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The main event responsible for the deposition of tungsten at Panasqueira was closely associated with strong tourmalinisation of the wall rocks. Tourmaline is coeval with a W-rich rutile (up to 8–10 wt% W), and both minerals record an early introduction of W in the system, just before the main W deposition. U-Pb dating of the rutile by LA-ICP-MS yielded an age of 305.2 ± 5.7 Ma, which is 6 to 10 m.y. older than the K-Ar age of 296.3 ± 1.2 Ma obtained on muscovite, which was therefore not coeval with wolframite. Major and trace element concentration variations in tourmaline record fluid mixing between two end members, both considered to be of metamorphic derivation on the basis of rare earth element profiles. Evidence is reported for a fluid rich in Co, Cu, Pb, Sc, Sr, V, Cr, Nb, Ta, and Sn interpreted to be of local origin (e.g., well equilibrated with the host formations) and a fluid rich in Li, F, Fe, Mn, and W inferred to be of deep origin and related to biotite dehydration. The second fluid carried the metals (in particular Fe and Mn) that were necessary for wolframite deposition and that were not necessarily inherited from the wall rocks through fluid-rock interaction. Micrometer-scale variations in tourmaline and rutile crystal chemistry are indicative of pulsatory fluid input during tourmalinisation., The main event responsible for the deposition of tungsten at Panasqueira was closely associated with strong tourmalinisation of the wall rocks. Tourmaline is coeval with a W-rich rutile (up to 8–10 wt% W), and both minerals record an early introduction of W in the system, just before the main W deposition. U-Pb dating of the rutile by LA-ICP-MS yielded an age of 305.2 ± 5.7 Ma, which is 6 to 10 m.y. older than the K-Ar age of 296.3 ± 1.2 Ma obtained on muscovite, which was therefore not coeval with wolframite. Major and trace element concentration variations in tourmaline record fluid mixing between two end members, both considered to be of metamorphic derivation on the basis of rare earth element profiles. Evidence is reported for a fluid rich in Co, Cu, Pb, Sc, Sr, V, Cr, Nb, Ta, and Sn interpreted to be of local origin (e.g., well equilibrated with the host formations) and a fluid rich in Li, F, Fe, Mn, and W inferred to be of deep origin and related to biotite dehydration. The second fluid carried the metals (in particular Fe and Mn) that were necessary for wolframite deposition and that were not necessarily inherited from the wall rocks through fluid-rock interaction. Micrometer-scale variations in tourmaline and rutile crystal chemistry are indicative of pulsatory fluid input during tourmalinisation.

The central Jebilet massif, part of the North African Variscan Belt, hosts significant polymetallic sulphide mineralization. It is generally considered syngenetic and has many features of volcanogenic massive sulphide (VMS) mineralization. However, some characteristics are not compatible with a classic VMS model and two alternative scenarios for formation have been proposed. Our preliminary research favours a complex, multi-stage development of the sulphide deposits. Uncertainty as to the critical processes controlling the mineralization and lack of agreement on a genetic model inhibit regional exploration. We identify the key knowledge gaps regarding sulphide mineralization in the central Jebilet and outline a research program to address these, with the ultimate aim of improving regional mineral exploration targeting and unlocking the economic potential of this relatively undeveloped district.

The central Jebilet massif, part of the North African Variscan Belt, hosts significant polymetallic sulphide mineralization. It is generally considered syngenetic and has many features of volcanogenic massive sulphide (VMS) mineralization. However, some characteristics are not compatible with a classic VMS model and two alternative scenarios for formation have been proposed. Our preliminary research favours a complex, multi-stage development of the sulphide deposits. Uncertainty as to the critical processes controlling the mineralization and lack of agreement on a genetic model inhibit regional exploration. We identify the key knowledge gaps regarding sulphide mineralization in the central Jebilet and outline a research program to address these, with the ultimate aim of improving regional mineral exploration targeting and unlocking the economic potential of this relatively undeveloped district.

Moisture-induced swelling and shrinkage deformation is a well-known phenomenon for clayey material. To quantify this deformation a fully coupled thermal-hydro-mechanical (TH2M) model based on the modified effective stress concept has been developed and implemented in the numerical code OGS. Three laboratory tests have been analysed using this concept. All specimens for the tests are argillaceous rocks, from different underground laboratory sites (Tournemire, Bure, France, and Mont Terri, Switzerland). The laboratory tests are performed under different conditions; the first test is free swelling/shrinkage and other two tests are with uniaxial loading. The relative humidity is also controlled. Only one test is carried out under varied temperature conditions. In this study the anisotropic swelling/shrinkage deformation wais simulated and analysed using this modified effective stress concept.

The three sulphide-bearing quartz veins come from three different geological settings: intensely migmatised banded gneisses on the edge of a granitic batholith; closely associated with a shallow, highly differentiated late Hercynian porphyritic leucogranite intrusion; and spatially related to a porphyritic monzogranite intrusion surrounded by heterogeneous leucogranite facies. Veins are structurally controlled and the three deposits consist of a set of subparallel veins. Fluid inclusion studies were carried out and discussion is presented of the relationships between hot complex carbonic-aqueous solutions and W-bearing mineralisation., The three sulphide-bearing quartz veins come from three different geological settings: intensely migmatised banded gneisses on the edge of a granitic batholith; closely associated with a shallow, highly differentiated late Hercynian porphyritic leucogranite intrusion; and spatially related to a porphyritic monzogranite intrusion surrounded by heterogeneous leucogranite facies. Veins are structurally controlled and the three deposits consist of a set of subparallel veins. Fluid inclusion studies were carried out and discussion is presented of the relationships between hot complex carbonic-aqueous solutions and W-bearing mineralisation.

