铁建造(Iron Formation,简称IF,TFe>15%)的概念由James(1954)首次正式提出,它最早是指美国Superior地区前寒武纪地层中的一类富铁的沉积岩(Van Hise&Leith,1911)。条带状的铁建造成为BIF(Banded Iron Formation),它曾被称为铁英岩、条带状赤铁石英岩和碧玉铁质岩等。BIF建造构成了铁矿的重要组成部分。
The Xialu chert, which contains abundant biological information, were investigated by major element analysis, micro-Raman, SEM and EPMA. The results show that SiO2 content of chert is 84.12%-93.08%, averaging 89.84%. The close packed structures of low degree crystallinity of quartz indicate the hydrothermal origin. SiO2 of modern hot springs exhibit loose silica pellets and nodular, beaded structures. Under polarization microscope, the presence of biological skeleton structures indicate that biological activities are involved in the hydrothermal deposition, which correspond to the geochemical characteristics: w(SiO2)/ w(K2O+Na2O), w(SiO2)/w(Al2O3) and w(SiO2)/w(MgO), with average values of 295.29, 68.88 and 284.45, respectively. SiO2 is enriched in the organism(radiolarian) centers, the degree order of SiO2 within the biologic structures is much higher than that of outside. The impurity minerals albites are formed earlier than the original deposition. Kaolinites, feldspars and mixture of organic materials display lower degree of crystallinities and accumulate as vermicular aggregates.
Precambrian banded iron formation(BIF)is one of the most important mineral resources in China,mostly abundant in the North China Craton(NCC)with relatively less common in South China.Since the BIF and siliceous rocks both originated from chemical deposition,the syngenetic BIF and Siliceous rocks can help evaluate their environment of formation.We examine here the mineralogy and geochemistry of siliceous rocks associated with the Tieshanmiao Formation BIF,aiming to decipher the conditions of formation of both BIF and Siliceous rocks in the Wuyang area in the NCC.Analysis of the geochemical characteristics of whole rock shows that the Si O2 content of the siliceous rock ranges from 90.11%to 94.85%and is relatively high overall.Trace element contents of Ba and U are also high,the Ba/Sr ratio ranges from 3.89 to 25.28 and the U/Th ratio ranges from 0.09 to 0.20.Finally,theΣREE value of rare earth elements ranges from 57.03 ppm to 152.59 ppm,and these indexes all indicate that siliceous rock resulted from hydrothermal deposition.Plots of Al2 O3-Si O2,Si O2/(K2 O+Na2 O)-Mn O2/Ti O2 and Mn-10×(Cu+Co+Ni)-Fe in discrimination diagrams also verify this interpretation.However,both the Mg O content,ranging from 0.16 to 0.32,and the Fe/Ti ratio,ranging from 2.50 to 9.72,suggest that terrigenous material was added during the depositional process.Major and trace element parameters of siliceous rock,such as the Al/(A1+Fe+Mn)ratio(from 0.81 to 0.93),Mn O/Ti O2(from 0.00 to 0.17),Al/(Al+Fe)(from 0.82 to 0.93),Sc/Th ratio(from 0.21 to 0.50),U/Th(from 0.09 to 0.20),(La/Yb)N(from 0.83 to 3.04),and the(La/Ce)N(from 0.01 to 0.02)all imply that the siliceous rock formed in a continental margin.In addition,the Sr/Ba ratio from 0.08 to 0.26,theδCe value from 0.31 to 0.90,and theδEu value from 0.14 to 0.58,all indicate that the siliceous rock was formed at a relatively deeper water depth and under weak hydrodynamic conditions.Siliceous rock and BIF formed in the same geological setting,with the Si O2/(K2 O+Na2 O)ratio of siliceous rock
LI HongzhongHE JunguoLIANG JinYANG FeiZHAI MingguoZHANG LianchangVoudouris PANAGIOTIS
BIF的成因是长期困扰学术界的重要科学问题。目前,学术界提出了BIF具有火山喷发作用(阎鸿铨,1985)、陆源风化物沉积作用(Zhai and Windley,1989)、热水喷流沉积作用(Zhang et al.,2012)和生物参与下的沉积作用(Li et al.,2011)等观点。然而,BIF在沉积后期经历了漫长的地质演化历史,复杂的变质改造作用势必会为BIF物源的示踪带来影响。另一方面。
