云南兰坪-思茅盆地大平掌铜多金属矿床同位素地球化学与成矿年代学

Isotope Geochemistry and Mineralization Age of the Dapingzhang Copper-polymetallic Deposit in Lanping-Simao Basin, Yunnan Province

大平掌铜多金属矿床是近年来在兰坪 -思茅盆地新发现的一个重要矿床 .该矿床产出于火山岩中 ,其构造背景及矿床地质特征均比较复杂 ,整个矿床由上部层状或透镜状的块状硫化物矿体和下部脉状及浸染状的矿体组成 .该矿床脉状矿体中石英流体包裹体Rb_Sr等时线年龄为 ( 1 1 8± 1 2 )Ma ,初始锶同位素组成为 ( 0 .70 80 0 7± 0 .0 0 0 0 3 7) ;流体包裹体研究表明成矿流体具有中低盐度 ( 2 .0～ 8.0wt %NaCl)和中低温度 ( 90～ 2 0 0℃ ) ;矿床中主要硫化物矿物的δ34S值在 - 0 .9‰～ 3 .5‰之间 ,集中在零值附近 .结合对以往的氢 -氧同位素和铅同位素研究结果的综合分析可以看出 ,大平掌矿床与区域内的脉状铜矿具有相似的地球化学特征 .因此认为大平掌铜多金属矿床为形成于燕山期的热液矿床 。

The Dapingzhang copper_polymetallic deposit was discovered in Lanping_Simao Basin in recent years. This deposit is hosted in volcanic rocks with complicated structure and geological setting. The orebodies include an upper stratiform and lentiform massive sulfide orebody and a lower vein type and dissiminated sulfide orebody. The fluid_inclusion Rb_Sr isotopes of quartz in the vein type orebody yield an isochron age of (118±12) Ma,with an initial Sr isotopic composition of (0.708 007±0.000 037). Fluid_inclusion analysis shows that the ore_forming fluid is of low to middle salinity(2.0～8.0 wt% NaCl), and of low to middle temperature (90～(200 ℃).) The δ^(34)S values of pyrite and chalcopyrite minerals in this deposit range from -(0.9 ‰) to 3.5 ‰, concentrating at about 0. Available H_O isotope results give δ^(18)O_(H_2O) values of -2.5 ‰ to +(2.9 ‰,) and δD values of -84 ‰ to 59 ‰ for the ore_forming fluids. Our modeling and water/rock ratio calculations using these available hydrogen_oxygen isotope data suggests an evolved meteoric water origin for the ore_forming fluids, instead of mixture of seawater and magmatic fluids. The Pb isotope composition shows radiogenic Pb sources with ()^(206)Pb/()^(204)Pb>18.310, ()^(207)Pb/()^(204)Pb>15.489, ()^(208)Pb/()^(204)Pb>37.811, and the data plot in between the orogeny and mantle Pb evolution curves in Zartman Pb isotope diagram, which suggests Pb from sedimentary rocks and mantle_derived volcanic rocks in the basin contributed to this deposit. The Pb and other metals in the deposit may have derived through water_rock interaction by basin fluid circulated in the host rocks in the basin. We found that the Dapingzhang deposit shares many similar geochemical characteristics with vein_type copper deposits in the Lanping_Simao Basin. In conclusion, we suggest that the Dapingzhang copper_polymetallic deposit is a hydrothermal deposit formed during Yanshanian period, but not a massive sulfide deposit of submarine exhalative_sedimentary origin as proposed by several previous researchers.