Geochronology of Mexican mineral deposits. I: the San Martín polymetallic skarn, Zacatecas

Resumen

The Zn-Pb-Cu(-Ag±Au)
San Martín deposit in northwestern Zacatecas is one of the most
economically important and biggest skarns in Mexico. Mineral
associations in this deposit belong to the sulfide skarn type (with
rather “classical” prograde and retrograde zones) and contain
peripheral subepithermal to epithermal veins. Re-Os ages were
obtained for two molybdenite samples from deep Cu-Zn-Pb±Mo±Bi±Ag
retrograde mineralization in these deposits, of 43.7 ± 0.3 and 44.0
± 0.2 Ma, which correspond essentially to the same age. These are 1
to 3 Myr younger than preexisting K-Ar ages for the granitic stock
that generated these mineral deposits. Prograde mineral associations
are hereby interpreted to have been directly associated with this
intrusion. Therefore, we may calculate a simple decrease rate in
temperatures of hydrothermal fluids between ~100º and > 300 ºC per
million years from prograde to retrograde skarn associations.
Subepithermal and epithermal veins, however, remain
undated.