The San Martín Mining District lies in the Sombrerete municipality, western Zacatecas State (Fig. 1). Geographic coordinates of the Industrial Minera México, S. A., San Martin Unit are 23°40′N and 103°45′W. A paved road leading east from the installations for 6 km joins Federal Highway 45 (Panamericana) at the Zacatecas-Durango leg; the town of Sombrerete with its railroad station is about 10 km south of the junction. There is a small-aircraft landing field 15 km west of the mine. The area of the installations is at an elevation of 2,600 m; climate is temperate-humid with relatively severe winters; temperatures fluctuate between a low of −6 °C in winter and a high of 30 °C in summer.