Introduction to Carlin Gold Deposits – For Geologists: Twin Creeks, Cortez, Gold Quarry, Goldstrike

INTRODUCTION
Welcome to the Geological Society of Nevada’s 2010 Symposium Field Trip, Introduction to Carlin Gold Deposits – for Geologists. 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: Twin Creeks on the Getchell trend, Cortez on the Battle Mountain-Eureka trend, and Gold Quarry and Goldstrike on the Carlin trend. Although different formations host ore on each trend, the primary host rocks are early 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 which has a characteristic “fuzzy” appearance (Fig. 1). Although the ore can appear different in different deposits, this is a function of variable oxidation of different host rocks. Alternatively, microscopic ore-stage mineralization from all deposits is nearly identical in appearance and chemistry. Ore-stage alteration accompanying the formation of pyrite includes decarbonatization, silicification, and fine-grained kaolinite and/or illite. This ore and gangue assemblage appears to have formed in response to fluid-rock reaction and sulfidation of host rock Fe. As pyrite precipitated, bisulfide-complexed elements adsorbed on the precipitating pyrite and were incorporated into the pyrite structure. These trace elements typically include, in addition to Au, significant As as well as lesser Sb, Hg, Tl, Cu and Te. During system collapse late-ore stage minerals, that can include orpiment, realgar, pyrite, marcasite, stibnite, quartz and calcite, precipitated in open space in response to dilution of ore fluids by meteoric water and system cooling. Ore fluids accessed the ideal host rocks via deep, high angle fractures and structures as the region experienced a transition from compression to incipient extension during the Late Eocene. Key features that characterize all of Nevada’s Carlin deposits are shown in Table 1.
The three-day field trip will leave Reno on Friday morning, May 14, and we will spend our first evening in Winnemucca and our second evening in Elko. From Reno, we will drive to Newmont’s Twin Creeks deposit on the Getchell district, which is hosted primarily by Cambrian-Ordovician Comus Formation. After touring Twin Creeks we will drive to Winnemucca where we will spend the night. On Saturday, May 15, we will travel to Barrick’s Cortez deposit on the Battle Mountain – Eureka trend where Barrick geologists will lead us on a tour of Cortez. In the afternoon we will drive to Newmont’s Gold Quarry deposit, a large low-grade deposit located on the Carlin trend. Following this visit we will head to Elko where we’ll spend the evening. On Sunday, May 16 we will drive back to the Carlin trend to Barrick’s Goldstrike property, a huge high-grade Carlin deposit. Following this visit we will head back to Reno.