Dolores is located in the Sierra Madre Occidental volcanic belt, which comprises calc-alkaline batholiths and volcano sedimentary rocks of the Lower Volcanic Series and ignimbrites of the Upper Volcanic Series.

The San Francisco fault and its footwall host most of the mineralization at Dolores. The immediate footwall and hanging wall of the fault forms a 500 metre wide northwest-striking corridor of igneous intrusions.

Low sulphidation epithermal silver and gold mineralization is hosted in north-northwest trending hydrothermal breccias and sheeted vein zones in the order of 5 metres to 10 metres wide. Most high grade mineralization occurs along three major structures. Silver and gold mineralization identified on the surface lies over an area 4,000 metres long and up to 1,000 metres wide.

The highest grade mineralization occurs within the San Francisco Breccia, a well-defined and continuous hydrothermal breccia and stockwork zone that occurs in the immediate footwall of the San Francisco fault. The breccia trends further away from the fault towards the north until it joins a second major breccia zone known as the Alma Maria Breccia.

Hydrothermal breccias carry the highest silver and gold grades and pass outward into vein stock works. The veins are thin, rarely over 30 mm, and tend to occur as sheeted swarms. Economically mineable grades occur where the veins are sufficiently closely spaced.

Mining at Dolores is by standard open pit methods using shovels, loaders, and haul trucks and by underground methods using sub level open stoping and ramp haulage to the surface.

High grade ore will be stockpiled on the run of mine pad, separated from the medium grade ore. The high grade feed will be tipped into a bin using front end loaders at a rate of 5,600 tpd. The feed will be recovered from the bin by a vibrating grizzly feeder and delivered to a primary jaw crusher (Metso C-160), and the discharge conveyed to a vibrating screen. Screen oversize will pass through a cone crusher (Metso HP-800) in a closed loop to produce a rod mill feed with a P80 of 12.8 mm. The screen undersize will report to a conveyor and radial stacker for depositing onto a fine feed stockpile. The material will be recovered from this stockpile via multiple reclaim feeders and a reclaim tunnel conveyor system to feed the rod mill (Nordberg 13’ by 18’8”).

The rod mill discharge will be pumped in closed circuit to a Vertimill/vibrating wet screen circuit. The screen oversize will report back to the Vertimill and the screen undersize will be pumped to a surge tank for feed to the pressure filters. This closed loop circuit is estimated to produce a P80 of 0.425 mm (35 mesh) product.

Heap leach flow sheet
Broken ore is trucked to the crushing plant and crushed to a particle size of P80 6.3 mm in a three stage crushing circuit with a recirculating load at a nominal rate of 16,200 tpd. The crushed ore is sampled at regular intervals by a sample tower, conveyed to the leach pads via an overland conveyor system, and placed on the pads using portable grasshopper conveyors and a radial stacking system. Sodium cyanide solution is prepared in the Merrill-Crowe plant, pumped to the leach pads, and applied using drip and sprayer systems.

Metal recoveries are a function of solution flow rates, cyanide concentration, and time, and the metal leaching period can cover years, continuing as subsequent lifts are placed on the pads. Gravity collectors and/or vertical perforated steel pipes (well risers) fitted with internal pumps transfer the pregnant leach solution containing the dissolved silver and gold from the bottom of the leach piles to the pregnant solution ........