Superimposed mineralization and emplacement paleodepth of the Laozuoshan Cu-Au deposit in the Jiamusi block: Evidence from garnet U-Pb geochronology and mineral geochemistry

Abstract

The Laozuoshan deposit is a large superimposed Cu-Au deposit composed of three ore zones (eastern, middle, and western). A large number of studies have revealed that the middle zone and eastern zone are skarn gold deposits formed by the contact metasomatism between a Late Paleozoic granodioritic magma and Mashan Group carbonate strata, while the western zone is a porphyry copper-gold deposit related to the hypabyssal emplacement of an Early Cretaceous dioritic-granodioritic magma. Therefore, is there mineralization related to this Early Cretaceous dioritic-granodioritic magmatism in the middle and eastern zones? To determine whether there was Early Cretaceous magmatism in the middle and eastern zones, diorite, diorite porphyrite and granodiorite porphyry, which are closely related to the skarn copper-gold mineralization in the eastern zone, were selected for study in this paper. Accurate garnet U-Pb isotope dating and chemical analysis of minerals reveal the following: (1) The formation age of the skarn copper-gold mineralization (104 Ma) is basically consistent with the emplacement time of the dioritic magma (103–106 Ma), which occurred in the Early Cretaceous. (2) The high temperature (722–802 °C), weak oxidation–reduction conditions (Δ FMQ = −4.82 ∼+0.24; log(ƒO2) = −21.2 ∼ −19.0), high water content (5.69–7.13 wt%) and shallow emplacement characteristics of the Early Cretaceous dioritic magma indicate that this magma had the potential to provide thermogenic minerals. (3) The geochemical characteristics of the skarn minerals reveal that the Early Cretaceous mineralization of the Laozuoshan deposit represents a calcareous reducing skarn copper-gold metallogenic system. Combining the mineralization and alteration characteristics of the three zones and previous work, this study further concludes that the Laozuoshan deposit is a superimposed metallogenic system involving a Late Permian skarn Au metallogenic system (eastern zone and middle zone) and an Early Cretaceous porphyry (western zone)-skarn (eastern zone and middle zone) Cu-Au system. The deep part of the mining area has the potential for searching for porphyry Cu-Au deposits.