摘要
位于滇西兰坪盆地西缘的茅草坪矿床是近年来新发现的一个脉状铜矿床,与该区其他脉状铜矿不同,它发育大量热液电气石。本文对矿床电气石的发育特征和化学成分进行了研究,以此探讨电气石生长和成分的控制因素、成矿流体来源及成矿过程中氧逸度的变化。茅草坪矿床铜矿体由石英-碳酸盐-硫化物脉及伴生的热液蚀变晕组成。电气石可分为两类:较早沉淀的蚀变晕电气石(T1)和较晚沉淀的脉体内电气石(T2)。T1电气石与石英共生,出现在变碎屑岩(T1-Q)和大理岩(T1-M)中,矿物强烈定向生长,长轴平行于围岩的剪切面理,颗粒相对细小,此阶段无含铜硫化物生成;T2电气石伴有黄铜矿生成,矿物长轴多与围岩的剪切面理斜交、呈弱定向生长,或不定向地生长,矿物颗粒相对粗大。由此反映在矿床热液矿化过程中,控制热液矿物生长的构造应力由强剪切到弱剪切及剪切停止的趋势变化。电子探针分析结果表明,所有电气石均属于碱性族,主体为镁电气石。T2电气石在背散射图像下存在明暗环带,与Ti、Fe、Mg含量的差异有关,也反映该期成矿流体的物理化学条件呈波动变化。茅草坪电气石的化学组成主要受流体成分控制,但T1-Q电气石相对富Al,T1-M电气石相对富Ca,表明围岩对蚀变晕电气石成分有一定影响。矿床以镁电气石为主的特征,暗示成矿流体不可能直接来自未经历水-岩相互作用的壳源花岗岩岩浆水。T2电气石较T1电气石具有高的Fe^(3+)/(Fe^(3+)+Fe^(2+))值,表明矿床成矿流体从早期到晚期可能是一个氧逸度升高的过程。结合区域脉状铜矿流体特征、金属组合、围岩和蚀变特征,推测茅草坪矿床及滇西脉状铜矿床迁移Cu的流体可能是CO_2缓冲p H值的还原性流体,在矿化部位流体氧逸度升高很可能是含铜硫化物沉淀的重要机制之一。
bonate-sulfide veins and associated Cu-barren hydrothermal alteration halos. Two types of tourmaline have been identified： （i） tourmaline in the alteration halos （T1）, which was formed relatively early; （2） tourmaline in the veins （T2）, which was formed relatively late. T1 tourmaline is anhedral, fine-grained, and aligned parallel to the mylonitic foliation d host rocks. It can he divided into two subtypes： metaclastic-hosted tourmaline （T1-Q） and marble-hosted tourmaline （T1-M）. T2 tourmaline is euhedral, coarse-grained, and sub-parallel to the my- lonitic foliation of host rocks or grows along or adjacent to the boundaries as comb tourmaline aggregates. The growth characteristics of T1 and T2 tourmaline suggest that the shear stress during the formation of tourmaline at Maocaoping was evolving from strong shearing through weak shearing to no shearing. Electron microprobe analyses of the tourmaline at Maocaoping show that all the tourmaline belongs to alkali group and dravite-schorl series, with dravite being dominant. T2 tourmaline has oscillatory zoning in back-scattered electron （BSE） images, suggesting their growth in an environment where physical and chemical properties fluctuated. The composition of tourmaline at Maocaoping is mainly controlled by the composition of the fluid phase. However, T1-Q tourmaline is slightly enriched in Al, and T1-M tourmaline is slightly enriched in Ca, suggesting that their compositions were slightly influenced by their host rocks, respectively. Tourmaline in Maocaoping is mainly dravitic and falls in 4, 5, 6 regions of the Al-Fe-Mg diagram, implying that the ore-forming fluid could not be derived from magmatic water of S-type granite with no interaction with the pathway rocks of fluid. The significantly higher Fe^3＋ /（Fe^3 ＋ ＋ Fe^2 ＋ ） ratios in T2 tourmaline than in T1 tourmaline imply that the pre-ore stage had rela- tively low oxygen fugacity than the ore stage. These features, combined with the characteristics of ore-forming fluid and alteration, metal assemblages, and the litbology of host rocks in the other vein Cu deposits in western Lanping Basin, suggest that a reduced, CO2-buffered pH value fluid （reduced sulfur〉oxidized sulfur） was re- sponsible for Cu transportation in the vein Cu deposits, and increasing oxygen fugacity of fluid might have played an important role in the Cu precipitation.
出处
《岩石矿物学杂志》
CAS
CSCD
北大核心
2016年第1期124-138,共15页
Acta Petrologica et Mineralogica
基金
国家自然科学基金资助项目(41273050
41373049
41320104004)
中国地质调查局地质调查项目(1212011220908)~~
关键词
电气石
成分
流体
脉状铜矿床
茅草坪
tourmaline
chemical composition
fluid
vein Cu deposit
Maocaoping
