Geological, Tectonic Evolution Characteristics and Uranium Mineralization of the Damara Orogenic Belt in Namibia
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摘要:
纳米比亚达马拉造山带是新元古代—早古生代泛非造山活动在西南非洲的体现,笔者系统梳理达马拉造山带内地质单元、岩浆作用、变质活动、构造动力学机制和铀矿成矿作用特征。该造山带主要由北部地体、北带边缘、北部带、中央带、南部带、南带边缘及南部前陆7个地质单元组成。依据板块运动特征,将其构造演化划分为板内裂谷期(750 Ma)、持续扩张期(730~600 Ma)、洋陆俯冲期(580~560 Ma)、俯冲碰撞期(550~540 Ma)及碰撞晚期(530~460 Ma)5个阶段。造山带内赋存大量的铀矿资源,主要形成于510~490 Ma,其成因与碰撞晚期构造及岩浆活动密切相关,成矿专属性特征明显。根据对现有资料的分析及总结,笔者认为富U的前达马拉基底是白岗岩型铀矿成矿物质的主要来源,成矿母岩浆是同化混染与分离结晶共同作用的结果,构造活动为富U岩浆的侵位及富集沉淀提供有利场所。
Abstract:The Damara orogeny in Namibia is part of the Neoproterozoic to early Paleozoic Pan–African orogeny in Southwest Africa. This paper systematically combs the characteristics of geological units, magmatism, metamorphism, tectonic dynamics mechanism and uranium mineralization in the Damara orogenic belt. The orogenic belt is mainly composed of seven geological units: the northern terrane, the northern margin, the northern zone, the central zone, the southern zone, the southern margin, and the southern foreland. Based on the characteristics of plate movement, the tectonic evolution of this orogenic belt has been divided into five stages, mainly including intraplate rift (750 Ma), continuous expansion (730~600 Ma), ocean–continent subduction (580~560 Ma), subduction collision (550~540 Ma) and late collision (530~460 Ma). This orogenic belt is endowed with plenty of uranium resources, mainly formed at 510~490 Ma, closely related to tectonic and magmatic activities in origin, with a peculiar metallogenic specialization. According to the analysis and summary of existing data, this article believes that the pre–Damara basement, which is rich in uranium, is the main source