Multiple mineralization events in the Zacatecas Ag-Pb-Zn-Cu-Au district, and their relationship to the tectonomagmatic evolution of the Mesa Central, Mexico

Abstract

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 occurs in both low- and intermediate-sulfidation styles. 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 Ma (adularia 40Ar/39Ar isochron age from Veta Grande of 42.36 ± 0.18 Ma; 2σ, MSWD = 0.76). Low-sulfidation Au-(Ag) mineralization occurs in the N–S-trending El Orito vein system, which yielded an adularia 40Ar/39Ar isochron age of 29.19 ± 0.20 Ma (2σ, MSWD = 1.8). These ages and the differences in structural orientation indicate that the two styles of epithermal mineralization are temporally and tectonically unrelated.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 (hedenbergite and garnets), 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 phyllic wallrock alteration (quartz, sericite-illite). The veins consist of quartz, calcite, dolomite, and ankerite with variable amounts of base metal sulfides (sphalerite, galena, pyrite, minor chalcopyrite, and rare acanthite and stromeyerite).The Veta Grande epithermal mineralization was emplaced in two main stages of Ag-rich quartz veining, with narrow selvedges of phyllic (quartz-sericite) wallrock alteration. Stage I consist of quartz, calcite, and minor adularia intergrown with pyrite, followed by sphalerite, galena, and lesser chalcopyrite, acanthite, pyrargyrite, and jamesonite. Stage II mineral paragenesis is similar to stage I but is characterized by amethystine quartz and contains less abundant sulfide minerals.The ore mineral paragenesis of the El Compas vein, within the El Orito System, consists of quartz, adularia, calcite, and chalcedony with minor pyrite, followed by rare aguilarite, naumannite, electrum, and native gold.The skarn-type and intermediate-sulfidation mineralization is coeval with Eocene subduction-related magmatism in the Zacatecas area, which is constrained by zircon U-Pb ages for igneous rocks between 51 and 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 volcanism in central Mexico, under transtensional 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.