云南姚安铅银矿床假白榴石斑岩空间分布和组构特征

Spatial distribution and fabric features of pseudoleucite porphyry from the Yao'an Pb-Ag deposit in Yunnan Province

姚安铅银矿床位于金沙江-哀牢山断裂带东侧的富碱斑岩带内,主要发育正长斑岩、粗面岩、假白榴石斑岩、含斑正长细晶岩等一套杂岩体,其中正长斑岩为主要容矿围岩。通过野外地质勘查、显微镜鉴定、电子探针及X射线微区衍射分析,对假白榴石斑岩的空间分布、岩石矿物学特征进行了综合研究,结果表明,姚安铅银矿床假白榴石斑岩多以脉状、角砾状或呈小岩枝产出,出露在正长斑岩质火山角砾岩的外围,主要分布在角砾岩型铅银矿(化)体的南西侧、F4断裂的北东盘,在空间上与角砾岩筒及外围断裂有关;该矿床的假白榴石具两个世代,早期为灰绿色浑圆粒状假白榴石斑岩的假白榴石斑晶,富Al,晚期为灰-灰白色四角三八面体假白榴石斑岩的假白榴石斑晶,富Si、Na,二者均蚀变成钾长石、高岭石、石英等含水富硅矿物;姚安假白榴石斑岩形成于印度板块与欧亚大陆俯冲碰撞后的板内拉张环境;假白榴石斑岩形成晚于粗面岩、正长斑岩,早于含斑正长细晶岩;假白榴石斑岩与铅银矿(化)体无直接成因联系。

There exists a suite of complex porphyries in the Yao＇an Pb-Ag deposit, such as syenite porphyry, trachyte, pseudoleucite porphyry, and phenocryst-bearing syenite aplite, with the syenite porphyry being the host rock. Through an integrated study of the spatial distribution and petrological and mineralogical characteristics of pseudoleucite porphyry through field geological survey, microscopic identification, electron microprobe analysis and X ray diffraction analysis, the following results have been obtained：① Pseudoleucite porphyry is outcropped in the periphery of syenite porphyritic volcanic breccia in the forms of veins, breccia or small apophyses and is mainly distributed in the southwest of breccia-type orebodies and the northeast of F4 fault. Pseudoleucite porphyry is associated with breccia pipe and peripheral fault in space; ② Pseudoleucite has two generations. The early pseudoleucite phenocryst is in the grayish green round form rich in Al, whereas the late pseudoleucite phenocryst, which exhibits high Si and Na, is grayish white tetragonal trisoctahedron phenocryst. Both of them have been replaced by altered minerals such as potash feldspar, kaolinite, and quartz; ③ Pseudoleucite porphyry was formed in an intraplate extensional environment after the collision between India continent and Eurasia continent; ④ Pseudoleucite porphyry was formed later than trachyte, syenite porphyry and earlier than phenocryst-bearing syenite aplite. It had no direct genetic connection with galena mineralization.