of ore–forming materials of the alaskaite type uranium deposit. The parent magma related to mineralization is the result of combined action of assimilation, contamination and fractional crystallization. The tectonic activity provides a favorable site for the emplacement, enrichment and precipitation of the uranium–rich magma.
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Keywords:
- Damara orogenic belt /
- tectonic evolution /
- uranium ore /
- Namibia
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铀矿作为战略性矿产资源,广泛应用于工业、国防、核电及医学等领域(Cuney et al.,2008;蔡煜琦等,2015;吴涛涛等,2018;耿涛等,2023)。全球铀矿资源分布不均,供需失衡较为严重,特别是中国铀矿资源禀赋较差。尽管中国铀矿的找矿潜力巨大,但短期内难以满足快速增长的工业需求,也使得中国铀矿对外依存度不断攀升(张金带等,2012;蔡煜琦等,2015;唐超等,2017;陈军强等,2021)。
非洲铀矿资源丰富,储量约占世界铀矿总储量的20%,其中纳米比亚是非洲最大的铀资源生产国,约占全球天然铀产量的10%(宋继叶等,2022;张晓等,2023;朱清等,2023)。与此同时,纳米比亚也是中国最大的海外铀资源来源地之一。近年来,国内外众多学者从矿床类型(Nex et al.,2001;Kinnaird et al.,2007;高阳等,2012;陈金勇等,2013,2017;顾大钊等,2016)、成矿时代(Nex,1997;Longridge et al.,2008;Freemantle, 2010;王生云,2013;陈金勇等, 2014)、铀矿物质来源(Nex et al.,2002;陈金勇等,2014;范洪海等,2015;黄冉笑等,2021,2022)等不同角度对纳米比亚铀矿进行研究,提出地层、构造及岩体控矿等多种成矿模式及找矿模型,在成矿规律和矿产分布方面取得诸多进展。然而,达马拉(Damara Belt)造山带演化过程与铀成矿作用尚未被系统分析,其区域铀成矿作用研究仍显不足。笔者通过对达马拉造山带的物质组成、岩浆作用、变质–变形、区域构造演化及典型矿床等方面内容进行梳理总结,进一步总结归纳达马拉造山带构造演化与铀成矿作用的关系,以期为中国地勘单位开展铀矿勘查提供基础地质资料与技术支持,服务于国际矿业产能合作及“一带一路”倡议。
1. 达马拉带的物质组成及内部构造单元
泛非造山作用(820~500 Ma)是地球演化史中的重要构造事件,大量板块和地体经聚合及增生作用形成了一系列大陆造山带(Coward,1983;Unrug,1992;Chakraborty et al.,2023),是冈瓦纳(Gondwana)大陆重要的组成部分之一。泛非(Pan–African)造山带横贯非洲大陆,自东向西主要包括莫桑比克带(Mozambique Belt)、赞比西带(Zambezi Belt)、卢弗里安弧形构造带(Lufilian Belt)和达马拉带等(Grantham et al.,2003,2019;Oriolo et al.,2017;孙宏伟等,2019,2021;Sun et al.,2021;许康康等,2021)。达马拉造山带位于非洲西南端,形成于新元古代—早古生代(650~460 Ma),是北部刚果(Congo)克拉通与南部喀拉哈里(Kalahari)克拉通碰撞的产物(图1a)( Coward,1983;宁福俊等,2018;Goscombe et al.,2018)。达马拉造山带主要是由陆内分支、Gariep带和Kaoko带3部分组成,其中Kaoko带向北延伸至安哥拉境内(Prave,1996;Poli et al.,2001)。在纳米比亚境内,达马拉造山带主要有2个分支,正北方向的海岸线分支和北东向的陆内分支(Kinnaird et al.,2007;Fan et al.,2017),文中主要介绍其陆内分支部分。
