青海尕林格矽卡岩铁矿富Cl角闪石矿物地球化学特征与其成矿意义

本文通过研究青海尕林格矽卡岩型铁矿床内成矿阶段富Cl角闪石的矿物学特征,来反演岩浆-流体-矿物三者间的演化关系。尕林格富Cl角闪石应属于镁绿钙闪石亚族,化学成分特征显示:0.3Mg/(Mg+Fe2+)0.69,Si6.25apfu,0.681%Cl3.161%。XCl与XMg显示具有很好的负相关性,而XK和X2+Fe则与XCl显示具有正相关性。这些阴阳离子间的线性关系与角闪石的晶体结构特征密切相关。对于Cl-交代OH-进入角闪石分子结构可以用经验公式表示为ln(XCl/XOH)amp=ln(fCl/fOH)fluid+A·[4]Al·Fe2+/RT+B/RT,其中A和B为常数。因此,当所有富Cl角闪石的成分与流体在同一温度和相对不变fCl/fOH下平衡时,则角闪石中的ln(XCl/XOH)对其成分[4]Al·Fe2+投影应为线性关系,这一特点可理解为阳离子成分对Cl的加入的影响随流体成分的改变而改变。尕林格富Cl角闪石环带成分从核部到边部具有XCl先降低后升高的特点,这与OH-比Cl-更易交代进入分子结构有关。当角闪石和与之平衡的流体进入相对封闭环境时,随着角闪石的结晶,流体中Cl的含量逐渐增加导致角闪石环带边缘XCl明显比核部高。晶体和液体间的微量元素分异不仅受到晶体结构的控制,而且流体中的挥发分也是控制微量元素分异的主要因素。富Cl角闪石中Cl的含量对REE的分异同样也有影响,REE元素的分配系数随着流体中Cl含量的降低而升高。通常认为硅酸质岩浆早期结晶分异出来的高温高盐度流体是重要的载矿流体,因其含有大量的Cl-并且pH值较低,有利于Fe的Cl络合物进行长期迁移。当流体遇到偏碱性的碳酸盐地层导致流体pH值升高,或与天水混合形成低温低盐度流体时,Fe的Cl络合物就会发生解离沉淀。而此时流体中fOH升高,从而导致OH-交代进入早期矽卡岩矿物中形成角闪石等退化蚀变矿物。

The significance of mineralization and geochemistry of the Cl-rich amphiboles from the Galinge skarn iron deposit in Qinghai Province

The Galinge skarn iron deposit, one of the large skarn deposits in the west of China, is located in the middle of the contact zone between the Qimantag orogen and the Qaidam basin, and possesses abundant potential values. This paper aimed to make specific research on the mineralogy of the Cl-rich amphiboles from the Galinge deposit so as to investigate the association of melt, fluid and minerals. The Cl-rich amphibole belongs to the magnesian hastingsitic subgroup, and its geochemical characteristics are 0.3< Mg/(Mg+Fe²⁺) <0.69; Si <6.25 apfu; and 0.681% < Cl <3.161%. The content of X_Cl shows the positive linear relation with the X_Mg, and the X_K and X_Fe²⁺ show the strong inverse relation with the X_Cl. The linear relation between the cation and the chlorine can be interpreted by the crystal structure consideration of Cl-rich amphiboles. In respect of the substitution between the Cl^- and OH^-, the Cl incorporation is expressed empirically by the function of ln (X_Cl/X_OH)_amp=ln (f_Cl/f_OH)_fluid +A·^[4]Al·Fe²⁺/RT+B/RT, where A and B are constant. Therefore, if all Cl-rich amphiboles are in equilibrium with the coexisting fluid at the same temperature and if each amphibole is in equilibrium with a fluid of relatively constant f_Cl/f_OH, the ln(X_Cl/X_OH)amp versus ^[4]Al·Fe²⁺ should show a linear correlation, and the effect of the cation substitution is contributed by the structure constraints for ^[4]Al and by a chemical constraints for Fe²⁺. The existence of amphibole zones implies that the X_Cl values decrease first and then increase from the core to the edge, since OH is preferentially incorporated relative to Cl. Therefore, when the free fluid phase is gradually consumed as a consequence of continued hydration/chloridisation reactions, the Cl^- content of the fluid should be increased. The crystal/liquid partitioning behavior of the trace elements is controlled by the framework of the lattice, and the volatile concentration in the fluid is also a key factor. The decrease of the Cl concentration in the fluid is attributed to the increase of the REE partition coefficients. Generally, the high saline hydrothermal fluid exsolved from the magma in the early crystallization stage is the dominated reservoir of metals because of the high Cl^- content and low pH value. The Cl^- plays an important role in the transportation of metals. When the ore fluid is reacted with an alkaline carbonate wall rocks, or mixed with meteoric water, the chlorine complex of iron would decompose and precipitate. Meanwhile, the increased pH value makes the f_OH of fluid ascend, which is beneficial for the incorporation of OH into the amphibole.