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SEG Foundation Student Field Trip No. 16

Carlin-Type Gold Deposits of Northern Nevada July 15–24, 2017

The 16th SEG Foundation Student Field Trip spent the better part of 10 days visiting eight mines

that are classic examples of Eocene Carlin-type mineralization, two Carlin exploration districts,

and two operations recently idled and looking to restart. Seventeen students from 11 countries,

three professional mentors, and two trip leaders made up the group. This trip investigated

“classic” Carlin-type deposits, in which gold is distributed “ionically” in pyrite in carbonaceous,

calcareous host rocks with limited evident spatial association with intrusive rocks. In addition,

participants examined Carlin-type deposits in similar facies of sedimentary rocks, but having

clearer spatial and temporal relationships to intrusive porphyries, which may be analogs of

intrusion-related systems that exist at significant depth below classic Carlin-type deposits.

After a comprehensive introductory briefing on the characteristics of deposit geology, the

group departed for Elko and spent the first several days visiting classic northern Carlin trend

deposits in the Bluestar district, including Goldstrike, and the Leeville, Tusc, and Gold Quarry

deposits in the Lynn and Maggie Creek districts. Barrick Gold’s Goldstrike mine is a massive

open-pit mining complex with mineralization particularly well developed in the Devonian

Popovich Formation. This formation, and particularly the basal Wispy member, contains

bioturbated, slope facies carbonate rocks that were highly permeable and reactive with and

receptive to acidic ore-forming solutions. The huge deposit reflects the ideal intersection of

high-angle structures that fed ore fluids into upright anticlines of highly reactive and structurally

prepared host rocks. Furthermore, this architecture was located beneath the Roberts Mountains

thrust, which diverted fluids laterally, enhancing fluid-rock reaction and mineralization. Deep

refractory mineralization currently being mined presents complex metallurgical challenges and

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ore types must be properly blended for dispatch to the heap leach, autoclave, or roaster for gold

recovery. The group also toured Barrick’s mill facilities and spent time at the core shed,

focusing on drill core that cut the Popovich Formation, the ideal host in this part of the Carlin

trend.

Examination of “classic” Carlin-type mineralization continued as the group headed south

on the northern Carlin trend to Newmont’s Leeville and Tusc deposits and also viewed a blast

during an overview of the Gold Quarry mine. The Leeville deposit is an underground operation

challenged by a lack of any consistent visible features that allow ore to be distinguished from

waste. The dominantly stratabound ore, fed by high-angle structures, occurs along a prominent

NW-trending fault corridor that extends from the Carlin mine (exhausted) through Leeville to the

Tusc deposit. Owing to a small difference in stratigraphic sequences defined independently by

Barrick and Newmont, Newmont places Leeville ore predominantly in uppermost Silurian-

Devonian Roberts Mountains Formation, although the ore occurs in approximately the same

rocks as at Goldstrike, where they are called the Wispy Member at the base of the Popovich

Formation. As at Goldstrike, alteration includes decalcification, silicification, and argillization.

Newmont’s Gold Quarry deposit differs from most northern Carlin trend deposits in that

mineralization is primarily hosted by the informally named Devonian Rodeo Creek formation

and is dominantly structurally controlled at the intersection of two prominent structures. The

Rodeo Creek is relatively unreceptive and, although the deposit is very large, grade is lower than

grades typical of the Popovich Formation. A tour of the now-dormant Tusc open pit highlighted

mineralization controlled by contractional structures.

From the northern Carlin trend, the group headed south and spent a day touring the Gold

Standard Ventures exploration prospects in the Pinon Range. Here, the classic Carlin setting is

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replicated to a large degree, although this region lies on the edge of the paleocontinental shelf

and east of the Roberts Mountains thrust fault. The facies are different but the loci for

mineralization is the same: receptive facies capped by aquitard units that focused upwelling ore-

forming solutions laterally away from feeder faults into receptive strata forming stratigraphically

controlled mineralization. The Pinon deposit, discovered during 1988 by Newmont, is a direct

analogue of Newmont’s Rain mine, straddling the Devonian-Mississippian boundary and

exhibiting dissolution collapse breccia ore control. The Dark Star prospect is of similar vintage,

but occurs upsection in Pennsylvanian-Permian carbonate and siliciclastic rocks, and is

characterized by a different style of mineralization that is dominated by control from subvertical

feeder-faults.

The dormant open pits of the Eureka district at the south end of the Battle Mountain-

Eureka trend were the focus on day 4. The Lookout Mountain and Windfall deposits are

significantly smaller and lower in grade than the deposits of the Carlin trend. They occur in

Cambrian and Ordovician rocks near the central axis of the Eureka culmination, a regional-scale

anticline trending N-S through the district. Significantly, the deposits are east of the Roberts

Mountains thrust and the presence of Eocene volcanic rocks within the open pits indicates that

mineralization likely formed close the surface rather than at a few kilometers depth, as occurs for

the Carlin trend deposits. Mining ceased at the deposits during the 1980s after they produced a

few hundred thousand ounces of gold from highly reactive and “sanded” Hamburg Dolomite

where capped by low permeability Dunderberg shale in another example of a receptive facies

beneath an aquitard unit. The contact also shows evidence of paleo- and more recent karsting.

The day ended at the overlook of Barrick’s Ruby Hill open pit with views of the pit wall failure

that terminated mining activity in 2013, after production of ~1.5 Moz of gold.

