Piergiorgio Rossetti

The Oligocene Biella pluton (western Alps, Italy): new insights on the magmatic vs. hydrothermal activity in the Valsessera roof zone

— The composite Biella pluton is part of an Oligocene volcano-plutonic complex whose origin is connected to the Alpine subduction-collision processes and that emplaced at shallow crustal levels within the eclogite-facies rocks of the Austroalpine Sesia-Lanzo Zone. In the roof zone of the pluton, small satellite igneous bodies are set within the SesiaLanzo country rocks, close to the main Biella pluton, and range in composition from quartz alkali feldspar syenite, quartz monzodiorite and monzogabbro, to quartz diorite and gabbronorite. Their geochemical features, including the REE patterns, are coherent with the calc-alkaline to shoshonitic affinity recognized in the whole volcano-plutonic complex. Field and petrographic data suggest that these bodies represent earlier crystallization/differentiation products of the Biella primary magma(s), which underwent contact metamorphic recrystallization during the multistage emplacement of the main pluton. Tourmalinebearing hydrothermal breccias and different types of hydrothermal veins (including quartz-plagioclase-, quartz-tourmalineand ankerite-quartz-sulphidesbearing veins) occur within both the intrusive rocks (satellite bodies + the main pluton) and their SesiaLanzo Zone country rocks. Both field relationships and vein assemblages suggest a close connection between the late-magmatic evolution of the Biella pluton and the multistage, boron-rich hydrothermal activity. riassunto. — Il plutone di Biella fa parte di un complesso vulcano-plutonico, legato all’evoluzione tardo-collisionale delle Alpi Occidentali, che si e messo in posto a livelli crostali relativamente superficiali all’interno delle rocce in facies eclogitica della Zona Sesia-Lanzo, appartenente al Dominio Austroalpino. Nelle rocce originariamente a tetto del plutone, sono presenti piccole intrusioni satelliti che presentano un ampio spettro composizionale, variabile da quarzo sieniti alcalifeldspatiche, quarzo monzodioriti e monzogabbri a quarzo dioriti e gabbronoriti. I loro caratteri chimici, inclusi i pattern delle Terre Rare, sono consistenti con l’affinita da calcalcalina a shoshonitica del complesso vulcanoplutonico. Le osservazioni di terreno e petrografiche suggeriscono che tali corpi rappresentano dei prodotti di cristallizazione/differenziazione precoci a partire dai magmi primari del corpo intrusivo principale e che hanno subito una ricristallizzazione metamorfica di contatto connessa con la messa in posto dell’intrusione di Biella. All’interno delle rocce intrusive (sia dei corpi satelliti sia del plutone principale) e della Zona Sesia-Lanzo incassante sono presenti brecce idrotermali ricche di tormalina e diversi tipi di sistemi di vene idrotermali (tra cui * Corresponding author, E-mail: piergiorgio.rossetti@unito.it 224 P. rossetti, a. agangi, d. Castelli, M. Padoan and r. ruFFini vene a quarzo e plagioclasio, a quarzo e tormalina, ad ankerite, quarzo e solfuri). Le relazioni di terreno e le paragenesi di vena e di alterazione suggeriscono uno stretto legame tra l’evoluzione tardo-magmatica del plutone di Biella e almeno gli stadi precoci dell’attivita idrotermale multifasica, caratterizzati dalla circolazione di fluidi con elevato contenuto di

Fragments of the Western Alpine Chain as Historic Ornamental Stones in Turin (Italy): Enhancement of Urban Geological Heritage through Geotourism

In Piemonte, stone has always been the most widely used raw material for buildings, characterizing the architectural identity of the city of Turin. All kinds of rocks, metamorphic, igneous, and sedimentary, are represented, including gneisses, marbles, granitoids, and, less commonly, limestones. The great variety of ornamental stones is clearly due to the highly composite geological nature of the Piemonte region related to the presence of the orogenic Alpine chain and the sedimentary Tertiary Piemonte Basin. This paper provides a representative list of the most historic ornamental stones of Piemonte, which have been used over the centuries in buildings and architecture. The main stones occurring in Turin have been identified and described from a petrographic and mineralogical point of view in order to find out the corresponding geological units and quarry sites, from which they were exploited. This allows the associated cultural and scientific interest of stones to be emphasized in the architecture of a town which lies between a mountain chain and a hilly region.

Sulfur isotope evolution in sulfide ores from Western Alps: assessing the influence of subduction-related metamorphism

Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed microtextural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of microtextural investigations with δ34S values from in situ ion probe analyses within individual pyrite and chalcopyrite grains allowed evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic recrystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the submillimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic reequilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.

