U redox fronts and kaolinisation in basement-hosted unconformity-related U ores of the Athabasca Basin (Canada): late U remobilisation by meteoric fluids

Proterozoic basement-hosted unconformity-related uranium deposits of the Athabasca Basin (Saskatchewan, Canada) were affected by significant uranium redistribution along oxidation–reduction redox fronts related to cold and late meteoric fluid infiltration. These redox fronts exhibit the same mineralogical and geochemical features as the well-studied uranium roll-front deposits in siliclastic rocks. The primary hydrothermal uranium mineralisation (1.6–1.3 Ga) of basement-hosted deposits is strongly reworked to new disseminated ores comprising three distinctly coloured zones: a white-green zone corresponding to the previous clay-rich alteration halo contemporaneous with hydrothermal ores, a uranium front corresponding to the uranium deposition zone of the redox front (brownish zone, rich in goethite) and a hematite-rich red zone marking the front progression. The three zones directly reflect the mineralogical zonation related to uranium oxides (pitchblende), sulphides, iron minerals (hematite and goethite) and alumino-phosphate-sulphate (APS) minerals. The zoning can be explained by processes of dissolution–precipitation along a redox interface and was produced by the infiltration of cold (<50°C) meteoric fluids to the hydrothermally altered areas. U, Fe, Ca, Pb, S, REE, V, Y, W, Mo and Se were the main mobile elements in this process, and their distribution within the three zones was, for most of them, directly dependent on their redox potential. The elements concentrated in the redox fronts were sourced by the alteration of previously crystallised hydrothermal minerals, such as uranium oxides and light rare earth element (LREE)-rich APS. The uranium oxides from the redox front are characterised by LREE-enriched patterns, which differ from those of unconformity-related ores and clearly demonstrate their distinct conditions of formation. Uranium redox front formation is thought to be linked to fluid circulation episodes initiated during the 400–300 Ma period during uplift and erosion of the Athabasca Basin when it was near the Equator and to have been still active during the last million years. A major kaolinisation event was caused by changes in the fluid circulation regime, reworking the primary uranium redox fronts and causing the redistribution of elements originally concentrated in the uranium-enriched meteoric-related redox fronts.     

Geochronology of unconformity-related uranium deposits in the Athabasca Basin,Saskatchewan, Canada and their integration in the evolution of the basin

The importance of geochronology in the study of mineral deposits in general, and of unconformity-type uranium deposits in particular, resides in the possibility to situate the critical ore-related processes in the context of the evolution of the physical and chemical conditions in the studied area. The present paper gives the results of laser step heating ⁴⁰Ar/³⁹Ar dating of metamorphic host-rock minerals, pre-ore and syn-ore alteration clay minerals, and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U/Pb dating of uraninite from a number of basement- and sediment-hosted unconformity-related deposits in the Athabasca Basin, Canada. Post-peak metamorphic cooling during the Trans-Hudson Orogen of rocks from the basement occurred at ca 1,750 Ma and gives a maximum age for the formation of the overlying Athabasca Basin. Pre-ore alteration occurred simultaneously in both basement- and sandstone-hosted mineralizations at ca 1,675 Ma, as indicated by the ⁴⁰Ar/³⁹Ar dating of pre-ore alteration illite and chlorite. The uranium mineralization age is ca 1,590 Ma, given by LA-ICP-MS U/Pb dating of uraninite and ⁴⁰Ar/³⁹Ar dating of syn-ore illite, and is the same throughout the basin and in both basement- and sandstone-hosted deposits. The mineralization event, older than previously proposed, as well as several fluid circulation events that subsequently affected all minerals studied probably correspond to far-field, continent-wide tectonic events such as the metamorphic events in Wyoming and the Mazatzal Orogeny (ca 1.6 to 1.5 Ga), the Berthoud Orogeny (ca 1.4 Ga), the emplacement of the McKenzie mafic dyke swarms (ca 1.27 Ga), the Grenville Orogeny (ca 1.15 to 1 Ga), and the assemblage and break-up of Rodinia (ca 1 to 0.85 Ga). The results of the present work underline the importance of basin evolution between ca 1.75 Ga (basin formation) and ca 1.59 Ga (ore deposition) for understanding the conditions necessary for the formation of unconformity-type uranium deposits. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Hydrothermal alteration of monazite in the Precambrian crystalline basement of the Athabasca Basin (Saskatchewan, Canada): implications for the formation of unconformity-related uranium deposits

