The polymetallic quartz-antimonite vein system is located 3-4 km NW and NE of Fojnica. The area consists of Lower Palaeozoic green schist facies metasediments formed under 3-5 kbars and 350°-450°C, intruded by Upper Palaeozoic metarhyolites on which K-Ar crystallization age of 256 Ma was obtained. K-Ar measurements indicate three metamorphic phases: (1) 343 Ma (phyllite); (2) 92 to 121 Ma (metarhyolite) and (3) 37 to 46 Ma (ottrelite schist). The biggest Cemernica vein, NW-SE stretching, can be traced along strike for 1500 m, dipping 30°-70° to the NE for more than 350 m. It was strongly faulted by NE-SW trending faults into four separate blocks. Structure is massive and banded, rarely brecciated. The remnant ore contains (in%) 2.5 to 8.6 (aver. 4.4) Zn, 0.6 to 8.6 (1.9) Sb, trace to 2.0 Hg; < 1.0 W and (in ppm) 74 to 196(156) Ag, trace to 0.7 Au. Paragenesis: quartz (50-95% of ore mass), antimonite, sphalerite, cinnabar, chalcedony, jamestonite, berthierite, boulangerite, plagionite, ferberite predominate over siderite, arsenopyrite, pyrrhotite, marcasite, pyrite. Scarce or accessories are metacinnabar, barite, tetrahedrite, galena, chalcopyrite, realgar, quartz II and gold. The 34S in Sb2S3 is +2.6‰, +3.71‰ in ZnS. Antimonite contains (in ppm): 250Ag; 300Pb; 145Cu; 540As; sphalerite 5Ag; 200Pb; 125 Sb; 70As. Reflectivity power of Sb2S3 (in%): E 35.0; D 31.9; C 29.9. Parameters of anisotropy: angle <lambda> -2°45' to +12°32'; <eta>2-<eta>1 -0.0372 to +0.0848; <chi>2-<chi>1 -0.0081 to + 0.0081; R2/R1 0.71 to 1.33; R2/R1 (in%) E 5.19 to 9.91; D 4.77 to 9.02; C 4.7 to 8.47.
The fluid inclusion in quartz from the Cemernica vein are aqueous and aqueous-carbonic. Their P-T-X characteristics developed during the complex geological evolution of the MBSM. P-T-t metamorphic loop, constructed on the basis of metamorphic phase analysis, bears imprints of regional metamorphism, caused by Alpine collisional processes. Isochores for the aqueous FI-s (calculated by Zhang and Franz equation of state (EOS), using Flincor, 1.4 Version) intersect the retrogressive part of the P-T loop. The same type of the FI-s have been already determined in quartz and hyalophane within schists in the neighbouring area of Busovaca, which grew up in the open clefts, developed during exumation of the hot metamorphics (TmCO2, -60 to -57.2°C, Tm clath, +2.5 to 6.5°C, ThCO2, +14.6 to 29.2°C; ThTot 300 to 322°C). Ore forming fluids are preserved as well (L+V), Th 150-230°C, Te -25°C, Tm ice -16.6 to -11°C, salinity 14.9-20.2 wt% equ. NaCl, and (L+V), Te -31.2°C, 16.9 wt% equ. NaCl, Th 330°C). Analytical data indicate polyphase mineralization, presented in detail.
Oxygen isotope compositions have been determined for olivine and co-existing chromium spinels from dunites and chromite-platinum ores of the Nizhny Tagil (within the Uralian Platiniferous Belt) and Uktus (Middle Urals) massifs. The Mg/(Mg+Fe) ratio in olivine varies from 0.96 (from chromitite) to 0.88 (Uktus dunite) and has a positive correlation with the same ratio in spinel. In almost all samples oxygen is slightly enriched in heavy isotopes compared to primary mantle material (18O = 4.8-6%). Average values of 18O for olivine and spinel are 7.6% and 6% respectively. This could be attributed to the influence of small volumes of crustal material involved in the magmatism associated with island arc subduction. The island arc affinity of the Ural-Alaskan-type mafic-ultramafic complexes is proven by the redox state of the olivine-spinel equilibrium (1.7-4.7 log fO2 above FMQ) determined by the modified Ballhaus-Berry-Green oxygen geobarometer (Ballhaus et al., 1991). The oxygen fugacity varies from a more reduced state (1.7-2.3 log fO2 FMQ) typical for the Uktus dunite and chromitite to the more oxidised Nizhny Tagil dunite (2.1-3.1 log fO2 FMQ) and the higher oxidised Nizhny Tagil chromite-platinum ores (3.2-4.7 log fO2 FMQ). The temperature of the olivine-spinel equilibrium was estimated by well known geothermometers (Roeder et al., 1979; Fabries, 1979; O'Naill et al., 1982; 1987) and modified by Chashchukhin et al. (1997). Temperatures for NT are 1260° for Pt-free chromites, 1050-1030° for dunites and 980-930° for Pt-bearing chromites. Chromites from Uktus yield temperatures between 1450 and 1150°. Redox conditions and relatively low temperatures of formation of the chromites in chromite-platinum ores indicate their formation at the latest magmatic stage, which is in agreement with some geological data but is in contrast to other evidence.
As elsewhere in the French Massif Central, the Late Variscan stage in the La Chataigneraie area is characterized by abundant granites, including porphyritic granites dated around 335-315 Ma (Veynazès, Entraygues, Marcolès, Soulaques) and leucogranites dated around 304 Ma, and by the deposition of As-W mineralization in the schists of the Enguialès-Leucamp district. A stable (O, H) isotopic study was realised on the granites and mineralization, in order to constrain their relationships.The 18O whole-rock values range from +9.7 and +12.5‰ (SMOW) for the porphyritic granites and from +12.5 to +13.0‰ for the leucogranites. These are very similar to the 18O values for granites derived from a continental protolith (Rossi et al., 1988) and are in agreement with the Nd-Sr geochemistry on granites (Downes et al., 1997) and leucogranites (Williamson et al., 1996) from the French Massif Central.The high-temperatures of 500-600°C determined from the quartz-biotite oxygen isotopic thermometry in both porphyritic granites and leuco-granites, correspond to subsolidus temperatures but imply late-magmatic hydrothermal phenomena. The calculated isotopic compositions of fluids in equilibrium with quartz and biotite give a 18OSMOW of +9.7 to +9.9‰ and a DSMOW of -40 to -30‰ for the porphyritic granites, and a 18OSMOW of +9.9 to +10.2‰ and a DSMOW around -50‰ for the leucogranites. The 18O values in both granites are similar to those of magmatic waters accompanying 18O -rich magmas, but the D in the porphyritic granites suggests a slight contribution of metamorphic waters during the subsolidus crystallization. In the wolframite-quartz-tourmaline± muscovite veins of the Enguialès deposit, and in most of the W-barren pegmatitic veins, formation temperatures of 500-550°C were determined from the quartz-tourmaline oxygen isotopic thermometry. The calculated isotopic compositions of fluids in equilibrium with quartz and tourmaline give a 18OSMOW of +9.8 to +10.2‰ and a D around -50‰. The formation temperatures and isotopic signatures of the fluids are thus similar to those of the leucogranites, which strongly suggests a genetic link between the As-W mineralization at Enguialès, the pegmatitic veins and the late leucogranites intruding the porphyritic granites. This interpretation is supported by the Ar-Ar ages around 304±3 Ma obtained on muscovites from the mineralized veins of the Enguialès deposit, the pegmatitic veins and the leucogranites (Monié et al.,1999).
Rossi P, Cocherie A, Meyer G, Fouillac AM & Autran A, Mineral Deposits within the European Community. Special Pub. of the Society for Geology Applied to Mineral Deposits. Boissonnas J. and Omenetto P. (Eds), 6, 3-28, (1988).
Downes H, Shaw A, Williamson BJ & Thirwall MF, Chem. Geol, 136, 99-122, (1997).
Williamson BJ, Shaw A, Downes H & Thirwall MF, Chem. Geol, 127, 25-42, (1996).
