Hua-Ying Liang

Mesozoic large magmatic events and mineralization in SE China: oblique subduction of the Pacific plate

SE China is well known for its Mesozoic large-scale granitoid plutons and ore deposits. In SE China, igneous rocks with intrusion ages between 180 and 125 Ma generally become progressively younger towards the NE. More specifically, 180–160 Ma igneous rocks are distributed throughout a broad area, with mineralization ranging from Cu–Au and Pb–Zn–Ag to W–Sn; 160–150 Ma plutons are present mainly in the Nanling region and are associated with the large-scale W–Sn mineralization; younger igneous rocks occur in the NE area that has many fewer deposits. These can be plausibly interpreted as reflecting a southwestward subduction followed by a northeastward rollback of a subducted oceanic slab, in rough agreement with contemporaneous drift of the Pacific plate. Consistent with this scenario, SE China contains three Jurassic metallogenic belts distributed systematically from NE to SW: (1) a Cu–(Au) metallogenic belt in the NE corner of the South China Block, represented by the Dexing porphyry Cu deposits; (2) a Pb–Zn–Ag metallogenic belt in the middle, represented by the Lengshuikeng Ag and Shuikoushan Pb–Zn deposits; and (3) the famous Nanling W–Sn metallogenic belt in the SW. The distribution of these metallogenic belts is analogous to those in South America where Fe deposits are distributed close to the subduction zone, followed by porphyry Cu–Au deposits and Pb–Zn–Ag deposits in a medial zone, and Sn–W deposits distant from the trench. Inasmuch as quite a few late Mesozoic Fe deposits occur in the Lower Yangtze River Belt to the NE of the Cu–Au deposits in SE China, the distribution of late Mesozoic deposit belts in SE China is identical to that in South America. Therefore, southwestward subduction of the Pacific plate and the corresponding slab rollback are proposed here to explain the distributions of the late Mesozoic (180–125 Ma) magmatism and the associated metallogenic belts in SE China.

Geochemical Constraints on Adakites of Different Origins and Copper Mineralization

The petrogenesis of adakites holds important clues to the formation of the continental crust and copper ± gold porphyry mineralization. However, it remains highly debated as to whether adakites form by slab melting, by partial melting of the lower continental crust, or by fractional crystallization of normal arc magmas. Here, we show that to form adakitic signature, partial melting of a subducting oceanic slab would require high pressure at depths of >50 km, whereas partial melting of the lower continental crust would require the presence of plagioclase and thus shallower depths and additional water. These two types of adakites can be discriminated using geochemical indexes. Compiled data show that adakites from circum-Pacific regions, which have close affinity to subduction of young hot oceanic plate, can be clearly discriminated from adakites from the Dabie Mountains and the Tibetan Plateau, which have been attributed to partial melting of continental crust, in Sr/Y-versus-La/Yb diagram. Given that oceanic crust has copper concentrations about two times higher than those in the continental crust, whereas the high oxygen fugacity in the subduction environment promotes the release of copper during partial melting, slab melting provides the most efficient mechanism to concentrate copper and gold; slab melts would be more than two times greater in copper (and also gold) concentrations than lower continental crust melts and normal arc magmas. Thus, identification of slab melt adakites is important for predicting exploration targets for copper- and gold-porphyry ore deposits. This explains the close association of ridge subduction with large porphyry copper deposits because ridge subduction is the most favorable place for slab melting.

High Oxygen Fugacity and Slab Melting Linked to Cu Mineralization: Evidence from Dexing Porphyry Copper Deposits, Southeastern China

The Dexing porphyry Cu deposit is the largest Cu deposit in eastern China, with total reserves of 8.4 Mt Cu. The Dexing porphyries have geochemical characteristics typical of adakites: they are similar to examples in the Circum-Pacific Belt and in the Lower Yangtze River Belt but different from adakites from the Dabie Mountains and the Tibetan Plateau. Ce4+/Ce3+ and values calculated from zircon trace-element compositions vary from 495 to 1922 and from 0.51 to 0.82, respectively, and reflect high oxygen fugacity similar to that measured in or inferred for porphyry Cu-Au deposits in the South America. The high oxygen fugacity is consistent with abundant anhydrite and magnetite-hematite intergrowths in the porphyry, which indicate that the highest oxygen fugacity of Dexing porphyry reached the hematite-magnetite buffer. Based on the geochemical characteristics and the drifting history of the Pacific Plate, we propose that the Dexing adakitic porphyries formed through slab melting, most likely during subduction of an aseismic ridge in the Pacific Plate in the Mid-Jurassic.

Shapinggou: the largest Climax-type porphyry Mo deposit in China

Shapinggou porphyry Mo deposit is the largest Climax-type Mo deposit in China and probably also in the world, with total proven Mo reserves of over 2.2 million tonnes at an average grade of 0.17%. It is located in the western Dabie Mountains, along the eastern extension of the East Qinling Mo mineralization belt. Similar to the majority of Mo deposits in the Qinling Mo belt, it is located north of the Triassic suture between the north and south China blocks. The orebody is mainly hosted in Cretaceous high-K granitic porphyry and explosive breccia, with potassic, silicification, and sericite-pyrite alterations. Fluorite is common in Shapinggou, indicating high-F content. The porphyry is closely associated with a large quartz syenite intrusion. Re–Os dating of molybdenite yielded an isochron age of 111.1 ± 1.2 Ma for the mineralization. Zircon U–Pb ages are 111.7 ± 0.8 Ma and 111.9 ± 0.6 Ma for the granitic porphyry and quartz syenite, respectively. Shapinggou is similar to the well-known Climax and Henderson Mo deposits in terms of geochemical characteristics and alterations, etc. We propose that Mo-rich ore-forming materials accumulated underneath the Shapinggou region during the convergence of the North and South China blocks, whereas the final mineralization was triggered by asthenosphere upwelling induced by Pacific Plate subduction.

