Mineralization is hosted in two veins with a general strike of N60W, dipping between 45 to 60 degrees southwest. The host rock of the veins is comprised of the Zacatecas formation, a mid to late Triassic turbidite sequence, consisting of beds of dark grey, green and grayish-green shale, sandstone, conglomerate and siltstone. The veins extend over a length of 2.5 to 3.0 kilometers, and to a depth of at least 150 to 200 meters, as determined from available drilling data (Rodriguez et al., 1987). Each of the Rosario I and Rosario II veins vary in width from 0.4 to 2.5 meters. Mineralization appears to have occurred in three stages, with the third stage precipitating the silver bearing minerals. This deposit has been described as a mesothermal vein system based on temperature (195 to 338°C), although mineralization is typical of other deposits in Mexico which are described as epithermal. Abundant sulfides, sphalerite pyrite and galena with subordinate amounts of chalcopyrite, arsenopyrite and hematite, can be found within the veins. Additional minerals identified through electron microscopy include freibergite, tetrahedrite, boulangerite, pyrargyrite, argentite, stromeyerite, and diaphorite (Rodriguez et al., 1987).

The cutoff for resource reporting is based on underground vein mining with long hole or shrinkage stoping with milling and flotation.