The multiple mineralizations and geodynamic settings of the Laozuoshan Cu–Au deposit in the Jiamusi Massif, NE China: Zircon U–Pb geochronological, elemental and Hf isotopic geochemical evidence

Abstract

Situated in the centre of the Jiamusi Block, the Laozuoshan deposit is a large-sized deposit with composite of vein and lenticular Au orebodies and veinlet-disseminated Cu-Au orebodies. It is characterized by two types of mineralization, skarn Au mineralization and porphyry Cu-Au mineralization. The former occurs along the contact zone between granodiorite and Proterozoic metamorphic rocks in the eastern and central ore zones, while the latter is hosted in granodiorite porphyry and diorite in the western ore zone. In this study, we present the results of geochronological and geochemical analyses of these rocks which show the following features. (1) Zircon U–Pb dating results show that the ages of granodiorite, diorite and granodiorite porphyry are 264.6±2.6 Ma, 103.2±1 Ma and 104.6±1.8 Ma, respectively. (2) Whole-rock elemental compositions indicate that the granodiorite shows potassium-rich adakitic rock properties, and the diorite has typical island arc calc-alkaline magmatic properties. They are all characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs). (3) The in-situ Hf isotope analysis results show that the granodiorite and diorite have εHf(t) values of −3.7 to −8.7 and −0.4 to 10.7, respectively, corresponding to TDMC ages of 2040–2500 Ma and 630–1650 Ma, respectively. These results indicate two periods of mineralization in the Laozuoshan deposit. Skarn mineralization occurred in the late Permian and was a result of metasomatism between partial melting magma that originated from thickened Paleoproterozoic lower crust and Proterozoic marble. Porphyry mineralization took place in the Early Cretaceous and was a result of the shallow emplacement of mid-Neoproterozoic mantle-derived magma with the addition of crustal materials. In terms of geodynamic settings, skarn Au and porphyry Cu-Au mineralization occurred in a compressional setting formed during the gradual closure of the Paleo-Asian Ocean, and a crustal-thinning setting formed from Eurasian Plate-ward subduction, retreating, and rollback of the Paleo-Pacific Plate, respectively.