Study On The Formation Environment And Geochemistry Of Lower Cambrian Black Shale Series, Northern Guizhou Province, China

As is known to us, there are many series of black rock scattered in the south of China, which are theimportant carriers of PGE, Ag, Au, Ni, Mo, V etc., and often generate large-scale or super-large-scale metaland non-metal deposits, such as Ni-Mo-PGE deposit, V-Cu-U deposit, phosphorus deposit, barite depositetc. Black shale series is also one of hydrocarbon source rocks of sedimentary rocks and closely related tofossil energy. Research on the black shale series has become a hot area in geology, which is of significancein theory and economy. Northern Guizhou is the typical region where the Lower Cambrian black shaleseries developed, wherealot of research has been done by many geologists through the methods ofpaleontology, stratigraphy, geochemistry and metallogeny while the research through combining organicgeochemistry with elemental geochemistry is not frequently used. In light of petromineralogy, organicgeochemistry and elemental geochemistry features of the black shale series in this area, on the basis of thecomposition of the black shale series, this paper mainly discusses on the formation environment, source ofelements, mode of occurrence and enrichment rules.Through field observation, rock-mineral analysis interior the lab and referring to the former studies,it proposes that the lithological sequences of the Lower Cambrian black shale series can be divided intosuch 5 layers from the bottom to the top as black quartzite (lower silicalite), phosphate (phosphorite),metal-rich shale (Ni ore bed and Mo ore bed), black striation ampelitic-argillaceous silicalite (uppersilicalite), and black ampelitic ledikite shale.In order to get the features of the composition of the black shale series, we used X-ray powderdiffraction and SEM-EDAX analysis. Black shale series of Niutitang formation of lower Cambrian ismainly composed of siltite, shale, stone-like coal, quartzite, carbonatite, phosphorite, baritic rock etc, andthe main mineral assemblage contains quartz, ledikite, chlorite, phosphorite, pyrite, barite, calcite etc, andsulfide minerals in Ni-Mu multielement enrichment layer mainly include pyrite, sphalerite, arsenopyrite,molybdenite, millerite, placodine, dobschauite, chalcocite, chalcopyrite and galenite etc.According to tectonic background, lithostratigraphic and paleontological characteristics, it is affirmedthat lithofacies paleogeography of the black shale series occured in Zunyi area of Guizhou province is anearshore-offshore facies, which means a shallow-water continental shelf, from where towards east itevolved gradually into a slope-basin area, which means a deep-water continental shelf:Deep fault become the channel of mineral matter and hot bittern seawater sediments as well as magmatic intrusion can provide the mineral matter and"Hot Resource". On the basis of the research data,we believe that at lower Cambrian and before, a series of deep fault grows in the investigated region withsome long-term activities, along which strong volcano eruption and magmatic intrusion occurred,controlling the development of regional geological tectonic and the distribution of minerals, which playedthe key role in the multi-element enrichment of the black shale series.Tests of the contents of organic carbon and organic sulfur of rock samples are made to find out theorganic abundance of the black shale series. The average content of organic carbon and organic sulfur is5.481% and 4.532%, the highest content up to 9.29% and 23.30%, respectively. The organic of the blackshale series takes their origin from algae and fungi biology. The organic sulfur and pyrite indicate aclosed-semi closed anoxic and reducing environment.The degree of organic maturation of black shale series can reflect its degree of thermal evolution.The component of chloroform bitumen"A"has the distribution of high saturated hydrocarbon, lowaromatic hydrocarbon and high nonhydrocarbon, that characterized in sapropelic organic matters evolvedfrom lower hydrobiont which are rich in lipids and proteins. The kerogen macerals of the black shale seriesare mostly formed by sapropel, and the kerogen belongs to the type"â… "(sapropel type), closely related tofungi and algae. Confirmed by what the organic carbon content and vitrinite reflectance indicated, the blackshale belonged to comparatively good hydrocarbon source rocks, with a high maturity of organic matters,reaching to the stage of oil-generative threshold and high maturity (or alternating action).Biomarker compounds are detected on the analysis of the gas-facies chromatogram of saturatedhydrocarbon, according to which we can discuss the evolvement of organic molecule compounds ingeologic bodies at the molecule level. The black shale series has an evidently high value of main peakcarbon and lower∑nC₂₁^-/∑nC₂₂⁺ and (nC₂₁+nC₂₂)/(nC₂₈+nC₂₉) ratios, which is possibly related to theeffect of late diagenesis.In the black shale series, OEP is 1.04～1.12, approximate tol at the odd evenpredominance, having no (or little) odd even predominance. It indicates that the organic matters weremainly from lower oceanic planktons. Additionally, the Pr/Ph ratio (0.80～1.18) of the black shale seriesless than or approximate to 1, reflects a weak reducing sedimentary environment.The differences of the C-isotopic composition in all kinds rock samples were regarded as the changeof the environment when paleo-ocean sediment started.δ¹³C_PDB ratio of the black shale [0]series isbetween -23.74‰and -31.63‰. It generally belongs to the type"L"amorphous kerogen and is richer inlight C-isotope. In section, from phosphate, Ni ore bed to Mo ore bed, theδ¹³C value decreases gradually, which indicates that the sea level lies in rising period and anoxic and reducing environment graduallyincreased, theδ¹³C value of Mo ore bed decreases to the minimum, representing a maximum transgressionperiod; and later, the increasingδ¹³C value of the black shale denotes a degressive sea level, companyingwith a decreasing anoxic and reducing environment.