| Popis: |
Porphyry Cu deposits can be classified at the global scale as oceanic subduction- and continental collision-types based on their tectonomagmatic settings. In order to further characterize the possible differences in the geochemical and isotopic features of these two types of porphyry Cu deposits, Li-Hf isotopes were investigated together with bulk rock major/trace element concentrations and geochronology, for porphyritic granitoids related with four porphyry Cu deposits. The Baogutu porphyry Cu-Au deposit, Xinjiang Province, China, and the Atlas porphyry Cu-Au deposit, Philippines, are taken to represent the oceanic subduction-type, whereas the Demingding porphyry Cu-Mo deposit, Tibet Province, China, and the Grasberg porphyry Au-Cu deposit, Indonesia, are taken as proxies for the continental collision-type. The Li isotopic compositions of the Baogutu granodiorite shows a limited range of + 3.2 to + 6.5‰ (avg. + 4.6 ± 1.7‰, n = 13) in δ7Li values, whereas the diorite from Atlas have δ7Li values of + 2.6 to + 7.7‰ (avg. + 5.9 ± 2.8‰, n = 16), i.e., slightly higher than the Baogutu granodiorites, probably reflecting the involvement of subducted marine sediments. The relatively homogeneous and MORB-like δ7Li values (~+3 to 5‰) of the Baogutu and Atlas porphyries, together with depleted zircon eHf(t) values for Baogutu (+11.8 to + 17.5, avg. + 14.5, n = 19) and Atlas (+7.1 to + 10.6, avg. + 9.0, n = 40), are suggestive of an affinity with oceanic plate, and/or partial melting of the mantle wedge in response to oceanic subduction. By contrast, the δ7Li values of the Demingding monzogranite and quartz porphyry, and the Grasberg quartz monzodiorite vary more broadly, ranging from −0.7‰ to + 10.3‰ (avg. + 2.5 ± 5.9‰, n = 19) and from −0.3 to + 7.3‰ (avg. + 3.7 ± 4.6‰, n = 8), respectively. These two localities also show variable zircon eHf(t) values ranging from −5.4 to + 5.7 (avg. + 2.8, n = 20) and from −20.0 to −1.0 (avg. −10.7, n = 42), respectively. The large variations of δ7Li and eHf(t) values at Demingding and Grasberg are generally similar to those of mid- to lower continental crust (δ7Li ~+3.3‰ and eHf(t) −20.0 to + 5.7), and are thus consistent with their source regions, which are interpreted to reflect continental collisional settings. The Li-Hf isotopic compositions of the four porphyry intrusions show that tectonic settings have dominant effects on the Li-Hf isotopic compositions of the related magmatic rocks, prior to any redistribution by porphyry Cu deposits-related hydrothermal alterations, suggesting that Li-Hf isotopes systematics can partly act as tracers of the source regions related to tectonic settings. |
Full Text from ScienceDirect