Porphyry copper and skarn fertility of the northern Qinghai-Tibet Plateau collisional granitoids

Abstract: The recent discovery of numerous large porphyry Cu deposits in the southern Qinghai-Tibet Plateau shows that porphyry Cu deposits can be hosted in magmatic suites in collisional settings. However, in contrast, only a few small porphyry Cu deposits have so far been discovered in association with collision-related granitoids in the northern Qinghai-Tibet Plateau. This raises questions about the origin of collision-related magmas and their mineralization potential. In this contribution, we comprehensively synthesize whole-rock geochemical and isotopic data on collision-related intrusions from the Qimantagh Metallogenic Belt in the northern Qinghai-Tibet Plateau, which hosts many skarn polymetallic deposits but only a few, small porphyry Cu-Mo deposits. This, combined with newly obtained zircon trace element data, provides a high-quality database that can yield insights on the nature and origin of the magmatic suites as well as their fertility in terms of Cu mineralization. Two volumetrically dominant intrusive suites are identified in the Qimantagh Metallogenic Belt: 435-370 Ma granitoids (Suite I) and 245-196 Ma granitoids (Suite II). They formed during syn- to post-collisional stages of the Caledonian and Hercynian-Indosinian Orogenies, respectively. In contrast, arc magmatic rocks are relatively scarce. Both Suites I and II are characterized by low zircon Ce/Ce* and Eu/Eu* values, low whole-rock Sr/Y and Eu/Eu* values with arc-like features (e.g., depletion of Nb and Ta). Furthermore, both suites display some evolved Sr-Nd-Hf isotopic values (e.g., eNdi = -8.1 to 0.1), with the majority of samples characterized by Paleo- to Meso-Proterozoic two-stage Nd and Hf model ages. These features suggest that the parental magmas of the two suites were probably derived from subduction fluid-modified mantle sources which underwent significant crustal AFC processes during magma ascent. The relative scarcity of arc magmatic rocks and the prevalence of collisional magmatism during the Caledonian and Hercynian-Indosinian Orogenies in the Qimantagh Metallogenic Belt can be explained by pre-collisional, flat-slab subduction and subsequent slab breakoff during collision, the later triggering asthenosphere upwelling and extensive magmatism in collisional settings. Compared to fertile plutons in some large porphyry Cu deposits worldwide, especially those in the Gangdese Metallogenic Belt, the two suites in the Qimantagh Metallogenic Belt have low magmatic oxidation states and low water content which inhibited the formation of large porphyry Cu deposits. Thus, skarn polymetallic deposits (probably as well as small porphyry Cu-Mo deposits) rather than large porphyry Cu deposits should be targeted during future mineral exploration in the northern Qinghai-Tibet Plateau, since such deposits do not necessarily need a parental magma with high oxidation state and water content.