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The Pirquitas tin-silver mine is located at an elevation of 4100 m in the Puna de Jujeña region in the Province of Jujuy in northwestern Argentina, ~200 km NW of San Salvador de Jujuy.
(#Location: 22° 42' 1"S, 66° 29' 10"W)
Placer gold prospecting during the early 1930's in the Orosmayo River drainage system, including the Collahuaima River that passes east of Pirquitas, led to the discovery of economic quantities of cassiterite in the gravel deposits of the Pircas River, east of the old town of Pirquitas. These tin placers were dredged between 1933 and 1949. A few years after the alluvial tin mining began, the bedrock source of the cassiterite further up the valley of the Pircas River was discovered, and by 1936 a number of silver and tin lode deposits were being mined by small underground workings. Over the next half century, as many as twelve mines operated in the district, making Pirquitas the largest historical producer of tin and silver in Argentina. The principal mines were the San Miguel, Chocaya, Llallagua and Potosí. By the 1970's most of the deposit was owned by a family controlled company, Sociedad Minera Pirquitas Picchetti and Cia S.A.
Estimated production from the vein systems amounts to 777.6 t of silver and 18,200 t of tin, with a further 9100 t of placer tin.
In 1995, the Argentine subsidiary of Sunshine Mining and Refining Company acquired the deposit and surrounds and undertook a comprehensive exploration program. In 2002 and 2004, Silver Standard acquired Sunshine Argentina in two tranches to full ownership and undertook further exploration and proving under the control of the subsidiary Mina Pirquitas, Inc.
The mine achieved commercial production on December 1, 2009 as a silver mine with zinc credits. It is an open-pit mine utilising a fleet of 100 tonne trucks and three loading units. Ore is crushed and treated in a gravity pre-concentrator prior to processing in a conventional mineral flotation plant. The silver and zinc concentrates produced from the plant are shipped to third party smelters.
For details of the tectonic, regional geological and metallogenic setting see the Central Andes and Bolivian Orocline, and the Andean Tin Belt records.
The Pirquitas deposit occurs in the Puna belt, an elevated plateau in the Andean orogenic zone. The host succession in this belt comprise uplifted and folded Ordovician to Devonian marine meta-sedimentary rocks overlain by Cretaceous to Middle Miocene continental and marine sedimentary and volcaniclastic rocks, intruded by mafic to intermediate igneous bodies. Sub-horizontal Late Miocene to Pleistocene andesitic to dacitic lavas and ash-flow deposits unconformably overlie this sequence.
The Pirquitas deposit is hosted by Ordovician rocks of the Acoite Formation, a suite of marine sandstone, siltstone and minor shale beds. These rocks have undergone low-grade metamorphism and crop out within fault-bounded, uplifted structural blocks to the southwest and east of the mine. Late Ordovician to Early Devonian compressive tectonism strongly folded of the Palaeozoic sedimentary sequence, and induced a well-defined axial planar cleavage. The same event generated high-angle thrust faults.
In the Pirquitas district, the Acoite Formation is composed of beds of fine to medium grained lithic wacke, ranging from <1 m up to several tens of metres in thickness, interbedded with greywacke siltstone and less abundant black shale layers that range in thickness from a few centimetres up to several metres. Structurally underlying this sequence to the northeast is a suite of continental sedimentary rocks, mainly hematite-stained arkosic sandstone intercalated with thin polymictic conglomerate beds and cream-coloured reworked dacitic tuff units, inferred to belong to the shallow ENE dipping, Early to Middle Miocene Tiomayo Formation. A few kilometres to the east, a medium-grained granodiorite intrusion forms the small mountain of Cerro Galan, representing the only substantial intrusive rock body in the area of the mine.
The sequence that immediately hosts the mineralisation at Pirquitas comprises a north south striking, dark grey, dirty quartzite, grading to a clean grey quartzite, with interbedded shales and mudstones. The mudstones are often massive with only very broad bedding. In places finely laminated shale with alternating black and pale grey laminae that range from less than 1 mm to 1 cm in thickness, is obvious within the mudstones. In the mine area the sequence comprises around 70% lutites and 30% quartzite.
At Pirquitas the host sequence is folded into a series of north-south to NNE-SSW trending anticlines and synclines with shallow-plunging axes and wavelengths ranging from tens of metres up to a maximum of a few hundred metres. Dips are up to 70° on the limbs of these structures. Axial planar cleavage is well developed in the Palaeozoic rocks, especially the siltstone and shale beds. However, the well-formed cleavage does not appear to have controlled the quartz hosted Ag-bearing Fe-Zn-Sn-Pb sulphide veins, although a minor amount of weakly-auriferous quartz veining does appear to be deposited along cleavage planes.
