西昆仑赞坎地区古元古代化学沉积岩系的地球化学及微区特征研究

Study on geochemistry and micro-area characteristics of Paleoproterozoic chemical sedimentary rocks from Zankan area,West Kunlun,China

赞坎铁矿是西昆仑地区典型的铁矿床之一,该矿区内存在一套共生的化学沉积岩系"铁矿层和硅质岩"。本研究对该化学沉积岩系的宏观地球化学特征和微观矿物学组构及矿物地球化学特征进行了分析和探讨,研究结果支持赞坎铁矿为遭受了改造的沉积变质型铁矿并形成于类似大陆边缘海相环境。显微镜下,富铁矿石主要包含石英、磁铁矿和硅酸盐矿物并具有明显的条带状构造,而硅质岩则表现为细-微晶石英颗粒构成的紧密堆积结构。矿石富铁条带的XRD和EBSD分析结果表明,该矿石富铁条带内的主要矿物为磁铁矿、辉石和微量磷灰石,磁铁矿颗粒的晶胞参数为a=b=c=8.394A,z=8并吻合沉积变质型铁矿的特征。LA-ICPMS分析结果表明,磁铁矿内杂质元素的氧化物平均含量按照Cr_2O_3(2764×10^(-6))、Al_2O_3(2494×10^(-6))、V_2O_5(1494×10^(-6))、SiO_2(1255×10^(-6))、TiO_2(589.7×10^(-6))、MnO(162.5×10^(-6))、MgO(114.9×10^(-6))依次降低,其地球化学示踪结果吻合沉积变质型铁矿的特征。全岩地球化学分析结果显示,硅质岩的SiO_2含量70.58%~81.78%、Ba含量平均372.4×10^(-6)、U含量平均2.21×10^(-6)、ΣREE值平均97.37×10^(-6),这些特征总体上吻合热水成因,而部分样品偏高的Al_2O_3含量和∑REE值及无明显Eu异常则指示了非热水成因陆源沉积物的贡献;硅质岩的Al/(A1+Fe+Mn)值0.68~0.96、MnO/TiO_2值0.00~0.05、Al/(Al+Fe)值0.74~0.97、Sc/Th值2.22~7.07、U/Th值0.33~0.86、Al_2O_3/TiO_2值平均32.24吻合,这些指标指示其形成于类似大陆边缘海相的沉积环境;硅质岩的Ni/Co平均3.10吻合富氧环境,Sr/Ba值平均0.23指示该硅质岩形成环境的水动力总体偏弱,这说明赞坎地区硅质岩及铁矿的原始形成环境相对富氧且水动力总体偏弱。由于遭受了后期的构造-岩浆事件改造,赞坎铁矿的矿石地球化学特征与典型BIF存在略微的差异,但其岩石学、宏观地球化学和微区矿物学特征均在总体上吻合沉积变质型BIF的特点。综上所述,赞坎铁矿属于早前寒武纪的沉积变质型BIF铁矿,该铁矿形成于类似大陆边缘海相环境并遭受了后期构造-岩浆事件的改造。

Zankan iron ore deposit is one of the typical iron ore in West Kunlun area, China. There exist series of chemical sedimentary iron ore layers and siliceous rocks in the Zankan ore area. In this study, we studied the geochemical- and microfabric characteristics as well as mineral geochemistry of these chemical sedimentary rocks, whose analytical results supported that the ore deposit was sedimentary metamorphic genesis and were formed in the continental marginal sea with subsequent influence. Under the microscope, the iron-rich ore has obvious banded structure with quartz, magnetite and pyroxene, whereas siliceous rocks are characterized by close packed structure of fine grain and mierocrystalline quart. The XRD and EBSD analyses shows that the iron-rich iron bands of the iron ore are mainly composed of magnetite, pyroxene and trace apatite. The cell parameters of magnetite are a = b = e = 8. 394 and z = 8, which quite agree with the characteristics of sedimentary metamorphic iron ore. LA-ICP-MS analysis shows that, the average oxide contents of impurity elements in magnetite reduce from Cr203 （2764 ×10 -6 ）, A1203 （2494 × 10 -6 ）, V205 （ 1494 × 10 -6 ）, SiO2 （ 1255 × 10 -6 ）, TiO2 （589. 7 × 10 -6 ）, MnO （ 162. 5×10 -6 ） to MgO （ 114. 9 × 10 -6 ）. These mineral characteristics are consistent with sedimentary metamorphic iron ore. According to the geochemical characteristics of total bulk of siliceous rocks, the SiO2 ranges from 70. 58% to 81.78% with Ba, U and ∑REE averaged 372. 4 ×10-6, 2. 21×10-6 and 97.37 × 10 -6, respectively. These characteristics agreed with hydrothermal sediments and are strongly supported by the low erystallinity degree of quartz with close packed structure. However, some of the siliceous rocks showed relatively high Al2O3 content and ∑REE value with unclear Eu abnormality and denoted the contribution of non-hydrothermal sediments from the continent. The depositional environment for the chemical sedimentary rocks is continental marginal sea and is supported by the siliceous rocks with A1/（ A1 ＋ Fe ＋ Mn） value 0. 68 - 0. 96, MnO/TiO2 value 0 N0. 05, A1/（A1 ＋ Fe） value 0. 74 -0. 97, Sc/Th value 2. 22 N7. 07, U/Th value 0. 33 -0. 86 and A1203/ TiO2 value averaged 32. 24. The Ni/Co value of siliceous rocks averaged 3.10 is consistent with oxygen-rich environments, while the Sr/Ba value averaged 0. 23 reveals weak hydrodynamic force during the deposition of the siliceous rocks. They indicate that the original formation environment of siliceous rocks and iron ore in Zankan area was relative rich in oxygen with weak hydrodynamic force. The geochemical characteristics of ore in Zankan area show slight differences from typical BIF due to the subsequent tectonic-magrnatic events. However, the petrology, geochemistry and mineralogical characteristics above agree well with the typical BIF. Thus, the Zankan iron ore deposit belongs to the Paleoproterozoic sedimentary metamorphic iron ore of BIF, which was formed in the continental marginal sea and suffered the influence of the subsequent tectonic-magmatic events.