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.
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.
Goldstrike - underground mines
Gold mineralization in the underground mine is subdivided into East Banshee, West Banshee, Meikle, South Meikle, (East) Griffin, Extension, West Griffin, Rodeo, Barrel, West Rodeo, and North Post deposits and sub-deposits. The sulphide mineralization is associated with silicification and argillization, and there is little or no oxide mineralization. In sulphide mineralization, the gold is intimately associated with very fine-grained pyrite and marcasite. Associated sulphide minerals include arsenopyrite, realgar, orpiment, and stibnite. Gangue minerals include quartz, calcite, and barite. Realgar and orpiment are generally low in abundance; however, these minerals are locally common in stockwork veinlets, fracture fillings, and breccia matrices.
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.