滇西北普朗斑岩型矿床钾硅酸盐带的形成及与矿化关系研究

The Relationship between Mineralization and the K-Silicate Zone in the Pulang Porphyry Deposit,Northwest Yunnan

云南普朗超大型斑岩铜金矿床位于我国西南三江特提斯构造域义敦岛弧南部的中甸岛弧内。为了探讨该矿床钾硅酸盐带矿物特征、形成机制及与矿体的关系,为下一步深部和外围找矿勘查工作提供依据,本研究开展了二长斑岩X射线衍射光谱(XRD)和长石类矿物主量成分测试。结果显示赋矿二长斑岩中钾长石平均含量为26.2%,斜长石为26.7%,石英和黑云母分别为~16.1%和~14.4%。在系统野外调查和室内观察的基础上,根据其形成机制,本研究将该矿床含矿二长斑岩中的长石划分为三类,即岩浆期结晶的原生长石(原生斜长石Pl1和原生钾长石Kfs1)、次生长石(热液蚀变成因,Pl2和Kfs2)和热液结晶长石(Pl3和Kfs3)。Pl1具有高铝(Al_(2)O_(3)=22.61%~27.27%)和钙(CaO=3.68%~9.29%),低硅(SiO_(2)=56.09%~63.89%)和钠(Na_(2)O=6.31%~9.27%)的特征;Pl3具有高硅(SiO_(2)=67.07%~68.97%)和钠(Na_(2)O=11.02%~11.99%)的特征;Pl2中SiO_(2)、Al_(2)O_(3)、CaO和Na_(2)O含量变化较大,主要分布于Pl1和Pl3之间,呈现出二者混合的特点。Kfs1~Kfs3元素含量变化较小,SiO_(2)、K_(2)O、Na_(2)O和Al_(2)O_(3)分别为62.75%~64.69%、15.42%~16.89%、0.20%~1.03%和17.93%~19.19%。与原生钾长石相比,次生钾长石中钾含量没有增加反而减少,说明流体中钾可能相对较为亏损,这可能是普朗矿床钾硅酸盐化带中次生钾长石发育较弱的主要原因。青磐岩化的叠加改造,特别是黄铜矿±绿帘石±绿泥石脉发育显著提高了该矿床的铜品位,增加了铜储量。

The Pulang superlarge copper-gold deposit in Yunnan Province is located in the Zhongdian Island Arc in the southern part of the Yidun Island Arc in the Sanjiang Tethys tectonic domain in southwest China.The mineral assemblage of the K-silicate zone is mainly biotite+quartz+epidote+chlorite,which is obviously different from the typical K-feldspar+biotite+quartz assemblage.Propylitization significantly reworked the K-silicate zone of the Pulang deposit.XRD results show that the average contents of K-feldspar,plagioclase,quartz and biotite in the mineralized monzonite porphyry are 26.2%,26.7%,~16.1%and~14.4%,respectively.Based on systematic field and indoor observations,the feldspar in the mineralized monzonite porphyry of the deposit is classified in this study into three types according to its formation mechanism,namely primary feldspar(including primary plagioclase:Pl1 and primary K-feldspar:Kfs1),secondary feldspar(Pl2 and Kfs2)and hydrothermal feldspar(Pl3 and Kfs3).Pl1 has high contents of aluminum(Al_(2)O_(3)=22.61%~27.27%)and calcium(CaO=3.68%~9.29%)and low contents of silicon(SiO_(2)=56.09%~63.89%)and sodium(Na_(2)O=6.31%~9.27%).Pl3 has high silicon(SiO_(2)=67.07%~68.97%)and sodium(Na_(2)O=11.02%~11.99%)contents.There is a mixing characteristic in the contents of SiO_(2),Al_(2)O_(3),CaO and Na_(2)O in Pl2.The contents of SiO_(2),K_(2)O,Na_(2)O and Al_(2)O_(3)in Kfs1 and Kfs3 are 62.75%~64.69%,15.42%~16.89%,0.20%~1.03%and 17.93%~19.19%,respectively.Compared to primary K-feldspar,the potassium content in secondary K-feldspar is decreased,indicating that potassium in the fluid may be relatively depleted.This may be the main reason for the weak development of K-feldspar in the K-silicate zone of the Pulang deposit.Propylitic alteration overlying the K-silicate zone,in particular the development of chalcopyrite±epidote±chlorite veins,has significantly improved the copper grade of the deposit and increased the copper reserves.