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La croissance des cristaux de quartz dans le filons sur l'exemple du filon de La Gardette (Bourg d'Oisans) et des filons du massif du Mont-Blanc

Abstract : The aim of this work is to !ind out relationships among quartz crystal features on the one hand, external and internaI morphology, impurity, size of the unit cell and, on the other hand, their conditions of growth. Two types of deposits have been chosen for this study : the "La Gardette" quartz vein (Bourg d'Oisans) and the crystal clefts of the Mont- Blanc massif. Both are usually classified as "alpine type" veins . Their crystals however are very different : in "La Gardette", the structure is lamellar (WEIL, 1930 ; BAMBAUER, BRUNNER and LAVES, 1961), while it is macromosaic (WEIL,1930 ; LAEMMLEIN, 1937) in the Mont-Blanc massif. The problem has been approached on three different paths : - In the first chapter, description of the crystals : observation of the external and internaI morphology, determination of the impurity content and unit cell size. - Characterization of the growth fluid media by the study of fluid inclusions (Chap. II). The method was to observe, under the microscope the phase transformations during heating and freezing. - Investigation of the geological environment of quartz veins. In the case of "La Gardette", the cristalline basement and its sedimentary cover have been investigated, as the age of the vein is post- triassic . For the Mont-Blanc clefts. the study is limited to the granite and its modifications at the contact of the clefts. The results of this research deal with the three following subjects : quartz crystal growth, deep- seated fluid geochemistry and regional geology of these quartz deposits. a) Quartz crystal growth : The lamellar quartz crystals are impure crystals which have grown alternatively at an high and low rate and have buried various quantities of impurities in relation with these rates. Quartz with a macromosaic structure are very pure crystals. The facies difference between both crystal types (long or thick) is explained by the difference in the impurity content. When the crystal buries many impurities, a preferential decrease of the growth rate of the prism face is observed, which leads to elongated crystals. At "La Gardette", the reason of this burial of many impurities has not been established with certainty : two parameters indeed join together their effects : temperature and nature of the fluid medium of growth . At"La Gardette" , temperature was probably lower than in Mont -Blanc massif and the fluid phase was not in equilibrium with silicate mineraIs, but with carbonate bearing sediments. b) Geochemistry of deep-seated fluids : The contribution of this work comes partly from the discovery of late fluids rich in CO2 during the retromorphosis of the Mont-Blanc granite. It may be inferred from this fact that the temperature during retromorphosis was much higher than actually indicated by the homogeneization of aqueous fluid inclusions in smoky quartz and reaches the temperatures usually believed to be within the range of the greenschist facies. ln this case homogeneization temperature indicates only the density of the fluid and is therefore more useful as a geological barometer than as a therrnometer. c) Geology of the Oisans area and Mont-Blanc massif. In the Bourg d'Oisans area, the density of fluids included in triassic and liassic mineraIs is so high that we must admit the presence, between Upper Lias and the present time, of a thick sedimentary cover which has now disappeared. This conclusion is in agreement with the existence of a schistosity in liassic rocks of the Ornon syncline and with the discovery by MARTINI and VUAGNAT (1965) of a zeolite facies west of the external crystalline massives. The "La Gardette" quartz vein was consequently formed in the depth from a material (fluids, silica, sulfides) deriving essentially from the above lying sediments. In the Mont-Blanc massif, it is shown that, in addition to the evidence of high pressures, CO2 has played a disturbing role during the retromorphosis. It induces an acidity which determines the equilibrium between some mineraIs, particularly between orthoclase and muscovite . These considerations lead to an open system retromorphic model in which temperature is less important than the composition of the fluid phase.
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Submitted on : Wednesday, November 23, 2011 - 8:44:16 AM
Last modification on : Friday, February 1, 2019 - 4:47:24 PM
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  • HAL Id : tel-00643862, version 1



Bernard Poty. La croissance des cristaux de quartz dans le filons sur l'exemple du filon de La Gardette (Bourg d'Oisans) et des filons du massif du Mont-Blanc. Minéralogie. Université de Nancy, 1967. Français. ⟨NNT : 1967NAFS0052⟩. ⟨tel-00643862⟩



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