The La Colorada mine, Sonora, Mexico: An Oligo–Miocene Au–Ag epithermal system overprinting a Late Cretaceous porphyry Cu–Mo deposit

The La Colorada mine is characterized by a low–sulfidation epithermal Au–Ag vein system hosted within Paleozoic siliciclastic rocks and a Late Cretaceous (~75–69 Ma) volcano–plutonic complex. While this Au–Ag mineralization has been intermittently mined since 1740, the exact timing of its emplacement has remained unconstrained. Existing Ar–Ar geochronology on alteration minerals indicates an emplacement age ca. 23 Ma. However, we provide evidence for a previously undocumented, poorly preserved porphyry Cu–Mo system centered on a 72.4 Ma rhyolitic porphyry. Re–Os molybdenite dating from sulfide veinlets yielded a consistent age of 72.3 Ma. Consequently, this study redefines the La Colorada deposit as an Oligo–Miocene epithermal Au–Ag system overprinting a Late Cretaceous Cu–Mo porphyry system. Fluid inclusion petrography reveals a distinct evolution: the porphyry stage is characterized by high–salinity, L+V+S inclusions in quartz veinlets, indicating boiling of magmatic–hydrothermal fluids. In contrast, the epithermal event is dominated by liquid–rich, two–phase inclusions with lower salinities, consistent with a cooler, more diluted environment. Sulfur isotope signatures, indicated as δ³⁴S values, provide further constraints on the ore–forming fluids. Molybdenite from the porphyry stage yielded a δ³⁴S of +6.55 ‰, suggesting a predominantly magmatic sulfur source with moderate heavy–sulfur enrichment. In contrast, sulfides from the epithermal overprint exhibit δ³⁴S values ranging from ~0 to –3.02 ‰, consistent with a typical magmatic–hydrothermal signature and minimal interaction with external sulfur reservoirs.