达马拉造山带(陆内分支)南北宽约为400 km。依据地层、断裂及主要线性构造、变质程度、岩浆活动、地质年代和航磁异常等特征的不同,该造山带自北向南划分为北部地体、北带边缘、北部带、中央带、南部带、南带边缘及南部前陆7个部分(图1b)( Martin et al.,1977;Corner,1983;Goscombe et al.,2005)。中央带以Omaruru断裂为界又分为北中央带和南中央带2部分,位于Omaruru断裂和Okahandja断裂之间的南中央带是纳米比亚铀矿床的主要分布区,包括Husab铀矿、Rössing铀矿及Valencia铀矿等大型铀矿床均处于该构造区块(宁福俊等,2018;Shanyengana et al.,2020;黄冉笑等,2022)。
北部地体主要以厚层碳酸盐岩和磨拉石建造构成;北带边缘和北部带被一逆断层分开,北带边缘岩性与北部地体相似,以碎屑沉积岩建造为主;北部带内发育较多的花岗质碱性岩浆岩(Henry et al.,1990)。中央带是整个构造带内岩浆活动最为发育的区域,以大量发育花岗质侵入岩和穹盆构造为显著特征(Stanistreet et al.,1991)。其中,北中央带主要以碎屑沉积岩、片岩、石英岩、片麻岩及花岗侵入岩为主,而南中央带则由高温–低压变质沉积岩类、广泛发育的花岗岩及伟晶岩侵入体、基底杂岩和北东向–北北东向穹窿组成(Downing et al.,1981;Miller,1983a)。南部带是指Okahandja断裂与Gomab断裂之间的区域,主要由新元古代的Nosib群和Swakop群碎屑岩及变质岩组成,局部发育少量火山岩、蒸发岩,并伴有少量铁镁质火山活动(Kukla et al.,1991)。南部边缘带是由航磁测量解译出的断裂或由线性构造带推测而来,主要发育一套硅质岩及碳酸盐岩(Corner,1983)。南部前陆由Naukluft推覆杂岩体以及上覆于喀拉哈里克拉通基底杂岩之上的碎屑沉积岩组成(Corner,1983)。
2. 区域地层
区域内地层主要由早期的Nosib群和晚期Swakop群组成,Nosib群不整合于Abbabis基底杂岩体之上(Nash et al.,1971;Coward,1983;Longridge et al.,2008;高阳等,2012)。Nosib群为裂谷–填充序列,由下部的Etusis组和上部的Khan组构成,Etusis组主要岩性为石英岩、长石砂岩及少量砾岩;Khan组主要由泥岩、钙质泥岩及碳酸盐岩组成。Swakop群主要为一套深水沉积序列,底部为Rössing组,岩性变化较大,包括大理岩、砂砾岩、石英岩、泥质片岩和董青片麻岩等均有出现(Coward,1983;Longridge et al.,2008);其上为Chuos组,由Sturtian期冰碛岩和条带状铁质建造共同组成(Hoffmann et al.,1996);Chuos组之上为Karibib组,由大理岩、钙质泥岩及少量泥质片岩组成;顶部为Kuiseb组,岩性以泥岩、泥质片岩为主(Hoffmann et al.,2004)。
3. 达马拉带的岩浆作用
达马拉造山带内岩浆活动十分发育,整体出露面积约为75000 km2,主要以发育大量中酸性侵入岩及少量基性岩脉为特征,其中90%以上为花岗岩,其余为花岗闪长岩类、钙碱性辉长岩及少量基性岩脉(Haack et al.,1982;Miller,1983a;Kisters et al.,2004)。
