Underground gold and silver mining operations are carried out at the different ore zones in the district. The operations employ underground cut-and-fill and long-hole stoping techniques, with LHD equipment feeding trucks to the mills. Primary access takes advantage of the topographic relief, with adits driven at river level to intersect the veins at depth.
The ore is transported from the mining areas to the processing plant in Tayoltita. The current operations employ approximately 1,100 people.
The Tayoltita area includes the oldest operating mine in the San Dimas area. The main access is a 4.4 kilometre tunnel from a portal approximately 400 metres northeast of the Tayoltita mill.
The mining method employs mechanized cut-and-fill mining on vein mineralization using waste rock as backfill. The veins vary from 1 to 3 metres in width and generally dip at 75-80 degrees. Production drilling is largely completed with jackleg drills depending on the vein thickness. Ore is hauled from the stoping areas using LHD equipment, then by rail haulage to surface through the main access tunnel. Tayoltita has the lowest overall operating costs and cut-off grade in the area mainly due to the efficient ore transport system.
The development of the San Luis tunnel to connect the San Antonio area to the Tayoltita area enabled the development of several additional veins of variable width dipping at 75 degrees. This area is currently being developed as an important new source of ore for Tayoltita.
Arana Hanging Wall Area
Main access to the Santa Rita mine is by an adit located three kilometres to the northeast of the Tayoltita mill site. The vein dip varies from sub vertical to as low as 35 degrees which allows for a room and pillar mining operation in some of the flatter lying areas. Ore is hauled by LHD equipment to either an internal shaft or directly to rail haulage on the main access tunnel. The shaft employs a double drum hoist with 2.2 tonne skips. San Dimas plans to connect the rail haulage to the Tayoltita tunnel, which will considerably reduce ore transport costs. Currently, ore is loaded at the portal for surface haulage to the Tayoltita mill.
Central Block Area
The Central Block area, currently the main mining area at San Dimas, is located 2 kilometres northwest of Tayoltita. It is linked to it by the San Luis Tunnel, which provides easy access to the area, as well as integration of support services.
Mining operations at Central Block vary according to the width of the veins, since they range from one to six metres. All are exploited by conventional cut and fill. Ore haulage is by a combination of LHD equipment and trucking.
Ore is to be transported through the San Francisco Tunnel, which was completed in the second half of 2009.
Sinaloa Graben Area
The Sinaloa Graben area, a recently discovered high-grade exploration area, is located 2 kilometres west of the Central Block. The tunnels that serve as the main haulage levels for the Sinaloa Graben and Center Block areas were completed during the first quarter of 2010.
Exploration results to date indicate that the Sinaloa Graben could contain higher grade mineralization than the adjacent Central Block and with larger widths. Primero is focused on increasing the known mineralization in the Sinaloa Graben, which represents the future of the San Dimas mine.
Milling operations are carried out at Tayoltita. The Tayoltita mill processes ore from throughout the district and has a current throughput capacity of 2,500 tonnes per day. The mill facility is a conventional operation that employs cyanidation and zinc precipitation for recovery of gold and silver.
The Tayoltita mill has undergone a series of plant expansions over its operating life. In December 2005, crushing capacity was increased to 2,100 tonnes per day, as well as improvements to the chemical treatment / leaching area. When Primero acquired the San Dimas Mine in 2010, a thorough optimization and expansion program began at the mine. This led to a two phase expansion plan for the mill and processing plant. In the first quarter of 2014 phase 1 was completed brining the throughput of the mill to 2,500 tonnes per day. On August 7th 2014 Primero announced its decision to construct phase 2 of the expansion plan which will bring throughput to 3,000 tonnes per day. The expansion is expected to be completed by the second quarter of 2016 and will increase production to approximately 215,000 gold equivalent ounces.
The Tayoltita mill employs fine crushing and single stage ball milling to achieve a fine grind before passing into the leach circuit. Leaching is completed in a series of tanks providing 72 hours of leach residence time. The pregnant solution is recovered in a CCD circuit with the gold and silver recovered from solution in a zinc precipitation circuit.
Tailing is pumped by a newly installed single stage pumping station to the tailings impoundment area in a box canyon east of the mill site. The gold and silver precipitate is refined using an induction furnace to produce 1,000 ounce gold and silver dore bars.