Predictive assessment of the asbestos content in the Western Italian Alps: an essential tool for an effective approach to risk analysis and management in tunneling operations and muck reuse

The modern approach to the design and management of tunnel excavation, and muck reuse, can be influenced to a great extent by the possible presence of rock formations containing asbestos minerals. This situation creates problems concerning the protection of the workers’ health and the expectable environmental criticalities, while a drastic re-consideration of the muck destination is necessary. Since, in the case of carcinogens, corrective action following exposure or dispersion is not acceptable, detailed knowledge of the characteristics of the material to be excavated is all the more essential. Only on this basis it is possible to design the tunneling operations in a prevention through design approach, so that the tunnel driving techniques, fittings and technologies, and special equipment and modus operandi, can grant, where necessary, minimized risk conditions, and make a correct decision on whether, how and when these measures must be fully activated, since such an approach involves a remarkable impact on costs and work organization. In the Western Alps a detailed identification and quantification of the asbestos content in rocks is a difficult task, due to the fact that the asbestos in the host rocks, mainly serpentinite, shows a highly variable distribution, typical of ophiolitic belts throughout the world, as it is mostly associated with joints and shear zones. The possibilities of a predictive assessment of the asbestos content in the formations to be excavated are here discussed, with special reference to the reliability of the achievable results.

The magnetite ore districts of the southern Aosta Valley (Western Alps, Italy): a mineralogical study of metasomatized chromite ore

Abstract In the southern Aosta Valley (Italian Western Alps), several massive magnetite bodies occur within serpentinized ultramafic rocks belonging to the Mesozoic meta-ophiolite nappe. The ultramafic rocks consist of lherzolite with minor dunite bodies and show a high pressure metamorphic overprint. The results of a multi-methodological study, based on optical microscopy, electron microprobe analysis and single-crystal X-ray diffraction, are reported here in order to give new insights into (1) the mineralogy and crystal chemistry of spinels and silicates and (2) the genesis of the massive magnetite bodies. Chromium-rich relict cores inside the magnetite grains suggest a derivation from primary chromite concentrations. The major-element behaviour shows the presence of two chromite types: a Cr2O3-rich (Al2O3-poor) type and a Cr2O3-poor (Al2O3-rich) type. Magnetite ore deposits probably represent the product of transformation from a chromite proto-ore which formed in ultramafic rocks pertaining to an ophiolite suite. The transformation of chromite to magnetite occurred during multiple stages: the premetamorphic setting of the ultramafics and the petrographic evidence suggest that metasomatism started before the onset of the alpine metamorphism and was active during the early alpine, eclogite-facies metamorphic overprint related to a subduction process under high fluid activity.

Past and present circulation of CO2-bearing fluids in the crystalline Gran Paradiso Massif (Orco Valley, north-western Italian Alps): tectonic and geochemical constraints

Abstract In the Orco Valley, inside the Gran Paradiso Massif, 3 main fault systems are present: ( a) E–W striking faults dipping at 45–60° to the N, ( b) high-angle NW–SE striking faults, and ( c) high-angle NE–SW to NNE–SSW striking faults. The E–W striking faults and the interposed NW–SE-faults appear to represent a cogenetic structural association related to a larger scale transtensional shear zone, while the NE–SW faults are probably inherited by an older discontinuous deformation stage. Breccia bodies or veins, mostly consisting of carbonate (siderite–ankerite±calcite)+quartz with sulphide–Au mineralisation, recording a multistage mesothermal evolution, occur along both E–W and NW–SE fault systems. Three water types are recognised: type I, Ca 2+ –HCO 3 − waters, with minor SO 4 2− ; type II, (Ca 2+ /Na + )–HCO 3 − waters varying towards Na + –(HCO 3 − /Cl − ) waters; and type III, Mg 2+ –HCO 3 − waters. Type I and type III groundwaters are freshly recharged waters, only slightly exchanged with rocks. Type II includes waters which come into contact with carbonate fracture fillings, and geochemical modelling indicates that dissolution of carbonates along fractures is the main process controlling the groundwater chemistry. These waters evolve in a system open to uprising CO 2 , and their strongly negative δ 13 C CO 2 suggests a substantial organic component in the CO 2 discharge. In the past, CO 2 -bearing fluids were likely responsible for the formation of Fe-bearing carbonate fracture fillings. The persistence through time of the CO 2 flux in the region has important implications for the reconstruction of the Alpine tectonic evolution and deep structure.

Adularia-sericite gold deposits of Marmato (Caldas, Colombia): field and petrographical data

Abstract At Marmato (Caldas, Colombia), epithermal gold mineralization is associated with volcanic to subvolcanic (dacite to andesite) bodies intruding a late Miocene sedimentary sequence and older basement rocks. Mineralization occurs in dilational veins related to a post-magmatic brittle deformation event, possibly connected with reactivation of the Cauca-Romeral fault system. Within the vein system, a vertical zonation is observed. In the lower part the veins are mainly composed of adularia + pyrite ± quartz ± calcite. In the upper part the dominant assemblage is calcite + quartz + pyrite + sphalerite + arsenopyrite + galena + chalcopyrite, and minor argentite, pyrrhotite, polybasite and freibergite. Data from drill cores show that gold is principally related to the hydrothermal veins and dissemination in the wall rock is of minor importance. Wall-rock alteration around the veins is mainly represented by a strong, often complete, sericitization, with minor silicification. The sericitization post-dates an earlier propylitic alteration event, unrelated to gold deposition. The hydrothermal circulation was characterized by ascent of low salinity fluids with a low CO2 content focused along a structurally controlled fracture system. Boiling was a major controlling factor in deposition in the lower part of the system, while in the upper part mixing also occurred.