Fresh itabirites display rather high gold concentrations which range from 30 to 40 ppb. In laterites, the gold content remains relatively high but may vary significantly from the bottom to the top of the hills, and within the different horizons from 20 to 60 ppb. Thus, fresh and altered itabirites may have constituted significant source rocks for gold. At the hydrothermal stage, gold deposition seems to be restricted to the veins characterised by the presence of C-H-O-S fluids characterised by rather low oxygen fugacity (around that fixed by the Ni-NiO oxygen buffer) whilst no gold was found in the quartz-specularite veins characterised by high oxygen fugacity (greater than that fixed by the haematite-magnetite buffer) and sulphate rich solutions., Fresh itabirites display rather high gold concentrations which range from 30 to 40 ppb. In laterites, the gold content remains relatively high but may vary significantly from the bottom to the top of the hills, and within the different horizons from 20 to 60 ppb. Thus, fresh and altered itabirites may have constituted significant source rocks for gold. At the hydrothermal stage, gold deposition seems to be restricted to the veins characterised by the presence of C-H-O-S fluids characterised by rather low oxygen fugacity (around that fixed by the Ni-NiO oxygen buffer) whilst no gold was found in the quartz-specularite veins characterised by high oxygen fugacity (greater than that fixed by the haematite-magnetite buffer) and sulphate rich solutions.

The cathodoluminescence mode of SEM appears to be a powerful tool for determining the degree of microfracturing, the volume of mineral healing and the relationships between ore minerals and ore fluids in specific networks of microfractures. The study of different gold bearing quartz veins has shown that most ore fluids have migrated in specific opened networks during late stages of brittle deformation. Early milky quartz or fine grained quartz generations appear as passive traps as a function of their late microfracturing, and their crystallisation has no relationship with the gold enrichment., The cathodoluminescence mode of SEM appears to be a powerful tool for determining the degree of microfracturing, the volume of mineral healing and the relationships between ore minerals and ore fluids in specific networks of microfractures. The study of different gold bearing quartz veins has shown that most ore fluids have migrated in specific opened networks during late stages of brittle deformation. Early milky quartz or fine grained quartz generations appear as passive traps as a function of their late microfracturing, and their crystallisation has no relationship with the gold enrichment.

The Hercynian belt in Europe hosts several gold districts with numerous showings and a few mined deposits. Most of the economic concentrations formed during late Hercynian deformational stages affecting the basement. Gold bearing quartz veins are characterised by a multistage ore deposition during specific stages of microfracturing and fluid migration which affected the previously deposited matrix of the quartz vein. Data issued from a multidisciplinary approach involving structural studies from the regional to the microscopic scale, physical-chemical reconstruction based on mineral assemblages and fluid inclusion studies together with thermodynamic calculations (P, T, fO2, fS2, pH), show that the previous "shear zone models" are inadequate for a precise reconstruction of the process of gold enrichment., The Hercynian belt in Europe hosts several gold districts with numerous showings and a few mined deposits. Most of the economic concentrations formed during late Hercynian deformational stages affecting the basement. Gold bearing quartz veins are characterised by a multistage ore deposition during specific stages of microfracturing and fluid migration which affected the previously deposited matrix of the quartz vein. Data issued from a multidisciplinary approach involving structural studies from the regional to the microscopic scale, physical-chemical reconstruction based on mineral assemblages and fluid inclusion studies together with thermodynamic calculations (P, T, fO2, fS2, pH), show that the previous "shear zone models" are inadequate for a precise reconstruction of the process of gold enrichment.

Most of the hydrothermal uranium (U) deposits from the European Hercynian belt (EHB) are spatially associated with Carboniferous peraluminous leucogranites. In the southern part of the Armorican Massif (French part of the EHB), the Guérande peraluminous leucogranite was emplaced in an extensional deformation zone at ca. 310 Ma and is spatially associated with several U deposits and occurrences. The apical zone of the intrusion is structurally located below the Pen Ar Ran U deposit, a perigranitic vein-type deposit where mineralization occurs at the contact between black shales and Ordovician acid metavolcanics. In the Métairie-Neuve intragranitic deposit, uranium oxide-quartz veins crosscut the granite and a metasedimentary enclave. Airborne radiometric data and published trace element analyses on the Guérande leucogranite suggest significant uranium leaching at the apical zone of the intrusion. The primary U enrichment in the apical zone of the granite likely occurred during both fractional crystallization and the interaction with magmatic fluids. The low Th/U values (< 2) measured on the Guérande leucogranite likely favored the crystallization of magmatic uranium oxides. The oxygen isotope compositions of the Guérande leucogranite (δ18Owhole rock = 9.7–11.6‰ for deformed samples and δ18Owhole rock = 12.2–13.6‰ for other samples) indicate that the deformed facies of the apical zone underwent sub-solidus alteration at depth with oxidizing meteoric fluids. Fluid inclusion analyses on a quartz comb from a uranium oxide-quartz vein of the Pen Ar Ran deposit show evidence of low-salinity fluids (1–6 wt.% NaCl eq.), in good agreement with the contribution of meteoric fluids. Fluid trapping temperatures in the range of 250–350 °C suggest an elevated geothermal gradient, probably related to regional extension and the occurrence of magmatic activity in the environment close to the deposit at the time of its formation. U-Pb dating

