Skarn geochronology and genesis of the Wanlongshan Zn-Sn polymetallic deposit in southeast Yunnan: constraints from garnet and sphalerite composition, and in-situ S isotopes

Abstract

The Wanlongshan Zn-Sn polymetallic deposit represents a significant deposit discovered recently in the world-class Dulong super-large Zn-Sn-In polymetallic ore field, situated in proximate to the Tongjie-Manjiazhai ore deposit. The stratiform to lenticular ore bodies are hosted within siliceous carbonate rocks of the Cambrian Tianpeng Formation’s second member and exhibit structural control by interlayer fault zones. Despite its importance, the precise timing of magmatic-hydrothermal activity and the associated ore-forming processes remain incompletely constrained, generating considerable debate regarding its genesis. To address these uncertainties, this investigation employs an integrated analytical approach combining garnet U-Pb geochronology, trace element geochemistry of garnet and sphalerite, and in situ sulfur isotope systematics. Geochemical analyses reveal that the garnets are predominantly grossularitic in composition, characterized by distinctive LREE depletion and HREE enrichment patterns that likely reflect the interplay between crystal-chemical constraints and adsorption mechanisms. The subdued europium anomalies suggest mineral formation under conditions of low oxygen fugacity within a weakly oxidizing to reducing environment that was neutral to mildly acidic, where hydrothermal metasomatism proceeded primarily via diffusional processes under restricted water/rock ratios. Geochronological constraints from garnet U-Pb dating yield an age of 88.1 ± 3.3 Ma (MSWD = 0.98, n = 17), which correlates remarkably well with molybdenite Re-Os ages from the mining district, ore formation ages at Manjiazhai, and the emplacement chronology of the Laojunshan granite. This temporal coincidence strongly indicates synchronicity among skarnization processes, Zn-Sn mineralization, and granitic magmatism. Trace element distributions in sphalerite reveal substantial enrichment in Fe, Mn, and In, with concomitant depletion in Ga, Ge, and Sn. Elemental ratios including Fe/Zn, Zn/Cd, and Ga/In in sphalerite indicate intermediate to high-temperature mineralization conditions. Multiple substitution mechanisms have been identified: Fe, Mn, and Cd incorporate through simple isomorphic replacement of Zn; Pb occurs predominantly as microinclusions; Cu enters as chalcopyrite solid solution; silver likely substitutes through coupled mechanisms involving either Ag⁺ + Ga³⁺ → 2Zn²⁺ or 2Ag⁺ + Ge⁴⁺ → 3Zn²⁺; gallium exhibits minimal incorporation; while indium predominantly substitutes via Cu⁺ + In³⁺ ↔ 2Zn²⁺.The trace element signature of sphalerite differs markedly from patterns characteristic of Mississippi Valley-Type (MVT), Volcanic-Hosted Massive Sulfide (VHMS), and Sedimentary Exhalative (Sedex) Pb-Zn deposits, instead exhibiting geochemical affinities with established skarn deposits worldwide. In situ sulfur isotope analyses yield δ³⁴S values ranging from 0 to 3.8 ‰ (mean 2.1 ‰, n = 38), consistent with predominant derivation from magmatic reservoirs, confirming that sulfur in the ore-forming fluids originated primarily from magmatic-hydrothermal sources. Collectively, these multiple lines of evidence—chronological, mineralogical, geochemical, and isotopic—provide compelling support for the classification of the Wanlongshan Zn-Sn polymetallic deposit as a classic skarn-type mineralization system.