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Accessory Mineral Alteration in Peraluminous Granites at the Hydro~Hermal Stage: a Review

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Accessory Mineral Alteration in Peraluminous Granites at the Hydro~Hermal Stage: a Review RENDICQNTI DELtA SOCIETA ITAU ... N... 01 MINERALOG(Ir. E PETROLOGI.... 1988. 'obL 4}.2. pp. 49';1·'08 Accessory mineral alteration in peraluminous granites at the hydro~hermal stage: a review MICHEL C ... THELINEAU CREGU me! G.S. CNRS.cREGU, B.P. 23, '4501, Vandoeuv~-les·Nancy, France ABSTRACT. - U. Th and REE distribution in (notamment ['uraninite et la monazite) et de leur peraluminous granites is essentially cOntro!1ed by remp[acement par des mineraux accessoires neoformcs accessory minerals such as monazite. uraninite, apatite (so[utions solides complexes. carbonates, phosphates). and. to a lesser degree, zircon. Although the nature and La cristalliution de mineraux neoformes, pn!sentant des distribution of such minerals are comroUed by magma solubwtes plus impO'lantes que: les mineraux acc;essoires fe1ltute$, they Jruly be significantly affected by $ubsolidus d'origine magmatique. est tres importante pour le alteration. Thus. different examples of hydrothermal lessivage u1tcrieur des eiCmenlS traces par des fluides aJteration have shown that trace e1emmlS may exhibit hydrothermaux. De telles altcrations ou neoformations conlrlsted behlviour. due to differential stability of dc minenux accenoires depend: 1) des facteurs primlry accessory minerals (especially uraninite and controllant la pm;:olltion des fluides a travcrs les roches monnite) and their replacement by authigenic phases (rapport eaufrochc:, degri de microfracruration .... ); 2) la (complex solid solutions, C1IrbonatC$ and phosphates). nature des minerlux accessoires, et en particulier la Crystallization of authigenic minerals having higher solubwtc des minmaux accessoires d'originc: magmatique so[ubilities than the magmatic accessory minerals is en condition hyd ro therm ale (stOOe de dissolution­ extremely important for further leaching of trace alteration); 3) la composition des fluides et les elements by hydrothermal fluids. Such alteration of changements intervenant dans les conditions physico­ magmatic accessory mineTals in granites and dissolution· chimiqucs lors de sradel uitetieurs de depot. recrystallization processes depeoo on: 1) fanou controllina fluid percolation through the roc:k (water/roc:k Man db: MincrlUX acccssoires. U Th, Terres rares, ratio, degree of microfracturing. etc). 2) nature of the alteration hydrothc:rmale. leucogranite peralumineux . accessory minerals and especially the solubility of the minerals of magmatic origin under hydrothermal conditions (dissolution-alteration stage), 3) fluid composition and changes occurring in the physical­ Introduction chemical conditions during the later deposition stage, Key words: Accessory minerals, U, Th. REE, The distribution of U, Th and REE in Hydrothermal alteration, Peraluminous granite. peraluminous granites is firsdy controlled by the magmatic stage. These trace elements, as well as Zr, P, etc., are not incorporated in the main rock· forming minerals because of the RESUME. - La distribution et les teneurs de U, Th ct low partition coefficients between the magma des tCITes rares dans les granites peralumineux est and minerals such as quartz, feldspars or essentiellement oontroll~ par les mi~raux accessoires. phyllosilicates, but in accessory minerals such tels que la monazite, J·uraninite. l'lpatite et. a un moindre dcgre, le zircon. Bien que la nature et la as uraninite, monazite, zircon or apatite. distribution de tels mincraux accessoires soit d'abord Consequently, their content in the whole rock contron~ par les call1Ctcristiquc:s du magma, dies is firstly dependant on the factors which peuvent ctre affc:ct~s de maniere significative par les control the distribution and nature of the attentions subsolidus. Ainsi, differents exemples d'a1teration hydrothc:rmaie ont montre que les ciCments accessory minerals. Thus, knowledge of the en traces peuvent avoir un comportemc:nt tlis COntrastc. mineralogy and thermodynamical properties en fonction de la stabilite differentieUe de lcurs portcurs of accessory minerals, which may bear more 500 CATIIEUNEAU M than 80% of the above mentioned trace distribution of accessory minerals crystallizing elements, is the key to understand the from the melt depend on four group of geochemical features of granites. parameters (PAGEL, 1981, 1982a, b; CUNEY For many years, most of the accessory et al. , 1979; CUNEY and FRlEDRlcH, 1987; minerals, except uraninite, have been CATIiEUNEA U, 1987c): the trace element considered as fairly unsoluble and metastable contents (1) and chemical features (2) of the under hydrothermal and supergene magma, (3) the degree of magma evolution, conditions. This assumption led to consider (4) the