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La Voluntad

Neuquen, Argentina

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The low grade La Voluntad porphyry copper-molybdenum deposit is located 74 km SW of the city of Zapala in Neuquén Province, Argentina (#Location: 39° 12' 41"S, 70° 36' 19"W).

Regional Setting

  The oldest rocks outcropping in the district are quartz-biotite schists, biotite-muscovite, gneisses and migmatites of the Piedra Santa Formation (Digregorio and Uliana, 1980) of the Palaeozoic metamorphic basement, confirmed to be Devonian-Carboniferous, based on K-Ar datings of shales nearby by Franzese (1995) . These rocks are intruded by the Chachil Plutonic Complex (Leanza, 1990), composed of granites, granodiorites, tonalites, tonalite porphyry and quartz-diorite. This intrusives complex has been allocated a Carboniferous to lower Permian age, based on their stratigraphic relationships and a radiometric dating in the La Voluntad intrusive (K-Ar, 281±4 Ma; Sillitoe 1977) and a Re-Os molybdenite age of mineralisation at La Voluntad (~316 to 312 Ma; Garrido et al., 2008). Upper Permian to Middle Triassic rhyodacitic volcanics of the Choiyoi Formation (Stipanicic et al., 1968) unconformably overlie the Plutonic Complex. These are, in turn, overlain by andesites of the Lapa Formation (Groeber, 1956), and black shales and mudstones of the Molles Formation (Weaver, 1931), red mudstones Formation of the Challacó (De Ferraris, 1947), all of Jurassic age. These are followed by Pleistocene Basalts of the Maipo Formation (Yrigoyen, 1972) and fluvial deposits and landslides (Garrido and Dominguez, 1997).
  La Voluntad is the oldest porphyry copper deposit so far recognised in the Andes, and its occurrence indicates the presence of an active Permo-Carboniferous magmatic arc, with associated porphyry style mineralisation, that trends SE south of Cerro Chachil, obliquely diverging from the Andean Range to the south.
  For more detail on the setting, see the Southern Andes and Patagonia Terrane record

Geology

  In the deposit area, granodiorite is intruded by tonalite and numerous felsic (aplitic) and dark dykes (diabasic), all belonging to the Chachil Plutonic Complex (Leanza, 1990). This intrusive complex is intruded into the metamorphites of the Palaeozoic Piedra Santa Formation.
  A grey granodiorite occurs in the northern and southeastern sections edges of the deposit area, and has a texture that varies from porphyritic to equigranular, composed of feldspar, quartz, biotite and amphibole, with feldspar phenocrysts that reach sizes of up to 2 cm.
  A tonalite 'stock' that covers an area of ~0.5 km2, intrudes the granodiorite, and is more resistant to erosion. It is characterised by its dark grey colour where fresh, and its clear and reddish tones where it has been subjectec to hydrothermal alteration. The tonalite ranges from porphyritic with phaneritic to equigranular. In the SW of the deposit area, where it is light grey in colour, the diorite is equigranular, medium grained, and principally consists of plagioclase, quartz and biotite. Elsewhere it is porphyritic and composed of phenocrysts of quartz, plagioclase and biotite in a phaneritic matrix of the same minerals. The quartz phenocrysts are anhedral, sometimes with graphic texture, whilst oligoclase is subhedral and zoned, and the alkali feldspars have perthitic textures. The subhedral biotite, which is sometimes chloritised, contains epidote and opaque minerals following cleavages. Muscovite occurs locally as an accessory mineral and exhibits kink banding.
  Locally, the tonalite contains rounded xenoliths of granodiorite up to 30 cm in diameter which are banded, and composed of quartz, plagioclase and alkaline feldspar and contain biotite bands. A steep, 30° trending fault intersects the tonalite intrusive. This fault zone comprises a 50 cm wide breccia zone composed of angular clasts of tonalite, rotated and cemented by quartz and sulphides.
  An ovoid, 50 x 25 m pegmatite occurs within the tonalite, containing quartz and coarse biotite. The biotite is unaltered, whilst the quartz has deformation textures. SW of the tonalite, there is an irregular barren quartz body, interpreted to represent a late stage event.
  Light aplitic dykes, composed of quartz, alkali feldspar, plagioclase and muscovite, and dark dykes that vary from dolerite to tonalite in composition cut the tonalite and granodiorite with a wide range of orientations. Another breccia is tabular, and composed of fragments of quartz, alkali feldspar and plagioclase in a finer grained matrix of the same composition.
  The tonalite is overlain by a small, irregular outcrop of a fine-grained porphyritic rhyolite with phenocrysts of sanidine texture, oligoclase and biotite in a sparse feldspar matrix.

