Jingwen Mao

Highly metalliferous carbonaceous shale and Early Cambrian seawater

We report evidence for the seawater origin of an extremely metal-enriched sulfide- and organic carbon–rich marker bed in a transgressive Early Cambrian black shale sequence along the passive margin of the Yangtze platform. The element concentration pattern in this marker bed suggests that it formed in a sediment-starved, stratified basin with a euxinic water column below an oxic surface layer. Biological activity was high in the surface layer, which was resupplied by communication with oxic oceans. The extremely low terrigenous input and the sulfate-reducing environment in the deeper part of the basin led to exceptionally high metal enrichments by factors of ∼107 with respect to modern seawater. The composition of the sulfidic rocks reflects the composition of the Early Cambrian oceans. The molybdenum isotope ratio suggests that during this time <35% of marine Mo was deposited in oxic sediments, and that suboxicanoxic marine environments were more widespread during the Early Cambrian than today.

Native gold and native copper grains enclosed by olivine phenocrysts in a picrite lava of the Emeishan large igneous province, SW China

Abstract A native gold bleb found in an olivine phenocryst in a picrite lava from the Emeishan Large Igneous Province (ELIP) may be the first documented case of the transport of gold as a distinct precious metal phase in a mantle-derived magma. Four picrite layers have been recognized in the lower part of the volcanic succession in the Lijiang area, in the western part of the ELIP. The native gold bleb was found enclosed in an olivine phenocryst in the second picritic layer of a basalt-picrite succession in the ELIP. The gold bleb is spheroidal, about 30 μm in diameter, consists of pure gold, and does not contain any other elements. In addition, native copper grains were also discovered in the serpentinized olivine phenocrysts, and native zinc and moissanite (SiC) were separated from an ~20 kg sample of picrite. The paragenesis of these minerals suggests that the primary magmas were S-unsaturated. The native gold and native copper grains are considered to be xenocrysts from the mantle, transported to shallow depths by a rising plume, and then captured by the picritic melts. The discovery of native gold and native copper grains provides direct evidence that the gold in the hydrothermal gold deposits and the native copper deposits in the ELIP lavas ultimately may be derived from a mantle plume.

Geodynamic settings of Mesozoic large-scale mineralization in North China and adjacent areas

Based on the summary of the highly precise datings of the metal deposits and related granitic rocks in North China craton and adjacent areas, such as the molybdenite Re-Os datings,40Ar-39Ar datings of mica, K-feldspar and quartz, some Rb-Sr isochrons, and the SHRIMP zircon U-Pb dating and single grain zircon U-Pb dating, we suggest that the large-scale mineralization in North China craton and adjacent areas take place in three periods of 200-160Ma, 140Ma±, and 130-110Ma. Their corresponding geodynamic settings are proposed to be the collision orogenic process, transformation of the tectonic regime, and delamination of the lithosphere, respectively, in light of analyzing the Mesozoic geodynamic evolution in the North China craton.

K-Ar Dating, Geochemical, and Sr-Nd-Pb Isotopic Systematics of Late Mesozoic Mafic Dikes, Southern Jiangxi Province, Southeast China: Petrogenesis and Tectonic Implications

A geochemical and isotopic study was carried out on late Mesozoic mafic dikes in southern Jiangxi Province in order to clarify their petrogenesis and explore tectonic implications. K-Ar dating results show that these mafic dikes intruded during Jurassic to Late Cretaceous (147-79 Ma) time. Dominantly sub-alkaline in composition, these dikes share several geochemical features: they are significantly enriched in large-ion lithophile element (LILE, e.g., Rb, Ba, Sr) and slightly enriched in light rare-earth elements (LREE), and are variably depleted in high-field-strength elements (HFSE, e. g., Nb, Ta, Ti). However, mafic dikes intruded in the Late Jurassic to Early Cretaceous (147-139 Ma) are also characterized by weaker Nb and Ta negative anomalies compared with dikes intruded in the mid-Late Cretaceous (118-78 Ma). Age-corrected Sr-Nd isotopic ratios show relatively wide ranges for (87Sr/86Sr)I from 0.7042 to 0.7130, and -10.8 to +5.9 for εNd(T). 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios for these dikes range from 17.819 to 18.400, 15.472 to 15.730, and 37.711 to 38.787, respectively. Geological, geochemical, and isotopic evidence suggests that the mafic dikes in southern Jiangxi Province were not significantly affected by crustal contamination. We consider them to have originated from dominantly EM-2 type lithospheric mantle ± minor asthenospheric mantle (MORB or DMM). The geochemical and Sr-Nd-Pb isotopic characteristics of basalts and mafic dikes, together with geochemical and isotopic data from granitoids, volcanic rocks, and rift basins in Southeast China suggest that this region was dominated by a lithospheric extensional tectonic regime in the late Mesozoic. This study, integrated with published geochemical and isotopic data for upper Mesozoic and Cenozoic basaltic rocks, allows us to postulate that lithospheric thinning in Southeast China continued until the end of Late Cretaceous time, and was accompanied by a shift in the mantle sources of basaltic rocks.

