摘要
程潮铁矿床是长江中下游成矿带最大的矽卡岩型铁矿床,位于鄂东南矿集区北缘,矿体主要产于鄂城杂岩体与下三叠统大冶组碳酸盐岩的接触带。本文报道程潮铁矿床不同产状磁铁矿的LA-ICPMS(激光剥蚀等离子体质谱)微量元素分析结果,以讨论磁铁矿的微量元素组成和变化规律及其与成矿环境的关系,为深入认识程潮铁矿床的矿床成因及成矿演化提供重要制约。研究结果显示,程潮铁矿床与成矿有关岩体(花岗岩)中的岩浆磁铁矿(副矿物)与内矽卡岩和外矽卡岩中的热液磁铁矿在显微结构和微量元素组成上具有显著差异。花岗岩中的磁铁矿不发育环带,也没有遭受后期热液的交代。内矽卡岩以发育具有振荡环带的富Si磁铁矿为特征,而外矽卡岩中的磁铁矿则以无明显环带的富Mg磁铁矿为主。内矽卡岩和外矽卡岩中的原生磁铁矿都遭受了广泛的热液交代作用而形成次生磁铁矿,即具有明显的磁铁矿溶解-再沉淀作用。与矽卡岩矿石中的热液磁铁矿相比,岩体中的副矿物磁铁矿具有高得多的V、Ti、Ni、Cr、Co及Ga等亲铁元素(相容元素)和较低的Si、Al、Mg、Sr及Ba等亲石元素(不相容元素)。这种岩浆副矿物磁铁矿和矿石矿物磁铁矿微量元素组成的系统差异表明,程潮铁矿床属于典型的矽卡岩型矿床,而非矿浆型铁矿。另一方面,内矽卡岩和外矽卡岩矿石中的原生磁铁矿也具有明显不同的微量元素组成:前者具有较高的V、Ti、Ni、Cr、Ga、Sr和Ba等,而后者具有较高的Sn、Zn、U和Sn/Ga、Zn/V及Co/Ni比值,表明外矽卡岩中的磁铁矿受地层围岩成分的影响较大,而内矽卡岩中的磁铁矿则更多地受岩浆流体组分的控制。对原生磁铁矿和次生磁铁矿微量元素的系统分析结果表明,在磁铁矿的溶解-再沉淀过程中,内矽卡岩中的次生磁铁矿相对原生磁铁矿具有显著不同的微量元素组成,前者的Si、Al、Mg、Ti、Sr及Ga等元素含量大大降低,而Zn、V、Mn、Pb、Th及U等元素含量则升高,Co、Ni、Sn及Ba等元素基本不变。外矽卡岩中的原生磁铁矿被交代形成次生磁铁矿时,Mg、Mn、Al、Zn及Co等元素发生丢失,Pb、U、Nb及Sr等元素发生富集,而Sn和Ga的含量基本不变。根据不同产状、不同结构和不同成因磁铁矿的微量元素分析可以得出以下认识:(1)程潮铁矿床属于典型的热液成因,矿区不存在矿浆型铁矿;(2)磁铁矿的溶解-再沉淀作用使大部分微量元素发生明显亏损,少部分元素发生富集,表明流体交代对磁铁矿的微量元素组成具有显著影响;(3)对磁铁矿微量元素组成的研究可为磁铁矿及矽卡岩型铁矿床的矿床成因和成矿演化提供重要信息;对磁铁矿的原位微量元素分析必须建立在详细的显微结构研究基础上,对发生过溶解-再沉淀作用的磁铁矿,需要分别对原生和次生磁铁矿区域进行高精度、高分辨率的微量元素分析,只有这样才能对磁铁矿及含磁铁矿矿床的成因进行正确的分析和认识。
The Chengchao iron deposit is located in the northern Daye district of the Middle-Lower Yangtze Metallogenic Belt in eastern China. Mineralization of the deposit is mostly localized in the contact zones between marine carbonates of the Lower Triassic Daye Formation and the E＇ cheng intrusive complex consisting of granite and quartz diorite intrusions. In this paper, we present in situ LA-ICPMS trace element analyses of magnetite from the Chengchao iron deposit to provide a better understanding in the origin and evolution of the deposit. This study involves three types of magnetite ： （ 1 ） magmatic magnetite from the ore-related granitoid intrusion that lacks oscillatory zoning and replacement texture, （2） high Si magnetite with oscillatory zoning from mineralized endoskarn, and （3） high Mg magnetite from magnetite-dominated exoskarn. Magnetite grains from the mineralized endoskarn and exoskarn were all extensively re-equilibrated with late stage hydrothermal fluids forming secondary magnetite domains, a process termed as dissolution- reprecipitation. Magmatic magnetite grains have significantly higher contents of siderophile and compatible elements such as V, Ti, Ni, Cr, Co, Ga and extremely lower hthosphile and incompatible elements including Si, AI, Mg, St, and Ba compared to the hydrothermal varieties from the skarn ores. Systematic compositional variations are also observed between magnetite grains from the endoskarn and exoskarn ores： the former is marked by much higher contents of V, Ti, Ni, Cr, Ga, Sr, and Ba and lower Sn, Zn, U abundances, with lower Sn/Ga, Zn/V and Co/Ni ratios. This study also shows that most of the trace elements in the Si- and Mg-rich magnetite have been significantly leached during the subsequent dissolution-reprecipitation process. Compared to the primary magnetite from the endoskarn, the secondary magnetite domains, which resulted from the dissolution-reprecipitation process, are characterized by much higher Si, Al, Mg, Ti, Sr and Ga, lower Zn, V, Mn, Pb, Th and U, and similar Co, Ni, Sn and Ba. However, secondary magnetite domains from the exoskarn contains much higher Pb, U, Nb and Sr, lower Mg, Mn, Al, Zn and Co, and similar Sn and Ga relative to their primary varieties. Results from this study confirm that the Chengchao iron deposit is of hydrothermal origin, rather than being crystallized from immiscible iron oxide melts as previously suggested. An additional implication of this study is that intensive textural characterization should be conducted before in-situ compositional analysis of magnetite grains so that trace elements of magnetite domains of different origins can be precisely determined; high-precision compositional data combined with textural evidence are of critical importance in better understanding the genesis and evolution of magnetite and magnetite-bearing ore deposits.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2014年第5期1292-1306,共15页
Acta Petrologica Sinica
基金
科技部973项目(2012CB416802)
中央高校基本科研业务费专项资金(CUG120102
1210491A05)
地质过程与矿产资源国家重点实验室科技部专项经费(MSFGPMR201205)联合资助
作者简介
胡浩，男，1987年生，博士生，矿产普查与勘探专业，E—mail：haohueug@gmail．com.
通讯作者：李建威，男，教授，矿床学专业，E—mail：jwli@cug．edu．cn
