投稿时间：2013-03-29
修订日期：2015-01-27
网络发布日期：2015-05-06
中文摘要:东非的肯尼亚-坦桑尼亚的宝石级铬钒钙铝榴石(又称察沃石), 均产于新元古代莫桑比克变质带的钙质硅酸盐石墨片麻岩中, 形成于泛非造山运动(650~500 Ma)过程中。察沃石原生矿床主要为区域变质矿床(或变成矿床), 其次为接触交代(矽卡岩)矿床, 而次生矿床为沉积砂矿床。在察沃石形成必须具备的5个控制因素:地层、岩性、有用的致色离子、高级变质作用和构造中, 地层(特别是石墨片麻岩地层)和岩性(特别是蒸发岩)是含察沃石结核形成的关键控矿因素, 而构造(特别是大地构造作用)则是矿化石英脉形成的关键控矿因素。Feneyrol等(2013)将察沃石矿床划分为3个类型8个亚型:Ⅰ型-结核(包括:NI亚型, 由初始硬石膏-石膏凝结而成;NII亚型, 由初始重晶石凝结而成)、Ⅱ型-石英脉(包括:ⅡA亚型, 位于转折端;ⅡB亚型, 位于鞍部;ⅡC亚型, 位于交代区)和Ⅲ型-砂矿(包括:ⅢA亚型, 残积砂矿;ⅢB亚型, 坡积砂矿;ⅢC亚型, 冲积砂矿)。东非察沃石矿床的成因过程可以概括为4期: ① 变质原岩沉积期(900~700 Ma), 在"莫桑比克洋"的海岸萨布哈环境中, 形成了由蒸发岩矿物和有机质构成的变质沉积岩的原岩;② 区域变质变形成矿期(700 ~ 500 Ma), 为主成矿期, 这些沉积岩发生了角闪岩相或麻粒岩相变质作用, 伴随着变质沉积地台发生韧性剪切作用, 形成了多相变质热液成矿系统, 主要发育结核状和石英脉状两种矿化作用;③ 接触交代(矽卡岩)成矿期(500~450 Ma), 东非造山运动晚期有超镁铁质岩浆侵入, 辉石岩与碳酸盐类围岩接触带发生矽卡岩矿化作用;④ 机械沉积分异成矿期, 原生矿床经风化、搬运、聚集成砂矿。对比研究表明, 在中国四川和新疆的矽卡岩体中有望找到具有经济价值的察沃石矿床。
Abstract:Tsavorites in East Africa, including the gem-quality vanadian (±chromian) grossular in Kenya and Tanzania, are hosted in calc-silicate graphitic gneisses along the Neoproterozoic Mozambique metamorphic belt. Most of the primary tsavorite deposits belong to metamorphogenic deposits formed during the Pan-African orogeny (650~500 Ma), with some belonging to skarn deposits. Tsavorite occurs either as nodules (Type Ⅰ, including two sub-types: Type NI-initially gypsum and anhydrite concretions and Type NII-initially barite concretions), or in quartz veins (Type Ⅱ, including three sub-types: Type ⅡA-located at the hinges of the sheared isoclinal folds, Type ⅡB-located within "saddle reef" structures and Type ⅡC-located within metasomatic zones), or in placers (Type Ⅲ, including three sub-types: Type ⅢA-eluvial deposits, Type ⅢB-colluvial deposits and Type ⅢC-alluvial deposits) (Feneyrol et al., 2013). There are five factors controlling the formation of tsavorites (Pohl et al., 1978), i.e., regional lithostratigraphy, lithology, chromophorous elements, high-grade metamorphism and structure. The nodules and quartz veins of primary tsavorite deposits are controlled by lithostratigraphy (especially graphitic gneiss) - lithology (especially evaporite) and structure (especially tectonism), respectively (Feneyrol et al., 2013). The genetic model of the tsavorite deposits in East Africa consists of four stages as follows: ① depositional stage of the metamorphic protoliths at 900~700 Ma, forming the protoliths of meta-sedimentary rocks composed of evaporite minerals and organic matter in a marine coastal sabkha environment of the Mozambique Ocean; ② main mineralization phase of regional metamorphism - deformation at 700~500 Ma: these sedimentary rocks were metamorphosed into the amphibolite or granulite facies, accompanied by deformation of the metasedimentary platform units with ductile shearing during the Pan-African orogenesis, which led to the formation of multiphase metamorphic hydrothermal metallogenic system, with the development of tsavorites in primary deposits either in nodule (type Ⅰ) or in quartz vein (type Ⅱ); ③ mineralization phase of the contact metasomatism (skarn) at 500~450 Ma, during which ultramafic intrusions occurred at the late stage of East African orogeny, leading to skarn mineralization along the contact between clinopyroxenite and carbonate-wall rock; ④ mineralization phase of physical-sedimentational differentiation, during which the primary deposits experienced weathering, transportation and aggregation to form the placer (type Ⅲ). A comparative study shows that the tsavorite (gem-quality chromian grossular) deposits could be found in the skarn zones in Sichuan and Xinjiang of China.
keywords:geology geological characteristics ore-controlling factor metamorphic skarn genesis tsavorite East Africa
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中图分类号：P619.24+4
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基金项目:本文为地质遗迹标本采集与购置项目(1211311381001)和地质矿产调查评价项目(1212011220038)联合资助成果
引用文本：
吕林素,彭艳菊,李宏博,周振华,毛冰,高芯蕊,杨森林.2015.东非宝石级铬钒钙铝榴石(察沃石)矿床成因模式探讨[J].矿床地质,34(2):404~422LÜ LinSu,PENG YanJü,LI HongBo,ZHOU ZhenHua,MAO Bing,GAO XinRui,YANG SenLin.2015.A tentative discussion on genetic model of gem-quality vanadian-chromian grossular (tsavorite) deposits in East Africa[J].Mineral Deposits34(2):404~422
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