青海同仁双朋西金铜矿矿床地质特征及矿床成因

摘要

在翔实的野外工作基础上,对青海省同仁县双朋西金铜矿床矿石与围岩的微量元素、稀土元素、流体包裹体和同位素地球化学特征展开了系统研究.研究结果表明,金矿石和花岗闪长岩的稀土元素配分模式具有相似性,均为轻稀土元素富集型,具负铕异常,基本上无铈异常,暗示它们之间存在成因联系；成矿流体为中高温、低盐度、中等密度、中等压力流体,且以含碳质为特征；铅、硫同位素结果均反映出成矿物质主要来源于深源.据此认为区内矿化是与中高温岩浆热液作用有关的矽卡岩型金铜矿化,属于印支-燕山期同一构造-岩浆作用的产物.%The Shuangpengxi gold deposit, as one of the most important gold deposits in northwest China, is located in the Tongren ore concentration area of Qinghai Province. Gold mineralization occurs within skarn that lies between granodiorite and marble. The ore bodies are in lenticular and veinlike forms. Ores are mainly composed of chalcopyrite, pyrrhotine and pyrite. Gangue minerals comprise diopside, garnet, calcite and sericite. The main types of rock alterations are skarnization, silicification, carbonatization, brecciation and ferritization. The tectonic setting, material source and genesis of the ore deposit are discussed on the basis of trace elements, REE, fluid inclusions and isotope geochemistry of ores and wall rocks as well as characteristics of the ore deposit. Studies show that ores and granodiorite have similar chondrite-normalized REE patterns and are enriched in light REE with a moderate Eu negative anomaly but no Ce anomaly. Microthermornetric homogeniza-tion measurements of the fluid inclusions show that temperatures are from 215℃to 468℃ , corresponding salinities vary from 1.40% to 13.40% NaCleq, and densities vary from 0.55 g/cm3 to 0.85 g/cm3. The metallo-genic pressure is 2.25 Mpa-20.61 Mpa. And the ore- forming fluids belong to NaCl-KCl-CaCl2-H2O types, especially enriched in CO2, CH4 and C2H2. Sulfur isotopic compositions of sulfides from ores show that the values of S 34S from the early depositional ores range from 2.2%o to 7.0%o. The 208Pb/204Pb, 207Pb/204Pb and 206pb/204pb ratios of ores range from 3819 to 38_53i 15-581 to 15.641 and 18.058 to 18.710, respectively.According to the isotopic data, it is considered that the ore-forming materials were derived from the crust and theupper mantle. Studies show that the Shuangpengxi ore deposit might be a skarn gold deposit related to hypo-and mesothermal processes and belongs to the acid and intermediate-acid magmatic activities of Indo-Chinese and Yanshanian period.