根据成岩时代及岩体特征,前人将达马拉造山带内岩体划分为5个侵入序列(赵希刚等,2015;刘晨阳,2016)。①Trakkopje序列(601±79 Ma),由花岗闪长岩、花岗岩、石英二长岩等组成的岩套,花岗岩体常以岩基形式产出,岩石多呈斑状结构,片麻理十分发育。②Hakskeen序列(516±23 Ma),主要由红色花岗岩组成,多分布在Rössing穹隆北部,以小岩体、岩脉和层状侵入体产出,岩石以等粒结构和红色为特征。③Gawib序列(500~490 Ma),主要由花岗岩、浅色花岗岩和伟晶岩组成,分布在Rössing穹隆以东地区,岩石多具斑状结构且黑云母含量较多,叶理构造较为发育。④Donkerhuk序列(458±25 Ma),主要分布在Rössing穹隆以东地区,岩性包括灰白色中粒黑云二长花岗岩和棕褐色含斑钾长花岗岩,常以大型岩基形式出现。⑤Rössing序列(542~468 Ma),主要以花岗岩、淡色花岗岩及花岗伟晶岩为主,岩石多具中粗粒结构或伟晶状花岗结构,地球化学特征显示其碱含量高,淡色花岗岩主要产出于背斜及穹隆构造附近,脉状侵入体则主要分布于Rössing穹隆区。
由于区内淡色花岗岩(又称白岗岩)中赋存大量铀矿资源,前人对其开展了大量研究,根据成岩时代、结构构造、矿物成分和矿化特征划分为至少6类(Nex,1997; Longridge et al.,2008;Freemantle,2010;王生云,2013;陈金勇,2014)。
A型:浅灰白色、淡粉色,细–中粒结构,以白色长石为主,副矿物很少,主要侵入于Khan组,LA–ICP–MS锆石U–Pb数据显示其成岩年龄为(547.4±3.6)Ma(王生云,2013)。
B型:白色,中粗粒结构或伟晶结构,含有石榴子石、电气石,以石榴子石为标志矿物,主要产出于Chuos组和Karibib组,LA–ICP–MS锆石U–Pb数据显示其成岩年龄为(537.8±4.3)Ma(王生云,2013)。
C型:白色、浅红色,中粗粒结构或伟晶结构,以含电气石和磁铁矿为标志,主要产出于Khan组和Etusis组,LA–ICP–MS锆石U–Pb数据显示其成岩年龄为(525.4±2.6)Ma(王生云,2013)。
D型:白色,中粗粒结构或伟晶结构,烟灰色石英十分发育,富铀矿物大量出现,分布最为广泛,主要侵入于Rössing组、Khan组、Karibib组和Kuiseb组,成岩年龄为(508±2)Ma(SHRIMP锆石U–Pb)(Briqueu et al.,1980;Longridge et al.,2008)~(497±5.5)Ma(LA–ICP–MS锆石U–Pb)(王生云,2013)。
E型:淡红色,细粒至伟晶状结构,可见烟灰色石英,以发育氧化晕圈为标志,圈外为粉红色,圈内为灰白色,主要产出于Khan组和Rössing组,其成岩年龄为(500±10)~(494±8)Ma(SHRIMP锆石U–Pb)(Jacobet al.,2000)。
F型:红色,粗粒至伟晶状结构,以发育红色巨晶钾长石、乳白色石英为主要特征,副矿物为磁铁矿,主要侵入于Etusis组和Khan组。对于其成岩时代存在不同认识,Jacob等(2000)认为其与E型花岗岩成岩时代基本一致,但陈金勇等(2014)获得的LA–ICP–MS锆石U–Pb数据显示其成岩年龄为(511.4±4.3)Ma,早于D型花岗岩。
达马拉期后岩浆侵入活动较弱,仅见一些粗玄岩和细晶岩沿断裂分布。此外,中生代冈瓦纳大陆的裂解和地幔柱活动期间达马拉带内发育少量基性岩脉,多切穿早期形成的白岗岩体,成岩时代与D型白岗岩受到后期热液改造作用的时间相近(陈金勇等,2014),加之基性岩浆在部分伟晶质岩浆演化过程中对铀矿化的富集作用(黄冉笑等,2022)。因此,认为基性岩浆的活动与D型白岗岩铀矿化富集有着密切的关系(范洪海等,2015;陈金勇等,2017;黄冉笑等,2022)。
4. 达马拉带的变质–变形过程
达马拉造山带内岩石经历的变质作用程度不同,主要以高温–低压变质活动为主,变质作用从西向东呈逐渐变低的趋势,一般为角闪岩相,在靠近大西洋沿岸的地区可以达到麻粒岩相(Hartmann et al.,1983; Masberg et al.,1992;Goscombe et al.,2004;Miller et al.,2008)。
