川南普格上二叠统玄武岩葡萄石-石英晶洞中矿物组合及热液演变特征

CHARACTERISTICS OF MINERAL ASSEMBLAGE AND HYDROTHERMAL FLUID EVOLUTION IN PREHNITE-QUARTZ GEODE OF UPPER PERMIAN EMEISHAN BASALT IN PUGE COUNTY,SOUTHERN SICHUAN PROVINCE

利用冷热台、激光拉曼光谱仪、电感耦合等离子体质谱仪等仪器设备,对川南普格跨山上二叠统玄武岩葡萄石—石英晶洞中的矿物进行系统的测试分析,揭示出晶洞中热液的来源与演变特征。晶洞中的主要矿物为葡萄石和石英,有少量的硅铁灰石、绿帘石、绿纤石以及赤铁矿等,葡萄石、石英以及硅铁灰石有不同的世代,它们生成的顺序依次为:第1世代浅紫色石英+赤铁矿→第2世代石英+第1世代葡萄石→第3世代石英+第2世代葡萄石+第1世代硅铁灰石→第4世代石英+第2世代硅铁灰石+绿帘石+绿纤石。除浅紫色石英外,其它各世代石英晶体内部均呈现出一定的环带,浅紫色石英中的流体包裹体含有一定量的二氧化硫,而其它三个世代石英中的流体包裹体中均含有二氧化硫和甲烷等气体。晶洞中的热液属于低盐度(8.28%~10.61%NaCl_(eqv))、低温(141.7~188.2℃)型热液,且热液的温度呈现出先升后降(140→190→158℃)的变化趋势。第1世代葡萄石晶体较小,浅黄绿色,单晶体呈板柱状,集合体为葡萄状;第2世代葡萄石呈蓝绿色,单晶体为板状,集合体呈灯笼状。不同世代葡萄石中微量元素含量的变化趋势一致,但第2世代葡萄石中Cr,Ni的含量(分别为301.09×10^(-6)和329.61×10^(-6))高于第1世代葡萄石中Cr,Ni的含量(分别为199.08×10^(-6)和247.57×10^(-6))。葡萄石—石英晶洞中的初始热液是在玄武岩结晶分异晚期,富含二氧化硫的挥发性组分进入玄武岩裂隙和气孔中形成。随着玄武岩浆的冷却,浅紫色石英以及赤铁矿从晶洞中富含二氧化硫和Fe(Ⅲ)的热液中结晶析出,后期深部岩浆热液将玄武岩下伏的碳酸盐岩中富含甲烷和二氧化硫等成分的流体带入到葡萄石—石英晶洞中,导致晶洞附近的玄武岩发生蚀变,岩石蚀变释放出的Cr,Ni等成分随流体进入晶洞,引起晶洞中的温度升高以及晶洞热液中甲烷、二氧化硫以及Cr,Ni等含量的变化。随着石英和葡萄石的大量晶出,晶洞环境逐渐由氧化向还原转变,从而形成了晶洞中不同的矿物。

Utilizing cold and hot stage,laser Raman spectrometer,inductively coupled plasma mass spectrometer and other instruments,the minerals in the Permian-basalt prehnite-quartz geode in Kuashan of Puge County,southern Sichuan are systematically measured and analyzed,and the origin and evolution characteristics of the hydrothermal fluid in the crystal are revealed.The minerals in the geode are mainly prehnite and quartz,with a small amount of babingtonite,epidote,pumpellyite,hematite,etc.The prehnite,quartz and babingtonite have different generations and the order of their generation is:the first generation of light purple quartz+hematite→the second generation quartz+the first generation prehnite→the third generation quartz+the second generation prehnite+the first generation babingtonite→the fourth generation quartz+the first generation prehnite+the 2 nd generation babingtonite+epidote+pumpellyite.Except for the light purple quartz,all other generations of quartz crystals show a certain ring zone.The fluid inclusions in the light purple quartz contain a certain amount of sulfur dioxide,while the fluid inclusions in the other three generations of quartz all contain sulfur dioxide and gas such as methane.The hydrothermal fluid in the geode has low salinity(8.28%to 10.61%NaCl_(eqv))and low temperature(141.7-188.2℃),and the temperature of the hydrothermal fluid goes through a normal distribution(140→190→158℃).The first-generation prehnite is small-sized,light yellow-green colored,with plate-column shaped crystal form,and the aggregate is grape-shaped.The second-generation prehnite is blue-green colored,with plate-shaped crystal form and lantern-shaped aggregate.The variation trend of trace element contents in different generations of prehnite is consistent,but the contents of Cr and Ni in the second generation prehnite(301.09×10^(-6) and 329.61×10^(-6),respectively)are higher than those in the first generation prehnite(199.08×10^(-6) and 247.57×10^(-6),respectively).The initial hydrothermal fluid in the prehnite-quartz geode was formed in the late stage of basalt crystallization and differentiation,when the volatile components rich in sulfur dioxide entered the basalt fractures and pores.With the cooling of the basalt magma,light purple quartz and hematite crystallized in the geodes from sulfur dioxide and Fe(Ⅲ)rich hydrothermal fluids.Later deep magmatic-hydrothermal fluids enriched the carbonate rocks beneath the basalts.Fluids such as methane and sulfur dioxide were brought into the vine-quartz geode,resulting in the alteration of the basalt near the geode,and the Cr,Ni and other components released from the rock alteration entered the geode with the fluid,causing the temperature rise and content changes of methane,sulfur dioxide,Cr,Ni,etc.in the geothermal fluid.With the mass crystallization of quartz and prehnite,the ore-forming environment gradually changed from oxidation to reduction,resulting in different minerals of different generations in the geode.