低级变质岩在热液蚀变过程中的微量元素地球化学行为——以赣东北银山地区双桥山群为例

Geochemical behavior of trace element during hydrothermal alteration in low-metamorphic rock: a case study for Shuangqiaoshan Group in Yinshan area, northwestern Jiangxi province, China.

对江西银山地区双桥山群绢云母千枚岩及其原岩的稀土及微量元素的研究表明,热液蚀变过程中它们的地球化学行为十分复杂:热液蚀变作用并不使REE淋滤降低,反而导致ΣREE较其原岩普遍升高,但岩体接触带附近蚀变围岩的ΣREE则低于原岩。蚀变岩出现Eu亏损,∑LREE/∑HREE值降低。定量计算显示,∑REE总升幅中有29％～45％是由围岩质量迁移引起的表观浓缩效应,而另外的55％～71％则是流体带入了REE;在绢云母千枚岩中绢云母可能是REE的主要寄主矿物相,REE主要以吸附作用的形式固着在矿物的表面或含有可交换电价的晶体结构层(四面体层和八面体层)的层面上;参与蚀变作用的热液∑LREE/∑HREE值低、Eu强正异常。流体REE的带入可能是造成蚀变围岩较原岩轻重稀土比值降低的主要因素。热液蚀变作用使岩石的Eu被还原成更易活动的Eu2+而活化迁出,导致蚀变岩的Eu负异常更显著;围岩蚀变作用 造成Sr普遍降低,Ba显著升高,而Rb含量相对稳定,这是由其主要寄主矿物相的变化特征所致;Hf、Th、U、V、Cr、Co、Nb、Mo、Ta、Zr表现为惰性,成矿元素Cu、Pb、Zn、Ag和Sn等大量带入,Y、Sc活化迁出。

Rare earth elements and other trace elements in altered phyllite and its fresh counter part were determined inorder to examine behavior of the trace elements during hydrothermal alteration in Shuangqishan group, a set of low-metamorphic rock in Yinshan polymetallic area of northwestern Jiangxi province. It is demonstrated that behavior of the trace elements during alteration were extremely complicated. The trace element characteristics are as follows: (1) Instead of being leached, REE contents were higher in the altered phyllite in a magnitude range from 8. 6% to 54. 8% than in their fresh counterparts, excepting samples on the contact with lower REE concentrations. Less than 45% of the increment REE contents were caused by the 'condensed' effect of mass transfered of major elements such as Si, Na, Mg etc. removed from the rock, and larger than 55% of which should be responsible for the addition of REE into the rock from hydrothermal fluid that caused rock alteration. Significantly lower contents of REE in samples on the contacts of intrusion were ascribed to the dilution caused by much more higher contents of REE barren-mineral such as quartz, pyrite and sphalerite in the rock. (2) The acting hydrothermal fluid was characterized by low ΣLREE/ΣHREE ratios and in a reductive state, resulting in a less partition between LREE and HREE in altered phyllite and evident loss of Eu that caused by Eu3+ was reduced into more mobile Eu2+ and was removed from phyllite. Dissolution of feldspars may also be responsible for part of ΣLREE/ΣHREE ratio decreased in altered rock by causing selective mobility of LREE and removed out. (3) LIL elements such as Rb, Sr, Ba show different behavior due to their hosting phases changed in different ways. Hf, Th, U, V, Cr, Co, Nb, Mo, Ta and Zr show immobility during hydrothermal alteration. Y and Sc were carried off while the ore-forming elements such as Cu, Pb, Zn, Ag and Sn were introduced into altered phyllite by hydrothermal fluid during hydrothermal alteration.