安徽庐枞盆地酸性蚀变岩帽地质地球化学特征研究

酸性蚀变岩帽是浅成低温热液系统演化的产物,形成于酸性高氧化性流体的化学条件下;在高硫化型浅成低温热液金矿床中广泛发育,是该类矿床的显著识别特征。通过对酸性蚀变岩帽的野外地质特征、矿物共生组合和地球化学特征研究,能较好阐明浅成低温成矿热液系统的特征、性质、发生和发展演化及成矿作用过程。庐枞矿集区是长江中下游成矿带重要的矿集区之一,盆地内广泛发育以明矾石为特征蚀变矿物的酸性蚀变岩帽,面积超过30km~2,指示盆地内高硫化浅成低温热液系统的存在。目前为止,前期工作主要针对明矾石矿床地质特征和明矾石资源储量进行,该酸性蚀变岩帽的地质地球化学特征研究尚未开展。本次工作通过对酸性蚀变岩帽系统的野外采样、全岩地球化学分析和短波红外光谱测试分析技术(PNIRS测试)分析,确定其主要赋存在砖桥组火山岩中,组成矿物为石英、明矾石、高岭石、地开石,此外有少量绢云母、伊利石、珍珠陶土、叶蜡石、褐铁矿,极少数的叶腊石和黄钾铁矾等,在钻孔深部存在浸染状和半自形粒状黄铁矿。由于受到地表风化剥蚀和不同热热中心的影响,水平方向从矾山明矾石矿床向外围发育石英+明矾石带、石英+高岭石/地开石+明矾石带、石英+高岭石/地开石带、硅化带以及最外围的泥质带即高岭石±绢云母±伊利石带。根据酸性蚀变岩帽的矿物组合和主量元素特征,可将其分为三类:硅质蚀变岩、明矾石蚀变岩和粘土蚀变岩。硅质蚀变岩中SiO_2含量发生明显的富集作用,其余主量元素(K_2O、Na_2O、Al_2O_3、Fe_2O_3、P_2O5)含量显著降低;明矾石蚀变岩和粘土蚀变岩具有相似的地球化学特征,SiO_2、Al_2O_3、Fe_2O_3、P_2O_5元素含量范围变大,K_2O和Na_2O含量降低,且Na_2O降低更加明显;而钛为不活泼元素,在岩石发生蚀变过程中TiO_2含量变化很小。矾山地区的酸性蚀变岩帽的产状、蚀变类型、地球化学特征受构造和地层的双重控制。

Geological and geochemical characteristics of lithocap in the Lu-Zong Basin, Anhui, China

Lithocap is widespread in high-sulfidation epithermal gold deposits, and have obvious features to explore this deposit. Through studying geology, mineral assemblages and geochemistry of lithocap, it''s better to understand the characteristics, developing and mineralization procedures of hydrothermal fluids. The Lujiang-Zongyang ore cluster is an important part of the Middle-Lower Yangtze mineralized belt. Lithocap represented by alunite is widespread in Lu-Zong Basin. The host lithocap has an area of >30km², indicating Lu-Zong Basin maybe exist high-sulfidation epithermal systems. All previous studies in the area concentrated on the alunite deposit geology and alunite mineral reserves, geochemical characteristics need further research. Our study aims to determine the mineral assemblages present in the lithocap, and their geochemical characteristics. Lithocap is located in Zhuanqiao Formation, associated minerals are quartz, kaolinite, dickite, minor sericite, illite and nacrite, jarosite and hematite, and few pyrite. The alunite and associated clay minerals are zoned horizontally away from Fanshan, assemblages are quartz+alunite, quartz+kaolinite/dickite+alunite, quartz+kaolinite/dickite, and kaolinite ±sericite ±illite outwards from the Fanshan district. Based on whole rock and SWIR analysis, lithocap can be divided into 3 types:siliceous altered rocks, alunite altered rocks and clay altered rocks. Except for SiO₂, other major elements (K₂O, Na₂O, Al₂O₃, Fe₂O₃, TiO₂ and P₂O₅) decrease in the siliceous rocks as a result of acid leaching. Other altered rocks show similar geochemical characteristics, but the contents of SiO₂, Al₂O₃, Fe₂O₃ and P₂O₅ have a wider range; K₂O and Na₂O decrease, particularly Na₂O, whereas TiO₂ contents are mostly unchanged. Lithocap in Fanshan district is located in caldera, controlled by both volcanic structures and strata.