Microanalytical studies of basement rocks below the Athabasca sandstone basin indicate that monazite is the dominant uranium-bearing mineral in the study area. Drill core samples of hydrothermally altered basement show that monazite is commonly altered to a Th–silicate phase, and uranium has been significantly mobilized. On average, 75% of the uranium bound to monazite is leached out during monazite alteration. In contrast, no substantial mobilization of uranium from detrital minerals (e.g. zircon) has yet been observed in the Athabasca sandstones. It is suggested that hydrothermal alteration of granitic rocks (especially potassic pegmatoids and potassic orthogneisses) of the sub-Athabasca basement, represents the most important uranium source for the unconformity-type deposits. Received: 3 December 1999 / Accepted: 24 May 2000     

Chemical brecciation processes in the Sue unconformity-type uranium deposits, Eastern Athabasca Basin (Canada)

Chemical brecciation in sandstone is common in many unconformity-type uranium deposits of the Athabasca Basin, and is expressed in some of them as ball zone breccia. Ball zones are composed of rounded argillized sandstone fragments, varying in size from several centimeters to 1 m, wrapped in a clay matrix. The Sue C open pit provided a unique opportunity to map and to study such ball zones. Here, they were up to 5 m wide with a 20–30 m vertical extension. They were mainly observed along a reverse fault controlling the Sue A and B uranium deposits, and were well developed at intersections with dextral NE-trending structures. Their maturity, characterized by the matrix percentage, increased toward the unconformity and at fault intersections. They are characterized by massive quartz dissolution, hematite leaching, (Ca,Sr,LREE)Al-phosphates crystallization and replacement of dickite by illite. Illite composition indicates formation temperatures of 240–280 °C, close to peak diagenesis conditions in the basin. Mass balance calculations show that V, K, Rb, B, LREE, Mg, Cr, Sr, U and Y were added and Si and Fe leached out with up to 85% volume loss.Ball zones were initiated by tectonic fracturing in sandstone during reverse faulting. Consecutive permeability increase induced basement fluid circulation in the sandstone with quartz dissolution along fractures. With a silica saturation of the fluid of 90%, a minimum fluid/rock ratio of 38,000 is obtained. The rounded morphologies of the breccia fragments are attributed to a diffusion-limited regime of dissolution. The resulting increase of clay content led to self-sealing of the hydrothermal system. Seismic reactivation may have been periodically rejuvenated the permeability. These processes seem to be coeval with the formation of structurally controlled high-grade unconformity-type uranium mineralization. Formation of the ball zones required probably more than 1 million years.     

Migration of brines in the basement rocks of the Athabasca Basin through microfracture networks (P-Patch U deposit,Canada)

The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U deposits in the world, are the result of massive basinal fluid migrations in the basement rocks.This study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths (hundred metres below the unconformity) and their chemical modification (salinity increase) by interaction with basement lithologies. These processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity-type mineralization.Microfracture networks, which opened during the basinal brine stage (ca. 1600–1400 Ma) are interpreted as sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing. The stress field at that stage (σ₁ = N130–150 °E, subvertical) partly reopened the earlier microcrack networks (σ₁ = N80–110 °E and N130–150 °E, subvertical) issued from the Trans-Hudson Orogeny late retrograde metamorphic stage (ca. 1795–1720 Ma). The circulation of the two types of fluids (carbonic and brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity-related uranium mineralization in the Athabasca Basin.     

