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Famatina mining district - Nevados de Famatina, La Mejicana

La Rioja, Argentina

Main commodities: Cu Au Mo Ag
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The Famatina mining district, which covers an area of 35 km2 contains several small Cu-Mo-Au mineralised porphyry stocks and a set of vertically extensive high-sulphidation epithermal Cu-Au-(As-Sb-Te) veins, and is located in the Famatina Range, La Rioja province, northwestern Argentina (#Location: 29° 00' 30"S, 67° 46' 41"W ).

Magmatic-hydrothermal mineralisation is of Early Pliocene above the incipient flat-slab region of the southern Central Andes more than 400 km from the modern Chilean trench. Ore-grade epithermal mineralisation in the La Mejicana vein system is 1.5 km to the northwest of, and above the Nevados de Famatina porphyry system.

The Nevados de Famatina porphyry contains:
    300 Mt @ 0.37% Cu, 0.06% Mo, 0.6 g/t Ag, 0.3 g/t Au,
while the proven reserves of the high-sulphidation epithermal veins is:
    0.25 Mt @ 1.1% Cu, 64 g/t Ag, 6 g/t Au.
Past production in this region yielded >1 Mt averaging 3% Cu, 80 g/t Ag, 11 g/t Au from >50 veins (Pudack, et al., 2009).

The sequence in the Famatina mining district begins with marine sedimentary units of the Cambrian Negro Peinado Formation (equivalent to the Puncoviscana Formation elsewhere in northern Argentina), which have undergone low-grade metamorphism during the Lower Ordovician, followed by the intrusion of Phaneritic granites of the Ordovician Ñuñorco Formation. Upper Palaeozoic continental sediments of the Agua Colorada and Patquía Formations unconformably overlie all of these rocks. Numerous dacitic stocks and dykes of the so-called Mogote Formation were subsequently intruded into the sedimentary rocks during the Early Pliocene (5.0 ±0.3 Ma), although there is no evidence of related volcanism in the area. However, ash-flow tuff of the El Durazno Formation on the eastern flank of the Famatina Range is probably coeval with these dacitic porphyries. Limonite-cemented conglomerates and clast-supported breccias of the Pleistocene Cueva de Peréz Formation and unconsolidated Quaternary alluvial and colluvial deposits locally cover earlier units.

The oldest regional faulting in the Famatina Range strikes NE and NW and was formed during the Famatinean orogeny. North-south trending aligned fault blocks are the dominant structures, bounded by steeply dipping reverse faults, and are the result of the Andean orogeny, as are the major north-south-trending faults and fold axes. Most of the dacitic porphyries, which are the main host for the Nevados de Famatina Cu-Mo-Au system, were emplaced along these pre-existing north-south-trending structures. In contrast, the east-west faults, which are stress relief zones after the main tectonic event, are structurally less important on a regional scale but appear to control the high-sulphidation epithermal Cu-Au veins. Ar-Ar plateau ages indicate that intrusive activity may have persisted for >1 m.y., from ~5.3 ±0.1 to 4.0 ±0.1 Ma as multiple pulses of subvolcanic intrusion, probably related to a single granodioritic magma chamber at greater depth, forming several porphyry stocks with similar magmatic texture and mineralogy.

Hydrothermal vein types recognised in the porphyry system are A-veinlets and B-veins, which are locally crosscut or reactivated by QSP veins (which are D-veins). The high-sulphidation epithermal veins formed after cessation of intrusive activity at the present levels of exposure, although these late veins occur in proximity to the porphyries and related dykes. Each of the porphyry stocks has a single and apparently consistent succession of vein stages, although the correlation of vein stages between the intrusions remains speculative.

A-veinlets are composed of equigranular quartz with variable amounts of K-feldspar, biotite, magnetite, ilmenite, hematite and anhydrite, and range in thickness from 1 to about 20 mm in all dacitic porphyries, extending into the adjacent of the Negro Peinado Formation metasedimentary rocks.   B-veins are composed of granular quartz, molybdenite and pyrite, with minor orthoclase, magnetite, biotite, rutile, ilmenite, hematite, chalcopyrite, bornite and late chalcocite. Vein centres are locally vuggy with parallel and slightly wavy vein walls. They range from 2 to 60 mm in thickness and have been observed in all dacitic porphyries as well as in the adjacent sedimentary wall rocks and are commonly associated with potassic alteration envelopes grading upward into texturally destructive sericitic alteration assemblages. The sulphide minerals occur either in vein centres or fill fractures cutting the coarser grained veins. Crosscutting relationships strongly suggest B-veins generally postdate A-veinlets.

