Work by Taylor and Johnson (2010) in the USGS Professional Paper 1763 indicated that the Greens Creek deposit displays a range of syngenetic, diagenetic, and epigenetic features that are typical of volcanic massive sulfide deposits (VMS), sedimentary exhalative (SEDEX), and Mississippi Valley-type (MVT) genetic models. Based on those observations the investigators indicated that the Greens Creek mineral deposit was a ‘hybrid’ type with elements of several deposit models.

Due to variations in mineralization, structural complexity, and spatial location, the Greens Creek mineralization is segregated into nine separate mineralized zones. In order from easternmost and highest elevations to westernmost, the zones are:

• East
• West
• 9A
• Northwest West
• Upper Plate
• 5250
• Southwest
• 200 South
• Gallagher.

On a gross deposit scale the mineralization trends N 30° W and plunges to the south at approximately 20°. The East Zone outcrops at the eastern edge of the mineral deposit, dips to the west, and transitions into the West Zone near a tight F2 fold where the mineral horizon transitions from a nearly flat orientation to a nearly vertical wall dipping steeply to the west. The East and West zones are bounded on the west by the Maki Fault system which offset the mineral horizon to the north in a dextral sense. The western deformation boundary of the Maki Fault tends to be a continuous fault splay which is called the Kahuna Fault. The mineralization hosted inside the fault zone are called the 9A Zone.

West of the Kahuna Fault, the Northwest West Zone represents the offset portion of the West Zone. Above and to the south of the Northwest West Zone is the main trend of mineralization which includes the Southwest Zone followed by the 200S Zone further down plunge. The 5250 Zone is the along the upper mineralized trend evidenced in the East Zone, just offset across the Maki-Kahuna Fault system.

The Gallagher Zone lies to the west of the 200 South Zone and is west of a second major dextral fault zone known as the Gallagher Fault. Offset on a post-mineralized dike swarm, the trend of the 200S Zone into the Gallagher Fault and the similar structural and chemical styles between the southern 200S and Gallagher mineral zones all indicate that the Gallagher Zone is simply the fault offset of the 200S Zone.

The mine is primarily accessed via the 920-level portal in the same general area as the mill, stockpile pad and administration building. This portal also serves as the primary air intake. A second portal adjacent to the 920 Portal serves as a secondary escapeway and an additional air intake. A third portal located at the 1350 elevation serves as the primary ventilation exhaust and an additional escapeway. There is also a raisebored ventilation raise to the 1350 portal area which serves as an additional exhaust.

The working areas are accessed via ramp. Most ramps are connected with crosscuts at various locations, therefore most working areas have multiple options for equipment access in the event a particular ramp is blocked for rehab or utility work. Two of the ramp systems have a single route for mobile equipment access. These ramps feature laddered escapeway raises to enable airflow and a secondary means of egress.

Mine production and development are undertaken with modern mechanized trackless equipment. Ore and waste are hauled to the surface using trucks. Backfill is achieved using a combination of three methods: paste pumped from the underground paste plant, cemented tailings trucked and jammed and into the heading, and development waste placed into the area to be backfilled. In addition to the conventional trackless mining equipment, the Greens Creek fleet contains one LHD capable of semi-autonomous operation and another LHD capable of teleremote operation. This equipment enables production activities to continue during the shift change and post-blasting periods when no personnel are allowed underground.

The current production rate is 2,300 t/d of which approximately 2,000 t/d is produced by cut-and- fill with the remaining 300 t/d from longhole stoping. Longhole Mineral Reserves are projected to be depleted approximately halfway through the remaining mine life, at which point the mine plan calls for 2,300 t/d of production from cut-andfill mining.

The Greens Creek mill produces three saleable flotation concentrates and a gravity concentrate. A carbon concentrate is produced as part of the process but is discarded as part of tailings.

A gravity circuit utilizing spiral concentrators treats a bleed stream from the grinding circuit cyclones. It produces a final gravity concentrate that is further processed offsite. Lead concentrate is produced in a rougher-cleaner circuit with re-grinding of the cleaner feed. The lead concentrate is relatively low grade, at approximately 35% lead, but carries a large proportion of the silver in mill feed.

Zinc concentrate is produced in a rougher-cleaner circuit, also with re-grinding, using lead rougher tailings as feed. The zinc concentrate typically contains 46 to 50% zinc, which is a normal grade, and considerably less silver than the lead concentrate.

Bulk concentrate is produced in a complex circuit which has as feed cleaner tailings from both the lead and zinc circuits. It is a relatively low- grade zinc concentrate, at 30% zinc, with a smaller amount of lead and some silver. Bulk concentrate has a relatively limited market so lead and zinc concentrates production is preferred over that of bulk.

