secondary-ion mass spectrometer. Detrital zircons from the metasediments of the Belozerskaya Formation of the greenstone belts of the Middle Dnieper block are usually dated within 3000–3100 Ma, while individual grains have an age of 3200–3300 Ma. This indicates that the sediments were derived mainly from proximal volcanic source with minor contribution of the basement material (Aul’skaya Group). The metasediments of the Soroki greenstone structure of the Azov block contain mainly zircons with ages within 3500–3600 Ma, except for scarce grains having the ages older than 3700 Ma. Zircon cores are overgrown by granulitic rims dated at approximately 3300 Ma. A wide scatter in ages and Th/U ratios in the zircons indicate that they were derived from rocks of different composition and age. Obtained data suggest significantly wider distribution of the Paleoarchean crust within the Azov block of the Ukrainian shield than was previously assumed.     

The early crust beneath the Azov Domain of the Ukrainian Shield: Isotopic-geochronological and geochemical investigations of detrital zircons from metasedimentary rocks of the Fedorovka structure

The results of isotopic-geochronological investigations of detrital zircons from metasediments of the Fedorovka structure in the Azov Domain of the Ukrainian Shield by SIMS and LA-ICP-MS methods are considered. The data obtained are compared with the results of isotopic dating of zircons from metasediments of the Soroki greenstone structure of the Azov region. All the examined samples yielded zircons with ages exceeding 3600 Ma. The geochemical properties of the oldest zircons indicate that they originate immediately from tonalite rocks. The obtained data imply substantially wider development of the Paleoarchean crust in the Ukrainian Shield than was previously thought.     

Arsenic in groundwater from mineralised Proterozoic basement rocks of Burkina Faso

This study describes the hydrogeochemistry and distributions of As in groundwater from a newly investigated area of Burkina Faso. Groundwaters have been sampled from hand-pumped boreholes and dug wells close to the town of Ouahigouya in northern Burkina Faso. Although most analysed groundwaters have As concentrations of less than 10 μg L⁻¹, they have a large range from <0.5 to 1630 μg L⁻¹. The highest concentrations are found in borehole waters; all dug wells analysed in this study have As concentrations of <10 μg L⁻¹. Skin disorders (melanosis, keratosis and more rare skin tumour) have been identified among the populations in three villages in northern Burkina Faso, two within the study area. Although detailed epidemiological studies have not been carried out, similarities with documented symptoms in other parts of the world suggest that these are likely to be linked to high concentrations of As in drinking water. The high-As groundwaters observed derive from zones of Au mineralisation in Birimian (Lower Proterozoic) volcano-sedimentary rocks, the Au occurring in vein structures along with quartz and altered sulphide minerals (pyrite, chalcopyrite, arsenopyrite). However, the spatial variability in As concentrations in the mineralised zones is large and the degree of testing both laterally and with depth so far is limited. Hence, concentrations are difficult to predict on a local scale. From available data, the groundwater appears to be mainly oxic and the dissolved As occurs almost entirely as As(V) although concentrations are highest in groundwaters with dissolved-O₂ concentrations <2 mg L⁻¹. The source is likely to be the oxidised sulphide minerals and secondary Fe oxides in the mineralised zones. Positive correlations are observed between dissolved As and both Mo and W which are also believed to be derived from ore minerals and oxides in the mineralised zones. The discovery of high As concentrations in some groundwaters from the Birimian rocks of northern Burkina Faso reiterates the need for reconnaissance surveys in mineralised areas of crystalline basement.     

Recurrent mesoproterozoic continental magmatism in South-Central Norway

We report U–Pb dates and Lu–Hf isotope data, obtained by LAM-ICPMS, for zircons from metamorphic rocks of the Setesdalen valley, situated in the Telemark block south of the classic Telemark region of southern Norway. The samples include infracrustal rocks from the metamorphic basement, metaigneous rocks and metasediments from the Byglandsfjorden supracrustal cover sequence, and metaigneous rocks which intruded the whole succession. The main crustal evolution took place from 1,550–1,020 Ma, beginning with the emplacement of juvenile tonalitic melts; the contribution of older crustal material increased with time. Around 1,320 Ma, further addition of juvenile material occurred, involving both mafic and felsic melts, metamorphism and deformation. Acid magmas with high FeO*/MgO were intruded at 1,215 Ma, coinciding with underplating elsewhere in South Norway. The period starting at 1,215 Ma is represented by supracrustal rocks, principally metarhyolites with minor mafic material and immature sediments of the Byglandsfjorden Group. The crust generation processes ended with the intrusion of diorites and granodiorites at 1,030 Ma, late in the Sveconorwegian orogeny. Regional processes of metamorphism and deformation (around 1,290 and 1,000 Ma) can be related to the assembly of Rodinia. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Stratigraphy,structure and geochronology of ore leads in the Matsitama Schist belt of Northern Botswana

