赣西石竹山硅灰石矿床石榴子石特征及其地质意义

Characteristics and geological significance of garnet in Shizhushan wollastonite deposit,western Jiangxi Province

江西省石竹山硅灰石矿床位于钦杭成矿带东段的萍乐坳陷带内,是世界资源储量规模最大的硅灰石矿床。为了厘清区内成矿流体性质及进一步找矿潜力,本文根据矿物间的穿插关系和光学特征,将该矿床的石榴子石分为早(GrtⅠ)、晚(GrtⅡ)两个世代,在详细的野外工作基础上,利用电子探针(EMPA)和LA⁃ICP⁃MS原位微区分析技术,对石竹山矿床中的石榴子石开展了系统研究。结果表明,石竹山硅灰石矿床石榴子石属于钙铝榴石-钙铁榴石固溶体系列,早期形成的石榴子石(GrtⅠ)Fe含量更高,且震荡环带发育,而晚期形成的石榴子石(GrtⅡ)则发育复杂环带。石榴子石GrtⅠ和GrtⅡ都亏损大离子亲石元素K、Rb、Ba、Sr等,而Nb、Ta、Zr等高场强元素相对含量较高,U含量与∑REE成正比。GrtⅡ的∑REE较低(12.92×10^(-6)~21.12×10^(-6)),具有较平坦的稀土元素配分模式,轻微富集轻稀土元素(Ce、Pr、Nd、Sm),微弱负铕异常;GrtⅠ的∑REE相对较高(13.78×10^(-6)~73.52×10^(-6)),更富集轻稀土元素(Ce、Pr、Nd、Sm),呈右倾的稀土元素配分模式,负铕异常明显。推测石榴子石GrtⅠ形成于相对开放、水岩比值较高、高氧逸度的酸性体系中,GrtⅡ则形成于相对封闭、水岩比值较低、高氧逸度的中酸性体系中。此外,将石榴子石的端员组分和微量元素特征与国内外不同矿种的接触交代型金属矿床进行对比,结果显示该矿床存在成Cu、Fe、Mo矿体的潜力。

The Shizhushan wollastonite deposit in Jiangxi Province is the world's largest reserve of wollastonite deposits and is located in the Pingle Depression of the eastern Qinhang metallogenic belt.In order to clarify the nature of ore⁃forming fluids and further prospecting potential in the area,this paper divides the garnet of the deposit into early(GrtⅠ)and late(GrtⅡ)generations according to the interpenetration relationship and optical character⁃istics between minerals.On the basis of detailed field work,this paper systematically studied the garnet in Shizhush⁃an deposit by using electron probe analysis(EMPA)and LA⁃ICP⁃MS in⁃situ microanalysis technology.The results show that the garnets in Shizhushan wollastonite deposit belong to the grossularite to andradite solid solution series.The early generation garnet(GrtⅠ)has higher Fe content and developed oscillatory zoning,while the late genera⁃tion garnet(GrtⅡ)has complex zoning.GrtⅠand GrtⅡare depleted in large ion lithophile elements(LILE)such as K,Rb,Ba,Sr,etc.,while high field strength elements(HFSE)such as Nb,Ta,Zr are relatively high,and U content is proportional to∑REE.The∑REE of GrtⅡis low(12.92×10^(-6)~21.12×10^(-6)),with a flat REE distri⁃bution pattern,slightly enriched in LREE(Ce,Pr,Nd,Sm),and weak Eu anomaly.The∑REE of GrtⅠis high⁃er(13.78×10^(-6)~73.52×10^(-6)),and it is more enriched in LREE(Ce,Pr,Nd,Sm)than GrtⅡ.The distribution pattern of rare earth elements is“right⁃inclined type”,and the negative Eu anomaly is obvious.GrtⅠis formed in an acid system with relatively open,high water⁃rock ratio and high oxygen fugacity.GrtⅡwas formed in a relatively closed intermediate⁃acid system with low water⁃rock ratio and high oxygen fugacity,while GrtⅡis formed in a rela⁃tively closed intermediate acid system with low water rock ratio and high oxygen fugacity.In addition,the composi⁃tion and trace element characteristics of garnet are compared with those of contact metasomatic metal deposits of dif⁃ferent minerals in domestic and abroad.The results show that the deposit has the potential to form Cu,Fe and Mo ore bodies.