Gunnison Project - North Star
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The in situ recovery Gunnison Project and North Star porphyry copper deposit that it exploits are located in Cochise County, Arizona, ~100 km east of Tucson and 2.5 km southeast of the historic Johnson Camp mining district.
Although no direct mining had historically been undertaken over of the North Star deposit, the district has been the subject of early copper, zinc, silver and tungsten mining beginning in the 1880’s, extending to the present day. Between 1882 and 1981, the district produced 11 Mt of ore containing 5440 t of Cu, 45 600 t Zn, 5900 t of Pb,22.4 t Ag, and minor Au (Keith et. al., 1983). The most significant mines were the Republic and Moore mines 1 to 2 km NW of North Star. Copper-oxide mineralisation was mined 2.5 km NW of North Star from the Johnson Camp open-pit operation since 1975, most recently by Nord Resources Corporation from 2008 until 2010.
The North Star deposit was discovered as the result of drilling magnetic anomalies to test for skarn mineralisation in the 1960s by private holders. Several companies, including Cyprus Mines and Superior Oil undertook extensive drilling programs, magnetic and IP surveys, metallurgical testing, hydrological studies, In-situ Recovery (ISR) tests, and preliminary mine design. By early 1974 several million tonnes of low-grade, acid-soluble copper mineralisation had defined. By 1977, the deposit had been returned to the private holders. No further substantial work was undertaken until Magma Copper optioned North Star in 1993 and drilled additional holes, undertook limited hydrological studies, and calculated a copper-oxide resource. Magma’s interest in the project was for ISR of the copper-oxide resource. Metallurgical test work indicated greater than 70% ISR recovery was possible. Shortly after being acquired by BHP-Billiton in 1997, Magma relinquished the project. Subsequently, in 1997, North Star was optioned by Phelps Dodge who drilled further holes on the periphery of the deposit, but in 1998 withdrew from the agreement. After compiling all the data from previous testing, AzTech Minerals Inc. acquired an option for the Project in May 2007 and then carried out pre-mining environmental and anthropologic surveys. In June 2010 AzTech Minerals was merged with Excelsior Arizona who became the sole owner of the property. Permitting of the mine was completed in October 2018 and construction commenced in December of the same year.
The North Star copper deposit is located in southeastern Arizona, within the Mexican Highland section of the southwest Basin and Range province. The province characteristically comprises fault bounded mountains that typically have large intrusive cores, separated by deep Tertiary and Quaternary
gravel filled basins. Thee deposit is located on the eastern slopes of the Little Dragoon Mountains, an isolated, fault bounded, up thrown Basin and Range horst block.
The surrounding rocks range from the Proterozoic Pinal Schist to Holocene sediments. The southern section of the Little Dragoon Mountains is predominantly composed of the Eocene Texas Canyon Quartz Monzonite, whilst the Pinal Schist and the Palaeozoic sedimentary units that regionally host copper mineralisation dominate the northern half.
The Pinal Schist, the oldest rocks in the area, comprise greenschist to amphibolite facies Palaeoproterozoic sandstones, shales and 1710 to 1675 Ma felsic volcanic flows. These rocks are unconformably overlain by 1.32 Ma Mesoproterozoic Apache Group conglomerates, shales and quartzite that were subsequently intruded by dolerite sills. The Apache Group is unconformably overlain by the Palaeozoic rocks that host the mineralisation including the:
• Middle Cambrian Bolsa Formation - Red-brown to white quartzite, which at Gunnison is a red-brown to white quartzite with green hornfels;
• Upper Cambrian Abrigo Formation - Shale, impure limestone and sandy dolomite, which at Gunnison is converted to a garnet-epidote-pyroxene-amphibole skarn and calc-silicate hornfels;
• Upper Devonian Martin Formation - Dolomite with some shale and sandstone, which at Gunnison is altered to a diopside-garnet skarn with diagnostic magnetite;
• Lower Carboniferous Escabrosa Limestone (Lower Mississippian) - A cliff forming limestone and dolomite, that at Gunnison is contact metamorphosed to garnet rich skarns and calc-silicate hornfels, and marble;
• Mid Carboniferous Black Prince Limestone (Lower Pennsylvanian) - Limestone with thin shale at the base, that in the deposit area is a pyroxene rich calc-silicate hornfels and skarn, and marble;
• Upper Carboniferous Horquilla Limestone (Middle Pennsylvanian) - Limestone with abundant thin beds of shale, that in the deposit area is a pyroxene rich calc-silicate hornfels and skarn, and marble;
• Lower Tertiary Texas Canyon Quartz Monzonite - a coarsely porphyritic intrusion, with coarse 1 to 10 cm potassium feldspar phenocrysts. It is exposed as a ~11 x 6 km, NE-SW elongated body that is on the western side of the skarn mineralisation and east dipping sedimentary sequence. At its southern end, the intrusion forms a sill between the Lower Abrigo Formation and the Bolsa Quartzite. At the northern end of the deposit, the intrusion commonly occurs as thin dykes and sills which cut the strata in numerous locations. It has been dated at 50.3±2.5 Ma (Livingston et. al., 1967), whilst another eight determinations range from 55.0 to 49.5 Ma (Reynolds et. al., 1986). ;
• Upper Tertiary and Quaternary - Stream laid gravels, with unconsolidated boulders, sand and silt.
