Insights Into the Magmatic and Metallogenic Evolution of North-Central Mexico from Geochemistry and Geochronology of the Fresnillo, San Julian, and Orisyvo Precious-Metal Districts

Abstract

Fresnillo is one of the largest silver districts of the world, Orisyvo is the largest single gold ore body of the high-sulphidation epithermal class discovered in Mexico, and San Julian is an emerging Ag-Au district displaying two different overlapping ore types. Detailed geochemical and geochronological data from San Julian and Orisyvo is provided for the first time.Fifteen new ages from U-Pb geochronology in zircons reported from Fresnillo provide details over three felsic magmatic pulses from 47.6 to 30.8 Ma and one mafic in the Miocene. Two new 11.8 and 18.6 Ma 40Ar-39Ar adularia ages are reported from two veins emplaced in NE-trending faults associated with Basin and Range extension, which crosscut the NW-oriented Jarillas-Valdecañas vein. Nineteen new ages from U-Pb geochronology in zircons from San Julian document 4 magmatic pulses, the oldest associated with the late Cretaceous to early Paleocene arc, followed by three 43.8 to 30.5 Ma felsic flow-dome complexes and associated volcanic products. Disseminated silver mineralization occurs at the onset of the Eocene pulse at ~42 Ma, and low-sulphidation epithermal veins were emplaced near the end of the Oligocene pulse, with 40Ar-39Ar adularia ages adularia from 33-34 Ma and 31.5 Ma. Nine new ages from U-Pb geochronology in zircons document three magmatic pulses, one at 69.8 Ma, a second fertile dacite porphyry pulse at 46.0 to 47.3 Ma, and a late 34.3 to 27.7 Ma post-mineral pulse. 187Re-187Os geochronology in molybdenite gave a mineralization age of 47.9 Ma, within the uncertainty range of the 47.3 Ma U-Pb age of its host rock. Two 40Ar-39Ar geochronology attempts were made to date alunite from the Orisyvo lithocap but failed due to Ar loss, providing a minimum age of 44.2 Ma. Whole rock Nd and Hf in zircon isotopic data were used to document the evolution of crust-mantle input to the magmas at the three study areas, which included mantellic incursions associated with mineralization at Orisyvo and at the San Julian late-stage veins. The evolution of the regional tectonic regime over time and its influence in the associated structural setting at upper crustal levels are discussed, especially in relation to their implications in the formation of ore deposits at the district scale and to the metallogenic evolution at the regional scale. Emphasis is placed on the possibilities for multiple mineralization events, which are favorable in Central Mesa due to its complex metallogenetic history.