Hydrothermal uranium veins associated with Hercynian leucogranites show important variations in their mineralogical, chemical and structural features in relation to the host rock lithology. Comparisons between the deposits at Vendee, Limousin and Erzgebirge show that open faults and subsequent mineralisation are concentrated at boundaries between competent and more plastic lithologies, mineralising fluids cause wall-rock alteration characterised by the removal of Si from granite and of Ca, Mg and Fe from metamorphic and basic rocks, and the chemical and structural characteristics of wall-rocks are important controls on the mineralisation., Hydrothermal uranium veins associated with Hercynian leucogranites show important variations in their mineralogical, chemical and structural features in relation to the host rock lithology. Comparisons between the deposits at Vendee, Limousin and Erzgebirge show that open faults and subsequent mineralisation are concentrated at boundaries between competent and more plastic lithologies, mineralising fluids cause wall-rock alteration characterised by the removal of Si from granite and of Ca, Mg and Fe from metamorphic and basic rocks, and the chemical and structural characteristics of wall-rocks are important controls on the mineralisation.

A comprehensive study of alteration minerals, especially clays developed around or within French hydrothermal uranium ore bodies (Vendee and Margeride) was carried out. The results of optical examinations, classical methods of clay mineralogy and electron microprobe determination, demonstrated the general feature of potassic minerals as a product of the reaction between ore forming fluids and host rocks. The six stages of alteration following the typical metallogenic sequence are described. The complex alterations in K-smectites, mixed layer minerals and adularia can be explained by potassic metasomatism by fluids in disequilibrium with the minerals of the granitic host rocks. The vareity of alteration products in each alteration zone indicates incomplete reactions, of the solid and chemical potential., A comprehensive study of alteration minerals, especially clays developed around or within French hydrothermal uranium ore bodies (Vendee and Margeride) was carried out. The results of optical examinations, classical methods of clay mineralogy and electron microprobe determination, demonstrated the general feature of potassic minerals as a product of the reaction between ore forming fluids and host rocks. The six stages of alteration following the typical metallogenic sequence are described. The complex alterations in K-smectites, mixed layer minerals and adularia can be explained by potassic metasomatism by fluids in disequilibrium with the minerals of the granitic host rocks. The vareity of alteration products in each alteration zone indicates incomplete reactions, of the solid and chemical potential.

The uranium deposits are located near a muscovite leucogranite, which is associated with shearing zones developed by the Hercynian tectogenesis. The mineralisation consists of a pitchblende filling of veins, with subordinate sulphides. Structural studies show the ore developed in an open fault network. The structural setting is the result of a well defined tectonic phase resulting in east-west shear zones and preferential faulting in certain metamorphic series. The influence of mechanical factors seems to be essential in the deposition of the uranium., The uranium deposits are located near a muscovite leucogranite, which is associated with shearing zones developed by the Hercynian tectogenesis. The mineralisation consists of a pitchblende filling of veins, with subordinate sulphides. Structural studies show the ore developed in an open fault network. The structural setting is the result of a well defined tectonic phase resulting in east-west shear zones and preferential faulting in certain metamorphic series. The influence of mechanical factors seems to be essential in the deposition of the uranium.

Prior to mining, the Neves Corvo deposit had around 300 000 000 tonnes of total sulphide-rich rocks, with over 42 000 000 t of Cu ores grading 7.6% Cu and 47 000 000 t of Zn ores grading 6.2% Zn. Several main types of mineralisation are present: massive sulphides, rubane copper and rubane tin ores which occur in the hangingwall of the massive sulphides, and fissural sulphide mineralisation in shales and acid tuffs of the footwall rocks. Studies of aqueous carbonic fluid inclusions from cassiterite crystals in the rubane tin ore indicate that this ore formed as early Sn stockworks, with at least some of the veins generated during the metamorphic reworking of the Neves Corvo deposit. Oscillatory pressure fluctuations between lithostatic and suprahydrostatic pressures at constant depth and continuous pressure decrease, as a result of uplift, are thought to be the mechanisms responsible for pressure variations reflected by different isochores typical of those for quartz fluid inclusions., Prior to mining, the Neves Corvo deposit had around 300 000 000 tonnes of total sulphide-rich rocks, with over 42 000 000 t of Cu ores grading 7.6% Cu and 47 000 000 t of Zn ores grading 6.2% Zn. Several main types of mineralisation are present: massive sulphides, rubane copper and rubane tin ores which occur in the hangingwall of the massive sulphides, and fissural sulphide mineralisation in shales and acid tuffs of the footwall rocks. Studies of aqueous carbonic fluid inclusions from cassiterite crystals in the rubane tin ore indicate that this ore formed as early Sn stockworks, with at least some of the veins generated during the metamorphic reworking of the Neves Corvo deposit. Oscillatory pressure fluctuations between lithostatic and suprahydrostatic pressures at constant depth and continuous pressure decrease, as a result of uplift, are thought to be the mechanisms responsible for pressure variations reflected by different isochores typical of those for quartz fluid inclusions.

