The Genesis of Beryllium Mineralisation in the Saxi Deposit, SW China: Key Controls From High V/Cr Wall Rocks and Fluid–Rock Interaction

ABSTRACT

This study investigates the key factors controlling Be mineralisation in the Saxi deposit (Laojunshan area, SE Yunnan, SW China), which uniquely hosts gem‐quality beryl (emerald). A fundamental challenge in understanding beryllium mineralisation lies in deciphering the spatial and temporal coupling between Be‐rich fluids and chromophore‐bearing lithologies rich in vanadium and chromium. This challenge is expressed as the source paradox in emerald formation: beryllium typically originates from highly evolved felsic melts, whereas essential chromophores are derived from mafic‐ultramafic or metasedimentary rocks. Systematic comparisons of wall rocks and micas from Saxi and nearby deposits reveal relatively high concentrations of V, Cr and Be, with peak V/Cr ratios occurring exclusively in the Saxi wall rocks. These geochemical signatures suggest that the lithological composition of the wall rocks—particularly their high V/Cr ratios and elevated Be concentrations—is a key control on Be enrichment at Saxi. Furthermore, comparative analyses of wall rocks and micas proximal to and distal from beryl‐bearing veins show that proximal samples exhibit significantly lower V, Cr and Be concentrations, as well as lower V/Cr ratios. This pattern suggests efficient extraction of these elements into ore‐forming fluids via fluid–rock interaction, which represents a second key mechanism for Be mineralisation at Saxi. Comparative analysis shows that distal biotite is rich in V (avg. 4529 ppm), Cr (avg. 477 ppm) and Be (avg. 34.5 ppm). In contrast, proximal altered biotite exhibits significant depletion in these elements (V avg. 1610 ppm; Cr avg. 263 ppm and Be avg. 13.9 ppm), indicating that biotite is the primary source of chromophores and Be for emerald mineralisation. In summary, this study highlights two synergistic controls on Be mineralisation at Saxi: (1) V‐ and Cr‐enriched, mica‐bearing metamorphic wall rocks, in which V–Cr‐rich biotite serves as the primary source of chromogenic elements and Be and (2) extensive fluid–rock interaction that facilitates metal mobilisation. These findings provide a novel exploration criterion for regional Be prospecting, suggesting that exploration should target areas with metasedimentary wall rocks characterised by high V/Cr ratios and abundant biotite.