Summary:
The Goldstrike deposits are Carlin style deposits. 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 gold mineralization is associated with silicification, argillization, and sulphide mineralization with little or no oxide mineralization remaining. In sulphide ore, the gold is intimately associated with very fine-grained pyrite and marcasite and is refractory. Over time, the pyrite oxidized, freeing the gold and making its extraction relatively easy, as in the historic Post Oxide deposit. 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.
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%.
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 underground (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.
UNDERGROUND ZONE DIMENSIONS
Banshee: Length - 1,750 ft, Thickness - 10 to 200 ft, (Fault and brecciated dykes);
Liberty: Length - 1,500 ft, Thickness - 10 to 150 ft, (Upper Mud and Wispy units, dykes and breccias along faults);
Meikle (main): 1,600 ft, Thickness - 10 to 380 ft, (Fault and breccia);
South Meikle: Length - 2,000 ft, Thickness - 10 to 70 ft, (Flat, faulted & brecciated dykes);
Extension: Length - 1,030 ft, Thickness - 10 to 70 ft, (Fault, brecciated dykes);
Griffin: Length - 1,100 ft, Thickness - 10 to 250 ft, (Fault, brecciated, dykes and Bazza Sand);
Griffin Bx: Length - 1,080 ft, Thickness - 10 to 150 ft, (Fault and brecciated);
West Griffin: Length - 1,400 ft, Thickness - 10 to 60 ft, (Bazza sand unit, Fault);
Barrel: Length - 1,600 ft, Thickness - 10 to 130 ft, (Upper Mud and Wispy units);
Rodeo: Length - 2,350 ft, Thickness 10 to 200 ft, (Upper Mud and Wispy units, dykes and breccias along faults);
North Post: Length - 2,400 ft, Thickness - 10 to 140 ft, (Wispy subunit, dykes and breccias along faults).