青海玉树东莫扎抓铅锌矿床流体包裹体研究

Fluid inclusion constraints on the origin of Dongmozhazhua Pb-Zn ore deposit,Yushu area,Qinghai Province

东莫扎抓铅锌矿床位于青藏高原金沙江缝合带和班公湖-怒江缝合带夹持的羌塘地体东北缘,是目前"三江"北段铅锌铜银多金属矿带中铅锌资源储量最大的矿床,代表着大陆碰撞造山带中一种新的铅锌矿床类型。对该矿床地质特征和成因类型的研究有助于理解区域铅锌铜银多金属成矿规律,对区域找矿具有重要意义,而以流体包裹体研究为载体的成矿流体性质研究则对确定矿床的成因类型意义重大。基于此,本文在矿区地质调查和矿床地质研究基础上,系统开展了东莫扎抓铅锌矿床流体包裹体研究。包裹体岩相学观察发现,东莫扎抓铅锌矿床各阶段流体包裹体类型简单,仅为以液相为主的气液两相液体包裹体。显微测温工作表明各期流体并非连续演化的产物,成矿前白云石化阶段、重晶石阶段流体温压条件相似,具中低温度(140～160℃)、低盐度(0.0%～2.0%NaCleqv.)特征,多金属硫化物阶段流体为主要成矿流体,具低温度(120～140℃)、高盐度(26.0%～28.0%NaCleqv.)特征,成矿后方解石化阶段流体具中高温(220～240℃)、低盐度(6.0%～8.0%NaCleqv.)特征。单个流体包裹体激光拉曼探针分析和群体包裹体成分分析表明各期流体成分相似,气相主要为H2O、CO2、N2,并含少量CO、CH4、C2H2、C2H4、C2H6等还原性气体及少量O2,液相为Ca2+-Mg2+-Na+-Cl-SO42-F-体系。对以上数据分析发现流体主要为长距离迁移的盆地卤水来源,并有封存在碳酸盐岩地层中的蒸发浓缩海水和大气降水的参与,流体演化过程中,矿区环境发生弱还原性-氧化性-弱还原性的变化。早期中低温低盐度流体白云石化围岩地层,使岩石致密度变小,孔隙度增大,为金属矿物的最初沉淀创造了空间;中期低温高盐度盆地卤水在盆地中经过长距离迁移,淋滤了地层中大量Ca2+、Mg2+、Pb2+、Zn2+等阳离子,成为富含金属离子的外来流体;矿区碳酸盐岩地层中封存的蒸发浓缩海水中的硫酸盐被细菌还原,在矿区圈闭构造中汇聚成富含还原硫的本地流体;二者在矿区发生混合是东莫扎抓铅锌矿床硫化物沉淀的主要机制。35Ma年前后东莫扎抓铅锌矿床形成时,玉树地区由于逆冲推覆和走滑断层活动活跃,区域地壳不稳定,逆冲推覆构造的发生使矿区深度发生间歇性变化,走滑断层活化早期逆冲断层,为流体从拆离滑脱带中进入矿区提供构造通道。东莫扎抓Pb-Zn矿床地质特征及成矿流体特征与世界上MVTPb-Zn矿床具很大相似性,归纳其成因类型为碰撞造山逆冲推覆带中类MVTPb-Zn矿床。这种类型的矿床在中国青藏高原大陆碰撞造山带陆续得到发现,代表了大陆碰撞造山带中一种新的矿床类型,成为青藏高原寻找Pb-Zn矿床的一个新的找矿方向。

Dongmozhazhua deposit,located in the northeast margin of Qiangtang terrane which is between Jinshajiang suture zone and Bangonghu-Nujiang suture zone,is a Pb-Zn deposit found in Pb-Zn-Cu-Ag ore-belt in 'three rivers' areas,and represents one type of hydrothermal deposits in thrust-nappe systems in continental collision orogenic belts. Now,this deposit has the largest resource quantity in the region. Studying on the geological characters and genetic type of this deposit is helpful for us to understand ore-forming rules of the regional Pb-Zn-Cu-Ag ore-belt and look for new deposits. And understanding the ore fluid properties which can be obtained by studying the fluid inclusions is much helpful for us to understand the genetic type of this deposit. So,based on detailed field work for geology of Dongmozhazhua Pb-Zn ore deposit and detailed observation of fluid inclusion sections of ores,the writers have done systematic work on fluid inclusions in this deposit. According to the fluid inclusion petrography there is just one type of fluid inclusions-liquid fluid inclusions in Dongmozhazhua Pb-Zn ore deposit. Based on the results of microscopic measuring temperature work,fluids before,at and after ore-forming stage are not continuous and they have different temperatures and salinities. Fluid at dolomitization stage and barite stage which are before the ore precipitation has middle-low temperatures (140~160℃) and low salinities (0.0%~2.0% NaCleqv.). While fluid at the ore precipitation stage has low temperatures (120~140℃) and high salinities (26.0%~28.0% NaCleqv.). At the calcite stage after the ore forming the fluid has middle-high temperatures (220~240℃) and low salinities (6.0%~8.0% NaCleqv.). Based on the results of gas chromatographic (GC) analysis,liquid chromatographic (LC) analysis and laser Raman spectrometry (LRM) analysis,the gas compositions of group and single fluid inclusions are mainly H2O,CO2,N2,a little CO,CH4,C2H2,C2H4,C2H6 and a little O2 and the liquid part of ore forming fluid belongs to Ca2+-Mg2+-Na+-Cl--SO42--F-system. By analyzing the data above,the fluids were considered mainly derived from basin brine which had moved for long distances in the basin,meteoric water and evaporated seawater which was kept in the carbonate strata. The fluids changed from weak reducing to oxidizing and then to weak reducing again and the sulfides precipitated more in the oxidizing environment. Before ore-forming stage the fluid with middle-low temperature and low salinity went into the mining area and dolomitized the carbonate rocks. The process of dolomitization made rock porosity increase and prepared first spaces for ore precipitation. At ore-forming stage the basin brine with low temperature and high salinity leached lots of metal cationic ions,such as Ca2+,Mg2+,Pb2+ and Zn2+ and so on from the regional strata when it moved in the basin and became external fluid rich in metals. By biogenic sulfate reduction (BSR),the sulfates in evaporated seawater kept in the carbonate strata generated many reduced sulfurs and this kind of fluid converged gradually in the mining area. Then the above two kinds of fluids mixed and the sulfides precipitated. This is the main mechanism of ore forming in this deposit. Around 35Ma when Dongmozhazhua Pb-Zn ore deposit formed the crust in Yushu area was not stable because of strong thrust and slip faults activities in the region. The thrust faults made the depths of the mining area changed in different ore-forming stage which was one factor for ore precipitation and the slip faults made the thrust faults move again which prepared structure paths for fluids to go into the mining area. The characters of geology and ore-fluids of Dongmozhazhua Pb-Zn ore deposit are much similar to those of MVT Pb-Zn deposits in the world. So the writers classify Dongmozhazhua Pb-Zn ore deposit as a MVT-like Pb-Zn ore deposit which is controlled by thrust-nappe system in orogenic belt. Resent years,this kind of Pb-Zn ore deposits found in Tibetan plateau has been reported several times and it becomes a new type of Pb-Zn ore deposits in continental collision orogenic belts. It also designates a new direction for exploration in Tibetan plateau.