Regional Greenschist Facies Metamorphism Research Articles

A Fluid Inclusion and Stable Isotope Study at the Loulo Mining District, Mali, West Africa: Implications for Multifluid Sources in the Generation of Orogenic Gold Deposits

The world-class Loulo mining district (15.5 Moz resource), in the Birimian terrane of western Mali, contains a range of mineralogically diverse styles of orogenic gold mineralization. The district is distinguished by As-rich orebodies as at Yalea, as well as tourmaline-bearing, Fe-rich, orebodies as at Gara. New fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization, and point towards the role of multifluid sources (metamorphic and magmatic) in the formation of orogenic gold deposits. Microthermometric and laser Raman studies from Yalea Main and two other similar orebodies (Loulo-3 and Baboto) reveal the dominance of coexisting CO 2 -N 2 ±CH 4 (type 1) and H 2 O-NaCl (type 2) fluid inclusions. These inclusions show evidence of fluid unmixing from reduced (quartz-faylite-magnetite, QFM, buffered), low-salinity (≤10 wt % NaCl equiv), CO 2 -rich-H 2 O-NaCl-N 2 ± CH 4 primary ore fluids. The combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270° to 340°C at 1.4 to 1.8 kbar. The P-T-X state of the Yalea-style mineralizing fluids, along with the δ 18 O fluid compositions of 8.8 to 10.7‰, is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane. Similar fluid compositions were previously reported elsewhere in the Birimian crust of West Africa and in other orogenic gold districts worldwide. The precipitation of gold from the H 2 S-rich metamorphic fluid is primarily linked to phase separation of the ore fluid, which is controlled by P-T fluctuations and/or fluid-rock interaction with carbonaceous host sedimentary rocks (confirmed by low δ 13 C values of −21.7 to −15.8‰). Fluid inclusion investigations from Gara and a similar style orebody (Yalea North) indicate the presence of coexisting CO 2 ± N 2 ± CH 4 (type 1) and mixed-salinity (5–21 wt % NaCl equiv) CO 2 -rich-H 2 O-NaCl inclusions (type 3). Inclusion assemblages also contain common oxidized (hematite-magnetite, HM, buffered), high temperature (>400°C), hypersaline (∼35–50 wt % total dissolved solids), metalliferous (Na+Fe+Ca+Cu+Ni+W+ Pb+Zn), multiphase H 2 O-rich-CO 2 -NaCl-FeCl 2 inclusions (type 4). This inclusion type has not been previously reported in other Birimian terranes. The composition of the brines, along with carbon isotope data ( δ 13 C of −14.4 to −4.5‰), suggests a magmatic input to the Gara-style hydrothermal system. The coexistence in the fluid inclusion assemblages of the magmatic brines with Yalea-style, CO 2 -rich, metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing. The interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the Gara-style orebodies. This includes the growth of widespread multi-stage Fe 3+ -rich tourmaline (B-rich granite source) and sodic alteration, and ore assemblages consisting of abundant nickeloan pyrite, (REE)-phosphates, Ni ± Co ± Pb ± Zn minor/trace sulfides, and scheelite. Gold deposition in the Gara-style hydrothermal system is related to physical and chemical changes of the two fluids during mixing (e.g., decreases in f O2 and T in the brines and retrograde boiling of the CO 2 component in the metamorphic fluids, a “salting out effect”).

Geological characteristics and metallogenesis of the shilu Fe-ore deposit in Hainan Province, South China

The Shilu Fe-ore deposit, a large-scale hematite-rich ore deposit in Hainan Province, South China, contains proven reserves of over 460Mt Fe-ore at 51% FeO, 4.07Mt Co-ore at 0.29% and 6.65Mt Cu-ore at 1.18%. The deposit is largely hosted within the Meso- to Neoproterozoic (ca. 0.85–1.30Ga) Shilu Group, a suite of metamorphosed neritic siliciclastic rocks and sedimentary carbonates, in which original sedimentary textures and structures are elegantly preserved. The Shilu Group, which hosts banded iron formations (BIFs), locally called amphibolitic itabirites, comprises six lithostratigraphic sequences. Fe-rich ores mainly reside in pyroxene–amphibole rocks of the middle sixth sequence, whereas the Co–Cu ores are located about 30–60m below the Fe-ore horizons and are primarily hosted by banded or impure dolostones of the lower sixth sequence. These ores are present mainly as lens-like or sheet-like bodies trending along troughs and/or transitional zones from limbs to troughs of the NW–SE trending synclinorium, and generally show banded or massive structures with lepidoblastic, cryptocrystalline or microcrystalline textures. Occasional oolitic and blastopsammitic textures are present, indicating an original sedimentary source for the Shilu Fe-polymetallic ores. At least two stages of deformation and associated metamorphism can be recognized. D1 deformation corresponds to a NW–SE trending synclinorium with associated regional greenschist facies metamorphism. D2 deformation records an early NW–SE orientated contraction (D2a), followed by doming with apparent skarnization (D2b) and finally transitional to a nearly E–W orientated extension (D2c), resulting in various amounts of folding, faulting and mineral recrystallization.Radiogenic isotopic analyses conducted on ores and host rocks record three metamorphic ages, including two Sm–Nd isochron ages of 830±16Ma and 212.9±6.6Ma, and a 40Ar–39Ar plateau age of 132.08±1.57Ma corresponding to D1, D2b, and D2c, respectively. Combined with previous U–Pb SHRIMP ages from ca. 960Ma to ca. 1300Ma and isotopic geochemical data (Xu et al., 2009), as well as other regional metamorphic events, an updated four-stage metallogenic model is proposed here: (1) deposition of the BIF-type ore source horizons between ca. 830 and 960Ma, (2) formation of a metamorphic sedimentary-type ore deposit from ca. 830–360Ma, (3) refinement of the deposit due to D2 deformation between ca. 250 and 210Ma and (4) superposed mineralizing stage formed by magma-related hydrothermal fluids between ca. 130 and 90Ma. The Shilu Fe-ore deposit can thus be better regarded as a structurally and hydrothermally reworked and re-enriched BIF-type ore deposit.