A number of deformation events are evident in the project area. Early, Pre-Apache Group deformation of the Pinal Schist produced isoclinal folding with steep to overturned fold axes and a generally northeastern structural trend. Minor deformation occurred in the late Proterozoic and between the end of the Palaeozoic and the beginning of the Cretaceous. Post Palaeozoic, to pre-Cretaceous deformation produced steep NE-to east-striking faults with offsets of as much as hundreds of metres.
The Late Cretaceous to Paleogene Laramide Orogeny encompassed both the mineralisation related intrusion, and the subsequent Basin and Range extension.
This deformation was normal to the Pre-Apache Group deformation, with structures striking in a northwesterly direction. Older faults were reactivated and modified. Folding and thrust faulting are common features of the Laramide deformation.
Lithological units, both regionally and in the deposit area, strike at ~NNW to NW (~320 to 350°) and dip at 30 to 45°NE. There is a dominant regional normal fault fabric of 10 to 30° ('Northeasters'), dipping at 70 to 75°SE, with a subordinate trend of 60 to 120° striking ('Easters'), dipping 3 to 50°S with some higher angle reverse faults dipping 75°S. These structures are overprinted by abundant 320 to 350° striking reverse faults, joint sets, and normal faults which range in dip from 35°NE, sub-parallel to bedding, to 75°NE. The reverse faults strike parallel to the long axis of the deposit. All of these fault systems have only apparently been responsible for minor displacements in the North Star area, although numerous sheared and brecciated faults, generally filled with copper-oxide mineralisation, cut through the deposit. Late-stage 70 to 110° striking vertical faults at the north end of the deposit contain local zones of high grade copper-oxide mineralisation. The latter are related to the two episodes of block faulting prior to the Quaternary that created the Basin and Range topography that dominates the current landscape and postdates hypogene mineralisation. The structures detailed above are observed in both the Johnson Camp and North Star deposit areas.
Alteration and Mineralisation
The Texas Canyon quartz monzonite intrudes the Palaeozoic host rocks along the western margin of the deposit, and has altered wide zones of these lithologies to calc-silicate and hornfels assemblages, accompanied by extensive low grade copper sulphide mineralisation.
This alteration generally has a zonation outward from the stock of garnet-wollastonite-idocrase, diopside, tremolite to chlorite-talc (Kantor, 1977). The Martin Formation grades from a wollastonite-diopside rich rock near the porphyry, to a distal diopside-tremolite-actinolite assemblage, and finally to dolomite. The Abrigo Formation characteristically has garnet-actinolite-epidote-diopside alteration with some biotite hornfels near the porphyry, and this grades to a distal tremolite alteration, and then into peripheral un-metamorphosed limey shale. Quartz-orthoclase-carbonate ±magnetite and chalcopyrite veins are characteristic of the lower Abrigo Formation where it is mineralised (Zimmerman, et al., 2017).
Within the Gummison Project area, significantly mineralised host rocks include the Abrigo and Martin formations and, less so, the Horquilla Limestone and lower sections of the Escabrosa Limestone. Mineralisation is also occurs in the Bolsa Quartzite and Precambrian basement rocks.
Hypogene copper mineralisation accompanies calc-silicate skarn alteration that has replaced carbonate rocks adjacent to the Texas Canyon quartz monzonite.
The deposit at North Star is overlain by un-mineralised basin fill that varies from 100 to 250 m in thickness. Oxidation has persisted below these cover rocks to a depth of ~500 m below the surface, producing a supergene assemblage of dominantly chrysocolla with minor tenorite, copper oxides and chalcocite. This Cu oxide mineralisation is hosted within the calc-silicate skarn alteration as fracture coatings and vein fillings, mainly as chrysocolla. The remainder of the oxide mineralisation is found as replacement patches and disseminations. Cu-oxide mineralisation is found over a strike length of 3400 m, width across strike of up to 900 m, and is more than 275 m thick in places.
Hypogene Cu sulphide mineralisation was preferentially formed in the proximal (higher temperature) skarn facies, particularly within the Abrigo and Martin Formations, and within structurally complex zones. Three styles of sulphide mineralisation have been recognised within the skarns, in decreasing order of abundance, namely: i). fracture coatings and vein fillings; ii). distinct quartz-orthoclase-carbonate ±magnetite and chalcopyrite veins that are 0.2 to 10 cm wide that have retrograde haloes of chlorite, actinolite and epidote (Weitz, 1976) as well as stringers and veinlets of chalcopyrite and bornite, and iii). disseminations.