Monié et al, J. Conf. Abs., 4, (1999)
At the end of the hercynian orogenesis, the St Sylvestre granite complexe (NW Massif central, France) is submitted to significant hydrothermal activity producing alteration (episyenitization) and metal deposition (U, Au, Sn-W). The ore and alteration paleo-fluid pathways are rather well known only in restricted zones of the mining areas. They were studied at the massif scale thanks to a systematic study of 3D geometry of fluid inclusion planes and of fluid composition using microthermometry and raman spectroscopy on fluid inclusions (GeoFrance 3D program). The results show that outside mineralized area, the fluids circulated among three main sets of microfissures: i) aquo-carbonic fluids (NNW-SSE, ±NW-SE microfractures) related to late-magmatic events, similar to those linked to As (Au) deposition in some intragranitic faults and in the metamorphic surrounding rocks, and trapped under temperatures around 450°C, and pressures ranging from lithostatic values (120 MPa) down to slightly supra-hydrostatic (70 MPa), at a depth of around 4.5 km; ii) aqueous vapours and liquids, showing evidences of heterogeneous trapping (EW microfractures) similar to those considered as linked to the granite episyenitisation in the La Crouzille mining district, and iii) low temperature aqueous fluids attesting of a cooling of more than 200°C after the previous fluid stages.
The first event (aquo-carbonic fluids) is caracterised by elevated temperatures at rather low pressures (high structural level linked to the uplift of the hercynian basement), indicating a high geothermal gradient (80-90°C/km). Such a gradient is similar to those observed at proximity of intrusions in present-day geothermal field such Larderello (Tuscany) or Geyser (California). This confirms the close space-time relationships between the migration of the aqeuous-carbonic fluids and the late hercynian magmatic intrusions (310-305 Ma) which are known to intrude the St Sylvestre massif some kilometers south to the studied zone. Other fluid events seem to be related to the income of meteoric fluids in the system, followed by local fluid immisbility, and cooling, these processes being common features of geothermal areas. During the three main fluid events, unmineralized and unaltered zones of the granite pluton have underwent fluid migration which is attested by the presence of healed microfissures containing the fluids recognized elsewhere in ore bearing faults.
The Upper Carboniferous deposits north of Argentat (Corrèze) dated Upper Stephanian by biostratigraphical methods (Doubinger, 1957), and commonly known as Argentat Basin, are distributed in four outliers of limnic sediments preserved in elongated N160°E trending block. The outliers exhibit a 200 to 300 m stacking of metric to plurimetric, fining upward sequences made up of coarse to very coarse conglomerates and sandstones with subordinates siltstones and thin coal horizons. They have been deposited in a proximal braided river environment, most likely in a high mountains relief. Current directions, mainly based on pebbles imbrication orientation measurements, indicate a complex internal palaeogeography with a northern part where general palaeodrainage is flowing north-westward, whereas, in the south, measurements show a distinctive south-east trending palaeocurrent. Reconstitution of the palaeodrainage pattern are hampered by the present structuration of the outcrops and will require further studies to be clarified. One of the four outliers, located to the west of Le Laurent hamlet, turned out to be the remnants of a complex torrential palaeovalley occupied by a braided river flowing eastward. Conglomerates have yielded numerous pebbles of Au-As hydrothermalized orthogneiss, known in the metamorphic basement, to the west of the Laurent palaeovalley (Bouchot et al., 1999). This palaeoconglomerate is the second Upper Carboniferous occurrence of Au-As mineralised pebbles signalled in the French Massif central and it proves that, in this part of the variscan crust, the main hydrothermal Au-As event is older than Upper Stephanian (i.e. older than 300 to 290 Ma).
Doubinger J., Bull. Soc. Geoil, France, (6), VII : 271-279, (1957,).
Bouchot et al., J. Conf. Abs., 4, (1999).
The Bourneix-Lauriéras mineralized structures are NE-trending faults hosting gold-bearing quartz lenses, formed under "mesothermal" (350°C, 100 MPa) to "epithermal" (250±50°C, hydrostatic pressure) conditions, the latter associated with the main gold-sulphosalt depositing event (Essarraj, 1992). Mineralized structures are hosted by the "Lower Gneiss Unit". Considering the regional trends of As anomalies, the hypothesis of a link between the mineralized BLSZ and the great Argentat fault, 50 km to the east, has been presented (Roig et al., 1997)): mineralizing fluids, drained by the Argentat fault, would have percolated through the "Lower Gneiss" aquifer. The geometry of fluid circulation around the BLSZ was constrained by sampling of metamorphic rocks of the "Lower Gneis Unit" (quartz lenses in micaschists and augen gneisses), late pegmatites and post-kinematic granites, a few kms away from the mineralized structures. Two sets of fluid inclusion planes (FIP) predominate in rocks: a N40-70°E, 50-60°N set, mainly present in the quartz lenses; and a "N-S" set (N160-30°E, 60°E to 60°W), mainly present in pegmatites and granites; two subordinated sets are also observed: a group of "flat" FIP (N110-130°E, 20-30°N), and a sporadic set of "E-W" FIP (N80-110°E, 60°N to 70°S). Most of the fluid inclusions are two-phase aqueous, homogenizing in the liquid phase. However, in a few cases, some fluid inclusions of the N40-70°E set display a low-density volatile component (CO2>>N2>CH4) and homogenize in the vapour phase, up to 400°C. The salinities of the dominant fluids are variable, from 14% to 1.7% wt. eq. NaCl, corresponding to a mixing trend between a hot saline fluid and a cooler low-salinity fluid, under fluctuating pressures, as recorded by Th variations at a given salinity, at an estimated depth of c. 5 km. The temperatures were comprised between 360 and 290°C. Observed fluids display similarities with those described by Essarraj (1992) for the early gold stage event at Lauriéras. It may thus be concluded that ore fluids percolated the Lower Gneiss Unit through microfracture sets, and that the hydrothermal system affected larger volumes than the major fluid pathways, i.e. the major fault zones.
The precise hydrothermal fluid composition of the latest mineralization stage of the crustal scale As-Au-Sb hydrothermal event, emplaced around 300 Ma, is an important marker in the reconstitution of the end of the Variscan orogeny. This hydrothermal event could be divided into three main stages (e.g. Bouchot et al, 1998):
AsW-rich stage 1, characterized by complex CO2-CH4-H2S-N2-(H2O-NaCl) fluids;- Au-rich stage 2, with chemical diversifications such as Zn-Pb-Cu or Bi-Pb-Ag-Zn associated with H2O-NaCl fluids;- Sb-rich stage 3, poorly documented, for which complex low saline and CO2-rich fluids characterized by temperatures higher than 260°C are reported (Brioude-Massiac, Brill,1983).
In order to characterized the fluids related to the Sb-rich stage, a fluid inclusion study was carried out on the stibnite of the symptomatic 'Les Brouzils' deposit (Vendée, Massif Armoricain, France). In the studied samples, the gangue is composed of three successive quartz facies: a thin band of comb quartz along the vein walls, arsenopyrite-bearing microcrystalline quartz and end-sequence euhedral macrocrystalline quartz. Stibnite-carbonate occurs in cavities or in late veinlets crosscutting the quartz gangue.
This microthermometric study was performed by using an infrared light source (~2500nm) in order to characterize P-T-X precipitation conditions of stibnite. Primary two-phase, aqueous fluid inclusions are elongated along {110} or {010} of stibnite crystals. Salinities and homogenization temperatures are low, ranging between 3.5 and 4.7 wt.% NaCl eq., and between 140 and 160°C, respectively. These results coupled with quartz texture suggest shallow level condition for stibnite deposition.
Two other fluid types are identified in stibnite in secondary inclusion trails perpendicular to {110} or {010}. The first one, exclusively in stibnite, is characterized by a low salinity (1.35 to 2.2 wt.% NaCl eq.), an average homogenization temperature of 215°C and the presence of included solid phases. The latest fluid type, identified in secondary plans in stibnite and in macrocrystalline quartz is characterized by H2O, probably KCl and an non-identified volatil component and an average temperature of 180°C.
This study shows that infrared microscopy is a powerful tool for obtaining direct informations about fluids involved in stibnite mineralization and demonstrates that fluids in gangue record latest hydrothermal events.
