Goldrush: Lessons Learned from the Latest Giant Gold Deposit Discovered in Nevada
Published:January 01, 2013
Kevin D. Creel, Mark A. Bradley, 2013. "Goldrush: Lessons Learned from the Latest Giant Gold Deposit Discovered in Nevada", Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings, M. Colpron, T. Bissig, B. G. Rusk, J. F. H. Thompson
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Goldrush is a Carlin-type sedimentary rock-hosted disseminated gold deposit located within the Cortez mining district on the Battle Mountain-Eureka trend, Nevada, USA. Goldrush is the third giant gold deposit (>310 metric tons Au or 10 Moz Au) discovered in the district after Pipeline (1991) and Cortez Hills (2002), and contains a measured and indicated resource of 59.8 Mt @ 4.35 g/t and an inferred resource of 39.2 Mt @ 4.52 g/t as of the end of 2012. Goldrush is concealed beneath unmineralized Paleozoic rocks as well as Tertiary and Quaternary postmineral tuffs, volcaniclastic sediments, and gravel ranging from more than 100 m to more than 300 m thick. The mineral system is tabular and continuous over a thickness of up to 70 m, a width of up to 250 m, and extends along strike for at least 4,000 m. Gold mineralization occurs within extensive zones of decarbonatization and silicification spatially associated with a stratigraphic horizon containing fossiliferous debris flows in thrust-faulted and folded Devonian carbonate rocks. The system is marked by a large stratiform silicified and sulfidized breccia horizon from 15 to 70 m thick that extends more than 7 km on a north-northwesterly strike; the strike length and continuity of this breccia zone make Goldrush unique compared with other Great Basin Carlin-type gold deposits. Gold occurs as submicroscopic inclusions within fine-grained pyrite, similar to other Carlin-type gold deposits in Nevada. The Goldrush discovery is attributed to a multiyear program utilizing open-pit and field mapping, detailed field, drill hole, and geochemical observations, and relogging of historic drill holes to construct new district- and deposit-scale geologic models. Barrick Exploration management provided strong support via a systematic, model-driven assessment process and funded deep drilling that ultimately resulted in the discovery. Persistence also played an important role as the discovery emerged over several years.
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Tectonics, Metallogeny, and Discovery: The North American Cordillera and Similar Accretionary Settings
The northern Pacific Rim—for the purposes of this contribution—comprises the Mesozoic and Cenozoic magmatic-arc and associated terranes of eastern China, Korea, Japan, the Russian Far East, Alaska, Yukon, British Columbia, the western United States, and Mexico. This ~1,800-km-long segment of the Pacific Rim is marked by a broad spectrum of metallogenic environments and mining jurisdictions, which combine to dictate where and how exploration is conducted and the overriding character of any resulting discoveries.
This summary report commences with a brief metallogenic overview of the northern Pacific Rim, with particular attention paid to the world-class Mesozoic and Cenozoic ore deposits that define the region’s premier metallogenic provinces. This is followed by a summary of the relative attractiveness of the region’s various mining jurisdictions, as recorded by recent exploration activity. The major discoveries made along the northern Pacific Rim, particularly during the past half century, are then placed in this metallogenic and regulatory context as a basis for determining the successful exploration methodologies employed. This discovery track record is then used to predict what the future of exploration in this vast and varied region may hold.
Much of the northern Pacific Rim, from eastern China and the Russian Far East in the northwest through Alaska to western parts of Canada, the United States, and Mexico in the southeast (Fig. 1), is characterized by a complex array of oceanic, accretionary prism, magmatic arc, and back-arc basin terranes and associated microcontinental blocks accreted to the North China, Siberian, Hyperborean, and North American cratons, mainly during Mesozoic times (Coney et al., 1980; Campa and Coney, 1983; Kojima, 1989; Nokleberg et al., 2005; Yakubchuk, 2009). The metallogeny of these tectonic collages is dictated by various combinations of pre-, syn-, and postaccretion ore-forming events, the last of which are generally preeminent, except in British Columbia (Nokleberg et al., 2005; Nelson and Colpron, 2007).
Although the Meso-Cenozoic metallogeny of the northwestern and northeastern Pacific quadrants displays some similarities, it is the contrasts that are most marked. The main contrasts stem from the preeminence of tin, tungsten, and antimony in eastern China, Korea, Japan, and the Russian Far East and of copper and silver in Western Canada, the conterminous United States, and Mexico. Nonetheless, both the northwestern and northeastern Pacific quadrants are exceptionally well endowed with gold and molybdenum deposits. The northeasternmost Russian Far East, Alaska, and Yukon Territory display elements of both northwestern and northeastern Pacific metallogeny (Fig. 1). These metallogenic contrasts between the northwestern and northeastern quadrants result in China being the world’s leading producer of tungsten, tin, bismuth, and antimony, mostly from its eastern Mesozoic metallogenic province.