Per. Mineral. (2006), 75, 2-3, 141-150 http://go.to/permin An International Journal of O PERIODICO di MINERALOGIA MINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY, established in 1930 ORE DEPOSITS, PETROLOGY, VOLCANOLOGY and applied topics on Environment, Archaeometry and Cultural Heritage Characterization and genetic inferences of arsenates, sulfates and vanadates of Fe, Cu, Pb, Zn from Mount Cervandone (Western Alps, Italy) Alessandro Guastoni1*, Federico Pezzotta2 and Pietro Vignola3 1 Department of Mineralogy and Petrology, Università di Padova, corso Garibaldi, 37 – 35137 Padova (Italy) 2 Department of Mineralogy, Museo Civico di Storia Naturale, corso Venezia, 55 – 20121 Milano (Italy) 3 CNR-IDPA Istituto per la dinamica dei processi ambientali, via Mario Bianco, 4 – 20131 Milano (Italy) Abstract. — Mount Cervandone is among the destabilization of accessory minerals hosted in these most renowned mineral deposits in the Alpine chain dikes and consequently the hydrothermal fluids and type locality of several very rare REE-arsenates. assimilated light elements (Be, B, F), high field The present study allowed for the characterization strength elements (Y, Nb, Ta) and rare earth elements and description of the assemblages of a number of (La, Ce, Nd, Th, U) and allowed the crystallization of arsenates, sulfates and vanadates of Fe, Cu, Pb, several rare and exotic minerals. Zn that occur with a number of rare minerals of B, Be, As, Nb, Y, REE on the Italian slope of Mount Key Words: Arsenates, hydrothermal fluids, Mount Cervandone. The mineral deposits of Cervandone Cervandone, NYF-pegmatites, quartz fissures, mainly occur in alpine quartz fissures and pegmatitic sulfates, vanadates. dikes. The pegmatitic dikes are hosted by very fine grained two-mica leucocratic gneisses. Locally the pegmatite dikes are cross cut by quartz veins. The Riassunto. — Il Monte Cervandone è uno dei assemblages reported in this study indicate that principali depositi mineralogici presenti sulla catena arsenates, sulfates and vanadates of Fe, Cu, Pb, Zn Alpina e località tipo di numerosi arseniati di terre are mainly hosted in quartz veins and only occur rare. Il presente studio ha permesso di caratterizzare e sporadically in pegmatitic dikes. In the pegmatite descrivere numerosi arseniati, solfati e vanadati di Fe, dikes the Be, As, Y and REE minerals occur with Cu, Pb, Zn che si rinvengono associati a rari minerali higher frequency than in the quartz veins. During di B, Be, As, Nb, Y e REE sul versante italiano del the Alpine event we account for a “scenario” where Monte Cervandone. Il giacimento mineralogico del swarms of NYF (niobium, yttrium, fluorine) granitic Cervandone è principalmente associato a fessure pegmatites intruded the leucogranitic-aplites. It is di quarzo di tipo alpino e filoni pegmatitici. I filoni unequivocal that the pegmatites show a strong NYF pegmatitici sono ospitati in gneisses leucogranitici geochemical signature represented by the occurrence a due miche. Localmente i filoni pegmatitici sono of an unique mineralogy. intersecati da filoni quarzosi. Le associazioni Following these premises we expected that during mineralogiche riportate in questo studio indicano the Alpine event As-enriched hydrothermal fluids che gli arseniati, i solfati e i vanadati di Fe, Cu, Pb, circulating through pegmatite dikes allowed the Zn sono principalmente presenti nei filoni quarzosi mentre risultano essere sporadiche nelle pegmatiti. I minerali contenenti Be, As, Y and REE si rinvengono * Corresponding author, E-mail: alessandroguastoni@ in concentrazioni più elevate nei filoni pegmatitici unipd.it mentre sono presenti con minore frequenza nelle vene 142 A. Guastoni, F. Pezzotta and P. Vignola quarzose. Nel corso dell’evento alpino è possible 1995; Praeger and Crumbach 2004; Cuchet et al. ricostruire uno scenario dove sciami di pegmatiti 2005). granitiche NYF (niobio, ittrio, fluoro) hanno intruso Field observations established the mineral i leucograniti aplitici. Difatti è inequivocabile che le deposit of Cervandone is mainly hosted in alpine pegmatiti registrano una forte impronta geochimica a quartz fissures and pegmatitic dikes. Based on NYF determinata da una mineralogia assolutamente these observations, this study expected to relate the caratteristica. formation of a number of rare minerals of B, Be, As, Su queste premesse si può ritenere che durante Nb, Y, REE, that occur with arsenates, sulfates and l’evento alpino fluidi idrotermali arricchiti in arsenico vanadates of Fe, Cu, Pb, Zn with the circulation of siano circolati nei filoni pegmatitici permettendo la hydrothermal fluids. During the Alpine event As- destabilizzazione di una serie di minerali accessori enriched hydrothermal fluids, circulating through presenti