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Baja de la Alumbrera

Argentina

Main commodities: Cu Au
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The Bajo de la Alumbrera porphyry copper-gold deposit (Alumbrera) is located within the northern Sierras Pampeanas in the eastern Andes Mountains of Catamarca province in northwestern Argentina. It lies 145 km to the northwest and 150 km southwest of the Provincial capitals of Catamarca and Tucuman respectively.

It is on the eastern margin of the Miocene to Pliocene belt of porphyry copper mineralisation, some 250 to 300 km to the east of the main belt of Chilean porphyry deposits. The deposit is centred on a two phase dacite-porphyry stock emplaced within a large andesitic strato-volcano. It has a central cylindrical core of K-silicate alteration with high grade Cu-Au zones rich in quartz and magnetite, surrounded by sericite and supergene kaolinite, with an external propylitic zone. The supergene profile is immature and economic ore is principally hypogene.

The Alumbrera deposit formed in a tectonically favourable location within a major arc-oblique wrench fault system, the Tucuman Transfer Zone. Initial andesitic volcanism deposited on crystalline Lower Palaeozoic basement, and subsequently emplaced dacitic subvolcanic stocks are directly related to eastward subduction of the Nazca oceanic plate beneath the western continental margin of South America. Structural preparation and shallowing of the angle of subduction of the Nazca plate - related to the arc-normal Juan Fernandez Ridge on that plate - probably aided the ascent of calc-alkaline oceanic arc-related magma into the Tucuman Transfer Zone.

The commencement of volcanism was approximately coincident with the shallowing of the angle of subduction beneath the northern Sierras Pampeanas. Intrusion of the mineralised porphyries was contemporaneous with the development of thick-skinned shortening which produced uplift of the Sierras de Aconquija basement block to the southeast at between 10 and 5 Ma. Structural degradation of the crystalline basement brought about by the development of a broad asthenospheric wedge above the descending Nazca plate, aided the development of block uplift caused by generally east-west shortening at that time.

Ar-Ar dating of the various mineralised intrusive phases indicates intrusive activity lasted for approximately 270 000 years. The upper age of mineralisation is interpreted to be at the onset of feldspar destructive alteration (phyllic-argillic styles) at 6.75 ± 0.09 Ma. The presence of an apparently older, unmineralised phase (the Los Amarillos Porphyry - LAP) adjacent to the mineralised porphyries suggests that the evolution from unmineralised to mineralised magma may have been quite rapid. However, due to the highly altered nature of the LAP, no clear genetic relationships have been established between these porphyries.

Mineralisation was focussed on the intrusive centre and the surrounding andesitic host rocks. Reconstruction of the original geometry of the deposit indicates that the cluster of porphyry stocks and dykes that define the intrusive centre formed a sub-circular body with a diameter of around 500 m, while the overall dimensions of the mineralised system (at greater than 0.15% Cu) was approximately 800 x 800 m. The vertical dimension of the mineralisation is less easily measured, but was probably in the range of 800 to 1000 m. Approximately 3.36 million tonnes (Mt) of copper and 409 tonnes of gold were deposited within this volume.

Fluid inclusion studies indicate that deposition of mineralisation seems to have been strongly controlled by cooling of the mineralising fluids, with sulphide phases being formed as the fluid cooled below a 400 to 360° C temperature threshold.

Mineralisation was accompanied by the near simultaneous formation of quartz-magnetite and potassic alteration in the porphyries and potassic alteration of the adjacent andesites. An unmineralised propylitic halo developed in the andesites beyond the limit of potassic alteration. Feldspar destructive (phyllic ± argillic) alteration post-dated the mineralisation and probably formed in response to degradation of the thermal plume and consequent induction of increasingly acidic phreatic water into convection cells adjacent to the intrusives. Feldspar destructive alteration is usually accompanied by decreasing grade, suggesting that at Alumbrera this alteration stage remobilised and removed mineralisation.

Structural rotation of the northern Sierras Pampeanas during the Upper Miocene to Lower Pliocene resulted in the reactivation of earlier structures in a transpressional regime that caused dismemberment of the porphyry mineralisation at Alumbrera. Strong alteration of the earliest of the major faults suggests that structural disruption occurred during the development of feldspar destructive alteration, and may have been responsible for the termination of this alteration event. Subsequent movement resulted in displacement of the ore blocks across both normal and reverse faults.