Gold-antimony-arsenic mineralisation occurs in the acid tuffs of the Villeranges Basin. Combined studies of mineral parageneses and fluid inclusions allow the determination of conditions of alteration and ore formation. Three alteration phases are distinguished. In the first, pervasive fluid circulation induces the alteration of Fe-Mg phases into chlorite at temperatures around 280 degrees C. The second is a potassic alteration, affecting the previous alteration minerals, forming white micas (phengite followed by illites) and ankerite, associated with Sb, As and Au. Illite formation is related to ore crystallisation: ore-forming fluids are aqueous, of low salinity, and their trapping temperature is about 200 degrees C. Calcite represents the latest stage and forms at around 80 to 100 degrees C. Mineralisation took place in a cooling hydrothermal system; aqueous fluids of intermediate temperature and low salinity are important for Sb, As and Au transport and deposition, and their more general significance as sources of gold mineralisation is discussed, including the problematic relationship with carbonic fluids., Gold-antimony-arsenic mineralisation occurs in the acid tuffs of the Villeranges Basin. Combined studies of mineral parageneses and fluid inclusions allow the determination of conditions of alteration and ore formation. Three alteration phases are distinguished. In the first, pervasive fluid circulation induces the alteration of Fe-Mg phases into chlorite at temperatures around 280 degrees C. The second is a potassic alteration, affecting the previous alteration minerals, forming white micas (phengite followed by illites) and ankerite, associated with Sb, As and Au. Illite formation is related to ore crystallisation: ore-forming fluids are aqueous, of low salinity, and their trapping temperature is about 200 degrees C. Calcite represents the latest stage and forms at around 80 to 100 degrees C. Mineralisation took place in a cooling hydrothermal system; aqueous fluids of intermediate temperature and low salinity are important for Sb, As and Au transport and deposition, and their more general significance as sources of gold mineralisation is discussed, including the problematic relationship with carbonic fluids.

The tungsten deposit is spatially related to a monzogranite intrusion which is surrounded by leucogranites. The deposit provides good evidence of hydrothermal transport and deposition of wolframite, followed by sulphides, and highlights the important role played by complex carbonic-aqueous fluids. Fluid inclusion data, mineral chemistry and mineral assemblages were used to estimate fO2, fS2 and pH at the pressure and temperature conditions of ore deposition., The tungsten deposit is spatially related to a monzogranite intrusion which is surrounded by leucogranites. The deposit provides good evidence of hydrothermal transport and deposition of wolframite, followed by sulphides, and highlights the important role played by complex carbonic-aqueous fluids. Fluid inclusion data, mineral chemistry and mineral assemblages were used to estimate fO2, fS2 and pH at the pressure and temperature conditions of ore deposition.

Regional soil geochemical exploration has resulted in the discovery of the Mokrsko deposit, central Bohemia, Czech Republic, with reserves of over 100 tonnes of gold, at an average grade of 2 g/t. The relative chronology between fluid migration, deformational events and metal deposition was investigated by combining field observations and laboratory studies using reflected-transmitted light microscopy and SEM-cathodoluminescence imagery, together with a detailed study of fluid inclusions. At least three major stages of brittle micro-deformation have affected the Mokrsko-Celina rocks resulting in fissural fluid migration through regular and dense networks of extensional structures., Regional soil geochemical exploration has resulted in the discovery of the Mokrsko deposit, central Bohemia, Czech Republic, with reserves of over 100 tonnes of gold, at an average grade of 2 g/t. The relative chronology between fluid migration, deformational events and metal deposition was investigated by combining field observations and laboratory studies using reflected-transmitted light microscopy and SEM-cathodoluminescence imagery, together with a detailed study of fluid inclusions. At least three major stages of brittle micro-deformation have affected the Mokrsko-Celina rocks resulting in fissural fluid migration through regular and dense networks of extensional structures.

Two kinds of tailings were investigated: granite type rocks from the Vendee, Limousin and SW Massif Central, including barren rocks, heap leached ores and cover materials; and sedimentary type rocks from the Lodeve deposit. Systematic investigations of newly formed products resulting from the interaction of acid waters and U-mine tailings show that a fine matrix cements the rock blocks. It is composed of Fe-sulphate or hydroxides which may or may not be associated with smectites. These minerals crystallise from the percolating waters in specific zones of the mine tailings, especially at the base of the tailing pile. Experimental and numerical simulations of the processes were attempted., Two kinds of tailings were investigated: granite type rocks from the Vendee, Limousin and SW Massif Central, including barren rocks, heap leached ores and cover materials; and sedimentary type rocks from the Lodeve deposit. Systematic investigations of newly formed products resulting from the interaction of acid waters and U-mine tailings show that a fine matrix cements the rock blocks. It is composed of Fe-sulphate or hydroxides which may or may not be associated with smectites. These minerals crystallise from the percolating waters in specific zones of the mine tailings, especially at the base of the tailing pile. Experimental and numerical simulations of the processes were attempted.

