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湘西大溶溪钨矿床矽卡岩矿物的矿物学、 地球化学特征及其形成机制
投稿时间:2019-08-06  修订日期:2019-12-12  点此下载全文
引用本文:张龙升,彭建堂,林芳梅.2020.湘西大溶溪钨矿床矽卡岩矿物的矿物学、 地球化学特征及其形成机制[J].地质论评,66(1):113-137,[DOI]:.
ZHANG Longsheng,PENG Jiantang,LIN Fangmei.2020.Mineralogical, geochemical characteristics and formation mechanism of skarn minerals in the Darongxi tungsten deposit, western Hunan[J].Geological Review,66(1):113-137.
DOI:10.16509/j.georeview.2020.01.008
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作者单位E-mail
张龙升 1)华北地质勘查局综合普查大队河北廊坊065201 2) 中南大学地球科学与信息物理学院教育部有色金属成矿预测重点实验室长沙410083 Email:zls2320@163.com 
彭建堂 2) 中南大学地球科学与信息物理学院教育部有色金属成矿预测重点实验室长沙410083 3)中国科学院地球化学研究所矿床地球化学国家重点实验室贵阳550002 Email:jtpeng@126.com 
林芳梅 1)华北地质勘查局综合普查大队河北廊坊065201 2) 中南大学地球科学与信息物理学院教育部有色金属成矿预测重点实验室长沙4100834)河北华勘地质勘查有限公司河北廊坊065201  
基金项目:注: 本文为国家自然科学基金资助项目(编号:41972090、41473043)、国家重点研发计划项目(编号:2016YFC0600207,2018YFC0603500)的成果
中文摘要:大溶溪钨矿床为湘西地区一层控型白钨矿矿床,白钨矿主要分布于矽卡岩和石英网脉中。在详细野外地质调查和室内镜下观察的基础上,利用EPMA和高精度LA ICP MS测试技术,对该矿主要矽卡岩矿物的矿物学和地球化学特征进行了研究,并初步揭示了其形成机制。研究表明,大溶溪矿区含钨矽卡岩为还原型类矽卡岩,矽卡岩矿物主要为辉石和石榴子石;辉石为透辉石—钙铁辉石—锰钙辉石系列;石榴子石以钙铝榴石为主,但锰铝榴石+铁铝榴石含量也较高。该区辉石REE具有总量较低、轻重稀土元素分馏不明显、正Ce和正Eu异常的特征;而石榴子石REE具有较高的稀土总量,呈现明显的重稀土元素富集、轻稀土元素亏损、负Ce异常、强烈正Eu异常的特点。辉石Ce和Eu异常可能与其形成时的水/岩反应及氧化还原条件有关。REE进入石榴子石的方式以REE3+等价置换Al3+的形式为主,正Eu异常主要为Eu2+等价置换钙铝榴石Ca2+所致,而负Ce异常则反映出热液沉淀正Ce异常辉石之后热液体系中贫Ce元素。该区含钨矽卡岩主要为热液与含锰灰岩在平衡机制下发生扩散交代作用所形成;而含钨石英网脉内发育的辉石、石榴子石等钙硅酸盐矿物,则是热液沿裂隙、孔隙对富钙变质砂岩进行交代的产物。矽卡岩形成过程中,相对贫Mn的辉石和石榴子石主要受交代流体作用控制,形成于一种相对高温、中等水/岩比的条件下;而相对富Mn的辉石和石榴子石,则可能是形成于靠近围岩一侧的相对低温、低水/岩比的环境中,部分锰质来自被交代的围岩。
中文关键词:矿物学特征  地球化学特征  形成机制  矽卡岩矿物  大溶溪钨矿床  湘西
 
Mineralogical, geochemical characteristics and formation mechanism of skarn minerals in the Darongxi tungsten deposit, western Hunan
NameInstitution
ZHANG Longsheng1) Exploration Unit of North China Geological Exploration Bureau, Langfang, Hebei, 065201; 2) School of Geosciences and Infophysics & Key Laboratory of Nonferrous Metals Metallogenic Prediction of Ministry of Education, Central South University, Changsha, 410083
PENG Jiantang2) School of Geosciences and Infophysics & Key Laboratory of Nonferrous Metals Metallogenic Prediction of Ministry of Education, Central South University, Changsha, 410083; 3) State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550002
LIN Fangmei1) Exploration Unit of North China Geological Exploration Bureau, Langfang, Hebei, 065201; 2) School of Geosciences and Infophysics & Key Laboratory of Nonferrous Metals Metallogenic Prediction of Ministry of Education, Central South University, Changsha, 410083;4) Hebei Huakan Geological Exploration Co., Ltd. Langfang,Hebei, 065201
Abstract:Objectives:The Darongxi tungsten deposit, situated in Xuefeng mountain of the Ghiangnania terrain between the Yangtze and Cathaysian blocks, western Hunan, is a typical strata bound scheelite deposit where the W mineralization is hosted by skarn and quartz stockworks along the contacts among metasandstone, marble and slate in the Lower Member of Nantuo Formation belonging to Nanhuan System. The strata bound scheelite skarns and stockworks in this deposit contain amounts of garnets and pyroxenes which are generated by the diffusion metasomatism between host rocks and the fluid deriving from the intrusion of the Dashenshan granitic stock. This work tries to provide some advices and evidences for how to reveal the formation mechanism of stratiform and strata bound tungsten skarn in the future. Methods: As the major skarn minerals, garnet and clinopyroxene