The Diaoquan Ag-Cu polymetallic skarn mineralization in central North China Craton: Timing, source and genetic model

摘要

The North China Craton (NCC) has been the focus of investigations on metallogeny in relation to craton destruction. The Diaoquan Ag-Cu polymetallic skarn mineralization is one of the large Ag deposits in central NCC. Here we present new mineralogical, geochronological and geochemical data from this deposit with the objective to gain an insight into the magmatic hydrothermal processes in relation to decratonization. The timing of polymetallic mineralization in this deposit is constrained as ca. 130 Ma by phlogopite Ar-Ar and molybdenite Re-Os dating. The physico-chemical conditions of the magmatic evolution estimated from amphibole, biotite, magnetite-ilmenite mineral thermobarometer indicate that the monzonite and porphyritic granite magma developed in a magma chamber at a palaeoburial depth of 2-3.3 km, and was subsequently emplaced at 1-2 km palaeodepth. The oxygen fugacity conditions were relatively high (> Delta NNO + 1.6 buffer) in the early stage and rapidly dropped (< Delta FMQ buffer). The major and trace element features suggest magma mixing as a possible mechanism that generated the different intrusions. Sr-Nd-Pb and zircon Hf isotope data indicate that lower crust is the major source of the magma and that part of metallogenic elements might have been sourced from mantle derived components. We constructed the magmatic hydrothermal system as follows: (1) mafic magma formed by the partial melting of thinned mantle lithosphere; (2) the mafic magma underplating resulted in the partial melting of the lower crust, and generated granitic magma; (3) mixing of the mafic and felsic magmas generated the intermediate magma; (4) ore-forming fluids from the magma chamber migrated into the contact zone through fractures, resulting in Ag-Cu mineralization by skarnization; and (5) Mo-bearing fluid separated from the granite magma deposited the molybdenite in the inner contact zone.