The genetic relationship between hydrocarbon systems and Mississippi Valley-type Zn–Pb deposits along the SW margin of Sichuan Basin, China

The Chipu Mississippi Valley-type (MVT) deposit is located on the southwest (SW) margin of the Sichuan Basin. Occurrence of plentiful organic matter (bitumen) at this deposit and abundant hydrocarbon reservoirs in the SW Sichuan Basin implies a link between lead–zinc mineralization and hydrocarbon systems in this area. The high δ34S values of most metal sulphides from the different ore stages suggest that H2S-bearing gases and/or thermochemical sulphate reduction (TSR) by organic matter could have been the source of reduced sulphur involved in ore formation. Sulphides with small positive to negative δ34S values can be attributed to organically bound sulphur at the Chipu deposit. Carbon and oxygen isotopic compositions from sparry carbonates suggest mixing of organic carbon with seawater-derived carbon in the mineralization process. From the early to the later ore stages, δ13CPDB values of ore-hosting carbonates are increasingly more negative, which indicates strengthening of the TSR role during mineralization. Hydrogen and oxygen isotopes in fluid inclusions in the quartz gangue indicate the provenance of the ore-forming fluids in the hydrocarbons. Moreover, some extremely low hydrogen isotope values suggest the addition of hydrogen from the same source. The low H/C ratios and high non-hydrocarbon component of the bitumen (Zhang et al. 2010) also suggest that the organic matter may have been involved in TSR and subjected to a strong oxidation by ore-bearing fluids. This study attempts to explain the lead–zinc mineralization process and the role of organic matter in it. As there is a demonstrable relationship between the evolution of the hydrocarbons and regional lead–zinc mineralization along the SW edge of the Sichuan Basin, we propose a possible model in which the MVT mineralization coincided with the degradation of hydrocarbon reservoirs due to the large-scale migration of basinal fluids, most likely driven by the late Indosinian orogeny in response to the closure of the Palaeo-Tethys Ocean.

Early Cretaceous magmatism and ore mineralization in Northeast China: examples from Taolaituo Mo and Aobaotu Pb–Zn deposits

The recently discovered Taolaituo porphyry Mo deposit and Aobaotu hydrothermal vein Pb–Zn deposit are both located in the Great Xing’an Range, Northeast China. Here we present new zircon U–Pb ages, whole-rock geochemical and Pb isotopic data, and molybdenite Re–Os ages for these two deposits. The Mo mineralization in the Taolaituo area occurred in quartz porphyry, which yields zircon U–Pb ages ranging from 138.5 ± 0.8 to 139.1 ± 0.5 Ma. Fine-grained granite representing pre-mineralization magmatic activity was formed at 145.2 ± 0.5 Ma. Molybdenite Re–Os dating indicates that Mo mineralization occurred at 133.8 ± 1.2 Ma. In the Aobaotu deposit, the ore-related granodioritic porphyry has a zircon U–Pb age of 140.0 ± 0.4 Ma. These geochronological data indicate that these magmatic and hydrothermal activities occurred during the Early Cretaceous. The mineralogical and geochemical features of the Taolaituo and Aobaotu granitoids suggest they can be classified as A1-type within-plate anorogenic granites and I-type granites, respectively. The Pb isotopic compositions suggest a mixed crust–mantle origin of the granitoids in these two deposits. The Taolaituo granitoids were formed by the partial melting of lower crust and crust–mantle interaction, with subsequent fractionation of apatite, feldspar, Ti-bearing phases and allanite or monazite. In contrast, the Aobaotu granites were derived primarily from lithospheric mantle that had been transformed or affected by the addition of subduction-related components. Combined with the regional geology, tectonic evolution and available age data from the literature, our results suggest that the Early Cretaceous (140–100 Ma) was likely to be the most important peak period for metallogenic mineralization in Northeast China. The Taolaituo and Aobaotu deposits formed under an extensional environment at an active continental margin in response to subduction of the Palaeo-Pacific oceanic plate.