达马拉造山带内发育多期次的褶皱、断裂、韧性剪切带以及穹窿构造,并且这些构造作用相互叠加,共同组成了现今较为复杂的构造面貌。对于区内的构造期次,目前争论较多,包括2期、3期和4期等不同划分(Jacob et al.,1974;Coward,1983;Miller,1983b;Oliver et al.,1994;Anderson et al.,1997;Poli et al.,2001;Kisters et al.,2004;Johnson et al.,2005;Ward,2009)。综合前人研究,笔者认为主要包括3期(D1、D2、D3)变形过程,并伴随大量的岩浆侵入活动(D2和 D3构造变形事件对白岗岩的侵位机制具有重要的影响)。D1期主要集中于580~560 Ma,以发育平行于层理的叶理和断裂构造、层内南东向的平卧褶皱及低角度逆断层为主要特征,部分黑云母和石英呈定向排列,石榴石和堇青石等矿物出现(Kasch,1983a,1983b;Miller,1983b,2008;Steven, 1993;Poli et al.,2001;Kisters et al.,2004);D2期主要集中于550~540 Ma,以发育北北东向的直立紧闭褶皱、平卧褶皱及低角度逆断层为主要特征,矽线石和堇青石出现,花岗质岩浆活动十分发育(Steven,1993;Poli et al.,2001;Miller,2008);D3期主要集中于535~500 Ma,以发育大规模开阔褶皱和大型南东向直立褶皱、北东走向穹隆、高角度逆断层和逆冲断层为主要特征,尖晶石和堇青石出现,并伴随大量花岗质伟晶岩的形成(Kasch,1983a,1983b;Kisters et al.,2004)。
由于区内覆盖严重且构造叠加活动强烈,很难在地表观测到线性构造断裂现象(主要依靠航空物探资料解译识别),但穹窿构造十分发育,也是达马拉造山带内的典型构造特征。关于穹窿构造的形成机制,存在多种解释,包括褶皱作用(Smith et al.,1961;Smith,1965;Coward,1983)、花岗质基底的底劈作用(Ramsberg,1972)及塑性地层的滑脱作用(Oliver,1994)等多种说法。尽管对于穹隆构造的形成机制认识不同,但均认为穹窿构造在富铀岩浆的运移和就位过程中发挥了重要作用( Jacob,1974;Coward,1983;Kasch,1983a,1983b;Miller, 1983a;Oliver,1994;Anderson et al.,1997;Poli et al.,2001;Kisters et al.,2004;Johnson,2005;Ward,2009)。
5. 达马拉带的区域构造演化史
达马拉造山带整个过程形成时代约为 750~460 Ma(Miller,2008),经历前造山期(以陆内裂谷和扩张作用为主)、造山期(以俯冲和陆陆碰撞作用为主)及造山后期,以喀拉哈里克拉通和刚果克拉通的碰撞而告终。前人对其演化过程进行研究(Martin et al.,1977;Martin,1983;Miller,2008;Anthonissen,2009),认为其主要经历了以下5个阶段(图2)。①早期(840~750 Ma)由于地幔柱活动导致板块处于伸展环境,逐渐形成多个裂谷系统(图2a),同期形成早期的裂谷火山–沉积岩系,包括碎屑沉积岩、双峰式火山岩及碱性侵入岩等。②随着拉张作用的进行(730~600 Ma),软流圈物质上涌,地壳底部发生部分熔融作用,上地壳大规模拗陷,接受巨厚的碎屑沉积,并伴有拉斑玄武岩的形成(图2b)。③580~560 Ma,岩石圈破裂后由于重力影响,洋壳下沉,开始发生由南向北的A型俯冲作用(图2c),下地壳发生部分熔融,并伴随早期的花岗质岩浆侵入活动(D1)。④随着俯冲作用的不断进行(550~540 Ma),喀拉哈里克拉通与刚果克拉通发生陆陆碰撞(图2d),在地表形成大量褶皱、断裂及逆冲推覆构造,导致早期岩石发生变质作用,同时有新的花岗质岩浆侵入(D2)。⑤530~460 Ma(图2e),进一步的挤压作用导致地壳加厚,达马拉构造带开始隆升,热效应增强,变质变形活动再次覆盖早期岩石并发育大量花岗质岩浆活动(D3)。与此同时,在结晶分异和同化混染作用的共同影响下,淡色花岗岩及花岗伟晶岩中发育大量富U矿物,并在有利部位形成了多处铀矿床。
