AGES OF EPITHERMAL DEPOSITS IN MEXICO: REGIONAL SIGNIFICANCE AND LINKS WITH THE EVOLUTION OF TERTIARY VOLCANISM

We present an analysis leading to a new space-time classification of the Mexican epithermal deposits based on new data, interpretations from seven deposits from central Mexico, and a new age for the La Guitarra deposit, Temascaltepec district. From the latter, adularia from a barren stage of mineralization and from quartz bands of the main ore stage, together with orthoclase phenocrysts from host monzogranites (with a known late Laramide age) were analyzed by the ⁴⁰Ar/³⁹Ar method. The ages obtained from adularia samples are 32.9 ± 0.1 Ma and 33.3 ± 0.1 Ma, corresponding to the middle part of Lower Oligocene. These ages are slightly younger than that of the ignimbrite succession exposed next to the study area, and suggest that the deposit is related to the crystallization of comagmatic intrusive rocks. Contrary to previous assumptions of a Miocene age for some epithermal deposits in southern Mexico, the ages from La Guitarra show that south of the Trans-Mexican Volcanic Belt these deposits may be Oligocene or older. Also, the interpretation of a genetic link between intrusive rocks and epithermal mineralization is consistent with previous work on the deposit. To evaluate the obtained ages in a regional context we analyzed the geographic distribution and age of Tertiary epithermal deposits in central Mexico. Our study shows that mineral deposits coincide with the main volcanic pulses of the Sierra Madre Occidental (SMO). The epithermal deposits can be thus divided into three main groups: (1) between ~48 and ~40 Ma, (2) between ~36 and ~27 Ma, and (3) between ~23 and ~18 Ma. Within the first group are the Batopilas and Topia deposits that record the oldest ages for epithermal deposits in Mexico. Deposits of this age are hosted by the SMO Lower Volcanic Supergroup and are related to the Laramide magmatism. Many of the epithermal deposits in Mexico occur within the second time span in a NW-SE belt from Chihuahua to the Mexico and Guerrero states. They are related to the main episode of ignimbrite flare-up of the SMO. The third group includes the Bolaños, San Martín de Bolaños, Pachuca-Real del Monte deposits plus, possibly, seven other epithermal deposits analyzed in this work located in Zacatecas, Sinaloa, Nayarit and Jalisco. These deposits are related to the last ignimbrite flare-up of the SMO with a general WNW-ESE distribution, recently documented in the southern SMO.