Emigrant gold deposits are contained primarily within the Lower Mississippian-age Webb siltstone and Devonian-age Devils Gate limestone.

Gold occurs in shallow west-dipping tabular bodies at or near the contact of the Webb siltstone and underlying Devils Gate limestone (unconformity), with secondary occurrence of gold along the Emigrant Fault. Gold mineralization is present near the surface. A small percentage of ore occurs in the Mississippian-age Chainman siltstone and Fresh Webb siltstone.

The Emigrant Fault occurs along the western margin of the Emigrant gold deposit. The fault strikes north-I0 degrees-east and dips 80 to 85 degrees west. The fault separates the Chainman siltstone in the hanging-wall (above fault plane) with the Webb siltstone and Devils Gate limestone in the foot-wall (below fault plane) (Thoreson I99I; Lapointe et at al. 1991). Although mineralization commonly occurs adjacent to the Emigrant Fault, elsewhere mineralization lies as much as 3,000 feet east of the fault. The Emigrant Fault is a localizing structure for hydrothermal fluids that migrated up the fault, and outward into adjacent sediment to form disseminated low grade gold deposits. Mineralization extends l2,000 feet along a north-south trend parallel to the fault, and thins away from the fault.

In the vicinity of the Emigrant ore deposits, siltstone and sandstone are argillaceous, fractured, silicified, bleached, and iron oxide stained (Bentz et al. I983). Most of the ore proposed for mining is completely oxidized, with pyrite converted to limonite and hematite. A small percentage of ore is unoxidized carbon sulfur refractory rock (Chainman siltstone and Fresh Webb siltstone).

The Emigrant open pit ceased mining operations in December 2018 while residual leaching of gold continues.

The majority of the underground ore as well as higher-grade surface refractory ores are processed through the roaster (Mill 6), which consists of a grinding circuit, roasting circuit and a conventional carbon-in leach circuit. Mill 6 processed approximately 3.4 million tons of ore in 2016. Higher-grade surface oxide ores are processed by conventional milling and cyanide leaching at Mill 5. Additionally, Mill 5 operates as a flotation mill treating lower grade, non-carbonaceous, sulfidic refractory ore to produce a gold/pyrite concentrate. Mill 5 processed approximately 5 million tons of ore in 2016. Lower-grade surface material with suitable cyanide solubility is treated on one of four heap leach pads.

The circuits at the South Area include Mill 5, Mill 6, and two heap leach pads served by a single plant. Mill 5 is a pyrite floatation plant that processes sulfide and oxide ores that are ground in a sag mill and ball mills. The material is then sent to the floatation circuit where the sulfides are floated and dried for later processing in the autoclave or roaster. The oxide material remaining after floatation is sent to a set of carbon-in-leach (CIL) tanks for gold recovery. Mill 6 consists of a double rotating mill and a roaster; sulfide material is fed to the roaster where the sulfides are volatilized. Material leaving the roaster is sent to a set of CIL tanks at Mill 5 for processing. Tailings from both CIL circuits is combined and sent through a Caro’s Acid cyanide destruction circuit before disposal in the Mill 5/6 TSF. Gold-bearing solution from the SAL (i.e., Property and Non-Property Pads) drains to a series of pregnant ponds. Solutions from the pregnant ponds are pumped via pipeline to carbon-in column (CIC) circuits at South Area plant. Loaded carbon is transferred to the carbon handling facility and refinery for further processing.

The circuits at the North Area include a heap leach pad and plant. The gold-bearing solution from the North Area Leach drains to a series of pregnant ponds. Solutions from the pregnant ponds are pumped via pipeline to CIC circuits at the North Area plant. Loaded carbon is transferred to the carbon handling facility at the South Area and refinery for further processing.