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.

The mineralization is stratigraphically controlled and typically found at the contact between the phyllites (stratigraphic footwall) and the argillites (stratigraphic hanging wall). Due to the intense structural deformation, mineralization may be tightly folded into the phyllite or argillite packages such that the original stratigraphic relationships are unclear. In rare cases there may be areas where the mineralized materials are stratigraphically above the phyllite/argillite contact but still proximal to it.

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.

East Zone
The East Zone outcrops at the discovery “Big Sore” gossan and extends down-dip to the west until it is deformed and offset by the D2.5 Klaus Shear at depth or by the Maki Fault at its southern extent. The mineralization occurs along the phyllite/argillite contact and varies from 1 to 30 feet (0.3-9 m) in thickness.

At the surface the mineralization dips at 60–80° to the west with the argillite on the bottom or eastern side. The dip shallows with depth to near-horizontal as a result of F2 folding. Where the mineral body terminates into the Maki Fault drag folding has rotated the mineralization nearly 900 to 850 feet along strike. This geometry indicates that the entire Greens Creek deposit is on an overturned major antiform with stratigraphic up being down.

West Zone
The West Zone is the down-dip extension of the East Zone below the Klaus Shear, and from 75 to 1100 ft in elevation. While quite variable, the overall trend strikes N 30°W for over 2,500 ft (762 m) of strike length and 1,025 ft of vertical extent (75 to 1,110 ft). The thickness is also highly variable from less than 10 ft (3 m) to over 300 ft (91 m) in the central portions.

9A Zone
The 9A Zone is the most structurally dismembered zone at Greens Creek as it lies within the Maki- Kahuna Fault Zone. The general orientation of the mineral body is striking to the northwest and dipping steeply to the west but many internal fault splays cut mineralization at differing orientations. In plan, mineralized widths range between less than 5 ft (1.5 m) up to 100 ft (30 m).

Northwest West Zone
The Northwest West Zone is an extension of the West and East zones with the 9A Zone tying the three together through the Maki-Kahuna Fault system. The structural setting is dominated by a pair of recumbent F2 folds. The upper fold is an argillitecored syncline while the lower fold is a phyllite-cored anticline. Mineral types and mineralization are similar to what has previously been described for the West Zone, with MFB and MFP dominate with some WSI and WCA intermixed.

In the Northwest Zone some mineralization is located up to 100 ft off the mine contact into the hanging wall argillite. Mineral types are a mixture of mostly massive and white-siliceous material types with lesser carbonate, baritic material and mineralized argillites. This zone is particularly rich in zinc, iron and copper with lower silver relative to most of the Greens Creek deposit, again because this is part of the main core of the deposit encompassing the lower East, West, 9A, and Northwest West zones.

Upper Plate Zone
The Upper Plate Zone is located at the far northern end of the Greens Creek deposit and above the northwest West Zone. It is a smaller body representing the top of the Northwest West Zone which was caught up in a very large F2 fold located on the bottom of a major D2.5 shear known locally as the Upper Shear Zone. The recumbent fold has an amplitude of over 3,000 ft with an argillite core no more than 200 ft thick. Mineralization is found mostly on the upper and lower contacts of the fold but does in places cross into the argillite core.

5250 Zone
Immediately west of the Kahuna-Maki Fault system is a lower temperature lens of barite-rich mineralization which links to the north with the Northwest-West Zone mineral body. This lens, known as the 5250 Zone is continuous for up to 1,200 ft (366 m) along a N300W trend and represents the uppermost mineralization trend at Greens Creek.

The mineral types are dominated by white baritic material (WBA) with lesser massive mineral and minor amounts of carbonate and siliceous mineral types. The silver grades are typically higher than average for the Greens Creek mineral bodies while zinc, lead and gold are below average. The mineralized material occurs along the phyllite/argillite mine contact and trends approximately N 35° W. The interpretation shows two limbs of a fold; the western limb dips generally 30º to the west/southwest and the eastern limb dips more steeply at approximately 80º.

