Passobreve vein system, Sagliano Micca, Biella Province, Piedmont, Italyi

Regional Level Types
Passobreve vein system	Group of Veins
Sagliano Micca	Commune
Biella Province	Province
Piedmont	Region
Italy	Country

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

The hydrothermal system crosscuts different types of rocks and, in the Passobreve area, it consists of veins which used to be exploited for copper, mainly during the 19th century, as testified by the presence of small trenches and tunnels.

The oldest veins (Type I) crops out only within the Sesia-Lanzo Zone rocks (metapelite, orthogneiss, metabasite) and are represented by subvertical veins striking N75° to N110°. These veins, mainly consisting of K feldspar ± tourmaline, are up to 1.5 cm thick and few metres long. K feldspar is Ba-enriched and occurs both as aggregates of xenomorphic to subidiomorphic grains, and as isolated idiomorphic crystals with rhomboherdal shape (adularia). The thinner veins are homogeneous and made up by fine-grained K feldspar with minor chlorite, carbonate (mostly ankerite), quartz and accessory titanite, pyrite, and chalcopyrite. The thickest veins are simmetrically zoned with an external whitish portion, showing the same texture and composition of the thinner veins, and an inner black portion, consisting of tourmaline and very minor carbonate, quartz, and chalcopyrite.
Type II are the most important veins and consists of tourmaline, quartz, carbonate, and sulfides as black to reddish veins occurring in both the Sesia-Lanzo Zone rocks and Oligocene volcanics. In the Passobreve area, the veins are subvertical and mostly strike N80° to N110°. They are millimetre to decimetre thick, and up to tens of metres long. Meso- to microstructural relationships and mineral assemblages indicate differents domains within a single vein and suggest a multistage evolution of the Type II veins. Types IIa and IIb domains have been observed only within the Sesia-Lanzo Zone rocks, whilst Type IIc domains occur in both the Sesia-Lanzo Zone and volcanic rocks. 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. 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, galena, tetrahedrite, scheelite, Bi sulfosalts, 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. In the volcanics, anhedral quartz and chlorite grow in the centre of the vein and carbonate at the rim. In the very centre, some open vugs are lined with fine-grained idiomorphic crystals of quartz. 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, galena, tetrahedrite, and Bi sulfosalts. Locally, chalcopyrite and Bi sulfosalts develop graphic-like structures, or Bi sulfosalts occur as small veins crosscutting pyrite-chalcopyrite lenses. Hessite, tetradymite, arsenopyrite, and Fe-poor sphalerite also rarely occur.
Type III veins consist of subvertical fine-grained carbonate + quartz ± chlorite veins striking N170-200°. Both in the Sesia-Lanzo Zone and volcanic rocks, 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.

Fossils

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References