La Granja


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The La Granja porphyry copper deposit is located within the Western Cordillera of the Peruvian Andes in Northwestern Peru, 30 km south-east of the Cañariaco deposit, ~120 km NW of Cajamarca, ~700 km north-west of the capital, Lima, and around 100 km to the north-east of the Pacific coast port of Chiclayo (#Location: 6° 21' 43"S, 79° 7' 29"W).

The La Granja porphyry copper deposit is located towards the centre of the Cajamarca Mineral Belt in the Western Cordillera of the northern Peruvian Andes, a generally NNW-SSE to NW-SE trending belt of Oligocene to Miocene porphyry copper deposits that extends for 350 km from Cajamarca in the south to the Ecuadorian border and includes two geochemically distinct groups of deposits along this trend namely: i). porphyry Cu-Mo deposits which include La Granja, Michiquillay, El Galeno, Cañariaco and Rio Blanco; and ii). porphyry Cu-Au deposits which include Cerro Corona, Minas Conga and La Carpa.   These systems are mostly associated with dacite to monzonite to diorite intrusions, which intrude basement rocks of Upper Jurassic to Lower Cretaceous quartzites, limestones and mudstones of the Goyllarisquizga Formation and Early Tertiary sequences of andesitic to dacitic lavas and tuffs of the Llama and Porculla Formations which together comprise the Lower Calipuy Group.

For detail of the broad regional setting and geology of the Peruvian Andes, and the location of La Granja within this framework, see the separate   Peruvian Andes   record.

Basement in the region comprises Precambrian to Early Palaeozoic pelitic schists of the Olmos Complex which includes Precambrian metamorphic rocks, overlain by Ordovician shales and sandstones. These are unconformably overlain by Permian conglomerates, sandstones and volcanic flows and tuffs, which are in turn overlain by Late Triassic to Early Jurassic marine sedimentary intercalated with minor volcanic units of the La Leche Formation. These are succeeded by the Early to Late Jurassic volcano-sedimentary sequence of the Oyotún Formation. The Mesozoic rocks were deposited in ensialic, extensional, marginal basins related to inferred eastward subduction, which extend the length of the Andes. During the Latest Jurassic to Early Cretaceous, the region was uplifted and eroded by the mid-Cretaceous Mochica tectonic phase. By the late Early Cretaceous, subsidence resulted in an eastern sub-basin bounded to the east by the basement Marañon Arch, and the deposition of 2 to 3 km of Cretaceous strata. The oldest of these sedimentary sequences are thick regionally extensive deltaic sandstones with shales and coal, and a thin marine limestone which collectively form the up to 700 m thick, Early Cretaceous Goyllarisquisga Group that unconformably overlies the older rocks. From the close of the Early Cretaceous to the middle of the Late Cretaceous, a marine transgressive sequence of up to 1500 m of marls, shales and limestone were deposited across the region, correlated with the Albian Inca-Chulec-Pariatambo Formation package. Sedimentation ceased abruptly at the beginning of the Early Tertiary when the basin was deformed by the Late Paleocene Incaic I (59 to 55 Ma) and Middle Eocene Incaic II phases (43 to 42 Ma), which resulted in the formation of a foreland thrust and fold belt with SW-dipping, NE-verging thrust sheets, and the development of open, upright folds. Some thrusts were reactivated and folded during the Quechua 1 orogenic pulse (17 Ma). These periods of activity were accompanied by the eruption and deposition of andesitic and rhyolitic volcanic units of the Eocene and Miocene Llama and Porculla Formations, which together comprise the Calipuy Group. A partly porphyritic diorite probably was related to this volcanism. This episode was followed by uplift and erosion and then by renewed magmatism and volcanic activity with the eruption of the 12 to 10 Ma Yanacocha volcanic complex and the Middle to Late Miocene Huambos Formation which capped the stratigraphic sequence in the region.

La Granja is a porphyry copper deposit with associated mineralised breccias and skarns. The main intrusives are dacitic feldspar-quartz porphyry stocks, which according to the USGS MRdata database (viewed 2016), were emplaced between 14 and 10 Ma, and comprise two adjacent clusters of intrusions, the CPB and the Mirador porphyry (Rio Tinto, 2013). These stocks, with the breccias, form irregular diatremes in the country rocks. The dacite emplacement metamorphosed limestones to skarns and formed the breccias, whilst the associated hydrothermal fluids created the hypogene mineralisation and alteration assemblages.

According to Schwartz (1982), the composite stock complex has an exposed area of 2 x 2.5 km and is highly altered. Where least altered, the host dacitic feldspar-quartz porphyry appears to be composed of 5 to 15% quartz phenocrysts, accompanied by relict textures of 20 to 30% feldspar phenocrysts in a 50 to 200 µm matrix of phyllosilicates and quartz in nearly equal proportions. In the northwestern to western section of the stock, this porphyry grades into a marginal, less altered biotite-granodiorite porphyry with 1 to 10 mm phenocrysts of 15 to 30% plagioclase, 3 to 25% biotite, 10 to 20% quartz and 3 to 15% K feldspar in a usually <100 µm siliceous matrix. Towards the southeastern and southern sides it merges instead into a hornblende-granodiorite porphyry which has 1 to 6 mm phenocrysts of 15 to 30% plagioclase, 15 to 25% quartz and 5 to 20% hornblende in a 100 to 300 µm matrix of quartz and potash feldspar with subordinate plagioclase and hornblende. The porphyry mass intrudes a 25 to 60° east dipping sequence composed of Cretaceous, rhyolite, andesite, sandstone/quartzite and limestone (Schwartz, 1982).

