Geologia

The Pisco formation in southern Peru consists of a thick succession of fine sandstone, siltstone, and diatomaceous and tuffaceous mudstone, with interspersed hardgrounds, very thin layers of phosphate pebbles, very thin to thin layers of volcanic ash, rudstones and calcite-and dolomite-cemented sandstones and siltstones. The succession records an abundant and diverse fauna of marine mammals, fish, pinnipeds, and other vertebrates, as well as bivalves and cirripeds. This sedimentary succession represents the last marine transgression that extended throughout the Pisco basin. The base of the Pisco formation transgressive sequence commonly consists of a layer of phosphate nodules and igneous boulders. Phosphate clasts are sub-rounded to rounded very coarse sand to small cobbles, but mostly pebble size, occasionally with small pits and borings. Locally, this layer is a fine sandstone with burrows, hardgrounds, and carbonate-cemented intraclasts, characterized by Glossifungites ichnofacies. Fossils are abundant and include shark teeth, fish bones, articulated bivalve shells, disarticulated oysters, and articulated and disarticulated marine mammal bones. Especially remarkable in this basal layer is the occurrence of igneous boulders. They are sub-rounded to rounded, sub-spherical to spherical clasts of granite and diorite/andesite, in sizes that vary widely between 25 cm and 2.5 m in diameter, with smooth surface and few borings. These boulders also occur in other horizons higher in the section within layers of phosphate pebbles, shell beds and/or rudstones with a matrix consisting of mono-mineral associations with shell fragments. Overlying this basal layer there are layers of brown to greenish, massive, well-sorted fine sandstones interspersed with thin layers of fossiliferous fine sandstone concretions (calcareous or dolomitic). These sandstones are locally flaser-bedded and have abundant current structures. The abundant fossil content in the concretions consists of bivalves (Dosinia, Anadara), marine mammal bones (baleen whales, seals, dolphins), shark teeth, gastropods and fish scales. Some levels of concretions lack fossils. Shell beds occur repeatedly at regular intervals in some sections, including layers of Turritella, Dosinia, oysters, and other shells, often containing abundant marine mammal bones. Overlying the sandstone is a thick succession of siltstone with abundant current, load, and scour-and-fill structures, hummocky cross-stratification, and parallel lamination. In some exposures, these siltstones are massive. Diatomaceous and tuffaceous mudstones, both massive and with rough parallel lamination, occur on top of some sections, interspersed with thin layers of volcanic ash, phosphate or siltstone pebbles. The mudstone layers lack invertebrate fossils but locally have high abundance of vertebrate fossils, especially marine mammals. The succession of fining-upward sediments of the Pisco formation suggests a marine transgression with deepening of the basin. Bored hardgrounds and intraclasts indicate surfaces of exposure and erosion in the foreshore, intertidal zone, on which shell, barnacles, whale skeletons and igneous boulders accumulated. The overlying fine sandstones suggest deposition in a relatively high-energy environment in the shoreface. Siltstones with cross-bedding and hummocky cross-stratification indicate storm deposition within the lower shoreface to offshore-transitional environment, above storm wave base. The diatomaceous and tuffaceous mudstones are evidence of the deepening of the basin and deposition in the offshore zone. Abstract Book of 31 st IAS Meeting of Sedimentology, 22

Zircon LA-ICP-MS and CA-TIMS and molybdenite Re-Os geochronology for the El Abra porphyry copper deposit (Chile) document a ~8.6 Ma protracted magmatic-hydrothermal history. Initial magmatic events occurred at ~45 Ma, and continued to ~36 Ma. An ~1.8 m.y. magmatic lull is recorded in the early stages of magmatism, which is characterized by equigranular rocks from the Pajonal suite. Magmatism from ~41 to ~36 Ma, corresponding to the El Abra Granodiorite Complex, is more recurrent, ending in a ~1.4 m.y. period of porphyritic intrusions, which are coeval with Cu-Mo mineralization. Late porphyritic units reveal subtle zircon inheritance likely associated with early magmatic pulses. The young stages of magmatism indicate system reju-venation based on plagioclase phenocryst compositions, suggesting that the porphyry system is ultimately linked to a less evolved magma. Although there are multiple porphyry and veining events, molybdenite Re-Os ages define a focused mineralization episode from 36.34 to 36.18 Ma. Molybdenite Re-Os ages from nearby related prospects belonging to the El Abra cluster expand the mineralization period in the district up to ~1.0 m.y., suggesting a multistage, long-lived hydrothermal system. Integration of the molybdenite Re-Os dates and those from previous works at the El Abra and Toki clusters, accompanied by available tectonic reconstruction along the Domeyko fault system suggest the development, by late Eocene times, of a single, large (~30 km across) porphyry copper cluster, which was subsequently separated by an offset of ~35 km along the West fault.