Active uplift of the Sierras Pampeanas and consequent rapid erosion prevented the development of significant secondary oxide or supergene mineralisation at Alumbrera.

The pre-mining measured resource was: 695 Mt @ 0.51% Cu, 0.66 g/t Au, with a high grade core of 118 Mt @ 0.64% Cu, 0.92 g/t Au.
Production to the end of 2004 amounted to: 220 Mt @ 0.66% Cu, 0.86g/t Au.
Remaining reserves + resources at the same date were: 404 Mt @ 0.47% Cu, 0.51g/t Au.

The mine is operated by Minera Alumbrera Limited which is owned by Xstrata plc 50%, Wheaton River Minerals Ltd. 37.5 % and Northern Orion Resources Inc. 12.5 %.

The most recent source geological information used to prepare this summary was dated: 2006.    
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:
Brown S C,  2005 - A Review of the Geology and Mineralisation of the Alumbrera Porphyry Copper-Gold Deposit, Northwestern Argentina: in Porter, T.M. (Ed), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide,   v.1 pp. 115-131
Guilbert J M  1995 - Geology, alteration, mineralisation and genesis of the Bajo de la Alumbrera porphyry copper-gold deposit, Catamarca Province, Argentina: in Pierce F W, Bolm J G (Eds),  Porphyry Copper Deposits of the American Cordillera Arizona Geol. Soc.   Digest 20 pp 646-656
Harris A C, Allen C M, Bryan S E, Campbell I H, Holcombe R J and Palin J M,  2004 - ELA-ICP-MS U-Pb zircon geochronology of regional volcanism hosting the Bajo de la Alumbrera Cu-Au deposit: implications for porphyry-related mineralization: in    Mineralium Deposita   v39 pp 46-67
Harris A C, Bryan S E and Holcombe R J  2006 - Volcanic Setting of the Bajo de la Alumbrera Porphyry Cu-Au Deposit, Farallon Negro Volcanics, Northwest Argentina: in    Econ. Geol.   v101 pp 71-94
Harris A C, Dunlap W J, Reiners P W, Allen C M, Cooke D R, White N C, Campbell I H and Golding S D,  2008 - Multimillion year thermal history of a porphyry copper deposit: application of U–Pb, 40Ar/39Ar and (U–Th)/He chronometers, Bajo de la Alumbrera copper–gold deposit, Argentina: in    Mineralium Deposita   v43 pp 295-314
Harris A C, Golding S D and White N C  2005 - Bajo de la Alumbrera Copper-Gold Deposit: Stable Isotope Evidence for a Porphyry-Related Hydrothermal System Dominated by Magmatic Aqueous Fluids: in    Econ. Geol.   v100 pp 863-886
Proffett J M  2003 - Geology of the Bajo de la Alumbrera Porphyry Copper-Gold Deposit, Argentina: in    Econ. Geol.   v98 pp 1535-1574
Sillitoe R H  1995 - Baja de La Alumbrera, Argentina: in Sillitoe R H  Exploration and Discovery of Base- and Precious-Metal Deposits in the Circum-Pacific Region During the Last 25 Years Metal Mining Agency of Japan    pp 21-23
Ulrich T, Gunther D, Heinrich C A,  2001 - The evolution of a Porphyry Cu-Au deposit, based on LA-ICP-MS analysis of fluid inclusions: Bajo de la Alumbrera, Argentina: in    Econ. Geol.   v96 pp 1743-1774
Ulrich T, Heinrich C A  2001 - Geology and alteration geochemistry of the Porphyry Cu-Au deposit at Bajo de la Alumbrera, Argentina: in    Econ. Geol.   v96 pp 1719-1742

   References in PGC Publishing Books: Want any of our books ? Pricelist
Brown S C, 2005 - A Review of the Geology and Mineralisation of the Alumbrera Porphyry Copper-Gold Deposit, Northwestern Argentina,   in  Porter T M, (Ed),  Super Porphyry Copper and Gold Deposits: A Global Perspective,  v1  pp 115-131
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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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