Results show that trapped fluids in specific networks of healed or sealed microfissures frequently show extreme fluctuation in their density although many chemical variables (salinities, CH4/CO2 ratios) remain nearly constant. Such features result from diverse causes: changes in P, T and structural levels, fluid immiscibility, post trapping changes and superimposed trapping in the same quartz grains due to intense and repeated microfissuring. The use of fluid inclusion data appears especially useful to test models depicting the control of fluids on the deformation of rocks (such as the necessary conditions for faults to behave as fluid-pressure-activated valves) and models of gold deposition (such as those considering fluid phase separation). Results show that there are some limitations to the general use of such models., Results show that trapped fluids in specific networks of healed or sealed microfissures frequently show extreme fluctuation in their density although many chemical variables (salinities, CH4/CO2 ratios) remain nearly constant. Such features result from diverse causes: changes in P, T and structural levels, fluid immiscibility, post trapping changes and superimposed trapping in the same quartz grains due to intense and repeated microfissuring. The use of fluid inclusion data appears especially useful to test models depicting the control of fluids on the deformation of rocks (such as the necessary conditions for faults to behave as fluid-pressure-activated valves) and models of gold deposition (such as those considering fluid phase separation). Results show that there are some limitations to the general use of such models.

A method is presented of relating fluid pathways (geometry of the palaeo open space) to the bulk brittle network (microfracture system) for each stage in a sequence of deformational events. It is shown that by the study of oriented FIP, each stage of fluid migration may be characterised by specific P-T conditions and geometry of the palaeo open space., A method is presented of relating fluid pathways (geometry of the palaeo open space) to the bulk brittle network (microfracture system) for each stage in a sequence of deformational events. It is shown that by the study of oriented FIP, each stage of fluid migration may be characterised by specific P-T conditions and geometry of the palaeo open space.

Qualitative and quantitative analysis of the gold distribution within some As-rich pyrites has been carried out. Some strong enrichments in gold and arsenic within the crystals attest fluctuations in the fluid chemistry and may be interpreted, in some cases, as diffusion after crystal growth. The study of one of these pyrite concentrates by 197Au Mossbauer spectroscopy has shown that the gold is in a combined state, different from the chemical state formerly identified in arsenopyrite. Quantitative calculations from RAE images allows a better knowledge of the gold distribution between its bearing minerals., Qualitative and quantitative analysis of the gold distribution within some As-rich pyrites has been carried out. Some strong enrichments in gold and arsenic within the crystals attest fluctuations in the fluid chemistry and may be interpreted, in some cases, as diffusion after crystal growth. The study of one of these pyrite concentrates by 197Au Mossbauer spectroscopy has shown that the gold is in a combined state, different from the chemical state formerly identified in arsenopyrite. Quantitative calculations from RAE images allows a better knowledge of the gold distribution between its bearing minerals.

Hydrothermal alterations which occur in granites may favour subsequent ore deposition or reworkings. The dequartzification of two-mica granites, which produces important reservoir-rock for pitchblende deposition, and the later clay alteration were modelled using computer codes DISSOL and THERMAL., Hydrothermal alterations which occur in granites may favour subsequent ore deposition or reworkings. The dequartzification of two-mica granites, which produces important reservoir-rock for pitchblende deposition, and the later clay alteration were modelled using computer codes DISSOL and THERMAL.

Present characteristics of Witwatersrand mineralisation reflect 1 000 Ma of depositional processes and post-burial diagenetic/metamorphic activity. Fluid and mineralogical constraints suggest that the (Au(HS)2)- complex has limited solubility under conditions pertaining to post-burial fluid migration, but the (AuCl2)- complex has potential for gold transport at temperatures greater than 350 degrees C. The ubiquitous uraninite-kerogen association is characterised by a paragenetic sequence involving early detrital uraninite concentration with later fixation of hydrocarbon residues around uraninite accumulations. The present mineral association in the Witwatersrand Basin can best be accommodated by an early accumulation of detrital minerals and subsequent remobilisation probably during a number of different stages., Present characteristics of Witwatersrand mineralisation reflect 1 000 Ma of depositional processes and post-burial diagenetic/metamorphic activity. Fluid and mineralogical constraints suggest that the (Au(HS)2)- complex has limited solubility under conditions pertaining to post-burial fluid migration, but the (AuCl2)- complex has potential for gold transport at temperatures greater than 350 degrees C. The ubiquitous uraninite-kerogen association is characterised by a paragenetic sequence involving early detrital uraninite concentration with later fixation of hydrocarbon residues around uraninite accumulations. The present mineral association in the Witwatersrand Basin can best be accommodated by an early accumulation of detrital minerals and subsequent remobilisation probably during a number of different stages.

