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The Huanuni tin deposit is located in the Pantaleón Dalence province of southwestern Bolivia, 275 km SSE of La Paz and 45 km SE of Oruro, capital city of the department of Oruro, in the central portion of the Bolivian Andes (#Location: 18° 16' 53"S, 66° 50' 2"W).
The Huanuni deposit was discovered in 1745, and in 2013, was the most important Sn producer in Bolivia, with a capacity of ~3000 t of Sn per day.
The Huanuni district is located in the Central Ranges of the Eastern Cordillera of the Andean orogen. The Eastern Cordillera is composed of sedimentary sequences that range from Ordovician to Cretaceous in age and comprise thick black shales, siltstones, limestones, sandstones, slates and quartzites (Arques et al., 2013) and lies within a zone of NNW-SSE trending regional folds and thrusting (McQuarrie & DeCelles 2001). The host sequence has only been affected by very low grade regional metamorphism.
For details of the tectonic, regional geological and metallogenic setting see the Central Andes and Bolivian Orocline, and the Andean Tin Belt records.
The sequence in the district is as follows, from the base:
Cancaniri Formation - Greywacke. Thickness >300 m.
Huanuni Formation - Micaceous sandstone and phyllite. Thickness 0 to 90 m.
Llallagua Formation - Shale with minor sandstone overlying a basal sandstone to quartzite unit with lesser shale. Total thickness 500 m.
Siluro - Devonian
Uncia Formation - comprising a >m thick 'Black shale' and ~400 m thick 'Brown shale'.
Catavi Formation - Arenites.
Morococala Formation - rhyolite lavas and tuffs. Quartz porphyry dykes.
Intrusions of Miocene acid stocks and domes are found in the central part of the Eastern Cordillera, and are partly covered by ignimbrite fluxes of upper Miocene age, and to the East of the mining area, an suite of ~north-south trending porphyrytic dykes, outcrops over an interval of more than 1.5 km (Arques et al., 2013).
The succession is folded into a broad anticline in the Huanuni area. To the south of the Huanuni River the Llallagua Formation rests unconformably on the Cancaniri Formation. In some places, remnants of the Huanuni Formation are found below this unconformity between the two formations (Arques et al., 2013).
Mineralisation at Huanuni occurs within quartz veins that are hosted by Silurian quartzites and quartz-rich sandstone with lesser intercalated shale of the lower member of the Llallagua Formation (Ahfeld and Schneider-Scherbina 1964). The only other units within the mineralised area are the Huanuni Formation in the bottom of the mine, and part of the Uncia Formation on the flanks of the anticline to the east and west, whilst a series of Tertiary quartz porphyry dykes cut the upper parts of the Llallagua Formation and the basal Uncia Formation to the east of the mine. The highest ore concentrations are located in fractures, joints and veins. Mineralisation is only developed within quartzite beds and only in those quartzites that are well fractured, whilst veins that cut shales are usually unmineralised (Arques et al., 2013).
The deposit comprises polymetallic mineralisation with Sn, W, Pb, Ag and Zn, that has a lateral zoning, with the high-temperature minerals like cassiterite in the central part of the deposit, at Cerro Pozokoni, decreasing outward to moderate-temperature minerals such as galena or sphalerite in the peripheries of the deposit. Silver and lead are found to the west at Porvenir, and to the south at Maria Francesca. Antimony, with minor associated Pb-Zn-Ag, is found to the north east. The tin-mineralised zone covers a SE elongated area of 1500 x1200 m. The most significant tin-bearing vein, the 'Veta Grande', is up to 0.5 m in width. In the peripheral Zn rich zone, sphalerite can be economically exploited from veins with a width of 10 to 70 cm (e.g., the Suerte mine) and 25 cm (as at a the Bonanza mine). These veins also contain Sn and Ag minerals (Arques et al., 2013).
Tin mineralisation in Veta Grande has a vertical zonation that changes from pyrite-pyrrhotite-cassiterite at the top, passing downward to pyrite-pyrrhotite-cassiterite-chalcopyrite and sphalerite and finally to pyrite-pyrrhotite-marcasite, whilst arsenopyrite is found at depth (Ahfeld and Schneider-Scherbina 1964). Stannite tends to form after cassiterite. The tin ores are massive and often display geodic infilling. Mineralisation occurs principally as coarse cassiterite (2 to 10 mm sized crystals) within dog-tooth coarsely crystalline vuggy quartz veins. Common gangue components are quartz, kaolin, pyrite, pyrrhotite and minor tourmaline, occasional chlorite, fluorite and often siderite. These veins usually have sharp boundaries with the enclosing rocks and range in size from 10 to 30 cm in width. Some veins are banded, with a central siderite band, bounded by pyrite, with cassiterite on the margins, all within a quartz gangue. The latter veins are not common. Vein textures are similar at Bonanza and Suerte, although these veins may be enriched in silver minerals and the mineral sequence becomes complex. Silver may be distributed in galena, or as discrete phases such as freibergite, matildite, cervelleite and others. Tourmalinisation is rare. It is best developed to the south west of the Huanuni mine in the Harrison area and is restricted to the veins, with little or no penetration of the wallrocks (Arques et al., 2013; Mine visit 1977).
Downstream of the mine, in the Huanuni River, extensive tin bearing alluvials are developed. These are the Playa Verde Alluvials. Placer workings extend for about 7.5 km downstream and over a width from 200 m to 1 km. Bedrock is at between 15 and 20 m below the surface with grades averaging about 105 g/m3 Sn. Grades of up to 300g/m3 are known over significant areas. There are small scale artesinal workings within this river as far as 15 km downstream from Huanuni.
The mine is operated by the state owned COMIBOL, which declared there were 20 Mt of economic reserves at an unspecified grade remaining in April 2017.
The most recent source geological information used to prepare this summary was dated: 2013.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
References to this deposit in the PGC Literature Collection:
Arques F, L., Amoros, A.C., Torres, D.A., Cueva, B.T., Torro i Abat, L., Melgarejo, J.V., Condori, R., Guevara, N. and Arce, O., 2013 - The Huanuni Sn-W-Pb-Zn-Ag vein deposits, Bolivia: Structure and mineralogy: in 12th SGA Biennial Meeting 2013, Proceedings, v.3, pp 1236-1238|
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