西秦岭阳山金矿带安坝金矿床流体包裹体特征及矿床成因

Characteristics of Fluid Inclusions and the Mineralization Process of the Anba Gold Deposit in the Yangshan Gold-Ore Zone of the West Qinling

安坝金矿是近年来在西秦岭地区阳山金矿带探明的一超大型矿床,大地构造位置处于勉略缝合带内,其产出主要受区域逆冲挤压断裂带控制。矿石类型可划分为蚀变岩型和石英脉型,以前者为主,主要矿石矿物为黄铁矿、毒砂、辉锑矿、自然金、银金矿等,围岩蚀变以硅化、绢云母化、绿泥石化、碳酸盐化为主。成矿过程可划分为黄铁矿-绢云母阶段(Ⅰ)、黄铁矿-毒砂-石英阶段(Ⅱ)、毒砂-黄铁矿-石英阶段(Ⅲ)、自然金-辉锑矿-石英阶段(Ⅳ)、方解石-石英阶段(Ⅴ)等5个阶段。不同阶段石英中均含有大量的流体包裹体,多数直径3~10μm,以富CO_2、含CO_2和水溶液3种类型为主,总体为中低温(110~330℃)、低盐度(0.62~9.34%Na Cl.eq)的流体。单个流体包裹体激光拉曼分析表明,流体以CO_2、H_2O为主,不同阶段含金石英脉石英δD变化范围为-83‰^-107‰,δ^(18)O变化范围为-0.31‰~11.95‰。从成矿早阶段到晚阶段,流体包裹体大小、CO_2含量、温度逐渐降低,盐度先升高后降低。成矿早阶段以下地壳的变质流体为主,而后经历了减压沸腾作用,压力降低,盐度增高,导致金大量沉淀。稳定同位素特征反映主成矿阶段以变质流体为主,成矿晚期发生低温流体的注入。总之,阳山金矿成矿流体经历了长期的演化过程,亦揭示金成矿作用的复杂性。

The Anba gold deposit,a super-large one of the Yangshan gold-ore zone discovered recently in the West Qinling orogenic belt,lies in the Mianlue suture,of which the occurrence is controlled by regional thrust faults. Its ores can be classified into alteration and quartz vein types,and the former is dominant. Major ore minerals include pyrite,arsenopyrite,antimonlte,native gold and electrum. Wall rock alteration types observed at the Yangshan ore field include silicification, sericitization, cbloritization and decarbonation. The mineralization process can be divided into the pyrite-sericite stage （I）, pyrite-arse- nopyrite-quartz stage （II）, arsenopyrite-pyrite-quartz stage （III） ,native gold-stibnite-quartz stage （IV） and the calcite-quartz stage （V）. Different stages of quartz contain plenty of fluid inclusions with three to ten ~m in size. Three main fluid inclusion types （rich-CO2 ,CO2 and aqueous） are recorded based on their morphology, number of phases at room temperature and their phase behavior on cooling. There are three types of fluid inclusions：overall as medium-low temperatures ranging from 110 to 330℃ ,low salinity with 0.62 to 9.34% NaC1. eq. Compositions of individual fluid inclusions by laser Raman spectroscopy are mainly COz and H20. Mass spectrometer analysis shows that SDs2o and 6Is OH2o of the fluid inclusions at different mineralization stages in auriferous quartz veins are - 83‰ to - 107‰ and - 3.08‰ to 0.17‰,respectively. The size, CO2 content,and homogenization temperature of fluid inclusions de- crease gradually,while salinity increases firstly and then decreases accompanying with the ore fluids from the early to the late stage. The ore fluids in the early stage are characterized by metamorphic fluid coming from the lower crust, and then experiencing decompression boiling leading to gold precipitation. Data of stable isotope also show the major ore-forming stage fluids were metamorphic in origin,and the late-stage fluids were injected by low temperature fluid. O- verall,the ore-forming fluids of the Anba deposit experienced a long-term evolvement process,indicative of the complexity of gold mineralization.