Petrogenesis and Mo prospecting significance of Sadaigoumen granites on the northern margin of the North China Craton

Abstract

The Sadaigoumen porphyry Mo deposit lies on the northern margin of the North China Craton. Molybdenum mineralization occurs mainly as veins and veinlets, or is disseminated within alkali feldspar granite and granite porphyry, which yield weighted-mean 206Pb/238U ages (±2σ) of 243.7 ± 1.6 to 242.2 ± 2.0 Ma and 240.0 ± 1.7 Ma, respectively. ReOs dating of molybdenite from Stage 3 mineralization yields an isochron age of 238.0 ± 1.9 Ma. The Sadaigoumen ore-related granites have petrological, mineralogical, and geochemical characteristics of I-type granite. Ore-related granites exhibit low (87Sr/86Sr)i ratios of 0.710256–0.711463, negative εNd(t) values of −16.4 to −15.4 with corresponding TDM2 (Nd) ages of 2061–1870 Ma, and negative εHf(t) values of −21.2 to −11.2 with corresponding TDM2 (Hf) ages of 2614–1983 Ma, indicating that the primary magma was derived by partial melting of ancient lower crust. In chondrite- and primitive-mantle-normalized trace element diagrams, the Sadaigoumen ore-related granites are characterized by moderate enrichment of light rare earth elements (REE) relative to heavy REE, moderate to strong negative Nb, Ta, Zr, Hf, and Ti anomalies, and increasing Rb/Sr ratios with decreasing Sr content, all attributed to fractional crystallization. Fractionation of amphibole, muscovite, and possibly minor biotite and garnet (and/or zircon) produced the variations in most elements. Together with the regional geology, the data imply that the Sadaigoumen ore-related granites and corresponding mineralization formed in a syn-collision to post-collision transitional setting during the Early–Middle Triassic. These fractionated Triassic alkaline granites or syenogranites potentially contain new porphyry Mo deposits.