Goldrush deposit is part of the Cortez deposits, which are Carlin-type sedimentary rock-hosted deposits.

Exploration currently reveals that the deposit has a maximum thickness of 76 m, a width of about 425 m, and extends along strike for approximately 5,275 m. The deepest significant intercept is currently at 1,435 masl. The Goldrush system remains open to the north into Fourmile, to the southeast, and in multiple directions in the Ken Balleweg (KB) Domain.

Exploration to date shows that mineralization at Goldrush is primarily stratigraphically controlled, occurring within subunit 5 of the middle Wenban Formation and to a lesser extent along the contact between the Horse Canyon and Wenban formations.

Mineralization is associated with low-angle thrust faults and their related hanging wall antiforms that typically concentrate gold along fold hinges and/or east-dipping limbs. Gold occurs within extensive zones of decarbonatization and silicification within the Devonian rocks.

The system is marked by a large stratiform silicified and sulfidized breccia horizon from 15– 70 m thick that extends more than 7,000 m on a north–northwesterly strike.

Mineralization
The main gold-bearing mineral is micrometre-sized arsenian pyrite that occurs as individual grains or as rims on pre-existing pyrite. Mineralization is primarily double-refractory and highly preg-robbing with both sulfides and active carbon present in the orebody. Mineralization exhibits low Ag:Au ratios with enrichment in arsenic, antimony, mercury, and thallium, and low base metal contents.

Seven mineralized domains are defined, from north to south, Crow’s Nest, Red Hill, KB North, KB South, 3½, Ranch, and Meadow.

The proposed method for mining the majority of the Goldrush deposit is small to moderate sized longhole stopes with cemented paste fill. The stopes will be extracted on a transverse primary/secondary system with (where possible), a continuous mining front. The mining direction is not fixed since the geotechnical and geometric conditions vary with each sub-area within the Goldrush complex.

Due to the mine access infrastructure being located on the western edge of the deposit, all cross cuts are mined from west to east. Considering this factor and the requirement to maintain a consistent mining front, the primary stopes will be extracted in a continuous sequence from east to west, top-down (where applicable) and from south to north. Once the second level of primary stopes are extracted and backfilled extraction of the secondary stopes can commence. The secondary stopes effectively follow the primary sequence.

Based on Mineral Reserves only, approximately 13% of the ore is produced from ore drive and drift-and-fill drive development, with the remaining 87% produced from longhole production stopes.

Each of the sub-areas within Goldrush were planned to be extracted in a regional sense from south to north. The five sub-areas were identified as:
• Red Hill;
• Crow’s Nest;
• 3½;
• Ranch;
• Meadow.

Mining is planned to commence in the Red Hill Zone as this will be the first area to be accessed from the twin declines. Test stoping began in November 2021 at the Red Hill Zone, which is expected to continue in 2022.

The main features of the final mine design are:
• Transverse primary–secondary longhole open stoping (LHOS) mining top-down under an engineered paste fill. This is a non-entry mining method. Level spacing is 20 m (floor to floor);
• Footwall drives mined the full strike length of the mining horizon and provide primary ventilation from which stope fans initiate the secondary ventilation network;
• Stope sizes have been determined using the geotechnical models (MRMM) correlating Q’ to their modelled stable stope dimension. As a result, stope lengths range from 15– 40 m;
• Inclined ore passes are linked into every footwall drive to enable effective materials handling;
• Designated trucks loading loops are provided below the mining areas with the exception of Red Hill;
• Truck loading areas are designed to permit effective autonomous operation of loaders reducing labor commitment;
• Primary vent raises are designed to provide defined vent districts. These will be mined as blind sinks and hence will save considerable time by not relying on underground access to be completed;
• An underground crush and conveyor system is designed to be installed adjacent to the south decline to handle all ore and waste to surface from the base of the twin declines. This will be critical in keeping the truck fleet to a sustainable size and hence, minimizing risk of heavy vehicle traffic congestion underground;
• Coarse ore and waste bins are designed to provide a buffer within the materials handling system;
• Production activities will use larger-scale equipment (LH621 loaders or equivalent) and TH663 trucks to ensure rapid turnover of stopes. The option to automate a material percentage of the production cycle is recommended, the designs have taken this into consideration.