The host sediments are cut by a series of east-west trending sulphide veins on the walls of the east-west trending Pircus Valley which cuts the sequence. A system of sulphide-rich veins cuts the axial surfaces of the folds and the related cleavage fabric at high angles. The ore veins are restricted to the crests of anticlinal structures and are usually normal to both bedding and anticlinal axes. The host rocks carry about 2% pyrite throughout, occurring on joint and fracture surfaces, in irregular hairline veinlets, and as laminae where individual planes are defined by unconnected strings of crystals. The black mudstone and shales have a higher content of sulphides than the quartzites where only thin veinlets and disseminations of sulphides are known.
Two main vein sets are recognised in the mine:
• Vein Set 1 - the dominant set, with orientations strike close to 105° and a steep dip to either the south or north. Veins in this set include the Potosí, San Miguel, Chocaya-Oploca, San Pedro, Llallagua, Chicharron and Colquiri veins. The Potosí Vein is the largest in the mine, with a strike length of ~500 m and maximum thickness of 2.5 to 3 m. The other veins of this set more typically have strike lengths of between 50 and 150 m, with average widths of 30 to 50 cm. The larger of these veins, such as the Potosí Vein, include localised matrix supported breccias with angular clasts of quartz-sericite altered wallrock in a matrix of iron and Zn±Sn-Ag-Cu sulphides.
• Vein Set 2 - which comprises secondary veins, including the Veta Blanca and Colquechaca veins. These lie north of the Potosí Vein and trend NW-SE.
These fracture and breccia-hosted veins consists of Fe and lesser Zn sulphides with accessory cassiterite and a large variety of Ag-Sn-Zn(-Pb-Sb-As-Cu-Bi) sulphides and sulphosalts. Crystalline quartz, along with chalcedony in the upper levels of the system, and kaolinite are the main gangue minerals in the veins and mineralised breccias. The main sulphides, specifically pyrite, pyrrhotite, sphalerite and wurtzite, form colloform bands parallel to vein margins, which together with crustiform and drusy vein textures suggest that the mineralisation is epithermal in origin. The vein textures imply that the mineralisation was deposited from relatively low temperature hydrothermal fluids within about 500 m of the palaeosurface. However, mineralogical evidence suggests that the initial temperature of the mineralising fluids was possibly greater than 400°C.
In addition to the major veins, sets of sheeted sulphide veins and veinlets occur in association with disseminated mineralisation. These are sulphide veins, which are often banded with thin bands of cassiterite set in fine sulphides. Individual veins from 2 to 15 cm thick can carry up to 30 kg/t Ag and 20% Sn with separations of 1 to 15 m, developed in zones of the order of 200 x 100 m in plan, with vertical dimensions of up to 200 m. The Pirquitas open pit exploits such a set of sheeted veins between the Potosí and San Miguel veins, where the sheeted veins comprise a 120 to 140 m wide north-south trending swarm, with a maximum east-west strike of ~300 m. They usually have no quartz, only sulphide and cassiterite. The Potosí Vein is on the northern margin of the pit, with the Chocaya Vein system to the south of the pit.
Within the sheeted vein zones, mineralisation is also disseminated within the host sedimentary rocks. Parts of the underground workings have been channel sampled. Examples of 150 m widths sampled with 1 m long channels give bulked grades of 0.243% Sn and 75 g/t Ag. If the veins are excluded the grade is 0.197% Sn.
A further ore type, typical of that developed in the Oploca section of the deposit, comprises pockets of semi massive sulphides with dimensions of the order of 20 x 20 x 30 m which occur within zones of intense intra-formational folding on the limbs of anticlines. These pods apparently average around 7 kg/tonne Ag and 1.1% Sn. They grade upwards into low grade galena and sphalerite.
The dominant sulphides within the ore zone veins is pyrite and pyrrhotite, with lesser sphalerite arsenopyrite and wurtzite, accompanied by cassiterite and a wide variety of other sulphides and sulphosalts. The silver minerals are tetrahedrite, argentite, polybasite, pyrargyrite and proustite. Ten tin sulphides have been recognised. These minerals are found in all ore types. In the disseminated mineralisation tin occurs as fine 50 µm cassiterite and as sulphides.
Malvicini (1978) interpreted paragenetic observations of relationships between the more than 26 sulphide and sulphosalt phases to indicate two main episodes of mineralisation, the first of which comprises two stages.
• Stage 1 of the first event was dominated by pyrite and pyrrhotite, with accessory cassiterite and arsenopyrite. Pyrrhotite is interpreted to have crystalised first, with partial replacement by pyrite and later by Zn and Sn sulphides. Quartz was deposited episodically with these early-stage fluids. A granular cassiterite was deposited during this stage in the upper levels of the larger veins, with the grain size of the Sn oxide diminishing downwards from ~300 to 20 µm in lower levels of the veins. Fine, granular cassiterite disseminations are also found in wallrock adjacent to sulphide-rich veins (Malvicini, 1978).
