陕西南郑马元铅锌矿区灯影组白云岩地球化学特征

Geochemical Characteristics of Dengying Dolomite in the Mayuan Lead-zinc Orefield,Nanzheng,Shaanxi

灯影组白云岩是南郑县马元铅锌矿的赋矿围岩。本文通过野外调研和主、微量及稀土元素的分析研究,探讨了赋矿围岩白云岩的沉积环境。白云岩主量元素 CaO 为23.15%～30.54%,MgO 为16.53%～22.05%,SiO2为0.43%～26.06%,钙镁含量及比值都接近纯白云岩（CaO 为30.4%,MgO 为21.8%,CaO/MgO=1.39）的值,表明灯影组地层主要为白云岩。Al2 O3、K2 O、Fe2 O3、TiO2含量极低,表明灯影组白云岩基本不受陆源组分的影响。因此,利用微量、稀土元素判别沉积环境是可靠的。白云岩微量元素 V/Cr、Ni/Co、V/（V＋ Ni）比值分别小于2、5、0.6,U/Th 比值介于1.89～230.00,均值75.24,后太古代页岩标准化后稀土元素具有中稀土富集、轻稀土与重稀土相当、正 Eu 异常及正 Ce 异常的特征。V/Cr、Ni/Co、V/（V＋Ni）比值及正 Ce 异常表明灯影组白云岩沉积时水体处于氧化环境,异常高的 U/Th 比值及正 Eu 异常暗示灯影组白云岩沉积时水体受海底喷流热卤水的影响。

Dolomite of the Dengying formation is the surrounding rock of the Mayuan lead-zinc orefield.This paper discusses the sedimentary environment by field work and analyses of major,trace elements and REEs.The major elements CaO is 23.1 5%～30.54%,MgO is 1 6.53%～22.05%,SiO2 is 0.43%～26.06%,the contents of CaO and MgO and their ratio are extremely close to pure dolomite,indicating that dolomite in the Dengying formation is pure dolomite.The contents of Al2 O3 、K2 O、Fe2 O3 、TiO2 are extremely low indicates that dolomite of the Dengying formation had not been contaminated by terrigenous components.The ratios of V/Cr,Ni/Co,V/（V ＋ Ni）of Dengying dolomite are less than 2,5,0.6,respectively,U/Th ratios are of 1 .89 to 230.00,with an average of 75.24.REEs are characterized by MREEs enrichment,comparative amounts of LREE and HREE,positive Eu and Ce anomalies.The ratios of V/Cr,Ni/Co,V/（V＋Ni）and Ce positive anomaly suggest an oxidation water envi-ronment when the Dengying formation was deposited.The extraordinarily high ratio of U/Th and Eu positive a-nomaly indicate the sea water may have been influenced by hydrothermal fluids.