Petrography, Geochemistry, and Genesis of the Sanchakou Contact-Metasomatic Tremolite Jade Deposit, Qinghai Province

The Sanchakou tremolite jade deposit in Qinghai Province represents a significant contact-metasomatic jade resource. Unlike typical contact-metasomatic (D-type) deposits—commonly associated with intermediate-to-acidic intrusions and magnesian carbonate protoliths—the Sanchakou deposit is genetically linked to a relatively SiO2-poor gabbro intrusion and dolomite. This atypical geological setting raises fundamental questions concerning the sources of ore-forming components and the nature of metasomatic processes involved. To address these questions, this study integrates petrographic observations, whole-rock major- and trace-element geochemistry, and Sr isotopic analyses of tremolite jade, host dolomite wallrocks, gabbro, and their hydrothermally altered counterparts to constrain the origins of Ca, Mg, and Si and to reconstruct the metasomatic evolution of the system. Our results reveal a multi-sourced contribution to ore formation: Ca is derived exclusively from dolomite; >98% of Si originates from an external crustal fluid; and Mg is supplied predominantly (77%) from dolomite, with the remaining 23% sourced from altered gabbro. Strontium isotopic data further confirm the involvement of a gabbro-derived fluid characterized by elevated 87Sr/86Sr ratios but low Sr concentrations. A well-defined, continuous metasomatic sequence—comprising silicification → diopsidization → tremolitization → nephritization—is clearly documented. The deposit formed in a post-collisional extensional tectonic setting, driven by heat-induced double metasomatism. This study clarifies the provenance and evolutionary pathways of ore-forming materials in this atypical contact-metasomatic system and provides a genetic framework for analogous jade deposits associated with mafic intrusions.