华北克拉通北缘元古宙大庙Fe-Ti-P矿床的挥发份组成和C-H-O同位素研究

Volatile components and C-H-O isotopic compositions of Proterozoic Damiao Fe-Ti-P oxide deposit in the northern margin of the North China Craton

华北克拉通北缘～1.74Ga大庙斜长岩杂岩体赋含有大型Fe-Ti-P矿床。采用分步加热质谱法分别测定了块状Fe-Ti矿石、块状Fe-Ti-P矿石和浸染状矿石中磁铁矿、磷灰石和斜长石释出的挥发份组成、含量以及C-H-O同位素组成。依据矿物的释气总量变化特征将释气过程分为三个阶段:200～400℃、400～800℃和800～1200℃。根据不同类型矿石中矿物在不同释气阶段的挥发份组成、含量以及C-H-O同位素组成,可将含矿岩体中的流体类型分成四种:(1)斜长石800～1200℃阶段释出的变质流体,主要以H2O、N2+CO和CO2为主;(2)斜长石400～800℃阶段释出的幔源流体组分,主要以H2O、H2、CH4和CO2为主;(3)磁铁矿400～800℃阶段释出的地表流体,主要以H2O、CO2、SO2和H2S为主;(4)所有矿物200～400℃阶段释出的代表后期次生包裹体的组分,以H2O和CO2为主。矿石中的磁铁矿相对富集H2O和CO2等流体组分,是在相对氧化的条件下结晶的;而斜长石释出气体中H2和CH4的含量较高,是在相对还原的环境中结晶的。这反映出斜长石可能是在岩浆早期相对还原的条件下结晶的,而磁铁矿是在岩浆演化晚期相对氧化的条件下结晶的,这一过程的转变可能与岩浆演化晚期,岩浆中的挥发份逐渐聚集以及地表流体的加入有关。岩浆中H2O和CO2等流体含量的不断增加导致岩浆的氧逸度逐渐升高,最终造成磁铁矿的大量结晶并富集成矿。

The-1.74Ga Damiao massif-type anorthosite complex in the northern margin of the North China Craton hosts a large Fe-Ti-P oxide deposit.Volatiles that were trapped in magnetite,apatite and plagioclase of massive Fe-Ti ore,massive Fe-Ti-P ore and disseminated ore were measured by step heating mass spectrometer at three releasing temperature intervals 200-400℃,400-800℃ and 800-1200℃ to obtain major components and C-H-O isotopic composition.Four types of fluid were identified from the volatiles：（1）metamorphic fluid released from plagioclase at 800-1200℃ temperature interval,mainly composed of H2O,N2＋CO and CO2;（2）mantle-derived fluid released from plagioclase at 400~800℃ temperature interval,mainly composed of H2,H2O,CH4 and CO2;（3）surface water released from magnetite at 400~800℃ temperature interval,mainly composed of H2O,CO2,SO2 and H2S;and（4）secondary fluid released from minerals at 200-400℃ temperature interval,mainly composed of H2O and CO2.Magnetite in both massive Fe-Ti ore and massive Fe-Ti-P ore contains high contents of H2O and CO2,indicating a relatively oxidized condition due to concentration of early-formed fluids and surface water at late stage of magma fractionation sequence,whereas plagioclase in disseminated ore contains abundant H2 and CH4,indicating a relative reduced.It is proposed that the parental magma from which the Damiao anorthosite complex formed may have fractionated under a relatively reduced condition at the beginning.The mantle fluids and surface water may have been involved in the later stage of the magma fractionation and fractioned to increase the oxygen fugacity of the magmas,which in turn,triggered the accumulation of magnetite and apatite to form the Damiao Fe-Ti-P oxide deposit.