Alteration and Mineralisation

  Alteration and mineralisation are predominantly developed within the tonalite and to a lesser degree in the granodiorite. Alteration is principally potassic, phyllic and propylitic, with the phyllic style being the best developed. The hypogene mineralisation comprises an early weak, pervasive phase associated with the potassic alteration, and a later, more intense pulse that produced a set of veinlets with the phyllic alteration.
Potassic alteration is pervasive and best manifested in the central part of the tonalite, characterised by an assemblage of biotite, subordinate K feldspar, sericite and quartz, with trace rutile and apatite. Apatite replaces primary magmatic biotite. Alteration is texture preserving and pervasive, with biotite being very fine grained and difficult to distinguish macroscopically. The secondary biotite is subhedral and brown, and is found in the matrix of the tonalite, as very small fibres (30 to 40µm), nested between other rock-forming crystals. K feldspar is very rare, occurring in the matrix forming a mosaic with quartz and occasionally replacing plagioclase. The potassic assemblage is associated with apatite, pyrite and chalcopyrite.
  Veinlets of biotite, biotite-K feldspar-quartz; biotite-quartz, and quartz-feldspar cut the tonalite. The latter also cut the pervasive biotite and potassic alteration veinlets. These veinlets ranging from microns to millimeters in width, with some as wide as 2 cm, composed of quartz, feldspar and biotite. One quartz-K feldspar-sericite veinlet was observed to cut and aplite dyke, but is cut, in turn, by a quartz veinlet.
Propylitic alteration is not well developed and is composed of chlorite, calcite, epidote and zeolites. Chlorite is dominant, replacing biotite and amphibole in the tonalite, and is associated with epidote. Calcite occurs in veinlets with subordinate quartz.
Phylliic alteration is characterised by light colours are superimposed on the potassic alteration, and is principally composed of quartz and sericite. It is not pervasive, and highly variable in intensity, commencing with an incipient replacement of plagioclase and biotite, up to the formation of a quartz and sericite mosaic with up to 10 vol.% pyrite, mainly occurring as fracture controlled veinlets. The veinlets vary in thickness from microns in the centre of the tonalite, to as much as 30 cm on the peripheries. The following vein types have been recognised:
  - Sericite occurring as microscopic monomineralic veinlets surrounding plagioclase phenocrysts and cutting hydrothermal biotite and biotite altered plagioclase;
  - Quartz filling veins cutting potassic altered aplite dykes and also appear to post-date phylllic alteration, possibly representing a late silicification event;
  - Quartz and sericite, occurring as as anastomosing veinlets with quartz centres, enclosed by sericite selvages, that cut potrassic altered tonalite;
  - Quartz and pyrite, filling fractures and microfractures, as banded quartz and pyrite;
  - Quartz, molybdenite and chalcopyrite, filling fractures, composed of abundant quartz with traces of molybdenite and weak chalcopyrite;
  - Quartz, sericite, sulphides and wolframite veins filling fractures, with an incipient fine grained replacement of quartz and sericite on the wallrocks, culminating in comb quartz with open spaces, and associated pyrite, chalcopyrite, arsenopyrite and sphalerite. In other veinlets, molybdenite is present along the vein edges, with pyrite, chalcopyrite and sphalerite. These veinlets cut others where wolframite is associated with sericite and quartz chalcopyrite and molybdenite.
  The phyllic alteration is limited to veinlets and superimposed on the potassic alteration, and occurs as at least two sets of veinlets.

  Mineralisation associated with the weak pervasive potassic alteration is dominantly disseminated chalcopyrite, pyrite and trace molybdenite, and as veinlets with subordinate biotite.
  The most important mineralisation is associated with the phyllic alteration, with a paragenesis from arsenopyrite → pyrite → trace bornite → sphalerite and abundant chalcopyrite (associated with pyrite) → wolframite → a second generation of arsenopyrite.
  Vein densities vary from a maximum of 0.13 veinlets per metre in the central part of tonalite intrusive, decreasing outwards to the periphery where veinlets are spaced at ~ 50 cm at a distance of 1 km from the centre.
  Fluid inclusions suggest moderate to low salinity fluids, with temperatures between 420 to 240°C, with sporadic boiling during the main mineralisation.
  Five Re-Os molybdenite ages from four samples representing three different vein types (i.e., quartz-molybdenite, quartz-sericite-molybdenite and quartz-sericite-molybdenite±chalcopyrite-pyrite) are identical within error and were formed between ~312 and ~316 Ma in the upper Carboniferous (Garrido and Dominguez, 1997).
  Supergene mineralisation is poorly developed, represented by malachite, chalcocite, covellite, hematite and limonite. Chalcopyrite is replaced by chalcocite and covellite, and fine hematite prisms replacing bornite. The limonite (goethite) is reddish brown, associated with pyrite and biotite. Malachite is presented as patinas and filling joints, colloidal textures and with calcite.

Resources

  Singer et al., (2008) quote a resource estimate of   250 Mt @ 0.15% Cu.

The most recent source geological information used to prepare this summary was dated: 2008.    
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.


La Voluntad

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Garrido, M. and Dominguez, E.,  1997 - Geologa, alteracin hidrotermal e inclusiones fluidas del yacimiento del tipo cobre porfdico La Voluntad, Provincia de Neuqun, Argentina: in    Revista Geologica de Chile   v.24, pp. 91-108.
Sillitoe, R.H.,  1977 - Permo-Carboniferous upper Cretaceous and Miocene porphyry copper-type mineralization in the Argentine Andes: in    Econ. Geol.   v.72, pp. 99-109


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