安徽东至兆吉口铅锌矿床的地质和地球化学特征及成因

Geological and Geochemical Characteristics and Genesis of the Zhaojikou Lead-Zinc Deposit in Dongzhi County,Anhui Province

安徽省东至县兆吉口铅锌矿床是近年来在江南过渡带上新发现的一处大型铅锌矿床。矿体受NNE向东至断裂及其次级张扭性裂隙控制。矿石主要呈脉状、细脉-网脉状充填于中元古界蓟县系木坑组浅变质碎屑岩中,亦见矿脉穿切细晶闪长岩脉。矿石的矿物组合主要为闪锌矿+方铅矿+黄铁矿+石英+方解石。矿石结构以交代结构、交代残余结构和填隙结构为主;矿石构造主要为脉状和网脉状,局部块状或团块状。围岩蚀变主要为硅化、黄铁矿化和碳酸盐化。矿床主成矿阶段流体包裹体类型以富液相气液包裹体(V_(H_2O)+L_(H_2O))为主,均一温度为110~275℃,盐度为0.18%~12.85%NaCleq,密度为0.57~1.03 g/cm^3,成矿压力为24.4~61.9 MPa,成矿深度为1.0~2.5 km,显示成矿流体为低温、低密度、中-低盐度的流体;流体包裹体液相成分反映成矿流体为CaSO_4-Na Cl-H_2O体系。矿石中石英δ^(18)O值为12.7‰~15.9‰,换算为成矿流体的δ^(18)OH_2O值为-2.7‰~0.8‰,δD值为-81.5‰^-70.7‰,显示成矿流体为深源岩浆水与大气降水的混合溶液。矿脉中的方解石δ^(13)CV-PDB值为-8.08‰^-7.73‰,δ^(18)OSMOW值为8.49‰~9.44‰,反映成矿的炭质主要来自深部岩浆。综合成矿地质背景、矿床地质和地球化学特征,可以认为,兆吉口铅锌矿床是一个受断裂构造控制的、与燕山期中酸性岩浆作用密切相关的浅成低温热液脉状矿床。

The Zhaojikou lead-zinc deposit is located in the Jiangnan transition belt of Anhui province. The ore-body is controlled by regional structures of the NNE-trending Dongzhi fault and its secondary tensional shear fractures. The occurrence of ores is vein, veinlet, and stockwork in the low metamorphic clastic rocks of the Mukeng Formation of the Mesoproterozoic Jixian Syetem. The ore mineral assemblage includes sphalerite, galena, pyrite, quartz, and calcite. The ore is characterized by metasomatic texture, metasomatic relict and filling texture, vein, stockwork and massive structure. The types of wall-rock alteration are mainly silicification, pyritization, and carbonatization. There are three types of fluid inclusions in the Zhaojikou deposit, mainly the gas-liquid fluid inclusions. Study of the fluid inclusions in quartz in various mineralization stages indicates that the ore fluids belong to low-temperature（110-275 ℃）, lowdensity（0.57-1.03 g/cm^3）, low- to medium-salinity（0.18%-12.85% NaCleq）, the CaSO_4-Na Cl-H_2O hydrochemical type and formed in a low-pressure（24.4-61.9 MPa） and shallow（1.0-2.5 km） environment. The δ18OSMOW values of the quartz in the main mineralization stage change from 12.7‰ to 15.9‰, and the δ^18H_2O values vary between-2.7‰ and 0.8‰, with the δDV-SMOW values ranging from-81.5‰ to-70.7‰, which reflects that the ore-forming fluids was magmatic water with input of meteoric water. The δ^13V-PDB values vary from-8.08‰ to-7.73‰, and the δ^18SMOWvalues ranging from 8.49‰ to 9.44‰ for calcites show that the carbon and oxygen of calcites are sourced mainly from magma. Combined with the geological evidence and analytical results, we conclude that the Zhaojikou lead-zinc deposit is of low temperature epithermal type that is controlled by fault system and related to the Yanshan magma activity.