Scaluggia vein and breccia pipe system, Sagliano Micca, Biella Province, Piedmont, Italyi

Regional Level Types
Scaluggia vein and breccia pipe system	Group of Veins
Sagliano Micca	Commune
Biella Province	Province
Piedmont	Region
Italy	Country

Tourmaline-rich ore-bearing system of multistage hydrothermal veins and breccia pipes well exposed along both banks of the Cervo stream to the southwest of Scaluggia, part of a larger system which continues southeast in front of Passobreve.

The hydrothermal system crosscuts different types of rocks and, in the Scaluggia area, it consists of three breccia pipe and some veins.

The oldest veins (Type I) of the hydrothermal system crop out only in the Passobreve area and are not present here.
In the Scaluggia area, Type II veins occur in the Sesia-Lanzo Zone rocks (metapelite, orthogneiss, metabasite) and consists of tourmaline, quartz, carbonate, and sulfides. Approaching the contact with the Valle del Cervo Pluton, the vein system strikes N20-N50°, dipping northwest 40-50° and southeast 45-80°. The veins, which are up to tens of metres long, can attain the tickness of ca. 50 cm close to the contct with the pluton. Meso- to microstructural relationships and mineral assemblages indicate differents domains within a single vein and suggest a multistage evolution of the Type II veins. Type IIa domains consists of tourmaline and very minor quartz, sulfides (euhedral pyrite and anhedral chalcopyrite), and titanite. Type IIb domains are the most important ore-bearing domains of Type II veins and consist of variable amount of fine-grained tourmaline, quartz, and ore minerals, with minor chlorite and accessory K feldspar, apatite, and titanite. Ankerite may also occur. Ore minerals (idiomorphic pyrite armoured by anhedral chalcopyrite, minor tetrahedrite, scheelite, and arsenopyrite) are commonly arranged as centimetre-thick pods, but also occur as either disseminated grains or small stockworks. Silver-rich tetrahedrite forms irregular patches cementing the gangue minerals. Scheelite occurs as fractured idiomorphic crystals, whose fractures are filled by pyrite and chalcopyrite. Arsenopyrite occurs as either disseminated idiomorphic grains and anhedral inclusions in pyrite. Fine-grained sphalerite and thorite are locally present. Where the Type IIb domains crosscut the Sesia-Lanzo Zone metabasites, the only ore minerals are magnetite and minor pyrite. Type IIc are medium-grained tourmaline-free domains consisting of quartz and carbonate, with minor albite, chlorite, and titanite. Major ore minerals are chalcopyrite, pyrite, tetrahedrite, scheelite, and sphalerite. In veins accross Sezia-Lanzo Zone metapelite and orthogneiss, euhedral quartz develops perpendicular to the vein walls (comb texture) and/or to Type IIa and IIb clasts (cockade-like texture), whilst carbonate occurs in the inner part. Ore minerals are arranged as mm to cm thick irregular lenses elongated parallel to the vein direction. Pyrite forms idiomorhic fractured crystals cemented by chalcopyrite, and tetrahedrite. Locally, chalcopyrite develops graphic-like structures. Tetradymite, arsenopyrite, and Fe-poor sphalerite also rarely occur.
Type III veins consist of subvertical fine-grained carbonate + quartz ± chlorite veins striking N170-200°. They occur as up 1 cm thick and 5 m long veins always crosscutting Type I and Type II veins. They are reddish in colour and consist of carbonate with minor quartz and sulfide pods. In some places, veins become darker in colour, due to the presence of fine-grained chlorite. The sulfide pods are made of idiomorphic fractured pyrite cemented by chalcopyrite and tetrahedrite with a graphic-like fabric. Outside the pods, chalcopyrite and tetrahedrite may also occur interstitially to carbonate and quartz.

Three tourmaline-rich breccia pipes have been recognised in this area as elongated, lens-shaped to irregular bodies, up to two metres thick and ten metres long. While the pipe closer to the pluton shows a broad elliptical shape, the other two anasthomose laterally to brecciated veins that narrow laterally. At the contact with the breccia pipes, the wall rock is strongly brecciated. The two pipes closer to the pluton are black in colour and are characterised by rounded clasts of hydrothermal quartz embedded in a fine-grained tourmaline matrix. The matrix is composed of tourmaline alone, or in association with quartz and sulfides (pyrite, chalcopyrite, and arsenopyrite). By contrast, the pipe further away from the pluton is reddish and composed of irregularly shaped tourmaline ± quartz clasts set in an abundant matrix of carbonate, quartz and minor pyrite, chlacopyrite, and arsenopyrite.

Fossils

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References