Greens Creek is a portal accessed mine that utilizes conventional rubber-tired mining equipment, and drill and blast techniques. Production mining is primarily executed using cut and fill and drift and fill methods, supplemented by longhole stoping where orebody geometry permits.

The orebody is complex which has resulted in each of the nine mining zones being unique in size and shape. Each requires differing levels of mine development infrastructure which is included in the mine plan. Ore handling is performed with a fleet of underground haulage trucks and scoop trams or load-haul-dump units (LHDs). Waste is either trucked out of the mine to the waste disposal area or is placed in previously mined-out stopes when available. All LHDs are equipped with remote operating capability and can be operated from an operations room on surface. Production areas are backfilled with either paste fill, created from concentrator tailings, or cemented or uncemented rock depending on future strength requirements.

Fresh air is fed into the mine via the 920 level access portal and distributed through a series of internal ramps and raises, and exhausts through the 1350 level portal and the 2853 surface raise. A ventilation on demand (VOD) system is currently in place in a limited number of headings and is planned to be extended to the remainder of the mine.

The LOM plan is based on a 2,300 stpd production rate continuing through to the end of mine life in 2035. Ore grades remain relatively stable through the mine life with silver grade ranging from a 10.6 oz/ton Ag to 11.9 oz/ton Ag.

Mined ore is delivered to the mill stockpile near the portal by underground haulage trucks. It is dumped into a “build” stockpile, which is regularly blended by the pad operator using either a loader or bulldozer. The pad operator at the same time uses the loader to feed the mill feed bin with material from the immediately adjacent “mill feed” stockpile, which consists of ore that has been previously delivered and blended. The mill feed stockpile is used until it is exhausted, at which point the build stockpile is designated the “mill feed” pile and is used to feed the mill.

The old mill feed area then becomes the build stockpile area and mine haulage is henceforth dumped there for blending. The two stockpile areas are thus alternated between the build and mill feed roles. A fresh mill feed stockpile can vary in size between as little as 1,000 tons (907 tonnes) to as much as 15,000 tons (13,608 tonnes) and so have a feed life ranging from less than a day to several days, depending on relative mine production rates and mill feed rates.

A Caterpillar 980 loader is used to transfer blended material to a fixed grizzly with 15-in (38-cm) square apertures located above a dump pocket with a 60-ton, 35-minute capacity. A hydraulic rock- breaker is used to break the small volume of oversize and a 48-in (122-cm) apron feeder is used to regulate the flow of grizzly undersize material onto a 48-in (122-cm) belt feeding the semi- autogenous grind (SAG) mill. The apron feeder speed is controlled to maintain target SAG mill feed rate of 85–110 wet tons/h (77– 100 tonnes) based on the feed belt weightometer output. A 16 x 5-ft (4.9 x 4.5-m) Marcy SAG mill is operated in closed circuit with a primary vibrating screen with 10- mm apertures.

Secondary Grinding
Primary screen undersize gravitates to the ball mill cyclone feed box where it combines with the discharge from the 900 hp 11 x 13-ft (3.3 x 4-m) Marcy overflow ball mill, before being pumped to a cluster of five 10-in (25-cm) diameter Warman Cavex cyclones. Two-inch diameter forged steel balls are added to maintain a target mill power draw of 600 kW. Four cyclones are usually in operation at 2,300 tons per day (2,087 tonnes/day); with the underflow from one cyclone being diverted through the gravity circuit for free gold recovery prior to return to the cyclone feed pumpbox. Cyclone overflow at 48 to 54% solids has a particle size range of 80% passing 70–85 µm and 95% passing 140 to 160 µm. Liberation of flotation feed is sufficient for recovery to low grade rougher concentrates, but not for production of final concentrates, which requires re-grinding prior to the cleaner circuits.

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 ........