摘要
在德国哈茨山ＳｔＡｎｄｒｅａｓｂｅｒｇ热液ＰｂＺｎ矿床的方解石脉中，发现了一个方解石巨晶。对该方解石三个不同方向上的Ｃ和Ｏ同位素分析发现，方解石晶体内部存在明显的ＣＯ同位素环带。通过对方解石同位素环带的理论模拟，发现在方解石生长过程中，有三种流体参与了作用。它们分别为Ａ：温度约为６０℃的近地表流体，δ１３Ｃ＝－１８５‰，δ１８Ｏ＝０‰；Ｂ：温度约为１４０℃的深源流体，δ１３Ｃ＝－７０‰，δ１８Ｏ＝＋１００‰；Ｃ：温度小于２０℃，δ１８Ｏ≤０‰，δ１３Ｃ≥－１４０‰的大气降水。作用过程有四个阶段：①方解石在流体Ａ中等温生长；②流体Ｂ开始与流体Ａ在封闭体系内均匀混合；③流体Ｂ缓慢注入的同时，发生了一次性流体Ｃ的快速不均匀混合；④流体Ｂ继续与Ａ缓慢混合，混合体系开放。流体混合的发现。
A calcite crystal, sized in 4×6×10cm, was found in the calcite veins of the St Andreasberg hydrothermal Pb-Zn deposits in the Harz Mountains, Germany Detailed investigations of carbon and oxygen isotopic compositions were carried out at different locations within the calcite crystal in the following three drilling profiles: A,( 1 010)→(10 1 0); B, (0001)→(000 1 )and C,(0 1 10)→( 1 101) For all of the three profiles, carbon and oxygen isotopes vary synchronously and both show evidently isotopic zoning, in which two zonations [( 1 010)→(10 1 0) and (0001)→(000 1 ) profiles] are symmetrical and the other one [(0 1 10)→( 1 101) profile] is asymmetrical According to theoretical modeling of the carbon and oxygen isotopic zonations of the calcite crystal, three kinds of fluid are identified to be responsible for the formation of the isotopic zonations: A, subsurface fluid with temperature about 60℃, δ 13 C=-18 5‰ and δ 18 O=0‰; B, deep-seated crustal fluid with temperature about 140℃, δ 13 C=-7 0‰ and δ 18 O=+10 0‰; C, meteoric-hydrothermal fluid with temperature less than 20℃, δ 18 O0‰ and δ 13 C-14 0‰ The evolution history of the fluids effecting the calcite crystal can be divided into four stages: ① Calcite grew in fluid A with a constant temperature of 60℃; ② Fluid B mixed with fluid A homogeneously in a closed system At this stage, the temperature of the mixed fluid increased from 60℃to 90℃, and δ 13 C and δ 18 O values increased from -18 5‰ and 0‰ to -16 3‰ and 3 8‰, respectively; ③ At the time when fluid B fluxed into the system slowly, fluid C was added instantaneously, which made the carbon and oxygen isotopic compositons and temperature of local fluid different from those of the predominant fluid The difference was erased gradually after a period of time at the end At this stage, the temperature of the mixed fluid decreased from 90℃to 20℃ firstly and then increased to 80℃, and δ 13 C value increased from -16 5‰ to -14 0‰ and δ 18 O value decreased from 3 8‰ to 0‰ then increased to 4 0‰ again; ④ Fluid B continued to mix with fluid A slowly, but the mixing system then became open so that the temperature of the system increased slowly to about 95℃ and then was kept at this temperature, and δ 13 C and δ 18 O values of the mixed fluid increased to -10 7‰ and 7 0‰ at the end, respectively The recognization of mixing of hydrotheraml fluids in different fashions shed light on the genetic mechanism of the St Andreasberg ore deposits
出处
《高校地质学报》
CAS
CSCD
1999年第2期183-196,共14页
Geological Journal of China Universities
基金
国家自然科学基金
中国科学院专项基金
关键词
方解石晶体
碳氧同位素
铅锌矿床
热液矿床
fluid mixing, isotopic zoning, calcite crystal, Pb-Zn ore deposit