Almost all of the composite feldspar-quartz porphyry stock has been intensely altered by phyllic-argillic and advanced argillic assemblages over an area of around 7 km2, affecting parts of the granodiorite porphyry, country rock andesites, rhyolites and sandstones. This area is surrounded by a halo of propylitic alteration, silicified quartzite and skarn (Schwartz, 1982).

The phyllic-argillic zone comprises pervasive replacement, disseminations and veinlets of quartz-sericite and clay minerals, and from 1 to 10% pyrite, with local chlorite and rare tourmaline, with the original silicates being almost entirely replaced and the original textures obliterated (Schwartz, 1982).

The overprinting advanced-argillic alteration, which is largely restricted to the feldspar-quartz porphyry, resulted from the thorough leaching of alkali and alkaline-earth metals and has developed an assemblage of aluminium silicates, quartz and diaspore, with the complete obliteration of the original rock textures. The final assemblage includes andalusite-quartz, andalusite-pyrophyllite-quartz, andalusite-sericite-quartz, diaspore-pyrophyllite-quartz and sericite-pyrophyllite-quartz. The development of these assemblages has released silica, with the areas of high andalusite and diaspore having the most quartz. The areas of the best developed advanced-argillic alteration also have the greatest pyrite, with from 2 to 12% (Schwartz, 1982).

The propylitic alteration halo is best developed in the andesitic, granodioritic and dioritic country rocks. It is weak, with an assemblage that includes chlorite, epidote, clinozoisite, calcite, anhydrite and clay minerals, and has not masked the original textures. The surrounding country rocks have been altered to varying degrees, with the pure quartzites having been metasomatised in places with the addition of pyrite, the silty sandstones have been modified by sericite-clay and the limestones converted to calc-silicate rocks with garnet, actinolite, wollastonite, epidote and clinozoisite (Schwartz, 1982).

Hypogene mineralisation is dominated by chalcopyrite, with subordinate bornite, occurring mainly as disseminations, with lesser fracture fillings and veinlets. The associated pyrite varies from 1 to 12% as disseminations, and in areas of higher concentrations, as fracture filling and veinlets. The chalcopyrite content varies inversely to pyrite. Molybdenite, which is commonly found as fracture coatings, is rarely in contact with other sulphides, and is independent of pyrite. In general, hypogene grades vary from 0.3 to 0.8% Cu in the sercite-clay zone, although in the advanced argillic zone it may be as low as 0.05% Cu. Locally, veins of massive enargite and tennantite are found. Rutile is a common mineral, while magnetite occurs as disseminations and in veins on the periphery of the deposit (Schwartz, 1982).

The entire central portion of the deposit is predominantly occupied by advanced-argillic alteration, with lesser sericite-clay, and corresponds to a leached cap that varies in thickness from a few metres in eroded valleys to as much as 200 m below old preserved erosion surfaces. Sulphides have been almost entirely oxidised to limonites, with only very rare copper oxides remaining. Copper has been depleted to between 3 and 10% of the hypogene grade, and Mo to half. Iron has been enriched in some parts of the leached capping, whereas gold remained stable. The coincidence of low Fe, high Mo and high Au contents with a high Cu/Fe ratio in the capping indicates the approximate centre of the underlying orebody. Also, high K, Rb, Mg and low Sr indicate high-grade copper ore. Despite the intense weathering that has produced the thick leached capping, the distribution of Mg, Ca, Na, K, Rb and Sr can still be correlated with different phases of hydrothermal alteration (Schwartz, 1981). Leaching is only weak to absent in the propylitic zone (Schwartz, 1982).

The underlying 25 to 250 m thick supergene sulphide enrichment blanket is separated by a thin transition zone which is commonly <2 m thick. The supergene enrichment corresponds to sericite-clay alteration, and is thickest between the base and crest of the main hill. The supergene sulphide assemblage is dominated by chalcocite and covellite which replace chalcopyrite and bornite. The supergene ore carries from 0.5 to 2.0% Cu in the sericite-clay zone (Schwartz, 1982).

InfoMine (2008) quote the resources at La Granja at 3.5 Gt @ 0.55% Cu.

JORC compliant ore reserves and mineral resources at La Granja at 31 December, 2015 (Rio Tinto Annual Report, 2016) were:
    Proven + probable ore reserves - Nil.
    Indicated resource - 130 Mt @ 0.85% Cu.
    Inferred mineral resource - 4.19 Gt @ 0.50% Cu.
  TOTAL measured + indicated + inferred mineral resources - 4.32 Gt @ 0.51% 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 Granja

  References & Additional Information
   Selected References:
Schwartz M O,  1982 - The porphyry copper deposit at La Granja, Peru: in    Econ. Geol.   v77 pp 482-488

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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