氢氧同位素体系成矿流体示踪若干问题

SEVERAL QUESTIONS ON TRACING ORE FORMING FLUID BY USING HYDROGEN AND OXYGEN ISOTOPE SYSTEM

综合分析国内外氢氧同位素研究进展结合部分热液矿床成矿流体的氢氧同位素示踪体系的可靠性,认为该方法存在以下问题:(1)通过石英矿物δ18 O计算流体的δ18O,同位素平衡温度厘定具有主观性,并且石英与流体之间的同位素平衡状态是否达到很难确定。同样的石英矿物δ18 O通过不同的平衡温度计算,得到不同的流体δ18 O,得出晚期大气降水混合的模糊认识;(2)石英中除流体包裹体外,由于晶格结构缺陷而含有部分结构水。在热爆裂法提取流体包裹体时石英中结构水的释放严重影响石英中流体包裹体δD测试结果。大量的已发表数据没有明确指定热爆裂温度,因此无法评价石英结构水对测试结果的影响程度;(3)石英作为流体包裹体寄主矿物对流体包裹体封闭性不佳,流体包裹体捕获后容易与外界流体发生物质交换,使流体包裹体δD不能代表成矿流体同位素组成,这直接挑战了石英作为寄主矿物进行流体包裹体δD测定的可靠性;(4)通过含差劲基热液蚀变矿物δD计算成矿流体δD,由于高温下压力对D/H分馏的影响难以定量评价,已有平衡方程大多忽略压力对D/H同位素分馏的影响,使得计算结果意义不明确。因此硫化物中流体包裹体的δD-δ18 O系统测试,需要辅以微量气体比值、He-Ar等惰性气体同位素方法,才能明确指示热液矿体成矿流体来源。

Abstract. On the basis of review of recent advances on tracing ore forming fluid by using widely adopted δD vs δ^18O system,attempt is made to discuss the reliability of δD vs δ^18O system in tracing ore forming fluid of hydrothermal mineral deposits. Following questions related to the problems were proposed. （1） Water δ^18O calculation from quartz δ^18O is suspected in the subjective determination of isotopic equilibrium temperature; because it is difficult in determining whether the oxygen isotope equilibrium between quartz and water is achieved. Sometimes, the same quartzδ^18O data produce different δ^18O liquid and result in different temperatures, which obtain a confused recognition of mixture of ore fluid with meteoric water in later stage. （2） Duo to crystal defects, quartz usually contains structural bonded water which is able to release from quartz in thermal decrepitating process. The released structural water is likely to alter significantly the δD measurement of inclusion fluid in quartz. However, most of the published data did not illuminate clearly the explosion temperatures. This prevent the evaluation how much the structural OH has been involved. （3） Quartz is not a well closed host mineral for fluid inclusion it is easy to exchange with outside materials. The mixed liquid can not represent the original ore forming fluid and therefore, the reliability of δD measurement from fluid inclusion in quartz is doubtful. （4） The δD calculation from hydrothermal OH bearing silicate minerals are ambiguous in tracing ore forming fluid, because the pressure effect on D/H fractionation between silicate minerals and water at high temperatures are not taken into consideration. It is recommended that systematic analysis and effort on δD-δ^18O from inclusion fluid in sulfide should be explored, coupled with minor volatile ratios and He-Ar isotope compositions,to trace the origin of ore forming fluid of hydrothermal ore deposits.