Near-surface lithogeochemical halo as an aid to discovery of deeply buried unconformity-type uranium deposits, Athabasca Basin, Canada

Unconformity-type U deposits are being found to depths of more than 400 m where graphitic rocks underlie the Proterozoic sandstone unconformity. Exploration to date involves drilling weak electromagnetic conductors that reflect the graphitic basement. The deposits are believed to have formed through circulation of heated formation waters which discharged essentially vertically above the deposits. Geochemically detectable zones of alteration are reported for cross-sections through the Deilmann, Midwest, and McClean deposits. Information on the areal dimensions of near-surface haloes was required to justify the drilling of grids of shallow holes along conductors or favourable structures as a cost-effective exploration alternative to deep holes, and to define grid spacings.Orientation surveys were undertaken by sampling the uppermost 10 m of sandstone in a selection of holes from the Cigar Lake, Dawn Lake, and Wolf Lake areas where the deposits are, respectively, 400–430 m, 100–160 m, and 180–210 m below surface. The first two deposits have published reserves of 385 and 30–40 million pounds U₃O₈, respectively, while Wolf Lake is much smaller.Well-developed anomalies lie vertically above the richest portion of the Cigar Lake deposit in an area 150 × 600 m and a weak anomaly in many elements continues over the 1800-m length that was sampled. Total and leachable U are 13 to 8 times background with maximum values of 53 and 26 ppm, respectively. Anomalies of 14 to 8 times background are also present for V, Na₂O and Zn, and 5 to 3 times background for Sr, Y, Pb, Th, As, and P.Of the four Dawn Lake mineralized zones, only the 11B Zone was studied, with partial coverage of the 11A Zone. Anomalies 50–100 m by 250–600 m in size are often displaced up to 100 m northeast of the 11B Zone. B, MgO, Pb and both total and leachable U anomalies are 16 to 8 times background with Y, V and As 7 to 5 times background.By contrast, at the Wolf Lake deposit, only irregular variations occur for most of these elements. The unusual patterns may reflect post-alteration tectonism of the sandstone overlying the deposit as regional thrust faulting occurs nearby.Detection of near-surface lithogeochemical haloes using grids of shallow holes would facilitate discovery of deep deposits and reduce expensive, deep drilling.     

Nanoscale relationships between uranium and carbonaceous material in alteration halos around unconformity-related uranium deposits of the Kiggavik camp,Paleoproterozoic Thelon Basin,Nunavut, Canada

Concentrations of 7% U and 1% Cu were identified in massive, brecciated, and amorphous carbonaceous materials (CM) characterized by strongly negative values of carbon stable isotopes (δ¹³C = − 39.1‰ relative to PDB). The anomalies are restricted to clay alteration halos developed in Neoarchean Woodburn Lake group metagreywacke that is the predominant host of unconformity-related uranium (U) deposits in the Kiggavik exploration camp. Petrographic and microstructural analyses by SEM, X-ray Diffraction, HRTEM and RAMAN spectroscopy identified carbon veils, best described as graphene-like carbon, upon which nano-scale uraninite crystals are distributed. CMs are common in U systems such as the classic Cretaceous roll-front deposits and the world-class Paleoproterozoic unconformity-related deposits. However, the unusual spatial and textural association of U minerals and CM described herein raises questions on mechanisms that may have been responsible for the precipitation of the CM followed by crystallization of U oxides on its surfaces. Based on the characteristics presented herein, the CMs at Kiggavik are interpreted as hydrothermal in origin. Furthermore, the nanoscale organization and properties of these graphene-like layers that host U oxide crystallites clearly localized U oxide nucleation and growth.     

The role of the thermal convection of fluids in the formation of unconformity-type uranium deposits: the Athabasca Basin,Canada