Access
Early access to the Goldrush underground was initiated in 2016 when twin exploration declines received authorization by the BLM together with the construction of limited supporting surface infrastructure. The twin declines (5.5 x 5.5m) were driven from a portal located on the western flank of the Cortez Mountains.

At the end of the existing twin exploration declines, twin inclines break away, and two additional declines will be cut to the north and south to provide the main arteries for accessing the complete strike length of the Goldrush deposit. All of the declines were designed at 5.5 m wide x 5.5 m high with an arched profile with stockpiles located nominally at 150 m apart. Due to the generally low-quality rock mass conditions, care will need to be taken in development blasting to avoid unnecessary overbreak that would result in additional scaling and support.

Materials Handling
Ore and waste rock from the Goldrush project will be loaded onto the trucks from storage bins at the base of the twin declines via haul trucks and/or LHDs. Ore and waste rock will be transported out of the mine via a conveyor belt system in the south decline. Ore and waste will be metered, from the bin, by an apron feeder and then discharged to a belt conveyor. This belt conveyor will carry material to a diverter gate where waste rock will be conveyed to a truck loading bin and/or pile that will be loaded into trucks via a surface loader. Trucks will be loaded to carry ore to the range-front decline ore pad and waste to the Canyon waste rock facility.

Backfill
The proposed paste fill source will be from a relatively local tailings dam source. This fill medium is expected to have the correct particle size distribution and hence with the addition of a binder, will form an accepted engineered backfill solution. Based on the proposed paste fill, the extraction sequence can be top-down, since extracting stopes under paste fill is a recognized practice when mining in poor ground conditions.

ROM ore is dumped into the crusher dump pocket at the mill and discharges onto a vibrating grizzly. Grizzly oversize material is crushed in a jaw crusher. Grizzly undersize material and the jaw crusher discharge are conveyed to the coarse ore stockpile. The coarse ore is conveyed to the SAG mill and lime is added to the belt based on ore requirements. The grinding circuit includes one SAG mill and one ball mill. Discharge from the SAG mill is screened on one of two vibrating screens.

Screen oversize is conveyed to a cone crusher where it is crushed prior to being recycled to the SAG mill feed. Screen undersize and ball mill discharge flow by gravity to the hydrocyclone feed pump box. The slurry is pumped from the pump box to the hydrocyclones. The ball mill is operated in closed circuit with the hydrocyclones to produce the target grind size for the CIL circuit, which is nominally 150 µm. The underflow from the hydrocyclones is recycled to the ball mill feed; the overflow from the hydrocyclones discharges into an agitated surge tank, which feeds the grind thickener. Cyanide is added to the surge tank to being the leaching process prior to the grind thickener. Milk of lime can also be added to the surge tank if the lime silo is down, and dry lime cannot be added to the belt.

Ore from the Cortez Complex can either be processed on site in the oxide processing facilities
or transported to the Goldstrike roaster, Goldstrike autoclave, or Gold Quarry roaster, parts of the Carlin Complex, for refractory ore treatment.

HIGHER-GRADE OXIDE ORE MILLING
Mill No. 2, or the Pipeline mill currently includes crushing, semi-autogenous grind (SAG) mill, ball mill, grind thickener, carbon-in-column (CIC) circuit for the grind thickener overflow solution, CIL circuit, carbon stripping and reactivation circuits, and a refinery to produce gold doré. Plant throughput can reach up to 16,330 t/d depending on the hardness of the ore being processed. The plant is permitted for an annual total of 4.9 Mt/a.

An appreciable amount of gold is dissolved in the grinding circuit, so a CIC circuit is used to recover gold from the grind thickener overflow solution. The grind thickener underflow is pumped to the CIL circuit, which consists of eight CIL tanks, 1 ........