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Age, paleotectonic setting, and common Pb isotope signature of the San Nicolás volcanogenic massive sulfide deposit, southeastern Zacatecas State, central Mexico Danielson, Thomas J.


The San Nicolas deposit is a world class polymetallic volcanogenic massive sulfide (VMS) deposit located in southeastern Zacatecas State, central Mexico. The location of the deposit in a metallogenic province previously thought to be barren with respect to VMS mineralization, coupled with the large size of the deposit, make understanding the framework within which the deposit formed important for further mineral exploration efforts in the region, as well as for providing a more complete understanding of the geology of the region. A geological, geochronological, lithogeochemical, and isotopic study was undertaken to address the age of the volcano-sedimentary succession which hosts the San Nicolas deposit and the paleotectonic setting in which it formed. U-Pb zircon dating of felsic volcanic units within the volcano-sedimentary succession indicates that the volcanic rocks, and by extension, the associated sulfide mineralization, formed in Late Jurassic time, at -148 Ma. The succession comprises Upper Jurassic submarine, mafic and felsic flows, volcaniclastic rocks, and hypabyssal intrusions. A limited number of thinly bedded, finegrained clastic sedimentary rocks are locally intercalated with the volcanic units. Volcanic rocks within the lower part of the succession (stratigraphically below and interfingering with sulfide mineralization) display geochemical and isotopic characteristics which are consistent with formation within a back-arc basin. Geochemical and isotopic characteristics of volcanic rocks within the upper part of the succession (stratigraphically above VMS mineralization) reflect a change in paleotectonic setting to a relatively juvenile volcanic arc environment. Isotopic and geochronologic constraints suggest that the entire package was formed above attenuated continental crust, perhaps as old as Precambrian in age. In a separate, but related study, sulfide Pb isotope ratios were examined as a means of distinguishing between styles of mineralization within central and southwestern Mexico. Pb isotopic compositions from syngenetic and epigenetic mineralization in the region form two distinct fields, suggesting that Pb isotopic compositions can be used as an inexpensive and effective tool for discriminating between syngenetic and epigenetic mineralization. The consistency of Pb isotopic compositions of sulfide minerals from syngenetic and epigenetic deposits may also indicate a similar basement for all of central and southwestern Mexico.

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