Gravity Circuit
A gravity circuit is operated to improve overall gold recovery. Two banks of eight double-start spirals are installed for roughing, with a single bank of two double-start spirals for open circuit first-stage cleaning. First stage cleaner spiral concentrates report to a second stage single-start cleaning spiral to produce a final gravity concentrate.

The final gravity concentrate is passed through a vibrating screen to remove relatively coarse (plus 30 mesh) material from the finer product comprising most of the gravity concentrate. The coarse fraction contains a significant amount of tramp copper wire fragments, which tend to interfere with off-site processing by intensive cyanidation. It is planned to treat the relatively small volume of coarse materially separately, simplifying and improving the treatment of the fine fraction.

The final gravity concentrate is collected and stored in barrels to be shipped to an off-site toll facility where it is treated using intensive cyanidation to produce precious metals. The gravity concentrates typically recover roughly 15- 20% of the gold in mill feed and less than 1% of the silver.

Flotation Circuits
All flotation is carried out in conventional Outokumpu mechanical flotation cells, unless otherwise noted.

Cyclone overflow is diluted to a range of 52% to 45% solids before gravitating to a carbon flotation circuit that consists of one 8-ft (2.4- m) diameter column. Thiscolumn removes naturally- floatable material such as graphite, carbonaceous pyrite and layer silicates (primarily clays and talc) from the mill feed and direct it to final tailings.

The first lead circuit flotation takes place at a pH of 8.5 in one 7-ft (2.1-m) diameter column. The concentrate from this column is high enough grade to be sent to the overall lead concentrate as a “scalped” lead concentrate product that is combined with the lead cleaner concentrate.

Zinc depression in the lead circuit is accomplished by pH control accompanied with the use of zinc sulfate in lead roughers. Zinc sulfate is used in the lead cleaners for both zinc and iron depression. Lead rougher tailings are conditioned with lime and then copper sulfate ahead of zinc roughing.

Lead roughing takes place at a target pH of 8.0 to 8.5 in banks of three 300-ft3 (8.5-m3) conventional flotation cells each followed by two 20-m3 tank cells. Carbon dioxide is added to the SAG mill, cyclone overflow, and lead rougher conditioner to stabilize the pH of the lead rougher. A low-grade 20% Pb–18% Zn lead rougher concentrate is reground to 80% passing 20 µm with a tower mill, and then cleaned at a pH of 7.2 in ten 100-ft3 (2.8-m3) cells.

The lead cleaning circuit comprises three stages in closed circuit. Several options have been installed on the lead cleaning circuit. There are options to run the circuit as a three-stage cleaner or as a two-stage cleaner plus scavenger. Lead cleaner tailings are sent to the lead cleaner scavenger rougher cells at a pH of 7.2 to 7.5. The lead cleaner scavenger rougher concentrate is cleaned in closed circuit and returned to the lead cleaners. Lead cleaner scavenger rougher tailings report as feed to the zinc–bulk rougher circuit by way of the swing cell circuit which are three Woodgrove cells.

The lead cleaner scavenger tailings report to a conditioner tank where pH is raised to 10.5 using lime and copper sulfate is added to activate the zinc for flotation. This material is then pumped to the swing cells which consist of three 3-ft diameter x 10-ft tall Woodgrove SFR flotation cells in series. The concentrate from the swing cells is added to the zinc rougher concentrate to report to the zinc cleaner circuit and keep as much zinc as possible in the zinc circuit. The swing cells tailings the reports to the bulk flotation circuit to recover as much valuable mineral that remains into the bulk concentrate.

Zinc roughing is carried out at a pH between 10.0 and 10.5 in a 7-ft diameter x 30-ft high (2.1 x 9- m) zinc rougher column, followed by five 300-ft3 (8.5-m3) cells in series with three 100-ft3 (2.8-m3) cells. Zinc rougher tailings form the majority of the final tailings flow.

Rougher concentrate is reground to 80% passing 20 µm with a tower mill before being fed to three- stage zinc cleaning at a pH of 10.5 to11.0. Zinc cleaner tailings join the swing cell tailings to feed the 7-ft diameter x 30-ft high (2.1 x 9-m) bulk rougher column, the tailings from which feed nine 10-ft3 (2.8-m3) bulk rougher cells. Bulk rougher tailings are directed to final tailings, while bulk rougher concentrate is cleaned once in three 100-ft3 (2.8-m3) cells in closed circuit with the rougher to produce final bulk concentrate. Zinc cleaner cell capacity can be redistributed from three stages to two stages of cleaning during periods of high-zinc head grades.