First U–Pb SHRIMP zircon and 40Ar/39Ar ages of metarhyolites from the Afyon–Bolkardag Zone, SW Turkey: Implications for the rifting and closure of the Neo-Tethys

The Afyon–Bolkardag Zone (ABZ) is one of the most important metamorphic zones in Turkey. This zone consists of metasedimentary and metavolcanic rocks, which have undergone regional greenschist facies metamorphism. The metavolcanic rocks are dominated by rhyolite, dacite, and trachyandesite. In the Ilgin (Konya) area, the low grade metavolcanics were originally rhyolitic lavas and tuffs. We present new precise Sensitive High-Resolution Ion MicroProbe (SHRIMP) 206Pb–238U age analyses of zircons and 40Ar/39Ar age data of phengite from metarhyolites that enable a more precise reconstruction of the ABZ. SHRIMP analyses of euhedral or sub-euhedral magmatic zircons from two metarhyolite samples yield weighted means 230±2Ma and 229±2Ma. These ages are interpreted as formation age of rhyolitic volcanics, implying they were formed in the Late Triassic time (Carnian). We suggest that these Upper Triassic magmatic rocks were formed in an extensional tectonic setting on the northern active margin of Gondwana, which led to the development of the northern branch of the Neo-Tethys. 40Ar/39Ar dating of two phengite samples from metarhyolites gave plateau ages of between 63.7±0.1Ma and 62.6±0.1Ma. These dates are interpreted as the metamorphic age of these volcanic rocks, and as the age of metamorphism that affected the ABZ. Consequently, these earliest Tertiary ages also constrain the timing of the closure of the Neo-Tethys.

The contact aureole of the South Mountain Batholith in Halifax, Nova Scotia: geology, mineral assemblages, and isograds

The contact aureole at the eastern margin of the South Mountain Batholith (Halifax Pluton) underlies most of the city of Halifax, Nova Scotia. The Halifax Group in the study area includes two lithological units, the Cunard and Bluestone formations. Before intrusion, both had been affected by greenschist facies regional metamorphism and deformed into northeast–southwest-trending, regional upright folds associated with a strong slaty cleavage. Contact metamorphic isograds trend obliquely across the Halifax peninsula, at a high angle to regional structural trends. At 2.5–3 km from the intrusive contact, sparse cordierite spots mark the outer limit of the contact aureole. The biotite-in isograd is marked by the development of biotite within chlorite + muscovite stacks inherited from regional metamorphism. Pyrrhotite is the dominant sulphide mineral throughout the contact aureole. With increasing metamorphic grade, assemblages in both units are marked by increasing modal abundance of cordierite and biotite, with K-feldspar variably developed within ca. 600 m of the contact. However, there is a marked difference in the distribution and appearance of andalusite between the two units. In aluminous pelites of the Cunard formation, idioblastic chiastolite appears before biotite more than 1500 m from the contact. In the less aluminous Bluestone formation, andalusite is present only within ca. 500 m of the contact, where it forms xenoblastic, spongy crystals. In both units, the assemblage andalusite + biotite + K-feldspar ± cordierite is developed near the contact, with local fibrolite and evidence of incipient partial melting. Petrographic constraints suggest pressure–temperature conditions at the contact of ca. 2.5–3.0 kbar (1 kbar = 100 MPa) at ca. 650 °C.

Extreme element mobility during transformation of Neoarchean (ca. 2.7 Ga) pillow basalts to a Paleoproterozoic (ca. 1.9 Ga) paleosol, Schreiber Beach, Ontario, Canada

This study reports petrographic, major and trace element, and oxygen isotopic data for a subaerial weathering profile at Schreiber Beach, Ontario, Canada. The weathering profile developed on Neoarchean (ca. 2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (ca. 1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation. Rocks at Schreiber Beach have been classified as pillow cores, pillow rims, hyaloclastites, weathered red basalts, and weathered brown to green basalts. There are gradual textural, mineralogical, and geochemical transitions from unweathered basalts to intensely weathered hematite-bearing basalts with stratigraphic height.Rocks at Schreiber Beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the Neoarchean. Following the subaerial weathering, they were buried and re-metamorphosed to greenschist facies in the Paleoproterozoic. Given the preservation of primary igneous textures and trace element patterns in pillow cores, they are used as a protolith to estimate element mobility during seafloor alteration, metamorphism, and subaerial weathering. Chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major (Si, Ca, Na, Fe, Ti, K) and trace (LILE, REE, HFSE, and transition metals) elements during Neoarchean seafloor alteration. Alteration of these rocks at relatively low temperatures is indicated by enrichment in 18O (δ18O=+10.4 to +12.7‰) relative to fresh basalt (~+5.5‰).Subaerial weathering resulted in significant textural, mineralogical (e.g., hematite precipitation), and geochemical variations in metamorphosed basalts, causing additional redistribution of most major and trace elements and further enrichment in 18O. The highest δ18O values (+16.1 to +17.1‰) occur at the silica-rich base of the weathering profile. Many elements, including LILE, REE and HFSE, are strongly enriched (>100%) or depleted (up to 96%) relative to their original abundances. Alkaline metasomatism, which is a characteristic of Precambrian paleosols, is also evident at Schreiber Beach, where extreme enrichment in K (280 to 1000%), Rb (390 to 660%), and Cs (1160 to 2560%) occurs in the weathering profile.

