Research On Depositional Feature And Mineralization Mechanism Of The Upper Devonian Manganese Deposit In Southwestern Guangxi Area

The Southwestern Guangxi area is belonge to Youjiang Basin. There were twomanganese deposit basins formed because of the active of NE trending and NWtrending strike slip faulting in the Late Devonian. They are the Jingxi-DaxinManganese Deposit Basin （Xialei-Longbang Manganese Deposit Basin） and theGuangnan-Funing manganese deposit basin. This paper focuses on the Jingxi-DaxinManganese Deposit Basin （Xialei-Longbang Manganese Deposit Basin） which is a riftbasin formed because of combining effect of the NW trending Guangnan-Daxin riftand the NE trending Xialei-Lingma rift.The Late Devonian is the most important period in manganese deposition to theSouthwestern Guangxi area. The upper Devonian can be devided into non-Manganese-bearing Sequences and Manganese-bearing Sequences. Thenon-Manganese-bearing Sequences named Rongxian Formation is characterized bythe massive carbonatite. The Manganese-bearing Sequences include the LiujiangFormation and Wuzhishan Formation, which are characterized by siliceous rocks,cherty limestone and argillaceous rock. The manganese contents are different to theLiujiang Formation and Wuzhishan Formation. The Liujiang Formation is low inmanganese without primary manganese deposit with industrial value, and the Tuhusupergene Manganese Ore Deposits formed during the process of oxidation andinfiltration. And the Wuzhishan Formation is rich in primary Carbonate-SilicateManganese Ore Deposit.The sedimentary facies type of the Late Devonian in Southwestern Guangxi areaconsists of semi-restricted platform, slope and platforms basin facies. Thesemi-restricted platform consist subfacies of lagoon, depressional platform andplatform-margin shoal. The lagoon have standing water circumstances and shallowwater body, developed rocks main in muddy limestone and dolomicrite, lime dolostone.The water body of depressional platform wae deep relatively, and developed rocks ofargillaceous limestone, nodular limestones and siliceous rocks. The water body inplatform-margin shoal is high in energy relatiively. Ooid shoal and bioclastic shoaldeveloped on the platform-margin shoal. The sediments are rich in benthos of corals, brachiopods, crinoids, stromatoporoids and shells. The slope consist subfacies of theupper slope and the lower slope. Its sediments are argillaceous limestone, nodularlimestones and micrite with few fossils. The platforms basin facie was beep in seawater, and low in water energy. Its sediments are mainly in siliceous and argillaceous,and primary Mn ore layer developed. There wer two semi-restricted platform developin study area on the left and right. Between the semi-restricted platforms was the trenchelongated. And stage of alternate platform-basin developed in the study area. Theplatforms basin facie was an ideal location to deposition of manganese.The upper Devonian Manganese Ore Deposit in Southwestern Guangxi area canbe devided into some Manganese Ore zones. Different Manganese Ore zones havesimilar characteristics. The Wuzhishan Formation developed2³manganese ore bedsof different hickness. The Manganese Ore is fairly common in massive, banded, cored,fumarolic, amygdaloidal structures. The Manganese Ore fall into two categories:pisolitic or oolitic structure and banded Manganese Ore. The colour of the ManganeseOre is rich which contains amaranth, rosiness, rose hermosa, off-white, sage green,yellowgreen, blue grey, gray black and so on.Study on the Manganese-bearing Sequences of Wuzhishan Formation inJingxi-Daxin manganese deposit basin show that the susceptibility of manganese orebeds are different to the surrounding rocks. The susceptibility of the surrounding rocksis low in common, absolute superiority in1²0(10^-6SI). But the susceptibility of themanganese ore beds is very high to several hundreds1²0(10^-6SI), even high tohundreds of thousands (10^-6SI). And in respectively, the susceptibility of the secondand third manganese ore beds is clearly higher than the first manganese ore bed, whichshows that the second and third manganese ore beds are higher in manganese than thefirst manganese ore bed. The susceptibilities of the Manganese-bearing Sequencesindicate that susceptibility can be as an effective index in discriminate non-Manganese-bearing Sequences and Manganese-bearing Sequences, and can be used indeep exploration.Pisolitic-oolitic structure is the important structural feature to the manganese orein Southwestern Guangxi area. There many types of pisolite and oolite. ThePisolitic-oolitic manganese ore can be devided in two types after the microstructure,mineralogical and geochemistry analysis. The first type is the manganese ore in whichpisolite and oolite incoherent; the boundary between the pisolite, oolite and thesubstrate show gradual transition; and the oolites do not connected with each other; thesubstrate is massive structure and lack of bedding structure. This type of manganeseore may be of hydrothermal origin. The second type is the manganese ore in which thepisolite and oolite show concentric laminar; and the oolites often connected with eachother; the substrate developed bedding structure. This kind of manganese ore may beformed from colloidal coagulation. There are two origin of manganese in study area:hydrothermal origin and terrestrial sources. There are siliceous rocks, cherty limestone and siliceous mudstones. thesesiliceous rocks have close relationship with the manganese ore. After microstructuraland geochemistry analysis, the siliceous rocks formed in continental margin. Theorigin of siliceous rocks contains hydrothermal origin and terrestrial sources. And thereare also biological forces. Results show that the origins of siliceous rocks andmanganese are in common.The deposition of manganese in study area was controled jointly by sedimentaryfacies, palaeogeography and hydrothermal system. Palaeogeographical stage ofalternate platform-basin was the base to the deposition of manganese carbonate. Therelatively closed and alkaline environment in trench was a good medium condition forthe enrichment of manganese. And the abundant of manganese from the hydrothermalsystem on the sea floor was the material basis to the manganese ore deposition.