剪切带型金矿中金沉淀的力化学过程与成矿机理探讨

剪切带型金矿是一种重要的金矿床类型,有关该类型金矿的成因问题已开展了大量研究,但对于剪切带中金的沉淀析出机制和成矿过程仍存在较大争议,对于赋矿部位的构造属性与矿床关系的研究尚显薄弱。对此笔者整理分析了近年来国内外有关剪切带型金矿的研究进展,并结合我国胶东金矿的研究实例,运用断层阀和力化学理论分析发现,无论是脆性还是韧性剪切带,无论是脉型还是蚀变岩型金矿,其成矿的关键部位均与构造应力集中而导致的脆性破裂(特别是R、T、R’破裂的产生)和碎裂作用以及(多期)岩体侵位密切相关,并且脆性破裂所导致的压力骤降从而引发流体闪蒸的力化学过程可能是造成金沉淀析出成矿的有效机制,其中多期岩体侵位所提供的流体是成矿的物质基础。此外,从国内外实例可以看出,剪切带中的脆性破裂不仅发生于脆性或脆韧性构造域,也可以发生在较深层次的韧性构造域中,尽管韧性域中产生脆性破裂的原因还不十分清楚,但这可能是韧性剪切带成矿的关键机制之一。最后,综合岩体、流体、剪切带三者对成矿的耦合作用,文中提出剪切带型金矿的成矿机理为:(多期)岩体侵位-热液活动-构造剪切-应力集中-脆性破裂(碎裂)产生-压力骤降-流体闪蒸-元素(金)析出,如此循环往复方可形成大型剪切带型金矿。

Discussions on the stress-chemical process of gold precipitation and metallogenic mechanism in shear zone type gold deposits

Most gold deposits in the world are related to shear zones, and the genesis of shear zones type gold deposits has long been a major concern in the geological study, while the precipitation mechanism of gold remains controversial and the cause at the ore-localized portions is seldom considered. This paper firstly sorts out and analysis the research progress of mineralization in recent years combining with the case studies of Jiaodong gold deposits. Using fault-valve model and stress-chemical theory, this paper finds that the ore-localized portions have close links to brittle fractures or joints (especially R, T and R'' joints) and (multi-stage) pluton emplacement whether in brittle or ductile shear zones or vein type or altered rock type gold deposits, which are resulted from stress concentration. And the stress-chemical process, which is associated with the extreme decrease of fluid pressure and flash vaporization during the abrupt rupture, may be the effective mechanism of gold precipitation. In addition, fluids derived from multi-stage pluton emplacement are material basis of mineralization. Hence, this paper proposes that the stress-chemical process is the key process for gold precipitation in shear zones, which is from stress concentration, brittle fracturing, sudden reduction of fluid pressure, flash vaporization to gold precipitation. Additionally, the structural level of fracturing is not limited to brittle or brittle-ductile shear zones found from cases in domestic and overseas study, it can also occur in ductile shear zones though the concrete reason is not clear, which may be the critical metallogenic mechanism to ductile shear zones. Depending on above characteristics, the metallogenic mechanism in shear zones can be summarized as (multi-stage) pluton emplacement, hydrothermal fluid action, shearing action, brittle fracturing, sudden reduction of fluid pressure, flash vaporization and (gold) mineralization, which is a cycling process making large deposits eventually. And from this mechanism we suggest that the relatively concentrated portions of stress in shear zones, like the striking-turning positions in strike-slip faults, the dip turn-to-gentle positions in normal faults and turn-to-steep positions in thrust faults, are favorable areas for mineralization.