In the global production of uranium, ~18% belong to the unconformity-type Canadian deposits localized in the Athabasca Basin. These deposits, which are unique in terms of their ore quality, were primarily studied by Canadian and French scientists. They have elaborated the diagenetic–hydrothermal hypothesis of ore formation, which suggests that (1) the deposits were formed within a sedimentary basin near an unconformity surface dividing the folded Archean–Proterozoic metamorphic basement and a gently dipping sedimentary cover, which is not affected by metamorphism; (2) the spatial accommodation of the deposits is controlled by the rejuvenated faults in the basement at their exit into the overlying sedimentary sequence; the ore bodies are localized above and below the unconformity surface; (3) the occurrence of graphite-bearing rocks is an important factor in controlling the local structural mineralization; (4) the ore bodies are the products of uranium precipitation on a reducing barrier. The mechanism that drives the circulation of ore-forming hydrothermal solutions has remained one of the main unclear questions in the general genetic concept. The ore was deposited above the surface of the unconformity due to the upflow discharge of the solution from the fault zones into the overlying conglomerate and sandstone. The ore formation below this surface is a result of the downflow migration of the solutions along the fault zones from sandstone into the basement rocks. A thermal convective system with the conjugated convection cells in the basement and sedimentary fill of the basin may be a possible explanation of why the hydrotherms circulate in the opposite directions. The results of our computations in the model setting of the free thermal convection of fluids are consistent with the conceptual reasoning about the conditions of the formation of unique uranium deposits in the Athabasca Basin. The calculated rates of the focused solution circulation through the fault zones in the upflow and downflow branches of a convection cell allow us to evaluate the time of ore formation up to the first hundreds of thousands years.     

Permo–Triassic unconformity-related Au-Pd mineralisation, South Devon, UK: new insights and the European perspective

An integrated mineralogical-geochemical study of unconformity-related Au-Pd occurrences within and around the Permo–Triassic basins of southwest England, UK, has confirmed the importance of low temperature (86±13°C), hydrothermal carbonate veins as hosts for the mineralisation. Fluid inclusion data for the carbonate gangue, supported by stable isotope (¹³C and ¹⁸O) and radiogenic (⁸⁷Sr/⁸⁶Sr) data, have identified three principal fluids: (1) a reducing calcic brine [>25 wt% salinity, <0.5 NaCl/(NaCl+CaCl₂)] originating in the sub-unconformity basement and an expression of advanced mineral–fluid interaction; (2) an oxidising sodic brine [~16 wt% salinity, >0.9 NaCl/(NaCl+CaCl₂)] originating in the post-unconformity red beds under evaporitic conditions, and (3) an oxygenated, low salinity groundwater (<3 wt% salinity). The sodic brine is reasoned to be the parent metalliferous fluid and to have acquired its enrichment in Au and Pd by the leaching of immature sediments and intra-rift volcanic rocks within the local Permo–Triassic basins. Metal precipitation is linked to the destabilisation of Au and Pd chloride complexes by either mixing with calcic brines, dilution by groundwaters or interaction with reduced lithologies. This explains the diversity of mineralised settings below and above the unconformity and their affinity with red bed brines. The paucity of sulphide minerals, the development of selenides (as ore minerals and as mineral inclusion in gold grains), the presence of rhodochrosite and manganoan calcites (up to 2.5 wt% Mn in calcite) and the co-precipitation of hematite and manganese oxides are consistent with the overall high oxidation state of the ore fluids. A genetic model is proposed linking Permo–Triassic red beds, the mixing of oxidising and reducing brines, and the development of unconformity-related precious metal mineralisation. Comparison with other European Permo–Triassic basins reveals striking similarities in geological setting, mineralogy and geochemistry with Au, Au-Pd and selenide occurrences in Germany (Tilkerode, Korbach-Goldhausen), Poland (Lubin) and the Czech Republic (Svoboda nad Úpou and Stupná). Though the known Au-Pd occurrences are sub-economic, several predictive criteria are proposed for further exploration.Editorial handling: B. Lehmann     

Easily altered minerals and reequilibrated fluid inclusions provide extensive records of fluid and thermal history: gypsum pseudomorphs of the Tera Group, Tithonian-Berriasian, Cameros Basin

