THE RELATION BETWEEN WIDESPREAD O-18 DEPLETION PATTERNS AND PRECIOUS-METAL MINERALIZATION IN THE TAYOLTITA MINE, DURANGO, MEXICO

The delta(18)O values of the wall rocks in the vicinity of the Tayolti ta mine were shifted to significantly lower values as a result of inte raction with geothermal fluids of predominantly meteoric origin. The a reas of strongest O-18 depletion (delta(18)O(rock) < 4 parts per thous and) form a north-northwest-south-southeast trend coinciding with the areas of Au-Ag mineralization in the vein system. At a detailed level, however, there is only minor isotopic evidence for interaction betwee n the vein fluids and the wall rocks. This finding is confirmed by the oxygen isotope compositions of mineral separates from the rocks and q uartz and calcite from the veins. Chlorite, epidote, and feldspar in t he wall rock have delta(18)O values too high to have formed in equilib rium with the same fluids that formed the vein minerals. Further, exce pt for chlorite close to major vein structures (less than or equal to 2 m), the delta(18)O values of neither chlorite nor epidote were signi ficantly shifted by interaction with the later fluids. The delta(18)O values of feldspar do appear to have partially equilibrated with the l ater fluids. By estimating the delta(18)O values of the evolving fluid s from analyses of the other minerals, we have calculated that the deg ree of oxygen isotope shift of the feldspars is consistent with a peri od of fluid circulation lasting 10,000 to 100,000 years at a Dare). fl ow velocity of 10(-9) to 10(-11) m/s. This indicates that although per vasive fluid flow through the rocks may have occurred during the forma tion of the veins, it was extremely limited and had little effect oil the regional delta(18)O(rock) patterns. The coincidence of the veins w ith the areas of strong O-18 depletion of the rocks must, therefore, b e an indication that there was a single, long-lived episode of hydroth ermal activity that remained focused in the same vicinity throughout t he life of the system.