Origin and metamorphic evolution of magnesite-talc and adjacent rocks near Gemerská Poloma, Slovak Republic

*Corresponding author Magnesite and talc deposit Gemerska Poloma is located in Early Palaeozoic rocks of the Gemericum, Slovakia. It experienced both Variscan and Alpine greenschist-facies regional metamorphism and was overprinted by contact metamorphism connected with the intrusion of a Permian granite. As the origin of talc in Mg-carbonate rocks from this locality is interpreted as a result of either regional or contact metamorphism, relation of talc formation to these two processes is critically discussed. The estimated P–T conditions (6–7 kbar at 400 oC), reached during Alpine metamorphism, were obtained from the Permian metagranite that was not affected by the Variscan metamorphic event. The calculated P–T conditions were verified using various thermobarometers and pseudosections for biotite- and garnet-bearing assemblages in the metagranite.

Metamorphic evolution of the contact aureole of the Dipilto Batholith, Eastern Chortis Terrane, Nicaragua

*Corresponding author We examined the tectono-metamorphic evolution of the contact aureole around the Cretaceous Dipilto Batholith in the Eastern Chortis Terrane (Nicaragua, Central America). Three distinct metamorphic phases were distinguished. The first phase represented greenschist-facies regional metamorphism (M 1 ) accompanied by a prograde deformation (D 1 ). The Cretaceous contact-metamorphic event (M 2 ) produced the peak-metamorphic mineral assemblages Wo + Di + Grs + Ves + Qtz + Pl + Kfs in the calc-silicate rocks and Bt + Qtz + Ab + And ± Crd ± Gr ± Chl ± Ms ± Kfs in the metapelites. The estimated P–T conditions near the contact with granitoids are 500–650 °C and ~ 0.2 GPa and coexisting fluids were relatively poor in CO 2 (X CO2 below 0.15). The emplacement of the Dipilto Batholith at shallow crustal levels was rarely accompanied by weak deformation of the metasediments (D 2 ). During cooling and exhumation, the contact aureole was locally affected by deformation (D 3 ) and retrograde metamorphism (M 3 ) again under greenschist-facies conditions.

Flux rates for water and carbon during greenschist facies metamorphism

The time-averaged flux rate for a CO2-bearing hydrous fluid during greenschist facies regional metamorphism was estimated to be 10–10.2 ± 0.4 m3 m−2 s−1 by combining (1) Peclet numbers obtained by ...

Origin of Witwatersrand gold: a metamorphic devolatilisation–hydrothermal replacement model

Goldfields extend for 300 km around the margin of the Archaean Witwatersrand Basin of South Africa associated with regional greenschist facies metamorphism and deformation. Metamorphic mineral assemblages involving pyrophyllite-chloritoid are associated with gold in all goldfields, reflecting low pressure and 300 to 400°C conditions, and indicating high geothermal gradients. The origin of Witwatersrand gold can be explained by metamorphic devolatilisation to generate auriferous fluids beneath and outside the Witwatersrand Basin, followed by passage of these fluids along large structures and into the Basin. In this model, generation of the fluid is a consequence of the transition to amphibolite facies of mafic rocks such as Archaean greenstone belts, as in . In a grain-by-grain scale devolatilisation process, the lower temperature minerals have released their H2O, CO2 and H2S to form a metamorphic fluid which at the time of its formation has already dissolved gold from these unenriched rocks (say at the ∼2 ppb Au level). A substantial volume of auriferous fluid is inferred given that regional metamorphism is on the scale of thousands of cubic kilometres, all evolving a few per cent of fluid. In the model, this auriferous fluid migrated from the immediate source region along grain boundaries, then in shear zones, and into the Witwatersrand Supergroup via the major thrust faults adjacent to all goldfields. Once in the Supergroup, fluid migration was via bedding planes and especially unconformity surfaces, along cross-cutting faults, shear zones and reef packages. The few thicker shale units and especially the overlying Archaean Ventersdorp basalt pile acted as barriers to fluid flow and facilitated fluid focussing. Deposition of gold from this auriferous fluid occurred where there was abundant carbon and/or iron. The distribution of these two elements was pre-determined by sedimentary distribution, and diagenetic migration. Many bedding features were preserved or mimicked during the hydrothermal replacement process because the volumetric strain in most of the sequence was low. Critical to the success of the hydrothermal process in producing gold deposits were the widespread local concentrations of C and Fe on unconformity surfaces, the reef packages, the major thrust faults adjacent to each goldfield, source rocks of greenstone-like character that were once at (sub)-greenschist facies grade with assemblages that included chlorite–calcite–pyrite, and a regional metamorphic event of high geothermal gradient. Moreover, critical to the exposure and preservation of the goldfields has been the limited erosion of the Basin since the hydrothermal deposition of the gold. The metamorphic devolatilisation–hydrothermal replacement model predicts many features of Witwatersrand ores, and can explain quantitatively the origin of its ∼100 000 t gold endowment. Calculations suggest that there is adequate gold at background (2 ppb) levels in inferred source rocks to form the necessary auriferous hydrothermal fluids; in contrast, there appears to have been insufficient particulate gold to form a significant placer accumulation. Without a viable source region for detrital gold, and indeed no plausible sorting mechanism to produce such deposits, it would be unwise to accept a placer model.

