Gold depletion in the Jiangnan Gold Metallogenic Belt in South China: Evidence for large scale fluid movement in the upper crust*

A controlled-source audio-frequency magnetotelluric (CSAMT) survey has been carried out to investigate potential iron (Fe) and polymetallic (Pb-Zn-Cu) deposits in Longmen region, which is one of the main metallogenic belts in southern China. Conducting geophysical surveys in this area is quite difficult due to mountainous terrain, dense forest, and thick vegetation cover. A total of 560 CSAMT soundings were recorded along twelve surveying lines. Two-dimensional Occam’s inversion scheme was used to interpret these CSAMT data. The resulting electric resistivity models showed that three large-scale highly conductive bodies exist within the surveying area. By integrated interpretation combined with available geologic, geophysical, and geochemical data in this area, three prospective mineral deposits were demarcated. Based on the CSAMT results, a borehole penetrating approximately 250-m depth was drilled at the location of 470 m to the northwest end of line 06, defined with a massive pyrite from the depth of 52–235 m with 7%–16% Fe content, as well as locally high-grade Pb-Zn- and Ag-Ti-bearing ores.

Download Full-text

Separation of Fe from Mn in the Cryogenian Sedimentary Mn Deposit, South China: Insights from Ore Mineral Chemistry and S Isotopes from the Dawu Deposit

Minerals ◽

10.3390/min11050446 ◽

2021 ◽

Vol 11 (5) ◽

pp. 446

Author(s):

Zhiming Xu ◽

Chengquan Wu ◽

Zhengwei Zhang ◽

Jinhong Xu ◽

Xiyao Li ◽

...

Keyword(s):

South China ◽

Large Scale ◽

Early Stage ◽

Sulfur Isotopes ◽

Mineral Chemistry ◽

Manganese Carbonate ◽

Separation Mechanism ◽

Geochemical Properties ◽

Stage Separation ◽

Fe Content

Manganese and Fe have similar geochemical properties in the supergene environment. Separation of Mn and Fe is an important process for the formation of high-grade sedimentary manganese deposits. Large-scale manganese carbonate deposits (total reserves of approximately 700 Mt) were formed during the interglacial of the Sturtian and Marinoan in South China. The orebodies are hosted in the black rock series at the basal Datangpo Formation of the Cryogenian period. The Fe contents in ores range from 1.15 to 7.18 wt.%, with an average of 2.80 wt.%, and the average Mn/Fe ratio is 8.9, indicating a complete separation of Mn and Fe during the formation of manganese ores. Here, we present element data of manganese carbonates and sulfur isotopes of pyrite from the Dawu deposit, Guizhou, China, aiming to investigate the separation mechanism of Mn and Fe and the ore genesis. The Fe in ores mainly occurs as carbonate (FeCO3) and pyrite (FeS2). The Mn, Ca, Mg and Fe exist in the form of isomorphic substitutions in manganese carbonate. The contents of FeCO3 in manganese carbonates are similar in different deposits, with averages of 2.6–2.8 wt.%. The whole-rock Fe and S contents have an obvious positive correlation (R = 0.69), indicating that the difference of whole-rock Fe content mainly comes from the pyrite content. The δ34SV-CDT of pyrite varies from 40.0 to 48.3‰, indicating that the pyrite formed in a restricted basin where sulfate supply was insufficient and the sulfate concentrations were extremely low. Additionally, the whole-rock Fe content is negatively correlated with the δ34S values of the whole-rock and pyrite, with correlation coefficients of −0.78 and −0.83, respectively. Two stages of separations of Mn and Fe might have occurred during the mineralization processes. The reduced seawater became oxidized gradually after the Sturtian glaciation, and Fe2+ was oxidized and precipitated before Mn2+, which resulted in the first-stage separation of Mn and Fe. The residual Mn-rich and Fe-poor seawater flowed into the restricted rift basin. Mn and Fe were then precipitated in sediments as oxyhydroxide as the seawater was oxidized. At the early stage of diagenesis, organic matter was oxidized, and manganese oxyhydroxide was reduced, forming the manganese carbonate. H2S was insufficient in the restricted basin due to the extremely low sulfate concentration. The Fe2+ was re-released due to the lack of H2S, resulting in the second-stage separation of Mn and Fe. Finally, the manganese carbonate deposit with low Fe and very high δ34S was formed in the restricted basin after the Sturtian glaciation.