The relative chronology and detailed chemistry of palaeofluids circulating at the base of the Kombolgie sub-basin were investigated in the East Alligator River uranium district. Three distinct fluids were distinguished: a Na-rich brine that corresponded to a diagenetic fluid at around 150 degrees C; a Ca-rich brine probably corresponding to residual evaporitic brine; and a low-salinity fluid heated in the basement before injection into the base of the sandstone cover. Hydraulic brecciation of the sandstone was associated with a pressure decrease favouring fluid mixing and subsequent cementation. H2 and O2 from water radiolysis, detected in the vapour phase of some fluid inclusions and especially in low-salinity ones from quartz breccias above mineralised areas, may constitute indicator gases linked to significant U concentrations deeper in the basement rocks., The relative chronology and detailed chemistry of palaeofluids circulating at the base of the Kombolgie sub-basin were investigated in the East Alligator River uranium district. Three distinct fluids were distinguished: a Na-rich brine that corresponded to a diagenetic fluid at around 150 degrees C; a Ca-rich brine probably corresponding to residual evaporitic brine; and a low-salinity fluid heated in the basement before injection into the base of the sandstone cover. Hydraulic brecciation of the sandstone was associated with a pressure decrease favouring fluid mixing and subsequent cementation. H2 and O2 from water radiolysis, detected in the vapour phase of some fluid inclusions and especially in low-salinity ones from quartz breccias above mineralised areas, may constitute indicator gases linked to significant U concentrations deeper in the basement rocks.

Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits, Grovelas and Penedono, together with a deposit linked to sub-vertical structures in silicified metasediments at Tres-Minas and several intra-metamorphic occurrences at Vila Pouca de Aguiar. They all have similar mineralogy and have undergone similar deformation and flow of similar fluids under comparable pressure-temperature conditions. Independent of location or host rocks, three successive crystallisation stages have been recorded during the formation of gold-bearing structures: development of milky quartz veins; deposition of clear quartz with local arsenopyrite and pyrite; and intense microfissuring of earlier vein fillings, plus crystallisation of native gold, electrum, sulphides, native Bi and sulphosalts., Mineralogical, fluid inclusion and geochemical studies were made on two intra-granitic gold deposits, Grovelas and Penedono, together with a deposit linked to sub-vertical structures in silicified metasediments at Tres-Minas and several intra-metamorphic occurrences at Vila Pouca de Aguiar. They all have similar mineralogy and have undergone similar deformation and flow of similar fluids under comparable pressure-temperature conditions. Independent of location or host rocks, three successive crystallisation stages have been recorded during the formation of gold-bearing structures: development of milky quartz veins; deposition of clear quartz with local arsenopyrite and pyrite; and intense microfissuring of earlier vein fillings, plus crystallisation of native gold, electrum, sulphides, native Bi and sulphosalts.

Reconstruction of the fluid composition in the system Na-Ca-Mg-Cl showed that two types of brine are responsible for the main quartz cements, an NaCl-rich brine which is interpreted as a primary formation water that was expelled from bedded evaporites, and a CaCl2-rich brine which is considered to have formed during the interaction between the NaCl-rich brine and Ca-rich minerals in the basement and was introduced into the fault system and mixed with the NaCI-rich brine during the critical stage of U deposition. Temperature and pressure drops, combined with the effects of brine mixing, appear to be key factors for the main stages of quartz cementation and U deposition., Reconstruction of the fluid composition in the system Na-Ca-Mg-Cl showed that two types of brine are responsible for the main quartz cements, an NaCl-rich brine which is interpreted as a primary formation water that was expelled from bedded evaporites, and a CaCl2-rich brine which is considered to have formed during the interaction between the NaCl-rich brine and Ca-rich minerals in the basement and was introduced into the fault system and mixed with the NaCI-rich brine during the critical stage of U deposition. Temperature and pressure drops, combined with the effects of brine mixing, appear to be key factors for the main stages of quartz cementation and U deposition.

Eluvial concentration of platinum-group minerals (PGM) has developed in the south of New Caledonia as a result of the weathering of mafic and ultramafic cumulate. The Pt/Pd ratio evolution from the bottom to the top of the weathering profile indicates chemical mobilisation of Pd in a supergene environment. SEM studies of the residual PGM collected in the weathering profile and chemical characterisation by electron microprobe show a preservation of the structure and chemical signature in the core of the grains of the fresh PGM, indicating that the PGMs are residual and are affected by a dissolution process that leads to Pd and Fe lixiviation and relative Pt enrichment in smaller and smaller isoferroplatinum particles. Secondary dispersal of the particles occurs as a result of a mechanical process in the weathering mantle and on the hill slopes., Eluvial concentration of platinum-group minerals (PGM) has developed in the south of New Caledonia as a result of the weathering of mafic and ultramafic cumulate. The Pt/Pd ratio evolution from the bottom to the top of the weathering profile indicates chemical mobilisation of Pd in a supergene environment. SEM studies of the residual PGM collected in the weathering profile and chemical characterisation by electron microprobe show a preservation of the structure and chemical signature in the core of the grains of the fresh PGM, indicating that the PGMs are residual and are affected by a dissolution process that leads to Pd and Fe lixiviation and relative Pt enrichment in smaller and smaller isoferroplatinum particles. Secondary dispersal of the particles occurs as a result of a mechanical process in the weathering mantle and on the hill slopes.