Groix Island is well known in the Variscan belt for its large occurrences of glaucophane schist and prasinite boudins inside micaschists. The glaucophane schists and prasinites are estimated to be former alkaline and tholeiitic basalts probably dragged in a subduction zone and subsequently sliced and partly retrograded in greenschists, presumably during their uplift and obduction. The micaschists are considered to be former pelitic continental sediments metamorphosed at 420 Ma (Peucat and Cogné 1977; Bernard-Griffiths et al. 1986). Discrete manganese-concentrations appear in different sites on the island. On the south-east coast between Locmaria and Pointe des Chats, Mn is weakly concentrated in ribbons, decimeter-thick, of a garnet-bearing quartzite included in glaucophane schists. These quartzites are composed of Mn-rich almandine garnet (Py3Alm46Sp34Gr17) with quartz, magnetite, ilmenite, rutile, a ferric omphacite and rare glaucophane. They correspond to metachert (former radiolarites ?) in which eclogite "microboudins" were scattered. In the south-east and near the "sables rouges" beach, in the east part of the island, high Mn-concentrations occur as centimetric to metric boudins of garnetites and quartzite included in micaschists. In that case, they are mainly composed of nearly pure spessartite, quartz, ilmenite, rutile and locally piemontite. The spessartite exhibits frequent Ca-, Mn-, Mg- carbonate inclusions (200 µm) and local euhedral low Al-content core (16% Al2O3). Among these high Mn-concentrations, a black, dense and metric boudin with a peculiar mineralogy has been recovered. Its decimeter-thick crust is composed of successive millimetric layers of spessartite and jacobsite, Fe-Mn hydroxides, pyrophanite and scarce REE arseniates. The core displays an unzoned assemblage, devoid of Fe-oxides, and dominated by tephroite, Mn-humite, rhodocrosite (with CaO Å7-8%), Al-poor garnet (25% Al2O3) of calderitic type and pyrophanite with rhodonite, zircon and As-bearing fluorapatite accessories. The spessartite overgrowth clearly postdates the tephroite, rhodocrosite and calderite paragenesis, through an intermediate crystallization of a high-Al calderite (or low Al-bearing spessartite Å12% Al2O3) and rhodonite. The presence of As-rich minerals support an exhalative origin of these Mn- and carbonate-rich, Al- poor lenses. The similar Mn-ores which occur within or in the vicinity of glaucophanitic rocks in the "schistes lustrés" (Alps), in the Franciscan zone and in the Cyclades, and the experiments on calderite show that the parageneses exposed are compatible with subduction zone environments. At Groix Island, these high-Mn concentrations likely result from initial hydrothermal activity. They have preserved HP relics and three steps of a retrograde metamorphic path, as registered by the metabasalts and sediments of a probable accretionary prism.
Microscopic investigations of La Zarza "stockwork" ores reveals that, following the ductile deformation of early quartz Q1 (± chlorite I± barite) and pyrite veins (i.e. the primary feeder stockwork which display macroscopic deformation) and disseminations, occurred the post-kinematic crystallisation of a polymetallic sulphide paragenesis, including arsenopyrite, sphalerite, chalcopyrite and galena (± Au). Arsenopyrite and sphalerite were first formed, associated with partial recrystallisation of deformed Q1 (Q2), whereas the other sulphides were cemented by newly deposited quartz (Q3) (± chlorite II ± barite). Although there is evidence of primary fluid inclusions in early Q1, these inclusions were decrepited during the Variscan tectonometamorphism, and therefore, the information relative to the deposition of the massive sulphide ores was lost. All the observed fluids are post-kinematic, occurring either as undeformed fluid inclusion planes (FIP) or as primary inclusions in QII and QIII. Similar observations were made at Tharsis. A detailed microthermometric and Raman study of fluid inclusions shows that the post-kinematic minerals (QII, QIII and polymetallic sulphides) crystallized from aqueous fluids with moderate salinities (around 4± 2 (up to 9) wt.% eq. NaCl), exhibiting sometimes a very low density volatile component (CO2 or CH4). Homogenization temperatures vary from 320°C (early recrystallized quartz QII) down to 180°C (quartz III+ cpy tension gashes). As these fluids clearly post-date the ductile deformation of the massive sulphides, they are interpreted as retrograde late Hercynian fluids. The highest estimated P-T conditions (50-200 MPa; 300-350°C) are consistent with the peak metamorphic conditions described in the South Iberic Pyritous Belt. The recorded fluid circulation begun probably under P-T conditions close to the peak metamorphism, but subsequent fluid movements were apparently favoured by decompression after a pressure drop linked either to the basement uplift and/or tectonic events. Although the degree of redistribution, transport distances, and degree of metal enrichment with respect to primary concentations cannot be deduced from the present study, the data demonstrate that retrograde metamorphic fluids played a significant role in textural, and mineralogical changes which affect the ores in the surroundings of the massive sulphide bodies.
This work has been carried out within the framework of GdR "Métallogénie" (Theme I), thanks to BRGM and SEIEMSA, and especially with the help of J.P. Milesi.
Exploration strategies have generally considered Grenvillian gneiss complexes sterile. Discovery of the extensive Cu-Au hydrothermal system in the Bondy gneiss complex in western Quebec led to a ressessment of their economic potential. Making use of mineralogy and geochemistry, we established (1) the nature of gneiss protoliths hosting the hydrothermal system as volcanogenic, (2) the paleo-environment of the gneiss complex to be a volcano-plutonic edifice formed in arc environment, and (3) the hydrothermal activity to be polyphased with early VMS-type alteration and later Cu-Au precipitation in chemical traps. We illustrate that the coarse grain-size of high-grade metamorphic rocks is a tool for field identification and characterisation of hydrothermal systems, and that despite the metamorphism, original REE and HFSE features are preserved.
In high-grade gneiss complexes, recognition of stratigraphic markers is difficult, but the coarse-grain size of minerals and their proportions permit to trace geochemical variations, and draw an alteration zonation pattern right in the field. Relative K, Ca, Al, Mg, Fe, Cu and B enrichment are mapped by unusual contents of (1) biotite, (2) clinopyroxene or plagioclase, (3) sillimanite, cordierite and garnet, (4) bronzite, (5) magnetite, (6) chalcopyrite and (7) tourmaline/kornerupine. Major elements data confirm these relative enrichments. A fossil chlorite alteration zone is recorded by the MgO enrichment in cordierite-orthopyroxene gneiss and aluminous gneiss, that grades into CaO leached orthopyroxene-magnetite rich gneiss and then into a K2O alteration zone with biotite-rich garnetite and K-feldspar rich gneiss. Magnetite-rich units are the main host for mineralization.
In this Cu-Au hydrothermal system, the typical relationship between Au and As is not observed. As As require Cl-complexing agents to be soluble, while Au is soluble both in F and Cl-based fluids, the hydrothermal fluid was certainly F-based. This is compatible with the sulphides REE-profiles which do not display the typical positive Eu anomaly inherited from sulphide deposition from Cl-brines. By contrast to VMS deposists, Cl-complexing agents were not a major ligand.
In some alterated facies, we observe an enrichment in Zr and the development of V-shaped REE profiles. Such anomalous signatures imply rock interaction with reducing hydrothermal solution. Such profiles, produced by a leaching of LREE and MREE coupled with an increase in HREE, are reported in porphyry environment with propylitic, argilitic and phyllic alteration, and are related to F and/or CO3 complexing agents. These analogs point toward the Cu-Au mineralization to be related to a F-rich orthomagmatic fluid derived from felsic plutons suggesting a porphyry environment.
In high-grade terrains, remobilization of trace elements during metamorphism is common. However, despite variable degree of LILE depletion, the REE, HFSE and Nd isotopic data of unaltered facies preserved the original patterns of their magmatic precursors, and suggest a calc-alkaline island-arc volcanism with local intra-arc rifting.
The St-Georges d'Hurtières deposit is located in the Belledonne Massif, External Crystalline Massifs of the Alps. The host rock is the "Série satinée" a thick Lower Paleozoic schist sequence. It has been affected by at least two orogenic events during the Variscan with ductile deformation and greenschist facies metamorphism and during Alpine times with brittle deformation. Two mineralisation events are recognized. An early Mn-rich siderite vein, which is discordant to the schistosity and contains barren quartz and barite in the upper orebody. The second event consists of quartz veins containing chalcopyrite, pyrite, galena, sphalerite and tetrahedrite, whereby siderite is partly replaced by secondary ankerite. In addition, disseminated barite and galena occur within a Triassic limestone lens immediately above the "Série satinée".