Download Full-text

A 7-Year Climatology of Warm-Sector Heavy Rainfall over South China during the Pre-Summer Months

Atmosphere ◽

10.3390/atmos12070914 ◽

2021 ◽

Vol 12 (7) ◽

pp. 914

Author(s):

Tao Chen ◽

Da-Lin Zhang

Keyword(s):

South China ◽

Heavy Rainfall ◽

Large Scale ◽

Convective Available Potential Energy ◽

Potential Temperature ◽

Vertical Wind Shear ◽

Precipitable Water ◽

Atmospheric Parameter ◽

Low Level ◽

Warm Sector

In view of the limited predictability of heavy rainfall (HR) events and the limited understanding of the physical mechanisms governing the initiation and organization of the associated mesoscale convective systems (MCSs), a composite analysis of 58 HR events over the warm sector (i.e., far ahead of the surface cold front), referred to as WSHR events, over South China during the months of April to June 2008~2014 is performed in terms of precipitation, large-scale circulations, pre-storm environmental conditions, and MCS types. Results show that the large-scale circulations of the WSHR events can be categorized into pre-frontal, southwesterly warm and moist ascending airflow, and low-level vortex types, with higher frequency occurrences of the former two types. Their pre-storm environments are characterized by a deep moist layer with >50 mm column-integrated precipitable water, high convective available potential energy with the equivalent potential temperature of ≥340 K at 850 hPa, weak vertical wind shear below 400 hPa, and a low-level jet near 925 hPa with weak warm advection, based on atmospheric parameter composite. Three classes of the corresponding MCSs, exhibiting peak convective activity in the afternoon and the early morning hours, can be identified as linear-shaped, a leading convective line adjoined with trailing stratiform rainfall, and comma-shaped, respectively. It is found that many linear-shaped MCSs in coastal regions are triggered by local topography, enhanced by sea breezes, whereas the latter two classes of MCSs experience isentropic lifting in the southwesterly warm and moist flows. They all develop in large-scale environments with favorable quasi-geostrophic forcing, albeit weak. Conceptual models are finally developed to facilitate our understanding and prediction of the WSHR events over South China.

Download Full-text

3-D Qp and Qs Seismic Attenuation for the Central Alps and their Foreland

10.5194/egusphere-egu21-8028 ◽

2021 ◽

Author(s):

Federica Lanza ◽

Tobias Diehl ◽

Donna Eberhart-Phillips ◽

Marco Herwegh ◽

Donat Fäh ◽

...

Keyword(s):

