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Llallagua, Siglo Venti, Siglo XX, Catavi, Uncia

Bolivia

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The Llallagua or Siglo Venti or Uncía Mine at Catavi is a major porphyry tin deposit located in the Altiplano of Bolivia, approximately 300 km southeast of the city of La Paz and 80 km southeast of Oruro.

Significant exploitation of the Llallagua deposits only commenced towards the end of the 19th century, much later than at the other major deposits in Bolivia due to the lack of silver within the host Salvadora Stock and the accompanying veins.

For details of the tectonic, regional geological and metallogenic setting see the Central Andes and Bolivian Orocline, and the Andean Tin Belt records.

Geological Setting

The geological setting of the Llallagua deposits is as follows, from the base:

Silurian
Concaniri Formation, >650 m thick - Massive sparsely pyritic black to grey greywacke.
Unconformity
Llallagua Formation,
250 m thick - Massive coarse sandstone overlain by alternating sandstone (to quartzite) and shale.
Siluro Devonian
Oruro or Uncia Formation (Pampa Shale), 750 m thick - Thin bedded grey to black shale with minor sandstone intercalations.
Devonian
Catavi Formation, 50 m thick - Fine to medium grained fossiliferous sandstone with shale intercalations. The sandstone is overlain by a >250m thick dark shale which is known as the Ventilla Shale in the Llallagua area.
Cretaceous
Condo Conglomerate - Coarse red conglomerate
La Puerta Formation - Pink, cross bedded, medium to coarse grained quartz sandstone.
Miocene
Salvadora Stock - Intrusive rhyolitic to dacitic feldspar-quartz porphyry dated at 20.6± 0.35 Ma. There are other similar stocks in the district.
Pliocene (?) -
Las Frailes Formation - dacitic and rhyolitic lavas and tuffs.
Quaternary - Moraine and alluvium.

The Salvadora Stock cuts the Cancaniri and Llallagua Formations on the western limb of a regional anticline, close to the fold axis.

Mine Geology

At Llallagua economic tin mineralisation is closely associated with the Miocene Salvadora Stock dated at 20.6±0.35 Ma which was intruded into a NNW-SSE trending anticline of Silurian metasediments.   The emplacement of the stock was probably controlled by a set of north to NW oreiented dextral strike-slip faults.

The Salvadora stock is oval shaped, with outcrop dimensions of 1730 x 1050 m. The stock tapers downwards such that it is only 1000 x 700 m some 800 m below the surface. It is composed of two major phases. The outer is a rhyodacitic porphyry grading to an igneous breccia near the outer margin with quartz and feldspar phenocrysts from 2 to 4 mm in diameter set in a fine brown to grey matrix. The breccia clasts occupy around 20% of the zone, ranging from a few mm to 20 cm and are angular to subrounded (Dietrich, et al., 2000).   An example of this zone visited (1977) has horizontal dimensions of 200 x 100 m and extends 280 m vertically. It comprises angular to subangular, to in places subrounded, fragments of feldspar porphyry, intrusive rhyolite, and sediments ranging from 2 or 3 cm to 15 cm across. These are set in a matrix of highly altered fine sand to silt sized material made up of porphyry and lesser sediment fragments.

The inner zone or core od the stock, which covers a surface area of some 1800 x 500 m, is a coarsely porphyritic feldspar-quartz-biotite porphyry of a similar rhyodacitic composition, but having sharp contacts with, the outer zone. This inner porphyry is distinguished by coarse ex-orthoclase phenocrysts ranging from 1 to 2 mm in diameter to 3 x 1 or 2 cm. Quartz phenocrysts are less common (around 35% of the phenocrysts) and are usually only 1 or 2 mm in diameter, but occasionally are up to 4 mm across. Both the feldspar and biotite phenocrysts have been altered to fine grained quartz-sericite-tourmaline aggregates. The ground mass is a fine grey sericitised material. Phenocrysts make up about 40 to 50% by volume of the porphyry and are commonly broken into fine grained fragments. The porphyry carries exotic fragments of sedimentary and intrusive rocks, that are rounded to subrounded, with dimensions of the order of 1 x 3 cm, comprising up to 1% of the porphyry.

In addition to these two main lithotypes, crosscutting rhyodacitic dykes and hydrothermal; breccias make up the remainder of the four main rock types of the stock.

