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Carlin Trend

The Carlin Trend

The Carlin Trend is northwest alignment of sedimentary rock-hosted gold deposits in north-central Nevada that extends for approximately 80 km from Gold Standard’s Dark Star deposit in the southeast, to the Arturo-Dee deposit in the northwest.  More than 40 separate oxide and refractory gold deposits have been delineated along the Carlin Trend, and more than 26 open pit and underground mines have been developed (Teal and Jackson, 2002). Since the discovery of micron-size, disseminated gold mineralization in 1961, more than 84 million ounces of gold have been produced from the Carlin Trend (Muntean, 2016).  The Carlin Trend hosts one of the largest gold concentrations in the western hemisphere.  Resources and reserves on the Carlin Trend exceeded 31 million ounces of gold as of 2015 (Barrick, 2016 and Newmont, 2016).

Gold continues to be found on this productive trend.  Recent developments in the northwest and central parts of the trend include the discovery of the Arturo deposit, significant expansion of the Leeville and Turf deposits, development of the Exodus and Pete Bajo deposits, and expansions of the Genesis and Gold Quarry deposits1.  At the southeast end of the trend, recent work includes development of the Emigrant deposit1; and Gold Standard’s discoveries at the North Bullion and North Dark Star deposits, and expansion of the Pinion and Main Dark Star deposits2. Gold deposits occur in clusters and have a periodicity with respect to certain geological features – some of which are described below.

Since the inception of mining at the original Carlin Mine in 1965 to the early 1990’s, gold ore mined from open pits was processed at oxide mills or heap leach facilities.  The mid 1990’s was a transitional period where open pit mining for oxide and refractory mineralization continued concurrently with the development of underground mines from shafts or declines extending outward from existing open pits.  Underground development targeted higher-grade, typically carbon-sulfide refractory gold ore.  To process the refractory ore, two whole-ore roasters were constructed – one at the Gold Quarry mining operation (Newmont) and the other at Goldstrike (Barrick).  Both companies blend multiple refractory ore types from a variety of gold deposits to enhance efficiencies within the roasting process.  Currently a large variety of ore types from across the Carlin Trend are processed via conventional heap leaching, thiosulfate heap leaching, milling, roasting and autoclaving.

Disseminated gold deposits along the Carlin Trend are hosted in Paleozoic age (250 to 541 million year old) sedimentary rocks.  These rocks were deposited beneath the ocean surface on the continental shelf and slope on the western edge of the North American continent.  Preferred host rocks are limestone with or without quartz silt grains, making for a reactive and permeable rock.  Other sedimentary rock types host gold where permeability is enhanced by fracturing due to faults, folding or dissolution of the rock.

Three distinct periods of igneous intrusive activity are documented on the Carlin Trend.  Intrusive rocks ranging in composition from rhyolite to diorite have been dated from the Jurassic (145-201 million years old), Cretaceous (66-145 million years old) and Tertiary (Eocene 34-56 million years old).  These igneous rocks occur as stocks, dikes and dike corridors, and sills.  Four distinct magmatic centers are recognized on the Carlin Trend – each center focusing multiple pulses of magma that produced igneous-cored domes with hornfelsed halos (Ressel and Henry, 2006).  From northwest to southeast on the Carlin Trend these magmatic centers / domes include:  the Richmond Dome, the Maggie Creek Dome, the Rain Dome, and the Railroad Dome on Gold Standard’s Railroad-Pinion Project.  Importantly, host rocks occur at the surface or shallow depths within and marginal to these domes.  Based on extensive age dating of igneous rocks and hydrothermal alteration, and observation of cross-cutting relationships, the age of gold mineralization on the Carlin Trend best correlates with Eocene age igneous activity.

Although volumetrically minor, lamprophyre dikes have been identified within and proximal to many gold deposits along the Carlin Trend, including some of the gold deposits on Gold Standard’s Railroad-Pinion Project.  Lamprophyre dikes are indicative of deep tapping faults that may have served as conduits for gold mineralization.  The presence of lamprophyres serves as a link between the Carlin Trend and other large gold belts around the world.

