STABLE ISOTOPE STUDY OF NEW PROSPECT VEINS OF THE FRESNILLO SILVER DISTRICT, ZACATECAS, MEXICO

The Fresnillo Mining District in Zacatecas, Mexico, is a large, low-sulfidation epithermal-style deposit consisting of high-grade silver and significant lead and zinc. Over the past six years, several extensive veins have been discovered approximately 8 kilometers west of the “traditional” Fresnillo deposits, further enlarging this already massive silver district. To better understand the formation and genesis of the new veins, and their relation to previously known mineralization, reflected light petrography, sulfur stable isotopes, and fluid inclusion analysis was performed on 41 samples from 19 different vein intercepts collected from 5 of the new prospect veins.

Mineralogy within the veins is dominated by sphalerite, galena, pyrite, and arsenopyrite, with most silver present within silver sulfosalts. δ³⁴S values of the sulfides are relatively consistent, with average values of -4.6 ±1.4 ‰ for pyrite separates (n=25), -4.6 ± 0.8 ‰ for sphalerite (n=27), and -6.8 ± 0.9 ‰ for galena (n=16). Fluid inclusion microthermometry from 4 different drill hole intercepts of the Valdecañas-East vein provides homogenization temperatures of ore-stage quartz ranging from 230 to 280°C, and fluid salinities from 3 to 10 wt. % NaCl equivalent.

Using fluid inclusion temperatures of homogenization and mineral δ³⁴S, the δ³⁴S of H₂S of the ore forming fluid was determined. δ³⁴S_H2S values from pyrite (-7.5 to -4.5 ‰) were slightly lighter than the δ³⁴S_H2S from sphalerite (-6 to -4 ‰) and galena (-6 to -3.5 ‰).

The consistent mineral δ³⁴S values between the veins suggest that the veins were formed from a common source of sulfur present throughout the entire western vein area, and could be related to a single ore-forming event. However, the values are consistently lighter than what exists in the majority of other epithermal systems, and could represent an incorporation of different sources of sulfur. Unlike magmatic systems, which have H₂S sulfur values typically near 0 ‰, the Fresnillo veins are much lighter. The incorporation of sedimentary sulfide, which can contain light δ³⁴S values, could explain the anomalous values seen in the veins.