• Stage 2 of the first event is characterised by colloform bands of schalenblende, which at the Pirquitas deposit is a brownish botryoidal intergrowth of sphalerite and wurtzite. Stannite and other rare Sn-rich sulphides, with minor galena and a variety of sulphosalts of Sn, Pb-Sb (boulangerite, bournonite) and Ag (pyrargyrite, miargyrite, polybasite) were mainly deposited during this stage also. Kaolinite and drusy quartz were crystallised as gangue coeval with the ore mineralisation. Sulphosalt phases replaced grains of pyrrhotite, cassiterite, schalenblende and galena (Malvicini, 1978).
• The second event was responsible for the bulk of the Ag at the Pirquitas deposit, deposited during a low temperature hypogene mineralising phase. Ag occurs in the sulphosalt minerals freiburgite, pyrargyrite, miargyrite and polybasite, and as minute inclusions in argentopyrite. Rare sulphosalts of Ag-Sn-Ge are also recognised. This second mineralisation event is characterised by the presence of Ag, together with bismuth in the form of Bi sulphosalts (Malvicini, 1978).
The upper 50 m or less of the mineralised veins and sheeted veinlets are oxidised. Within this oxide zone, the Fe and Zn sulphides were replaced by Fe oxide minerals, mainly goethite and hematite, while the Sn sulphides and sulphosalts were replaced by the fibrous and acicular forms of cassiterite. Native Ag and the Ag halide cerargyrite are the main supergene Ag minerals in the deposit. Cu from stannite and very minor chalcopyrite forms covellite in the near-surface oxides zone. A variety of sulphates, including jarosite, alunite and few hydrous Zn sulphates, are also present in the oxidised mineralisation.
The oxidation of disseminated and veinlet pyrite and pyrrhotite is inferred to have produced acidic meteoric waters that caused rock-forming silicate minerals to be replaced by layered clays. It is this supergene argillic alteration that is seen as a large zone of creamcoloured 'bleached' rock surrounding the Pirquitas Mine area. From the base of oxidation, below the water table there is enrichment of Ag and to a lesser extent Sn over an interval of up to 40 m.
Alluvial deposits have been worked over a width of 1.5 km and down stream length of 2.5 km. High grade deposits from 0.25 to 1.5 m thick may average 1 to 5 kg/tonne while intervals of up to 30 kg/tonne are known. On the upstream side, the palaeo channel branches into two, one leading to the main workings to the south, the other to the north. The southern channel only yields tin, while the northern has coarse cassiterite and gold.
Reserves and Resources
NI 43-101 compliant mineral resources in May 2008 (Board et al., 2011) at a 50 g/t Ag cut-off, prior to the current mining operation, were:
Measured + indicated resources - 45.2 Mt @ 152.3 g/t Ag, 0.78% Zn, 0.17% Sn;
Inferred resources - 2.4 Mt @ 247.8 g/t Ag, 0.78% Zn, 0.07% Sn.
NI 43-101 compliant mineral resources as at September 30, 2011 (Board et al., 2011) at a 50 g/t Ag cut-off, were:
Measured + indicated resources
Open pit - 29.8 Mt @ 148.6 g/t Ag, 0.72% Zn, 0.22% Sn;
Stockpiles - 3.0 Mt @ 78.5 g/t Ag, 1.5% Zn, 0.11% Sn;
TOTAL - 32.8 Mt @ 142.2 g/t Ag, 0.79% Zn, 0.21% Sn.
Remaining NI 43-101 compliant ore reserves and mineral resources are (Silver Standard ore reserve statement, December 31, 2015):
Probable + probable reserves
Un mined - 2.99 Mt @ 171.9 g/t Ag, 0.25% Zn;
Stockpiles - 2.21 Mt @ 109.1 g/t Ag, 0.71% Zn;
Measured + indicated resources
Open pit - 13.67 Mt @ 122.4 g/t Ag, 1.01% Zn;
Underground - 2.34 Mt @ 241.1 g/t Ag, 4.11% Zn;
Stockpiles - 2.32 Mt @ 107.3 g/t Ag, 0.73% Zn;
Open pit - 0.79 Mt @ 87.3 g/t Ag, 1.88% Zn;
Underground - 0.94 Mt @ 202.0 g/t Ag, 6.97% Zn.
This summary is largely drawn (and paraphrased) from "Board, W.S., Kennedy, R.B. and Yoemans, T.J., 2011 - Technical Report on the Pirquitas Mine, Jujuy Province, Argentina; an NI 43-101 Technical Report prepared for Silver Standard Resources Inc., 220p." supported by information and observations from a technical visit to the mine in 1977.
The most recent source geological information used to prepare this summary was dated: 2011.
This description is a summary from published sources, the chief of which are listed below.
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