This study reports a complex fluid and thermal history using petrography, electron microprobe, isotopic analysis and fluid inclusions in replacement minerals within gypsum pseudomorphs in Tithonian-Berriasian lacustrine deposits in Northern Spain. Limestones and dolostones, formed in the alkaline lakes, contain lenticularly shaped gypsum pseudomorphs, considered to form in an evaporative lake. The gypsum was replaced by quartz and non-ferroan calcite (Ca-2), which partially replaces the quartz. Quartz contains solid inclusions of a preexisting non-ferroan calcite (Ca-1), anhydrite and celestine. High homogenization temperatures (T _h ) values and inconsistent thermometric behaviour within secondary fluid inclusion assemblages in quartz (147?C351°C) and calcite (108?C352°C) indicate high temperatures after precipitation and entrapment of lower temperature FIAs. Th are in the same range as other reequilibrated fluid inclusions from quartz veins in the same area that are related to Cretaceous hydrothermalism. Gypsum was replaced by anhydrite, likely during early burial. Later, anhydrite was partially replaced by Ca-1 associated with intermediate burial temperatures. Afterward, both anhydrite and Ca-1 were partially replaced by quartz and this by Ca-2. All were affected during higher temperature hydrothermalism and a CO₂-H₂O fluid. Progressive heating and hydrothermal pulses, involving a CO₂-H₂O fluid, produce the reequilibration of the FIAs, which was followed by uplift and cooling.     

Some problems concerning the study of fluid inclusions in porphyry copper (molybdenum) deposits of China

This paper reviews the types, homogenization temperatures, salinities, compositions, boiling and formation mechanisms of inclusions in porphyry copper (molybdenum) depositi of China. In addition, it discusses, in particular, pH conditions during quartz sericitization and potash-silicate alteration. If the homogenization temperatures, KC1 and NaCl concentrations and chemical compositions of K-feldspar and sericite are obtained, pH and a_k+ can be worked out with the equili brium equations and expressions. Research on the Yulong, Malasongduo and Dexing porphyry copper (molybdenum) deposits reveals that at the stage of quartz sericitization, potash silicate alteration and ore deposition, the activity values ofK′ in ore solutions are 0.1-25 mole, averaging 0.67 mole, and pH values range from 4.0–4.7.     

Radiogenic lead mobility in the Shea Creek unconformity-related uranium deposit (Saskatchewan,Canada): migration pathways and Pb loss quantification

The average Pb/U ratio of the Shea Creek unconformity-type uranium deposit has been estimated at 0.071±0.015. The calculation was performed on a volume enclosing the orebody to take into account the possible radiogenic lead migration within the ore zone. Despite this precaution, this ratio is significantly lower than the expected ratio (0.211) assuming a main U deposition around 1315 Ma, as suggested by previous UPb isotopic dating. Although part of the radiogenic lead can be trapped as galena within the orebody, about 60% of Pb have migrated more than 700 m away from the orebody, preferentially along the unconformity. To cite this article: P. Kister et al., C. R. Geoscience 336 (2004).     

New insights into nanomineralogy and geochemistry of Ni-laterite ores from central Greece (Larymna and Evia deposits)

Nickeliferous laterite ores from two typical central Greece deposits (Larymna and Evia), currently used in the LARCO GMMSA smelting plant to produce ferronickel, were characterized using a combination of diffraction, microscopic, and analytical techniques. X-ray diffraction patterns of various fractions, with emphasis to the clayey material (<2 μm), after glycolation and heating at 550 °C, indicated that both materials contain crystalline Fe³⁺ oxide (hematite) and chlorite-group phyllosilicates, whereas the Evia sample contains additional illite. Transmission electron microscopy investigations revealed that the LARCO laterite ores consist of complex nanoscale aggregates of the above-mentioned phases. Different Ni-bearing Mg-Fe-phyllosilicates (mainly chlorite-group minerals), occur in mixture with hematite. Nickel is present in all examined phases, and therefore the separation of pure Ni-phases, by physical or chemical methods, is practically unfeasible. Trace element bulk analyses showed that there no significant differences, with regarding to Ni content concentrations (ca. 0.6–0.7 wt.%), between the initial ore and the clay fraction, for both the Larymna and Evia laterites (ca. 30% and 26% wt.% enrichment respectively). However, the Larymna ore contains double quantities of Co and it is enriched in rare earths compared to Evia (ΣREE = 774 ppm and 76 ppm respectively), while Sc concentrations are comparable in both mining areas (64 ppm and 42 ppm respectively). Discrimination diagrams (e.g. Th/Sc vs. Zr/Sc and Ce/Ce* vs. Eu/Eu*) showed that LARCO laterite Ni-ores do not exclusively originate in ultrabasic -ophiolitic- rocks as previously considered.     

