Genesis of the Luanchuan serpentine jade, East Qinling, China: Insights from petrology, geochemistry, and U-Pb geochronology

• The age of 860 ± 8 Ma constrains the upper limit for the formation of the Luanchuan serpentine jade. • The δ 34 S values in the jade suggest a homogeneous sulfur source and a magmatic-hydrothermal origin. • The Sr compositions were inherited from metasedimentary components during fluid–rock interactions. • The jade was formed by regional magmatic-metamorphic processes involving multiple stages. The Luanchuan jade deposit in the East Qinling Orogen, central China provides crucial clues to the tectonic evolutionary processes of the southern margin of the North China Block (NCB). The jade is primarily composed of serpentine jade with subordinate associated nephrite/tremolite jade, where the former is distinctly in contact with metamorphosed gabbro. Significant debate remains, however, regarding the jade types’ spatial and temporal distribution and genesis. In this study, we integrate petrography, X-ray powder diffraction, geochemistry, S-Sr isotopes, and zircon U-Pb geochronology to elucidate the mineralogy, mineralisation processes, and tectonic setting of major serpentine jade occurrences. Our findings indicate that the ore bodies primarily underwent serpentine alteration, characterised by the mineral assemblage serpentine (antigorite)–dolomite–calcite–diopside–chlorite–pyrite. Pyrite shows a narrow range of δ 34 S value from 3.4 to 5.6 ‰, coupled with a Co/Ni ratio varying between 1.83 and 12.6, indicative of a magmatic-hydrothermal origin. Whole-rock geochemistry of the jade samples is characterised by a strongly negative Eu anomaly, a slight Ce anomaly, enrichment in light rare earth elements (LREEs), flat heavy rare earth elements (HREEs) patterns, along with significant U and Th enrichments, but depletion in Rb, Ba, Nb, and other trace elements. These characteristics indicate a close genetic link between the serpentine jade and its host dolomitic marble. Geochemical characteristics (Th/U, δU, and δCe ratios) indicate that the jade was formed in an oxidized environment, while the whole-rock Sr/Ba, Zr/Hf, and Nb/Ta ratios depict its formation in an alkaline formation environment. The 87 Sr/ 86 Sr values in the jade samples range from 0.706459 to 0.709109, typical of marine Sr components pointing to inheritance from metasedimentary components. Zircon U-Pb dating of the metagabbro yields a concordant age of 860 ± 8 Ma (interpreted as the crystallisation age) revealing a significant mid-Neoproterozoic magmatic event. In contrast, U-Pb dating of metamorphic zircon (with Th/U ratios of 0.02–0.24) from the jade (serpentinised marble) yields an upper intercept age of 1924 ± 14 Ma. The date is interpreted as corresponding to a Paleoproterozoic tectono-thermal event that triggered the regional metamorphism of marine carbonate rocks into dolomitic marble. Integrated geological investigations reveal that the Mg and Ca at the Luanchuan serpentine jade were primarily derived from dolomitic marble. While the sources of SiO 2 and H 2 O were associated with both metamorphic fluids and mafic magmatism, further confirmed by the comparatively close ratios of Eu/Sm and Sm/Nd between the metagabbro and samples of the Luanchuan serpentine jade. Our study emphasizes that the serpentine jade deposit primarily formed through multiple tectonic episodes, during which basic magmatic-hydrothermal fluids interacted with metamorphic processes, and the emplacement age of ca. 860 Ma is considered to interpret the upper age limit for the formation of the Luanchuan serpentine jade.