Isotope systematics indicates that Sr and Pb in the different mineralisations are predominantly derived from the "Série satinée" at different times. Early siderite and associated barite have 87Sr/86Sr ratios between 0.71695 and 0.72394 , within the Sr isotope range of the "Série satinée" during the Variscan. They yield variable Sr contents that may indicate mixing between two fluids or variation of the partition coefficient during vein formation. The later stage ankerite yield 87Sr/86Sr ratios of 0.72560 to 0.72787, and fall between the ratios of the siderites and the "Série satinée" during Alpine times, it supports an Alpine age for the quartz-sulfide veins. The siderites are strongly enriched in REE especially in LREE, this indicates an acidic and hot mineralising fluid. Barites associated with the early siderites and the ones hosted by the Triassic limestones have very distinct isotopic compositions (barite in siderite: 18O = 9.166 - 10.347; 34S = 26.3 - 28.8; 87Sr/86Sr = 0.71695 - 0.71868, barite in limestone; 18O = 7.036; 34S = 19.2; 87Sr/86Sr = 0.72262). The Pb isotope compositions of galena within the Triassic limestones (206Pb/204Pb = 18.388; 207Pb/204Pb = 15.677; 208Pb/204Pb = 38.779) are less radiogenic than the galenas and the chalcopyrites within the quartz-sulfide veins (206Pb/204Pb = 18.413 - 18.499; 207Pb/204Pb = 15.611 - 15.727; 208Pb/204Pb = 38.661-38.835). It indicates that mineralisation in the Triassic limestones and the veins in the "Série satinée" record different hydrothermal events.
Palaeozoic chains as the Variscan belt, the Urals or the Kazakhstan belt represent major metallogenic provinces on the Earth. Large ore deposits and widespread hydrothermal palaeofields show a complex interaction between heat flow, magma generation and structural evolution. Studies undertaken in the French Massif Central (GéoFrance 3D) show that the onset of the hydrothermal palaeofield corresponds to peculiar stages of the orogenic evolution. Two main metallogenical crises are defined in relation to the tectonic context:- At about 360 Ma (Devonian-Carboniferous boundary), the achievement of the Mesovariscan collision tectonics between Armorica and Gondwana is well marked by thickening of the lithosphere from Galicia to Bohemia. At the end of the exhumation of the previously buried lithosphere, extensional areas were developed either in back-arc or intracontinental rifts. Massive sulfide deposits (Cu, Zn, Sn) are formed at this stage in the Brevenne area and in the Iberian Pyrite Belt.- Between 325 and 305 Ma (Namuro-Westphalian) the Neovariscan collision, corresponding to the final closure of the Rheic ocean, changes to the late Variscan extension which will culminate by the development of crustal scale extensional shear zones associated with granitic-migmatitic dome emplacement. This granitic magmatism is the result of the involvement of a fertile hydrated juvenile crust herited from the earlier collision processes. During crustal thinning, associated folding and fracturing of the lithosphere returning to its normal thickness, control the emplacement and trapping of W-Au-As-Sb-bearing concentrations at different depths in the crust. Coal-basins of Stephanian age contain reworked primary Au- mineralizations.
These two metallogenical crises are suggested to belong to large-scale geodynamic events that would have affected the whole lithosphere at a given time. For the first crisis, crustal thickening followed by changes in tectonic regimes (extension) leads to new intracrustal thermo-mechanical conditions. The second crisis would be associated with enhanced exhumation and erosion rates within the Variscan belt. In both cases, partial melting of the intermediate crust was generated before and during the mineralization event, leading to strong perturbations of the thermal regime.
Preliminary thermal and mechanical models show that lithospheric-scale instabilities are likely to occur due to the changes in the boundary and internal conditions: - Partial melting occurring within only a few million years can be attributed to a new equilibrium thermal regime, related to a vertical redistribution of heat production. As a result, geometry of the brittle-ductile transition (BDT) can be strongly modified, as well as fractures density and location. - large scale crustal faults can cut through the BDT down to the depth of 20-30 km and favour for upward fluid suction from the granulitic crust to the upper crust and intermountain basins.
A new tectonic map of the Cévennes area in the para-autochtonous domain of the Hercynian belt of the French Massif Central is proposed on the basis of the identification of three lithological series, namely: i) the Cézarenque gneiss series, ii) the black micaschists series and iii) the sandstone-pelite unit. These three series form an imbrication of five tectonic units which overlie the unmetamorphozed Paleozoic unit of the Viganais to the south. To the north, the micaschist stack of nappes tectonically overlies a gneissic unit distinct from the "lower gneiss" nappe of the Massif Central. This "infra-micaschist" gneiss unit which outcrops in a tectonic window corresponds to the deepest domain outcroping in the Cévennes area. It is also an allochtonous thrust stack which ends in a wedge-like style below the Mt-Lozere pluton in agreement with gravimetric modelling.
The tectonic, metamorphism and magmatic evolution of the Cévennes area is divided into three stages.
1) S-SW shearing. Several deformation phases with a general southward vergence are responsible for nappe emplacement coeval to a MP/MT metamorphism dated by 40Ar/39Ar around 340 Ma.
2) Early anatexis. The infra-micaschist gneiss unit is reworked by a melting event under T<750°C and P>5 kb conditions older than the Velay migmatite. A radiometric date of 315±4 Ma might suggests relationships between plutonism and migmatization.
3) Namurian plutonism and extensional tectonics. During the postcollisional crustal thinning, around 315 Ma, granodiorite emplacement is controlled by E-W stretching. The plutons are the driving power of the hydrothermal convective flows responsible for the early formation of diffuse arsenopyrite grains in the thermal aureole. Gold bearing sulfides are afterward concentrated in quartz veins along brittle normal and strike-slip faults related to the last stage of pluton emplacement. Lastly, ore bearing quartz pebbles are eroded and sedimented in the Stephanian coal basin.
In the southern part of the Massif Central (France), ore deposits (W, Au, Sb) are genetically associated with the development of different tectonic and magmatic events at the end of the Variscan orogeny. In order to better decipher the sequence of the various stages that contributed to the gold concentration in the Cévennes and Châtaigneraie areas, a large set of laser-probe 40Ar/39Ar data has been obtained from different representative localities sampled during the Geofrance3D - 3D Mapping and Metallogeny of the Massif Central project. In the Cévennes, after an initial stage of crustal thickening, a regional HT/LP metamorphism developed in relation with an extensional tectonic episode dated at 330-325 Ma by mica ages from different metamorphic zones. The lack of internal age gradients within micas is interpreted to result from fast cooling after peak metamorphic conditions. This is immediatly followed by the emplacement of two main granitic bodies, the Mont-Lozère and Aigoual massifs, for which amphiboles and micas from these intrusives and their hornfelses yield cooling ages in the range 315-325 Ma, in agreement with some previous Rb-Sr and U-Pb ages. In the Châtaigneraie massif, the Veinazes porphyric granite also produces a biotite age close to 315 Ma which is considered as a minimum emplacement age. A first generation of ore deposits is associated with this magmatic activity as indicated by the age of hydrothermal muscovite at 313 ± 3 Ma from a Au-As-Sb vein (Pont de Rastel) in the Cévennes micaschists. However, most of these hydrothermal muscovites from the Cévennes (Travers, As-Au) and Châtaigneraie (Enguiales, As-W±Au±Bi) yield a narrow age range of 304 ± 2 Ma. In the Cévennes, the northern and western borders of the Mont-Lozère granite (Touril, St-Etienne du Valdonnez) are affected by greenschist facies mylonitisation for which synkinematic muscovite yields similar ages of 304 Ma. This indicates that gold concentration in the Travers postdates emplacement of the Mont-Lozère granite and is synchronous with some late shearing activity. In the Châtaigneraie, muscovites from late leucogranite bodies give ages of 306 ± 3 Ma which suggests in this place a genetic link between ore concentration and magmatic activity. The absence of age gradients within muscovite attests for a fast development of the hydrothermal assemblages. These results are used to propose a model of gold deposition at the end of the Variscan orogeny which involves two main hydrothermal stages: the first one associated with the emplacement of 315 Ma old porphyric granites (As concentrations ± W), the second one mainly related to emplacement of 305 Ma old leucogranites (Au then Sb concentrations).
