New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico

Abstract

The San Dimas district is a world-class silver-gold low-sulfidation epithermal deposit located in the central part of the Sierra Madre Occidental of Mexico, within the eastern part of the Gulf of California extensional province. Previous works assumed a single period of mineralization between ∼38.5 and 31.9 Ma, which is at odds with the existence of vein systems with two different orientations and Ag/Au ratios. We present a re-evaluation of this district based on new zircon U/Pb and apatite fission-track ages as well as petrographic and field observations of mineralization styles. Our study also includes two new prospective areas of Causita and Mala Noche located to the south of the main district.Within the Lower Volcanic Complex, we identify a Late Cretaceous volcanic succession (∼77 to 69 Ma) correlative with the Tarahumara formation from southern Sonora and coeval with the San Ignacio batholith exposed to the west. This volcanic succession hosts Au-rich mineralized sub-volcanic felsic bodies yielding slightly younger ages and is covered by Paleocene intermediate lavas with hypabyssal intrusions with ages around 48 Ma. A voluminous intrusive suite (Piaxtla batholith and associated dike swarms) was emplaced in the region between ∼49 and 44 Ma. Early extensional basins were filled by a continental sedimentary sequence (Palmas formation), which yielded U/Pb age peaks at 66 and 56 Ma from detrital zircons and a maximum depositional age of ∼43 Ma. The last magmatic activity, as in the rest of the Sierra Madre Occidental, consists of silicic ignimbrites and less basaltic lava flows clustered in two pulses of ∼31.5–29 Ma and 24–20 Ma.NNW–SSE extensional fault systems expose the mineralization and tilted all the succession prior to the emplacement of a ∼24 Ma ignimbrite package. This late Oligocene extension, associated to the early stage of the Gulf of California rift, is confirmed by apatite fission-track dating of samples from the Piaxtla batholith, which consistently indicate an episode of cooling at 25–23 Ma followed by a second episode at ∼12.5 Ma. Our absolute ages and geologic mapping allow to infer that an older, WSW–ENE trending normal fault system with up to 1 km of displacement must exist between the San Dimas district and the Causita area to the south. This fault system, currently buried beneath Oligocene–Miocene ignimbrites, may have controlled the intrusion of the Piaxtla batholith and played a crucial role in the preservation of large vein systems of San Dimas in a tectonic depression setting.The main epithermal mineralization is associated with two kinds of structures: Ag/Au veins with WSW–ENE to E–W orientation and Au/Ag veins with NNW–SSE to N–S orientation. The dominant Ag/Au veins slightly post-date the Piaxtla intrusive suite and partly recycled older felsic intrusion with porphyry mineralization. The NNW–SSE to N–S Au/Ag veins are most probably associated with the Oligocene silicic volcanism.