Summary:
The mineral deposits along the Carlin Trend form a suite of deposits known as Carlin type deposits.
Carlin mineral systems are a unique class of epigenetic ore deposits with enigmatic origins. They are unique due to the size and scale of deposits that formed exclusively within NE Nevada during the middle-Eocene period (42-35 Ma), establishing this area as a world-class gold complex. Carlin mineral systems are enigmatic due to their unknown, distal source of gold, as well as deposit characteristics and hydrothermal conditions that culminate to being atypical of most documented mineral systems.
The Goldstrike deposit, is the largest gold deposit on the Carlin Trend divided into three sub-deposits; Goldstrike OP, Goldstrike UG, and the Ren Project.
Mineralization is dominantly hosted within the Popovich, and lesser so within the Rodeo Creek and Roberts Mountain formations. Geometrically complex breccia development occurs in variably orientated structures, which concentrate the highest-grade mineralization. Jurassic dike swarms of varying compositions also utilize these structures and may also be a significant host to mineralization, particularly within the Goldstrike Open Pit. Consequently, disconcordant mineralization can be hosted within less favorable units such as the Bootstrap limestone.
The gold endowment at the Goldstrike deposit is uniquely large, attributed to its favorable structural preparation. The deposit is adjacent to the felsic Goldstrike stock that intruded during Jurassic extension resulting in a district-scale flexure of the regional north-trending axial trace of the regional Phase 1 anticline. Strain partitioning focused subsequent Jurassic dike swarms during a tectonic switch back to contraction reactivating local structures as dominantly oblique slip further enhancing permeability pathways. The largest structural feature that resulted was the district-scale PostGenesis fault zone along the steep limb of the Tuscarora anticline, where Mineralization is dominantly focused.
The culmination of deformation events created a significant volume of highly permeable, continuous structural zones often characterized by dissolution and fault breccia. Structural preparation thus hyper-focused intense fluid-rock interaction with highly reactive rocks concentrating large quantities of high-grade gold mineralization with great efficacy.
Mineralization is dominantly refractory in Goldstrike UG and the Ren Project. Significant oxide material is present in Goldstrike OP, with gold associated with secondary oxides. Weathering alteration extends up to 200 m in depth resulting in oxide mineralization, which overlies the refractory sulphides. Gold is alteration liberated through the chemical degradation of pyrite resulting in the formation of iron oxides and secondary sulphate minerals, which include goethite, hematite, jarosite, scorodite, alunite, and gypsum
The dimensions and orientations of the modelled mineralized domains for Goldstrike deposits:
Length - 300 – 4,500 m
Width - 30 – 490 m
True Thickness - 15 – 600 m
Approximate Strike Direction - NNW.
Goldstrike OP - 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%.
The gold bearing arsenian pyrite may be subdivided into coarse grained sulphides at ±200 µm diameter and fined grained at 10 µm to 20 µm, with the latter carrying proportionately much more gold. Gold at 0.05 µm to 0.1 µm is occluded in the iron sulphides. Approximately 10% to 20% of the gold is free, 20% to 30% is held in the fine-grained pyrite/marcasite, a few percent (generally less than 2%) is contained in coarse pyrite, and the balance is in very fine pyrite associated with clay.
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 UG (Meikle and Rodeo deposits)
The gold deposits of the Goldstrike underground mines are hosted in lower Paleozoic carbonates of the Devonian Rodeo Creek Formation (Drc), Devonian Popovich Formation (Dp), Devonian Bootstrap Limestone, and Siluro-Devonian Roberts Mountains Formations (SDrm), and series of highly altered dikes cross cutting stratigraphy.
Gold mineralization at the Goldstrike 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 and is wholly refractory ore. 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.
The orientation of the mineralization is different in each zone. East Banshee, Meikle, Meikle-East, Extension, Rodeo, North Post, and East-Griffin are characterized by steep and shallow angle east dipping mineralization. South Griffin, West Banshee, and part of Lower Rodeo are more moderately west dipping. South Meikle, West Griffin, and Barrel mineralization are relatively flat lying. Mineralization is commonly hosted throughout the stratigraphy column from the Bazza Sands in the Rodeo Creek Formation to the Roberts Mountains Formation with strong structural controls such as hydrothermal and collapse breccias, folding, and intrusive dikes and sills.