Pyrite and magnetite are texturally late and replace skarn minerals, whilst chalcopyrite was the last formed. Magnetite, the bulk of which (~90%) is hosted within the skarns, occurs as 0.2 to 0.5 mm disseminated euhedral to anhedral grains, and is closely associated with pyrite. Magnetite may comprise up to 5 vol.% of the skarn. The disseminated magnetite and magnetite bearing veins are interpreted to be the cause of the magnetic response of the deposit (Colburn and Perry, 1976).
Disseminations and veins of hypogene chalcopyrite-molybdenite also occur within the mineralised porphyry below and to the west of the skarn mineralisation at North Star. To 2017, only nine drill holes have intersected the quartz monzonite over lengths of >30 m. Most were mineralised with a best intersection of 88.09 m @ 0.31% Cu and 0.028% Mo, including 9 m @ 1.35% Cu. This mineralisation had not been fully investigated in 2017.
There is a strong fracture control to both oxide and sulphide mineralisation. The greatest width and density of fracturing and faulting is restricted to a zone centred on the intrusive contact, decreasing away from that contact into the less altered rocks to the east.
The initial formation of prograde skarn created a denser mineral assemblage and liberated CO2, leading to a volume reduction of the rocks. This in turn created significant fracturing, and consequently an increase in porosity and permeability, facilitating access by the later Cu-bearing fluids. Weitz (1976) calculated a 30% volume reduction in the skarn-altered portions of the Abrigo and Martin formations at North Star.
Oxide Cu also occurs in the hypogene-supergene transition zone, mainly along fractures and in quartz vein selvages as chrysocolla. Supergene Cu sulphide minerals, e.g., chalcocite are often associated with the oxide mineralisation in the transition zone, which is typically 30 to 60 m thick and strongly fractured and broken, similar to the oxide zone.
The main North Star resource outline covers a NNW-SSE elongated plan area of ~11 km and width of ~2 km.
Extraction and Treatment of Copper
The Project utilizes In Situ Recovery (ISR) methods to leach copper from the buried copper oxide deposit and extract the copper by conventional solvent extraction-electrowinning (SX-EW) technology. The ISR process involves injecting leach solutions acidified with sulphuric acid into the oxidised mineralisation to get soluble Cu into solution. Recovery wells pump the copper-bearing pregnant leach solution (PLS) to the surface for copper recovery by SX-EW into salable copper cathodes. The ISR extraction of copper will be from oxidised, mineralised bedrock that lies 90 to 240 m beneath the overlying alluvial basin fill. The basin fill is typically above the water table, with most of the oxidised mineralisation below the water table.
The host rocks to the North Star Cu deposit have been significantly fractured and jointed, resulting in broken ground that is below the water table (saturated zone) and permeable. The copper silicates and oxides preferentially occur as coatings on the fracture planes and as veinlets or matrix fill to broken fragments. Testing indicates this should result in preferential exposure of the copper minerals to leaching solution (lixiviant), thus reducing the amount of acid consumed by the un-exposed gangue rocks.
The Project development is planned to involve three progressively larger production 'stages' with capacities of 11 340, 35 000 and 56 500 t per annum respectively. Column tests suggest recovery of acid soluble copper ranges from 65% to >90% but was dependent on rock type, with the Lower Abrigo formation having the highest and shortest duration leach cycle and the Martin/Escabrosa column tests having the lowest recovery over the longest period. Tests also indicate 72 to 75% recovery of acid soluble Cu for the different rock units over a period of 4 years, equating to ~48% of total Cu.
Reserves and Resources
Published Ore Reserves and Mineral Resources at 7 July, 2015 (Excelsior Mining website, viewed December, 2018) at a 0.05% Cu cut-off were:
Probable Ore Reserve - 709.4 Mt @ 0.29% Total Cu;
Measured + Indicated Mineral Resource - 792 Mt @ 0.29% Total Cu for 2.263 Mt of contained Cu;
Inferred Mineral Resource - 170 Mt @ 0.17% Total Cu.
Note: Ore Reserves are included in Mineral Resources.
The information in this summary is largely drawn from: Zimmerman, R., Gustin, M., Roman, R.J., Prenn, N., Bartlett, D. and Drielick, T.L., 2017 - Gunnison Copper Project, Cochise County, Arizona, USA., Feasibility Study; An NI 43 101 Technical Report prepared for Excelsior Mining Corp. by M3 Engineering & Technology Corporation, 317p.
The most recent source geological information used to prepare this summary was dated: 2017.
This description is a summary from published sources, the chief of which are listed below.
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