Summary:
The Cortez deposits are Carlin-type sedimentary rock-hosted deposits. At Cortez, mineralization consists of submicron to micrometer-sized gold particles and gold in solid solution in pyrite.
A secondary deposit model that is applicable to the Cortez Complex is an intrusion-related gold model. Cortez mineralization occurs approximately between elevations 1,630–1,240 m, is approximately 400 m wide northeast-southwest by 1,219 m long northwest-southeast, and ranges in thickness from approximately 90–455 m. Mineralization is hosted within the Roberts Mountains Formation and the Hanson Creek Formation. A series of north-northwest trending and north-east trending faults cut the lithologies, with mineralization forming where these faults intersect shallow east-dipping thrust breccia zones (thrust duplexes). Most of the Cortez NW Deep higher-grade gold mineralization (<3.43 g/t Au) occurs in two zones lying between the 1,280-1,325 m elevations beneath the old Cortez open pit floor. One zone consists of an oxidized and strongly altered thrust zone within the Roberts Mountains Formation and the other is an unoxidized, sulfide-bearing thrust zone at the top of the Hanson Creek Formation. Mineralization becomes dominantly refractory at about 1,280-1,325 m elevation. Breccia gold mineralization is hosted in hydrothermally brecciated and fractured rocks that are spatially associated with the WSW dipping faults and attendant structures. Altered, matrix-supported breccia bodies contain the highest gold grades and are surrounded by “crackle” breccias and highly-fractured rock with moderate gold grades continuing outwards to less fractured rocks with lower grades. Most of the Breccia mineralization dips moderately southwest enveloping the west–southwest-dipping faults.
Pipeline
The main Pipeline deposit is a 15-90 m thick, tabular zone lying at about 150-180 m beneath the surface. The deposit dips at a low angle to the east and extends over an area of 230 m north-south by 460 m east-west. Drilling indicates the deposit extends to at least 427 m depth.
South Pipeline consists of two zones, a shallow zone starting at 20–46 m depth and a deep zone starting at 300 m. The shallow zone occupies an area of approximately 550 m by 610 m, north and east respectively, and exhibits both low-angle and high-angle structural controls on gold distribution. The deep zone occupies an area 60 m north-south by 180+ m east-west, is up to 76 m thick and is more closely associated with high-angle structures.
Within the Pipeline deposit, gold occurs in association with silica, pyrite, hematite, and illitic or sericitic matrix material. Gold grains are coarser in open spaces and in fracture fillings, and finer-grained when associated with silica, pyrite and hematite.
At South Pipeline, microscopic gold is commonly associated with silica, in replaced matrix and quartz veinlets, and in association with limonite after pyrite.
Mineralization is predominantly characterized by oxides, and sulphidic and carbonaceous refractory material. Carbon content in the deposits is highly variable and occurs generally in the Devonian Wenban Limestone and Roberts Mountain Formation.
Supergene alteration extends up to 656 ft depth resulting in oxide ores, which overlie 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.
Crossroads
Crossroads is to the south of, and along strike with, the Pipeline deposit, but is at a greater depth, consistent with the south–southeasterly dip of the host formations into the basin.
The deposit has dimensions of 1,700 m by 500 m along strike and extends from about 1,320 m to 890 m in elevation above sea level. The Crossroads deposit consists of two mineralized zones: an upper stratiform zone along the Horse Canyon-Wenban Formation contact and a deeper zone controlled by an east– northeast-striking, west dipping (20° to 25°) structural zone that cuts across stratigraphy.
Gold mineralization is associated with anomalous arsenic, antimony, and thallium. Gold occurs in solid solution within arsenian rims on hydrothermal pyrite in primary ore, and as sub-micron sized free gold particles in ores which have been oxidized. Common gangue minerals include pyrite, abundant calcite, oxide and arsenate minerals, as well as clays.
Gold Acres
The Gold Acres deposit lies just to the north-west of the Pipeline Open Pit, in an exposed window of the upper plate of the Roberts Mountain Thrust. The deposit has dimensions of 450 m by 900 m along strike and extends from approximately 1,650 m to about 1,430 m in elevation above sea level.
Two mineralizing phases are recognized. The first consists of skarn calc-silicates and sulfide assemblages; the second comprises gold mineralization within the imbricate fault zone. Some gold was also deposited in the high angle structures and along the Roberts Mountains Thrust above the imbricate fault zone. Gold is present as disseminated submicroscopic particles. Gangue minerals include pyrite, calcite, quartz, melanterite, azurite, jarosite, realgar and various base metal sulfides and calc-silicate minerals related to the skarns.
Robertson
The deposit is about 2285 m long, 910 m wide, and approximately 400 m thick. 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.
Cortez Hills Complex
The Cortez Hills Complex consists of two in-situ and connected Carlin-type orebodies with differing geometries and an exotic satellite deposit (Pediment deposit) that was eroded and redeposited adjacent to the sub-cropping Cortez Hills orebody.
The underground portion of the Cortez Hills deposit consists of the Breccia Zone, the Middle Zone and the Lower Zone (subdivided into Lower Zone A, B, C and D).
The conical-shaped Breccia Zone mineralization extends from a near-surface elevation of 1,783-1,240 m, terminating just east of the Middle Zone. It is approximately 300 m wide with a northwest trend, and varies in width from 75–580 m.
The Middle Zone occurs between elevations 1,318-1,166 m, is approximately 550 m wide northwest-southeast by 400 m long northeast-southwest, and ranges in thickness from 3-80 m.
The Lower Zone lies at an elevation of 1,298 m to the northwest and 933 m to the southeast, extends 1,310 m northwest-southeast, varies in width from 440 m in the north to 150 m in the south, and ranges in thickness from 20-80 m. An emerging area below Lower Zones C and D has mineralization extending to 759 m elevation along the Hanson Fault.
The Pediment deposit covered an area of 900 m x 180 m, ranged in elevation from 1,785– 1,615 m, and was about 75 m thick.
Breccia gold mineralization is hosted in hydrothermally brecciated and fractured rocks that are spatially associated with the Voodoo Fault and its attendant structures. Mineralization within the Middle and Lower Zones lies at depth to the west and southwest of the Breccia Zone, occurring as tabular, sub-horizontal to shallow dipping zones. Mineralization in the Lower Zone is typically refractory in the north, transitioning to dominantly oxide as the zone plunges deeper to the south.