Deposit Type
- Carlin-type
- Sediment-hosted
- Hydrothermal
- Stratabound (SMS)
Summary:
The mineral deposits along the Carlin Trend form a suite of deposits known as Carlin-type. Carlin deposits comprise stratabound disseminated gold mineralization hosted by Silurian-Devonian carbonate rocks that have been metamorphosed to varying extents. The deposits are hydrothermal in origin and are usually structurally controlled. The carbonate host rocks are part of an autochthonous miogeoclinal carbonate sequence exposed as tectonic windows beneath the Roberts Mountains allochthon. The allochthonous rocks are a sequence of lower Paleozoic dominantly siliciclastic eugeoclinal rocks that were displaced eastward along the Roberts Mountains Thrust over younger units during the Upper Paleozoic Antler Orogeny. The Carlin Trend is the largest concentration of gold deposits in North America. NGM and other companies have discovered over 40 deposits along the 64 kilometre long, north–northwest-oriented Carlin Trend. Gold deposits are generally hosted in a variable stratigraphic package of Ordovician through Lower Mississippian rocks. The preferential host rocks are autochthonous carbonate assemblage rocks that are now preserved in uplifted tectonic windows. All Carlin Trend gold deposits that have been discovered to date are either within the Bootstrap, Lynn, Carlin, and Rain tectonic windows, or proximal to them. Within specific deposits, Cretaceous and Tertiary dike swarms and a Jurassic-aged granodiorite stock (Goldstrike stock) may constitute as much as 15% of the mineralized material.
Host rocks are most commonly thinly-bedded silty or argillaceous carbonaceous limestone or dolomite, commonly with carbonaceous shale. Although less mineralized, non-carbonate siliciclastic and rare metavolcanic rocks can locally host gold that reaches economic grades. Felsic plutons and dikes may also be mineralized at some deposits. Deposits typically have a tabular shape and are stratabound, localized at contacts between contrasting lithologies but can also be discordant or breccia-related.
Mineralization consists primarily of micron-sized gold and sulfide grains disseminated in zones of siliciclastic and decarbonated calcareous rocks and are commonly associated jasperoids. Major ore minerals include native gold, pyrite, arsenopyrite, stibnite, realgar, orpiment, cinnabar, fluorite, barite, and rare thallium minerals. Gangue minerals typically comprise fine-grained quartz, barite, clay minerals, carbonaceous matter, and late-stage calcite veins.
Current models attribute the genesis of the deposits to:
• Epizonal plutons that contributed heat and possibly fluids and metals;
• Meteoric fluid circulation resulting from crustal extension and widespread magmatism;
• Metamorphic fluids, possibly with a magmatic contribution, from deep or mid-crustal levels; and
• Upper crustal orogenic-gold processes within an extensional tectonic regime.
Goldstrike - Betze-Post
The Betze-Post deposit, the largest on the Carlin Trend, is divided into sub-deposits which, from east to west, are the Deep Post, Post, Betze, West Betze, and Screamer. Other zones within the pit are North Betze, West Barrel, and North Screamer.
Five generations of pyrite mineralization have been recognized at the Betze-Post open pit. Early stages of diagenetic pyrite, and coarse-grained pyrite in the metamorphic aureole of the Goldstrike diorite, are barren. Early hydrothermal, very fine-grained pyrite and marcasite grains of the third generation are coated by a 25 µm thick rind and cut by micro-veinlets of arsenic and gold bearing pyrite of the fourth generation. Barren, late hydrothermal coarse-grained pyrite and marcasite is accompanied by barite and stibnite. Sulphides make up approximately 2% by weight of the ore, locally up to 20%.
Mineralization may be predominantly oxides, sulphides, or refractory or carbonaceous sulphides. Weathering alteration extends up to 200 metres in depth resulting in oxide mineralization, which overlies the refractory sulphides. Alteration has liberated gold by the destruction of pyrite and resulted in the formation of oxide and secondary sulphate minerals, which include goethite, hematite, jarosite, scorodite, alunite, and gypsum. The alteration is deepest in the Post deposit due to extensive fracturing and high pyrite content.
Gold Quarry
The mineralization of Gold Quarry is generally bounded on the northwest by the northeast-striking Chukar-Alunite Fault zone, and on the southeast by the north–northeast-striking Deep Sulfide Feeder Fault zone. Mineralization is preferentially located in the hanging wall of the Chukar-Alunite Fault Zone and in the footwall of the Good Hope Fault. Gold mineralization is disseminated, with higher gold grades concentrated adjacent to structures.
Tri-Star (Silverstar and Goldstar)
The gold deposits at the Tri-Star complex (formerly Genesis) are developed along the Tuscarora, Turquoise, and Ridge anticlines within the Lynn Window. They occur over an area of about 3.2 kilometres long by 1.9 kilometres wide. The Silverstar deposit occurs in the hinge of the Tuscarora anticline whereas the Bluestar Point, Bobstar, Goldstar, and Payraise deposits occur on the more western anticlines. The anticlines are intruded in the north by the 158 Ma Goldstrike intrusion, a thick composite diorite–granodiorite sill complex.
