Summary
Bajo de la Alumbrera forms an example of a series of world class copper-gold deposits that are hosted by potassic igneous rocks. The Bajo de la Alumbrera porphyry copper-gold deposit is situated in the Catamarca Province of Northwest-Argentina and belongs to the ten largest copper mines in the world; it is also one of the largest gold producers in South America.
The deposit is characterized by an alteration-zoning pattern with a central potassic core which grades outwards into a zone with propylitic alteration and an intermediate annular phyllic overprint. Porphyry copper-gold mineralization consists mainly of disseminated and/or vein-related chalcopyrite within the central potassic alteration zone which also hosts the highest gold grades (up to 2 ppm). Mineralization is genetically related to co-magmatic hypabyssal dacite domes with shoshonitic compositions which intruded the high-K calc-alkaline andesitic to latitic lava flows, breccia tuffs, and agglomerates of the Upper Miocene Farallon Negro Formation. The Farallon Negro Formation is interpreted as the remnants of a major stratovolcano some 16 km in diameter, with volcanism being controlled by sinistral pull-apart tectonics along a major NW-trending lineament.
The dacitic rocks associated with the porphyry copper-gold mineralization have porphyritic textures with phenocrysts of alkali feldspar, plagioclase, biotite, and very rare amphibole in a fine-grained feldspathic groundmass. The abundance of hydrous minerals, such as biotite phenocrysts and apatite microphenocrysts, is consistent with the volatile-rich nature of the melts. The rocks underwent moderate degrees of clinopyroxene-biotite-âpatite-titanite fractionation. Several samples are overprinted by weak to moderate potassic or carbonate-sericite alteration. Geochemically, the dacites are characterized by high LILE (e.g. K20 up to 7.16 wt%, and Rb, Sr, and Ba up to 145 ppm, 445 ppm, and 610 ppm, respectively), moderate LREE (La < 73 ppm and Ce < 140 ppm), and low HFSE (TiO2 < 0.92 wt%, Zr < 142 ppm, Hf < 4.6) which are typical for subduction-related potassic igneous rocks from a mature continental-arc setting. The rocks have evolved compositions with relatively low mg# < 53. Their high Ba/Nb and high Rb/Nb ratios (up to 87 and 31, respectively) clearly distinguish them from rocks derived from within-plate settings.
The rocks from Bajo de la Alumbrera contain average ratios of Fe2O3/(FeO+Fe2O3) as high as 0.49 reflecting the oxidized nature of their parental melts.
Zusammenfassung
Bajo de la Alumbrera ist ein Beispiel für eine bedeutende Gold-Kupfer Lagerstätte in Kalium-reichen Magmatiten. Die Kupfer-Gold Lagerstätte Bajo de la Alumbrera gehört zum sogenannten Porphyry-Typ und befindet sich in der Catamarca Provinz im Nordwesten von Argentinien. Bajo de la Alumbrera zählt zu den zehn größten Kupferminen der Welt und ist einer der größten Goldproduzenten in Südamerika.
Die Lagerstätte zeichnet sich aus durch eine klassische Zonierung der Alterationszonen, beginnend mit der zentral aufgeschlossenen `potassischen' Alterationszone (sekundärer Biotit-Magnetit-Orthoklas), die weiträumig von ‘propylitischer’ Alteration (sekundärer Epidot-Chlorit) umgeben wird. Beide Alterationstypen werden von einer spätgenetischen ‘phyllischen’ Alteration (sekundäre Tonminerale-Serizit-Pyrit) umgewandelt. Die Kupfer-Gold-Vererzung besteht überwiegend aus Kupferkies, der entweder feinverteilt in den Silikaten oder in Quarzadern innerhalb der potassischen Alterationszone vorkommt. Diese Zone beherbergt neben den höchsten Kupfergehalten auch die höchsten Gold-Gehalte von bis zu 2 ppm. Die Vererzung steht in direktem genetischen Zusammenhang mit subvulkanischen Dazit-Domen mit außergewöhnlich hohen Kalium-Gehalten, die die kalkalkalischen Andesite, Latite, Tuffbrekzien und Agglomerate der Ober-Miozänen Farallon Negro Formation intrudierten. Die Farallon Negro Formation stellt die erodierten Reste eines Stratovulkans mit schätzungsweise 16 Kilometern Durchmesser dar. Der Vulkanismus ereignete sich innerhalb eines ‘Pull-Apart’ Beckens, entlang eines großen Nordwest-streichenden Lineaments.
Die dazitischen Muttergesteine der Kupfer-Gold-Vererzung haben porphyrische Texturen mit Phänokristallen bestehend aus Kalifeldspat, Plagioklas, Biotit, und, sehr selten, Amphibol in einer fein- bis mikrokristallinen Grundmasse. Die Häufigkeit von Mineralen wie Biotit und Apatit reflektiert die hohen Gehalte an volatilen Phasen in der Schmelze. Die mineralogische Zusammensetzung der Gesteine wurde von einer moderaten Klinopyroxen-Biotit-Apatit-Titanit Fraktionierung mitgeprägt. Einige der untersuchten Dazite wurden von einer schwachen bis moderaten Biotit-Magnetit-Orthoklas oder Karbonat-Serizit-Alteration überprägt.
Die chemische Zusammensetzung der Dazite zeichnet sich aus durch hohe Gehalte an LILE (z. B. K20 bis zu 7.16 Gew%, Rb, Sr, und Ba bis zu 145 ppm, 445 ppm, und 610 ppm), moderate Gehalte an LREE (z.B. La < 73 ppm, und Ce < 140 ppm), und geringe Gehalte an HFSE (z.B. TiO2 < 0.92 Gew%, Zr < 142 ppm, und Hf < 4.6 ppm), die typisch sind für Kalium-reiche Magmatite von kontinentalen Subduktionszonen. Die untersuchten Gesteine sind relativ fraktioniert und besitzen geringe mg# < 53. Ihre hohen Ba/Nb und Rb/Nb Verhältnisse sind typisch für Kalium-reiche Magmatite aus Subduktionszonen und unterscheiden sie eindeutig von Gesteinen aus within-plate settings.
Die hohe Oxidationsstufe der Mutterschmelze reflektiert sich in den hohen Fe2O3/ (FeO + Fe2O3) Verhältnissen der untersuchten Gesteine von 0.49.
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Müller, D., Forrestal, P. The shoshonite porphyry Cu-Au association at Bajo de la Alumbrera, Catamarca Province, Argentina. Mineralogy and Petrology 64, 47–64 (1998). https://doi.org/10.1007/BF01226563
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DOI: https://doi.org/10.1007/BF01226563