Hydrocarbon-bearing fluid inclusions in the Drau Range (Eastern Alps, Austria): implications for the genesis of Bleiberg-type Pb-Zn deposits

Summary Hydrocarbons in ore minerals of Bleiberg-type Pb-Zn deposits and in authigenic quartz of the Drau Range (Eastern Alps) have been analysed by bulk-sample gas chromatography and fluorescence microscopy. Microthermometric data and the molecular composition of the hydrocarbons indicate a hot (120° to 130°C) pulse of migrating condensate-like hydrocarbons within Late Triassic sediments during Middle Cretaceous to Early Tertiary times. The molecular composition of hydrocarbons enclosed in authigenic quartz corresponds to the composition of hydrocarbons trapped in Bleiberg-type ore minerals, therefore a common fluid source is assumed. The hydrocarbons possibly played an important role as reducing agents in the late stage of sulfide precipitation.

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Kohlenwasserstoffhältige Flüssigkeitseinschlüsse im Drauzug (Ostalpen, Österreich) und ihre Bedeutung für die Genese der Pb-Zn-Lagerstätten vom Bleiberg-Typ

Zusammenfassung Die organo-chemische Zusammensetzung von Kohlenwasserstoffen, die in Erzmineralen vom Typus Bleiberg and in authigenen Quarzen des Drauzuges (Ostalpen) eingeschlossen sind, wurde durch gaschromatographische and fluoreszenzmikroskopische Untersuchungen charakterisiert. Mikrothermometrische Daten und die molekulare Zusammensetzung dieser Kohlenwasserstoffphasen weisen auf migrierende kondensatartige Kohlenwasserstoffe in den Sedimenten der Obertrias während des Zeitraumes zwischen der Mittelkreide und dem Alttertiär hin. In Erzmineralen der Pb-Zn-Vererzungen vom Bleiberg-Typ können Kohlenwasserstoffe nachgewiesen werden deren molekulare Zusammensetzung dem Chemismus von Kohlenwasserstoffen, die in authigenen Quarzen eingeschlossen wurden, entspricht. Es wird vermutet, daß diese Kohlenwasserstoffe während der späten Vererzungsphase als Reduktanten wirkten.

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With 6 Figures     

New insights into the origin of the Evate apatite-iron oxide-carbonate deposit,Northeastern Mozambique,constrained by mineralogy,textures, thermochronometry,and fluid inclusions

The Evate deposit represents the largest resource of apatite in south-east Africa (155 Mt. ore grading 9.3 wt.% P₂O₅) accumulated in up to 100 m thick magnetite-carbonate-apatite horizons conformable to the granulitic gneiss of the Monapo Klippe. Baddeleyite and zircon from early iron-oxide (magnetite, geikielite, spinel), apatite- and forsterite-bearing rocks have been dated to 590 ± 6 Ma using the LA-ICPMS U-Pb method, whereas monazites from anhydrite-apatite-carbonate rocks show a concordant U-Pb-Th age corresponding to 449 ± 2 Ma. Temperatures inferred from calcite-dolomite solvus data and graphite structural ordering span the interval from ≥ 815 to 276 °C. Primary and secondary fluid inclusions in apatite document calciocarbonatite melts associated with early apatite, and CO₂-bearing sulfate-chloride brines progressively diluted with low-salinity, probably metoric waters, towards ultimate stages of the deposit formation. The calciocarbonatite melts have initially coexisted with liquid nitrogen and later with sulfate-chloride brines mixed with N₂ ± CO₂ gas.Crystallization of spinel around baddeleyite by the mechanism of Ostwald ripening, nucleation of graphite spherules along pyrrhotite-carbonate boundaries, the occurrence of molybdenite, baddeleyite-to-zircon transformation, and high crystallization temperatures inferred from graphite structural ordering and calcite-dolomite thermometry suggest a magmatic origin of the early mineral assemblages. In contrast, microthermometric characteristics of primary aqueous inclusions in the late apatite and the presence of zeolites (thomsonite-Ca, mezolite) is diagnostic of a low-temperature hydrothermal crystallization.Formation of the early magnetite-apatite-forsterite assemblage is thought to be coeval with mafic alkalic intrusions of the Mazerapane Suite superimposed on the granulite facies metamorphism of the Monapo Klippe. The low-temperature, anhydrite-bearing mineralization was associated with the massive circulation of sulfate-rich brines along fractures activated during the Late Cambrian-Ordovician extension. Origin of the sulfate-rich brines may be genetically related either with the magmatic-hydrothermal differentiation, or with the remobilization of crustal evaporites.     

