Datation Re-Os sur pyrite et traçage des sources des métaux dans des gisements de type porphyre et épithermal neutre: Exemple des gisements de Bolcana, Troita et Magura, Monts Apuseni, Roumanie.

Abstract : Many porphyry-type (Cu-Au) and neutral epithermal (Pb-Zn and Au ± Ag) ore deposits are encountered in the region of the Apuseni Mountains, located at the foot of the Carpathian chain in the Western Romania. These deposits are related to a neogene andesitic volcanism. In order to demonstrate possible genetic relationships between the porphyry-type and neutral epithermal deposits, the Bolcana porphyry has been investigated since it is surrounded by a number of epithermal low-sulfidation veins with a Pb-Zn ± Au mineralisation. These veins are currently mined at the Troita and Magura sites. A structural analysis and a 3D modelling pf these deposits indicate that the geometry and orientation of fractures and mineralized vein are consitent both with direction of regional extension and with a NW-Se pregression of the different andesitic intrusions. In order to establish precisely the temporal relationship between the different ore deposits, a Re-Os dating method has been developped and applied on pyrite wich is ubiquitous in all of the deposits. This method enabled us to assign an age of 10.9 ± 1.9 Ma for the porphyry-hosted mineralization. The ages obtained for the epithermal systems are somewhat approximative as perturbations of the Re-Os system are observed for these environements. A fractionation of rhenium responsible for a significant enrichment in this element for the apical zone of the porphyry has been demonstrated. This enrichment is most probably related to a maximum boiling event , which may also explain a similar enrichment in arsenic for the pyrite in the same zone. The sources for the metals have been characterized at the district scale by combining two isotopic systems (Re-Os and Pb-Pb) on both pyrite and galena. The osmium data indicate that the Troita deposti has compostion which is similar to that of the Bolcana porphyry. In constrast the results obtained for the Magura deposits indicate the Re-OS system has in this case been perturbed due to a weathering of nearby ophiolites which probably contributed some osmium to the pyrite at Magura. Additional results obtained for common lead seem to confirm the osmium data, with an osmium isotopic composition which has a more important mantle-derived component than at Troita.
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Applied geology. Université Henri Poincaré - Nancy I, 2007. French


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Olivier Cardon. Datation Re-Os sur pyrite et traçage des sources des métaux dans des gisements de type porphyre et épithermal neutre: Exemple des gisements de Bolcana, Troita et Magura, Monts Apuseni, Roumanie.. Applied geology. Université Henri Poincaré - Nancy I, 2007. French. <tel-00267780>

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