New Mexico Geological Society Annual Spring Meeting — Abstracts

The San Francisco Mine is located one mile SE of Estacion Llano, which is 70 miles S of Nogales, Sonora, Mexico.

Gneisses and schists of Middle to Late Jurassic age of the greenschist facies outcrop in the area. These metamorphic rocks, which are derived from shales and graywackes, are intruded by Tertiary dikes of basaltic andesite composition and calc alkaline affinity. Small discontinuous and massive Au-bearing quartz-tourmaline veins crosscut the metamorphic rocks. One somewhat larger vein contains galena and pyrite.

Vein quartz samples from two localities, the San Francisco Mine and EI Manto areas, were selected for fluid inclusion studies. Two types of inclusions were observed, based on phases present at room temperature: Type I, a liquid rich, two phase (liquid + vapor) fluid inclusion; Type II, a three phase CO₂-bearing fluid inclusion with a CO₂ vapor, liquid CO₂, liquid H₂O phases. Homogenization temperatures (Th) for fluid inclusions of the San Francisco Mine area show two peaks, at 230 and 280 degrees C, meanwhile Th on fluid inclusions of the EI Manto area show a peak at 230 degrees C. The salinity of the inclusions for both areas range approximately between 4 and 10 equivalent weight % NaCI.

Two mineralizing fluids (F1 and F2) were responsible for two stages of quartz mineralization. Stage I was deposited at the San Francisco Mine area by fluid F1 at a temperature between 250 and 360 degrees C. Stage II was deposited at the San Francisco Mine and EI Manto areas by fluid F2 at a lower temperature, 200 to 250 degrees C.

Oxygen isotopes of the quartz vein samples support the existence of the two fluids. Stage I of mineralization has a δO-18 (H₂O) = 4.8 0/00, meanwhile stage II has a lower value of 2.7 0/00. These isotopic values are too light to be considered unaltered metamorphic, magmatic or sedimentary fluids. However, they could be explained by meteoric water, which has undergone SUbstantial exchange with country rocks during hydrothermal circulation.