德国哈茨山St.Andreasberg热液矿床方解石巨晶的碳氧同位素研究

在德国哈茨山Ｓｔ．Ａｎｄｒｅａｓｂｅｒｇ热液Ｐｂ－Ｚｎ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Ｃ－Ｏ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，δ＾１３Ｃ＝－１８．５‰，δ＾１８Ｏ＝０‰；Ｂ：温度约为１４０℃的深源流体，δ＾１３Ｃ＝-7.0％O，δ＾１８Ｏ=+10.0％o；C：温度小于20℃。δ＾１８Ｏ≤％O ，δ＾１3C≥ 14.0％o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，为认识St Andreasberg热液矿床的形成机制提供了颟的视野。

STUDY OF CARBON AND OXYGEN ISOTOPIC ZONING PROFILES IN A CALCITE CRYSTAL FROM THE ST ANDREASBERG ORE DEPOSITS IN THE HARZ MOUNTAINS, GERMANY

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.( 10l0)--(10 10): B (0001)--(000 1) and C.(0 110)--(1 101). For all of the three profiles，carbon and oxygen isotopes vary synchronously and both show evidently isotopic zoning，in which two zonations [( 1010)--(10 10) and (0001)--(000 1)profiles]are symmetrical and the other one[(0 l10)--( 1101)profile] is asymmetrical. According to theoretical modeling of the carbon and oxygen isotopic zonations of the calcite crysta1, three kinds of fluid are identified to be responsible for the formation of the isotopic zonations: A，subsurface fluid with temperature about 60℃，δ13C= -18.5‰ and δ180=0‰; B，deep-scaled crustal fluid with temperature about 140℃，δ13C= -7.0‰ and δ18O= +l0.0‰; C, meteoric-hydrothermal fluid with temperature, 1ess than 20℃，δ18O≤ 0‰ and δ13C≥ -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 clewed system. At this stage，the temperature of the mixed fluid increased from 60℃ to 90℃ and δ13C and δ18O 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 C value increased from -16.5‰ to -14.0‰ and δ180 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 and δ13C and δ18O values of the mixed fluid increased to -10.7‰ and 7.0‰ at the end，respectively. The recognization of mixing of hydrothermn1 fluids in different fashions shed 1ight on the genetic mechanism of the St. Andreasberg ore deposits.