The Mont-Lozère-Borne granitic complex, located in the Cévennes area (french Massif Central) is composed by the Pont-de-Montvert porphyroid granite and by the earlier leucocratic Bougès-Signaux granite. Its emplacement is dated at about 315 My and is coeval with the late-hercynian extension that affects the whole Massif Central at this period.From structural and gravity data acquired on the field by our team as part of the GéoFrance 3D program, a 3D model has been produced. It is based upon the combination of field geological observations with a series of cross sections constrained by detailed gravity studies. The geometrical model we propose, based on a Voronoi-type triangulation, indicates both the shape of the plutons and their relationships with the micaschistous and gneissic basement. Moreover, a forward computation of the 3D gravimetric effect of the model enables us to verify its pertinence.In agreement with the previous AMS and petrofabric studies (Fernandez, 1977; Faure et al., 1992; Talbot et al., this congress), the obtained model exhibits important variations within the general laccolithic shape. They can be imputed to the location of the root of the plutons and to the rheology of the surrounding rocks (granite or micaschists). This 3D model is a new step in the knowledge of the late-hercynian plutons from southern Massif Central and provides us a useful tool in the study of the hydrothermal paleofields associated to granite emplacement. In the light of this model, the settings of fluid flows in the country rocks as well as the mode of emplacement of the gold-arsenic-stibium mineralizations will be discussed.
Faure M, Pons J & Babinault JF, C. R. Acad. Sci. Paris, 315, 201-208, (1992).
Fernandez A, C. R. Somm. Soc. géol. Fr, 3, 137-140, (1977).
Talbot JY, Chen Y & Faure M, J. Conf. Abs., 4, (1999)
Individual fluid inclusions from quartz veins at Pont-de-Rastel and Alteyrac gold mines, (Cévennes, France) have been investigated for their major-and trace-element composition using synchrotron radiation X-ray fluorescence (SXRF). The analyses were performed on the beamline ID-22 Micro-FID (Fluorescence, Imaging, Diffraction) of the European Synchrotron Research Facility (ESRF), Grenoble, a third generation synchrotron light source. The experimental set-up ensured a focal spot at the sample position of 2x7 µm2 and a flux of 1010 ph/sec. As a focusing optic, a Fresnel zone plate (FZP) was positioned in the monochromatic beam. The energy window, in which fluorescence could be detected, was comprised between the low energy background of the Si-Li detector (1 keV) and the energy of the incident beam (15 keV). An excitation energy of 15 keV was chosen because it is just above the absorption edge of gold, thus optimizing the sensitivity for elements such as Au, As and Br.
Two main populations of fluid inclusions are present in the studied samples. These are : Type 1) two (H2O liquid and CO2 vapor) or three phase (H2O, CO2 liquid and CO2 vapor) water-dominated H2O-CO2 inclusions. Calculated H2O, CO2 and NaCl molar fractions range between XH2O = 0.97-0.45, XCO2 = 0.53-0.02 and XNaCl = 0.003-0.010; Type 2) two phase (H2O liquid + vapor) fluid inclusions containing less than 5 wt% equivalent NaCl.
The spectral analysis of the data obtained from individual inclusions indicates that type 1 inclusions are Zn- and Br-dominated aqueous brines with minor amounts of Fe ± Cu and that type 2 inclusions contain significant proportions of As, Cu and Fe ± Ca and Ni. In addition to these, we found a major enrichment in gold in one type 2 inclusion from a vein quartz at Pont-de-Rastel. Optical examination under the microscope show that the fluid inclusion is devoid of solid phases. Fluorescence X-ray scans and 2D-map collected above and around the inclusion revealed that the Au X-ray signal is clearly emerging from the fluid inclusion. A study is now underway to better constrain the distribution of gold in the studied samples. The preliminary data presented here indicates that highly-intense synchrotron source is ideally suited for unraveling the evolution of hydrothermal systems responsible for the concentration of metals of economic interest in the Earth's crust.
We have analyzed using the electron and nuclear microprobes (PIXE -PIGE), the trace-element-content of selected magmatic, metamorphic and hydrothermal minerals from two districts of the French Hercynian Orogen: Chataigneraie (CH) hosting two W vein-deposits and Cévennes (CE), West and East Massif Central, respectively. The samples are either related in space and/or time to magmatic events between 330-300 Ma (mineralized W-veins, tourmaline-rich rocks from CH), or unrelated to them.
(1) In CE, apatite crystallized at 14 km-deep during the Visean thrusting (Arnaud, 1997), and later at barrovian metamorphic conditions in the 330-300 Ma range (either co-precipitated with wolframite in veins, or in tourmaline-rich rocks). All these apatites contain Sr and Y in the 300-3000 ppm range and are F-rich. The late Hercynian apatites however always contain As >300 ppm, sometimes associated with Bi (150 ppm) and In at the detection limit level (LOD=300 ppm). (2) Ti-oxides from granite-related tourmaline-sulfide veins in the CH contain a wide range of major and trace elements: up to 3 wt.% W, 1000 to 8000 ppm Fe, Nb and Ta, 1500 ppm Zn, 800-1000 ppm Sn, 400-1200 ppm Zr, and In at the LOD. (3) Arsenopyrite from both W-veins and sulfide-tourmaline-rich rocks contain 1500 ppm Cu. The former may additionally contain 130 ppm Zn and about 400 ppm Au (2xLOD). Pyrites from W-veins contain Cu and As in the 2000-3000 ppm-range and 400 ppm W. Some pyrites with lower Cu, As and W-contents are enriched in Bi (2500 ppm). Finally, some other pyrites contain Ni, Cu and Se, as also found in pyrites from calc-silicate gneisses (CE) of probable metamorphic-metasomatic origin (Gibert et al., 1992), and in sulfide-poor tourmaline-rich rocks. (4) Tourmalines from leucogranites (probably hydrothermal), from tourmaline-rich rocks and W-veins are aluminous, both alkali- and proton-deficient. All samples contain Fe-rich tourmalines (0.55< Fe*=(Fe/Fe+Mg)at <0.75), with two additional generations in W- veins and tourmaline-rich samples (Fe*=0.5 and Fe*=.4). The trace-element content of tourmalines is (ppm) : 50<Ga<200, 140<Zn<300, 100<Sr<400, with additional W and As (Ti inclusions ?), with often F.
These trace element data reinforce the genetic and temporal link between W-(±Sn-Au) mineralizations and the leucogranites (Boissavy-Vinau, 1979). However, additional microstructural and FI studies show that the tourmaline-bearing rocks fossilize a major metasomatic/hydrothermal event contemporaneous with the leucogranite magmatism: at the late Hercynian, hot CO2-Fe-bearing fluids infiltrated the upper crust at the favour of intense fluid-assisted (micro)cracking. This thermal-metasomatic event accompanied and probably caused the rapid uplift of the Belt at the Stephanian.
Arnaud F, Unpubl. Thesis, INPL Nancy, (1997).
Gibert F, Moine B, Schott J, Dandurand J, Contr. Min. Pet, 112, 371-384, (1992).
Boissavy-Vineau M, Unpubl. Thesis, Univ. Paris VI, (1979).
Previous paleomagnetic studies of Visean volcanic and plutonic units from the northern Massif Central, the Vosges, Black Forest, Odenwald and Spessart, have shown that these massifs have undergone the same tectonic and hydro-thermal history after 340 Ma, which is the age of emplacement of the units and of the primary paleomagnetic directions Cn. The post-340 Ma story is the following: - a prominent compression phase leading to uplift, folding and magnetic overprinting (Cp directions) in the time range 335-330 Ma, - a 75°, clockwise rotation of the belt around 330 Ma, - large scale magnetic overprinting in relation with green-schist facies metamorphism from 328 to around 320 Ma (the B components), - a 45° clockwise rotation in the time range 320-310 Ma, - a less important overprinting phase (the A1 components) related to the magmatism at 310-300 Ma, - a last prominent overprinting phase in the Late Permian, around 260-250 Ma (the A components). In contrast to the previous overprints that are in general carried by magnetite, the latter A overprints result from an oxidizing phase leading to hematite.
In Limousin, various metamorphic rocks, tonalites and granites from 34 sites have been paleomagnetically investigated. 17 sites from a previous study are added to the results. The paleomagnetic story is the same as in the north-eastern massifs. All groups of directions are represented, with a predominance of Cp (335-330 Ma) and B directions (328-320 Ma). The Cp components are attributed to the last granitic magmatism. The B and later overprints result from the hydrothermal and tectonic activity that was recorded up to the latest Permian. A map, representing the location, the degree of overprinting and the age of the event, has been produced.
From the tectonic point of view, the Limousin has undergone the same motions as the central part of the Variscan belt. In addition, in the southern part of the massif, the C and part of the B components, have been tilted. This means that, in the latest Visean-Namurian, during the relaxation that followed the major clockwise rotation and during acquisition of the B overprints, the basement was tilted toward the south. This result agrees with the recent GeoFrance 3D seismic section that shows southward dipping units in southern Limousin.