Collision-related granitic magmatism in the Granites–Tanami Orogen, Western Australia

The Palaeoproterozoic Granites–Tanami Orogen (GTO) is a significant auriferous province located in the poorly exposed southwestern part of the North Australian Craton (NAC). U-Pb data from Sensitive High Mass Resolution Ion Microprobe (SHRIMP) studies, Sm–Nd isotopic data, geochemistry and petrological studies all suggest that granitic rocks in the Western Australian portion of the GTO were emplaced around ca. 1795 Ma broadly synchronous with gold mineralization. These are variably fractionated, peraluminous, and derived from, or interacted with, ca. 2600 Ma upper crustal material. Deformation, metamorphism and magmatism around 1795 Ma in the GTO were synchronous with the Stafford Orogeny in the Arunta Orogen of central Australia. This deformation was the product of north- to northeast-directed compression within the GTO, and is associated with greenschist facies regional metamorphism. Amphibolite facies metamorphism is restricted to the immediate aureole of certain granitic rocks. This process is clearly a thermal overprint of the regional greenschist facies metamorphism, and indicates that the granitic rocks were emplaced during peak regional metamorphism or soon after. The granitic rocks are inferred to have formed in response to convergence and amalgamation of the GTO and the northern part of the Arunta Orogen along an accretionary margin delineated by the Willowra Lineament, which extends for thousands of kilometres in central and western Australia. Synchronous ca. 1795 Ma orogenic gold mineralization and syn-collisional granitic rocks have been identified across extensive regions throughout the NAC (e.g. the Pine Creek and Halls Creek orogens) suggesting that the Palaeoproterozoic assembly of Australia was marked by a widespread, significant metallogenic event at that time.

Reactions destroying detrital monazite in greenschist-facies sandstones from the Witwatersrand basin, South Africa

Detrital monazite in sandstone and conglomerate from the Witwatersrand Supergroup, South Africa, displays reaction textures indicating that monazite has undergone replacement after sediment deposition, and most probably during regional greenschist-facies metamorphism. In sandstones with moderate- to high-Ca bulk compositions, detrital allanite is preserved in heavy mineral bands, whereas co-existing detrital monazite is replaced by secondary allanite, apatite and ThSiO 4. In sandstones with low-Ca bulk compositions, detrital monazite is replaced by florencite, apatite and ThSiO 4 or pseudomorphed by metamorphic monazite, apatite and Th-silicate. The monazite alteration phases are commonly intergrown with metamorphic quartz, chlorite and sericite. Large (~ 0.5 mm) grains of metamorphic monazite are present in the same samples as detrital monazite pseudomorphs. This suggests that REE are mobile on a scale of millimetres to centimetres in these rocks, and that detrital grains are a likely source of components for metamorphic monazite. In other low-Ca sandstones, monazite forms composite grains that comprise a core surrounded by a pore-filling, inclusion-rich rim. The monazite cores have pronounced negative Eu anomalies and higher concentrations of U, Th, Ca and Y than the rims. Previous SHRIMP U–Pb dating of the cores gives dates of ~ 3.0 Ga and ~ 2.85 Ga, indicating a detrital origin. The monazite rims lack Eu anomalies and generally have lower Th contents than the cores. They result from dissolution and reprecipitation of monazite, and the ages of the rims suggest that replacement occurred at 2.045 Ga in the West Rand goldfield and at 2.12 Ga in the Welkom goldfield. Our results indicate that detrital igneous monazite is unstable under mid-greenschist facies conditions (~ 350 °C) and may be replaced by low-Th metamorphic monazite, allanite, apatite, florencite and ThSiO 4, as well as matrix minerals such as quartz, chlorite and sericite.

The geology of the Neoproterozoic Swakop-Otavi transition zone in the Outjo District, northern Damara Orogen, Namibia