6. 达马拉带内部铀矿化特征
达马拉造山带内主要有白岗岩型和钙结岩型2种类型的铀矿床,前者是达马拉造山期后的富U岩浆和热液在有利的部位叠加富集成矿,以Rössing铀矿、Husab铀矿等为代表(高阳等,2012;左立波等,2017;Shanyengana et al.,2020);后者是富铀岩石在长期的干旱气候条件下,经过风化剥蚀所形成,其分布也与白岗岩型铀矿密切相关,以Langer Heinrich铀矿、Trekkopje铀矿为代表(顾大钊等,2016;陈秀法等,2021)。区内铀矿床以规模大、品位低、露天开采闻名于世。达马拉造山带内白岗岩型铀矿的控矿因素较多且特征十分明显,其中构造、岩浆岩和地层为主要的控矿要素,后期热液活动导致的热液叠加作用,对其成矿作用影响也十分显著(高阳等,2012;王生云,2013;陈金勇等,2014;范洪海等,2015;左立波等,2017;Shanyengana et al.,2020;黄冉笑等,2021)。
6.1 控矿要素
达马拉造山带内的铀矿床主要分布于穹窿构造边缘及北东–南西断裂的两侧,构造活动与铀矿化分布密切相关。构造运动中形成的断裂破碎带、构造转向处、剪切拖曳带、地层薄弱处、脆性–韧性剪切带及穹窿构造等区域为岩浆迁移和花岗岩侵位提供了重要通道,尤其是晚期构造运动中,对早期构造变质变形进行叠加,是最为有利的控矿构造(Miller,1983a; Oliver,1994;Anderson et al.,1997;Poli et al.,2001;Kisters et al.,2004;Ward,2009)。
达马拉造山带内的铀矿化与白岗岩密切相关,成矿期主要集中在造山运动晚期及后期。因此,碰撞造山期后(D3)产生的D、E型白岗岩对铀矿化具有直接指示意义,而其他白岗岩,尤其是俯冲碰撞期以前的花岗岩则未发生铀矿化,可见铀矿化岩体具有一定专属性(表1)(陈金勇,2014)。
表 1 达马拉造山带内主要铀矿产出层位特征表Table 1. Characteristics of main uranium mineralization horizons in Damara orogenic belt铀矿床 矿化白岗岩产出层位 白岗岩类型 矿化白岗岩类型 Rössing Khan组与Rössing组接触带及其组内 C~E D Husab Khan组与Rössing组接触带及Rössing组内部,少量分布于Chuos组内部 A~F D、E Etango Etusis组与Khan组及Khan组与Rössing组接触带 A~F D、E Hildenhof Khan组与Chuos组及Khan组与Rössing组接触带;Khan组及Rössing组内部 C~F D、E Ida Dome Khan组与Rössing组接触带及其各自组内 A~E D、E Holland’s Dome Khan组与Rössing组接触带,Khan组内部 C~E D、E Valencia Khan组与Rössing组及Karibib组与Kuiseb组接触带 A~F D、E Goanikontes Etusis组与Khan组,Khan组与Rössing组接触带,Khan组内部 B~F D、E 达马拉造山带内的铀矿床中赋矿白岗岩主要侵入于Khan组与Rössing组或Khan组与Chuos组的接触带(表1)(Nex,1997;Freemantle,2010;陈金勇等,2014,2017;黄冉笑等,2021),铀矿化受到地层控制特征明显。这主要是由于地层接触部位为构造薄弱处,利于岩体侵位及就位,同时富碳酸盐岩及大理岩地层可能在岩浆烘烤下发生脱碳效应,促进U元素的富集沉淀(陈金勇等,2017)并最终成矿。
6.2 流体特征
