• Author / Creator
    Zamora Vega, Osbaldo
  • Mineralization in the Zacatecas district is polymetallic (Ag, Zn, Pb, Cu, and Au) and occurs as skarn-type and epithermal veins formed in different metallogenetic stages. The oldest mineralization in the district is skarn-type, Cu-rich with lesser Zn-Pb-Ag, and is considered to be close in age to felsic dikes and plugs dated at ~51 Ma. Epithermal mineralization is present as both low- and intermediate-sulfidation deposits. Intermediate-sulfidation veins (the Veta Grande, Mala Noche, El Bote, and La Cantera veins) are polymetallic, Ag-rich, hosted in ESE- to SE-striking structures, and were formed at 42.36 ± 0.18 Ma (40Ar/39Ar adularia isochron age; MSWD = 0.76). The low-sulfidation system (El Orito) is Au-rich with minor silver, and lacks base metal sulfides; it is hosted by predominantly N–S-trending structures and was formed at 29.19 ± 0.20 Ma (40Ar/39Ar adularia isochron age; MSWD = 1.8), which indicates that these two styles of epithermal mineralization are temporally unrelated. The skarn-type and intermediate-sulfidation mineralization is broadly coeval with Eocene subduction-related magmatism in the Zacatecas area, which is constrained by zircon U-Pb ages for igneous rocks between 51–42 Ma. The emplacement of these magmas was controlled by the same regional-scale, ESE- to SE-trending, transtensional structures that controlled the skarn-type and intermediate-sulfidation deposits. This mineralization is thus interpreted to be related to the last stages of subduction-related volcanism in central Mexico, under neutral to mildly extensional stress conditions. In contrast, no nearby magmatism is clearly related to the Oligocene low-sulfidation system. However, its age and structural orientation (N–S), combined with a regional change in magma composition from Eocene calc-alkaline to Oligocene bimodal volcanism in central Mexico, suggest that the low-sulfidation mineralization is related to post-subduction continental extension processes, reflecting the beginning of Basin and Range tectonics.The mineral paragenesis of the Mala Noche deposit consists of early skarn-type Cu mineralization overprinted by later epithermal Pb-Zn-Ag veins. Skarn-type minerals include relicts of prograde silicate minerals (diopside, hedenbergite, and garnet), retrograde silicate minerals (ilvaite, grunerite, stilpnomelane, epidote, clinochlore), and ore minerals (chalcopyrite, pyrite, sphalerite, galena, magnetite, wolframite, and minor bismuthinite). Epithermal mineralization is characterized by layered to vuggy quartz veins and breccias, with variable amounts of base metal sulfides (sphalerite, galena, pyrite, minor chalcopyrite, and rare acanthite and stromeyerite) intergrown with quartz, calcite, dolomite, and ankerite. Fluid inclusion and stable isotope data, combined with mathematical modelling, indicate that ore formation, was caused by a magmatic fluid (~340°C, ~14 wt.% NaCl eq., 18O = +5 to +8 ‰), which boiled and mixed with isotopically exchanged, moderately saline ground water (~150°C, ~4 wt.% NaCl eq., 18O = +0.9 to +2.9).The Veta Grande mineralization was emplaced in two main stages of Ag-rich quartz veining. Stage I consist of pyrite, followed by sphalerite, galena, and lesser chalcopyrite, acanthite, pyrargyrite, and jamesonite, intergrown with quartz, calcite, and scarce adularia. Fluids from stage I (180°–260°C, 6–10 wt.% NaCl eq., 18O = +2 to +2.8‰) are interpreted to be isotopically exchanged formation waters that boiled during ascent and depressurization. Stage II mineral paragenesis is similar to stage I but contains less sulfides, shows supergene alteration, and is characterized by amethystine quartz. Stage II fluids are of bimodal composition (6¬–13 and <1 wt.% NaCl eq.) and show a wide range of homogenization temperatures (150°–280°C). Ore minerals are interpreted to have precipitated by boiling of the hot and saline, possibly magmatic fluid, end member (~320°C, ~13 wt.% NaCl eq., 18O = +4.6 to +10.1 ‰).The ore mineral paragenesis of the El Compas vein, which represents the El Orito System, consists of scarce pyrite followed by aguilarite, naumannite, electrum, and native gold with a gangue of quartz, adularia, calcite, and chalcedony. This mineralization was deposited in a single stage from a dominantly meteoric fluid boiling at a temperature of ~210°C, and with a salinity of <1 wt.% NaCl eq., and 18O = -1.4 and +3.1 ‰.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
  • Degree
    Doctor of Philosophy
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