Large Scale ◽

Hot Springs ◽

Western Alps ◽

Seismic Attenuation ◽

Upper Crust ◽

Central Alps ◽

Northern Switzerland ◽

High Q ◽

Velocity Models ◽

Q Model

<p>In the framework of the SeismoTeCH project, which aims at advancing our understanding of seismotectonic processes in Switzerland, we present the first 3-D attenuation model of the upper crust for the Central Alps and their northern foreland. The 3-D inversions derive the quality factor Q (1/attenuation) using path attenuation t<sup>&#8727;</sup> observations for 4,192 distributed earthquakes recorded on permanent and temporary stations, including both velocity and acceleration records for the period 2002-2019. We followed a procedure of gradational inversions, in which a series of inversions with increasingly grid complexity are performed, with the goal of obtaining a useful Q model everywhere despite the varied data distribution. The Qs and Qp results show large-scale features in the upper crust, which are consistent with a recently improved high-resolution velocity models of the same region and serve to refine the interpretations of crustal structures from Vp and Vp/Vs. For example, the foreland region of southern Germany and northern Switzerland show a low Q crustal block bounded by high Q regions in the uppermost layer between -2.5 and 2.0 km depth. This markedly correlates with the overlying surface geology, where low Q areas coincide with the Molasse Basin, and the transition between low and high Q regions outline the geological boundary between the Molasse and the Mesozoic sediments towards north and the Alpine front to the south. At depths ranging between 2.0 - 6.5 km, low Q is imaged along the Rhone valley in the Valais in southwest Switzerland. This region presents the transition between the Centrals and Western Alps and hosts the presently seismically most active fault zones. As the attenuation of fractured areas is enhanced by fluids, low Q values may relate here to distributed microfractures that produce greater fracture connectivity and permeability in a relatively higher strain-rate zone. These geophysical constraints seem to support crustal scale fluid flow along fracture networks as manifest by the prominent occurrence of hot springs in this area. On the other hand, the moderate-to-high Qs and Qp (400-800) along with low Vp/Vs ratio and high Vs observed in the external Aar Massif could be indicative of metamorphic processes leading to different Vp/Vs ratios compared to the basement in the northern foreland (Black Forest Massif), and possibly image the continuation of the massif 20-30 km further to the northeast. In combination with recently developed Vp and Vs velocity models, the developed 3-D attenuation models provide additional constraints in terms of composition and physical properties of the uppermost crust of the central Alps as well as crucial input for next generation seismic hazard models of Switzerland, allowing for a more realistic prediction of earthquake related ground motions.</p>

Download Full-text

Topographic Rossby Waves in the Abyssal South China Sea

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0187.1 ◽

2021 ◽

Author(s):

QI QUAN ◽

ZHONGYA CAI ◽

GUANGZHEN JIN ◽

ZHIQIANG LIU

Keyword(s):

South China Sea ◽

Group Velocity ◽

South China ◽

Large Scale ◽

Rossby Waves ◽

Western Boundary ◽

Horizontal Shear ◽

Boundary Current ◽

China Sea ◽

Topographic Rossby Waves

AbstractTopographic Rossby waves (TRWs) in the abyssal South China Sea (SCS) are investigated using observations and high-resolution numerical simulations. These energetic waves can account for over 40% of the kinetic energy (KE) variability in the deep western boundary current and seamount region in the central SCS. This proportion can even reach 70% over slopes in the northern and southern SCS. The TRW-induced currents exhibit columnar (i.e., in-phase) structure in which the speed increases downward. Wave properties such as the period (5–60 days), wavelength (100–500 km), and vertical trapping scale (102–103 m) vary significantly depending on environmental parameters of the SCS. The TRW energy propagates along steep topography with phase propagation offshore. TRWs with high frequencies exhibit a stronger climbing effect than low-frequency ones and hence can move further upslope. For TRWs with a certain frequency, the wavelength and trapping scale are dominated by the topographic beta, whereas the group velocity is more sensitive to the internal Rossby deformation radius. Background circulation with horizontal shear can change the wavelength and direction of TRWs if the flow velocity is comparable to the group velocity, particularly in the central, southern, and eastern SCS. A case study suggests two possible energy sources for TRWs: mesoscale perturbation in the upper layer and large-scale background circulation in the deep layer. The former provides KE by pressure work, whereas the latter transfers the available potential energy (APE) through baroclinic instability.

