Content uploaded by Ruidong Yang
Author content
All content in this area was uploaded by Ruidong Yang on Oct 19, 2017
Content may be subject to copyright.
New Evidence for Hydrothermal Sedimentary Genesis of the Ni-Mo Deposits in Black Rock Series of the Basal Cambrian, Guizhou Province: Discovery of Coarse-Grained Limestones and its Geochemical Characteristics
Abstract
: The molybdenum-nickel deposits in Shuidong District of Nayong County (Guizhou Province, Southwest China) are found mainly in black shale series of Lower Cambrian Niutitang Formation, which is another Mo-Ni-rich region besides Zunyi District (Guizhou province). Our systematic study on the Mo-Ni deposits in Tangjiaba of Nayong reveals that layered coarse-grained limestones, spherical beaded limestones concretions are hosted at the lower seam of the Mo-Ni deposits. Its strong negative carbon isotope anomaly (the carbon isotope value of the coarse-grained limestones varies from –2.148‰ to 8.223‰) is similar to that in the modern submarine black smoker chimney. The carbon in the coarse-grained limestones from black rock series of Nayong County might be deep source inorganic carbon. The seams, coarse-grained limestones, ore-bearing coarse-grained limestones and the roof and floor of the deposits are characterized by co-variation on the trace element spider diagram, showing good homology. The extraordinary enrichment of Ag, As and Sb resembles hydrothermal sedimentation. Pro-Earth's core elements Se is strongly enriched in Ni-Mo ore-bearing coarse-grained limestones. The ore-bearing rock series has an extremely low Th/U value (0.012–0.19); in the logU-logTh Cartesian Coordinates, the samples of the roof and floor of the deposits and ore-bearing coarse-grained limestones are found in the East Pacific tise; and the samples of coarse-grained limestones are found between the paleo-hydrothermal dedimentary area and the East Pacific tise. The chondrite-normalized rare earth element patterns of the Ni-Mo deposits show LREE enrichment, Ce negative anomaly, and Eu negative anomaly (which is supposed to be influenced by the deep magmatic processes in an extensional environment) resembles the rare earth element distribution patterns of the fluid and its sediments in modern submarine hydrothermal system. It proves that coarse-grained limestones is characterized by typical hydrothermal limestones, being closely related with the genesis of Mo-Ni deposits in Nayong County, which provides new evidence for hydrothermal sedimentary genesis of Mo-Ni deposit and negative carbon anomaly in the basal Cambrian on a global scale.
... 1,11,15−23 However, these studies usually do not distinguish the differences of the main ore-forming elements and have combined the metallogenic causes of Mo, Ni, and V. In some sections, V is often mineralized separately; 10,11 this kind of V deposit has a special sedimentary structure, which is different from Mo−Ni−V−PGE deposits. Therefore, the genesis of V deposits should be studied separately. ...
... As mentioned in the literature review, the rock association of carbonaceous rocks, siliceous rocks, and phosphorus nodules is a necessary condition for the genesis of V deposits in Lower Cambrian. 10,11,13,29 A noteworthy feature in black shale V deposits is that most of phosphorus in V-enriched layer is present in the form of phosphorite nodules. The observed phosphorus nodules in V deposits could be attributed to the upwelling current, 59−61 and it can be responsible for transporting elemental phosphorus from the seafloor to the relatively shallow water, which results in elemental phosphorus being deposited as phosphorus nodules. ...
