A Comparative study between the geochemistry of ore-bearing and barren calc-alkaline intrusions
Date
2001
Authors
Ballard, Julian Richard
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The Chuquicamata-El Abra district is located in the Antofagasta region of northern Chile, about 1300 km north of Santiago. The area is of particular geological interest because it hosts the giant Chuquicamata porphyry copper deposit, and at least four other porphyry systems. The geology of the district is varied and includes igneous, metamorphic and sedimentary rocks that range in age from Precambrian to Quaternary. Copper is currently being mined from Eocene to Oligocene age felsic porphyries at Chuquicamata and Radomiro Tomic, and from Eocene intrusions at El Abra.Further work is needed to establish the economic potential of Opache and Mansa mina, prospects that are located to the south of Chuquicamata. North and south of this district, Tertiary calc-alkaline intrusive complexes also host porphyry copper deposits, including Collahuasi, Escondida and Potrerillos.
This study examines the geochemical characteristics of ore-bearing calc-alkaline intrusions, and contrasts these with intrusions that are barren of ore. To achieve this objective, samples were collected from 21 intrusive units. Multiple samples from each unit were analysed for whole rock major and trace element compositions, zircon trace element compositions, platinum group element compositions and plagioclase core anorthite contents. In addition, U-Th-Pb zircon ages were determined for each unit, making it possible to sort the geochemical data into chronological order. The findings from this study are presented in four chapters, which follow a summary of the local geological setting.
First, this study examines the excimer laser ablation inductively coupled plasma mass spectrometry (ELA-ICP-MS) analytical technique and shows that it is a reliable method for dating young zircons by U-Th-Pb isotopes. To test this new technique, the three Chuquicamata mine porphyries were analysed by ELA-ICP-MS and sensitive high resolution ion microprobe (SHRIMP).Both techniques identify two discrete igneous events, which are discussed in Abstract relation to previous geochronology and used to help explain the anomalously large size of Chuquicamata.
Second, additional ELA-ICP-MS U-Th-Pb zircon ages are presented for the remaining intrusive units. Together with whole rock geochemical data and plate tectonic displacement rates, the ages are used to resolve a number of issues central to the understanding of the geological evolution of the district. In particular, the ages show that the Los Picas-Fortuna and Pajonal-El Abra intrusive suites were once part of a single batholith that was emplaced over ca. 5.5 m.y. Third, zircons from the each unit were analysed by ELA-ICP-MS for their trace element compositions. The zircon data, and additional whole rock geochemistry were then used to calculate individual Ce(IV)/Ce(III) ratios in zircon. The Ce(IV)/Ce(III) ratio depends predominantly on the magmatic oxygen fugacity at the time of zircon crystallisation. The results show that the highest zircon Ce(IV)/Ce(III) ratios, and hence most oxidising conditions, are associated with porphyry copper mineralization.
Finally, variations in the major and trace element data from the Los PicosĀ Fortuna/Pajonal-El Abra intrusive units were examined to help establish the controls on magmatic differentiation in the batholith. The whole rock data, and the geochemical modelling,show that porphyry copper mineralization is associated with high amounts of amphibole fractionation, and therefore high magmatic water contents, from intermediate compositions.
The most important finding from this study is the realisation that a single magma chamber may survive for an extended period of time if it is located deep enough in the crust to maintain a temperature close to that of its surrounding country rocks. The extended lifespan of such a chamber may be an important factor in controlling the generation of economic porphyry systems because it gives the magmas the chance to become oxidised and hydrous enough to concentrate ore metals.
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