[0]The temperature of thermal evolution of black shale series is of a great significance in expoundingrock-forming and ore-forming condition. The solid bitumen reflectance data of the black shale seriessamples is between 5.25% and 6.27%, vitrinite reflectance data between 3.645% and 4.455%, converted bythe experience formulas, and the heated temperature of black shale series between 92℃and 250℃.According to the color index of kerogen and micro-algae fossil, it is demonstrated that thepaleotemperature of the black shale series ranges from 60℃to 110℃. By combining with pyritethermoelectricity coefficient thermometry result (100℃～240℃) and inclusion equal thermometry result(226℃～230℃and 113℃～153℃) it shows that the heated temperature of the black shale series ofNiutitang formation is between 60℃and 250℃, and the max paleotemperature between 200℃and 250℃,which belongs to the middle-lower temperature sedimentary and rock-forming and ore-forming actions inLower Cambrian black shale series in northern Guizhou.The content of Au, Ag, PGE were tested for making clear of the source of these elements in the blackshale series. Platinum-group elements (including Au and Ag) is enriched in the metal-rich shale, especiallyin Mo ore bed of the black shale series. The PGE abundance, elements' interrelation, relative parametersand PGE patterns indicated that PGE sourced from hot bittern seawater sediments from magma, and highenrichments of PGE resulted from mixing of hot bittern seawater sediments and normal seawatersediments.The total REE content of the black shale series reaches its peak value (up to 315.12×10^-6) in Mo orebed of metal-rich shales, La/Sm＞1, representing mantle plume or abnormally style, which suggests thatmantle matters have been joined while forming the black shale series. The chondrite-normalized REEpattern is a right curve tilting of LREE enrichment.δEu andδCe range from 0.52 to 0.87 and from 0.68 to0.88, respectively, all of which is less than 1, and the negative anomaly of Eu and Ce showed that the blackshale series were deposited in an anoxic sea environment. The North America shale-normalized REEpattern shows negative anomaly of Ce, LREE＜HREE, which is nearly smooth, reflecting the characteristicof marine hydrothermal sediments. The enrichment layer of the multi-metal elements is the enrichment body of other trace-elements,such as Mo,Se,As,Sb,Tl,Ni,U,Pb,Bi,Cu,Cs,Zn,V etc. That the total content of the trace-elementsreaches the highest value (110759.11×10^-6) in Mo ore bed. The data that has been got,δU＞1, suggests areducing sedimentary environment. The high enrichment coefficient of trace-elements and the data that hasbeen got, U/Th＞1, indicate that they might be affected by comparatively strong hydrothermal activities.The enrichment characteristic of the trace-elements in rocks can integratively reflect the formationenvironment of sediments and rock-forming process. The black shale seires is rich in organic matters andclay minerals, mainly with the sulfide in Ni-Mo ore bed, which indicates that the occurrences of theelements are represented as the results of sulfide, organic matter bound and clay mineral adsorption. Basedon the research above and combining with the former research, this paper is to simply discuss theoccurrence of elements in black shale series, including PGE,REE,Ni,Mo,Au,Ag,V,U,Cu,Pb,Zn,As,Sb,Tl,Se etc.On the basis of organic geochemistry, elementary geochemistry and geologic structure background,this paper discusses the formation environment of black shale series and the enrichment rule of multi-metalelements. At earlier Cambrian of about 542Ma, when the north of Kuizhou province generated thepaleohydrothermal vents and -multiple submarine volcano eruption and magmatic intrusion occurred alongdeep fault, it caused deep"hot resource"upwelling, together with surface water, ground water and seawater, because of which the hot bittern seawater at certain temperature came into being, resulting in hotwater recycling or hot water activity. And in the process of flowing, the hot bittern seawater dissolved lotsof useful elements of the basic-ultrabasic rocks at Proterozoic Wuling period, that furthered the formationof the ore-bearing hot bittern seawater, which is the direct source of the multi-metal of the black shaleseries. On the belt of the resident and reducing platform margin slope (about 200m deep of water), a lot ofalgaes and planktonthes had been bred in surface layer water, and the biologies had a high reproducingability and provided with adequate organic matters for the seabed as they died and began to sink, whosedecaying and dispersed reliquiaes absorbed a great number of dissolved oxygen, resulting in an anoxic andreducing environment of water body of middle ocean and then the formation of black shale series, in whichthe organic matters is enriched, acting greatly on the movement and gathering of multi-metal elements.Therefore, comparison can be made between the anoxic event (or environment) of Earlier Cambrian of thisarea and that of global oceans. The mineralization of multi-metal elements had experienced several periodsof sedimentary stage, diagenesis stage, epigenetic stage and hypergene stage. The mineralization characterized in low temperature (below 200℃). Ever since the black shale series had begun in sediment, aperiod of reassembly (which represents diagenesis stage) and movement and enrichment (which indicatesepigenetic stage) had been put on platinum-group element (including Au and Ag) at the low temperature.The effect of the epigenetic stage had been posed only on places of certain depth next to the earth surface.Study on the formation environment and geochemistry of the black shale series can establish thefoundation for the genesis of the black shale series, enrichment model of multi-metal element anddiscussing mineral-generative mechanism of black shale series deposit and promote exploration of theblack shale series deposits, especially the platinum-group element deposit.