Jianghong Deng

Petrology, geochemistry, and tectonic significance of Mesozoic shoshonitic volcanic rocks, Luzong volcanic basin, eastern China

We studied the geochemical characteristics of three types of Mesozoic igneous rocks from the Luzong volcanic basin: basaltic trachyandesite at Shuangmiao, pyroxene monzonite at Bajiatan, and quartz-syenite (A-type granite) at Huangmeijian. Based on analyses of whole-rock major elements, all investigated rocks are enriched in K, Na, Ti, Al, but depleted in Ca, representing a shoshonitic series. Trace element analyses show that these rocks are characterized by enrichments of large-ion lithophile elements and high field strength elements. Positive Nb and Ta anomalies in the chondrite-normalized spider diagram indicate that the shoshonitic volcanic rocks share similar features with Nb-enriched basalts, which are different from normal island-arc volcanical rocks (they are typically strongly depleted in Nb and Ta). Bulk-rock chemical compositions and Sr–Nd isotopes indicate that the three types of igneous rocks are geochemically comagmatic, suggesting that the melts were derived from an enriched mantle reservoir. We postulate an extensional tectonic setting for the formation of Luzong volcanic basin, possibly related to subduction of a palaeo-Pacific plate beneath the east Chinese continent during the Yanshanian period (Cretaceous). Therefore, the petrogenetic features of those volcanic rocks as well as A-type granites in the Luzong basin indicate that the regional large-scale Fe–Cu–Au mineralization was associated with oceanic slab melting, but not delamination or recycling of the ancient lower continental crust, as previously proposed.

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.

Study of late-Mesozoic magmatic rocks and their related copper-gold-polymetallic deposits in the Guichi ore-cluster district, Lower Yangtze River Metallogenic Belt, East China

ABSTRACT The Guichi ore-cluster district in the Lower Yangtze River Metallogenic Belt hosts extensive Cu–Au–Mo polymetallic deposits including the Tongshan Cu–Mo, Paodaoling Au, Matou Cu–Mo, Anzishan Cu–Mo, Guilinzheng Mo and Zhaceqiao Au deposits, mostly associated with the late Mesozoic magmatic rocks, which has been drawn to attention of study and exploration. However, the metallogenic relationship between magmatic rocks and the Cu–Au-polymetallic deposits is not well constrained. In this study, we report new zircon U–Pb ages, Hf isotopic, and geochemical data for the ore-bearing intrusions of Guichi region. LA-ICP-MS U–Pb ages for the Anzishan quartz diorite porphyrite is 143.9 ± 1.0 Ma. Integrated with previous geochronological data, these late Mesozoic magmatic rocks can be subdivided into two stages of magmatic activities. The first stage (150–132 Ma) is characterized by high-K calc-alkaline intrusions closely associated with Cu–Au polymetallic ore deposits. Whereas, the second stage (130–125 Ma) produced granites and syenites and is mainly characterized by shoshonite series that are related to Mo–Cu mineralization. The first stage of magmatic rocks is considered to be formed by partial melting of subducted Palaeo-Pacific Plate, assimilated with Yangtze lower crust and remelting Meso-Neoproterozoic crust/sediments. The second stage of magmatism is originated from partial melting of Mesoproterozoic-Neoproterozoic crust, mixed with juvenile crustal materials. The depression cross to the uplift zone of the Jiangnan Ancient Continent forms a gradual transition relation, and the hydrothermal mineralization composite with two stages have certain characteristics along the regional fault (Gaotan Fault). Guichi region results from two episodes of magmatism probably related to tectonic transition from subduction of Palaeo-Pacific Plate to back-arc extensional setting between 150 and 125 Ma, which lead to the Mesozoic large-scale polymetallic mineralization events in southeast China.

Formation of an Iron – Copper – Gold (Uranium) Metallogenic System in the Luzong Volcanic Basin, Central Anhui

Iron oxide copper gold (IOCG) deposits, which was first formally defined by Hitzman et al. (1992), comprise a broad and ill-defined clan of mineralization styles. IOCG deposits contain abundant (>10%) iron oxides (magnetite and/or hematite) and economic grade of copper and/or gold (Sillitoe, 2003; Williams et al., 2005). Besides the copper and by-product gold, IOCG deposits may also contain appreciable amounts of cobalt, zinc, molybdenum, silver, rare earth elements, uranium and other elements. In fact, the iron copper gold uranium deposits in the Luzong volcanic basin, central Anhui have some close relationships. For example, the porphyrites have close relationships to the formation of copper (gold), which are mainly in the Shaxi diorite porphyrite; the iron deposits are closely related to the syenites, such as the Nine iron deposit and Dabaozhuang iron deposit; and the uranium mineralization is closely related to the A-type granite in the contact zone of Mesozoic sedimentary layers, such as the Huangmeijian uranium deposit. In this paper, we present a case study of the IOCG mineralization system in the Luzong volcanic basin, central Anhui Province.