The Ivrea Morainic Amphitheatre as a Well Preserved Record of the Quaternary Climate Variability (PROGEO-Piemonte Project, NW Italy)

In the Piedmont plain of NW Italy the Ivrea Morainic Amphitheatre (IMA) is a remarkable evidence of the Quaternary glaciations. It consists of a wide (505 km2) complex of lateral moraines (i.e. the Serra d’Ivrea), end moraines and kame terraces, encircling a 200 km2 wide flat internal depression above which a subglacially moulded rocky hills (the Colli d’Ivrea) elevates. The glacigenic succession ranges from the end of the Early Pleistocene (dated on palaeomagnetic basis) to the end of the Late Pleistocene (14C radiometric and 10Be exposure ages) (about 900–20 ky BP). The IMA has recently been parted into ten stratigraphical units, potentially correlable to the whole sequence of the main Quaternary glaciations recorded by the marine oxygen isotope stratigraphy. Natural (glacigenic deposits and forms) and archaeological (i.e. the Roman gold mines) features make the IMA a very interesting topic for a multidisciplinary research with educational, cultural and tourist purposes. Some recent and present activities for the land promoting are presented. A candidature to the UNESCO global geopark network is considered as a suitable and ambitious goal.

The Monviso massif and the Cottian Alps as symbols of the Alpine chain and geological heritage in Piemonte, Italy.

In order to promote geosite conservation in the project entitled ‘PROactive management of GEOlogical heritage in the PIEMONTE Region’, we propose a comprehensive study involving the Monviso Massif (MM) geothematic area, one of the most outstanding symbols of the Alps and particularly of the Cottian Alps. Specifically, at the MM, the inventory of a number of different geosites whose conservation and development require different geologic and some additional non-geological expertise is considered: (1) some of the best preserved ophiolites in the Alps and the associated Cu–Fe mineralizations; (2) the lithostructural units in the Germanasca Valley; (3) the first primary source of jade in the Alps at the MM and its importance in terms of Neolithic to Bronze Age-polished stone implements; (4) the world-famous minerals such as coesite and giant pyrope, as well as type localities for new minerals (including carlosturanite); (5) the area, now buried under a debris flow, where Hannibal is thought to have regrouped his army while crossing the Alps; and (6) the biodiversity of lichens, microfungi and cyanobacteria colonizing the ophiolites, which can give additional value for the environmental assessment and evaluation of the MM outcrops. Following geodiversity identification, further stages will be devoted to develop appropriate plans for geodiversity conservation in this area.

The Zn-Pb deposits of Casario (Ligurian Alps, NW Italy): Late Palaeozoic sedimentary-exhalative bodies affected by the alpine metamorphism

In the Ligurian Alps (South-Western Italian Alps), Zn-Pb deposits occur within late Palaeozoic meta-sedimentary units belonging to the Briançonnais Zone near Casario (Tanaro valley). Different types of sulphide-rich, lens-shaped mineralizations are recognized: sphalerite-galena massive sulphide bodies, pyrite-rich lenses and sulphide-rich quartz–carbonate-chloritoid granofels. Sulphide lenses and host rocks are affected by at least three ductile deformation phases and by a polyphase alpine metamorphism, whose climax conditions are estimated, based on P-T pseudosection calculations, at T = 300-325 °C and P = 0.55-0.60 GPa. In all the mineralized lenses the ore minerals are represented, in variable amount, by Fe-poor sphalerite, galena, pyrite and arsenopyrite (± tetrahedrite, chalcopyrite and pyrrhotite); the gangue consists of quartz, carbonate (sideritemagnesite ± rhodochrosite s.s.), Fe-chloritoid, muscovite-phengite and chlorite. The mineralizations are associated with chloritoid – carbonate micaschists displaying a finely bedded texture, with sharp between-bed compositional contrast, which suggests their exhalative origin. In spite of the tectono-metamorphic overprint, some pre-metamorphic features of the hydrothermal system are still recognized, like relics of the hydrothermal feeding system, primary growth textures and sulphide-rich microbreccias. These massive sulphide lenses, which share many characters with the SEDEX deposits, testify to the occurrence of an exhalative event of Upper Carboniferous age previously unrecognized in the Ligurian Briançonnais Unit.