内蒙古乌兰拜其铅锌矿床成因探讨:流体包裹体及矿石矿物S-Pb同位素、微量元素证据

A discussion on the genesis of the Wulanbaiqi Pb-Zn deposit,Inner Mongolia:Evidence from fluid inclusions,S-Pb isotopes and trace elements of ore minerals

乌兰拜其铅锌矿床是近年来在大兴安岭中南段地区新发现的一处中型铅锌多金属矿床,目前已探明铅锌储量约27万吨,其中Ⅰ号矿段为主要的组成矿段。本文以乌兰拜其矿床Ⅰ号矿段作为研究对象,试图讨论乌兰拜其铅锌矿床成矿流体性质、成矿物质来源、成矿地质背景及成因类型。乌兰拜其矿床Ⅰ号矿段矿体主要呈似层状赋存于白音高老组地层中酸性火山碎屑岩中。根据矿物的组构和穿插关系,初步将I号矿段成矿作用划分为:毒砂-黄铁矿阶段、多金属硫化物阶段和绢云母-绿泥石阶段等三个主要成矿阶段。其中矿床中主成矿阶段的石英、闪锌矿中的流体包裹体可分为气液两相包裹体(Ⅰ型)和含子晶多相包裹体(Ⅱ型)两类。石英中气液两相包裹体的均一温度范围为125~481℃,盐度为2.74%~7.02%NaCleqv;闪锌矿中的流体包裹体均一温度范围为110~327℃,盐度为2.90%~7.02%NaCleqv。分析计算得出的包裹体形成时的成矿压力为8.27~37.43MPa,平均压力为21.26MPa;成矿深度为0.28~1.25km,平均深度为0.71km。这表明乌兰拜其铅锌矿床的成矿流体具有中低温、低密度、低盐度的特征,其形成于压力较低,深度较浅的环境下。矿床主成矿阶段的多金属硫化物具有较小的δ34S值变化范围(-6.73‰~0.17‰,均值为-4.10‰),指示硫来源于深源岩浆。闪锌矿、黄铜矿Pb同位素(206Pb/204Pb为18.130~18.267,207Pb/204Pb为15.488~15.545,208Pb/204Pb为37.920~38.104)组成较为稳定。其μ值为9.27~9.28,ω值为33.46~34.02,暗示乌兰拜其矿床铅具有壳幔混合来源特征。乌兰拜其硫铅同位素组成与大兴安岭地区成矿物质为岩浆来源的金属矿床特征相似,与早白垩世火山岩浆活动关系密切,是深部岩浆热液逐步演化的产物。结合矿区与成矿关系密切的燕山晚期花岗岩,本次研究初步认为乌兰拜其铅锌多金属矿床是受燕山晚期中酸性火山岩-次火山岩和断裂控制的,与岩浆活动有关的中低温热液铅锌多金属矿床。

The Wulanbaiqi Pb-Zn deposit is a medium-sized Pb-Zn polymetallic deposit newly discovered in the middle-southern segment of Da Hinggan Mountains.At present,the proven Pb-Zn reserves of this deposit are about 270000 tons.The ore block I is the main block of the Wulanbaiqi deposit.In this paper,we take this block as the research object,trying to discuss its ore forming fluid properties,material source and genetic type.The orebodies in this block occur as stratoid in pyroclastic rocks of the Baiyingaolao Formation.Based on structures,mineral assemblages and interspersed relationships between ores,we divided the mineralization stage into the arsenopyrite-pyrite,the polymetallic sulfide and the sericite chlorite stages.The fluid inclusions in quartz and sphalerite in the main ore-forming stage can be divided into two types of gas-liquid two-phase inclusions(type I)and daughter bearing multiphase inclusions(typeⅡ).The homogeneous temperature range of gas-liquid inclusions in quartz is 125~481℃,and its salinity is 2.74%~7.02%NaCleqv;while homogeneous temperature of the inclusions of sphalerite is between 110~327℃,with a salinity of 2.90%~7.02%NaCleqv.The calculated ore-forming pressure of inclusions is 8.27~37.43MPa(averaged at 21.26MPa),and the calculated ore-forming depth is 0.28~1.25km(averaged at 0.71km),which indicates that the ore-forming fluid of the Wulanbaiqi Pb-Zn deposit has the characteristics of low temperature,low density and low salinity,and formed at low pressure and shallow depth.The polymetallic sulfide has a narrow range ofδ34S(-6.73‰to-0.17‰,with an average value of-4.10‰),suggesting that the sulfur of this deposit derives from a deep magma.The results of Pb isotopic compositions(206Pb/204Pb:18.130~18.267,207Pb/204Pb:15.488~15.545,and 208Pb/204Pb:37.920~38.104)of sphalerite and chalcopyrite is stable.Besides,its μ and ω values are 9.27~9.28 and 33.46~34.02,respectively,indicating that the Pb isotopes of the Wulanbaiqi deposit has the crust-mantle mixed source features.The S-Pb isotopic compositions of the Wulanbaiqi deposit are similar to those of the magma-derived polymetallic deposits in Da Hinggan Mountains,indicating its close relationship with the Early Cretaceous volcanic magmatic activity,i.e.,it is the product of the gradual evolution of deep magmatic fluid.Inferred from the fact that the Late Yanshan granitoids is closely related to regional mineralization,this study preliminarily believed that the Pb-Zn polymetallic deposit in the Wulanbaiqi deposit is controlled by the intermediate acid volcanic rocks,subvolcanic rocks and faults of Late Yanshan,which demonstrate that it is a middle-low temperature hydrothermal Pb-Zn polymetallic deposits.