Petrogenesis of the Dasuji porphyry Mo deposit at the northern margin of North China Craton: Constrains from geochronology, geochemistry and isotopes characteristics

Abstract

The Dasuji porphyry Mo deposit is located at the northern margin of North China Craton. Mo mineralization occurs mainly within the intrusions of alkali feldspar granite, granite porphyry, and quartz porphyry, which yield weighted mean 206Pb/238U ages (2σ) of 227.6 ± 1.5, 225.7 ± 1.5, and 224.3 ± 1.8 Ma, respectively. These ages are indistinguishable to the molybdenite ReOs isochron age of 223.9 ± 1.5 Ma (2σ). The Dasuji granites show the petrological, mineralogical, geochemical, and geotectonic characteristics of A-type granites. They exhibit low negative εNd(t) values (−16.7 to −12.6) with corresponding TDM2 (Nd) ages of 1705–2000 Ma, variable but mainly negative εHf(t) values (mostly −21.0 to −7.9) with corresponding TDM2 (Hf) ages of 1715–2728 Ma, and Pb isotopic compositions indicating that the primary magmas were dominantly derived from partial melting of ancient lower crust. In chondrite- and mantle-normalized trace element diagrams, all the granitic rocks show negative anomalies in Ba, Sr, and Eu, and a positive anomaly in Pb. The ratios of LREE/HREE, and (La/Yb)N and δEu values decrease from the alkali feldspar granite to the granite porphyry and quartz porphyry. These data are attributed to fractional crystallization processes. Fractionation of feldspar, biotite, and possibly K-feldspar produced the variations in major elements and Rb, Sr and Ba, and the distribution of REE are controlled mainly by fractionation of zircon, allanite, minor apatite, and possibly a small quantity of monazite. Together with the regional geology, the data infer that the Dasuji porphyry Mo deposit formed in a post-collision extensional setting during the Late Triassic.