Similar to the igneous activity, the Carlin Trend has been the subjected to multiple structural deformation events including emplacement of the Roberts Mountains thrust; two contractional phases that produced north, northwest, west-northwest and northeast trending folds; and, moderate to high-angle normal faults (Rhys et. al., 2015).  Gold mineralization occurs in structural culminations and intersections produced by convergence of folds, faults, and favorable, reactive lithologies (Rhys et. al., 2015).  Gold bearing fluids flowed upward into these favorable structural settings and were commonly trapped beneath or against an impermeable rock type or fault, much like a trap in an oil field.  At the deposit scale, collapse and hydrothermal breccias can be an important control on gold mineralization.

Gold was deposited from hydrothermal (hot water) fluids.  These hot, slightly acidic, metal-bearing solutions changed, or altered, the mineralogy and chemistry of the rocks they passed through.  The altered rock, “alteration halo”, surrounding these deposits is much larger in volume than the gold deposit itself, and serves as a guide in the exploration process for these deposits.  Important alteration types are listed below.

Decalcification (decarbonitization)

  • the removal of carbonate from calcareous rocks by slightly acidic gold-bearing fluids

Silicification

  • the addition of silica (quartz) through replacement or veining of a given rock type
  • “jasperoid” – a decalcified and pervasively silicified carbonate rock

Sulfidation

  • introduction of fine-grained, gold-bearing pyrite – often called “sooty sulfides” (e.g. pyrite, arsenical pyrite or marcasite)

Oxidation

  • may occur as a supergene or hypogene process whereby sulfide minerals (pyrite) are exposed to oxygenated fluids to form iron oxide minerals such as hematite, limonite or goethite
  • vertically-extensive oxidation zones may exceed 300m in some gold deposits
  • many Carlin Trend gold deposits are vertically-zoned where an oxidized gold zone transitions to a reduced gold zone at depth

Carbon

  • black, residual and/or hydrothermal carbon is common in reduced zones and on the margins of gold deposits

Dolomitization

  • dolomite or ferroan dolomite occurs as replacement flooding or as veins

Argillic Alteration

  • formation of clay minerals such as kaolinite, illite, montmorillonite, dickite

Common Accessory Minerals

  • barite, orpiment, realgar, stibnite

Certain elements typically accompany gold in Carlin systems. The elements include:  arsenic, antimony, mercury, thallium, barium, zinc and silver.  Concentrations of these pathfinder elements in soil, rock, and stream sediment sample surveys, and in drill samples, guide exploration for gold.

Microscopic gold occurs within rims on pyrite or as micron-size native gold grains.  Visible gold is very rare along the Carlin Trend and has only been documented at a few select locations.

Barrick Gold Corporation (Barrick), 2016, Annual Report 2015, March 24, 2016.

Muntean, J.L. 2016. Overview – The Nevada Mineral Industry 2014 in Nevada Bureau of Mines and Geology Special Publication MI-2014, Mackay School of Earth Sciences and Engineering, College of Science, University of Nevada, Nevada, USA, p 3-12.

Newmont Mining Corporation (Newmont), 2016, Newmont Reports Reserve and Resources with Notable Additions, Press Release February 17, 2016.

Ressel, M.W. & Henry, C.D., 2006, Igneous geology of the Carlin trend, Nevada: Development of the Eocene plutonic complex and significance for Carlin-type gold deposits: Economic Geology, v. 101, p. 347-383.

Rhys, D., Valli, F., Burgess, R., Heitt, D., Griesel, G. and Hart, K, 2015.  Controls of Fault and Fold Geometry on the Distribution of Gold Mineralization on the Carlin Trend, in New Concepts and Discoveries, Pennell, W.M., and Garside, L.J., eds., Geological Society of Nevada 2015 Symposium, Reno, Nevada, p. 333-389.

Teal, L. and Jackson, M., 1997. Geologic Overview of the Carlin Trend Gold Deposits and Description of Recent Deep Discoveries, in Carlin-Type Gold Deposits Field Conference edited by Peter Vikre, Tommy B. Thompson, Keith Bettles, Odin Christensen, and Ron Parratt, Society of Economic Geologists Guidebook Series, Volume 28, p 3-37.

 

1 Mineralization discovered elsewhere on the Carlin Trend may not be indicative of mineralization on Gold Standard’s Railroad-Pinion Project.

2 To date Gold Standard has not performed any preliminary economic or other study of the Pinion, Dark Star and North Bullion deposits.

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