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La pression à l'intérieur d'une inclusion augmente avec la température. Lorsque cette pression devient supérieure à la résistance des parois de l'inclusion, celle-ci décrépite. Des travaux antérieurs effectués sur des quartz synthétiques ou naturels, par décrépitométrie ou par microthermométrie, conduisent à des valeurs différentes de la pression de décrépitation. La taille des inclusions est un des facteurs considérés comme responsables de ces variations de la pression. De nouvelles valeurs ont été obtenues par microthermométrie sur des inclusions fluides biphasées de quartz synthétiques. L'épaisseur des lames est de 0,2 mm. Pour des inclusions de taille supérieure à 35 µm, la pression de décrépitation est 850 ± 50 bar, comme trouvée précédemment par Naumov et al. (1966). Pour des inclusions plus petites, la pression augmente jusqu'à 1 200 bar pour une taille de 12-13 µm. Pour des inclusions encore plus petites, des pressions de 2,7 kbar ont été obtenues sans aucune décrépitation et la dissolution des parois de ces inclusions a même été observée. Enfin deux applications de la décrépitation sont présentées.

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Description of an apparatus for measuring the temperature of micro-samples under the microscope in the range -180°C to + 600°C. Precision and accuracy are discussed. Use of this apparatus is foreseen in physics, chemistry, mineralogy and biology.

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The uranium deposits of the La Crouzille district which are the subject of this study (Margnac and Fanay mines) are located in the Saint-Sylvestre granitic plutonic complex, approximately 20 km north of Limoges (Haute-Vienne). The Saint-Syh'estre pluton in its entirety and the Saint-Sylvestre granite in particular were first reinterpreted on the basis of new observations made in the mines along the contact zone between the Brame and Saint-Sylvestre granites (Peny area). A_ similar origin for members of the SaintSylvestre plutonic complex, recognized earlier by Chenevoy (1958), was confirmed. Structural information (synthesized by Autran and Gulllot, 1974) and isotopic data (Duthou, 1977) are reinterpreted in a new tectonic framework. The genesis of the leucogranitic Saint Sylvestre pluton is believed to be comparable to that of Himalayan leucogranites.The existence of two granitic facies (Brame and Saint-Sylvestre) is due to magmatic and deuteric processes (muscovitization, albitization) operating within the same plutonic complex. These phenomena obliterated the primary features and caused a reequilibration of the initial catazonal paragenesis (sillimanite-orthoclase) to mesozonal conditions (muscovite-biotite).The eraplacement of the two granitic facies took place around 350 to 360 m.y. The age determination of 315 m.y. is now interpreted to be the result of partial reequilibration of Rb-Sr isotopes.The intrusion of lamprophyres at 285 m.y. corresponds with brittle fracture tectonic activity, in contrast to the earlier plastic deformation. A. similar sequence of magmatic and tectonic styles is found elsewhere in the Hercynian chain. Two fracture systems are present (EW-NS and NW-SE). Lamprophyric magmatic activity produced localized hot spots. This heat flux caused the reheating of the cold waters which impregnated the plutonic complex and produced a convective fluid circulation as in modern geothermal systems. The fluids were channeled into faults and fractures of the east-west-north-south fracture system causing the micaceous episyenitization of the granite (dissolution of quartz, total muscovitization of plagioclase and biotite, and partial replacement of orthoclase). A.n earlier type of episyenite (feldspathic episyenite produced by quartz dissolution, chloritization of biotite, and feldspathization of muscovite) is considered to have been produced during the plastic P4 deformation which folded the Saint-Sylvestre plutonic complex into an anticlinorium (NNE to NE fold axes).Pitchblende and pyrite mineralization immediately followed the emplacement of the lamprophyres and the micaceous episyenitization of the granite and a minimum age of 275 m.y., rather than 240 to 250 m.y., is accepted. The mineralization was precipitated from a CO2-rich fluid by the unmixing of complex CO2-H20 mixtures following a drop in pressure. The temperature of the solutions in the 132 episyenite column of the Margnac mine was approximately 345øC.A.fter pitchblende deposition, the fluid became progressively more water-rich during the...

The Henderson Cu-Au deposit is contained within the metaanorthosite of the Dore Lake Complex, located at the eastern end of the Chibougamau-Matagami greenstone belt. Two ore zones, one enclosed within a shear zone (zone B) and the other in a subsidiary structure (zone D) offer an ideal situation to study the correlation between the mechanical development of a shear zone and its subsidiary structure and the evolution of mineralizing fluids. A geomechanical interpretation demonstrates thatat the onset of the development of the shear zone, subsidiary fracture patterns are developed in second order faults in the following sequence. At peak strength Riedel shears (R and R') are formed which propagate out into the walls of the shear zones producing the zone D structure. After peak strength, restraint (P) shears are developed in the thrust attitude within the shear zone. Principal displacement shears (D) developed toward residual strength in the direction of movement. The continuation of the shear displacement gives rise to schistosities within the main shear zone. While the main shear zone and the P and D shears within the main shear zone were in continual movement, the subsidiary zone D structure, once formed retained a simple fracture pattern and moved little in comparison to the main shear zone. The above model makes necessary an examination of the mechanics of dilation which created openings for fluid movements and ore concentrations. The general principle involves the mechanics of the overriding of irregular planes and surfaces and the work performed by the normal pressure a during the dilation or contraction of the system. Application of the shear fracture dilatancy model explains the dilation mechanisms in zones B and D. Although the effect of interstitial fluid pressure is incompletely known, increasing dilation should bring about a reduction of the pore fluid pressure which will change the effective normal stress and aid fluid movement.Mineralizing fluids introduced into this shear environment show a similar pattern of evolution, with a wider range of homogenization and halite disappearance temperatures in the more active zone B than within zone D. Systematic variations of Ca/Na and Ca/Mg ratios in the fluid between zones B and D indicate migration and accompanying chemical changes of the fluid from zones B to D, in keeping with the evolution of the main shear zone and its subsidiary structure. These correlations suggest strongly that fluid inclusion studies can be useful for documenting geomechanical processes.