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From Eureka, the group headed north-northwest along the Battle Mountain-Eureka trend

to Barrick’s Cortez Hills deposit and the newly discovered Goldrush deposit. Cortez Hills is a

large open-pit and underground operation, exploiting high-grade mineralization in a funnel-

shaped collapse breccia in the Devonian Wenban Formation, which is correlative to the Popovich

Formation on the Carlin trend and the underlying Silurian-Devonian Roberts Mountains

Formation. Polylithic breccia clasts of varying facies, alteration, grade, and degree of oxidation

make up this deposit. Although located in host rocks that are slightly different from the north

Carlin trend, this large and high-grade deposit has characteristics very similar to the northern

trend. Goldrush may pose challenges to mining. The deposit lies on the flank of Mt. Tenabo,

above Cortez Hills and near the dormant Horse Canyon mine, and like Cortez Hills, it occurs

within Devonian Wenban. The solution collapse breccia deposit is rather linear and stratabound,

following an anticline. At close to 6 km in length, nearly a kilometer wide and, on average, 30 m

thick, it will require extensive underground development.

From the Cortez district, the group continued north to the Potosi (Getchell) district for

visits to the Twin Creeks and Turquoise Ridge mines. Newmont’s Twin Creeks mine is a

massive open-pit operation in its later years, hosted by dolomitic, calcareous shales and siltstones

of the Cambro-Ordovician Comus Formation and, to a lesser extent, in Pennsylvanian-Permian

Etchart Formation. Mineralization is best developed in the crest of the north-trending Conelea

anticline in sedimentary carbonate rocks interbedded with basalts. Underground mining of the

Vista shear zone recently started in structurally prepared and mineralized Ordovician Valmy

pillow basalts. Within the structure, Eocene-aged Carlin-type gold overprints Cretaceous base

metal sulfide mineralization. Nearby, Barrick’s Turquoise Ridge mine is also hosted in the

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Comus Formation in carbonate rocks, as subhorizontal lenses developed predominantly in the

hanging wall of the Getchell fault and along the contact of Comus with a dacite dike.

Mineralized Comus here is largely comprised of debris flows with soft sediment textures

formed at the base of the continental shelf. The deposit is highly altered, resulting in a host-rock

sequence that has very low rock quality, requiring careful mine planning to achieve safe and

economic extraction. Mineralizing solutions are interpreted to have travelled up along the

Getchell fault and into structurally prepared hanging-wall rocks and a high-grade breccia zone

developed within the sedimentary breccia unit adjacent to the dike. Some of the host units are

very high in organic carbon, requiring expensive metallurgical recovery processes.

The final day was devoted to the Battle Mountain district and mineralization associated

with intrusions at Phoenix and McCoy-Cove. Newmont’s Phoenix open-pit mine is located on

the south flank of Battle Mountain. This Au-Cu-Ag-Pb-Zn skarn system is centered on the 38

Ma Eocene Copper Canyon diorite porphyry stock. Paleozoic sediments within 3.2 km of the

stock have undergone extensive contact and hydrothermal metamorphism. A major N-S

structural corridor in the district played a role in the localization of the ores. Current reserves are

within the Pennsylvanian-Permian aged overlap assemblage of the Battle Mountain, Antler Peak,

and Edna formations. The nearby McCoy-Cove deposit, located in the Fish Creek Range,

southeast of Phoenix, was mined by both open-pit and underground methods until 2002. Since

2014, Premier Gold Mines has been exploring the property. McCoy is a gold skarn deposit

hosted in Triassic limestone intruded by an Eocene granodiorite stock. The nearby Cove Au-Ag

deposit is within a broad NW-trending anticline developed within the same Triassic carbonate

sequence. Gold-silver mineralization is characterized by early polymetallic sulfide veins and

manto that are overprinted by Carlin-style gold mineralization that is locally silver rich. The

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deposit is highly structurally controlled, with subvertical faults acting as pathways for

mineralizing solutions to ascend until they encountered and reacted with laterally extending

receptive and reactive sedimentary rocks. These deposits have been variously interpreted as

skarn or porphyry-related deposits, and are analogs of intrusion-related systems that exist at

significant depth below classic Carlin-type deposits.

These student field trips would not be possible without the preparation and support of

many individuals. We acknowledge the management and staff from all the involved companies

and operations visited, including Barrick Gold, Newmont Mining, Gold Standard Ventures,

Timberline Resources, and Premier Gold Mines. Mentors Kevin Heather (Regulus Resources),

Doug Kirwin (Consultant), and Borden Putnam (Principal Analyst, Mione Capital) contributed a

great deal to in-field and evening discussions. We also thank SEG staff and the SEG Student

Field Trip Program Committee (Borden Putnam, Rael Lipson and Brock Riedell) for invaluable

assistance, and SEG members for their generous financial support of this program.

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Caption: Trip participants from left to right: Henning Seibel, Thomas Dols, Jenna Kaempfer,

Borden Putnam, Fred Transburg, Connor Allen, Kevin Heather, Doug Kirwin, Mate Lesko,

Collette Pilsworth, James Davey, Nansen Olson, John Muntean, Soelena Wood, Rajarshi

Chakravarti, Natalia de Azevedo, Tuna Ercivan, Maria Cherdantseva, Victor Torres Pacheco,

Kathryn MacWilliam, Elizabeth Hollingsworth, Jean Cline, and Barrick geologists Chelsea

Raley and Amy Tuzzolino.