Twin Creeks is a sediment-hosted disseminated gold deposit.

The Twin Creeks deposit is hosted by black calcareous shales, siltstones, cherts, and basaltic hydroclastic tuffs of Early Ordovician age Comus Formation. Igneous rocks include sills and lava flows of basalts of tholeiitic-to-alkalic composition, and at least one sill of ultramafic composition. The rocks have been hydrothermally altered, including decalcification, silicification, dolomitization, and minor sericitization. The ore contains very high concentrations of As (avg. 5600 ppm, max 38.5 wt%), Sb (avg. 763, max 2.2 wt%), Hg (avg. 25 ppm, max 0.1 wt%, and a high Au/Ag ratio (>10:1). The deposit occurs within a north-south trending belt of gold mineralization that is at least 5.6 Km long and 300-450 m wide. It appears to be localized by a major, deep-seated structural zone (“suture”), and by favorable calcareous lithologies within a major north-west trending overturned anticline. The deposit is concealed under coalescing alluvial material consisting of three lithologic units. The lower unit is thin and discontinuous, composed of gravel containing chert, siltstone, shale and altered basalt. The middle unit is the most voluminous and composed of limestone gravel. The upper unit is fluvially reworked, windblown loess up to 7 m thick. Total overburden thickness is 12-200 m over the mine area. (Bloomstein, 1991).

Twin Creeks lies at the northwest edge of the Kelly Creek drainage and the southeast flank of the Dry Hills, an outlier of the Osgood Mountains within the Getchell Mining District (Figure 1.1). The Dry Hills and Osgood Mountains are comprised of tilted Miocene basalt, and a complexly folded and faulted sequence of early and late Paleozoic sedimentary and basaltic rocks that have been deformed during two major orogenic events.

A large Cretaceous granodiorite pluton underlies much of the east-central part of the Osgood Mountains, southwest of Twin Creeks and in close proximity to the Getchell Mine and Pinson Mine. Miocene basalt caps the northern Dry Hills, and both Eocene and Miocene volcanic rocks make up sizeable igneous centers to the east and north of Twin Creeks. Topography descends from a high point in the Dry Hills northwest of Twin Creeks, with a gradient of about 100 feet per mile, to the south and east. A blanket of young alluvium thickens southeastward, reaching a thickness of over 700 feet. The Mega gold deposit was overlain by 50-700 feet of unmineralized alluvium. Gold mineralization at the Twin Creeks occurs at the northeast end of the Getchell Trend containing several sediment-hosted Carlin-type gold deposits.

Disseminated gold mineralization in Lower Paleozoic calcareous sedimentary host rocks at Turquoise Ridge is a characteristic of Carlin-type gold. However, the Tmquoise Ridge ore body occurs at greater depth than many Carlin-type gold deposits. The Turquoise Ridge ore body is approximately 244 meters in width, 305 meters in length (Getchell Gold Corp., I 996), and extends from 420 meters to 840 meters below the current surface at Turquoise Ridge (Bercntsen et aL I 999).

At the Turquoise Ridge deposit, ore develops in metamorphosed carbonaceous and siliceous sedimentary rocks. The carbonaceous unit primarily consists of calcite and dolomite. Quartz is the main constituent of the siliceous unit, and minor microcline, illite, biotite, calcite, and dolomite are present. Assays indicate gold is present in altered host rocks of both types.

Three types of pyrite are present in Turquoise Ridge samples: (l) euhedral to subhedral diagenetic pyrite in unaltered host rocks, (2) euhedral to subhedral pyrite in pre-ore stage quartz veins, (3) finely crystalline pyrite in high-grade samples. Ore-stage pyrite crystals are rich in trace metals, particularly arsenic, and contain gold. Gold-bearing pyrite crystals are consistently present withinjasperoid quartz crystals, along pre-ore quartz and ore-stage jasperoid quartz crystal boundaries, and along fractures in oreore stage quartz crystals.

Pre-ore stage mineral assemblage consists of quartz, pyrite, and marcasite in veins that are spatially associated with a Cretaceous granodiorite intrusion. Pre-ore stage minerals in ore-grade samples are fractured and crosscut by ore-stage minerals.

Petrographic studies indicate a precipitation sequence of (I) pyrite with gold, (2) jasperoid quartz, (3) stibnite, (4) fluorite, (5) orpiment, (6) realgar, and (7) calcite during the gold hydrothermal event.

Turquoise Ridge Surface is the Newmont’s former Twin Creeks property.

Turquoise Ridge Surface uses conventional open pit mining methods including drilling, blasting, loading, and hauling.

Surface mining operations occur in two developed open pit areas: Vista Pit to the north and Mega Pit to the south. Vista Pit measures approximately 1.1 km north to south, 1.0 km east to west, and the current pit depth is over 213 m. Mining in 2019 occurred in Vista 8 (V8), which will continue to deepen the pit by another 91 m in the ultimate design. Mega Pit measures 4.0 km overall north to south, and approximately 1.6 km east to west. The current pit depth is over 365 m and will deepen by approximately 48 m in the ultimate design.

ROM higher grade oxide ore from the Turquoise Ridge Surface sources are blended for Au grade, hardness, and carbonate content and fed to the Juniper Mill. The Juniper mill consists of a variable speed SAG mill operating in closed circuit with a discharge screen. SAG mill product is fed to a ball mill operating in closed circuit with cyclones. 1.5 MW installed power;18 ft x 6.5 ft (6 m x 2 m) 900HP SAG (Variable speed drive), turns both directions, 11.5 ft x16.5 ft (3.8 m x 5.5 m) 1150 HP ball mill. Final product grind size is 90% - 200 mesh. Cyclone overflow product is fed to the neutralization circuit. Here the carbonate in the oxide ore is used to neutralize the acidic autoclave discharge slurry. The combined oxide slurry and autoclave discharge slurry are further neutralized with lime before treatment in the CIL circuit. The CIL circuit is used to concurrently leach gold from the ore and adsorb it onto activated carbon. The final tailings slurry is pumped to the tailings containment ........