The Formation Mechanism Of Datangpo Manganese Ore Deposits During Nanhua Period In South China And The Paleo-redox Conditions Of Nanhua Marine Basin

Two sets glacial diamictite-the older Tiesi’ao Formation and the younger Nantuo Formation, were widely occured in the Nanhua Basin, which are separated by the interglacial Datangpo Formation. The sedimentary sequences of the Datangpo Formation in the Yangtze block are mainly composed of Mn-carbonates, black shales, grey shales and grey siltstones, providing excellent geological records with establishing the changes in paleoenvironments during the Nanhua (Cryogenian) Period; meanwhile, the Mn-carbonates in basal Datangpo Formation is one of the most important manganese resources in China, which is of great economic importance. In order to understand the formation mechanism of manganese ore deposits during Nanhua periods in South China and the change in paleoredox conditions of Nanhua marine basin, in this dissertation, elemental and isotopic tracers were carried on Cryogenian succession (manganese ores, black shales, and pyrites) deposited in different water depth in Yangtze Block, South China. The main results were as following:1). Based on elemental analysis, the REE of all studied Datangpo Mn-carbonate ores display three common features:(a) Mn-carbonate ores show very high REE contents, which is higher than PAAS and Kalahari Mn-carboante ors deposited in anoxic conditions, and are slightly lower than modern seafloor manganese crusts or nodules (commonly with REE contents over1000ppm);(b) PAAS-normalized REE distribution patterns display depletion in light and heavy REE and relative enrichment in middle REE, show a hat-like pattern;(c)) unlike typical marine carbonates, but similar to modern seafloor manganese crusts and nodules, Datangpo Mn-carbonate ores display positive Ce anomalies. Meanwhile, the trace elements of Datangpo Mn-carbonate ores show obvious Mo and Co enrichments. All of the above characteristics suggeest that although now preserved as Mn-carbonate minerals, these deposits were initially precipitated as oxides or hydroxides, which were reduced and converted to Mn carbonates in the diagenetic environment through-coupling with oxidation of organic matter. Our carbon isotope results for Mn-carbonate ores provide further evicence to support this conclusion, the inorganic carbon isotope compositions of Mn-carboante ores range between-9.18%o--7.05%o with a mean of-8.53%o, indicating these carbons were of organic orgin.2). This dissertation pioneered to report pyrites iron isotope results during Nanhua (Cryogenian) periods, and also further provided pyrites sulfur isotope results from Datangpo Mn-bering sequences deposited in different water depth in Yangtze Block. Our sulfur isotope results further confirm pyrites with super-high δ34S-VDCT values are common phenomena during Nanhua (Cryogenian) periods. Pyrites iron isotope results indicate pyrites are overwhelmingly enrich in heavier isotopes, we suggest partial oxidation of ferrous iron to ferric iron and the following near-quantitative reduction and transformation of iron-oxides/hydroxides precursors into pyrite during diagenesis is responsible for the widely positive pyrite Fe isotope values, which further suggest a suboxic sedimental environment in the early Datangpo stage.3). Based on a newly completed drill-core in Daotuo super-large manganese ore deposit in East Guizhou province, we obtained the basic iron isotope evolution curve during Nanhua (Cryogenian) period by measuring pyrites selected successively from the middle Tiesi’ao diamictites to the upper Datangpo Formation. Iron isotope ratios (δ56Fe.-IRMM) of all the analyzed pyrites vary from-0.80%o to+1.19%o with a mean of+0.13%o, but show great stratigraphic variations, which can be divided into three intervals:Interval I is the upper part of the Tiesi’ao diamictite, characterizing by the negative δ56Fe values; Interval Ⅱ comprises the top of Tiesi’ao diamictite and the basal Datangpo Formation, characterizing by disappearance of negative856Fe values and emergence of widely positive δ56Fe values appeared to be characteristic for the time immediately after the rise of atmosphere oxygen during the Global Oxidation Event; Interval III is the middle-to-upper part of Datangpo Formation, which is characterized by the disappearance of widely positive δ56Fe values and the reemergence of relatively negative δ56Fe values.4). We obtained the basic sulfur isotope evolution curve during Nanhua (Cryogenian) period by measuring these pyrites described in3). to better our understanding of Nanhua (Cryogenian) sulfur cycles. Sulfur isotope ratios (δ34S-VCDT) of all the analyzed pyrites vary from3.8%o to69.7%o with a mean of48.2‰, among which pyrite sulfur isotopes in Tiesi’ao diamictites vary from30.8%o to67.5%owith a mean of55.1‰. An obviously decrease in pyrite δ34S-VCTD values was observed in the transition belt between Tiesi’ao Formation and Datangpo Formation, which was followed by a sharp increase to very high δ34S-VCDT values and persisted to the boundry of Datangpo black shales and grey shales.5). In order to further understand Cryogenian sulfur cycle perturbation events, we provide in-situ sulfur isotope analysis for pyrite framboids and euhedral grains using NanoSIMS. The results turned out to be intriguing, as the δ34S-VCDT values of the pyrite framboids are systematically15‰to20‰lower than those of co-occurring massive pyrite, and sulfur isotope distribution in a single framboids are not homogeneous.6). Pyrite sulfur and iron isotope characteristics suggest a stepeise oxygenation history of Nanhua (Cryogenian) marine basin, which may have triggered by a potential rise of atmospheric oxygen during Cryogenian period.