A multi-technical mineralogical,petrological and geochemical approach to the origin of ore mineralizations in the mafic - ultramafic rocks of the Ivrea-Verbano Zone, Piedmont, Northern Italy

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The topic of this thesis was the evaluation of mineralogical, petrological and geochemical characteristics of twelve Fe-Ni-Cu-PGM ore deposits, hosted by the Ivrea-VerbanoZone (IVZ), Northern Italy. The IVZ is interpreted as a section through lower continental crust and the upper mantle, which comprises the lithologies of: Main Gabbro (Sella Bassa deposit), Cyclic Units (Gula, Guaifola, Isola dep.), Ultramafic Pipes (Bec d’ Ovaga, Valmaggia, Castello di Gavala, Piancone la Frera and Fei di Doccio dep.), La Balma - Monte Capio ultramafic sill (Penninetto, Campello Monti and Alpe Laghetto dep.). The S isotopic study was conducted on representative samples of each of the twelve sulfide dep. Data on each stable S isotopes (32S, 33S, 34S and 36S) was gained on pyrrhotite and pentlandite via Secondary Ion Mass Spectrometry (SIMS), and the mass dependent (δ34SVCDT) and mass independent fractionation factors (Δ33S and Δ36S) were calculated. The mass indep. factors were used to interpret the silica and sulfide magma evolution of the investigated mantle lithologies, with their according sulf. dep. The mass dep. factors were used to identify the internal and/or external origin of the S forming the pyrrhotites and pentlandites. A common magma origin of the Main Gabbro and the Cyclic Units dep., a related but evolved origin for the Ultramafic Pipes dep., and an independent magma source for the La Balma - Monte Capio dep., is assumed. Contamination by external S is assumed for each Cyclic Units dep. (3), for the Fei di Doccio pipe dep. and the Penninetto dep. All these interpretations are confirmed by the direct correlation with the geological context of the single dep. U-Pb age data on zircons of the Fei di Doccio dep. was gained via Sensitive High Resolution Ion Micro Probe (SHRIMP II). 23 analyses were performed on two larger (up to 170µm) and five smaller zirc. (~30µm). The zirc. Are intimately intergrown with the sulfides and hydromagmatic phases. 277±6 Ma was interpreted as the true cryst. age of the pipe. The ages 277 to 251 Ma are interpreted as rejuvenated ages due to disturbance of the isotope equilibrium by fluids. Thermal rejuvenation was neglected by temp. calculations. This interpretation is in accordance with field observations and the dating of the regional magmatic event in the IVZ. The halogen (F, Cl, OH) and the trace element (REE, U, Th, Y) composition of 154 apatite minerals of each of the five Ultramafic Pipes of the IVZ were analyzed using a Jeol JXA 8200 Superprobe. A generally juvenile mantle character is proposed for the melts and the fluids involved in the formation of the ap. It is clearly evident that the fluids affecting the Ultramafic Pipes are not related to the metasomatic fluids of the Finero Complex. An influence by the metasediments of the adjacent Kinzigite Formation can be neglected. Complex de-mixing processes of fluids and vapors have to be taken in consideration. The Bec d’ Ovaga ap. preserved the magmatic signature best, the Valmaggia, Fei di Doccio and Piancone la Frera ap. were intermediately affected by coexisting fluids. The strongest Cl enrichment, with accompanying REE accumulation, is shown by the Castello di Gavala pipe. A new analytical-statistical technique for the compositional analyses of mineral phases by RAMAN spectroscopy was invented. This new technique was applied to apatites hosted by the Ultramafic Pipes of Bec d’ Ovaga, Castello di Gavala and Valmaggia. The results show a maximum use of the advantages of RAMAN spectroscopy, e.g. in-situ analysis, and analysis of very small minerals, and a very high level of precision, particularly if compared to standard el.-mic. analyses. This innovative technique is immanently important for further investigations as it opens a wide range for a non-destructive and easy-to-use chemical analysis of minerals and materials which possess a relationship between their chemical compositions and their Raman-active modes.


Translated title of the contributionEine multitechnische mineralogische, petrologische und geochemische Untersuchung zum Ursprung der Erz Mineralisationen in den mafischen - ultramafischen Gesteinen der Ivrea-Verbano Zone, Piemont, Norditalien
Original languageEnglish
Publication statusPublished - 2013