Fracturing and remobilization of Bi-rich melts during a late metallogenic stage of the Martinamor Sn–W system (Salamanca, Iberian Massif)

Abstract

Abstract Bismuth-rich melts are increasingly recognized as efficient collectors and transport agents for gold in hydrothermal systems, yet important questions remain regarding the processes controlling their generation, remobilization, and transport. The Martinamor Dome (Iberian Massif, Spain) hosts a long-lived granite-related Sn-W (± Bi-Au) mineral system that evolved between ca. 338–301 Ma and records multiple stages of structurally controlled hydrothermal mineralization, providing an ideal setting for investigating deformation-assisted metal remobilization. The youngest hydrothermal event (ca. 301–300 Ma) comprises an early cassiterite-bearing vein stage (V1) and a later Bi-Au-bearing, sulfide-dominated vein stage (V2), both formed during brittle deformation conditions under a strike-slip-dominated strain regime. Between these two stages, structural analysis indicates a shift from N-S to E-W maximum horizontal elongation, interpreted as the result of a σ2–σ3 stress permutation coupled with a decrease in differential stress. The sulfide stage records extensive remobilization of low-melting-point chalcophile elements (Bi, Ag, Te, Se, Pb, and Sb). Coupled dissolution-reprecipitation of early arsenopyrite generated sieve-textured arsenopyrite and contributed to the remobilization of low-melting point chalcophile elements (LMCE) into polymetallic Bi-rich melts, together with a likely contribution from the circulating hydrothermal fluid. Textural relationships and fluid inclusion data indicate the coexistence of Bi-rich melt droplets with a low-salinity H₂O-CO₂-CH₄ fluid at homogenization temperatures of 239–285 °C, under conditions compatible with their mechanical transport. Synchronous brittle deformation facilitated both melt transport and trapping, whereas the close association between electrum and Bi-rich droplets is consistent with Au scavenging by Bi-rich melts. These results demonstrate that brittle deformation, coupled dissolution-reprecipitation of arsenopyrite, and fluid-assisted metal remobilization were key controls on Bi redistribution during the late evolution of the Martinamor mineral system.