Research Article| March 01, 2008 The geology of the Neoproterozoic Swakop-Otavi transition zone in the Outjo District, northern Damara Orogen, Namibia Tom N. Clifford Tom N. Clifford School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, e-mail: cliffordt@geosciences.wits.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information Tom N. Clifford School of Geosciences, University of the Witwatersrand, Johannesburg, South Africa, e-mail: cliffordt@geosciences.wits.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2008, The Clay Minerals Society South African Journal of Geology (2008) 111 (1): 117–140. https://doi.org/10.2113/gssajg.111.1.117 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Tom N. Clifford; The geology of the Neoproterozoic Swakop-Otavi transition zone in the Outjo District, northern Damara Orogen, Namibia. South African Journal of Geology 2008;; 111 (1): 117–140. doi: https://doi.org/10.2113/gssajg.111.1.117 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The Neoproterozoic succession in the Outjo District of northern Namibia is a mainly sedimentary sequence that was deposited in the time-interval of approximately 800 to 600 Ma ago and was deformed and suffered mild greenschist facies regional metamorphism during Damaran times ca. 550 Ma ago. The map area occupies the lithofacies and tectonic transition from the highly deformed more eugeoclinal Swakop Group sequence to the south to the coeval, uniformly southward-dipping, miogeoclinal Otavi Group of the Fransfontein Ridge to the north. The oldest unit is a largely sandstone and biotite-muscovite phyllite succession tentatively equated with the volcanogenic Naauwpoort Formation (Nosib Group) that pre-dates the Swakop Group in the Summas Mountains to the southwest. The overlying Saturn Formation comprises two distinct but coeval lateral carbonate facies: a 300 m thick sequence of bedded dolomite with a 20 m thick median limestone; and massive biohermal dolomite of undetermined thickness. The conformably overlying Landeck Formation comprises a lower succession of interbedded sandstone, phyllite and local carbonate rocks, a distinctive middle iron formation marker horizon with rather rare overlying dolomite, followed by a siliciclastic-carbonate sequence. The combined succession is up to 450 m in thickness, and glacial diamictite is recorded at 30+ localities sited below, within and above the iron formation. However, in the more distal parts of the Landeck Formation in the Saturn Dome, glacial diamictictite has not been identified, the iron formation is overlain by a distinctive dolomite, and the uppermost 200 to 300 m of the Landeck Formation is characterized by four horizons of siliciclastic sediments with three intervening horizons of blue limestone that reflect the concordant transition to the overlying Bergfriede Formation. New chemical analyses are reported for graphitic dolomite, magnetite-calcite schist, iron formation, ferruginous limestone, dolomitic sandstone and biotite-muscovite phyllite of the Landeck Formation. The Bergfriede Formation consists almost entirely of limestone and dolomite and is characterized by major changes in facies and thickness, and five lateral facies end-members are recognized: I. Hankow Facies, a five-fold succession of limestone, dolomite and calcarenite up to 500 m thick; II. Tsuwandes-Zuwitsaub Facies, a three-fold sequence of limestone-dolomite-limestone >300 m thick; III. Uranus Facies, entirely dolomite generally 150 m thick; IV. Belina-Sophienhof Facies, a limestone-dolomite sequence, with a distinctive arenaceous limestone at the top, 100 to 400 m thick; and V. Mooilagte-Mooihoek Facies, largely dolomite-chert-limestone ≥1500 m thick, that is transitional to the dolomite-dominated upper Otavi Tsumeb Subgroup of the Fransfontein Ridge to the north. The overlying Okaua Formation is composed of a >3000 m thick sequence of muscovite-chlorite phyllite and calcareous sandstone that occupy, inter alia, the troughs of the regional Ugab and Uranus-Neuland Synclines; it is most likely correlative with the Mulden Group of the Otavi Mountainland. The Damaran regional structure of area is characterized by: D1 an early phase of southward-directed gravity nappes; D2 the regionally dominant northeast-southwest to east-west trending tight compressional upright or northward-verging folds, and local thrusts; and D3 late strike-slip faulting. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Copper mobility in the Eastern Creek Volcanics, Mount Isa, Australia: evidence from laser ablation ICP-MS of iron-titanium oxides

The Palaeoproterozoic Eastern Creek Volcanics are a series of copper-rich tholeiitic basalts which occur adjacent to the giant sediment-hosted Mount Isa copper deposit in Queensland, Australia. The volcanic rocks are often cited as the source of metals for the deposit. New laser ablation ICP-MS analyses of iron–titanium oxides from the basalts provide evidence for the local mobilisation of copper during regional greenschist facies metamorphism. This interpretation is based on the observation that copper-bearing magmatic titanomagnetite was destabilised during greenschist facies metamorphism, and the new magnetite which crystallised was copper poor. Petrological observations, regional geochemical signatures and geochemical modelling suggest that the mobilised copper was concentrated in syn-metamorphic epidote-rich alteration zones, creating a pre-concentration of copper before the main mineralisation event at Mount Isa. Geochemical modelling demonstrates this process is enhanced by the addition of CO2 from adjacent carbonate-rich sediments during metamorphic devolatilisation. Regional geochemical data illustrate elevated copper concentrations in epidote-rich zones (high CaO), but where these zones are overprinted by potassic alteration (high K2O), copper is depleted. A two-stage model is proposed whereby after metamorphic copper enrichment in epidote–titanite alteration zones, an oxidised potassium-rich fluid leached copper from the epidote-altered metabasalts and deposited it in the overlying sedimentary rocks to form the Mount Isa copper deposit. This ore-forming fluid is expressed regionally as potassium feldspar-rich veins and locally as biotite-rich alteration, which formed around major fluid conduits between the metabasalt metal source rocks and the overlying deposit host sequence. This model is consistent with the remobilisation of copper from mafic source rocks, as has been found at other world-class copper deposits.

