The Frisco mine is near the city of Parral, in southern Chihuahua, Mexico. It is worked by San Francisco Mines of Mexico, Ltd., and produces daily 1,500 metric tons of ore with a grade of 0.5 gram (0.015 ounce) of gold per metric ton, 150 grams (5 ounces) of silver, 5 percent of lead, 8 percent of zinc, and 0.6 percent of copper.The ore comes from about 32 fissure veins, which occupy faults of small displacement in a calcareous shale formation of probable Cretaceous age. The veins are arranged in a complex pattern whose description and explanation is the principal aim of the paper. From their attitudes and physical characteristics, the veins can be grouped into four sets of mutually parallel veins, each set parallel to a different imaginary plane. These four sets make up two systems of veins, each composed of two sets of conjugate shears. The two systems are believed to have resulted from two stages of deformation--distinct in orientation of stress but not necessarily separated by any large amount of time.The vein matter was probably introduced by a hydrothermal fluid during the second stage of deformation, when the fractures of the second system were formed and those of the first system were reopened. The walls and breccia fragments within the veins were silicified and silicated, with the formation of quartz, diopside, actinolite, epidote, and ilvaite. During the same general epoch, but somewhat later at any one place, the sulfide minerals sphalerite, galena, chalcopyrite, pyrite, and arsenopyrite were deposited--in large part as replacements of altered shale fragments. In most places, the last minerals deposited were quartz, fluorite, and calcite. The mineralogy is like that of skarn deposits and like that of the veins at nearby Santa Barbara, which Lindgren included in his hypothermal class.