Mineralization is preferentially developed in laminated silty limestone and bioclastic debris flows of the Popovich and Roberts Mountains Formations, but locally can also occur in contact metamorphosed calc-silicate hornfels, Rodeo Creek Formation siliceous mudstone, siltstone and calcarenite, Vinini Formation mudstone/quartz hornfels, and fractured Goldstrike intrusive margins. Gold deposits generally occur where mineralizing fluids exploited thrust faults, conjugate northwestand northeast-striking normal faults, and anticline hinge zones. The deposits have dimensions that range from about 183 to 457 metres by 61 to 183 metres. Mineralization can be stratabound locally but is typically discordant to the formations along faults with thicknesses ranging from 15 metres to 91 metres.
Emigrant
Gold mineralization at the Emigrant deposit is located along the flanks of the Emigrant antiform with the majority of the mineralization being concentrated on the western limb in a shallow, southwestdipping tabular orebody located at the contact between siltstones of the Mississippian Webb Formation and limestones of the Devonian Devils Gate Formation.
Leeville (West Leeville, Turf, Four Corners Deposits)
Gold deposits of the Leeville underground Complex are hosted by lower Paleozoic carbonates of the Devonian Rodeo Creek Formation (Drc), Devonian Popovich Formation (Dp), and Siluro-Devonian Roberts Mountains Formations (SDrm), and a series of highly altered, undifferentiated dikes cross cutting stratigraphy. Gold mineralization is controlled by a combination of lithology and structure. Intrusive rocks occur as dikes and sills, and locally host mineralization. Lithologic hosts include Rodeo Creek, Popovich, and Roberts Mountains Formations that vary between sub-horizontal to moderately folded. Mineralized material consists of 60 – 90% quartz, 5 - 20% kaolinite, 1 - 17% carbonate, and 3 – 7% pyrite. Gold mineralization in the Leeville complex is wholly refractory.
Exodus and Northwest Exodus
Mineralization, primarily hosted in the Devonian Popovich Formation (Dp), is bound on the west by the Castle Reef Fault (CRF) and restricted to the east by the Eastern Dike Swarm (EDS). This structurally controlled system follows near vertical structural fabrics and crosses stratigraphic boundaries. The highest gold values are located adjacent to, or within, the steeply dipping Big Green Dike (BGD), Castle Reef Intrusive (CRI), and Eastern Dike Swarm (EDS). These intrusive filled structures appear to be the main conduits for Exodus gold mineralization. Sediment hosted mineralization appears to have no correlation.
Mining Methods
- Truck & Shovel / Loader
- Drift & Fill
- Longhole stoping
- Cemented backfill
- Paste backfill
Summary:
The Carlin Complex has three major open pit operations including Goldstrike, Gold Quarry and Goldstar (part of the Genesis/Tri-Star Complex). All three are truck and shovel operations. Blasting is required and blast patterns are laid out according to material type using rock type designations of hard, average, soft, or a combination of the three. The pit design varies between 6.1 metre to 12.2 metre (20 to 40 ft) benches and, where possible, up to 18.3 metre (60 ft) benches in the ore, though mined in 6.1 metre (20 ft) cuts. Slopes vary based on location.
The current mine equipment fleet will be used throughout the mine life and is shared with the other mines at the Carlin Complex. The number of loading and hauling units allocated to each deposit varies depending on the operational needs from the mine plans. The equipment list also includes the
auxiliary equipment needed to support mining and the re-handling of the ore from the stockpile pad into the mill feeders.
The Carlin Complex has three major operating underground mines including Goldstrike underground, Leeville and the Portal Mines (including Pete Bajo and Exodus/Northwest Exodus). All mines utilize drift and fill and/or long-hole stoping and are accessed by shaft or portals. Ground conditions vary greatly in the different mining areas. Poor conditions in some areas are due to increased brecciation and/or alteration of original structures. Oxidation affects rock strengths in some areas and requires corrosion-resistant ground support. Generally low-strength rock conditions are the key factor in the mine design and mining method selection.
The underground mines utilize three forms of backfill including cemented rock fill (CRF), uncemented run of mine waste, and paste fill.
Secondary egress is provided through a series of escape raises and declines. In addition, there are refuge chambers strategically located throughout the mine in accordance with NGM’s Nevada refuge policies. The current underground production mobile equipment fleet across the Carlin Complex consists of load-haul-dump units, haul trucks, jumbos, longhole drills, bolters, and roadheaders. In addition, there are many function-specific utility vehicles and personnel carriers. The underground mining fleet can be shared across the different NGM operations as needed by the integrated mine plan.