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 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 ar ........

The relatively sub-horizontal geometry and the thickness of the mineralization suggested the use of room and pillar as the primary mining method. Longhole slashing, benching and partial pillar recovery would be employed as needed during the secondary extraction phase. A minimum mining height of 8 ft was included in the resource modeling to allow sufficient height for personnel and mechanized equipment. Areas with thickness over 15 or 20 ft can be mined using the secondary extraction methods listed above, or another variation of conventional room-and-pillar mining.

Mine design for primary extraction is based on 6.8 oz Ag/ton cutoff grade. Block grades for the stopes is the block diluted grade of the 10 ft x 10 ft x 8 ft block with a cutoff of 6.8 oz Ag/ton. The minimum mining height is 8 feet with basic heading size of 28 ft wide. Average height for the above cutoff resource base is 12 ft. Stopes were formed by successive re-blocking of the block model at higher cutoffs. The average stope width is estimated to be 140 ft from the selected resource blocks. Pillars are planned to be 24 ft by 24 ft. These stope dimensions yield a primary extraction ratio of 78%. Secondary extraction involve mining half of the pillars. Overall extraction increases to 89%, assuming half the pillar marterial is recoverable.

Once the primary extraction sequence in each work area is complete, secondary extraction would include combinations of: slashing pockets of back material ........

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