Andean Cordillera

Tags: andesite claystone dyke granite jurassic miocene paleogene permian rhyolite sandstone stock triassic

Description:

The Andean Cordillera extends for about 5,000 km along the western coast of South America, attaining a maximum width of about 700 km in the Central Andes of Bolivia.

Tectonism in the Cordillera varies both along strike and across the range; along-strike variations reflect changing plate geometry along the Pacific margin, whereas across-strike variations generally assigned to four sub-domains reflect the generally eastward migration of Andean arc magmatism and deformation through time.

In general terms, there are three units within each sub-domain, from west to east: a fore-arc zone, a magmatic arc, and a back-arc region. In the southern flat-slab sub-domain of the Central Andes (from 28°S to 33°30’S), the forearc zone is a steady rise to the crest of the Andes, which is formed by an inactive magmatic arc and thrust belt (Frontal Cordillera and Cordillera Principal). The Triassic magmatic (rift) arc has a general northwest–southeast trend. The foreland consists of an active, thin-skinned fold-thrust belt (Pre-cordillera) and zone of basement uplifts (Sierras Pampeanas, with altitudes ranging from 2,000 to 6,000 m).

The Altar Project is located in the Cordillera Principal. Basement rocks in the Altar region have been assigned to the Choiyoi Group, of Permo-Triassic age; the Choiyoi Group covers about 500,000 km2 in Argentina. It comprises an  upper and lower volcanic sequence, intruded by shallow-level plutons, stocks, and dyke-like bodies. The lower volcanic sequence comprises calc-alkaline andesite-dacites that represent the products of a subduction-related magmatic arc, which is overlain by an upper sequence ofperaluminous rhyolites, related to a period of post-orogenic extensional collapse. Composition of the volcanics trends from mafic to acidic through time. Both sequences are propylitically-altered and contain fracture-controlled epidote, chlorite, albite, and calcite veining.

The volcanic sequence was intruded by peraluminous A-type and S-type granites that are considered coeval with the rhyolitic volcanics and likewise typically exhibit lowgrade propylitic alteration. Generally, Jurassic marine sediments that consist of red-bed sandstones and claystones infill the Triassic rift, and unconformably overlie the Choiyoi Group; however Jurassic sediments are not known in the immediate surroundings of the Altar Project.

Within the Project area, rhyolitic ignimbrites and andesitic volcanics of the Pachon Formation overlie the Choiyoi basement sequence with age dates of 20 to 22 Ma (Miocene). The wider area of what comprises the Altar and Río Cenicero concessions is flanked by two significant regional north-south striking faults, referred to as the Pelambres Fault to the west, and the Río Teatinos Fault to the east of the concession area. The Pelambres fault limits the rocks of the Pachón formation against the paleogene Pelambres formation to the west. The Río Teatinos fault juxtaposes the Pachón formation against paleozoic to lower mesozoic metasedimentary and intrusive basement rocks to the east.

Deposits

Mineralizations

No mineralizations available.

Placers

No placers available.