矿床地球化学国家重点实验室知识库

The newly discovered Shimensi deposit is a super-large tungsten-copper (W-Cu) deposit with a metal reserve of 742.55 thousand tonnes (kt) W and 403.6 kt Cu. The orebodies are hosted in Mesozoic granites, which intruded the poorly documented Shimensi granodiorite belonging to the Jiuling batholith, the largest intrusion (outcrop > 2500 km(2)) in South China. Our new SHRIMP (Sensitive High Resolution Ion MicroProbe) zircon dating revealed that the granodiorite at Shimensi (ca. 830-827 Ma) was formed coeval (within analytical uncertainty) or slightly earlier than those in many other places (ca. 819-807 Ma) of the Jiuling batholith. The Neoproterozoic Shimensi granodiorite is peraluminous and high-K calc-alkaline, and contains low P content with no S-type trend (positive P2O5 vs. SiO2 correlation) displayed, thus best classified as peraluminous I-type. The I-type classification is also supported by the zircon REE patterns, largely (93%) positive epsilon Hf(t) (-0.87 to 6.60) and relatively low delta O-18 (5.8-7.7 parts per thousand). The Neoproterozoic Shimensi granodiorite was formed after the continental arc magmatism (ca. 845-835 Ma), but before the post-collisional S-type granite emplacement (ca. 825-815 Ma) in the Jiangnan Orogen. Therefore, we propose that the Shimensi granodiorite was formed in a collisional/early post-collisional setting. The delta O-18 increase from the Shimensi granodiorite to many younger (ca. 819-807 Ma) granodiorites (6.0-8.5 parts per thousand) in the Jiuling batholith probably reflects an increase of supracrustal rock-derived melts with the progress of collision. The Shimensi granodiorite contains low zircon Ce4+/Ce3+ and Eu/Eu*, suggesting a relatively reducing magma that does not favor porphyry Cu-Au mineralization. This left a high background Cu concentration (avg. 196 ppm) in the Neoproterozoic granodiorite, which may have contributed to the Mesozoic W-Cu mineralization, when the granodiorite is intruded and assimilated by the Mesozoic granites.