秦岭造山带西段八方山-二里河地区硅质岩的地球化学特征及其地质意义

陕西省凤县-太白(凤太)地区位于秦岭造山带的西段,是我国重要的金属成矿集中区之一.本文重点分析凤太地区中泥盆统古道岭组与上泥盆统星红铺组过渡部位的沉积成因硅质岩.它是八方山-二里河铅锌矿床的主要赋矿层位.硅质岩多为灰黑色,块状、层纹状、角砾状构造,致密坚硬,经受过轻微变质作用(主要是重结晶作用),主要组成矿物为细晶、微晶石英,含有少量的铁白云石、方解石、绢云母、炭质.根据地球化学分析,八方山-二里河地区沉积成因硅质岩的SiO_2含量均大于80%,最高者可达95.3%,TiO_2和Al_2O_3明显偏低,高FeO,Al/(Al+Fe+Mn)值落在0.13～0.64之间.大部分微量元素含量远小于地壳元素丰度,但Ba、As、Sb、B、Ag、Hg含量较高.在V/Y-Ti/V和Ti-V图解中,硅质岩样品投影在洋中脊→大陆边缘的洋盆位置,大部分落在远洋区域,部分靠近大陆边缘.样品稀土元素含量较低,∑REE仅为3.28×10_(-6)～20.66×10_(-6),经北美页岩标准化后的稀土元素配分曲线可以分为H和S两组.H组曲线左倾,富集重稀土(HREE),具有明显的热水成因特征.S组与H组稍有差异,是受非热水沉积作用影响所致.(La/Lu)_N介于0.36～1.99,多分布于0.36～0.70和1.00～1.99两个区间内,分别代表与洋中脊环境相似的的洋盆喷口或断裂附近环境和受一定陆源物质影响的远洋盆地或深海平原环境.(La/Yb)_N值介于0.32～2.21,多分布0.32～0.72和1.74～2.21两个区间内,地质意义与(La/Lu)-N类似.硅质岩样品的(La/Ce)_N值在1.04～3.28,多数大于1,平均为1.62,主要反映大洋盆地(或深海平原)环境,个别样品也表现出大陆边缘沉积特征.研究结果表明,尽管经历了后期成岩作用等,但硅质岩仍整体上较好地保持了初始沉积时的地球化学特征.

Geochemical characteristics and their geological implications of cherts from Bafangshan-Erlihe area in western Qinling orogen

The Fengxian-Taibai area,Shanxi Province,is situated in the western section of the famous Qinling orogene,central China,with rich mineral resources.This study deals with chert from the transitional zone hetween the Gudaoling(D_2)and the Xinghongpu(D_3)formations of the Fengxian-Taihai area.The chert formation is the ore-hosting strata of the Bafangshan-Erlihe metallic deposit.The chert is mainly dark gray in color,hard and highly dense,with massive,lamellar and hreceiated sedimentary structures.It was slightly metamorphosed(mainly recrystallized).Cryptomere-crystallitic quartz is the dominant mineral of the studied chert,with small amounts of ankerite,calcite,sericite,carbon and the likes.It is shown by geochemical analysis that the studied chert is higher than 80% in content of SiO_2,with the highest reaching 95.3%,but low in contents of TiO-2 and Al_2O_3.The value of Al/(Al+Fe+Mn)ranges in 0.13～0.64.Most of trace elements are much lower in content than the Clarke value,however,Ba,As,Sb,B,Ag and Hg are higher.Plotted in the diagram of V/Y～Ti/V and Ti～V,the samples of chert fall into the ocean basin area,from mid-oceanic ridge to the oceanic margin,with most in pelagic area and part near oceanic margin area.The content of ∑ REE ranges in 3.28×10~(-6)～20.66×10~(-6).The samples of chert may be divided into two groups(H and S)based on their NASC-normalized REE distribution pattern.The group H is characterized by NASC-normalized REE distribution pattern inclining to the different from the group H,implying the eontamination of non-hydrotherma]sedimentation on hydrothermal one.The value of (La/Lu)_N is scattered in 0.36～1.99,characterized by the occurrence of two peaks in 0.36～0.70 and 1.O0～1.99.The first peak represents the environment of the vents in the oceanic basins or deep fractures in mid-oceanic ridges,while the second one denotes the environment of far ocean basin or abysmal sea plain influenced by continental sources.The value of(La/Yb)_N ranges in 0.32～2.21,with most in the two peaks of 0.32～0.72 and 1.74～2.21.(La/Yb)_N has similar geological implications to(La/Lu)_N The value of (La/Ce)Nvaries in 1.04～3.28,with an average of 1.62 and most being higher than 1.00,indicating that the chert formed mainly in oceanic basin or abysmal sea plain,partly in the continental margin.It is concluded from the present study that the original geochemical characteristics of chert are well conserved despite of later reworking.