from the scheelite skarns and stockworks were studied. Based on the field investigation and the microscopic observation, the major elements and trace elments compositons of garnet and clinopyroxene were analysed by EPMA and LA ICP MS at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences (Guiyang).Determination of the major element compositions of garnet and clinopyroxene was performed by wavelength—dispersive spectrometry using an EPMA 1600 electron probe microanalyser, at an accelerating voltage of 25 kV, a beam current of 10 nA, a beam diameter of 10 μm, and using artificial synthesized silicate and oxide minerals as standards. The calculations are based on the Geokit (2010) excel program.Twenty eight trace elements of garnet and clinopyroxene were analysed by laser ablation inductively coupled plasma mass spectrometry (LA ICP MS) using Perkin Elmer Sciex ELAN DRC e ICP MS. The laser was operated with a 40μm diameter beam and ablating number frequency 120. Calibration was performed using NIST SRM610 glass as external standard, and BIG 1G, BHVO 2G, BCR 2G, GSE 1G, QC KL2 and QC ML3B standards, with 29Si and 44Ca, previously determined by electron microprobeanalysis, as an interal standard. Detection limit for LA ICP MS is below 0.1 ppm for most elements, and in run precision is <5%. The calculations are based on the ICPMSDataCal (V8.4) excel program.Results: In the Darongxi deposit, pyroxenes from tungsten bearing skarns and stockworks belong to diopside—hedenbergite —johannsenite series (Di11.5~67.1Hd10.6~81.2 Jo6.00~34.3), and garnets are mainly grossularite(Gro43.9~78.1And3.3~12.6 Spe+Alm10.7~50.3) with the relatively high spessartine+almandite component. The pyroxene is characterized by enriced in Zn, W and U, with LILE and HFSE deplated, low REE concentrations, no obvious fractionation between LREE and HREE, and positive Ce and Eu anomaly. Garnet exhibits strongly enriched in U, Zr, Ga, Nb, but deplated in Rb, Sr, Ba, which has higher REE concentrations, obvious HREE enrichment, negative Ce anomaly and strongly positive Eu anomaly. Incoporation of trace element and rare earth element into pyroxene and garnet are predominately controlled by their crystal chemistry. Positive Ce and Eu anomaly of pyroxene may be affected by the fluid—rock interaction and different redox conditions. Incoporation of REE into garnet follows along with REE3+substituting for Al3+and Eu2+substituting for Ca2+, whereas the negative Ce anomaly may be related to the mineralization fluid which is depleted in Ce due to the early crystallization of pyroxene.Conclusions: The tungsten bearing skarn of the Darongxi deposit belongs to the reduced skarnoid which is generated by the diffusion metasomatism between the hydrothermal fluid and the Mn rich limestone.However, the pyroxene and garnet from the stockwork orebody may be formed by the metasomatism between the fluid and the calc meta sandstone. During the formation of skarn, Mn depleted pyroxene and garnet resulted from the diffusive metasomatism under the condition of relatively high temperature and moderate W/R ratios, while Mn rich pyroxene and garnet trended to be deposited near the walk rock under low temperature and W/R ratios condition, and partial Mn maybe be derived from the metasomated wall rocks.
keywords:mineralogical characteristics, geochemical characteristics, formation mechanism, skarn minerals, Darongxi tungsten deposit, western Hunan
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