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The Santa María and Antares Zn-Pb(-Ag) skarn deposits in the Velardeña Mining District are located in central–NW Mexico. They lie 470 m apart along the contact between Oligocene felsic intrusions and Cretaceous limestones, and were developed during prograde, retrograde, post-ore (Santa María), and late stages. Firstly, the prograde stage was formed by fluids at¿~¿600 °C and 15 wt% NaCl equiv., and consists of garnet¿+¿wollastonite¿±¿clinopyroxene and biotite¿±¿K-feldspar assemblages. Secondly, the retrograde/ore stage was formed by fluids at 300–500 °C with salinities of 20–30 wt% CaCl2 (Santa María) and¿>¿40 wt% NaCl equiv. (Antares). It comprises assemblages of chlorite, amphibole, epidote, calcite, scapolite, quartz, sericite, adularia, fluorite, and muscovite associated with sphalerite, pyrite, galena, pyrrhotite, arsenopyrite, chalcopyrite, and Pb-Bi-Sb sulfosalts. Thirdly, the post-ore stage was formed by fluids at¿~¿400 °C and 20–30 wt.% CaCl2 and comprises poorly mineralized calcite veins. Fourthly, the late stage was formed by fluids at¿<¿300 °C and 20–30 wt.% CaCl2 (Santa María) and¿~¿15 wt% NaCl equiv. (Antares), and crystallized tetrahedrite-group minerals and pyrite¿+¿marcasite. d18Ofluid between¿~¿14‰ and 23‰ at Santa María and between¿~¿12‰ and 17‰ at Antares show a less-modified magmatic affinity for mineralizing fluids at Antares; d13Cfluid between 0‰ and –6‰ register recycling of sedimentary C. Moreover, sulfides with d34SVCDT between –3‰ and 2‰ reveal a magmatic source for S. Altogether, these data suggest that, at Santa María, magmatic-derived fluids actively interacted with the wall rocks, whereas at Antares the fluid-rock interaction was milder. In both deposits, metal deposition was triggered by the cooling and neutralization of ore-bearing fluids with carbonate rocks. Our 40Ar/39Ar dates for adularia of ca. 37.5 Ma place the deposits within the Eocene–early Miocene metallogenetic epoch of central–NW Mexico, during which other world-class skar, The Consejo Nacional de Humanidades, Ciencia y Teconología (CONAHCyT) supported N.C. with a grant during his M.Sc. studies, and this paper constitutes a substantial part of his associated research. This study was financially supported by means of personal allocations to UNAM academicians. Additional funding was provided by the CONACyT-SENER PT. 4.1 Gemex-EU research grant to E.G.P., and CONACyT-Ciencia Básica A1-S-14574 and PAPIIT IA-101419 research grants to Vanessa Colás Ginés (Departamento de Ciencias de la Tierra, University of Zaragoza)., Peer Reviewed, Postprint (published version)

Epithermal bonanza-type ores, characterized by weight-percent contents of e.g., gold and silver in a few mm to cm, are generated by mixtures of magmatic-derived hydrothermal brines and external fluids (e.g., meteoric) that transport a variety of metals to the site of deposition. However, the low solubilities of precious metals in hydrothermal fluids cannot justify the high concentrations necessary to produce such type of hyper-enriched metal ore. Here we show that boiling metal-bearing brines can produce, aggregate, and accumulate metal nanomaterials, ultimately leading to focused gold + silver ± copper over-enrichments. We found direct nano-scale evidence of nanoparticulate gold- and/or silver-bearing ores formed via nonclassical growth (i.e., nanomaterial attachment) during boiling in an intermediate-sulfidation epithermal bonanza. The documented processes may explain the generation of bonanzas in metal-rich brines from a range of mineral deposit types.