临界包裹体及其在金矿地质研究中的应用

临界包裹体具有在热动力过程中气液相比不变直至均一成一个流体相的特征.它在超临界流体中,当临界体积为一定值时才能形成.它可以在各类岩浆岩、火山岩、深变质岩以及铁、铜、金等多金属矿床中出现.利用临界包裹体的临界均一温度结合该流体体系的相图,可以获得如成矿流体体系的热力学状态、流体基本组分特征、临界压力、盐度、密度等成矿物理化学信息.通过多年研究发现,临界包裹体在中国北方深变质岩区的很多金矿床都有出现,它的临界均一温度都小于纯水的临界温度(374.2℃),表明成矿早期的超临界流体是由一种低盐度CO₂-H₂O体系组成,它可能来自古老的深变质岩.另一种较高盐度的成矿热液来自与矿床毗邻的中生代中酸性小岩体.它们通过超临界流体成矿地质作用,形成了具有中国特色的多源多期多成因金矿床.这在包裹体冷冻法及其成分分析和包裹体水的氢氧同位素分析的结果中也得到证实.

CRITICAL INCLUSION AND ITS APPLICATION IN GOLD DEPOSIT GEOLOGICAL STUDY

Critical inclusions are formed in ultra critical fluid only when the critical volume is definite Their gas/liquid ratio is invariant till it becomes homogeneous fluid in thermodynamic process They occur in all kinds of magmatic, igneous and hypometamorphic rocks and iron, copper, gold and other multi metal deposits By studying the critical homogeneous temperature and the phase graph of the fluid system,we can obtain the physicochemical information about the thermodynamic state of minerogenetic fluid system, the basic composition of fluid, critical pressure, salinity, density etc Through years’ research, the authors discover that many gold deposits occurred in hypometamorphic rocks in North China contain critical inclusions whose critical homogeneous temperature is lower than the pure water's (374 2℃),indicating that the ultra critical fluid in early minerogenetic stage is composed of a low salinity CO 2-H 2O system and derived from ancient hypometamorphic rock The other mineralization hydrothermal, with higher salinity, comes from Mesozoic acid intermediate rock bodies near the deposits The ultra critical fluid has then formed gold deposits with multi stage,multi origin and multi genesis features, which is proved by inclusion refrigeration, composition analysis and the H and O isotopic analysis for the water from inclusions