Deciphering the tectonic-geodynamic context of the gem-quality “noble serpentine” deposit formation combining microstructural, chemical and micro-Raman analyses in Palaeozoic olivine-bearing marbles and serpentine-hosting rocks (Pizzo Tremogge, Margna unit – Austroalpine, Val Malenco – Central Alps, Italy)

The gem-quality “noble serpentine” of Pizzo Tremogge (Val Malenco, Italy) is included in Palaeozoic olivine-bearing marbles, occurring in the Margna unit, Central Austroalpine domain. A detailed inspection of serpentine samples reveals the presence of the three serpentine species (lizardite, chrysotile and antigorite) occurring in different microstructural domains. Serpentine samples were investigated to obtain microstructural information preserved in the serpentine species by optical microscopy, X-ray powder diffraction, Electron Microprobe Spectrometry and Micro-Raman spectroscopy, the last of which is considered a reliable method for the identification of the different serpentine species. Two types of “noble serpentine” have been identified: the yellow-green type characterized by fine-grained aggregates in marbles and the green type localized in fibrous veins in association with calcite and quartz. The identification of the different serpentine types allowed us to recognize three stages of mineral crystallization characterized by: i) the formation of olivine-bearing marbles (Stage 1), ii) the growth of lizardite after olivine or within lizardite-rich veins (Stage 2) and iii) the replacement of lizardite by antigorite in marbles and by antigorite + clinohumite in veins (Stage 3). These stages clarify the complex geodynamic-tectonic evolution of the Margna-Malenco system. The Paleozoic olivine-bearing marbles (Margna protoliths, Stage 1) are subjected to extensional tectonics that brings the Margna protoliths close to the Malenco in a thinned-extended-type passive margin (Stage 2). Finally, an inversion of tectonics leads to the subduction and collision of the Margna-Malenco system during alpine convergence (Stage 3). All this suggests that gem-quality “noble serpentine” deposits may be related to different geological contexts.