流体包裹体研究显示,达马拉造山带内白岗岩中主要存在2期成矿流体,早期为高温低盐度热液,温度大多为470~530 ℃,盐度ω为3.55~9.60 wt%NaCl(均值为6.14 wt%NaCl),属岩浆晚期(主成矿期)热液。晚期为中–低温低盐度热液,温度集中于150~220 ℃,盐度ω为4.65~19.05 wt%NaCl(均值为11.5 wt%NaCl),为后期(叠加改造期)的热液(陈金勇,2014;范洪海等,2015)。
6.3 成矿物质来源
对于达马拉造山带内铀矿化的成矿物质来源存在较大争议,早期认为其成矿源岩为变沉积岩,即富U沉积岩在变质改造过程中导致U富集沉淀成矿(Smith,1965;Barnes et al.,1978)。后期随着研究的深入,对白岗岩为成矿源岩逐步达成共识(Marlow,1981;Brynard et al.,1988;Nex et al.,2001),但对于白岗岩的成因仍存在一定争议。Brynard等(1988)认为矿化白岗岩是红色花岗岩熔融的产物,是早期未发生熔体抽离的富U基底在深部重熔而形成的。Nex 等(2001)和陈金勇等(2014)则认为富U的前达马拉基底是白岗岩型铀矿主成矿期的成矿物质来源。黄冉笑等(2021)通过对E型伟晶岩矿物组成和化学性质演化规律的研究认为成矿花岗伟晶岩是同化混染与分离结晶共同作用的结果,并推测岩浆演化过程中基性组分(FeO、MgO、TiO2)的混入直接影响到相关U元素的沉淀富集。陈金勇等(2017)初步判断后期热液中U主要来源于原生铀矿物的再分配。
6.4 成矿模式
基于对Rössing铀矿、Husab铀矿及Valencia铀矿等典型铀矿床的研究,前人依据控矿因素的不同提出多种达马拉造山带铀矿的成矿模式,早期主要包括3种。①地层成因说,依据铀矿化主要分布在Rössing组与Khan组或Khan组与Chuos组接触界线附近(Jacob,1974;Marlow,1981)。②构造成因说,依据铀矿化主要产于断裂构造两侧、穹隆构造边缘或穹隆的转折部位等(Kinnaird et al.,2007;高阳等,2012;陈金勇等,2013)。③岩浆成因说,依据铀矿主要产出于白岗岩体附近或白岗岩体即为矿体(Berning et al.,1976;Nex et al.,2001;陈金勇,2013)。由于不同矿床所体现出的主要控矿特征不尽相同,其成因模式亦存在较大差异,但综合来看达马拉造山带内铀矿的形成更可能是岩浆–构造–地层等多重因素共同耦合的结果(高阳等,2012;陈金勇等,2013;Corvino et al.,2013;黄冉笑等,2022),前达马拉变质基底提供铀物质来源;地层与构造多因素控矿,多期次构造事件形成的断裂和穹窿分别为含U岩浆的运移和结晶沉淀创造了通道和空间,变沉积层内不同地层间氧化/还原属性的转变为岩浆内U元素的结晶沉淀和富集提供有利条件;后期热液作用导致早期铀矿体发生活化运移,在断裂破碎带重新富集成矿(图3)。
图 3 达马拉造山带铀矿成因模式图(据Corvino et al.,2013修改)Figure 3. Genetic model of uranium deposits in the Damara orogenic belt基于以上认识,前人进一步细化其成因模式(Corvino et al.,2013;陈金勇等,2014;范洪海等,2015;孙宏伟等,2020;黄冉笑等,2022),即在达马拉碰撞造山过程中由前达马拉基底和达马拉变沉积岩发生部分熔融首先形成斑状花岗岩、黑云母花岗岩和A、B、C、F型白岗岩(550~540 Ma)。随着软流圈的上涌(510~500 Ma),由前达马拉富U基底重熔形成的岩浆(也受分离结晶作用影响),沿断裂向上运移,侵入并就位于Rössing组与Khan组或Khan组与Chuos组接触部位(D、E型富U白岗岩),这些富含还原物质和大理岩的地层形成有效的氧化/还原障,导致U元素的富集沉淀,也是区内铀矿化的主要时期。约150 Ma,由于热液活动导致部分U发生活化运移,在断裂破碎带等有利部位富集形成沥青铀矿、脉状铀石等铀矿物。对于钙结岩型铀矿,则主要是由于新生代以来地壳抬升,富U地层和白岗岩体遭受风化剥蚀,并受地表水淋滤,沉淀固结形成硅钙铀矿、钒钾铀矿等次生铀矿物。