Although the feldspar porphyry is a single body at the surface, it decreases in size with depth to a number of smaller bodies. The largest of these is within the Animas area, where it has dimensions of 200 x 400 m some 400 m below the surface.

A number of kinds of coarse breccia are evident underground. One is a fault breccia, which is usually planar and up to several metres in thickness, and is composed of a brecciated mass of the host rocks that are cut by the fault in the immediate vicinity. The second type is the explosive or hydrothermal breccia mentioned above. These breccias occur as dike or pipe like bodies with a tourmaline and silica cemented matrix of crushed rock containing angular to subrounded clasts of both sediments and volcanics which make up around 40% of the rock.

Adjacent to the stock the sediments have the appearance of grey massive quartzites which are interpreted to be silicified greywacke. The stock has been altered throughout. The outer rhyodacite intrusive has been subjected to sericite-kaolin alteration, while the feldspar porphyry has been altered to sericite-tourmaline.

The rhyodacite and feldspar porphyry appear to have much less disseminated pyrite than the equivalent rocks at Potosi and Oruro.

The Salvadora Stock is cut by two north-south trending sub parallel fault structures, the Stanton and Diaz Faults.

Mineralisation

Within the Salvadora stock mineralisation occurs as disseminations of cassiterite, and as fracture coatings and anastomosing veins and veinlets of quartz-tourmaline-cassiterite-sulphides. The distribution of mineralisation is largely controlled by fractures, fault breccias and intrusive breccias.

As in the case of Oruro, but contrary to Potosi, the veins are composed mainly of sulphide with lesser quartz. The distribution of mineralisation has led COMIBOL geologists to propose the following sequence of mineralising events (1977):

• The development of a NE-SW vein system carrying cassiterite and sphalerite within the stock and intruded sedimentary rocks.
• The development of a NW-SE vein system with slightly more quartz and less cassiterite.
• The Stanton and Diaz faults apparently post date these veins. They appear to have acted as channelways for the introduction of further and economically more important mineralising solutions, which formed veins and disseminations.
• The explosive or hydrothermal breccia zones are interpreted as having developed late in the mineralising period allowing the introduction of the solutions which pervasively altered and mineralised the whole stock.

The early NW-SE and NE-SW trending veins are distributed over most of the stock. These are cut by the Stanton and Diaz fault zones which are extensively altered, pyritised and mineralised. In places the Stanton fault has pockets of ore with dimensions of several tens of metres with up to 25% Sn. Mineralisation is very patchy within the faults. The best grade ore within the mines is found between the Stanton and Diaz faults and adjacent to them. The main areas of extraction within the mine are centred on these faults particularly where a high density of cross cutting veins is developed normal to the fault zones. Grade falls off in these veins away from the faults. The vein systems, like the stock margins, converge with depth, although in some parts of the mine they begin to diverge again.

Cassiterite is dispersed through the hydrothermal breccias. In many places this mineralisation is not visually detectable. In other places there is strong disseminated medium to coarse pyrite with associated veins of pyrite and sphalerite. Where faults cross these breccia zones there are usually high grade sphalerite-cassiterite pockets.

Sections, but not all of individual breccia zones carry grades of from 0.5 to 1% Sn. The cassiterite occurs exclusively within the breccia matrix. Most of the Salvadora Stock is said to carry around 0.2% Sn, occurring as fine disseminated cassiterite "a few" microns in diameter. These grades apparently persist for up to 50 m outwards into the intruded sediments. Sulphide veins also pass outwards undiminished into the sediments for 50 m or so. Within the stock we were told that individual veins have haloes of higher grade disseminated mineralisation. Sulphide veins 10 to 30 cms wide are said to have 10 m wide zones on either side with about 0.6% Sn. There is no silver at Llallagua although the ore has significant tungsten. This tungsten is not extracted.

Practically all of the vein systems outcrop. At the surface the veins are represented by siliceous limonite over a width of a metre or more, with very high tin values.

Part of this summary derives from a mine visit by the author in 1977.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2000.    
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 & Additional Information
   Selected References:
Dietrich, A., Lehmann, B. and Wallianos, A.,  2000 - Bulk rock and melt inclusion geochemistry of Bolivian Tin Porphyry systems: in    Econ. Geol.   v.95, pp. 313-326.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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