云南普朗超大型斑岩铜矿床岩浆混合作用:熔融包裹体证据

Magma Mixing in the Giant Pulang Porphyry Copper Deposit, Yunnan Province: Evidence from Melt Inclusions

普朗矿床是云南中甸地区规模最大的斑岩铜矿床,其探明的Cu金属量达418万吨。普朗含矿岩体主要由石英闪长玢岩、石英二长斑岩和花岗闪长斑岩组成。野外及镜下观察显示,普朗岩体中含有大量镁铁质微粒包体。相比于寄主岩,包体明显富集角闪石和黑云母。在普朗岩体锆石中发现暗色和浅色两类熔融包裹体:暗色熔融包裹体多含黑色和白色两相,浅色熔融包裹体加热均一化后为灰白色。暗色熔融包裹体黑色相为富Fe熔体(SiO2<5.5%、FeO>80%)。加热均一化的浅色熔融包裹体为富Si熔体(SiO2>69%、FeO<2.2%),较全岩组分更酸性。此外,锆石中还含有Fe质与Si质含量相当的熔融包裹体(33.26%SiO2、40.58%FeO)。这些特征指示普朗岩体在形成过程中发生了岩浆混合作用。镁铁质富Fe熔体和长英质富Si熔体的混合,使后者获取了大量的硫和亲铜元素,还促使Cu、Au和Mo等成矿元素分配进入岩浆–热液流体,形成斑岩铜矿床。

The Pulang deposit is the largest porphyry copper deposit in the Zhongdian area of Yunnan province,with 4.18 Mt copper reserves.Ore-bearing rocks in the Pulang deposit include quartz diorite porphyrite,quartz monzonitic porphyry,and granodiorite porphyry.Field and microscopic observations show that the Pulang porphyries contain plenty of mafic microgranular enclaves(MMEs).Compared to the host rock,the MME are obviously enriched in hornblende and biotite.Two types of melt inclusions can be identified in zircon from the Pulang porphyries in terms of color.The dark melt inclusions contain black and white phases that are difficult to homogenize,whereas the light tint ones are offwhite under heating.The dark phases in dark melt inclusions are Fe-rich melt(SiO2<5.5%,FeO>80%).The light tint melt inclusions are the Si-rich melt(SiO2>69%,FeO<2.2%)after being heated to homogenous glasses,which are more acidic than the whole rocks.Moreover,the melt inclusions with similar Fe and Si contents(33.26%SiO2,40.58%FeO)are also identified in zircon.These characteristics indicate that magma mixing might have occurred in the magmatic process of the Pulang porphyries.Mixing of mafic Fe-rich melt will surely conveys large amounts of sulfur and chalcophile elements into the felsic melt and increase the potential of the magmatic-hydrothermal ore system and contribute to the formation of the giant porphyry copper deposit.