阿尔泰南缘克兰盆地的脉状金-铜矿化及其流体演化

Fluid inclusion study on gold-copper mineralization in Lower Devonian strata of the Kelan basin,Altay,China

阿尔泰山南缘泥盆纪克兰火山-沉积盆地蕴藏有丰富的VMS锌铅铜多金属矿床。自晚泥盆世至早二叠世末,阿尔泰山南缘为NE-SW向强烈挤压的构造环境,VMS矿石受到变形变质改造,脉状金铜矿化发育。金(铜)石英脉主要有2种产状:(1)白色-灰白色(硫化物)顺层石英脉(QI),产于韧脆性剪切带发育地段,呈细脉状或透镜状产于绿泥片岩、黑云片岩中;(2)斜切黄铁矿化蚀变岩、层状铅锌矿和变质岩产状的黄铜矿-黄铁矿石英脉(QII),与晚期的脆性构造有关。金(铜)石英脉的流体包裹体发育,按室温下相态特征有3类。第I类为含子矿物的高盐度包裹体(L-V-S型),子晶为NaCl,有时为KCl,包裹体呈孤立或无序分布,代表变质早期流体特征。一般NaCl子晶先消失(210～357℃),包裹体的最终均一温度369～512℃,其捕获温度与变质相的相平衡计算温度相当,反映了变质早期中高温热液活动的特征。第II类是富CO2包裹体,包括单相的碳质流体包裹体(LCO2、LCO2-CH4或LCO2-N2)和两相富CO2包裹体(LCO2-LH2O)2个亚类。碳质流体包裹体是常见类型,有时与LCO2-LH2O型伴生,在较晚期的黄铜矿-黄铁矿石英脉中表现为原生特征,而在较早的石英脉中常表现为次生特征。萨热阔布的碳质流体可分为纯CO2包裹体和CO2-CH4体系包裹体,纯CO2包裹体的固体CO2熔化温度(Tm,CO2)为-60～-56.5℃,CO2部分均一温度(Th,CO2)变化于-23～+31℃;密度一般为0.85～0.89g.m-3。CO2-CH4包裹体的Tm,CO2<-57℃,可低达-78.1℃,Th,CO2低达-33.7～-17.7℃,其密度高达1.01～1.07g.m-3。VMS矿床中晚期叠加的黄铜矿石英脉中碳质流体包裹体可分为贫CH4-N2和富CH4-N2的CO2-CH4-N2包裹体,贫CH4-N2的碳质包裹体Tm,CO2=-63.3～-57℃,Th,CO2=-27.5～+29.7℃;富CH4-N2的CO2-CH4-N2包裹体Tm,CO2=-83.4～-65.5℃,Th,CO2=-56.0～+16.9℃。铜金石英脉中与碳质流体共生的LCO2-LH2O型包裹体均一温度Th,total=205～370℃,略低于第I类高盐度包裹体的Th,total=369～512℃。据CO2流体高温高压相图估算包裹体的捕获压力至少为110～300MPa。金(铜)石英脉的主体在相当于445～566℃的高温条件下形成的,而金铜矿化则是在高于205～370℃、110～330MPa的中高温中深条件下发生的。流体包裹体的δ18O为7.54‰～11.84‰(QI)和3.82‰～7.82‰(QII),δD为-84.7‰～-98.2‰(QI)和-75.8‰～-108.8‰(QII)。结合地质特征和流体研究,说明成矿热液来源与区域变质及相关的岩浆活动有关。

The southern margin of the Altay had undergone intensive NW-SE compression since the Late Devonian to the Early Permian,which resulted in syn-orogenic metamorphism,deformation and hydrothermal mineralization.This was another important mineralizing epoch after the Early Devonian VMS mineralization.Not only had VMS ores undergone deformation and metamorphism,but also gold-copper-bearing sulfide-quartz veins formed during orogeny or post-orogeny.These quartz veins can be divided into two groups.The first group of quartz occurs as lentoid or streaked quartz veins which are parallel to the foliated structure of the biotite-chlorite or garnet-chlorite schist.The second group is chalcopyrite-bearing quartz veins which cut across chlorite schist and banded sulfides at small angles,representing a much younger,metamorphic-related overprinting event.There are three types of fluid inclusions in the quartz veins.The first type of fluid inclusions is chracterized by daughter mineral-containing inclusions（L-V-S type）,which are scarcely found and have a primary origin representing early metamorphism.They have the final homogenization temperatures of 369~512℃ after daughter mineral disappearring in 210~357℃.The trapping temperatures of the L-V-S inclusions may correspond to the temperatures calculated from the phase equilibrium of metamorphic minerals.The second type of fluid inclusions is characterized by CO2-rich fluids,including CO2-H2O fluid inclusions（LCO2-LH2O）and carbonic（CO2-CH4-N2）inclusions.Carbonic fluid inclusions are commomly observed;a few carbonic fluid inclusions may be of primary origin,and some inclusions may be pseudosecondary,especially in the chalcopyrite-quartz veins from the Sarekoubu deposit.A large number of these carbonic fluid inclusions are of secondary origin.There are two instances for melting temperatures of frozen phases（Tm,CO2）.The first group is pure CO2 inclusions,with Tm,CO2=-60--56.5℃ and Th,CO2=-23-＋31℃;while the second group is CO2-CH4-N2 inclusions with Tm,CO2 less than-57℃,to a minimum of-78.1℃,and Th,CO2=-33.7--17.7℃.According to a microthermometric study on some associated LCO2-LH2O inclusions,the trapping temperatures of carbonic inclusions at least range from 205℃ to 370℃.In this situation the lowest trapping pressures of CO2 fluids can be estimated to be 110~300MPa based on CO2 densities and a CO2 phase diagram at high P-T of Van den Kerkhof and Thiéry（2001）.The main quartz veins of gold-copper ore shoots were formed under 445~566℃,whereas gold-copper mineralization happened under 205-370℃ and 110-330MPa.The data of δ18O of fluid inclusions are 7.54‰-11.84‰（QI）and 3.82‰-7.82‰（QII）,whereas δD are-84.7‰--98.2‰（QI）and-75.8‰^-108.8‰（QII）.Based on field geology and fluid inclusion study,the source of ore-forming fluids is related with regional metamorphism and associated magmatism.