Late carboniferous, coal bearing intermontane basins are widespread in the Variscan French Massif Central. These basins are characterized by thick clastic sedimentary infillings, interbedded volcanic rocks and a close association with faults. Our focus for these basins is based on the fact that their study may aid understanding the geologic processes operating during the post-collisional stages of the Variscan orogeny. A precise determination of their formation ages should, therefore, help pinpoint the timing of the extensional processes affecting the orogen. This study presents combined geochronological results from the Bosmoreau Stephanian basin located at the northern end of the Argentat fault (U-Pb on zircon and K-Ar on illite) and from the shear zone associated with block movements along this fault (40Ar/39Ar on muscovite). Single zircon grains extracted from a fireclay at the base of the sedimentary sequence, 10 meters above basement, provide an age of 332±4 Ma interpreted as the age of eruption of the magma and of deposition of the airborne ash in the Bosmoreau basin. The fireclay contains also a limited amount of xenocrystic zircons having ages of ca. 390-400 Ma, and reflecting ages of basement components. Clay fractions extracted from a shale underlaying the fireclay yield a consistent K-Ar age of 342±9 Ma attributed to crystallisation of these minerals during diagenesis. Upwards, siltstones sampled at different stratigraphic levels provide Stephanian K-Ar ages ranging from 295±7 to 300±7 Ma, in good agreement with paleobotanical evidence. These results demonstrate that the Bosmoreau basin, consisting mainly of Stephanian sedimentary infillings, contains older sedimentary units and was actually initiated during the late Visean. The location of the basin in the hanging-wall blocks of the Argentat fault also oulines a genetic link between fault motion and basin opening. This was checked by 40Ar/39Ar dating of muscovite sampled in the shear zone associated with the fault. Muscovite yields 40Ar/39Ar plateau ages ranging from 335±4 Ma to 337±4 Ma, similar to the U-Pb and K-Ar ages obtained from rocks of the lower part of the sedimentary pile. These results indicate that magmatism and associated volcanism, block movement along crustal-scale fault, erosion and basin opening were contemporaneous and shed new light on the Visean-Westphalian synconvergent extension that was previously only described as a deep-seated event.
The Argentat Fault formed a major channelway for the circulation of arsenic-gold-bearing fluids in the crust at the end of the Variscan orogeny, as is shown by the hydrothermal Gold-300 event. The hydrothermal palaeofield, characterized by a strong As+Au anomaly in stream sediments, results from the dissemination of pyrite-arsenopyrite in the orthogneiss along the Argentat Fault (Bouchot et al., 1997). This disseminated mineralization, in places associated with gold-bearing quartz stockworks, was emplaced in the brittle domain following ductile movement of the Argentat Fault dated at about 330 Ma (Roig et al., 1997). The discovery of a palaeoconglomerate with reworked Au-As-bearing pebbles in the Argentat basin proves that the mineralization was trapped before deposition of the sediments. These Carboniferous rocks, attributed to the Upper Stephanian by biostratigraphical methods (Doubinger, 1957), accumulated in a synform of the metamorphic basement. According to Genna et al. (1998), this folding would have been synchronous with the sinistral reactivation of the Argentat Fault in the shallow brittle domain.
The mineralized conglomerate was identified near Le Laurent hamlet in a very steep palaeovalley (slopes of ~45°), some 300 m wide, that flowed from west to east oblique to the main palaeodrainage basin system. This valley is filled by coarse polygenic fluviatile conglomerates (matrix <15%, imbricated pebbles) of the braided river type. The pebbles (orthogneiss >> micaschist) are rounded and fairly well sorted with an average diameter of 15-25 cm (maximum 50 cm). According to these sedimentological parameters, the Laurent palaeovalley belongs to a braided network of the proximal high-mountain torrential valley type (Becq-Giraudon et al., 1999). Consequently, the source of the mineralized pebbles should be looked for to the west of the basin, in the metamorphic basement that contains large volumes of orthogneiss.
Most of the reworked orthogneiss pebbles show early pervasive sericitization with a network of thin pyritic quartz veinlets and later cataclasis, similar to the mineralized basement orthogneiss. Moreover, in the rare pebbles, the sericitization is associated with a strong dissemination of dominant pyrite (~30%) ± stibnite and traces of sphalerite and galena. An ICP-Au rock analysis of some 10 samples shows that the mineralized pebbles are characterized by an As-Au-(Sb) signature typical of the Gold-300 event, with grades of between 350 and 7000 ppb As, 15 and 50 ppb Sb and, for four of the samples, between 100 ppb and 1 g/t Au.
In substance, the presence in the Argentat basin of conglomerate with reworked Au-As-mineralized pebbles means that the hydrothermal Au-As event can be positioned between the ductile movement of the Argentat Fault at around 330 Ma, and the fault's brittle sinistral strike-slip reactivation subsynchronous with the filling of the Argentat basin at 300-290 Ma.
Becq-Giraudon, J. Conf. Abs. 4 (1999)
Bouchot et al, Chron. Rech. min., 528, 13-62, (1997)
Doubinger J, Bull. Soc. Geol, France, (6), VII : 271-279, (1997).
Genna et al. C.R.Acad. Sci. Paris, 327, 279-284, (1998)
Roig et al, Chron. Rech. min., 528, 63-69, (1997)
The Vaulry Sn-W-(Au) mineralization, located in the Blond rare metal granite (c. 310 Ma), consists in a set of subvertical N10-30°E quartz veins (2 to 10 cm wide), with sometimes greisen selvages. The sequence of deposition was: 1) barren quartz I, predominant as fracture filling, generally affected by subsequent ductile deformation; 2) quartz II-wolframite-cassiterite; 3) arsenopyrite-chalcopyrite-pyrite. Rare native gold grains have been found. The geometry of fluid migration was constrained by sampling along a 8 km long E-W profile in the Blond granite, starting from the Vaulry deposit to the east. Two major sets of subvertical fluid inclusion planes (FIP) are observed throughout: a N50±10°E set, mostly developed in the quartz I veins and their close surrounding granite (up to 500 m from the veins); and a N130± 10°E set, mostly developed in the granite. As a whole, the density of FIP decreases from the mineralized zones toward the barren part of the granite body. Mineralizing fluids were observed as primary inclusions in cassiterite and in undeformed quartz QII quartz with ore minerals. They are similar to the fluids observed in the N50±10°E set of FIP. These fluids are mostly aqueous, with moderate salinities (3.7 to 5.5% NaCl). A minor volatile component (1 to 2 mole% CH4 + N2) is sometimes present, up to 2 km from the mineralized quartz veins, at variance with many other Sn-W deposits in the Variscan belt, in which ore deposition was related to CO2 rich fluids. Ore deposition occurred at rather high temperatures around 350-400°C, probably in relation with a mixing between a fluid of moderate salinity and a low-salinity fluid, under fluctuating pressures (¾ 120 MPa). Direct determination of fluid composition in fluid inclusions by LA-OES is also in favour of a mixing process, involving a fluid characterized by elevated Na/Li ratios, typical of a possible magmatic component or of a fluid having strongly interacted with a granite, and an external component characterized by the presence of Ca and a low density volatile component. From an isotopic point of view 18O values for quartz ranging from 11.1 to 12.2), quartz crystallized from high temperature fluids equilibrated with crustal rocks in great excess.
This work is part of the Geofrance 3D program.
