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纪敏,赵新福,曾丽平,范田纬腾. 2018. 鄂东南铜绿山矿床石榴子石显微结构及微区成分对成矿过程的指示. 岩石学报, 34(9): 2716-2732
鄂东南铜绿山矿床石榴子石显微结构及微区成分对成矿过程的指示
作者单位E-mail
纪敏 中国科学院壳幔物质与环境重点实验室, 中国科学技术大学地球与空间科学学院, 合肥 230026
中国地质大学地质过程和矿产资源国家重点实验室, 中国地质大学资源学院, 武汉 430074 
 
赵新福 中国地质大学地质过程和矿产资源国家重点实验室, 中国地质大学资源学院, 武汉 430074 xfzhao@cug.edu.cn 
曾丽平 中国地质大学地质过程和矿产资源国家重点实验室, 中国地质大学资源学院, 武汉 430074  
范田纬腾 中国地质大学地质过程和矿产资源国家重点实验室, 中国地质大学资源学院, 武汉 430074  
基金项目:本文受科技部"973"项目(2012CB416802)和中央高校基本科研业务费专项资金(CUG140618)联合资助.
摘要:
      鄂东南矿集区铜绿山矿床是典型的矽卡岩型铜铁多金属矿床,矿体产出在铜绿山岩体与三叠系碳酸盐地层的接触带。尽管本矿床的研究程度很高,但对早期成矿流体的成分与演化及矿质富集沉淀等过程的精细制约依然比较欠缺。石榴子石在铜绿山矿床中分布广泛,本文对不同产状的石榴子石利用SEM显微结构、EPMA主量元素和LA-ICPMS微量元素分析,去探讨石榴子石的生长动力学及其对成矿过程的指示。大理岩和镁质外矽卡岩中的石榴子石均以钙铝榴石为主,端元成分比较均一,∑REE含量低(3.01×10-6~14.34×10-6),具有弱的Eu异常、轻微富集LREE的模式。钙质外矽卡岩和内矽卡岩中的石榴子石以钙铁榴石为主,端元成分变化较大,单颗粒从核部到边部Fe含量具有增加的趋势,其∑REE含量较高(22.71×10-6~806.8×10-6),具有明显的正Eu异常、极度富集LREE亏损HREE的模式。大理岩中的钙铝榴石环带不发育,表明在晶体生长过程中,界面反应速率很慢且物质迁移以扩散为主;而镁质外矽卡岩中的钙铝榴石可见振荡环带,表明界面反应速率比以扩散为主要方式的物质迁移快,这与白云石相对于方解石具有较低的反应吉布斯自由能,因此较难被热液消耗有关;钙质外矽卡岩中的钙铁榴石振荡环带清晰,该类石榴子石只有在界面反应速率较快且物质迁移方式以对流为主的情况下才能形成;内矽卡岩中的钙铁榴石的环带杂乱,尽管其形成的动力学模式与钙质外矽卡岩中的类似,但由于其距离岩体更近,热量迁移困难,在结晶生长受阻的同时由于对流提供的充足物质使得原先的环带发生溶解再沉淀进而形成结构混乱的再吸收环带。上述生长动力学模式也能够很好的对应Nb、Ta含量的脱耦和轻重稀土分异的特征。铜绿山不同产状的石榴子石中,外矽卡岩带中钙铁榴石Eu含量最高,而Eu2+与成矿金属元素一样可被Cl-络合的,其含量的高低可能指示了对应的成矿元素在成矿热液中的浓度。石榴子石环带自核部向边部Fe含量具有逐渐增加的趋势,反应干矽卡岩阶段从早到晚,热液中的金属成矿元素含量增加。
英文摘要:
      The Tonglushan Cu-Fe deposit is one of typical skarn-type deposits in southeastern Hubei metallogenic province. 13 ore bodies occur along the contact zone between the Tonglushan quartz diorite pluton and the Lower Triassic carbonate rocks. Although numerous studies have been made, the evolution of the early ore fluids and mechanism of precipitation of ore minerals is still unclear. Garnet is widespread in Tonglushan skarn deposit, and has a record of the composition, properties and evolution of the hydrothermal fluid. We perform SEM microtexture observation, EPMA major elements analysis and LA-ICPMS trace elements analysis to understand the growth kinetics of garnets and its implication for ore forming process. Garnets from marble and magnesian exoskarn zone are mainly grossulars, which are homogeneous and have low ∑REE (3.01×10-6~14.34×10-6) with LREE slightly enriched patterns with weak Eu anomalies. In contrast, andradites are dominate phases in calcareous exoskarn and endoskarn, which have a large range of variation and have Fe content increasing from core to rim in single grains. Their ∑REE are very high (22.71×10-6~806.8×10-6) and exhibit LREE-rich and HREE-depleted patterns with strong positive Eu anomaly. The grossulars in marble have no zonation, indicating that the rate of interface reaction is slower than mass transfer, which is mainly controlled by diffusion during crystal growth. Whereas the magnesian exoskarn grossulars show oscillatory zonings, suggesting that the rate of interface reaction is faster than diffusion dominated mass transfer. The mass transfer is relatively slow, which may be related to the lower reaction Gibbs free energy of dolomite than calcite. The andradites in calcareous exoskarn develop oscillatory zonings, which is related to the process of high rate of interface reaction and the advection dominated mass transfer. The zoning of endoskarn andradites is very chaotic, although it has growth kinetics similar to the andradites in calcareous exoskarn. However, the difficulties of heat transfer limit the crystal growth. Meanwhile, fluid advection is enhanced by dissolving the previous zoning to form absorbed zoning. This pattern of garnets growth kinetics can also explain the characteristics of Nb/Ta content decoupling and differentiation between LREE and HREE. In regard to the highest concentration of Eu in andradites of exoskarn zone, Eu2+, just like iron ion, could combine with Cl- to form stable complex, so the content of which could be an indicator of the corresponding metal ions in hydrothermal fluid. The content of Fe in garnets zonings increases from core to rim, indicating the content of metal ions increase from early to late prograde skarn stage.
关键词:石榴子石  环带  微量元素  生长动力学  热液演化  铜绿山矽卡岩矿床
投稿时间:2017-08-20  修订日期:2018-03-01
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主办单位:中国矿物岩石地球化学学会
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