地球化学

2025-03-04 02:43:12
推荐回答(1个)
回答1:

在哈图金矿西区钻孔中选取不同程度蚀变和矿化的含碳凝灰质泥岩(强蚀变、弱蚀变、新鲜、弱矿化),进行了成矿元素分析。结果表明,不同程度蚀变岩石中Au的浓度为0.79×10-9~15.64×10-9(表5-3)。与平均上地壳相比,哈图金矿西区不同程度蚀变的含碳凝灰质泥岩中Cu、Ag、As和Sb的浓度相对较高,而Sn的浓度明显较低,Au和Mo的含量变化较大,其他元素含量与平均上地壳接近(图5-22)。新鲜、中等蚀变以及强蚀变含碳凝灰质泥岩中Au的浓度具有相同的变化范围,而弱矿化岩石中Au的浓度略高,这说明哈图金矿西区含碳凝灰质泥岩并不是区内热液金矿化过程中Au的主要源区,而弱矿化岩石中具有相对较高的Au、As浓度,说明成矿流体本来含有一定浓度的Au和As(图5-22)。其他元素(如Cu、Zn、Ag、Bi、Co、Ni、Mo、W)在不同程度蚀变和矿化岩石中的浓度相近,说明含碳凝灰质泥岩也不是这些金属元素的主要源区。

表5-3 哈图金矿西区不同程度蚀变含碳凝灰质泥岩中成矿元素分析结果

注: Au含量单位为10-9,其他为10-6

在哈图金矿区-400 m矿段采集含细脉凝灰岩、凝灰岩(HT07b、HT10、HT11、HT21、HT30、HT33)以及不同程度蚀变的玄武岩(HT12、HT14、HT20、HT07a、HT08)。经手标本和镜下观察后选取新鲜样品进行全岩化学分析。先将样品破碎,选取较纯净、新鲜的石英-钠长石脉、石英-方解石脉以及不同程度蚀变的玄武岩。溶样处理操作在北京大学地球与空间科学学院教育部重点实验室中进行,使用中国科学院地质与地球物理所Finnigen MatICP-MS仪器测试,测试方法同Zhu et al.(2006)。所测量的石英-钠长石脉、石英-方解石脉以及玄武岩样品的微量元素含量和特征元素比值列于表5-4,矿脉和围岩的稀土元素配分模式如图5-23a,b所示,玄武岩的多元素图解如图5-23c所示。

图5-22 哈图金矿不同程度蚀变含碳凝灰质泥岩平均上地壳标准化的成矿元素图解(平均上地壳数据。据Taylor et al.,1995;Wedepohl,1995)

表5-4 哈图金矿蚀变岩型矿体中不同类型样品的微量元素含量

续表

注: 元素含量单位为10-6

玄武岩的球粒陨石标准化稀土元素配分模式为平坦型,稀土元素总量平均为33.10×10-6,(La/Yb)N平均值为0.90,轻重稀土分馏不明显。轻稀土元素和重稀土元素均无明显分馏,(La/Sm)N和(Gd/Yb)N平均值分别为0.92和1.04。δEu平均值为0.96,其中除样品HT07a具有弱的Eu负异常外,其他样品均无明显Eu异常。δCe平均值为0.97,无明显Ce异常。在原始地幔标准化图解上,玄武岩强烈富集Cs、Rb、Pb、Sr和Ba等大离子亲石元素,亏损Nb、Ta。

石英-钠长石脉(HT07b,HT10,HT33,HT21,HT11)的稀土配分模式为弱右倾式,稀土元素总量较低(平均为6.43×10-6),(La/Yb)N平均值为3.28,轻稀土弱富集。轻稀土元素无明显分馏,而重稀土元素有弱的分馏,(La/Sm)N和(Gd/Yb)N平均值分别为1.36和2.13。δEu平均值为1.38,具弱Eu正异常。无明显Ce异常;石英-方解石脉(HT30)的稀土配分模式为右倾型,稀土总量较低(平均2.91×10-6),(La//Yb)N平均值为7.17,轻稀土富集。轻稀土和重稀土元素均有弱的分馏,(La/Sm)N和(Gd/Yb)N平均值分别为3.14和2.14。具有明显的Eu正异常(δEu平均值为16.75)。无明显Ce异常(δCe平均值为0.91)。石英-方解石脉(HT30)的REE配分模式与石英-钠长石脉的REE配分模式相比,右倾程度增加,轻重稀土间分馏增强,Eu正异常非常明显,说明随着成矿作用过程的进行,成矿流体向轻稀土富集和Eu正异常增强的方向演化,晚期成矿体系的氧逸度增强。

图5-23a 不同程度蚀变玄武岩的球粒陨石标准化稀土元素配分模式

图5-23b 不同阶段矿脉的球粒陨石标准化稀土元素配分模式

图5-23c 蚀变玄武岩的原始地幔标准化图

石英-钠长石脉和石英-方解石脉富集LREE、亏损HFSE,Hf/Sm、Nb/La和Th/La基本小于1。前人的研究表明,富Cl的热液富集LREE,Hf/Sm、Nb/La和Th/La值一般小于1,而富F的热液富集LREE和HFSE,Hf/Sm、Nb/La和Th/La值一般大于1(Oreskes etal.,1990)。由此推断哈图金矿蚀变岩型矿体成矿流体中Cl含量高于F含量。Y和Ho具有相同的价态和离子半径,八次配位时,两者的离子半径分别为1.019×10-10和1.015×10-10(Shannon,1976),因此Y和Ho具有相同的地球化学性质,在很多地质过程中,Y/Ho不发生变化,接近于球粒陨石的Y/Ho比值,为~28(Bau et al.,1995)。哈图金矿蚀变岩型矿体中阶段形成的石英-钠长石脉的Y/Ho比值(30~50)较阶段Ⅴ形成的石英-方解石脉的Y/Ho比值要低得多(70),由此可以说明,蚀变岩型矿体成矿流体在演化过程中发生了一定分异,导致Y/Ho比值变化,这也可能是成矿晚期另一种流体(如深循环的地下水)加入的结果。

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