San Dimas

The San Dimas mining complex is located in western Durango at the border with Sinaloa and has five underground zones: San Antonio West, Sinaloa Graben, Central Bolck, Tayoltita and Arana Hanging Wall. The San Dimas deposit occurs in the central part of the Sierra Madre Occidental volcanic pile which contains rocks reflecting two separate periods of magmatism, which were active between 45-100 million years ago and 32-23 million years ago respectively. Three major stages of veining have been identified in the area, all of them containing variable amounts of mineralization.

San Dimas has been mined intermittently since 1757 but modern mining began in 1883 by San Luis Mining which operated the mine until it was acquired by Primero Mining in 2010. Underground gold and silver mining operations are now carried out at all the different ore zones in the district. The mine is operated through underground cut-and-fill and long-hole stoping methods. The main access to the mine follows the topographic relief and allows Primero Mining to intersect the vein at depth through adits driven at river level. The mine is operated through mechanized cut-and-fill on the veins where waste rock is used as backfill, with the width of mineralized veins ranging from 1-3 meters and most of them dipping at 75- 80 degrees. Most of the production drilling is performed with jackleg drills, although this varies depending on the thickness of the vein. The ore is hauled out of the mine using LHD equipment before being conveyed by rail to the surface using the main access tunnel. This well-structured ore transport system has been carefully crafted by Primero Mining to allow San Dimas to have one of the lowest overall operating costs and cut-off grades in the area. The mill processes ore from throughout the district and has a current throughput capacity of 2,500 t/d and employs cyanidation and zinc precipitation for recovery of gold and silver. The mill has undergone a series of plant expansions over its operating life, and employs fine crushing and single stage ball milling to achieve a fine grind before passing into the leach circuit. In December 2005, crushing capacity was increased to 2,100 t/d, but in the first quarter of 2014, phase 1 of a new expansion was completed, bringing the mill’s throughput to 2,500 t/d. This was followed by Primero Mining announcing in August 2014 that phase 2 was starting with the target of raising throughput to 3,000 t/d by 2016, by which point annual production should have increased to approximately 215,000 ounces of gold equivalent.