Chapter 15: Goldstrike Gold System, North Carlin Trend, Nevada, USA

The Eocene Goldstrike system on the Carlin Trend in Nevada is the largest known Carlin-type gold system, with an endowment of 58 million ounces (Moz) distributed among several coalesced deposits in a structural window of gently dipping carbonate rocks below the regional Roberts Mountains thrust. The 3.5- × 2.5-km Goldstrike system is bounded to the east by the Post normal fault system and to the south by the Jurassic Goldstrike diorite stock and is partly hosted in the favorable slope-facies apron of the Bootstrap reef margin that passes through the system. The carbonate and clastic sedimentary sequence is openly folded, cut by sets of reverse and normal faults, and intruded by the Jurassic Goldstrike stock and swarms of Jurassic and Eocene dikes, establishing the structural architecture that controlled fluid flow and distribution of Eocene mineralization. A proximal zone of permeability-enhancing decarbonatization with anomalous gold (>0.1 ppm) extends a few hundreds of meters beyond the ore footprint and lies within a carbonate δ18O depletion anomaly extending ~1.4 km farther outboard. The full extent of the larger hydrothermal system hosting Goldstrike and adjacent deposits on the northern Carlin Trend is outlined by a 20- × 40-km thermal anomaly defined by apatite fission-track analyses. The bulk of the mineralization is hosted in decarbonatized sedimentary units with elevated iron contents and abundant diagenetic pyrite relative to background. Gold is associated with elevated concentrations of As, Tl, Hg, and Sb, and occurs in micron-sized arsenian pyrite grains or in arsenian pyrite overgrowths on older, principally diagenetic pyrite, with sulfidation of available iron as the main gold precipitation mechanism. The intersection of a swarm of Jurassic lamprophyre dikes with the edge of the limestone reef provided a favorable deeply penetrating structural conduit within which a Jurassic stock acted as a structural buttress, whereas the reef’s slope-facies apron of carbonate units, with high available iron content, provided a fertile setting for Carlin-type mineralization. The onset of Eocene extension coupled with a southwestward-sweeping Cenozoic magmatic front acted as the trigger for main-stage gold mineralization at 40 to 39 Ma. All these factors contributed to the exceptional size and grade of Goldstrike.

North Bullion: Latest gold discovery on the Carlin Trend, Nevada

Gold Standard Ventures Corp. highlighted in their 2014 corporate presentation that “North Bullion and Central Bullion were both gravity- and CSAMT-led discoveries.” The North Bullion gold discovery demonstrates the continued effectiveness of geophysics as applied to gold exploration on the Carlin Trend of northeast Nevada. In combination with geologic understanding and target model development, geophysics provides a cost-effective exploration tool to guide exploration drilling. A sustained, incremental program is critical to realizing the full benefit geophysics brings to the exploration effort. In a program spanning five years, 3587 gravity stations and 51.7 line-km of controlled-source audio magnetotelluric (CSAMT) data have been acquired in a multistage approach. A major structure, called the Bullion Fault Corridor (BFC), hosts two known gold deposits over an 18-km strike length. Gravity, followed up with CSAMT, defines the BFC in detail, as well as hydrothermal alteration products related to the mineralization process. Drill and geologic confirmation is integrated with the geophysics to refine interpretations and adjust ongoing survey design. Application of gravity along the Carlin Trend has been actively used as a structural mapping tool for many years. However, combining the results with CSAMT is a more recent development, particularly along the Carlin Trend.

The Method for the Characterization of Carlin Trend Gold Ores

Carlin Trend gold ores are often refractory by conventional cyanid, for a number reasons:1) the gold is locked up within host sulfide minerals such as pyrite or arsenopyrite; 2) the ore contains carbonaceous compounds which are preg-robbing; 3) the gold is locked up in silicate minerals. An alternative technique called diagnostic leaching, originally developed by the Anglo American Research Laboratories in South Africa, has been used successfully to distinguish between a wide variety of different gold associations, including the proportion of preg-robbing gold in carbonaceous gold ores.