Download Full-text

Circulation Features Associated with the Record-Breaking Rainfall over South China in June 2017

Journal of Climate ◽

10.1175/jcli-d-17-0903.1 ◽

2018 ◽

Vol 31 (18) ◽

pp. 7209-7224 ◽

Cited By ~ 10

Author(s):

Jianqi Sun ◽

Jing Ming ◽

Mengqi Zhang ◽

Shui Yu

Keyword(s):

East Asia ◽

South China ◽

North Atlantic ◽

Large Scale ◽

Wave Train ◽

Western Pacific Subtropical High ◽

Early Summer ◽

Intense Rainfall ◽

Maritime Continent ◽

Anomalous Anticyclone

In June 2017, south China suffered from intense rainfall that broke the record spanning the previous 70 years. In this study, the large-scale circulations associated with the south China June rainfall are analyzed. The results show that the anomalous Pacific–Japan (PJ) pattern is a direct influence on south China June rainfall or East Asian early summer rainfall. In addition, the Australian high was the strongest in June 2017 during the past 70 years, which can increase the equatorward flow to northern Australia and activate convection over the Maritime Continent. Enhanced convection over the Maritime Continent can further enhance local meridional circulation along East Asia, engendering downward motion over the tropical western North Pacific and enhancing the western Pacific subtropical high (WPSH) and upward motion over south China, which increases the rainfall therein. In addition, a strong wave train pattern associated with North Atlantic air–sea interaction was observed in June 2017 at Northern Hemispheric mid- to high latitudes; it originated from the North Atlantic and propagated eastward to East Asia, resulting in an anomalous anticyclone over the Mongolian–Baikal Lake region. This anomalous anticyclone produced strong northerly winds over East Asia that encountered the southerly associated with the WPSH over south China, thereby favoring intense rainfall over the region. Case studies of June 2017 and climate research based on data during 1979–2017 and 1948–2017 indicate that the extremities of the atmospheric circulation over south Europe and Australian high and their coupling with the PJ pattern could be responsible for the record-breaking south China rainfall in June 2017.

Download Full-text

Eddy characteristics in the northern South China Sea as inferred from Lagrangian drifter data

Ocean Science ◽

10.5194/os-7-661-2011 ◽

2011 ◽

Vol 7 (5) ◽

pp. 661-669 ◽

Cited By ~ 26

Author(s):

◽

Keyword(s):

South China Sea ◽

South China ◽

Large Scale ◽

Northern South China Sea ◽

Temporal Distribution ◽

Height Anomaly ◽

Spatial And Temporal Patterns ◽

China Sea ◽

Eddy Characteristics ◽

Anticyclonic Eddies

Abstract. Cyclonic and anticyclonic eddies from large scale to submesoscale in the northern South China Sea (NSCS) have been statistically characterized based on the satellite-tracked Lagrangian drifters using our developed geometric eddy identification method. There are in total 2208 eddies identified, 70% of which are anticyclonic eddies. If the submesoscale eddies are eliminated, the other eddies in the NSCS will show a 1.2:1 ratio of the number of anticyclones (210) to the number of cyclones (171). The spatial distribution of the eddies is regional: in southwest of Taiwan, the number of anticyclones dominates the number of cyclones, and most of them are the submesoscale anticyclones with small radii; in contrast, the large and medium cyclonic eddies are a little more than the same scale anticyclonic eddies in northwest of Luzon. The temporal distribution of eddy number in the NSCS has a close relation with the Asian monsoon. The number of the large and medium eddies peaks during the winter monsoon, while the submesoscale eddies are apt to generate in the summer monsoon. The spatial and temporal patterns have a good agreement with the results of the sea surface height anomaly (SSHA). The maximum and mean tangential velocities of anticyclones (cyclones) are 40 (30) cm s−1 and 25 (15) cm s−1, respectively. The calculated normalized vorticities from drifters suggest that although the mesoscale eddies may be considered in geostrophic balance, ageostrophic dynamics and centrifugal effects may play an important role for the growth and decay of the mesoscale cores.