U-Pb Ages and Lu-Hf Isotopes of Detrital Zircons from Sedimentary Units across the Mid-Neoproterozoic Unconformity in the Western Jiangnan Orogen of South China and Their Tectonic Implications

The Jiangnan Orogen, located at the southeastern margin of the Yangtze Block, South China, records the complete history of assembly and evolution between the Yangtze and Cathaysia blocks. Neoproterozoic sedimentary sequences are widespread in the Jiangnan Orogen, with a regional angular unconformity separating the Sibao and Danzhou groups. U-Pb geochronology and Lu-Hf isotope analyses were carried out on detrital zircons across this unconformity and also the associated granites in the western Jiangnan Orogen in this study. Our new results, combined with previous data, indicate that the Sibao and Danzhou groups were deposited at 860–832 and 803–764 Ma, respectively. Thus, this unconformity was constrained to be 832–803 Ma. Detrital zircons from the Sibao and Danzhou groups have generally similar age populations, with three major peaks at 2000–1550, 1000–880, and 850–830 (760) Ma and one minor at 2600–2400 Ma. The most important generation of juvenile crust appears to have occurred at 2600–2400 Ma. Both recycling of ancient crustal materials and addition of juvenile mantle components took place in the time intervals of 2000–1550 and 1000–880 Ma. Detrital zircons in the age population of 850–830 (760) Ma were probably derived from proximal magmatic rocks in the western Jiangnan Orogen, reflecting fast erosion of newly formed igneous rocks. The Sibao and equivalents were deformed and intruded by granites posterior to the deposition of the Sibao and equivalent sequences after 832 Ma but before deposition of the Danzhou and equivalent units before 803 Ma. The pre-Danzhou deformation marked a collision along the southeast margin of the Yangtze Craton. The regional angular unconformity sealed the Jiangnan Orogeny, and thus the collision between the Yangtze and Cathaysia blocks was during 832–803 Ma. The Danzhou and equivalent units unconformably overlying the Sibao Group record the onset of rifting along the southeastern Yangtze margin beginning around 803 Ma.

Geological Characteristics and Sphalerite Rb‐Sr Ages of the Tiantang Cu‐Pb‐Zn Polymetallic Deposit, Guangdong Province and their Geological Significances

Zhao et al., 1990; Meinert, 1992, 1993; Mao et al., 1996, 2006, 2011; Misra, 2000), especially their economic importance is fully recognized (Boyle, 1979; Bache, 1987; Meinert, 1989; Sillitoe, 1989; Chen et al., 1992, 1997; Zhao et al., 1992, 1997; Chen, 1996). The large Tiantang Cu-Pb-Zn polymetallic skarn deposit of Guangdong Province occurs in the northeast edge of the Yangchun basin, which locates in the Qinzhou-Hangzhou metallogenic belt. It is hosted in the limestone of Devonian Tianziling, and the ore bodies occur in the skarn and skarnization marble as stratoids, lamellar, lenses and irregularity. Metallic minerals consist malinly of chalcopyrite, galena, sphalerite and pyrite while gangue minerals are composed of garnet, diopside, epidote, wollastonite, chlorite and so on. On the basis of geological field observations, the authors select main sulfides to analyze its S-Pb isotopic composition so as to trace and discuss the sources of ore-forming metal materials, and the age of the Tiantang Cu-Pb-Zn polymetallic deposit is determined by the Rb-Sr isochron method for sphalerite and three group of intergrowth mineral assemblage samples (pyrite and galena). S and Pb isotope show that the ore-forming material of the ore bodies in the Tiantang Cu-Pb-Zn polymetallic deposit was derived from crustmantle hybrid (Zheng et al., 2012). The Rb-Sr isochron convinced us that the metallogenic epoch is about 98 Ma (Zheng et al., 2013), and it is thus held that the Late Cretaceous is the main time for the formation of the Tiantang Cu-Pb-Zn polymetallic deposit. Above all, characteristics of Tiantang Cu-Pb-Zn polymetallic deposit shows that it is a typical skarn deposit, which is resulted from the replacement of the marble by granitoid magma. It is may be related that Chinese mainland including South China Plate was in a sustained stretch phase, which was caused by changing movement direction of the Pacific Plate after 135 Ma.

Characteristics and Significance of Chlorite from Xinliaodong Cu Polymetallic Deposit in Wastern Guangdong Province, China

Eastern Guangdong Province is located in the intersection part of Nanling E-W tectonic-magmatic belt and the southeastern China N-E volcanic belt (Xu et al., 1999). The Xinliaodong is a newly-discovered copper ore deposit in eastern Guangdong Province, the genesis and type of which remains unclear until this moment. Chlorite is one of the main ore associated alteration minerals, which perhaps genetically related to copper mineralization in this ore district. Based on the field geological survey, microscopic observation and electron microprobe analysis, we systematically studied the category, elemental composition, forming temperature, forming environment and the formation mechanism of the chlorites with various occurrences from the Xinliaodong Cu polymetallic deposit. This paper tries to conduct preliminary studies on the relationship between chlorite and mineralization.

Magmatic Cu-Ni sulfide deposits in northern Xinjiang, China: A review

On the basis of previous studies, this paper provides a preliminary summary of recent research on mafic-ultramafic intrusions and related magmatic Cu-Ni sulfide ore deposits in northern Xinjiang. The authors argue that there is still a need to study some key problems concerning the origin and tectonic settings of these intrusions and sulfide deposits.