At various times since its discovery the Minto deposit has been described as an example of Porphyry Copper, Volcanogenic Massive Sulphide (VMS), Redbed Copper, Magnetite Skarn (see discussion by Pearson and Clark, 1979) and Iron Oxide Copper Gold “IOCG”(Minto Explorations Ltd., 2003). Based on the preceding paragraph it is reasonable to say that the origin of the Minto deposit is enigmatic. Various workers (including the current authors) appear to have ascribed different interpretations for the most part based on their empirical observations, the background of the observer and the popular models of the day. The abundance of the high Cu/S mineraln bornite in a moderately oxidized magmatic system along with the obvious magnetite association suggests that Minto belongs to one of two recognized deposit types: Magnetite Skarn or Iron Oxide Copper Gold (“IOGC”). The lack of a typical calc-silicate skarn mineral assemblage seems to preclude the skarn deposit type, this appears to leave the IOCG model or alternatively it belongs to a previously unrecognized deposit type.

The host rocks to the Minto deposit were emplaced in a deep batholitic setting (exceeding 9 km deep to perhaps as much as 18-20 km deep), which is not considered to be the typical porphyry environment. The host is a moderately oxidized magma (Tafti and Mortensen, 2004) with widespread iron oxide (magnetite and hematite) mineralization. At least some of the hematite is supergene in origin but it is unclear if some hematite is also primary. There are very strong structural controls on ore mineral emplacement and there is no apparent genetic link to a specific phase of intrusion. Typical porphyry-type alteration zoning such as widespread propylitization, argillization, barren silicic core, or large barren pyritic halo is not recognized. Stockwork style, fracture or vein mineralization is also not present.

MintoEx geologists have been advised that some examples of IOCG mineralization (in personal communications) exhibit some similar characteristics and setting to Minto including Copperstone in Arizona, Candelaria in Chile, and Ernest Henry in Australia (Williams et al., 2005). From a genetic and structural prospective, albeit not size wise, the Sossego Deposit in Brazil may be a reasonable analog. While an IOCG origin for the Minto Deposit cannot be unequivocally demonstrated, MintoEx geologists are of the opinion that this style of deposit provides the most consistent model for their current level of understanding. However, the unique nature of this mineralization style and apparent lack of close analogs elsewhere suggests the Minto Copper-Gold deposits may represent an unrecognized mineral deposit type.

The M-zone, Area 118 and Area 2 zones were all mined using a longhole open stoping method, and the Minto East zone is mined in the same manner. All of these ore zones can be described as lenses of foliated and variably migmatized metamorphic rocks bounded at their hanging wall and footwall contacts by equigranular, undeformed granodiorite (eG) host rock. The metamorphic zones are typically 5-30m thick, and the grade within them varies from 0% to approximately 6% copper. These zones typically dip at 20° to 35°.

The mining method requires a series of parallel sill drifts to be developed along the strike of the deposit, following the footwall contact. From these sill drifts, typically 6m wide and 4.5m high, a top-hammer longhole rig drills rings of 3” up-holes into the deposit above, stopping at the hanging-wall contact.

To provide adequate void space for blasted muck when starting a new stope, 1.8m x 1.8m inverse raises are drilled. These are composed of six 6- inch reamed holes, which are left unloaded, surrounded by a pattern of eleven 3½-inch blast holes. Generally, each stope is initiated with one or more rings of blast holes on either side of the inverse raise; subsequent blasts increase the number of rings fired simultaneously to take advantage of the void space in each block.

After drilling is completed, the rings are loaded, blasted, and then mucked out from the sill drift, which serves as a drawpoint. The average blast size is 3,000 tonnes. Mucking is via remote-controlled load-hauldump machines (LHDs); all stopes are non-entry so that no personnel are exposed to the open stope. Ore is trucked to surface along the main ramp through the Minto South portal.

For the Area 118 zone and M-zone, production drift centerlines were 15m apart. From each 6m- wide sill drift, drill holes fanned out to blast a 10m wide stope, and 5m-thick rib pillars separated each stope, supporting the hanging wall. For the Area 2 zone, production drift centerlines are 20m apart and stope and pillar widths are varied based on the ore thickness (stope and pillar height). Typical stope widths are 15m and pillar widths are 5m.

The mining method does not use backfill; however, small quantities of development waste are sometimes placed in completed stopes to reduce haulage requirements.

The main ramp of the Minto South Underground measures 1,677m, extending to the currently active Area 2, 630 level. The first access to the Minto East zone is 2,440m in length. The upper ramp is 5.0m wide and 5.0m high; the ramp below the 690 level has been driven at dimensions of 5.0m wide and 5.5m high to provide additional clearance between vent ducting and haul trucks. This access is used for all ore and waste haulage, personnel/equipment access, and services. It is also used as an exhaust airway.

Re-muck bays are typically developed every 150 m along the decline to improve the efficiency of the development cycle; they are designed to hold two rounds of development muck. The re-muck bays have the same dimensions as the decline and are generally 15 m in length. Once they are no longer needed for development, they are repurposed as equipment storage, pump stations, drill bays, service bays, etc.

Ground support generally comprises 2.4m-long fully grouted resin rebar bolts on a 1.5m x 1.5m pattern with a 1.8 m bolt in the center for the back and 1.8m bolts for the walls. Welded wire screen is installed to within 1.5 m from the floor. Additional support is installed at all drift intersections.

A combination of 7, 8 and 10-yard LHD units and 42 tonne trucks are used for ore and waste haulage. The broken ore from the stopes is mucked by LHDs to remuck bays, or loaded directly onto trucks, which carry ore from the mine to a small stockpile adjacent to the portal. The surface mining fleet then takes ore to open pit stockpiles or to the mine crusher on a daily basis.

The ore is fed into the primary crusher and the Semi-Autogenous Grinding (SAG) mill where it is crushed and ground to reduce particle size. The ore enters a number of flotation circuits where copper, gold and silver minerals are separated out from other minerals. The concentrated ore is thickened and de-watered using thickeners and filters. The concentrate is moved to the concentrated storage shed where it is loaded onto trucks and exported via the Port of Skagway, Alaska, to overseas smelters for treatment and sale.