Geochronology and petrogenesis of volcanic rocks in the Laurani epithermal Au–Ag–Cu deposit, northern Bolivian Altiplano

• Published in: Journal of South American earth sciences Vol. 120; p. 104044
• Main Authors: Liu, Fei, Han, Run-Sheng, Zhao, Dong, Wen, Shu-Ming, Li, Wen-Yao, Wang, Jia-Sheng, Lei, Li, Guo, Yu-Xin-Yue
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.12.2022
• Subjects: Arequipa massif; Bolivian Altiplano; Grenvillian metamorphism; Hf isotope; Laurani Au–Ag–Cu deposit; Zircon U–Pb dating; Grenvillian metamorphism; Laurani Au–Ag–Cu deposit; Bolivian Altiplano; Zircon U–Pb dating; Hf isotope; Arequipa massif
• ISSN: 0895-9811; 1873-0647
• DOI: 10.1016/j.jsames.2022.104044

The Laurani Au–Ag–Cu deposit is a typical high-sulfidation (HS) epithermal deposit genetically related to volcanic rocks in the Bolivian Altiplano polymetallic belt (BAPB), yet few studies have been carried out on these volcanic rocks. Detailed petrographic observations show that the Laurani volcanic rocks comprise monzogranite porphyries, dacites, dacitic porphyries and tuffs. Zircon U–Pb dating demonstrates that they formed at 7.44 ± 0.15 Ma ∼7.65 ± 0.09 Ma in the late Miocene during the rapid uplift of the Altiplano. Geochemical features indicate these volcanic rocks belong to high-K calc-alkaline to shoshonite series, enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depleted in high field strength elements (HFSE) and heavy rare earth elements (HREE), with no Eu anomalies (Eu/Eu* = 0.80–1.11, mean = 0.95). We suggest that the magma sources of the Laurani volcanic rocks are primarily generated from the partial melting of the thickened lower crust, for: (1) No Eu anomalies indicate that plagioclase is absent in the residue, and the extensively depleted HREE and positively correlated high (Gd/Yb)N, (Dy/Yb)N, and (La/Yb)N ratios indicate that garnet is a dominant residual mineral in their source, which are consistent with the thickened lower crust; (2) 176Hf/177Hf ratios (0.282409–0.282544) and negative εHf(t) values (−13.15 to −8.37) confirm that the magmas are mainly evolved from the partial melting of the old lower crust; (3) Inherited zircons ages of 1.87–1.62 Ga are consistent with the basement ages of the Arequipa Massif between 1.9 Ga and 1.8 Ga, indicating that the magma sources are mainly generated from the partial melting of the Paleoproterozoic crust. These observations coincide with the thickened (60–80 km) felsic to intermediate lower crust beneath the Altiplano. Combined with the geodynamic process of rapid uplift of the Altiplano, we propose that the removal of the lower crust and/or mantle lithosphere and/or its interaction with the asthenosphere provide heat to contribute to the partial melting of the thickened lower crust to generate parental magma, then parental magma emplaces along the strike-slip Eucaliptus fault to form the Laurani volcanic rocks. Furthermore, numerous inherited zircon ages with a weighted mean of 1189 ± 30 Ma reflect the intense Grenvillian metamorphism in the Mesoproterozoic and the two ages 259.9 ± 5.11 and 231.7 ± 2.2 Ma indicate the Permian magmatic events in northern Bolivia. From this study, we infer that partial melting of the thickened lower crust during the rapid uplift of the orogenic belt may be an important magma formation mechanism. [Display omitted] •Zircon U–Pb dating demonstrates the Laurani volcanic rocks formed at ∼7.5 Ma.•The parental magma of the Laurani volcanic rocks generated from the partial melting of the thickened lower crust.•Inherited zircon ages indicate the existence of the Arequipa Massif and the Grenvillian metamorphism in the Central Andes.•Partial melting of the thickened lower crust is an important magma formation mechanism during uplift of orogenic belt.