Hydrothermal Alteration, Ore Fluid Characteristics, and Gold Depositional Processes along a Trondhjemite-Komatiite Contact at Tarmoola, Western Australia

Tarmoola is a structurally controlled Archean orogenic gold deposit hosted in greenschist facies metamorphosed komatiite and trondhjemite in the Leonora district of the Eastern Goldfields province, Yilgarn craton. High-grade (>1 g/t Au) orebodies are located in komatiite wall rock adjacent to the eastern and northeastern margins of the asymmetrical, north-south–striking, Tarmoola trondhjemite intrusion. Gold-bearing veins postdate trondhjemite emplacement (ca. 2700 Ma), quartz diorite dikes (ca. 2667 Ma), and regional greenschist facies metamorphism. Textures and crosscutting relationships in gold-bearing veins indicate two stages of hydrothermal fluid infiltration associated with a single gold-related hydrothermal event: a volumetrically dominant, but gold-poor, stage I fluid and a gold-rich stage II fluid. Gold-bearing veins contain stage I milky quartz and pyrite that are overprinted by stage II quartz-ankerite-muscovite-chalcopyrite-sphalerite-galena-gold-tellurides ± albite ± chlorite ± fuchsite ± epidote ± scheelite. Stage I hydrothermal alteration assemblages are different in trondhjemite and komatiite due to contrasting reactions between a common ore fluid and disparate wall-rock chemistry. Stage II fluid-wall rock interaction was minor compared to stage I and is indicated by the overprinting of stage I mineral assemblages by stage II microveins. Wall-rock alteration proximal to veins in trondhjemite is characterized by replacement of igneous plagioclase, amphibole, biotite, and metamorphic chlorite by hydrothermal quartz, muscovite, ankerite, calcite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold, whereas in proximal alteration in komatiite, metamorphic chlorite and talc are replaced by ankerite, quartz, muscovite, albite, chlorite, fuchsite, pyrite, chalcopyrite, sphalerite, galena, tellurides, and gold. The stage II fluid was enriched in H2O, CO2, Si, Ca, K, Na, S, Au, Ag, Cu, Pb, W, Bi, As, Mo, Zn, and Te. Based on fluid inclusion studies and stage II mineral equilibria, gold deposited from a homogeneous, neutral to slightly alkaline (pH 5.1–5.5), reduced, low-salinity (<5.5 wt % NaCl equiv) fluid that had a bulk composition of 78 mole percent H2O and 21 mole percent CO2, and trace amounts of CH4, C2H6, H2, Ar, H2S, and He. Gold deposition occurred at 300° ± 50°C and 0.5 to 3.0 kbars. Assuming lithostatic fluid pressures, gold precipitated at a 2- to 10-km depth. Stage II gray quartz δ 18Ofluid values range from 5.9 to 7.5 per mil, whereas δ Dfluid values calculated from the dehydration of muscovite grains and measured directly from bulk fluid inclusion analyses of stage II gray quartz have ranges of –9 to –35 and –27 to –28 per mil, respectively. Hydrothermal ore fluids were transported from greater crustal depths to the site of gold deposition during the district-scale D3 event by shallowly W dipping, reverse brittle-ductile shear zones in supracrustal rock and along the steeply east dipping trondhjemite contact. Associated subhorizontal east-west shortening caused the reactivation of the eastern trondhjemite margin and subparallel foliation, which facilitated the transport of hydrothermal fluids and the generation of gold-bearing veins and hydrothermal alteration zones in komatiite. East-west–striking fractures in trondhjemite aided the lateral migration of ore fluids away from trondhjemite margins and the formation of east-west–striking gold-bearing veins and broad alteration zones. Gold was most likely transported in the stage II fluid as bisulfide complexes. The sulfidation of trondhjemite and komatiite wall rock by the stage II fluid caused the destabilization of Au bisulfide complexes and gold deposition. Potassium, Ca, and CO2 metasomatism of komatiite wall rock may have enhanced gold deposition via the acidification of the stage II fluid. The physicochemical characteristics of the Tarmoola ore fluid and relative timing of gold mineralization are consistent with the Yilgarn-wide, 2650 to 2630 Ma gold event. The host-rock chemistry, vein, and proximal wall-rock alteration mineral assemblages, and fluid temperature and pressures during gold deposition are also similar to that of earlier (ca. 2755 Ma) gold deposits in the Leonora district.

Terrane assembly and geodynamic evolution of central–western Hoggar: a synthesis

After a review of the rock sequences and evolution of the eastern and central terranes of Hoggar, this paper focusses on the Neoproterozoic subduction-related evolution and collision stages in the central–western part of the Tuareg shield. Rock sequences are described and compared with their counterparts identified in the western and the eastern terranes exposed in Hoggar and northern Mali. The Pharusian terrane that is described in detail, is floored in the east by the Iskel basement, a Mesoproterozoic arc-type terrane cratonized around 840 Ma and in the southeast by Late Paleoproterozoic rock sequences (1.85–1.75 Ga) similar to those from northwestern Hoggar. Unconformable Late Neoproterozoic volcanosedimentary formations that mainly encompass volcanic greywackes were deposited in troughs adjacent to subduction-related andesitic volcanic ridges during the c. 690–650 Ma period. Abundant arc-related pre-collisional calc-alkaline batholiths (650–635 Ma) intruded the volcanic and volcaniclastic units at rather shallow crustal levels prior to collisional processes. The main E–W shortening in the Pharusian arc-type crust occurred through several stages of transpression and produced overall greenschist facies regional metamorphism and upright folding, thus precluding significant crustal thickening. It was accompanied by the shallow emplacement of calc-alkaline batholiths and plutons. Ages of syn-collisional granitoids range from 620 Ma in the western terranes, to 580 Ma in the Pharusian terrane, thus indicating a severe diachronism. After infill of molassic basins unconformable above the Pan-African greenschists, renewed dextral transpression took place in longitudinal domains such as the Adrar fault. The lithology, volcanic and plutonic suites, deep greenschist facies metamorphism, structures and kinematics from the Adrar fault molassic belt previously considered as Neoproterozoic are described in detail. The younger late-kinematic plutons emplaced in the Pharusian terrane at 523 Ma [Lithos 45 (1998) 245] relate to a Cambrian tectonic pulse that post-dates molasse deposition. The new geodynamic scenario presented considers several paleosubductions. The major east-dipping subduction, corresponding to the closure of a large Pan-African oceanic domain in the west (680–620 Ma) post-dates an older west-dipping “Pharusian” subduction (690–650 Ma?) to the east of the eastern Pharusian terrane. Such a diachronism is suggested by the 690 Ma old eclogites of the western part of the LATEA terrane of central Hoggar [J. African Earth Sci. this volume (2003)] that are nearly synchronous with the building up of the Pharusian terrane, thus suggesting that the 4°50 ′ lithospheric fault represents a reactivated cryptic suture.