Physicochemical formation parameters of hydrothermal mineral deposits: Evidence from fluid inclusions. II. Gold,silver, lead,and zink deposits

Information from a database, which was compiled and continuously updated by the authors of this paper and now includes information from 19500 publication on fluid and melt inclusions in minerals, is used to summarize results on the physicochemical formation parameters of hydrothermal Au, Ag, Pb, and Zn deposits. The database provides information on fluid inclusions in minerals from 970 Pb-Zn, 220 Au-Ag-Pb-Zn, and 825 Au-Ag deposits in various settings worldwide. Histograms for the homogenization temperatures of fluid inclusion are presented for the most typical minerals of the deposits. In sphalerite, most homogenization temperatures (1327 measurements) of fluid inclusions lie within the range of 50–200°C with a maximum at 100–200°C for this mineral from Pb-Zn deposits and within the range of 100–350°C (802 measurements) with a maximum at 200–300°C for this mineral from Au deposits. Data are presented on fluid pressures at Au (1495 measurements) and Pb-Zn (180 measurements) deposits. The pressure during the preore, ore-forming, and postore stages at these deposits ranged from 4–10 to 6000 bar. The reason for the high pressures during preore stages at the deposits is the relations of the fluids to acid magmatic and metamorphic processes. More than 70% of the fluid pressures values measured at Pb-Zn deposits lie within the range of 1–1500 bar. Au-Ag deposits are characterized by higher fluid pressures of 500–2000 bar (61% of the measurements). The overall ranges of the salinity and temperature of the mineral-forming fluid at Au-Ag (6778 measurements) and Pb-Zn (3395 measurements) deposits are 0.1–80 wt % equiv. NaCl and 20–800°C. Most measurements (～64%) for Au-Ag deposits yield fluid salinity <10 wt % equiv. NaCl and temperatures of 200–400°C (63%). Fluids at Pb-Zn deposits are typically more saline (10–25 wt % equiv. NaCl, 51% measurements) and lower temperature (100–300°C, 74% measurements). Several measurements of the fluid density fall within the range of 0.8–1.2 g/cm³. The average composition of volatile components of the fluids was evaluated by various techniques. The average composition of volatile components of fluid inclusions in minerals is calculated for hydrothermal W, Au, Ag, Sn, and Pb-Zn deposits, metamorphic rocks, and all geological objects. The Au, Ag, Pb, and Zn concentrations in magmatic melts and mineral-forming fluids is evaluated based on analyses of individual inclusions.     

The origin of Kibaran (late Mid-Proterozoic) tin,tungsten and gold quartz vein deposits in Central Africa: a fluid inclusions study

Composition and evolution of fluids depositing tin and tungsten ores in Kibaran quartz vein deposits allow the modelling of devolatizing evolved granites as their source at depth. Fluids forming gold quartz veins and breccias are different from the first, especially by showing characteristics of a high-pressure environment. All deposits are controlled by compressional deformation whose fading phases affect earlier formed veins. These findings lead to the conclusion that both anatectic melting resulting in intrusion of fertile granites, and the generation of fluids forming gold deposits are the final consequence of deep crustal metamorphism. The latter was caused by crustal thickening immediately preceding the metallogenetic climax.     

