Introduction to Carlin Gold Deposits – For Geologists

Welcome to the Geological Society of Nevada’s 2015 Symposium Field Trip, Introduction to Carlin
Gold Deposits. The many Nevada Carlin deposits are found within several trends and districts across a
large portion of northern Nevada. Although deposits extend across a large region, the ore stage
mineralogy and alteration assemblages at most deposits are incredibly similar, reflecting similar
formation processes during incipient extension, approximately 42 to 34 Ma. We will visit four classic
“Carlin-type” gold deposits that are located on three different trends: Turquoise Ridge on the
Getchell trend, Cortez Hills/Pediment on the Battle Mountain-Eureka trend, and Emigrant
and Gold Quarry on the Carlin trend. Although different formations host ore on each trend, the
primary host rocks are typically early to mid-Paleozoic carbonaceous and pyritic, silty calcareous
rocks that formed in response to continental rifting. Primary mineralization on all trends is
remarkably similar and consists of microscopic trace element-rich pyrite that is soft, relatively
unstable, and has a characteristic “fuzzy” appearance. Although the ore can appear different in
different deposits, this is largely a function of the presence of different host rocks and variable
oxidation of those rocks. Microscopic ore-stage mineralization and alteration from all deposits is,
alternatively, nearly identical in appearance and chemistry. Ore-stage alteration accompanying the
formation of pyrite includes decarbonatization or removal of primary carbonate minerals,
silicification in the form of jasperoid replacement of carbonate minerals, and fine-grained illite
and/or kaolinite replacement of earlier-formed alumino-silicate minerals. This ore and gangue
assemblage formed in response to reaction between acidic and fairly low temperature (~200-225°C)
ore fluids and reactive carbonates, and sulfidation of host rock Fe. Gold and other bisulfidecomplexed
trace elements in the ore fluid were liberated as sulfur combined with Fe in the rock to
form ore-stage pyrite. Ionic Au and other available cations were incorporated into the ore-stage
pyrite structure. These trace elements typically include significant As well as lesser Sb, Hg, Tl, Cu
and Te in addition to Au. During system collapse and cooling owing to eventual influx of meteoric
water, late-ore stage minerals potentially including orpiment, realgar, pyrite, marcasite, stibnite,
quartz and calcite, precipitated in open space. The tectonic trigger that initiated ore formation is
interpreted as related to the incipient transition from compression to tension during the Late
Eocene. Ore fluids accessed ideal host rocks via deep, high angle fractures and structures as old as
Precambrian that were reopened as the region was extended.
During this three-day field trip we will leave Reno on Friday morning, May 14, and spend two
evenings in Elko. From Reno, we will drive to Barrick’s Turquoise Ridge deposit on the Getchell
district, which is hosted primarily by Cambrian-Ordovician Comus Formation. After touring
Turquoise Ridge we will drive to Elko where we will spend the night. On Saturday, May 15, we will
travel to Newmont’s Emigrant deposit on the central Carlin trend where Newmont geologists will
lead us on a tour. In the afternoon we will drive to Newmont’s Gold Quarry deposit, a large lowgrade
deposit also located in the central Carlin trend. Following this visit we will head back to Elko
for the evening. On Sunday, May 16 we will drive to the Battle Mountain-Eureka trend to Barrick’s
Cortez property, a huge high-grade Carlin deposit. Following this visit we will head back to Reno.