physical-chemical conditions of magma first the whole rock contents in U-Th-REE crystallization. Figure 1 represents the factors as relatively good tracers of the magmatic controlling parageneses at the magma stage, processes, and second Th and REE as and a possible sequence of crystallization. relatively immobile in granites, except in The distribution of V, Tb, and REE does severe hydrothermal alterations. Thus, not depend only on the nature and abundance uraninite abundance has been mainly of each accessory mineral acquired at this investigated as a tool for the recognition of stage, but also on the trace element potential source rocks for uranium. distribution among each mineral. The V­ Consequently , peraluminous granites which content in monazite or thorite and the Th­ may be rich in uraninite, were considered as content in uraninite, for instance, may be favourable environments for hydrothermal highly variable according to magma chemistry leaching and deposition of uranium (RANCHlN, or magma evolution. Thus, as a given trace 197 1; MOREAU, 1977; FRlEDRlOt et al., 1987, element is heated by several minerals, detailed among others). mineralogical studies are required to establish In recent years, new data have been the complete distribution of its content in the obtained on the stability of monazite and rock. Figure 2 gives a schematic distribution zircon during metasomatic, hydrothermal of La and V among the different rock forming (CATHELlNEAU, 1987a, b) and ductile minerals in V-rich peraluminous granite. It structuring processes (GUlNEBERTEAU, 1986). shows that uraninite and monazite are major Such works have demonstrated that under constraints for V and La contents respectively specific conditions, accessory minerals may be in the rock, but that the V content in altered and U, Tb, REE leached. Thus, the monazite and zircon, or the La content in purpose of this paper is to investigate the role phylIosilicates and apatite cannot be ignored of hydrothermal fluids as factors controlling in the mass balance. the alteration of accessory minerals, and the consequent changes on the trace element geochemistry of granites. 11. Changes in accessory mineralogy at the hydrothermal Sklge The alteration of accessory minerals in l. Distribution of V, Th, REE among minerals granites during subsolidus fluid-rock At the stage of granite crystallization, interactions depends on (CATHEUNEAU, accessory minerals have two origins: 1987c): «1) the nature of the: mineral, and crystallization from the melt, and mechanical especially the solubility of the: mineral (Sds) transport by the magma of accessory minerals under the dissolution conditions, 2) the of external origin, as those inherited in solubility (Sf) of this mineral in the fluid relatively large amount from the partial which is a function of the initial concentration melting of metamorphic rocks. Thus, the of the elements such as V, Tb, REE and inheritance of zircon (POLDERVAART and ligands in the percolating fluid, 3) the ECKERMANN, 1958; HoPPE, 1965) and water/rock ratio and the fl ow regime in the monazite (MU-LER and MiTI1.EFELD, 1982; granite, 4) the exchange and interaction SAWKA et al., 1986) is relatively common since surface between the fluid and the minerals, the solubility of such minerals in magmas is which depends on the degree of relatively low. The nature, abundance and microfracturing and amount of microcracks ACCESSORY MINERAL All"ERATlON IN PERAWMINOUS GRANITES ElC. !MAGMATIC STAGE I l'<lrtial 1IIl'iting Magma differentiation \ Controlllng parameters , U. Th. REE. I inilt'ril.lm' r of Zr. P U.Th. REE eontent ll' tin'on '0, Aleallnlty I (low) Ca eontent Fig. 1. - M.in w:essory minerals aystallizing .t the magm.tic; $!age in pertiuminous granites (following d.l. from liter.ture .nd especially PAGEL (1981,1982.) .nd CUNEY and FII.IEDRlCH (1987)). in minerals. The higher the solubility differemial in-situ alteration of minerals, 2) difference (SrSds)' the larger the amoum of crystallization of authigenic minerals in other mineral which can be dissolved. Therefore, sites (microcracks, vugs, etc.) after moderate the most favourable conditions for the to important transport of demons in solution. dissolution of a given accessory phase may be predicted if the parameters constraining the 1) In-situ alteration of accessory minerals highest (SrSds) values are known. The two last factors (4 and 5) have an Accessory minerals of magmatic origin may extreme influence on the rate of alteration of be partially to entirely altered and replaced minerals. As the accessory minerals are small­ by authigenic minerals. Each accessory sized, scattered in granite minerals (feldspar, mineral has a specific behaviour under given biotite, quartz, etc.), and represem a very physical and chemical conditions, and will small volume of the rock, a high degree of transform into specific alteration products,
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