"The Pierina deposit is located in the Calipuy belt of Miocene volcanic rocks near Huaraz in Central Peru. Most of the mineralized zone is stratiform and hosted by a gently dipping unit of felsic ash flow tuffs, but some ore occurs within and on the borders of breccia dikes that are associated with a felsic prophyry intrusion underlying the south side of the deposit. Control at the regional scale is by northwest and northeast faults. The mineralization is of the high-sulphidation type, with ore mainly associated with a porous vuggy-silica altered rock surrounded by a zone of quartz-alunite-clay alteration. Over 95% of the known mineralization at Pierina is oxide. However, a sulphide feeder zone has been intersected in the south-central area of the deposit. Mineralization at Pierina (dated at 14.5 million years ago) was the result of a four-phase sequence of events beginning with acidic leaching of the pumice tuff, creating a porous and permeable host that allowed subsequent gold-and-silica-rich fluids to deposit pervasive mineralization. Later activity served to crack the brittle host and allow for the formation of late-phase, quartz-sulphide-gold veinlets. A final event of leaching (supergene and/or hypogene) helped remobilize and concentrate existing gold mineralization.
Mining - Rates The mining rate for the month of December, was 43,900 tons per day (tpd). A total of 2.7 million tons of material was moved in 1998. For 1999, the mining rate is estimated at 20 million tons or 54,000 tpd. The mining rate is to be increased in June 1999 to 64,000 tpd from 44,000 tpd. Mining activity is currently focused on two laybacks: layback zero mining the high-grade ore near surface, situated beside the crusher, and layback one primarily stripping waste material. Layback one is expected to provide a significant portion of the ore in 2000. Additional mining equipment, scheduled for delivery in mid-1999, will be used on layback one.
Mining - Unit Costs The cost per ton for the month of December was $0.81 per ton reflecting the short haul distance to the crusher. For 1999 and beyond, mining cost per ton is expected to be in the range of $0.85 to $0.90. The depth of the pit and the height of the heap leach pad impact the truck-haul cycle time and unit cost, which is the most significant component of mining costs.
Processing - Rates The crushing/processing rate is 21,500 tpd. It is expected to rise to 30,000 tpd in 2000 with the completion of the leach pad expansion. The processing rate for the month of December was 18,500 tpd.
Processing - Grade And Recovery Rates The average grade of ore placed on the leach pad was 0.25 ounces per ton (opt) in 1998. For 1999, the average grade of ore placed on the pad is expected to be 0.11 opt. Gold recovery rates to date have been consistent with the anticipated life-of-mine rate of 80%. Silver, which leaches at a slower rate, also appears to be following the expected recovery pattern of 23%.
Mine Description The Pierina Mine is a conventional open pit truck-and-shovel operation in which ore and waste are being mined in 30-foot benches. The Mine employs satellite tracking of haul trucks to ensure proper routing of material. The life-of-mine strip ratio is 1.3:1 (waste:ore) and it is anticipated mining will reach the maximum rate of 83,000 tons per day in 2001 with the final expansion of the mining fleet.
Process Description The process facilities consist of a valley-fill heap leach pad and a conventional Merrill-Crowe gold and silver recovery plant. The ore is stacked in a lined containment area behind a retention dam. A leach solution is applied to the top of the ore and allowed to percolate through the heap. As the solution migrates through the ore, it leaches the gold and silver from the rock and holds it in a solution. The gold-bearing solution ("pregnant solution") is collected at the base of the leach pad in the pore space within the heap. The pregnant solution is pumped to the gold recovery plant where suspended solids are removed and the solution is then treated in a conventional Merrill-Crowe precious metal circuit. The same valley-fill system was successfully used at Barrick's Mercur Mine in Utah. For 1999, an exploration program will test the feeder zone at depth discovered during the 1996-97 definition drilling program.
Processing - Unit Costs The crushing and leaching cost per ton of ore placed on the pad was $1.84 in 1998. The crushing and leaching cost per ton is expected to be in the $2.00 range in 1999 and in subsequent years."