Download Full-text

Origin of the Shangfang Tungsten Deposit in the Fujian Province of Southeast China: Evidence from Scheelite Sm–Nd Geochronology, H–O Isotopes and Fluid Inclusions Studies

Minerals ◽

10.3390/min9110713 ◽

2019 ◽

Vol 9 (11) ◽

pp. 713 ◽

Cited By ~ 3

Author(s):

Lü-Yun Zhu ◽

Shao-Yong Jiang ◽

Run-Sheng Chen ◽

Ying Ma

Keyword(s):

Fluid Inclusions ◽

Large Scale ◽

Tectonic Setting ◽

Critical Role ◽

Isotope Analysis ◽

Western Boundary ◽

Ionization Mass ◽

Tungsten Deposit ◽

Nanling Range ◽

Metallogenic Belt

The Shangfang deposit is a recently discovered large-scale tungsten deposit (66,500 t at 0.23% WO3), which is located near the western boundary of the Southeastern Coastal Metallogenic Belt (i.e., Zhenghe–Dafu fault), and adjacent to the northeast of the Nanling Range Metallogenic Belt. Unlike many other W–Sn deposits in this region that occur within or near the granites, the orebodies in the Sangfang deposit all occur within the amphibolite of Palaeoproterozoic Dajinshan Formation and have no direct contact to the granite. In this study, we carry out a thermal ionization mass spectrometer (TIMS) Sm-Nd isotope analysis for the scheelites from the orebody, which yields a Sm–Nd isochron age of 157.9 ± 6.7 Ma (MSWD = 0.96). This age is in good agreement with the previously published zircon U–Pb age (158.8 ± 1.6 Ma) for the granite and the molybdenite Re–Os age (158.1 ± 5.4 Ma) in the deposit. Previous studies demonstrated that the W–Sn deposits occurring between Southeastern Nanling Range and Coastal Metallogenic Belt mainly formed in the two periods of 160–150 Ma and 140–135 Ma, respectively. The microthermometry results of fluid inclusions in scheelite and quartz are suggestive of a near-isothermal (possibly poly-baric) mixing between two fluids of differing salinities. The H–O isotope results illustrate that the ore-forming fluids are derived from magma and might be equilibrated with metamorphic rocks at high temperature. The Jurassic granite pluton should play a critical role for the large hydrothermal system producing the Shangfang W deposit. Furthermore, the negative εNd(t) of −14.6 obtained in the Shanfang scheelite suggests for the involvement of the deep crustal materials. In general, subduction of the paleo-Pacific plate caused an extensional tectonic setting with formation of the Shangfang granites and related W mineralization, the geological background of which is similar to other W deposits in the Nanling Range Metallogenic Belt.

Download Full-text

Analysis of Severe Droughts in Taiwan and its Related Atmospheric and Oceanic Environments

Atmosphere ◽

10.3390/atmos10030159 ◽

2019 ◽

Vol 10 (3) ◽

pp. 159 ◽

Cited By ~ 1

Author(s):

Chih-wen Hung ◽

Ming-Fu Shih

Keyword(s):

South China Sea ◽

South China ◽

Large Scale ◽

Drought Index ◽

The South China Sea ◽

Severe Drought ◽

The South ◽

China Sea ◽

Dry Conditions ◽

Oceanic Environments

Drought is one of the important issues in climate studies. A drought index, Taiwan Meteorological Drought index (TMD index), was previously proposed and is applied here to identify historical severe droughts in Taiwan in order to clarify the corresponding large-scale backgrounds as a potential alert to the society in future. Through the TMD index, several historical severe drought cases in Taiwan are detected and characterized by significant seasonal variability in the annual cycle. Composites for large-scale atmospheric and oceanic environments over different periods within the dry season are conducted. From October to December, the colder sea surface temperature (SST) pattern of Pacific Meridional Mode (PMM) and the PMM-induced local anomalous anticyclones over the South China Sea are both in charge of the extremely dry conditions in Taiwan. From January to February, cold SST in the South China Sea and its adjacent oceans dominates local atmospheric conditions above these regions and creates an unfavorable environment for convection systems. From March to May, a massive anomalous anticyclonic circulation centering beside Alaska and extending its properties to East Asia and Taiwan generates a descending environment and in turn suppresses convection systems to develop. Therefore, the extremely dry conditions under this system are expected.

Download Full-text