The Iglesiente-Sulcis mining district (ISMD) in SW Sardinia is characterized by a number of small, shallow-level to subvolcanic granitoid intrusions. In their aureoles base metal deposits were mined for c3000 years. Scarce literature data assign these intrusions a Permo-Carboniferous age. Recent Sr-Nd-O whole rock isotope data on intrusions from Arburese [Secchi et al., 1996], 20 km N of ISMD, emphasize the ubiquitous role of hydrothermalism.To constrain the evolution of the mineralizing fluids in the shallow hydrothermal systems we analysed granitic and vein minerals by 39Ar/40Ar: 2 biotites from the almost barren granodiorites of Chia and Capo Pecora; 2 biotites and 1 K-feldspar from the mineralized leucogranites of Tiny and Perda Niedda; 1 celsian and 1 armenite from Su Zurfuru.All samples show discordant Ar/Ar age spectra. The biotites have low K concentrations (4.2 to 6.4% K), due to partial chloritization. By using three-isotope correlation diagrams [Villa, 1999; Villa et al., 1996] it is possible to unravel the primary magmatic age (or the rapid post-magmatic closure) from the chloritization age of =270 Ma. The primary magmatic ages appear to be different: for the barren granodiorites they are estimated at 320-330 Ma, for the two leucogranites = 310 Ma. K-feldspar recrystallized extensively and gives =270 Ma. Celsian and probably also armenite first formed around 270 Ma, and later underwent at least one further extensive disturbance from a low-Cl, high-Ba fluid.The isotopic record, if perused carefully, confirms the importance of the fluid circulation.The alteration events and the vein-filling occurred around 50-60 Ma later than magmatism, at a time when these shallow intrusives had no residual heat left to fuel the hydrothermal circulation system.Conclusions: 1. Removing the effects of alteration, the Chia and Capo Pecora granodiorites are found to be older than 320 Ma, pushing back the onset of magmatism in Sardinia. 2. Hydrothermalism is ubiquitous; probably it was contemporaneous around 270 Ma throughout the SW and temporally unrelated to the supply of magmatic heat.
Secchi F & D'Antonio M, Per Mineral, 65, 257-273, (1996).
Villa IM, J. Conf. Abs.4, (1999)
Villa IM et al, Contrib Mineral Petrol, 126, 67-80, (1996).
The Palaeozoic basement of north-eastern Montagne Noire coincides with the western margin of the so-called "Grand Causses" Mesozoic basin. As a consequence, the metallogenic framework of this region reflects imprints of the two successive geodynamic episodes which greatly differ from a tectonic setting point of view: localised late-orogenic extension associated with strike-slip faulting in the late Variscan time, widespread syn-rift phase associated withnormal faulting during the Mesozoic. Nevertheless, mineralisations are often telescoped as the same faults act repeatedly during the two successive episodes. A detailed analysis of such an imbrication has been carried out in the Bournac-La Rabasse district, combining paragenetic and geochemical (fluid inclusions and sulphur isotopes) data. Compositions of the mineralised fluids illustrate the physical-chemical state of the crust when metals were leached and were deposited. The late Variscan paragenesis is divided into 4 main successive stages: Fe-As stage, Zn stage, Pb-Sb stage, and a Sb stage. The associated aqueous carbonic (up to 6% eq. NaCl) fluids gradually drop in temperature from ca. 400°c to 150°C. The 34S values for sulphides vary from -3‰ to +3‰. These data reflect the conditions unherited from the hercynian dynamic metamorphism. The Mesozoic paragenesis is characterised by two main stages: an early siliceous/barytic stage followed by sulphide-dominated stage. Associated fluids are high-salinity (20-25% eq. NaCl) low-temperature (80°C to 170°C) fluids. The 34S values for barite, vary from +15 to +19‰. These conditions account for the Mesozoic rifting accompanied by widespread marine evaporitic deposition. The Mesozoic/late Variscan inprints differ in Bournac and La Rabasse deposits: weak remobilization of the late Variscan stibnite mineralisations leading to a sulfosalts stage deposition in the Bournac vein deposit, massive deposition of lead-zinc within Cambrian carbonates collapse -breccias in La Rabasse mine. Such a Mesozoic/late Variscan mineralisations superimposition has been investigated within other segments of the southern Massif Central. These examples suggest that contribution of the Mesozoic/late Variscan mineralising events has to be carefully addressed within the Variscan belt.
The Pataz gold district is situated in the Eastern Andean Cordillera along the east flank of the Marañón valley. It provides about 15% of the gold production of Peru with Au grades from 7 to 15 g/t. The gold is in quartz-sulfide veins at the edge of a 70 km long 329 ± 5 Ma old granodioritic batolith close to the contact with low-grade metamorphic Precambrian and Upper Paleozoic rocks. The gold-bearing veins occur mainly as N-S oriented metric to decametric structures with a moderate dip to the east and as undulating subhorizontal structures; they are typically emplaced along lithologic discontinuities, such as aplitic dykes, or are parallel to the bedding of the sedimentary units. The observed hydraulic breccias, shear zones, banded textures and wallrock slivers are typical brittle-ductile features of a shear zone hosted gold deposits. The entire vein system was formed under an oblique-sinistral shear during a roughly NW-SE compression.
The ore-forming event was dated by 40Ar/39Ar on sericites between 305 and 315 Ma. This age excludes a genetic link with the host batolith and is coeval with the deposition of molasses in Carboniferous basins. Vein formation is related to a local compression regime within a regional extensional setting. The auriferous lodes comprise a two-stage paragenesis with a first generation of pyrite and arsenopyrite followed by fracturation and filling with galena, sphalerite, electrum and gold. A decametric to metric pervasive hydrothermal alteration, mainly composed of quartz, sericite, fuchsite, carbonates and coarse pyrite, surrounds the mineralized veins. Gold and electrum precipitate from medium saline CO2-poor fluid(s) at 300 ± 50 ºC. Isotopic composition ( D, 13C, 18O, 34S, Pb/Pb) of the ore and alteration minerals is homogeneous along the 70 km long belt and indicates a major crustal-scale migration event around 310 Ma. The slightly less radiogenic lead signature of the ore relative to the batolith suggests as major lead source the intrusive rocks with some contribution of the lower crust. The sulfide-rich two-stage paragenesis, the medium saline CO2-poor fluid(s), the tectonic setting and the age of the Pataz gold deposits are similar to some European Hercynides shear zone gold deposits.
The epithermal gold deposit Kochbulak (C3-P1) in the Tjan-Shan Mountains is a rare example of variscan epithermal deposits and a unique one due to its specific mineralogy and formation comditions. Epithermal Au and Ag ores are deposited from the deluted aqueous solutions with predominant meteoric component, and are accompanied with intensive rock alteration. Following types of such deposits that differ from each other with mineral assemblages, rock alteration, and fluid composition are distinguished: Au-alunite-kaolinite (high sulfidation); Au-Ag-adular-sericite (low sulfidation), and Au-Te (roscoelite, or alkaline). The last one commonly relates to the alkaline granitic magmas and is divided into two groups of deposits with various geological position and mineral assemblages: hypabyssal Au-Te-quartz vein deposits (roscoelite type) and firstly distinguished Au-Te-sulfosalt subvolcanic one (Kochbulak, Uzbekistan).
The Kochbulak deposit (Au-Te-sulfosalt type) combines some features of the low- and high-sulfidation, and of the roscoelite type deposits. Specific for Kochbulak is the occurence of three types of ore solutions, that varies from stage to stage from highly mineralized Cl-Na-Ca to deluted Cl-Na-K. They contain significant fraction of magmatic components. The common anions are Cl- and somewhere SO42- and F. The predominant cation is Na+ (Na/K varies from 1 to10), and in gas CO2 and CH4 are common. T varies from >400 to <100°C; pH - from 2 - 3 to 6 - 6.5; fO2 corresponds to the Ni-NiO buffer, and fS2 corresponds to the interval betwen pyrite=pyrrhothite and bornite=pyrite equilibria.
The 18O values for ore forming solution correspond to the values for mixture of magmatic and meteoric waters, mixing in various proportons. However the role of effervessence in ore deposition is admitted as well. The source of some metals is proposed on the base of Pb isotope and Rb-Sr ratio determinations in ore minerals (galena and pyrite), metasomatic sericite, and magmatic plagioclases, and hornblende. (Kovalenker, Chernyshov, 199x). Determined values lays in very narrow field and correspond to the mantle-crust boundary magmatic chamber. The study is supported by the Russian Fond of Base Researches, Project No 98-05-64052.
