STABLE ISOTOPE STUDY OF NEW PROSPECT VEINS OF THE FRESNILLO SILVER DISTRICT, ZACATECAS, MEXICO
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. δ34SH2S values from pyrite (-7.5 to -4.5 ‰) were slightly lighter than the δ34SH2S 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.