滇西北雪鸡坪斑岩铜矿流体包裹体初步研究

Study of fluid inclusions in quartz veinlets in the Xuejiping porphyry copper deposit,Northwest Yunnan,China.

雪鸡坪中型斑岩铜矿床位于三江地区义敦岛弧南端的中甸弧,成矿斑岩为石英闪长玢岩和石英二长斑岩,属于印支期产物。含矿岩体蚀变分带明显,由中心向外发育强硅化带→石英绢云母化带→粘土化—石英绢云母化带→青磐岩化带,工业矿体赋存于斑岩体中心强硅化和石英绢云母化带内。矿化类型以网脉状矿化为主,细脉浸染状矿化不发育。本文对主要矿化阶段石英脉中的流体包裹体系统进行了包裹体岩相学、显微测温学和激光拉曼谱学研究,发现与成矿有关的流体包裹体可以分为水溶液包裹体、CO_2包裹体和含子矿物包裹体3类,子矿物主要为石盐、方解石、赤铁矿和少量 CaCl_2水合物及不透明硫化物。其中含子矿物包裹体均一温度为230～420℃,盐度为33.48%～75.40%NaClequiv.,密度为1.01～1.09g/cm^3。激光拉曼光谱分析表明,包裹体的液相成分主要为 H_2O,气相成分为 H_2O和 CO_2。早期水溶液包裹体和 CO_2包裹体共生,其均一温度相近,以及纯 CO_2包裹体的发现,指示成矿流体存在不混溶现象,这种不混溶是由原始岩浆流体"二次沸腾"作用产生的。CO_2相分离、温压条件降低和 pH 值升高是雪鸡坪斑岩铜矿硫化物沉淀的主要原因。晚期低温、低盐度的流体可能来源于大气降水与岩浆流体的混和,对矿化的意义不大。

The Xuejiping porphyry copper deposit is located in the Zhongdian arc of the southern Yidun island arc in Sanjiang district. The porphyries related to copper mineralization consist of Indosinian quartz-dioritic porphyrite and quartz-monzonitic porphyry. There are obvious alteration zonations for mineralized porphyry rocks. From the center to the outer parts of the porphyry body, rocks are experienced from strong silicification, quartz-sericite alteration, clay-quartz-sericite alteration, and propylitic alteration respectively. Copper ore bodies mainly hosted in strong silicification zone and quartz-sericite alteration zone in the center of the porphyry body. The copper mineralization occurred mainly in type of stockwork veining, with little in type of veinlet-dissemination. A series petrographic, microthermometric and laser Raman spectroscopic studies on fluid inclusions （FI） in various kinds of ore rocks from the Xuejiping deposit have been carried out in this paper. Three types of fluid inclusions have been observed. They are aqueous FI, COE-bearing FI and daughter mineral-beating FI respectively. Daughter minerals mainly include salt, calcite, hematite, with minor CaCI2 hydrate and opaque sulfide. The homogenization temperatures （ Th ）, salinities, and densities of daughter mineral-bearing FI are mainly in ranges of 230 -420℃, 33.48% - 75. 40% NaClequiv. , and 1.01 - 1.09g/cm^3 respectively. Laser Raman spectroscopy of inclusions indicate that the principal fluid component is H2O and the gas components are H2O and CO2. The coexistence of some aqueous FI and CO2-rich FI, with similar Th, and the existence of carbonic inclusions indicate the unmixing of ore-forming fluid generated by ＂ secondary boiling＂ of primary magmatic fluid. It is believed that the unmixing of CO2 and aqueous phases, the decrease of pressure and temperature and the increase of pH values of ore-foriming fluid are main factors leading to the sulfide deposition in the Xuejiping deposit. The fluid of low homogenization temperature and low salinity in inclusions of late stage quartz veins could be formed by the mixing of meteoric and evolved magmatic fluids, with insignificance to the mineralization.