The Lower Cambrian black shales of the Sansui vanadium deposits, located in South China, host a thin accumulation of Ni, Mo, V, and platinum group of elements (PGE). However, among them, the origin of V-bearing deposits remains controversial. To characterize the enrichment process of V-bearing deposits, samples of the mineralized layer and surrounding rocks from the Sansui area, South China, were investigated through bulk geochemical analysis and scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) analyses. There is a consistency in the change curves of Mo, Ni, and V from the Sansui V deposits, but the contents of elements show a great difference. This means the strong similarities in the metal sources but a difference in enrichment factors of Mo, Ni, and V. The presence of the tuff and the barite layer in the Sansui V deposits indicates that the formation of the associated V deposits was closely related to either a volcanic or hydrothermal activity. Analysis of geochemistry and sedimentation suggests a hydrothermal source of the metals, where the mineralization of V is related to clay and organic matter. Phosphorus nodules were observed at all sites of black shale V deposits in early Cambrian and were most likely related to the upwelling currents during the depositional period. The comparison with the Ni–Mo deposits highlights a stronger enrichment of clay in the V deposits. The V deposits are located in the lower part of the continental slope. Both organic matter and clay minerals are abundant in the Sansui deposits. However, the isomorphism of V–Al is promoted by the hydrothermal activity and suggests that the origin of V deposits is a multistage process.
... Several types of sedimentary structure, such as baritevein structure, eroding structure, barite-fragmental structure, barite-discus structure and barite-pillar structure, brecciated structure formed in hydrothermal venting channel phases (Yang et al., 2007). Several types of sedimentary structure, such as barite-banded structure, horizontal bed structure, thin siliceous, laminar structures formed in hydrothermally mixed synsedimentary phases (Gao et al., 2012). The two phase sediment structures were identified widely in barite orebeds in Zhenning and Tianzhu, Guizhou. ...
... Moreover, the typical hydrothermal mineral, including pyrite, chalcopyreaite, sphalerite, hyalophane were abundant in the Cambrian Tianzhu barite deposit, therefore, Mo Ni, V elements were abundant (Wei et al., 2012). But pyrite, chalcopyreaite, sphalerite, hyalophane were absent in the Devonian barite deposit with abnormally high-δ 34 S (+41.88‰ ...
... even all over the world. These deposits include Mo-Ni, V, PGE, barite deposits, etc. (Wei et al., 2012a), in which the Mo-Ni deposits mainly appeared in the so-called "multiple metal layer". The appearance of these rare elements and rare-earth elements is closely associated with the hydrothermal activities at the bottom of the sea. ...
In the study of the phosphate-bearing stratum at the bottom of the Cambrian system, the authors found that there occurred carbon isotope negative anomalies in the Taozichong section phosphate-bearing stratum in Qingzhen, Guizhou Province; they can be correspondingly compared with other synsedimentary carbon isotope negative anomalies both at home and abroad. The results showed that there occurred three negative anomalies of carbon isotopes in the Neoproterozoic-Early Cambrian Taozichong Formation, indicating that the temporal palaeo-oceanographical environment changed significantly, in which there happened two times of intensive carbon isotope variation, corresponding to creature extinction. Meanwhile, it is believed that the carbon isotope negative anomalies in phosphorites were caused by the ascending water mass of ocean current with the negative carbon isotopic composition of deep-ocean hydrothermal deposits.
Natural gas generation is the result of organic matter degradation under the effects of biodegradation and thermal degradation. Early Cambrian black shales in the Upper Yangtze Region are rich in organic matter and have shown great shale gas potentiality in recent years. Nevertheless, the enrichment mechanism and distribution of organic matter in these black shales between different sedimentary settings, such as intra-platform basin, slope, and deep basin, are still poorly understood. In this paper, based mainly on elemental geochemistry, a comprehensive study of the marine redox conditions, primary productivity, sedimentation rate, terrigenous input, hydrothermal activity, and water mass restrictions was conducted on the Early Cambrian Niutitang black shale in the Upper Yangtze Region. Our data showed that an intra-platform basin received a higher terrigenous input and that it deposited under more restricted conditions than the slope and deep basin settings. The primary productivity in the slope and deep basin settings was higher than that in the intra-platform basin setting. In the intra-platform basin, the productivity increased from its inner part to its margin. For the slope and deep basin settings, the high paleoproductivity generated large amounts of organic matter and its preservation was synergistically affected by the redox conditions. In contrast to the slope and deep basin, the preservation of organic matter in the inner part of the intra-platform basin was mainly controlled by redox conditions because the paleoproductivity in it was much lower than in the slope and deep basin settings. The intra-platform basin margin was the most favorable area for accumulating organic matter.
