Geological and geochemical characteristics of Permian tourmalinization at val trompia (southern Alps, northern Italy) and relationship with the orobic tourmalinites

Tourmaline breccias in the preAlpine basement in Val Trompia (Southern Alps, Northern Italy) are related to late Palaeozoic granites and are associated with Sn-W -bearing ore deposits. The breccias are rich in acicular tourmaline with variable textural and compositional characteristics. At least four stages of tourmaline crystallization are distinguished, mainly by Mg/Fe ratios. All tourmalines can be classified as «alkali group» varieties using recent nomenclature schemes. Compositions vary from schorl-dravite for earlier, main-stage type I and II coarse crystals, to highly Fe-enriched tourmalines, probably approaching the povondraite endmember (typical of oxidized hydrothermal environments), for type Ill and IV fine-grained tourmalines within late veinlets. Whole-rock REE geochemical data show variable, locally remarkable enrichments in LREE related to the action of hydrothermal fluids. Coarse tourmalines occurring in the cryptocrystalline groundmass of the Orobic tourmalinites, which are hosted in cataclastic zones along the tectonic contact between basement and Permian volcanosedimentary cover rocks in the Lake Diavolo area (southern Alps), have schorl-dravite compositions similar to type I and II, main-stage Val Trompia tourmalines. The Orobic tourmalinites were considered of metasomatic origin by previous studies. The Val Trompia tourmalinites can be interpreted as having formed by magmatic-hydrothermal fluids that produced metasomatic tourmalines with different compositions at different distances from the fluid source (magma). Compositional analogies between tourmalinites of the two areas may therefore reflect similarities between the tourmalinization processes that developed in relation to large-scale Permian magmatic-tectonic activity. The Val Trompia tourmalinites thus may represent deeper metasomatic products, closer to the magma chamber, whereas the Orobic tourmalinites would have formed from hydrothermal fluids injected along shallower faults during coeval volcanic activity.