Macro- to nanoscale investigation unlocks gold and silver enrichment by lead-bismuth metallic melts in the Switchback epithermal deposit, southern Mexico
Por:
Cano, N, González-Jiménez, JM, Camprubí, A, Proenza, JA, González-Partida, E
Publicada:
28 abr 2025
Resumen:
Lead-bismuth (Pb-Bi) minerals of the galena (PbS)-matildite (AgBiS2) series and lillianite homologs (Pb3-2xAgxBi2+xS6) are intergrown with electrum (Au-Ag alloy) and chalcopyrite (CuFeS2) in specific bands within a colloform-banded vein at the Switchback epithermal deposit in southern Mexico. A macro- to nanoscale study revealed that these minerals fill small (<200 mu m) cavities in the gangue minerals, showing curvilinear boundaries, bleb-like morphologies, and rounded nanoparticles (similar to 100 nm). These observations are consistent with growth in a molten system from a precursor Pb-Bi melt containing Au, Ag, and Cu. Minerals in the galena-matildite series typically display Widmanst & auml;tten textures (i.e., octahedral-like or basket-weave matildite(ss) lamellae in the galena(ss) matrix), which have been traditionally linked to the decomposition of a high-temperature solid solution. However, galena(ss) and matildite(ss) show nanoscale sinuous reaction fronts and replacement relicts ("islands") while maintaining the [011](Galena) parallel to [100](Matildite) crystallographic orientation relationship. This suggests a topotaxial growth of matildite(ss) mediated by coupled dissolution-reprecipitation reactions between galena(ss) and the metallic melt upon cooling. A similar scenario is proposed for Pb-Bi sulfosalt intergrowths, which replace galena-matildite and electrum and grow topotaxially along (200)(Galena). Collectively, these results suggest that Pb-Bi melts can exist in epithermal fluids, acting as precursors for the crystallization of ore minerals and being able to sequester precious metals. This model explains abnormally high-Au-Ag enrichments observed in some deposits that contain Pb-Bi ores.
Filiaciones:
Cano, N:
Univ Nacl Autonoma Mexico, Inst Geol, Ciudad Univ, Coyoacan, Mexico
Univ Nacl Autonoma Mexico, Lab Nacl Geoquim & Mineral LANGEM, Ciudad Univ, Mexico City 04510, Mexico
González-Jiménez, JM:
CSIC, Inst Andaluz Ciencias Tierra IACT, Avda Palmeras 4, Granada 18100, Spain
Camprubí, A:
Univ Nacl Autonoma Mexico, Inst Geol, Ciudad Univ, Coyoacan, Mexico
Univ Nacl Autonoma Mexico, Lab Nacl Geoquim & Mineral LANGEM, Ciudad Univ, Mexico City 04510, Mexico
Proenza, JA:
Univ Barcelona, Dept Mineral Petrol & Geol Aplicada, Fac Ciencies Terra, Marti i Franques S-N, Barcelona 08028, Catalonia, Spain
González-Partida, E:
Univ Nacl Autonoma Mexico, Inst Geociencias, Inst Neurobiol, Blvd Juriquilla 3001, Juriquilla 76230, Qro, Mexico
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