The Batang Group in the Zhaokalong area not only hosts an important Fe-Cu polymetallic deposit but also reveal important insights for the Late Triassic tectonism in the northern Sanjiang region. In order to delineate the tectonic setting and evolutionary process of the paleo arc-basin system, geochemical studies on the Batang Group strata have been carried out. The results suggest that andesite in the Zhaokalong area mainly belongs to the tholeiite series and is characterized by obvious fractionation of LREE and HREE, enrichment in LILE and depletion in HFSE, and a distinctly elevated δ34SCDT average of 10.5‰. The sandstone is classified as lithic sandstone, which is also characterized by obvious fractionation of LREE and HREE, as well as weak negative Ce and Eu anomalies. The limestone displays positive Eu anomalies, with δ13CPDB ranging from -1.3‰ to 4.4‰ and δ18OSMOW ranging from 14.6%‰ to 22.5‰. These results indicate that the andesite has a dual signature of both arc andesite and rift volcanic rocks, whereas the sandstone may be formed in an active continental margin, and the limestone could be deposited in a weak oxidizing shallow sea. The sandstone in the Zhaokalong area represents sedimentation in a platform slope facies, corresponding to the main stage of the Jinshajing oceanic basin subduction during the middle period of Late Triassic. The continental arc volcanic activity resulted from subsequent strengthened subduction, forming the andesite in the North Qiangtang back-arc basin. Afterwards, the limestone was formed after the cessation of magmatic activity. The information gleaned from the Batang Group strata helped constrain the evolution of the paleo-Jomda island arc and Jinshajiang oceanic subduction in the Late Triassic.
The upper Qigeblaq Formation (Fm) dolostones and the Yurtus Fm phosphatic cherts, black shales, limestones, and dolostones are widely distributed in the Precambrian/Cambrian transitional succession of the Aksu-Wushi area. Negative δ13C excursion above the Yurtus Fm/Qigeblaq Fm boundary was determined in this study. The pronounced negative carbon isotope excursion occurs in the phosphatic chert layers at the bottom of the Cambrian Yurtus Fm, below which the first appearance of the Asteridium-Heliosphaeridium-Comasphaeridium (AHC) acritarch assemblage zone. The δ13C curve of the lower part of the Yurtus Fm in the Aksu-Wushi area was found to be correlated with the early Cambrian δ13C curves of the Zhujiaqing Fm (Daibu Member), the lower part of the Yanjiahe Fm on the Yangtze Platform in China, the lower Tal Fm in India, the Sukharikha Fm in Siberia, and the upper part of the Tsagaan Oloom Fm in Mongolia through biostigraphy. The lower part of the Yurtus Fm in the Tarim Basin is at the Nemakit-Daldynian stage, and the Precambrian/Cambrian boundary of the Aksu-Wushi area may be located in the phosphatic chert unit which just below the first appearance AHC acritarch assemblage zone. The negative δ13C excursion (N1) across the Precambrian/Cambrian boundary in the studied section may have resulted from oceanic overturning and sea level rise.