Tectonic Setting of the Permo-Triassic Chiang Khong Volcanic Rocks, Northern Thailand Based on Petrochemical Characteristics

The inferred Permo-Triassic Chiang Khong volcanic belt is composed of felsic to mafic volcanic rocks and their pyroclastic equivalents. Almost all the least-altered mafic volcanic rocks are lava flows; a few might have occurred as dykes. These mafic volcanic rocks are non-foliated to weakly foliated, and mostly have porphyritic textures. The phenocrysts/microphenocrysts in porphyritic samples are commonly plagioclase, and may include clinopyroxene, olivine, Fe-Ti oxide, apatite and amphibole. The matrix of lava flows ranges texturally from felty to trachytic but a few samples show felty to ophitic/subophitic, and glassy textures, whereas that of possible dyke samples is holocrystalline. The primary matrix constituents are largely plagioclase and variable proportions of clinopyroxene, Fe-Ti oxide, amphibole, olivine, apatite, quartz, alkali feldspar and/or glass. All the studied samples have been subjected to greenschist-facies regional metamorphism. Chemically, the samples show narrow ranges of least-mobile incompatible-element ratios and range compositionally from dacite to basalt of tholeiitic series. These samples are chemically analogous to those of the Tertiary andesite from Sardinian Rift, Sardinia, Italy, particularly in terms of least-mobile incompatible-element ratios. Accordingly, the studied mafic volcanic rocks are interpreted to have formed in a continental volcanic arc. However, the problem related to the geometry of plate convergence, giving rise to the continental volcanic arc, still exists.

Late Archaean to Palaeoproterozoic geotherms in the Kaapvaal craton, South Africa: constraints on the thermal evolution of the Witwatersrand Basin

ABSTRACTThe c. 2.97–2.71 Ga Witwatersrand Basin located in the Kaapvaal craton of South Africa represents a remnant of a large Late Archaean sedimentary basin that hosts the world's premier gold deposit within a series of conglomerate horizons. Evidence of postdepositional gold mobility within these conglomerates associated with hydrothermal–metamorphic activity has led to speculation about the Late Archaean to Palaeoproterozoic geothermal gradients in the basin. We use surface heat flow and heat production data from rocks in the basin and its environs in order to calculate detailed temperature profiles for the central Kaapvaal craton that show that the steady state crustal geotherm during the Late Archaean and Palaeoproterozoic was relatively cool at 15–20 K km−1. The geotherm in the upper crustal strata is also largely unaffected by substantial increases in the heat flow into the base of the crust. Consequently, regional greenschist facies metamorphism of the basin sediments could only have been achieved during a transient thermal event that advected heat into the upper crust. The most likely candidate for this is the Bushveld magmatic event at 2.06 Ga.

Greenschist facies regional and contact metamorphism of the Thalanga volcanic-hosted massive sulfide deposit (northern Queensland, Australia)

The conditions of two separate greenschist facies metamorphic events have been reconstructed at the syn-genetic Thalanga massive sulfide deposit (northern Queensland, Australia) using scarce garnet–biotite- and garnet–chlorite-bearing samples from the felsic volcano-sedimentary host-rock succession. Regional metamorphism (peak temperature: 440 to 480 °C) generated a foliation defined by aligned chlorite, muscovite and biotite, and rare, euhedral spessartine–almandine(-grossular)-rich garnets. The emplacement of a post-tectonic diorite pluton was associated with contact metamorphism and scarce anhedral spessartine–almandine-rich garnet poikiloblasts crystallized within a ~650-m wide aureole at temperatures of ~500 °C. Both metamorphic stages occurred under pressure conditions of ≤3.5 kbar. Our study documents a useful method for geothermobarometric reconstruction of metamorphosed, syn-genetic ore deposits where calculations based on sulfide assemblages failed to produce unequivocal constraints. Furthermore, we show that hydrothermally altered footwall rhyolite have higher \( {\rm X}_{{\rm Mg}}^{{\rm chlorite}} \;{\rm and}\;{\rm X}_{{\rm Mg}}^{{\rm biotite}} \) than unaltered rocks, which may help to identify fossil, metamorphosed, hydrothermal systems associated with massive sulfide deposits similar to Thalanga.

