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This paper stems from the dissertations of E.F.G., G.H.A. and, partly, J.C.M.H., who acknowledge the support of CONACyT through their respective PhD (E.F.G.) and MSc grants (G.H.A. and J.C.M.H.). Funding for this work was provided by CONACyT through the research grants 155662 to A.C. and “GEMex: Cooperación México-Europa para la investigación de sistemas geotérmicos mejorados y sistemas geotérmicos supercalientes”, Peer Reviewed, Objectius de Desenvolupament Sostenible::12 - Producció i Consum Responsables, Postprint (author's final draft)

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)

The Velardeña mining district is economically the most important of Durango state. The ore deposits occur in different skarn zones developed within the intrusive contact between Mesozoic limestones and Eocene granitic stocks and dikes. The most important ore deposits are related to the Santa María dike and Reyna de Cobre porphyritic stock (separated from each other by 10 km). They occur as irregularly shaped replacement masses developed near the intrusive contact and have a skarn paragenesis dominated by calc-silicates and sulfides. The mineral assemblages show replacement textures and are dominated by calcic clinopyroxene (Di97-53Hd42-02Jh04-01) and garnet (Ad100-57Grs43-00) in the exoskarn, with wollastonite particularly abundant in the endoskarn. Hydrous silicates are actinolite, epidote, and chlorite, whereas sulfides include pyrite, sphalerite, pyrrhotite, galena, chalcopyrite, and sulfosalts. Scheelite, hematite, quartz, and calcite are also present. According to sphalerite geobarometry, the skarns formed at hypabyssal depths (~3–4 km). They developed by a succession of replacive mineralizing events, including (a) a prograde stage at temperatures from =470 to 335 °C in conditions of low f(CO2), followed by (b) a retrograde stage from 335 to 220 °C. There was a general increase in f(O2), accompanying the temperature decline during the formation of the system, which accounts for a process of mixing with cooler, oxidizing, and dilute water. During the retrograde stage, wollastonite calcic garnet and clinopyroxene formed. On the other hand, hydrous silicates, sulfides, sulfosalts, scheelite, and hematite crystallized during the retrograde stage. Skarn mineralization is crosscut by veins of calcite, fluorite, adularia, and sphalerite. The vein mineralization formed at temperatures below 200 °C. The different ore deposits of Velardeña constitute a telescoped skarn–epithermal mineral system., This project was sponsored by Industrias Peñoles, S.A. de C.V., and the stable isotope measurements were supported by the Catalan project 2014-SGR-1661. The SEM-EDS and WDS analyses and BSE images were obtained with the assistance of Carlos Linares. Teresa Pi Puig is thanked for the XRD analyses. We thank Thomas Bissig, Bernd Lehmann, Teresa Ubide and Lisard Torró i Abat and for helpful comments on the manuscript. We also thank the anonymous reviewers for constructive comments., Peer Reviewed, Postprint (published version)

© 2020. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0, Manganese mineralization at Montaña de Manganeso, San Luis Potosí state, consists of oxide ores that form sharp contacts with volcanic host rocks. The orebodies are generally in the form of veins and irregular masses, and locally as mineralized breccias. Petrographic analyses indicate that the mineralization is multi-episodic, with colloform and crustiform textures predominating. The X-ray diffraction and electron microscopy show that manganese oxides (todorokite, birnessite, pyrolusite, romanechite and cryptomelane) are the main ore minerals, while iron oxides (goethite and hematite) are accessory. The most common gangue minerals are calcite and quartz with subordinate amounts of barite. According to fluid inclusion microthermometry, the mineralization is associated with aqueous solutions of intermediate salinity (8–16 wt% NaCl equivalent) and temperatures between 101 and 140 °C. Stable isotope analysis of calcite (d13CPDB: -7.76 to -6.32‰; d18OPDB: -8.01 to -4.71‰) and barite (d34S: 7.9–13.6‰) shows a contribution of magmatic volatiles to hydrothermal fluids dominated by meteoric water (calculated d18Ofluid: 6.58–13.14‰, relative to SMOW). Argillic alteration is the most widespread hydrothermal alteration at Montaña de Manganeso and indicates fluid temperature below ~150 °C and near neutral pH. Much more local, advanced argillic alteration, revealed by the occurrence of kaolinite, is interpreted as a steam-heated overprint, which in turn suggests a boiling process that must have happened at greater depths. The Mn ores formed through a process of mixing of: (a) boiled-off hydrothermal fluids, with (b) cold, diluted meteoric water. Our results support a hot-spring deposit model, according to which Montaña de Manganeso corresponds to the shallowest portion of an epithermal system formed in a continental volcanic arc setting., This work was supported by the PAPIIT (DGAPA, UNAM) project IG100116 and forms part of J.M’s PhD dissertation. Carlos Linares López is thanked for the assistance in WDS analyses. Edith Cienfuegos Alvarado and Francisco Javier Otero Trujano are thanked for the carbon and oxygen isotopic analyses. Colleagues Andrea Hernandez Cervantes, Irvin González Romo and Urenia Navarro are thanked for their help during fieldwork and preparation of graphics. Prof. Nicolas Johannes Beukes is greatly thanked for access to the Central Analytical Facility of the University of Johannesburg were SEM imagery was carried out. We would also like to thank the three anonymous reviewers whose comments greatly improved the manuscript., Peer Reviewed, Postprint (author's final draft)