The black shale is widely and mostly in the base Cambrian of Guizbou Province. Therefore, to study the carbon isotope exercusions of Neoproterozoic-Cambrian boundary Province for geologist is difficult. Then, the 60 m thick base Cambrian consisting of mudsone interbedded thin limestone was discovered recently in Yingping, Fuguan County, Guizhou Province. This discovery is beneficial to study the ocean environment of the early Cambrian. The notable negative C isotope excur sion on Neoproterozoic-Cambrain boundary in Yingping Fuquan County, Guizhou Province compares with the most continous C isotope chemostratigraphy from the Xiaowaitaoushan Member of the Laolin section in Lunaqian village, Huize County, northern Yunnan Province, southwest China, and that from the upper Tsagaan Oloom Formation of southwest Mongolia, and that from the upper Salarrny Gol of northern Siberia, and that from Mackenzie Mt. Canada, and that from Iran, Oman, Poland and Britain. The notable negative carbon isotope excursion has been shown to be a powerful tool for the global division and correlation of Neoproterozoic-Cambrian boundary. The notable negative C isotope excursion in Yingping section, Fuguan County, Guizhou Province, which is located at 2 m thick succession over the Dengying dolostone, then, between this notable negative C isotope excursion and the Dengying dolostone has 8 m thick black shale and siliceous rocks bearing abundant Bradorida and Priaputida fossils, it is shown that ocean animals ware thriving while the black shale and siliceous rocks deposited, but the grey-green mudstone interbedded thin limestone with notable negative C isotope upon the black shale and siliceous rocks is deficient in fossils, and it is shown that mass extinct event at the Neoproterozoic-Cambrian closely relate to this global negative C isotope excursion, but not oxygen-deficient event.
The Chengjiang Biota was called a window of the "Cambrian explosion". The occurrences of the biota are restricted all along eastern Yunnan Province. Recently, Naronia, Archotuba conoidalis, Isoxys, Perspicaris, Lingulepis, Tsunyidiscus, Scenella, medusiform fossil, Leptomitus, Rhabdopleura fossil, Hyolithids, Zunyiphyton perelegans Yang et Zhao, 1999, Yuknessia sp., Longenema Ding, 1996, Sphaerocongregus variabilis were discovered in the Lower Cambrian Niutitang Formation of Zunyi County, Guizhou Province, China. Among them, Naronia, Archotuba conoidalis, Isoxys, Yuknessia sp., medusiform fossil and Leptomitus are members of Chengjiang Biota. The Biota from the upper part of the Niutitang Formation is called the "Zunyi Biota". It is the first time Chengjiang-type Biota have been found outside Yunnan Province. Additionally, the "Songlin Biota" from the base of the Niutitang Formation, containing abundant sponge fossils, Arthropoda and algal fossils, has been found at 40 m beneath the horizon of the "Zunyi Biota". Over the "Zunyi Biota", Early Cambrian Archaeocyathan fauna occur in the Mingxinsi Formation. Therefore, the discovery of the biota is of great importance to the study of evolution of Early Cambrian biota in Guizhou Province. Simultaneously, it is also important for the palaeoecological study of the Chengjiang Biota.
Marine iron manganese deposits show a greater chemical variability than was realized only 10 years ago. As a consequence correct conclusions regarding the genesis of a given deposit frequently require knowledge about several geochemical parameters and preferably about accumulation rates and spatial relations vis-a-vis e.g. spreading centers.
For old deposits the analogous interpretation is considerably more difficult, since evidence may be missing for a nearby spreading center in the past and since elevated accumulation rates may not be detectable. Hence petrographic and geochemical characteristics may be the only ones available for a genetic interpretation. I will here show that a combination of geochemical relations such as Fe/Ti — Al/(A1+Fe+Mn), Fe/Mn, Fe-Mn-(Co+Ni+Cu), U/Th and REE (rare earth element) data may yield conclusive genetic information.
The carbonate samples described in this paper were collected by Guangzhou Marine Geological Survey in April 2002 from the northern continental slope of the South China Sea using the tow-net samples. Petrology and geochemistry show the chimney-like carbonate is seep carbonate. The chimney-like carbonate possibly preserves MOA/SRB fossils, and consists of calcite, illite, quartz, pyrite, and an unknown mineral. The very light carbon isotopic compositions (-51. 24‰- -51:757‰) of the chimney-like carbonates suggest that their carbon was derived from methane microbial oxidization. The shale-normalized REE patterns of the chimney-like carbonates with characteristics of no of negative Ce and Eu anomalies, slightly LREE depletion and MREE enrichment are significantly different to those of the normal marine algal carbonate. This suggests that the chimney-like carbonates were formed in anaerobic condition. Above characteristics suggest that gas venting has occurred recently on the seafloor in the northeastern continental slopes of the South China Sea.