Oceanic and orogenic fluid–rock interaction in 18O/ 16O-enriched metagabbros of an ophiolite (Tinos, Cyclades)

Two spatial scales of fluid–rock interaction in an ophiolite suite are revealed by oxygen isotope and hydrogen isotope studies of metagabbros on the island of Tinos (Cyclades, Greece). Sequentially formed mineral generations in the metagabbros include relict igneous augite, hornblende of sub-seafloor hydrothermal origin, and actinolite and albite formed by regional greenschist-facies metamorphism during orogenesis. With the exception of augite (δ 18O=4.4–5.6‰), the metagabbros are characterized by unusually high δ 18O values: hornblende (5.8–7.4‰), actinolite (6.5–10.2‰), feldspar (14.6–14.9‰) and whole rocks (7.0–10.5‰). Hornblende δ D values range from −57 to −66‰. The high δ 18O values and the δ D range of the hornblendes are compatible with interaction of oceanic gabbro with seawater that had previously been enriched in 18O/ 16O (δ 18O=6.5–8‰) by isotopic exchange at moderate to high temperatures. The high degree of oceanic alteration in the layered gabbros, mass balance calculations of isotopic exchange, and field evidence for early oceanic thrusting suggest that seawater could have penetrated deeply into the ocean crust, becoming 18O/ 16O-enriched through isotopic exchange with gabbros at progressively increasing temperature. Upward, down-temperature flow of the high-δ 18O water would be very effective in elevating the δ 18O values of gabbros. The regional greenschist metamorphic overprint of the ophiolite, possibly the result of continued thrusting and piling up of nappes during obduction, is characterized by localized fluid–rock exchange. Actinolite in massive gabbroic layers has δ 18O values (6.5–7.2‰) close to those of the hornblende, whereas in deformed meter-sized gabbroic blocks the amphiboles have significantly higher values (8.4–10.2‰). Likewise, albite in the gabbroic blocks has high δ 18O values of ca. 15‰ that are ascribed to meter-scale exchange with 18O-rich fluids derived from dehydration reactions in low-temperature hydrothermally altered basaltic host rock enclosing the blocks. Deformation-enhanced permeability facilitated fluid infiltration in gabbroic blocks, whereas the relatively undeformed, and therefore less permeable, massive gabbros experienced minor interaction with fluids. The orogenic fluid–rock interaction thus represents local-scale redistribution of hydrous mineral components introduced during seafloor hydrothermal exchange.

A chemical model for the Devonian remobilization process in the Cambrian volcanic-hosted massive sulfide Rosebery Deposit, western Tasmania

The Rosebery deposit in western Tasmania is a polymetallic massive sulfide deposit hosted in felsic volcanics of the Cambrian Mount Read Volcanic belt. The deposit underwent upper greenschist facies regional metamorphism and related deformation during the Devonian Tabberabberan orogeny, resulting in folding, shearing, and faulting (thrusting) of the ore lenses. The south end of the Rosebery deposit has undergone metasomatic replacement related to a postorogenic Devonian granitoid pluton, whose existence is inferred from geophysical (gravity) data. Metasomatic mineral assemblages related to this pluton have been recognized transgressing Cambrian volcanic-hosted massive sulfide minerals at the south end of the Rosebery orebody. Cambrian volcanic-hosted massive sulfide mineralization at Rosebery consists of three primary zones: a lowermost pyrite-chalcopyrite zone, overlain by a sphalerite-galena ± pyrite zone, and further overlain by a massive barite ± carbonate zone. Three major Devonian replacement zones are also present (1) a magnetite-biotite ± chalcopyrite zone, (2) a pyrrhotite-pyrite zone, and (3) a tourmaline-quartz ± magnetite zone. Minerals such as fluorite, gamet, and helvite are also present in the replacement zones. Metal zonation studies indicate that zinc occurs dominantly in a blanketlike enrichment zone in the primary sulfide lenses of the Rosebery deposit. Gold is largely concentrated at the top of the ore lenses. In the transgressive pyrrhotite-pyrite replacement zone at the south end of the Rosebery deposit, zinc is conspicuously depleted but gold concentrations of more than 20 glt are observed. In contrast, the biotite-, magnetite-, and tourmalinebearing replacement zones contain low gold grades (generally less than 5.0 glt Au). Copper concentrations as high as 2 percent are observed in the biotite-magnetite zone, as well as in the pyrrhotite-pyrite zone. Microthermometry and laser Raman spectroscopy studies of fluid inclusions from tlle Devonian replacement zone indicate that the early biotite-magnetite and pyrrhotite-pyrite assemblages formed from the interaction of moderate- to high temperature (>330°C), saline (10-25 wt % NaCl equiv), CO2-bearing fluid with the original stratiform lead-zinc mineralization. The later stage tourmaline veining and associated replacement assemblages resulted from lower temperature (<300°C), less saline (<10 wt % NaCl equiv) fluid. The high-temperature, high-salinity, and CO2-bearing metasomatic fluid support the hypothesis of derivation of this fluid from Devonian granite below the Rosebery deposit. During the Devonian metasomatic event, gold and, to a lesser extent, copper were most likely remobilized as chloride complexes because of the high temperature and high salinity of the fluid. In comparison to copper and gold, zinc and lead were dissolved and removed, probably as chloride complexes.