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Loma Galena is the deposit with the highest Ag (6,410.8 t) and Pb (997,130 t) endowment in the world-class polymetallic Navidad district. Previous investigations show that Ag and other elements are hosted mainly by pyrite and marcasite but are invisible by conventional microscopy methods. By coupling laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) spot analysis with scanning and transmission electron microscopy (SEM, TEM) observations, we investigate the relationship between invisible Ag and associated elements in the sulfide hosts. Pyrite occurs as framboids and polyframboidal clusters, is massive, and occurs in association with marcasite, forming colloform aggregates. The size of framboids (>10 μm) suggests that their formation environment was suboxic. The dominance of colloform textures—composed by multiple and alternating micrometer-scale bands of fine-grained pyrite and coarser marcasite—indicates fluctuations in the pH and degree of saturation of the fluid during sulfide formation. This environment facilitated the incorporation of significant, albeit highly variable, trace element contents in the pyrite-marcasite bands, most importantly Pb, Ag, Mn, As, Zn, Co, Ni, Sb, Tl, and Cu, with minor Cd, Hg, Mo, Ge, and V. When the solid solubility limits for these elements in pyrite-marcasite were exceeded during sulfide growth from the hydrothermal fluid, micro- to nanosized mineral particles of key metals nucleated on the sulfide surfaces and were incorporated within the bulk sulfides. TEM data confirmed that Ag occurs as fine (<150 nm) Ag-bearing particles within the pyrite-marcasite bands. Distinctive Pb-rich and Cu-rich bands that texturally overprint the pyrite-marcasite suggest late-stage replacement processes. Rapid supersaturation during fluid mixing likely facilitated the incorporation of Ag and other metals both as micro- to nanoparticles and in solid solution within the sulfides. Our results help understand the trace element incorporation in pyrite and marcasite and may support strategies to improve Ag recovery across ore types.
