Simone Vezzoni

Mobilization of Tl-Hg-As-Sb-(Ag,Cu)-Pb sulfosalt melts during low-grade metamorphism in the Alpi Apuane (Tuscany, Italy)

We report the discovery of an exceptional assemblage of Tl-Hg-As-Sb-(Ag,Cu)-Pb sulfosalts showing textural evidence for their mobilization as melts in the barite–pyrite–iron oxide orebodies of the Monte Arsiccio mine (Alpi Apuane, Tuscany, Italy). The relative abundance of rare thallium sulfosalts (including three new mineral species), their peculiar textural features within the orebodies (e.g., migration along matrix grain boundaries, drop-like internal textures, low interfacial angles between sulfosalts and matrix minerals), and the overall high thallium content in pyrite from the entire mining district (to ∼900 ppm) make the barite–pyrite–iron oxide deposits of the Alpi Apuane a reference locality for studying low-temperature sulfosalt melts in low-grade metamorphic complexes (greenschist facies). Our study reveals how sulfosalt melting during low-grade regional metamorphism controls the redistribution of economically valuable and environmentally critical elements such as thallium in sulfide orebodies containing significant amounts of low-melting-point chalcophile elements.

Mercury-arsenic sulfosalts from the Apuan Alps (Tuscany, Italy). II. Arsiccioite, AgHg2TlAs2S6, a new mineral from the Monte Arsiccio mine: occurrence, crystal structure and crystal chemistry of the routhierite isotypic series

Abstract The new mineral species arsiccioite, AgHg2TlAs2S6, was discovered in the baryte-pyrite-iron oxide ore deposit exploited at the Monte Arsiccio mine, near Sant’Anna di Stazzema (Apuan Alps, Tuscany, Italy). It occurs as anhedral grains scattered in microcrystalline baryte, associated with cinnabar, laffittite, protochabournéite, pyrite, realgar, Hg-bearing sphalerite and stibnite. Arsiccioite is red, with a metallic to sub-metallic lustre. Minimum and maximum reflectance data for COM wavelengths in air are [l (nm): R (%)]: 471.1: 29.0/29.4; 548.3: 27.6/28.3; 586.6: 26.1/26.5; 652.3: 24.2/24.6. Electron microprobe analyses give (wt.%): Cu 0.78(6), Ag 8.68(21), Zn 0.47(27), Fe 0.04(1), Hg 35.36(87), Cd 0.20(5), Tl 18.79(33), As 10.77(19), Sb 4.75(10), S 18.08(21), Se 0.07(5), total 97.99(44). On the basis of SMe = 6 a.p.f.u., the chemical formula is Ag0.87(2)Cu0.13(1)Zn0.08(4)Fe0.01(1)Hg1.91(5)Cd0.02(1)Tl1.00(2) (As1.56(2)Sb0.42(1))∑1.98S6.12(6)Se0.01(1). Arsiccioite is tetragonal, I4̄2m, with a 10.1386(6), c 11.3441(5) Å, V 1166.1(2) Å3, Z = 4. The main diffraction lines of the powder diagram are [d(in Å), visually estimated intensity, hkl]: 4.195, m, 211; 3.542, m, 103; 3.025, vs, 222; 2.636, m, 114; 2.518, s, 400 and 303. The crystal structure of arsiccioite has been refined by single-crystal X-ray data to a final R1 = 0.030, on the basis of 893 observed reflections. It shows a three dimensional framework of (Hg,Ag)- centred tetrahedra (1 M1 + 2 M2), with channels parallel to [001] hosting TlS6 and (As,Sb)S3 disymmetric polyhedra. Arsiccioite is derived from its isotype routhierite M1CuM2Hg2TlAs2S6 through the double heterovalent substitution M1Cu+ + M2Hg2+ → M1Hg2+ + M2Ag+. This substitution obeys a steric constraint, with Ag+, the largest cation relative to Hg2+ and Cu+, entering the largest M2 site. The ideal crystal chemical formula of arsiccioite is M1HgM2(Hg0.5Ag0.5)2TlAs2S6. The crystal chemistry of the routhierite isotypic series is discussed. Finally, the distribution of Hg ore minerals in the Apuan Alps is reviewed.

Evidence of Permian magmatism in the Alpi Apuane metamorphic complex (Northern Apennines, Italy): New hints for the geological evolution of the basement of the Adria plate

Abstract The occurrence of metavolcanic rocks within the Paleozoic basement of the Alpi Apuane metamorphic complex has been known since long time. Among them, some massive porphyritic tourmaline-bearing rocks cropping out in the southern sector of the Alpi Apuane present some distinctive and peculiar features, differing from the better known middle Ordovician metarhyolites of the “Porfiroidi e scisti porfirici” Fm. The porphyritic tourmaline-bearing rocks belong to the recently proposed Fornovolasco Metarhyolite Fm. They are granular to porphyritic, with phenocrysts of quartz (often with magmatic embayment), pseudomorphosed feldspars, and mica (both biotite and muscovite), in a groundmass formed by quartz, white mica, albite, and K-feldspar. Tourmaline (schorl-dravite in composition) is an abundant accessory mineral, in some cases forming cm-sized spots. The studied rocks plot into the rhyolite field of the Total Alkali vs Silica classification diagram. They show a peraluminous nature, having an Alumina Saturation Index ranging from 1.3 and 3.2. Their trace-element signature is that typical of highly evolved orogenic magmas. Laser ablation-ICP-MS U—Pb datings on zircon suggest a Permian crystallization age (weighted average ages of the four samples ranging from 292 and 271 Ma), thus relating these rocks to a post-Variscan magmatism. This new dating represents the very first evidence of a Permian magmatism in the pre-Triassic basement of the Northern Apennines. The potential relationships between Permian felsic magmatism and the ore genesis in the Alpi Apuane metamorphic complex are also discussed.

Lateral extrusion of a thermally weakened pluton overburden (Campiglia Marittima, Tuscany)

The ascent and emplacement of magmas in the upper crust modify the local pre-existing thermal and rheological settings. Such changes have important effects in producing anomalous structures, mass extrusion, rock fracturing, and in some conditions, hydrothermal mineralizations. In the Campiglia Marittima area, detailed field mapping led to the reconstruction of a local deformation history that overlaps, chronologically and spatially, with regional extension. This local deformation was triggered at the Miocene–Pliocene boundary by the intrusion of a monzogranitic pluton beneath a carbonate sedimentary sequence. The emplacement of the pluton produced a perturbation in the rheological behaviour of the carbonate host rocks, producing transient ductile conditions in the very shallow crust. The carbonate rocks were thermally weakened and flowed laterally, accumulating downslope of the pluton roof, mainly toward the east. As the thermal anomaly was decaying, the brittle–ductile boundary moved progressively back towards the pluton, and large tension gash-shaped volumes of fractured marble were generated. These fractured volumes were exploited by rising hydrothermal fluids generating sigmoidal skarn bodies and ore shoots. This work presents the Campiglia Marittima case study as a prime example of structural interference between regional extensional structures and local, lateral mass extrusion in a transient ductile rheological regime triggered by pluton emplacement.