The Imiter Ag-Hg deposit is located in the Precambrian volcano-sedimentary formations of the Saghro inlier. The orebodies consist of NE-SW to EW veins and lenses hosted by Cryogenian black shales and greywackes and Neoproterozoic conglomerates, and are controlled by an EW fault network, the so-called Imiter fault. Mineralogical and palaeo-fluid geochemistry investigations show that the main Ag ore stage is related to circulation of deep-basinal sedimentary brines (Na-K-Mg) (salinity 16.7 to 26 wt% NaCl equiv., molar Cl/Br 330, d18O 2.15-2.35 per mille SMOW, and dD -53.8 to -65.5 per mille SMOW), at temperatures of about 180 to 220 degrees C and hydrostatic pressures. The main driving mechanism for Ag ore deposition is the dilution of ore-bearing brines by a low-salinity meteoric fluid containing a low-density volatile component (N2, CH4, CO2), at 180 to 220 degrees C, d18O -1.4 per mille SMOW, and dD -28.2 per mille SMOW. Silver content of the brines ranges from 2 to 30 mmol/kg solution (up to 3 200 ppm Ag, av. 900 ppm Ag), whereas the maximum Ag content found in dilute waters is about 0.4 mmol/kg solution (40 ppm). The ore-forming model proposed for the Imiter deposit is: Ag extraction from the basement by the penetration of deep-basinal brines, and deposition in a structural trap through fluid mixing with recharge fluids. This model is comparable to that described worldwide for the origin of Pb-Zn, F, Ba, and U deposits near unconformities between basement and sedimentary basins., The Imiter Ag-Hg deposit is located in the Precambrian volcano-sedimentary formations of the Saghro inlier. The orebodies consist of NE-SW to EW veins and lenses hosted by Cryogenian black shales and greywackes and Neoproterozoic conglomerates, and are controlled by an EW fault network, the so-called Imiter fault. Mineralogical and palaeo-fluid geochemistry investigations show that the main Ag ore stage is related to circulation of deep-basinal sedimentary brines (Na-K-Mg) (salinity 16.7 to 26 wt% NaCl equiv., molar Cl/Br 330, d18O 2.15-2.35 per mille SMOW, and dD -53.8 to -65.5 per mille SMOW), at temperatures of about 180 to 220 degrees C and hydrostatic pressures. The main driving mechanism for Ag ore deposition is the dilution of ore-bearing brines by a low-salinity meteoric fluid containing a low-density volatile component (N2, CH4, CO2), at 180 to 220 degrees C, d18O -1.4 per mille SMOW, and dD -28.2 per mille SMOW. Silver content of the brines ranges from 2 to 30 mmol/kg solution (up to 3 200 ppm Ag, av. 900 ppm Ag), whereas the maximum Ag content found in dilute waters is about 0.4 mmol/kg solution (40 ppm). The ore-forming model proposed for the Imiter deposit is: Ag extraction from the basement by the penetration of deep-basinal brines, and deposition in a structural trap through fluid mixing with recharge fluids. This model is comparable to that described worldwide for the origin of Pb-Zn, F, Ba, and U deposits near unconformities between basement and sedimentary basins.

Water-gas-salt fluids are the most common metal carriers and are the main agent in the extraction of ore metals from source rocks, transporting them in drainage zones and precipitating them in chemical and structural traps. Meteoric waters, seawater and basinal brines, and magmatic and metamorphic fluids at deeper structural levels, may predominate in distinct fluid reservoirs. They may interact at depth with host rocks and mix when geological processes favour mass and heat transfer via thermal gradients. High-temperature 350-650 degrees C magmatic hydrothermal systems may provide considerable amounts of precious and base metal resources and are often considered to be one of the main factors favouring economic concentrations of precious and base metals, but other widely varied processes occur at lower temperatures of 100-250 degrees C, particularly in domains along faults at the interface between the basement and the sedimentary cover. Almost complete reconstruction of the fluid chemistry of fluid inclusions, including pH, ligand concentration, temperature and redox processes, has resulted in a better understanding of ore-forming fluids and their role in the transport and deposition of ore minerals. The importance of processes that may change these parameters has resulted in developments in accurate micro-analytical techniques and thermodynamic modelling. The discussion is illustrated with micrographs or photographs from French talc deposit Trimouns, Moroccan Co-Ni-Ag-Au and Ag mines Bou Azzer and Imiter, American Mo mine Climax and Western Australian Au mine Daisy Milano., Water-gas-salt fluids are the most common metal carriers and are the main agent in the extraction of ore metals from source rocks, transporting them in drainage zones and precipitating them in chemical and structural traps. Meteoric waters, seawater and basinal brines, and magmatic and metamorphic fluids at deeper structural levels, may predominate in distinct fluid reservoirs. They may interact at depth with host rocks and mix when geological processes favour mass and heat transfer via thermal gradients. High-temperature 350-650 degrees C magmatic hydrothermal systems may provide considerable amounts of precious and base metal resources and are often considered to be one of the main factors favouring economic concentrations of precious and base metals, but other widely varied processes occur at lower temperatures of 100-250 degrees C, particularly in domains along faults at the interface between the basement and the sedimentary cover. Almost complete reconstruction of the fluid chemistry of fluid inclusions, including pH, ligand concentration, temperature and redox processes, has resulted in a better understanding of ore-forming fluids and their role in the transport and deposition of ore minerals. The importance of processes that may change these parameters has resulted in developments in accurate micro-analytical techniques and thermodynamic modelling. The discussion is illustrated with micrographs or photographs from French talc deposit Trimouns, Moroccan Co-Ni-Ag-Au and Ag mines Bou Azzer and Imiter, American Mo mine Climax and Western Australian Au mine Daisy Milano.

