This district, one of the most prolific silver producers in the world, is located in central Hidalgo State, about 100 km northnortheast of Mexico City (Fig. 1), and has remained in production almost continuously since its discovery in 1522. Almost 25 million ounces of silver and 134,000 oz of gold were produced from 1973 to 1981 from more than 2,000 m of underground mineworks. Past and present diamond-drillholes total more than 100 km. Accumulated production from discovery to 1982 is estimated at 1,311,418,000 oz Ag and 6,643,000 oz Au. The geology and development of the Pachuca District, set forth in detail by Fries and Geyne (1963), are briefly summarized below.