Robertson mine Project is a part of Cortez Complex.
Nevada Gold Mines includes Carlin, Cortez, Turquoise Ridge, Phoenix and Long Canyon. Barrick is the operator of the joint venture and owns 61.5%, with Newmont Corporation owning the remaining 38.5%.
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Summary:
The Cortez property is situated along the Cortez/Battle Mountain trend. The principal gold deposits and mining operations are located in the southern portion of Crescent Valley, which was formed by basin and range extensional tectonism.
Mineralization is sedimentary rock-hosted and consists of submicron to micrometer-sized gold particles and gold in solid solution in pyrite. Mineralization is disseminated throughout the host rock matrix in zones of silicified, decarbonatized, and/or argillized, silty calcareous rocks. The deposits range in length between 2,000 and 3,350 meters and range in width between 1,000 and 1,200 meters. Mineralization thickness can change significantly, up to 400 meters. Exploration from projects at Robertson and Goldrush suggest that the deposits can be in excess of 5,000 meters in length and 900 meters in width.
The intrusive-related deposit model is appropriate for areas with igneous intrusions, as the Robertson deposits are considered to be examples of intrusion-related gold mineralization.
The Robertson deposit is about 2285 m long, 910 m wide, and approximately 400 m thick. The deposit has been drill tested to about 420 m depth.
Robertson is an igneous-related gold system. Gold mineralization is found in Upper Plate siliciclastics of the Devonian Slaven and Silurian Elder formations, as well as inside Eocene intermediate composition igneous rocks, primarily diorite and granodiorite. Mineralization is primarily concentrated around the Tenabo Stock in three main areas: Gold Pan in the northwest, Porphyry in the east to northeast, and Altenburg Hill in the southeast. Gold mineralization overprints an initial contact metamorphic hornfels event and a subsequent chalcopyrite-pyrrhotite-pyrite-chlorite-actinolite skarn event.
A series of later-staged phyllic and propylitic alterations are present, primarily altering later faults, fractures and late-stage feldspar porphyry dykes.
Mineralization at Gold Pan can be separated into two zones: Gold Pan and 39A, which represent one gold depositional event but in two different host lithologies in the Slaven Formation. Mineralization at Gold Pan is controlled by a gently northeast-dipping metabasalt strata, which is preferentially altered to a biotite hornfels, with skarn alteration overprinting the hornfels. Mineralization at 39A is controlled by the 39A structure, a bedding sub-parallel thrust that is also gently northeast dipping. The 39A structure is preferentially skarn-altered, with gold most strongly present at and under the 39A structure.
Gold mineralization at Porphyry is found primarily in the Elder Formation, with lesser amounts present in the Slaven Formation, as well as the Eocene-aged Tenabo stock, a polyphase diorite and granodiorite intrusive stock. In the Elder Formation, and to a lesser extent the diorite, gold deposition is controlled by the endoskarn fault series, a series of north-south striking, moderately west-dipping faults. Inside the endoskarn, the initial hornfels and retrograde skarn events have been overprinted by the gold event, with gold mineralization strongest in and in near proximity of the structures and becoming more diffuse further away from the structures. Mineralization in the granodiorite at Porphyry is controlled by a series of shallowly west-dipping structures present inside the stock.
Gold mineralization at Altenburg Hill is present in the Elder Formation and the granodiorite phase of the Tenabo stock. Mineralization in the Elder Formation is controlled by a series of bedding sub-parallel granodiorite dyke emplacements, the majority of which dip shallowly to the east-northeast. Mineralization is also partially controlled by a series of roughly east-west- and northwest-southeast- striking high-angle faults, which cross both the Elder Formation and the granodiorite.
Gold itself is associated with bismuth and tellurium, and is commonly found in association with arsenopyrite and loellingite (FeAsS). Gold at Robertson is present as native gold, with minor electrum, and all gold present is free-milling.