Oxygen and carbon isotope analysis and fluid inclusion studies were carried out on samples from calcite-quartz veins in three goldfields, Welkom, Klerksdorp and Evander, in close vicinity to Au-bearing reefs. The sedimentary or mafic host rocks were also studied to investigate the effects of rock-fluid interactions. The results showed that the oxygen isotope composition of quartz in veins is buffered by the host sedimentary rocks so that it not possible to determine the nature of the original fluid. Carbon isotope analyses in quartz-calcite veins hosted by mafic rocks show a range of values indicating two distinct sources, independent of their location within the Witwatersrand Basin. One end-member is represented by organic matter found locally within the basin sedimentary rocks and the second end-member has a marine signature. The most likely candidate for the marine signature is the overlying limestones from the Chuniespoort Group. This indicates a basin-scale downward fluid circulation from the overlying rocks to the basin sedimentary rocks. The fluid must have circulated after ca. 2642 to 2432 Ma, which is the age of the Chuniespoort Group deposition., Oxygen and carbon isotope analysis and fluid inclusion studies were carried out on samples from calcite-quartz veins in three goldfields, Welkom, Klerksdorp and Evander, in close vicinity to Au-bearing reefs. The sedimentary or mafic host rocks were also studied to investigate the effects of rock-fluid interactions. The results showed that the oxygen isotope composition of quartz in veins is buffered by the host sedimentary rocks so that it not possible to determine the nature of the original fluid. Carbon isotope analyses in quartz-calcite veins hosted by mafic rocks show a range of values indicating two distinct sources, independent of their location within the Witwatersrand Basin. One end-member is represented by organic matter found locally within the basin sedimentary rocks and the second end-member has a marine signature. The most likely candidate for the marine signature is the overlying limestones from the Chuniespoort Group. This indicates a basin-scale downward fluid circulation from the overlying rocks to the basin sedimentary rocks. The fluid must have circulated after ca. 2642 to 2432 Ma, which is the age of the Chuniespoort Group deposition.

Mass balance calculations on fresh and altered rocks show that during the ore stage, hydrothermal fluids contributed H2O, CO2 and S (Au, Sb) to authigenic minerals and leached almost completely Na, Ba and Sr. Ore fluids are characterised by an aqueous composition, a rather low salinity, and temperatures ranging from 160 to 200 degrees C. Ore deposition occurred at a low pH, indicated by the complete alteration of the host rocks into a quartz-illite association, and at a low oxygen fugacity intermediate between that fixed by the haematite-magnetite and Ni-NiO oxygen buffers. Gold deposition probably resulted from a destabilisation of Au bisulphide of thioarsenide complexes during sulphide crystallisation. Thus, the Au-As-Sb ores result from a single hydrothermal stage and occur during a late stage of brittle deformation, significantly posterior to the continental basin formation and the acid volcanism. Ore fluid circulation is linked mostly to the fluid convection produced by the abnormal heat flow characterising the Marche-Combrailles shear zone., Mass balance calculations on fresh and altered rocks show that during the ore stage, hydrothermal fluids contributed H2O, CO2 and S (Au, Sb) to authigenic minerals and leached almost completely Na, Ba and Sr. Ore fluids are characterised by an aqueous composition, a rather low salinity, and temperatures ranging from 160 to 200 degrees C. Ore deposition occurred at a low pH, indicated by the complete alteration of the host rocks into a quartz-illite association, and at a low oxygen fugacity intermediate between that fixed by the haematite-magnetite and Ni-NiO oxygen buffers. Gold deposition probably resulted from a destabilisation of Au bisulphide of thioarsenide complexes during sulphide crystallisation. Thus, the Au-As-Sb ores result from a single hydrothermal stage and occur during a late stage of brittle deformation, significantly posterior to the continental basin formation and the acid volcanism. Ore fluid circulation is linked mostly to the fluid convection produced by the abnormal heat flow characterising the Marche-Combrailles shear zone.

The uranium deposits linked to the Hercynian leucogranites in Western Europe are generally supposed to be of hydrothermal origin; the U-Pb geochronological study of the mineralisation linked to the Mortagne leucongranite (South-Armorican massif) confirms this hypothesis and shows that the different ages of uranium deposition correspond to the described paragenesis. Certain ores, like that of the Retail deposit, which is typically intrametamorphic can indicate as many as three mineralising phases, separated by some considerable time: a first phase, between 420-390 Ma, related a caledonian metamorphic event; a second phase, between 345-310 Ma, of remobilisation of the previous phase, contemporary with the leucogranitic magmatism, which seems to have had a role in the transfer of mineralising fluids; and a third phase, between 290-270 Ma, after the cooling of the leucogranite., The uranium deposits linked to the Hercynian leucogranites in Western Europe are generally supposed to be of hydrothermal origin; the U-Pb geochronological study of the mineralisation linked to the Mortagne leucongranite (South-Armorican massif) confirms this hypothesis and shows that the different ages of uranium deposition correspond to the described paragenesis. Certain ores, like that of the Retail deposit, which is typically intrametamorphic can indicate as many as three mineralising phases, separated by some considerable time: a first phase, between 420-390 Ma, related a caledonian metamorphic event; a second phase, between 345-310 Ma, of remobilisation of the previous phase, contemporary with the leucogranitic magmatism, which seems to have had a role in the transfer of mineralising fluids; and a third phase, between 290-270 Ma, after the cooling of the leucogranite.