Early Neoproterozoic evolution of Southeast Pakistan: evidence from geochemistry, geochronology, and isotopic composition of the Nagarparkar Igneous Complex

ABSTRACT Rocks of the early Neoproterozoic age of the world have remained in discussion for their unique identity and evolutionary history. The rocks are also present in Sindh province of Pakistan and have been in debate for a couple of years. Yet, these igneous rocks have been studied very poorly regarding U-Pb and Lu-Hf age dating. The early Neoproterozoic rocks located in Nagarparkar town of Sindh have been considered as shoulder of Malani Igneous Suite (MIS) discovered in Southwest of India. The Nagarparkar Igneous Complex (NPIC) rocks are low-grade metamorphosed, mafic and silicic rocks. These rocks are accompanied by felsic and mafic dikes. Two types of granite from NPIC have been identified as peraluminous I-type biotite granites (Bt-granites), of medium-K calc-alkaline rocks series and A-type potash granites (Kfs-granites) of high-K calc-alkaline rocks series. Geochemical study shows that these Kfs-granites are relatively high in K and Na contents and low MgO and CaO. The Bt-granites have positive Rb, Ba, and Sr with negative Eu anomalies rich with HFSEs Zr, Hf, and slightly depleted HREEs, whereas Kfs-granites have positive Rb with negative Ba, Sr, and Eu anomalies and have positive anomalies of Zr and Hf with HREEs. In addition, these rocks possess crustal material, which leads to the enrichment of some incompatible trace elements and depletion of Sr and Ba in Kfs-granites and relatively high Sr and Ba in Bt-granites, indicating a juvenile lower continental crust affinity. Zircon LA-ICP-MS U-Pb dating of these granites yielded weighted mean 206Pb/238U ages ranging from 812.3 ± 14.1 Ma (N = 18; MSWD = 3.7); and 810 ± 7.4 Ma (N = 16; MSDW = 0.36) for the Bt-granites, and 755.3 ± 7.1 Ma (N = 21; MSDW = 2.0); NP-GG-01 and 736.3 ± 4.3 Ma (N = 24; MSWD = 1.05) for Kfs-granites, respectively. The Bt-granites and Kfs-granites have positive zircon εHf(t) values, which specify that they are derived from a juvenile upper and lower continental crust. Based on the geochemical and geochronological data, we suggest that the Bt-granites were formed through lower continental crust earlier to the rifting time, whereas the Kfs-granites were formed via upper continental crust, during crustal thinning caused by Rodinia rifting. These zircon U-Pb ages 812 to 736 Ma, petrographic, and geochemical characteristics match with those of the adjacent Siwana, Jalore, Mount Abu, and Sirohi granites of MIS. Thus, we can suggest that NPIC granites and adjacent MIS can possibly be assumed as a missing link of the supercontinent Rodinia remnants. Graphical Abstract

Geochemistry of Adakitic Rocks from the Shaxi Porphyritic Copper (Gold) Deposit, Central Anhui: Significance for Mineralization

The Lower Yangtze River Belt (LYRB) is economically one of the most important metallogenic belts in China, with more than 200 polymetallic (Cu–Fe–Au, Mo, Zn, Pb, Ag) deposits (Chang et al., 1991). Field investigations and chronological studies reveal that these deposits are closely associated, both spatially and temporally, with Early Cretaceous intermediate to felsic calc-alkaline intrusions (Xie et al., 2012; Xie et al., 2009). Geochemical studies further show that the host intrusions of porphyry Cu-Au deposits exhibit some distinctive compositional characteristics resembling modern adakites at convergent plate margins (Wang et al., 2006; Xie et al., 2009),as originally defined by Defant and Drummond (1990). What is the genesis of the adakitic rocks associated with the porphyritic Cu (Au) deposit in Shaxi? It is still need consideration. This paper focuses on the petrology and geochemistry of the Au-bearing porphyritic copper deposit to better understand metallogenic processes and associated intrusive bodies.

Petrology, Geochronology and Geochemistry of the early Cretaceous Ophiolite Complexes in Cebu Island, Central Philippines: A Clue to Mineralization

Ophiolite complexes, believed to represent exhumed oceanic lithosphere, are potentially useful markers in the reconstruction of ancient plate boundaries. Investigations of modern oceanic basins suggest that they formed and were tectonically emplaced in different ways. In addition, their geodynamic evolution was controlled by physicalchemical conditions specific of their particular tectonic setting although geochemical signature overprinting might be common than previously recognized (Parkinson and Pearce, 1998; Tamayo et al., 2001). Ophiolite complexes make up the bulk of basement lithologies in the Philippines. Origins of most ophiolites, including those from the Central Philippines, remain poorly understood. Exploratory studies have been done on the crust-mantle sequences in Panay, Bohol and to a certain extent of Cebu (eg. Falistino et al., 2003; Tamayo et al., 2001; Yumul et al., 2001). The geochronology of ophiolite complexes in Cebu island are weakly studied, only an age of K-Ar method was reported, yielded result of 108 Ma for the gabbros (Diegor et al., 1996). Our new geochronological and geochemical data from Cebu Island allow to well document the forming ages and genesis of the ophiolites, providing better constraints on tectonic reconstructions of the western portion of the Philippine Sea Plate prior to the installation of the West Philippine Basin.