The transformation from serpentinite to talcose rocks and its consequences. A pilot study in Cabo Ortegal, an ultramafic Massif in NW Spain

This study investigates the transformation of ultramafic rocks into serpentinite, carbonated serpentinite, talcose serpentinite, and pure talcose rocks within Cabo Ortegal ultramafic complex (NW Spain). We aimed to characterize mineralogical, geochemical, and isotopic changes driven by fluid-rock interactions and assess environmental implications. An integrated approach including Petrographic Microscopy, X-ray Diffraction, Transmission Electron Microscopy, Thermal Analysis, ICP–MS, and Stable Isotope (δ¹⁸O, δ¹³C, D/H) analyses was applied. Results reveal a stepwise evolution with SiO₂ and Al₂O₃ enrichment, depletion of Fe, Mg, Mn, and volatiles, and notable trace element variations (Sr, Li, Zr, Nb, Th, U). Isotopes indicate alteration mainly by metamorphic fluids, with minimal meteoric or marine influence. Talc formation occurs via low-temperature hydrothermal processes involving SiO₂- and CO₂-rich fluids, mobilizing potentially toxic elements (e.g., Cr, Ni, Co), raising environmental concerns near abandoned quarries. Although serpentinites have industrial potential, their heterogeneity from ongoing transformation complicates their use. Enhancing our understanding of serpentinite alteration is crucial for effective resource management and environmental protection. Although Cabo Ortegal has been extensively studied, the significance of serpentinite transformation remains insufficiently explored. The findings emphasize the need for detailed petrological and geochemical investigations to balance resource utilization with environmental stewardship in ultramafic terrains. This study demonstrates that talcose rocks are unsuitable for pharmaceutical or cosmetic applications but possess artisanal potential, promoting the tourism of a newly recognized UNESCO Global Geopark. Improving the scientific knowledge of the serpentinites and their evolution can help to increase the knowledge on the ultramafic massif but also may help to promote industry in the area as well as to preserve the environment.