Petrogenesis and Mo(W) mineralization of deep porphyritic granites in the Laowan ore field, Eastern Qinling, Central China

Abstract

The Laowan ore field of the Tongbai district is a crucial metallogenic belt within Eastern Qinling, including super large gold (Au) and large molybdenum (Mo) deposits. However, the Au and Mo metallogenic relationship and tectonic setting are still disputed. Petrology, geochemistry, and geochronology were conducted on the 3000 m scientific drilling hole, ZKE1012, south of the Laowan ore field. A concealed porphyritic granite with Mo mineralization was observed at a depth of 1717 m. The concealed porphyry comprises K-feldspar, plagioclase, quartz, biotite and minor magnetite, ilmenite, rutile, apatite, and zircon. Molybdenite, scheelite, pyrite, and chalcopyrite are immersed in the rock as veinlet-dissemination, showing typical porphyry mineralization characteristics. Zircon U–Pb data define that the deep rock was formed at 131.0 ± 2.1 and 133.2 ± 1.0 Ma, consistent with the molybdenite Re–Os age (130 Ma), indicating that the Mo mineralization was related to the porphyry. The rocks are characterized by high silica (SiO2 = 70.5–76.8 wt%), high alkali (Na2O + K2O = 8.55–9.25 wt%), and metaluminous to weakly peraluminous (A/CNK = 0.99–1.09). They are enriched with large ion lithophile elements (Rb, Y, Th, Ce, Sm, and Zr), depleted in Nb, Ba, and Sr, and show negligible Eu anomalies. The deep porphyries belong to I-type granite, and their zircon εHf(t) values are from − 11.2 to − 5.7. Therefore, the concealed porphyritic granite and its mineralization are the product of partial melting of the basements, mixing minor mantle materials caused by the subduction of the Paleo–Pacific Plate to Eurasia in the Early Cretaceous.