Zircon U-Pb dating of Shuiquangou alkaline complex intrusives, northwestern Hebei Province

SUPER large gold deposits related to alkaline intrusions have been found in America, Canada, Australia, Papua New Guinea and elsewhere.Dongping gold deposit with superlarge potentialhas also been found in Shuiquangou alkaline complex intrusions, northwestern Hebei Province, China. One of the keys to resolving the possible genetic relationship between gold deposits andore-host alkaline intrusions is to determine their formation ages. This note dates the Shuiquan-gou intrusions with zircon U-Pb method....

Studies on the genesis of adjacent Changkeng gold- and Fuwang silver-deposits, Guangdong Province, China

The Changkeng Au- and Fuwang Ag-deposits represent an economically significant and distinct member of the Au-Ag deposit association in China. The two deposits are immediately adjacent, but the Au- and Ag-orebodies separated from each other. Changkeng is hosted in brecciated cherts and jasperoidal quartz and is characterized by disseminated ore minerals. Fuwang, hosted in the Lower Carboniferous Zimenqiao group bioclastic limestone, has vein and veinlet mineralization associated with alteration comprised of quartz, carbonate, sericite, and sulfides. The Changkeng gold and Fuwang silver deposits overlap in homogenous temperature and salinity of fluid inclusions. The delta D-H2O delta O-18(H2O), delta C-13(CO2) and He-3/He-4 values of the fluid inclusions suggest the ore fluids of the Changkeng Au-ore come from the meteoric water and the ore fluids of the Fuwang Ag-ore are derived from mixing of magmatic water and meteoric water. The Changkeng gold- and the Fuwang silver deposits show different Pb isotope signatures, suggesting different sources of ore-forming material. Rb-Sr isochron age (68 +/- 6 Ma) and Ar-40-Ar-39 age (64.3 +/- 0.1 Ma) of the ore-related quartz veins from the Ag-deposit indicate that the Fuwang deposit formed during the Cenozoic Himalayan tectono-magmatic event. The adjacent Changkeng and Fuwang deposits could, however, represent a single evolved hydrothermal system. The deposits are alternatively the product of the superposition of two different geological events. Our work indicates that the Pacific Coastal Volcanic Belt in the South China Fold Belt has greater potential for Himalayan precious metal mineralization than previous realized.

Geochronological and geochemical study on the Yulong porphyry copper ore belt in eastern Tibet, China

The Yulong copper belt, along part of the Red River-Ailao Shan fault system and its northwestern extension in eastern Tibet, consists of five porphyry pipes that contain a total copper resource of over 8 million tons. The porphyries are characterized by high alkali content (K2O+Na2O > 6%), K2O/Na2O > 1, and marked negative Ti, Ta and Nb anomalies on mantle-normalized incompatible element diagrams. U-Th-Pb laser ICP-MS dating of zircons from the Yulong porphyries showed that they were emplaced over a 4.3 Ma period and that they Young systematically from northwest to southeast as follows: Yulong, 41.2 +/- 0.2Ma; Zalaga, 38.5 +/- 0.2Ma; Mangzong, 37.6 +/- 0.2 Ma; Duoxiasongduo, 37.5 +/- 0.2Ma; and Malasongduo, 36.9 +/- 0.4 Ma. We suggest that the source of the shoshonites was lower crust that was pushed into the mantle by the compressive component of transpressional movement on the adjacent Tuoba-Mangkang fault and that the compositional variation in the porphyries is due to mixing between magmas of different composition, generated by different degrees of partial melting of a heterogeneous source region.

Confirmation and significance of Caledonian metallogenic epoch for gold deposits in South China

Caledonian gold deposits are widely distributed in South China. They are developed in both South China Caledonian fold belt and adjacent Proterozoic Jiangnan uplift. The host rocks are Proterozoic metamorphosed microclastic rocks in the Jiangnan uplift and Proterozoic and Cambrian strata, as well as Chengjiang and Caledonian igneous bodies in the South China fold belt. The distinct differences between the Caledonian and the most developed Yenshanian gold deposits in South China are reflected in age and host-rock type, relations to Yenshanian magmatic activities, element association, mineral assemblage and gold deposit type. The studies have proven that the Caledonian epoch is a principal metallogenic period of gold deposits in South China. This conclusion is of very important enlightening significance in exploration of Caledonian gold deposits in South China as well as in other Caledonian fold belts and adjacent uplifts in China.

Reworking Intensity-A Key Factor Leading to the Formation of Superlarge Gold Deposits in Greenstone Belts and Metamorphosed Microclastic Rocks in China

The greenstone belt and metamorphosed microclastic rock-type superlarge gold deposits in China are hosted in metamorphic rocks and later intrusive bodies. Sedimentation, regional metamorphism and mineralization contributed a lot to the formation of the deposits, so did remelting magmatic process to some deposits, but the deposits were finally formed by reworking processes. The key factor leading to the formation of superlarge gold deposits is the reworking intensity, which for superlarge gold deposits is reflected by the large-scale reworked source rocks and even ore materials of various sources, strongly oxidized ore-forming fluids with a long and repeated active history and stable geothermal heat current. The factor which decides the reworking intensity is the network consisting of structures of different classes.