Precious metal production from veins of the Tayoltita mine totals more than 10 7 kg of silver and 2 X 10 5 kg of gold. The deposit, the largest in the San Dimas mining district, occurs in the central part of the Sierra Madre Occidental volcanic pile which contains rocks reflecting two separate periods of magmatism. K-Ar dating by McDowell and Keizer (1977) demonstrates that these were active between 100 and 45 m.y., and 32 and 23 m.y. ago. Encasing the veins are rhyolitic, andesitic, and granodioritic rocks of the older magmatic episode. Ignimbritic rocks of the younger period of magmatism postdate the mineralizing event, which occurred about 40 m.y. ago. With only one exception, the N 15 degrees W Arana fault vein, economic mineralization is restricted to fractures striking east-west to north-northeast. Regional normal faults trending north-northwest cut the rocks of both magmatic episodes and divide the district into tilted blocks dipping east.Supergene processes played an insignificant role in the formation of ore. Hypogene open-space fillings can be divided into three paragenetic stages: (1) quartz with or without abundant sphalerite, and galena with or without minor silver-gold values; (2) quartz with or without three mutually exclusive gangue suites all occurring with ore-grade (+200 ppm Ag and +2 ppm Au) precious metal values, but each with or without dark green hydrothermal chlorite and copper-lead-zinc sulfides: quartz-rhodonite/johannsenite-albite, quartz-adularia, and quartz-calcite; (3) white to amethystine vuggy quartz. Small amounts of pearceite, native silver, chalcopyrite, calcite, and zeolites were deposited in stage 3 vugs during the waning stage of the hydrothermal system.Silver and gold were deposited within a horizontal favorable zone minimally estimated to have been 600 m thick which essentially paralleled the untilted volcanic pile. The distributions of both metals are zoned vein by vein with respect to two small granodiorite stocks with Ag/Au varying from 44 to 105 with the lowest Ag/Au closest to the intrusions. Filling temperatures for quartz of stage 2 (250 degrees to 290 degrees C) and stage 3 (250 degrees to 310 degrees C) show little time-space variation of temperature during the mineralizing event. Indications of boiling in stage 3 quartz are found over a broad three-dimensional range. Salinities of the inclusion fluids range from 4.1 to 9.7 equivalent weight percent NaCl in stage 1, 3.3 to 8.4 weight percent in stage 2, and 1.9 to 9.7 weight percent in stage 3. Fluid inclusion data and field relationships indicate an approximate 400-m depth below the surface for the top of the favorable zone at the time of vein formation.Gas analyses of thermally decrepitated fluid inclusions indicate that water constituted over 99.5 mole percent of the liquid and gas phases with CO 2 comprising most of the remainder. The delta 18 O values of stages 2 and 3 vein quartz range from 3.9 to 4.8 per mil, suggesting that meteoric water dominated the hydrothermal system.