METALLOGENESIS OF THE EOCENE-OLIGOCENE SANTA MARIA AND ANTARES ZN-SKARN DEPOSITS, VELARDEÑA MINING DISTRICT, DURANGO, MEXICO

The Velardeña Mining District lies on the NE portion of Durango state, within Sector Transversal de Parras (STP), close to its contact with Sierra Madre Occidental and Mesa Central. The Santa María (SMB) and Antares (AMB) mineralized bodies are strategic deposits at the district and develop contiguously along the boundary between felsic stocks/dykes (Oligocene) and limestones (Cenomanian). In this investigation, both bodies were analyzed through petrography, mineral chemistry, fluid inclusions, and isotopic studies. The results suggest that they constitute a skarn system formed in three stages: prograde, retrograde, and veins. The prograde stage is represented by anhydrous skarns (garnet-vesuvianite-wollastonite) and biotite alteration, especially at AMB. In contrast, the retrograde stage formed mid-temperature minerals such as amphibole, chlorite, adularia, fluorite, and quartz, and grew coevally with the ores, made up by sphalerite, pyrite, arsenopyrite, pyrrhotite, molybdenite, chalcopyrite, galena, and sulfosalts. Calcite veins crosscut other events, carrying scarce arsenopyrite, pyrite, and fahlores.

Geothermometers revealed temperatures of 300-490 °C (ore stage) and ~370 °C (vein stage) concordant with homogenization temperatures from fluid inclusions of 325-450 °C. Microthermometry from SMB suggest a chemical system composed of H₂O-CaCl₂-NaCl-CH₄ for the fluids, with total salinities (NaCl+CaCl₂) of 15-34 wt.%. δ¹⁸O_VSMOW and δ¹³C_VPDB values show mixed sources between magmatic and marine carbonates, with a stronger magmatic affinity at AMB. Sulfur isotopes occupy a narrow range close to zero, typical of igneous sources.

The data indicate that mineralizing fluids were reequilibrated aliquots of a magmatic fluid that cooled and evolved into a neutral-alkaline and reduced system, triggering ore deposition. Dominant magmatic signatures at AMB and higher influence of the host at SMB indicate that both deposits formed within a single system, in which AMB is the proximal skarn and SMB the distal counterpart. Ore stage adularia yielded ⁴⁰Ar/³⁹Ar plateau ages at 37.5 ± 0.1 Ma and 37.6 ± 0.1 Ma, that agree with reported ages for intrusions at the district, and for other bodies along the STP. These ages place the deposits within the Eocene-early Miocene metallogenetic epoch in central-NW Mexico.