Gold deposits of the Mulatos district are considered as high sulfidation-state epithermal systems (Austin et al, 2004; Staude, 2001). Epithermal precious metal systems may be classified as high, intermediate, and low sulfidation styles. They are characterized by the sulfidation state of the hypogene sulfide mineral assemblage, and show general relations in volcano-tectonic setting, precious and base metal content, igneous rock association, proximal hypogene alteration, and sulfide abundance (i.e.; John, 2001; Sillitoe; and Hedenquist, 2003). Ore in all occurrences is of the type formed under epizonal conditions, that is, generally within 2 kilometers of the paleo-surface. Past workers have referred to high sulfidation systems as acid-sulphate, enargite-gold, or alunite-kaolinite systems.

Gold occurs in oxide, mixed oxide/sulfide, and sulfide ore types, with pyrite as the primary sulfide mineral. The deposits are amenable to cyanidation in all ore types, but gold extraction decreases with decreasing levels of oxidation.

Precious metal mineralization at Mulatos is associated with intense silicic alteration (mostly vuggy silica), advanced argillic alteration, and the presence of hydrothermal breccias. The original protolith (dacite porphyry flow/tuff, coarse grained volcaniclastites, Breccias), as indicated by surface mapping and core drilling, may have contained in the order of 2-3 percent sulfide as pyrite with various amounts of enargite and tetrahedrite. The principle gold bearing host rock is interpreted a favoured for mineralization due to relatively high primary porosity and its intense fracturing.

Gold mineralization within the Mulatos deposit occurs primarily within areas of pervasive silicic alteration of the volcanic host rocks, and to a lesser extent, within advanced argillic alteration assemblages proximal to silicic alteration. The gold-bearing advanced argillic zones are dominated by pyrophyllite or dickite alteration. Silicic rocks host approximately 80 percent of the contained gold within the deposit.

Free gold is commonly found in hematite-filled fractures. Gold also occurs in pyrite, as gold/silver tellurides, and possibly as a solid solution in some copper sulfide minerals. Supergene oxidation and perhaps remobilization and secondary enrichment of gold have been ongoing since the post-mineral volcanic cover was removed.

The open pit mining is a typical drill, blast, load haul operation with mining in the main pit being done with 9 m bench heights. The mine switched from a 6 m to 9 m bench about two years ago for improved productivity. A 9 m bench will be used at Victor and San Carlos and it is anticipated that a 3 m bench will be used at Yaqui and Pelon to improve selectivity. The open pit schedule calls for an average of 17,500 tonnes per day (tpd) of heap ore to be delivered to the primary crusher except during the rainy season (third quarter of the year) when the schedule is for 16,000 tpd of heap ore. The mill started in early 2012 at 500 tpd which is currently supplied by the open pit mining in the Escondida area of the main pit.

Underground mining planned at Mulatos consists of two primary mining methods, longhole open stoping with delayed backfill and modified drift and fill. The preferred sequence for longhole stopes is to mine from the bottom up. Thus the upper development or drill drift for one stope becomes the lower mucking drift for the stope above after filling the lower stope.

There are two primary recovery processes at MON. The 17,500 tpd heap leach facility is for the lower grade ore and the recently installed 1,000 tpd mill processing plant is for high grade ore.

Crushing for Heap Leach Process
Run-of-mine ore is delivered to the primary crusher feed hopper by rear-dump haul trucks. A static grizzly screen above the hopper limits the top size of rock fed to the crusher. Below the hopper, a vibrating grizzly feeder transfers ore. Grizzly screen oversize feeds the primary jaw crusher. Grizzly screen undersize joins the crusher product on the primary crusher discharge conveyor which feeds the primary crushed product to the second stage of crushing. The rock breaker is installed to serve the static grizzly. Tramp iron is removed from the crushed product by way of the magnet mounted above the discharge of the discharge conveyor.

Primary crushed product is fed to the secondary cone crusher. Secondary crushed product is discharged onto a conveyor and delivers ore to the scalping screens by conveyors. Screen oversize reports to the screen oversize conveyors which feed the radial stacker which discharges into the coarse ore stockpile. Screen undersize reports to the conveyors which discharges onto conveyor which ultimately directs ore to the fine ore stockpile.

The tertiary crushed product is discharged onto conveyor and fed to a vibrating screen. Screen oversize is transferred by conveyors and is crushed in the single quaternary cone crusher. The quaternary crushed product is discharged onto the crusher conveyor and re circulates back to the vibrating screen.

Crushing for High Grade Mill (Gravity Plant)
High grade ore which has been crushed to P80 75 mm, is diverted to a stockpile located near the gravity circuit. Ore is then withdrawn from the stockpile at the rate of 50 tph by one of three vibrating feeders located along a reclaim tunnel. The reclaim conveyor delivers ore to the gravity primary feed conveyor.

The primary feed conveyor is equipped with a weightometer and a belt magnet to remove tramp metal. The ore is delivered to the cone crusher, operating in open-circuit. The ore is discharged onto another belt conveyor where it is carried to the double-deck wet primary screen.

The wet primary screen oversize ore is transported via a conveyor to feed the vertical shaft impactor (VSI). The screen undersize reports to the rougher IPJ feed pump box. The VSI discharge is conveyed back to the primary screen feed conveyor.

There are two primary recovery processes at MON. The 17,500 tpd heap leach facility is for the lower grade ore and the recently installed 1,000 tpd mill processing plant
is for high grade ore.

Heap Leach Process
Ore is processed through four stages of crushing to a target crush size of 95 percent minus 3/8 inches.

Upon completion of crushing, the ore is delivered to the leach pad for stacking and leaching. An impermeable layer of plastic overlies a compacted clay layer beneath the leach heaps. Using a low-pressure irrigation sprinkler system, cyanide solution of low concentration is applied to the ore on the leach pad for gold extraction. Inter-lift liners are periodically installed on the pad to reduce percolation time and the time required to extract and produce gold from each lift of the heap. The gold-bearing solution is directed to the pregnant solution pond and pumped to the carbon-in-column circuit for gold recovery.

Gold and silver r ........