冀北东坪金矿床深部-外围的构造-蚀变-流体成矿研究

冀北东坪金矿田是我国首次在碱性杂岩体内发现的金矿床,曾被认为是与碱性岩有关的金矿床。近年来年代学数据表明,东坪-后沟一带金矿的赋矿碱性杂岩体形成于海西期,而成矿却主要发生在燕山期。金矿床严格受构造裂隙控制,构造-蚀变-流体成矿作用显著,钾长石化是最重要的蚀变。由未蚀变岩石向矿体和断裂带中心方向,典型的构造-蚀变-矿化分带依次为:0-原岩(二长岩、正长岩)带,I-微斜长石化带,II硅化绢云母化微斜长石岩带,III碎裂微斜长石岩带,及IV断层泥。从0带到III带,Au含量增加,Ag、Cu、Pb、Zn、Mo也略有增加。东坪金矿构造-蚀变-矿化阶段可分为4个:Ⅰ钾长石-石英脉阶段;Ⅱ黄铁矿-白色石英阶段;Ⅲ多金属硫化物-烟灰色石英脉阶段;Ⅳ晚期碳酸盐阶段。深部中段各阶段脉石英的流体包裹体研究表明, 在I、II、III阶段均发育富CO₂包裹体。第Ⅰ阶段钾长石石英脉L-V型包裹体均一温度(Th)为220.3~359℃,盐度1.1%~3.1% NaCleqv;H₂O-CO₂型包裹体Th在346.5~383.5℃。第Ⅱ阶段黄铁矿白色石英脉中L-V型包裹体Th范围是217.2~372.5℃,盐度在1.1%~5.7% NaCleqv;H₂O-CO₂型包裹体Th在241.2~396.7℃,盐度为2.2%~6.2% NaCleqv。第Ⅲ阶段的烟灰色石英脉中L-V型包裹体Th范围为158.2~350.5℃,盐度在0.7%~5.5% NaCleqv;H₂O-CO₂型包裹体Th范围在215.2~378℃之间,盐度范围在3.0%~6.0% NaCleqv。第Ⅳ阶段晚期石英脉L-V型包裹体Th范围为151.2~249.8℃,盐度在0.9%~8.3% NaCleqv。矿区外围转枝莲矿段的II阶段白色石英脉中包裹体的Th范围为220~416.2℃,III阶段烟灰色石英脉的Th范围为195.3~425℃。富金石英脉形成于中高温(＞300℃,可达400℃以上)、中深压力(70~160MPa以上)条件下。其成矿背景、热液蚀变、矿物共生组合及流体性质与典型的造山型金矿有一定的差别,归属于"与侵入岩有关的金矿床"更合理。

Structure, alteration, and fluid inclusion study on deep and surrounding area of the Dongping gold deposit, northern Hebei, China

The gold deposits in Dongping area,northern Hebei Province are firstly found among the alkaline complex in China.These gold deposits are often recognized as alkaline rock-related gold deposits because of surrounding rocks.In recent years,it is indicated that the alkaline complex formed in the Hercynian,but gold mineralization occurred in the Yanshanian stage through geochronology.These gold deposits are strictly controlled by the structure fractures and have a significant mineralization of structure-alteration-fluids.Alterations and mineralization of the Dongping gold deposit can be divided into four stages.They are(I) feldspar-quartz veins stage,(II) pyrite-white quartz stage,(III) sulphides-smoky gray quartz vein stage,and(IV) late carbonate stage.Zonation is clear in altered fractured ore-controlling rocks.There are five zones from unaltered syenite of footwall to centre of ore body:zone 0-the original rocks(monzonite,syenite) zone;zone I-microclinization zone;II-silicified sericited microclinite zone;III-fractured microclinite zone;and IV-fault gouge zone.From zone 0 to zone III,the content of Au increases,correspondingly,Ag,Cu,Pb,Zn,Mo also increase slightly.A study of fluid inclusion shows that CO2-rich inclusions both exist in the stage I and the stages II,III.Ore-forming fluids belong to H2O-CO2-NaCl hydrothermal systems with medium-high temperatures and low saltinities.CO2-rich inclusions gradually decrease from ductile to brittle transformation in ore-bearing tectonic zones.The homogenization temperatures of L-V,H2O-CO2 inclusions are 220.3～359℃,346.5～383.5℃in feldspar quartz veins of stage I,and the salinity of L-V inclusions is 1.1%～3.1% NaCleqv;the homogenization temperatures of L-V,H2O-CO2 inclusions are 217.2～372.5℃,241.2～396.7℃ in pyrite-white quartz veins of stage II,and the salinity is 1.1%～5.7% NaCleqv,2.2%～6.2% NaCleqv respectively;the homogenization temperatures of L-V,H2O-CO2 inclusions are 158.2～350.5℃,215.2～378℃ in smoky gray quartz veins of stage III,and the salinity is 0.7%～5.5% NaCleqv,3.0%～6.0% NaCleqv respectively;and those of L-V inclusions are 151.2～249.8℃ in the late quartz veins of stage IV,and the salinity is 0.9%～8.3% NaCleqv.In its surrounding area,Zhuanzhilian mine,the homogenization temperatures of fluid inclusions in white quartz vein(II) are 220～416.2℃,and those in dark grey quartz vein(III) are 195.3～425℃.Gold-rich quartz veins were formed in hypothermal fluids(T>300℃,up to higher than 400℃) and deep portion(pressures higher than 70～160MPa) .It is more resonable to consider the gold deposit in the Dongping area as intrusion-related gold deposits,based on geological setting,hydrothermal alteration,ore mineral assemblage,and characteristics of fluid inclusions.