滇西北北衙斑岩型金多金属矿床长石矿物学特征及成矿意义

Mineralogical characteristics of feldspar and its metallogenetic significance in the Beiya porphyry-type gold polymetallic deposit,NW Yunnan

北衙斑岩型金多金属矿床是西南＂三江＂成矿带新生代斑岩型矿床的典型代表,其成矿与喜马拉雅期富碱斑岩密切相关。本次研究对该矿床与成矿关系密切的石英正长斑岩进行了系统岩矿鉴定和长石矿物电子探针分析。电子探针分析结果表明,石英正长斑岩内斜长石An为6.04~14.42,Ab为80.03~93.37,Or为0.59~9.80,主为钠长石、更长石;原生正长石An为0~0.48,Ab为3.91~8.58,Or为90.40~95.91;蚀变成因钾长石An为0.27~0.83,Ab为20.56~37.45,Or为61.90~78.95,属于正长石、Na-正长石。本次研究认为斜长石蚀变为钾长石的过程中,K2O含量明显升高,而Na2O含量明显降低,而钾长石交代原生正长石的过程中,Na2O含量增高,K2O含量降低。根据成矿流体富K质特征,结合Au元素的地球化学特征,本研究认为在钾长石化过程中金被活化迁移,随着钾质沉淀以及成矿流体温度和压力降低,金发生沉淀并在有利部位富集成矿。

The Beiya porphyry-type gold polymetallic deposit is one of the typical Cenozoic porphyry deposits in Sanjiang metallogenetic area,northwestern Yunnan Province.The deposit is closely related to the Himalayan epoch alkali-rich porphyry.The quartz syenite porphyry related to porphyritic mineralization was studied through systematical identification of rock and minerals,and electron probe analysis of the feldspar mineral.The electron probe result shows that the range of An,Ab and Or of plagioclase in quartz syenite are 6.04-14.42,80.03-93.37 and 0.59-9.8 respectively,therefore,the plagioclase mainly is albite and oligoclase.The range of An,Ab and Or of primary orthoclase are 0.00-0.48,3.91-8.58 and 90.40-95.91 respectively.The range of An,Ab and Or of altered Kfeldspar are 0.27-0.83,20.56-37.45 and 61.90-78.95 respectively,therefore,the altered K-feldspar mainly is orthoclase or Na-orthoclase.The study result indicates that K2 O content increased and Na2 O content decreased apparently during the process of plagioclase altered into K-feldspar;In the contrast,in the process of K-feldspar replaced orthoclase,the content of Na2 O increased and K2 O decreased significantly.Based on the characteristics of potassium-rich ore-forming fluids rich in potassium,in combination with geochemical characteristics of Au element,it is concluded that the K-feldspar alteration plays an important role in the formation of the Beiya porphyry gold deposit.