L'INTRUSION DE LA HELLE ...

L'INTRUSION DE LA HELLE (HAUTES-FAGNES, BELGIQUE) ET LA MINERALISATION CUPRO-MOLYBDENIFERE ASSOCIEE: SYNTHESE BIBLIOGRAPHIQUE, DONNEES DES SONDAGES ET INTERPRETATION GLOBALE

ABSTRACT.
The Helle intrusion is located in the eastern part of Belgium and was discovered by the end of last century. Geochronology (U-Pb isotopic ratios ofzircon) indicates that the magma emplacement took place between Silurian and Lower Devonian (minimum age of emplacement: around 381 ± 16 Ma). Previous studies have shown that the Helle intrusion is a sill structure with most of its outcropping rocks characterized by a petrographic composition of quartz-diorite (tonalite). The low degree of thermic metamorphism related to the intrusion is reflected by a very thin aureole of hornfels. The rather large zone of pyritic rocks surrounding the igneous rocks was probably generated by an extensive hydrothermal (mainly a sulphidization) episod possibly contemporary of the thermic metamorphism phase. Finally, a main episode of regional metamorphism contemporary of the variscan orogeny induced a metamorphism of low-grade (ACF : paragenesis prehnite-chlorite-epidote) in the igneous rocks of la Helle.
In 1976-1977, at the initiative of Union Minière S.A., the concealed magmatic rocks were recognized by four drillings down to a depth of approximately 100 m. The Belgian Geological Survey supervised the study. Results are included in the present memoir divided into 3 parts.
The first part is devoted to a bibliographic synthesis of the geology and the mineralization of the Helle area. In a second part, results of lithological and petrographical observations are presented. The study under the microscope emphasizes a large variation of petrographic composition ranging from diorites to granodiorites and monzodiorites. Mineralogic variations seem to be related to mechanisms of magmatic differentiation (mainly fractional crystallization) but also to a complex post-magmatic hydrothermal alteration process, which mainly affected the superficial area of the intrusion. That zone is characterized by a secondary K-feldspar occurrence. The hydrothermal alteration also produced silicification and partial albitization with the development of a network of veinlets (locally mineralized) in the magmatic and surrounding Revinian metaquartzopelitic rocks. In this part, lithogeochemical logs of metallic elements are shown. A three dimensional reconstruction of the intrusion indicates that the structure corresponds to a laccolitic sill-shaped of: ± 500 m of NE-SW length, 125 m of maximal thickness and 30° SE dip. Small dykes are also connected to the main sill.
The third part concerns the petrological and metallogenical interpretations. The primary mineralization is mainly formed by sulphide components (chalcopyrite, bornite, molybdenite). The average Cu and Mo content of the igneous rock is low (Cu = 0,17 % ; Mo = 0,02 %). The low metal stock of the intrusion (Cu: ± 35 000 tons; Mo: ± 4 000 tons) is the consequence of a reduced volume of mineralized rock as weIl as common low metal contents. Metallogenically, this type of mineralization can be compared to the porphyry copper model although some important differences appear. The difficulty to distinguish the various concentric patterns of alteration suggests the smash of the alteration zones and is probably due to the small size of the intrusion.

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