Hot springs with associated Mn-Ag mineralization were identified in the intertidal zone of the Bahia Concepcion Bay of the central-eastern Baja California Peninsula. Hot spring activity is related to a system of NW-SE faults that are associated with the opening of the Gulf of California. The hot springs are hosted by Miocene volcanic and volcaniclastic units of the Comondu Group. The maximum discharge temperature recorded is 72 °C, with the pH ranging from 6.2 to 6.5 and salinity values reaching up to 27 g/kg. Veins, stockworks, breccias, crusts, and organosedimentary stromatolites of silica-carbonate and Mn oxides are associated with the hot springs. The Mn oxide and silica deposits show an association with Ag, As, Ba, Ca, Cu, Mg, Pb, Sb, Sr, Tl, V, W, and Zn. Acanthite and native silver occur along with pyrite in a mineral assemblage rich in opal-A, todorokite, romanechite, aragonite, high-Mg calcite, and barite. Three distinct mineral associations characterized by sulfides, sulfates, and oxides are recognized. The silver minerals formed late in the paragenesis. Textural evidence suggests a microbial mediation for the Mn mineral deposition. Pourbaix diagrams for Mn and Ag species were constructed for the physicochemical conditions of the coastal hot springs. These diagrams show that both Mn and Ag minerals are stable at low temperature (65 °C), and near neutral pH, under slightly reducing or oxidizing conditions. The characteristics of the subaerial to shallow submarine hot springs and the results of the geochemical modeling suggest that the hydrothermal system in the Bahia Conception Bay is formed through convection of meteoric waters, intermixed with some saline fluids or seawater.

The Tehuitzingo ultramafic body (Acatlán Complex, southern Mexico) is a strongly serpentinized harburgitic body that encloses chromitite bodies and blocks of eclogitic rocks. Hydrous retrograde metamorphism on chromitite bodies resulted in the formation of: i) partly altered chromite characterized by unaltered cores surrounded by Fe-rich and Al-depleted porous chromite containing chlorite; ii) porous chromite corresponding to a chromite that was entirely transformed to Fe-rich and Al-depleted porous chromite; and iii) zoned chromite formed by modified cores surrounded by Fe-rich non-porous chromite and magnetite rims. The content of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the cores of partly altered chromite preserves the magmatic fingerprint of back-arc-bearing chromitites, while the cores of zoned chromite are enriched in Zn, Co and Mn but depleted in Ga, Ti, Ni and Sc, and display a metamorphic signature. P-T pseudosections performed in the system CrMFASH allow us to constrain the temperature of formation of Fe-rich chromite by reaction of magmatic chromite with olivine between 584 and 449 °C; while the new thermodynamic calculations performed in the FMASH system shows that the Fe-rich non-porous chromite and magnetite rims in zoned chromites were formed at lower temperatures (i.e. 270–340 ºC and < 20 kbar). Temperatures of alteration obtained using our new thermodynamic model are in agreement with the qualitative estimation that use minor and trace elements in Fe-rich chromite (i.e. 560–700 ºC). These temperatures are more restricted than those estimated for chlorite hosted in partly altered (193–481 ºC) and porous chromite (158–255 ºC), and those reported in host serpentinite and associated eclogitic rocks (210–399 ºC). A comparison of P-T conditions estimated by our new thermodynamic models and the distribution of minor and trace elements in the different zones of chromites allow us to infer that the alteration took place during the hydrous

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The Acoculco geothermal system is hosted by a caldera complex located at the eastern portion of the Trans-Mexican Volcanic Belt. Surface manifestations are scarce and consist of low temperature, bubbling, acid–sulfate springs that are concentrated in two zones separated from each other by ~1750 m. In the northernmost one, there are conspicuous features suggesting recent, explosive, hydrothermal activity. Most of the rocks that crop out are tuffs and breccias that show pervasive hydrothermal alteration. Six SWIR-active minerals have been revealed by reflectance spectroscopy in the surface altered rocks: Opal, kaolinite, alunite, ammoniojarosite, buddingtonite and interstratified illite–smectite; they are indicative of alteration assemblages comparable to those reported in other geothermal systems and in epithermal deposits. Opal is the most widespread alteration mineral and occurs in association with tridimite and anatase. Kaolinite is also widely distributed but has a greater presence in the area with active surface manifestations, which, moreover, is the only area with alunite and ammoniojarosite occurrences. Buddingtonite occurs in the same zones as sulfates. As a complement to the alteration maps, we are proposing a simple method for mapping ammonium anomalies, based on total N analyses in altered rocks. Total N data show a discontinuous distribution within a wide range of variation (0.01–3.28 wt.%). All medium and high N values (>0.26 wt.%) plot in three clusters, two of which coincide with zones of gas emission. A deep circulating, active hydrothermal system cannot be ruled out in Acoculco although it may be confined to the deep caldera structure by an impermeable cap-rock of silicic and argillic alteration. In this scenario, hydrothermal fluids would reach the surface periodically, when explosive hydrothermal fracturing allowed it. The occurrence of hydrothermal explosion features and of acid–sulfate alteration supports this scenario. Therefore, further explor, Peer Reviewed, Postprint (published version)

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