Pre-Variscan metalliferous events in the geological history of the Bohemian Massif are related to the Neoproterozoic back-arc spreading at the northern edge of Gondwana (sediment- and volcanics-hosted Fe-(Cu-Zn-Pb) stratabound sulfide and barite deposits, alum shales and Mn-carbonate deposits), early Paleozoic rifting of the Gondwana margin during Middle Cambrian to Early Ordovician(basic-ultrabasic rocks-hosted Ni-Cu deposits, stratabound carbonate-hosted Fe-Zn-Cu-Pb mineralization) the Middle to Late Ordovician subduction and back-arc spreding (Fe-oxide and sulfide deposits, Fe-Pb-Zn-Ba sulfide mineralization)and the Middle Devonian distention giving rise to limited oceanic zones in the northern part of the Saxothuringian unit and in the Moravo-Silesian unit(iron ores of Lahn-Dill type, Fe-Cu-Pb-(Au) and Fe-Zn-Pb-(Ag) stratabound sulfide ores). Variscan subduction (395 - 370 Ma), crustal thickening, magmatism, transcurrent movements and formation of pop-up structures (370 - 345 Ma) in the inner part of the Variscan belt was not connected with the formation of ore deposits. Variscan metalliferous events are related to the (1) synconvergence extension in the inner part of the belt, (2) mid crustal melting and granite melt differentiation and (3) the late orogenic extensional collapse of the whole Variscan belt. The synconvergence extensional, locally transtensional or transpressional tectonic regime (345 - 325 Ma) resulted in a strong uplift, an increase in the heat flow and denudation of the Saxothuringian and Moldanubian units and in sinking of the Cadomian (Panafrican)-consolidated block of the Tepla-Barrandian unit (Bohemian unit). Rising and sinking block boundaries were healed by intrusions of tonalite-granodiorite plutons (345 - 335 Ma). The synconvergence extension stage in the inner part of the Variscan belt corresponds to the main metallogenic epoch for mesothermal, shear-zone hosted gold- and scheelite deposits and thrust-bound Sb mineralization. Decompression and radiogenic heating-induced melting of uplifted blocks (330 - 290 Ma) resulted in the formation of large, crust-derived granodiorite-granite plutons. The late-magmatic to early post-magmatic formation of greisen- and vein-type Sn, W, Mo and Li mineralization is related to the youngest, shallow intrusions of highly fractionated granites commonly enriched in trace elements. Late orogenic extensional collapse of the whole Variscan belt, which occured after reorganization of the regional stress field at about 305 Ma, is recorded by the opening of the coal basins in the Tepla-Barrandian unit (300 Ma) and in Saxothuringian and Moldanubian units (290 Ma). Wide range of Au-, Sb-(Au), Pb-Zn-Cu-Ag-sulfide vein-type deposits are associated with the initial stage of the opening of brittle structures: normal folds, normal folds with a strike-slip component or wrench faults. The same structures host the younger (280 - 270 Ma) hydrothermal uranium mineralization.
The lithological succession of the Seksaoua district (Western High-Atlas, Morocco) is composed, from the base to the top, by: calcareous schist, graphitic schist (black schist), dolomite, calcareous schist with volcanic intercalations, pelitic-sandstone and sericito-schist. These units are attributed to the Cambrian by facies analogy.
Mineralization is systematically localized within the black schist and dolomite levels in which it takes two different expressions: i) early massive pyrite syn-sedimentary bodies, parallel to the stratification; ii) Cu-rich stockwork mainly composed of quartz, pyrite, chalcopyrite, carbonates and other accessory minerals among them scheelite and brannerite. This stockwork is particularly well-developed when hosted by white quartz-grained dolomite interpreted to invade the base of the black schist unit during earlier stages of the stockwork formation. Because the economic rates of Cu is related to the stockwork instead of the massive pyrite bodies, its relationships with the structural features that characterized the studied area have been particularly examined.
The zone is affected by at least three tectonic events that are responsible for regional foliation developed under greenschist facies metamorphism. The first tectonics corresponds to SW-verging shearing event while the second one develops metre-scale knee-folds systematically overturned toward the East. Copper mineralization appears associated with the third tectonic event that develops, essentially within the black schist, detachment-type tectonics. The black schist is interpreted to play the role of a decollement level and the stockwork to form by breakdown of the dolomite in response to the ductile stretching within black schists. These relations provide good evidences for the contemporaneity between stockwork formation and decollement tectonics. In three other places of the district, Cu-occurrences are also described always in the vicinity of dolomite and black schist levels. Here also, indicators of ductile tectonics, ramp-flat thrust contacts and decollements have been observed at the interface dolomite / black schist. This demonstrates the significant role of this decollement-type tectonics in the mineralization process.
Because the third tectonic event kinematics is consistent with a NW-SE direction of compression that corresponds to the direction of the hercynian tectonics responsible for the Tichka granite emplacement, a genetic link between copper stockwork and the Tichka granite and consequently an hercynian age for the mineralization are proposed and discussed. A granitic source for the mineralization is also consistent with the occurrence of scheelite and brannerite in the Cu-stockwork paragenesis.
The South Iberian Pyrite Belt is well-known for its abundant massive sulphide deposits that are still intensively exploited for polymetallic mineralizations. The massive sulphide bodies were emplaced within a volcano-sedimentary unit (VS) that is part of a complex and thick flysch-type sequence. All the units are interpreted to be deposited in a submarine continental platform environment and were affected by the Variscan convergence. Relationships between deformation features and mineralizations have been herein analysed in order to constrain the place of the mineralizing events within the regional evolution.
Deformation - Two kinds of structures affect the studied area. The first type, corresponding to the south-verging thrusting event, comprises flat-lying tectonic contacts and metre-scale inclined folds overturned to the south. Folds, that are the best-expressed structures, have east-west trending and roughly horizontal axes. They exhibit a similar geometry (thickened hinge) and well-expressed north-dipping axial plane schistosity (S1). The second type of structure, developed more locally, is marked by a secondary schistosity cleavage (S2) with vertical or southward dipping. Its formation has been until now misunderstood. Because the S2 occurrences are frequently associated with ramp-flat south-verging thrust fault, we link the S2 formation to backthrust development. In such an interpretation, the S2 tectonic event is not a distinct phase but appears in the continuity of the D1 tectonics.
Mineralization - If the massive sulphide deposits were interpreted as formed during the deposition of the volcano-sedimentary units, mainly because of their systematic locations into the VS unit and their close relationships with volcanics, the observation of a well-oriented and regular stockwork within several places question the existence of only one mineralizing event. Within several open pits, relationships between this stockwork geometry and tectonic structures have been carefully examined. The Tharsis open pit and surrounding outcrops offer the best observations. Stockwork veins, observed within the vicinity of the "San Guillermo" and "Filon Norte" orebodies, are regular, well-expressed and contained by the S1 schistosity or S/C structures that were initiated during the south-verging tectonics. Within the "El Perrunal" open pit, a similar regular stockwork has also been observed although occurring parallel to the S2 cleavage.
Because of its closed relationships with deformation structures, the regular stockwork is interpreted as contemporaneous or late with respect to the hercynian tectonics. It was clearly distinguished from the earlier stockwork coeval with massive sulphide formation and deformed by D1 and D2 tectonics. This fact re-addresses the possible existence of a second hydrothermal event that could remobilize earlier sulphides and created a second generation of mineralization.
South West Iberia hosts a variety of mineral deposits not commonly matched in other parts of Europe. Many of these deposits, including massive sulphides, were formed during the transpressional stage of the Variscan Orogeny in pull-apart basins. The aim of this paper is to present a quantitative approach to the question what control transpressional tectonics has on ore forming processes in the South Portuguese Zone (SPZ). First results on the assessment of the regional geological setting of the massive sulphide deposits and their paleogeographic characteristics will be discussed.
For the approach adopted here the following methodology was selected:
1. Analysis of the tectonic evolution of the SPZ to facilitate the palinspastic reconstruction of the basin, to trace the synsedimentary extensional faults and to assess their role during Variscan deformation.
2. Integration of thermal data (vitrinite reflectance, fission track analysis, illite crystallinity) to quantify the magnitude and timing of basin heating during burial, fluid infiltration and exhumation.
3. Computer-aided basin modelling using stratigraphic, structural, thermal and petrophysical parameters to reconstruct and quantify basin subsidence history as well as regional variations of convective versus conductive heat flow.
First results of the structural analysis show that parts of the SPZ are of a complex geometry with duplex structures and fault-propagation folds, which was recently also mentioned by Quesada, 1998. Thermal data analysis from illite crystallinity measurements indicate the possibility of a strong dependence with the stratigraphic depth and, therefore, peak metamorphism during basin development and deepest burial. These analysis provide first quantitative data for reconstruction of basin geometry and basal heat flow.
Future synthesis and integration of data will include the compilation of geological sections in a comprehensive and regional model for the basin evolution, including data from structural and basin modelling studies. Further objectives are the definition of exploration criteria, e.g. formulation of genetic exploration modells for shale-hosted massive sulphides and stockworks and the development of pre-exploration intelligence and computer models.
Quesada, C, Mineralium Deposita, 33, 1-2, 31-44, (1998).
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