The QSP (D-) veins only contain minor amounts of bornite, idaite, chalcocite, chalcopyrite, covellite, sulphosalts, and sphalerite and tend to be thicker and more continuous than either A-veinlets or B-veins. They have relatively straight walls, and locally contain open spaces or vugs. Quartz is euhedral, with a glassy to glassy-white color, and was commonly precipitated on top of an older generation of epigranular quartz. Pyrite is common in vein centres or along vein margins and contains abundant sulphide inclusions, e.g., as chalcopyrite, bornite, and covellite. Sericite is present as selvages and locally fills open spaces between pyrite grains. The QSP veins invariably post-date A-veinlets and B-veins of the porphyry stockwork, and appear to be most abundant in proximity of the high-sulphidation epithermal veins and appear in some instances to grade upward into high-sulphidation epithermal enargite-famatinite-pyrite-alunite veins.

At La Mejicana, the high-sulphidation epithermal Cu-Au veins are steeply dipping and trend east-west, with widths of a few cm to 4 m. They are mainly hosted by Negro Peinado Formation metasedimentary rocks, extending to about 500 m above the currently exposed porphyry stock. The veins are composed of famatinite, enargite, pyrite and alunite in vari- able proportions, with minor quartz, kaolinite, dickite, pyrophyllite, tennantite, tetrahedrite, sphalerite, gold, tellurides, covellite and chalcopyrite. Quartz has a microcrystalline to jasperoid texture, commonly lining open vugs. Alunite is predominantly of hypogene origin, as indicated by its coarse grain size, bladed crystal habit, and its close association with the hypogene Cu minerals. Gold is present as free gold and in tellurides such as calaverite and sylvanite. Enargite and famatinite are the dominant sulphosalts and Cu-bearing minerals, forming massive granular intergrowths. Covellite is found either as rims on tennantite-tetrahedrite or as isolated grains within alunite. Pyrite, which is also common, usually constitutes the cores of grains with marcasite overgrowths. Chalcopyrite is a minor constituent, commonly occurring as inclusions in sphalerite and pyrite. Argentite, freibergite, and native silver have been reported at upper levels.

The porphyries are generally pervasively altered and weakly mineralised by a dense network of quartz veinlets overprinted by a stockwork of quartz-molybdenite-pyrite ± chalcopyrite veins extending into the wall rock. The related alteration sequence comprises an early potassic assemblage, surrounded by a limited propylitic zone at the southern and northeastern fringes of the district. Phyllic alteration occurs in the upper parts of the system and predominates in the vicinity of the La Mejicana ridge. Advanced argillic alteration includes an assemblage of alunite, kaolinite, dickite, pyrophyllite, diaspore and zunyite, and tends to be localised above the intrusive bodies, where it may obliterate previous alteration assemblages. Pervasively argillic altered rocks show a gradual transition into highly siliceous zones where quartz ± zunyite replaces all previous mineral assemblages. Subordinate supergene alteration resulting from sulphide weathering occurs throughout the district. QSP veins cross lithologic contacts, which causes subtle changes in vein mineralogy, morphology and related hydrothermal alteration. They are associated with sericitic assemblages, including quartz, sericite, pyrite, illite and rutile, which generally obliterates original rock textures and causes a characteristic bleaching of the host rocks. Residual vugs associated with this veining are commonly filled with typical advanced argillic mineral assemblages, including alunite, kaolinite, pyrophyllite, and aluminum-phosphate-sulphate (APS) minerals. Advanced argillic alteration is particularly abundant in the northwestern part of the mining district, typically forming during pre-mineralisation acid leaching in the epithermal environment. Texture-destructive quartz ± alunite alteration occurs discontinuously up to 1 m away from steep faults or fractures. The quartz is dominant fine grained, whereas alunite occurs as well-developed crystals exhibiting growth zones. Alunite becomes more abundant with increasing distance from the veins and in the higher parts of the epithermal vein system. It grades into an assemblage of quartz, pyrophyllite, or dickite, and alunite. Kaolinite-dominated bleaching extends up to 200 m away from veins and zones of more intense alteration and is also dominant in the highest parts of the deposits, accompanied by some barite and anhydrite.

The most recent source geological information used to prepare this summary was dated: 2009.    
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.


Famatina

  References & Additional Information
   Selected References:
Pudack C, Halter W E, Heinrich C A and Pettke T,  2009 - Evolution of Magmatic Vapor to Gold-Rich Epithermal Liquid: The Porphyry to Epithermal Transition at Nevados de Famatina, Northwest Argentina : in    Econ. Geol.   v104 pp 449-477


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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