The Matsitama schist belt in northeastern Botswana comprises an area of metasediments, notably quartzites, limestones, shales and amphibolites that are bounded by granites and gneisses. The belt lies southwest of the Rhodesian cration and north of the Limpopo mobile belt.Stratigraphic, structural and lead isotopic evidence indicates that the Matsitama metasediments are equivalent to the Shashi metasediments in the Limpopo belt. There is strong evidence that the Matsitama and Shashi metasediments stratigraphically underlie volcanic rocks of the Tati belt which have been correlated with Archaean schist belts of about 2700 Ma of Rhodesia. Therefore, the Matsitama and Shashi rocks are at least as old as the schist belts of the Rhodesian craton and may represent a shallow-water facies that occurs only in the Limpopo area.There is no structural evidence that the Matsitama and Shashi metasediments were deposited unconformably on basement rocks, although the presence of gneiss, amphibolite and ironstone pebbles in a Matsitama conglomerate, as well as the presence of orthoquartzites, shows the existence of a basement source region. However, the surrounding granites intrude the Matsitama and Shashi metasediments and all underwent several deformation phases.The structural history of the Matsitama rocks can be described in terms of five phases of deformation. The main cleavage-producing deformation phase, F₂, folded the rocks into a major synform and intensely deformed them. Before this, however, the rocks had been folded and thrust so that part of the succession shows downward-facing F₂ structures and there are possibly repetitions of the stratigraphy due to imbrication. Structures of the F₃ and F₄ phases fold the main cleavage but locally are sufficiently intense to modify the shape of the finite strain ellipsoid. There is a major ductile shear zone of F₄ age, south of which F₄ folds are tight, while to the north, F₄ deformation is negligible. All of these structures can be correlated with deformation phases in the Tati schist belt to the east and in the northern part of the Limpopo mobile belt.Lead isotope evidence suggests that mineralization in the Matsitama metasediments occurred at least 2200 Ma ago, and that leads from Dihudi/Thakadu and Messina, in the centre of the Limpopo belt, underwent a two-stage history of events at 2600–2700 Ma and 2000–2100 Ma ago, agreeing with other geochronological evidence. The leads from Matsitama and Messina are isotopically distinct from leads from the Rhodesian schist belts, which show evidence of transfer to the crust some 3500 Ma ago. The absence of this 3500 Ma-old lead from the Matsitama and Messina environments may indicate different crustal conditions and possibly the absence of the Rhodesian-type early basement.     

Structural and gold mineralizing evolution of the world-class orogenic Mana district,Burkina Faso: Multiple mineralizing events over 150 million years

The Mana district, located in the northern part of the Birimian Houndé greenstone belt in western Burkina Faso, is a world-class Paleoproterozoic orogenic gold district (∼8 Moz) including five gold deposits (Fofina, Nyafé, Siou, Wona-Kona and Yaho). These deposits are located in specific lithostratigraphic domains, and gold is controlled by various structural features. Deposit- and regional-scale mapping, intrusion age and geochemistry, as well as airborne aeromagnetic and electrical resistivity geophysical data, were used to decipher the tectonic evolution of each gold deposit and the district. Five deformational and four gold mineralizing events were recognized.The first deformation event (D1_MD: E-W oriented shortening) affected the metamorphosed volcanic and sedimentary rocks of the Lower Birimian group. This early deformation episode was correlated with the formation of gently N-plunging folds (F_1MD) and N-S-striking thrusts faults coeval with emplacement of the pre- to synkinematic Wona-Kona and Siou plutons dated at ∼2172 Ma, under greenschist facies metamorphism. The quartz-carbonate veins (V_1MD) at Fofina and Siou formed during D1_MD at Eoeburnean time, manifesting the first gold event at approximately ∼2172 Ma.The following deformation event (D2_MD: E-W oriented extension) is associated with the deposition of the Upper Birimian group (Mana basin) overlying the Lower Birimian group. The geometry of the Mana basin is controlled by the Mana and Maoula shear zones. The Tarkwaian-type rock formation overlying the Upper Birimian group, controlled by the Wona-Kona and Siou shear zones, is constrained at the end of D2_MD or at the beginning of the D3_MD event with a maximum deposition age at ∼2113 Ma.The third deformation event (D3_MD: E-W to WNW-ESE transpression) affected the entire supracrustal rock. Such event is correlated with the formation of map-scale F_3MD folds and dextral shear zones during the Eburnean orogeny (∼2113–2090 Ma). A second gold mineralizing event occurred during D3_MD and is manifested by quartz-carbonate veins (V_3MD) and disseminated sulfides at the Yaho, Fofina and Nyafé and possibly Wona-Kona deposits.The fourth deformation event (D4_MD: NNW-SSE transpression) is correlated with sinistral shearing along the major transcurrent faults and the development of asymmetric NNE-striking folds (F_4MD) associated with vertical fold axes. Syn-D4_MD mineralization is characterized mainly by a strong silicification (Si_4MD) with disseminated pyrite and arsenopyrite along the Wona-Kona shear zone and by tiny quartz-carbonate veinlets (V_4MD). This event is considered the main gold-bearing event in the western margin of the Mana district.The fifth and last deformation event (D5_MD) is brittle in character and was responsible for the formation of E-W subvertical crenulation cleavages and reverse faults under overall N-S shortening. This late deformation event is tentatively associated with a last gold event recorded as free gold associated with muscovite in brittle fractures developed in competent orebodies at the Wona-Kona and Siou deposits. This event could be as young as ∼2022 Ma, the age obtained from Ar-Ar datation of muscovite-schists at the Wona-Kona deposit.Our main contribution is that we decipher multiple gold mineralizing events at the district scale based on deposit- and regional-scale mapping. It is interpreted that gold was introduced as early as ∼2172 Ma and possibly as late as ∼2022 Ma during at least 3 or even 4 shortening tectonic events in a timeframe not yet recognized at the district scale for all the Birimian belts.