The Shafter silver deposit is considered an example of a polymetallic replacement deposit. Because of their irregular, but sharp contact with the enclosing carbonate host rocks, deposits of this type have been categorized as high-temperature, carbonate-hosted deposits.

Polymetallic deposits consist of massive lenses and (or) pipes, known as mantos or replacement orebodies, and veins of iron, lead, zinc, and copper sulfide minerals that are hosted by and replace limestone, dolomite, or other sedimentary rocks; most massive ore contains more than 50 percent sulfide minerals. Sediment-hosted ore commonly is intimately associated with igneous intrusions in the sedimentary rocks. Emplacement of these intrusions triggered ore formation and they host polymetallic veins and disseminations that contain iron, lead, zinc, and copper sulfide minerals. Some polymetallic replacement deposits are associated with skarn deposits in which host carbonate rocks are replaced by calc-silicate±iron oxide mineral assemblages. Most polymetallic vein and replacement deposits are zoned such that copper-gold ore is proximal to intrusions, whereas lead-zinc-silver ore is laterally and vertically distal to intrusions.

The Shafter deposit is hosted within the gently dipping beds of the Permian Mina Grande Formation, just below their contact with Cretaceous rocks. The reef-derived dolomite and limestone of the Mina Grande Formation were susceptible to differential weathering and karst activity at the upper level of the formation, and passageways for mineralizing solutions formed along facies contacts and bedding planes.

The mineralized material consists of a massive aggregate of medium-grained, vuggy silica stained with varying amounts of iron and manganese oxides. Mineralogy is fairly consistent within the district. The mineralization originally consisted of sulfide minerals, which are now almost thoroughly oxidized. Secondary minerals include iron and manganese oxides, acanthite, hemimorphite, descloizite, embolite, plumbojarosite, cerargyrite, native silver, cerussite, anglesite, and small amounts of covellite, chrysocolla, and possibly other copper minerals. Primary minerals include dolomite, calcite, quartz, pyrite, sphalerite, galena, argentite, chalcopyrite, covellite, molybdenite, and tetrahedrite. Silver occurs predominately as oxidized acanthite in fine-grained aggregates of quartz, calcite, and goethite, with lesser dolomite, hemimorphite, willemite, anglesite, galena, smithsonite, and sphalerite. Lead and perhaps zinc appeared to be more plentiful relative to silver in the outlying mines of the district than in the Presidio mine, although the outlying mines are scattered and were poorly developed so generalizations are difficult.

The relatively sub-horizontal geometry and the thickness of the mineralization suggested the use of variations of room-and-pillar and cut-and-fill mining methods with a minimum height of 8 feet to allow sufficient height for personnel and equipment. Areas with thickness over than 15 feet can be mined in two or more passes, or could be mined using post room-and-pillar mining, or another variation of the conventional room-and-pillar mining.

The Shafter mine processing facility proposed in this study will use whole-ore cyanide leaching to extract silver from the mineralization. Metal recovery will be accomplished using a standard Merrill Crowe CCD zinc precipitation method. Run-of-mine (“ROM”) material will be crushed to a nominal 1 inch size using a single jaw crusher for primary crushing and a cone crusher in closed circuit with a product screen. The crushing plant will operate on a single 12 hour shift, seven days a week, to replenish the crushed ore stockpile. The stockpile will have enough capacity to feed the milling operations, which will operate with two, twelve-hour shifts to continuously operate 24 hours/day and 7 days a week.

Milling to the final leach feed product size of 80 percent passing 74 microns will be achieved by a single ball mill in closed circuit with cyclones for classification. Cyclone overflow will feed into a pre-leach thickener. Thickened slurry, at 68 percent solids, will flow to the leach circuit where it will be diluted with returned filtrate from the zinc precipitation circuit and make up process water to a solids weight of 45 percent. The pre-leach thickener overflow will report to the process water tank for use in the grinding circuit and as wash water for the tailings filter.

The leach tanks are designed for 72 hour retention to achieve an extraction of silver at 82.4 percent. The slurry from the leach circuit will report to the CCD circuit using four thickeners for cleaning of the slurry of pregnant leach solution at an anticipated wash efficiency of 96.0 percent. The pregnant solution from the CCD circuit will flow by pumps to the deaeration vessel and then to the zinc precipitation circuit. Cleaned residue from the CCD circuit will be pumped to the tailings plate and frame filters for one final wash before the residue cake is conveyed to a tailings load out area where it will be hauled to a lined, dry-stacked tailings storage facility.

The zinc precipitation circuit will mix zinc with silver-bearing pregnant solution causing the silver to precipitate from solution. The silver precipitated slurry will be pumped through the zinc precipitation filters to capture the silver as a cake. The silver precipitated cake will be transferred to a retort for drying and to remove any contained mercury which will be collected for removal off site. The dried cake from the retort will then be mixed with flux and smelted in a gas fired furnace for pouring in silver doré. The silver doré will be stored in a safe until it is shipped off site to a refiner.