Formation of porphyry Mo deposit in a deep fault zone, example from the Dabaoshan porphyry Mo deposit in northern Guangdong, South China

Abstract

The Dabaoshan porphyry Mo deposit is located in the Wuchuan-Sihui fault zone, a deep fault system in South China. The Dabaoshan Mo mineralization occurs as disseminations and veinlets in the Dabaoshan porphyries and skarn-greisen associated with the Chuandu porphyritic monzogranite. Here we report the zircon LA-ICP-MS U-Pb ages of the porphyries mentioned above. We also present the major and trace elements, whole-rock Sr-Nd and the zircon Hf isotopic compositions of the Dabaoshan porphyries. The Dabaoshan porphyries are composed of weakly to strongly peraluminous monzogranite porphyry and granite porphyry. They yield ages of 166.3±2.0Ma (MSWD=1.9) and 166.2±2.7Ma (MSWD=2.7), respectively, slightly older than the Chuandu porphyritic monzogranite pluton which crystallized at 162.1±1.6Ma (MSWD=2.7). This is indicative of two distinct magmatic pulses in this region. The Dabaoshan porphyries have 64.42%–75.41% SiO2 and show an affinity of high-K calc-alkaline and shoshonite in the plot of Th vs. Co. They have [La/Yb]N of 14.6–35.0 and εNd(t) values ranging from −8.2 to −6.6. The εNd(t) values of the rocks are more negative than those for the Middle Jurassic mafic rocks (εNd(t)=−2.7 to +7.9) in the region, but similar to those for Proterozoic crust in the Cathaysia Block at ~165Ma. Zircon from the Dabaoshan porphyries has εHf(t) values ranging from −13.2 to −7.5 and two-stages Hf model ages (TDM2) from 1.7 to 2.0Ga, close to the TDM2 (~1.7Ga) of the basement in the western Cathaysia Block. The Dabaoshan porphyries have averaged Th/U of 5.0, similar to those for the Proterozoic crystalline basement (Th/U=5.3–5.4) in the Cathaysia Block, but higher than those for arc magmas (Th/U=1.5–3.0). Moreover, Nb/Ta ratios of the porphyries (average 11.8) are also similar to those of the Cathaysia Proterozoic crystalline basement (average 12.4). These geochemical features indicate that the porphyries were derived from high-degree partial melting of the crystalline basement in the Cathaysia Block. Based on the available geochemical data of the Dabaoshan porphyries, together with that of porphyry Mo deposits distributed along the Wuchuan-Sihui deep fault zone, it is concluded that the Dabaoshan porphyry Mo deposit is genetically related to the movements of the Wuchuan-Sihui deep fault induced by the subduction of the paleo-Pacific plate during the Jurassic.