Genetic and metamorphic conditions constrained by fluid inclusions from Paleoproterozoic (c. 1.8 Ga) magnesite ore deposits, NE Brazil

Precambrian magnesite occurrences hosted by metadolomites from the Orós belt, Ceará, Brazil, are part of a greenschist–amphibolite, metavolcano-sedimentary terrain, dated at 1.8 Ga, cut by Meso- to Neoproterozoic Brasiliano granites and Neoproterozoic basic sills. These rocks were affected by a shear zone between 580 and 500 Ma. The magnesite-bearing marbles can be grouped as medium-grained (1–9 mm) at the Riacho Fundo ore deposit or sparry magnesite (1–15 cm) at the Cabeça de Negro ore deposit. The sparry magnesite shows textural characteristics related to original sedimentary structures. Both types of magnesite-bearing marbles contain aqueous and aqueous-carbonic fluid inclusions that yield homogenization temperatures between 170 and 370 °C. Applying a pressure correction, these temperatures are compatible with the evolution from greenschist to amphibolite facies metamorphic conditions, as described in previous work on the Orós region. It also agrees with data in specialized literature on the metamorphism of carbonate rocks. Fluid inclusion distribution, composition, and physical-chemical characteristics suggest temperature increase, probably related to metamorphism on these rocks. The medium-grained magnesite records partial contamination of CO₂-rich inclusions by relict carbonaceous material (bitumen, hydrocarbons?) that favors, but does not confirm, a syngenetic sedimentary origin and could have caused the lowering of CO₂ melting point in these inclusions. Therefore, though textural evidence points to a sedimentary-diagenetic model, fluid inclusions record conditions of a metamorphic event.     

Genesis of the Sb-W-Au deposits at Ixtahuacan,Guatemala: evidence from fluid inclusions and stable isotopes

The Ixtahuacan Sb-W deposits are hosted by upper Pennsylvanian to Permian metasedimentary rocks of the central Cordillera of Guatemala. The deposits consist of gold-bearing arsenopyrite, stibnite and scheelite. Arsenopyrite and scheelite are early in the paragenesis, occurring as disseminations in pyritiferous black shale/sandstone and in argillaceous limestone, respectively. Some stibnite is disseminated, but the bulk of the stibnite occurs as massive stratabound lenses in black shales and in quartz-ankerite veins and breccias, locally containing scheelite.Microthermometric measurements on fluid inclusions in quartz and scheelite point to a low temperature (160–190°C) and low to moderate salinity (5–15 wt% NaCl eq.) aqueous ore fluid. Abundant vapour-rich inclusions suggest that the fluid boiled. Carbon dioxide was produced locally as a result of interaction of the aqueous fluid with the argillaceous limestone. Bulk leaching experiments and SEM-EDS analyses of decrepitated fluid inclusion residues indicate that the ore-bearing solution was NaCl-dominated. The ¹⁸O values of quartz, ankerite and scheelite from mineralized veins range from 19.7 to 20.5, 18.1 to 20.0 and 7.0 to 8.4 respectively. The average temperature calculated from quartz-scheelite oxygen isotopic fractionation is 170°C. The oxygen isotopic composition of the fluid, interpreted to have been in equilibrium with these minerals, ranged from 5.7 to 7.6, and is considered to represent an evolved meteoric water. Diagenetic or syngenetic pyrite has a sulphur isotopic composition of 0.5±0.3 which is consistent with bacterial reduction of sulphate. The ³⁴S values of arsenopyrite and stibnite range from –2.8 to 2.0 and –2.7 to –2.3 respectively, and are though to reflect sulphur derived from pyrite.The Ixtahuacan deposits are interpreted to have formed at low temperature (<200°C) and a depth of a few hundred metres from a low fO₂ (10^–49–10^–57), high pH (7–8) fluid. Arsenic was probably transported as arsenious acid, antimony and gold as thio-complexes and tungsten as the complex HWO ₄ ^– .A model is proposed in which a meteoric fluid, heated by a felsic intrusion at depth, was focused to shallow levels along faults. The interaction of the fluid with pyritiferous beds caused the deposition of arsenopyrite as a result of sulphidation and/or decreasing fO₂; gold probably co-precipitated with As or was adsorbed onto the arsenopyrite. The precipitation of stibnite was caused by boiling. Scheelite deposited in response to the increase in Ca²⁺ activity which accompanied interaction of the ore fluid with the argillaceous limestones.