SPATIAL VARIATION OF PHYSIO–CHEMICAL CONDITIONS OF THE HYDROTHERMAL FLUIDS IN FRESNILLO BASED ON FLUID INCLUSION STUDIES

We are reporting fluid inclusion data from two recently discovered Ag - Au veins in west Fresnillo district, Mexico. Jarillas and Valdecañas-East veins occur about 6 km and 8 km west of the Fresnillo mine, respectively, and share many mineralogical and textural similarities with other well-studied veins in the district such as Santo Niño, San Carlos and San Mateo. They are low sulfidation epithermal veins with textures typical of fissure filling and their mineralogy consists mainly of pyrite, arsenian pyrite, arsenopyrite, sphalerite, galena, pyrargyrite, acanthite, electrum, quartz, calcite, clay minerals, epidote and adularia.

Measurements from mostly primary liquid-rich inclusions at Jarillas (n=270) yields Th= 190°C to 295°C and 1 to 5 wt% NaCl eq. for calcite hosted inclusions and Th= 210°C to 260°C and 0.5 to 5 wt% NaCl eq. for quartz hosted inclusions. The minimum calculated pressures range from 5 to 70 bars with mode at 25 bars which corresponds to a paleo-depth of approximately 300 meters. For Valdecañas-East vein, west of Jarillas, we have measured primary liquid-rich inclusions hosted mainly in quartz (n=129) and have determined Th= 160°C to 310°C and 0.5 to 11 wt% NaCl eq.

Our new data from Jarillas and Valdecañas-East when compared with data from Santo Niño (Simmons 1986) shows a trend of progressive increase of average Th°C and salinity from Santo Niño Vein in the east, to Jarillas and then Valdecañas-East vein to the west. This trend in physio-chemical conditions of the hydrothermal fluids as a function of space will be used in conjunction with other geochemical tools to examine the genetic relations amongst veins and the district-wide fluid source and fluid flow.