in questi filoni e conseguentemente i fluidi the pegmatite dikes, destabilized pre-existing idrotermali, avendo assimilato elementi leggeri (Be, accessory minerals contained in HFSE-rich B, F), elementi ad elevata forza di campo (Y, Nb, (high field strength elements) pegmatites. These Ta) ed elementi delle terre rare (La, Ce, Nd, Th, pegmatites with a NYF geochemical signature U), hanno permesso la cristallizzazione di numerosi liberated Y, Nb, Ta and REE (La, Ce, Nd, Th and minerali rari. U). Low temperature hydrothermal fluids formed the quartz veins but also allowed the crystallization Parole chiave: Arseniati, fluidi idrotermali, monte of several rare minerals. Cervandone, pegmatiti NYF, fessure di quarzo, solfati, vanadati. Analytical methods Introduction Sixty specimens containing arsenates, sulfates and vanadates of Fe, Cu, Pb, Zn from Mount Cervandone and the Swiss Mount Cervandone were observed under a Wannigletscher (also named Cherbadung) are stereomicroscope in order to describe of each among the most renowned mineral deposits in the mineralogical assemblage. Additionally, a number Alpine mountain chain. Several very rare REE of samples were selected for chemical and\or arsenates including asbecasite (Graeser 1966), diffraction data in order to characterize the specific cafarsite (Graeser 1966), cervandonite-(Ce) mineral phases. (Armbruster et al. 1988), fetiasite (Graeser et al. Mineralogical analyses were performed 1994), gasparite-(Ce) (Graeser and Schwander utilizing X-ray powder diffractometers and 1987) and paraniite-(Y), (Demartin et al. 1994) “JEOL” 5610 LV scanning electron microscope were first described at these localities. The (SEM) equipped with a “Gresham” EDXS aforementioned mineralogy at Mount Cervandone spectrometer. Powder diffraction data were and at Wannigletscher were extensively collected with a “Panalytical X’Pert Pro” investigated in the past because of the very rare instrument operating at 40 kV and 40 mA with REE-arsenates which occur in this deposit and CuKα radiation at the Department of Earth the mineralization was described as a typical Sciences of the University of Milano-Bicocca. example of alpine quartz type fissures (Albertini Powder data for tyrolite were collected with 1991; Graeser and Albertini 1995). a “Rigaku” DMAX III X-ray diffractometer Arsenates, sulfates and vanadates of Fe, Cu, Pb, operating at 40 kV and 40 mA with CuKα Zn occur with many other mineral phases which radiation at the Department of Structural and include arsenates and phosphates of Y, REE Inorganic Stereochemistry of the University of (rare earth elements), oxides of Y, Nb, Ta, REE, Milan. Diffraction peak intensities were measured carbonates of Bi, REE and silicates of B, Be and in the 2θ range 5-80° except for cornubite (range Y that previous authors have described at Mount 5-110°); NIST Silicon 640c was used as an Cervandone and at Wannigletscher (Graeser and internal standard. Antlerite, linarite, mimetite, Stalder 1976; Albertini 1991; Graeser and Albertini olivenite, pharmacosiderite, strashimirite and Characterization and genetic inferences of arsenates, sulfates and vanadates of Fe, Cu, Pb, Zn ... 143 vanadinite reflections were indexed to obtain Along the north-east flank of “Bandiera” peak unit-cell parameters. Brochantite-2M1, cornubite a mineralogical deposit occurring in quartz veins and tyrolite unit cell parameters were refined was investigated. This deposit includes large with “Rietveld” method using “GSAS-EXPGUI” alpine type quartz fissures that may contain program (Larson and Von Dreele 2000; Toby museum quality specimens. Smoky quartz 2001). Due the scarcity of material available crystals coated by dodecahedral and octahedral for X-ray diffraction reliable cell parameters crystals of cafarsite were collected. A number for duftite could not be obtained. The scarcity of accessory mineral phases were also collected of material also precluded the collection of X- and they include weathered, brownish masses of ray diffraction data of mottramite. Nevertheless tennantite, agardite-(Y), asbecasite, chernovite- mottramite and mimetite were characterized by (Y), gasparite-(Ce), rutile and minor anatase. means of electron microprobe analyses (EPMA) Along the endocontact of the quartz vein with carried out at an accelerating voltage of 20kV, a the two-mica leucocratic gneisses, a number of filament current corresponding to 85 μA, a beam arsenates, sulfates and vanadates of Fe, Cu, Pb, diameter of 2μm (25 spot size) and count time Zn occur. Four characteristic assemblages were of 60 s per point analysis. Quantitative analyses observed and include: were performed on a polished section utilizing a brochantite-2M1 (Fig. 1) + cerussite + duftite series of natural standards (apatite for P; realgar