Toward a Global Carlin-Type Exploration Model: The Relationship between Eocene Magmatism and Diverse Gold-Rich Deposits in the Great Basin, USA

Late Eocene (~42-34 Ma) ore deposits of the Great Basin, U.S.A. have contributed the majority of this region’s precious- and base-metal production, including about
6438 tonnes Au (207 million oz), 38,879 tonnes Ag (1.25 billion oz), 21.7 Mt Cu, and
0.74 Mt Mo. Combined, these four metals would have a cumulative 2019EOY value of
about $489 billion (US), a remarkable 64% of which is attributable to gold, and 28%
to copper. In terms of precious metals, Eocene ore deposits have contributed about
75% of the 274 million ounces of total recorded Au produced in the Great Basin and
about 65% of the 1.93 billion ounces of recorded Ag produced. Gold production from
Eocene deposits of the Great Basin, which largely reflects the time since the Carlin
mine began producing in 1965, represents one third of the total U.S. gold production
since 1835 and an estimated 4% of all gold produced in the world.
Eocene ore deposits are restricted to the northern half of the Great Basin. Metal
production from Eocene deposits is strikingly partitioned in the Great Basin, with
81% of Au production derived from mines in north-central Nevada, and 99% of Cu
and 81% of Ag derived from mines in north-central Utah. Most Eocene Au production in Nevada is from three major mineral belts that host giant sedimentary rockhosted Carlin-type Au deposits: Carlin, Battle Mountain-Eureka, and Getchell. Three
major districts produced the bulk of Utah’s Cu, Mo, Au, and Ag production: Oquirrh,
Park City, and Tintic, with the vast majority of Cu, Mo, and Au coming from the supergiant Bingham Canyon porphyry Cu-Mo-Au mine (Oquirrh district). Eocene Au
deposits in northern Nevada contain more than 1.68 Kt of Au in reserves (54 million
troy ounces), and Bingham Canyon contains reserves of 2.7 Mt of Cu, 0.22 Mt of Mo,
and 0.15 Kt Au. This paper examines the strongly contrasting styles of mineralization
and metal endowment in Eocene ore deposits between Nevada and Utah.
Eocene gold deposits in Nevada are almost entirely disseminated in either calcareous sedimentary rocks or contact-metamorphosed and metasomatized sedimentary
rocks, and these deposits are collectively referred to here as sedimentary rock-hosted,
disseminated gold deposits (SHDG). Four styles of replacement-type SHDG occur in
northern Nevada: (1) gold skarns, (2) hornfels-margin gold, or distal disseminated
deposits, (3) Carlin-type gold deposits, and (4) shallow-formed Carlin-type deposits,
the latter closely linked to the Eocene surface. Of all Eocene gold deposits in Nevada,
“gold-only” Carlin-type deposits have contributed most to production, mainly from
eight giant deposits, although important Au production has come from all styles. The
four styles of SHDG are considered part of a continuum that span high-temperature
and relatively deeply emplaced to low-temperature and shallow-formed. SHDGs formed over a brief period from ~42–34 Ma, temporally and spatially coincident with
Eocene arc magmatism but predating large-magnitude Cenozoic upper-crustal extension. Individual districts mostly possess one predominant style, although more extensive mineral belts such as the Carlin trend and Battle Mountain-Eureka belt preserve
a range of deposit styles that reflects along-trend variations in the Eocene erosional
level.
Eocene deposits progressively change from Au- to Cu-dominant across the central to eastern parts of the Great Basin, respectively. This change in regional metallogeny mimics changes in the geochemical and isotopic character of magmatism across
the Eocene arc, although igneous rocks are mineralogically consistent and dominated
by intermediate composition andesite through dacite and their intrusive equivalents.
Some regional differences are rocks having high alumina saturation indices (ASI)
≥ 0.96), δ34S (> 7 ‰), and δ18O (10 ‰) and low oxygen fugacity (∆NNO ≈ 0–2; where
NNO is the nickel-nickel oxide experimental buffer) and copper contents in northcentral Nevada, compared with those having relatively low ASI, δ34S, and δ18O and
high oxygen fugacity and copper contents in north-central Utah. Initially oxidized Eocene mafic magmas in north-central Nevada interacted with significantly more crust
and crust that was relatively more carbonaceous and reduced than north-central
Utah magmas. Greater crustal interaction by Eocene magmas in north-central Nevada may stem from larger volumes of mafic magmas emplaced into the lower crust.
In addition, major variation in the thickness and composition of the middle and upper
crust from reduced and carbonaceous in the Neoproterozoic through Paleozoic slope
and basin setting in north-central Nevada to more oxidized, locally evaporite-bearing,
coarse clastic and carbonate shelf sequences in north-central Utah may have played a
fundamental role in modifying redox conditions of derivative magmas.
The range of Eocene SHDGs in northern Nevada defines a distinct intrusionrelated gold metallogeny that contrasts with deposits formed in many other continental arc settings, including the eastern Great Basin in Utah, which are typified by
porphyry Cu and Cu-Mo, polymetallic skarns and replacements, and high- and intermediate-sulfidation epithermal Au-Ag-(Cu) deposits. The reduced mineralogy and
geochemistry of ores and Au-dominant or Au-only character of SHDGs in northern
Nevada imply overall reduced ore fluids that fundamentally differ from highly oxidized fluids indicated for porphyry-related systems. Emplacement of the Eocene arc
far inboard of the plate margin into kilometers-thick carbonaceous slope and basinal
rocks of the Neoproterozoic through Paleozoic passive margin progressively reduced
and modified mid-crustal magmas from mafic to silicic compositions through assimilation. Thus, differentiated magmas may have been saturated in reduced sulfur, thereby forcing pyrrhotite (monosulfide solid solution) to precipitate, which partitioned Cu
over Au with consequent high Au/Cu ratios in derivative melts and ore fluids. More
favored, however, is that magmas remained sulfur undersaturated during most of
their ascent, and therefore maintained high mantle Cu and Au contents until interacting with carbonaceous and sulfide-bearing strata of the slope-facies passive margin at
high crustal levels (≤ 10 km). Such assimilation reduced magmas, caused sulfur and
other volatiles to saturate, and promoted fluid release, which due to fluid reduction,
strongly partitioned Au over Cu by a factor of ~100. Gold solubility and transport
to lower temperatures in magmatic-hydrothermal fluids would have been enhanced
by the unique buffering capacity coupled with the reduced nature of the slope and
basinal sedimentary units, allowing gold deposits to develop. Thus, continental arcs
emplaced into thick slope and basinal sedimentary sequences of passive margins are
considered a necessary component to form such reduced, gold-rich and copper-poor
ore deposits and are ideal terrains to explore for Carlin-type gold systems.
Key Words: Eocene, Great Basin, metallogeny, magmatism, Carlin-type, porphyry, gold,
copper