7. 典型矿床
7.1 Huseb铀矿
Husab铀矿位于纳米比亚共和国中西部Erongo行政区Swakopmund市东约50 km处,为中广核集团控股的海外大型铀矿床,铀资源量可达30万t(以U3O8计)(荣建锋等,2016;张怀峰等,2018)。
矿区内地层主要为Nosib群Khan组,Swakop群Rössing组、Chuos组、Karibib组和Kuiseb组(图4)。矿区内侵入岩主要以白岗岩为主,主要发育B、C、D、E和F型白岗岩(Nex,1997)。矿化岩体主要侵入Khan组与Rössing组不整合接触带及其上部的Rössing组内,少量侵入Chuos组内。矿体主要产出于D和E型白岗岩内部,锆石U–Pb数据显示其成岩成矿时代为(496±4.1)Ma(Cross et al.,2009)。
图 4 Husab铀矿Ⅰ号矿体剖面示意图(据荣建锋等,2016修改)Figure 4. Geological section of Zone 1 in Huseb uranium deposit矿体主要赋存于背斜转折端,呈似层状、脉状或透镜状,延伸方向大致与地层的走向或层理平行,没有明显的变形特征(荣建锋等,2016;张怀峰等,2018)。矿化蚀变主要包括高岭土化、蛇纹石化、绢云母化和绿泥石化等(刘晨阳,2016)。
Husab铀矿中铀主要呈独立铀矿物形式存在,铀矿物形式为铀的氧化物、铀钛氧化物及铀硅酸盐等,少量以类质同象的形式存在于钍矿物内。矿区内主要原生矿石矿物为晶质铀矿,其次为少量的钍铀矿、钛铀矿和铌钛铀矿,后期热液作用形成的矿石矿物主要为铀石,其次为硅钙铀矿和黄硅钾铀矿(Freemantle,2010;黄冉笑等,2021,2022)。
Husab铀矿为典型的白岗岩型铀矿床,后期遭受不同程度的热液叠加成矿和热液蚀变,是达马拉造山期内花岗质岩浆同化混染与分离结晶作用的产物(Freemantle,2010;黄冉笑等,2021,2022)。
7.2 Rössing铀矿
Rössing铀矿是世界上规模最大、开采时间最长(自1975年至今)的露天铀矿床之一,矿床位于纳米布沙漠中,在Swakopmund市北东约60 km处(韩军等,2021)。Rössing铀矿床具有储量大(2.81万t)、品位低(平均品位为0.03%)、可露采等特征(Berning et al.,1976;张晓康等,2015)。
Rössing铀矿床位于Rössing穹窿南部,矿区出露地层主要为Rössing组、Khan组、Etusis组(图5)。矿区内侵入岩主要以白岗岩为主,主要发育A~F型白岗岩。原生铀矿化和大部分次生矿化都集中产于D型白岗岩中,其成岩成矿年龄为(510±3)Ma(Basson et al.,2004)。白岗岩脉(矿体)宽度由几厘米到90 m不等,呈脉状或不规则的透镜状。矿化蚀变主要包括硅化、绢云母化、黄铁矿化、绿泥石化、伊利石化和高岭土化等(韩军等,2021)
图 5 Rössing铀矿地质图(据Berning et al.,1976修改)Figure 5. The geological map of Rössing mine矿区内铀的赋存形式以独立铀矿物为主,其中晶质铀矿、钍铀矿和钛铀矿等原生铀矿物约占69%,多为全自形或半自行晶粒状结构,浸染状构造;反应边状铀石、铀钍石、沥青铀矿、钒钾铀矿和硅钙铀矿等次生铀矿物约占31%,多呈隐晶结构、交代残余结构,脉状构造(Abrahams,2009)。
Rössing铀矿为白岗岩型铀矿床的典型代表,是在达马拉造山期岩浆形成演化阶段,由结晶分异作用形成铀预富集,而后在造山期后韧性变形作用下白岗岩再次重熔富集成矿(Corvino et al.,2013;韩军等,2021)。
8. 结论
(1)达马拉造山带形成于750~460 Ma,主要经历板内裂谷期(750 Ma)、持续扩张期(730~600 Ma)、洋陆俯冲期(580~560 Ma)、俯冲碰撞期(550~540 Ma)及碰撞晚期(530~460 Ma)5个阶段。
(2)达马拉造山带内的铀矿化与白岗岩密切相关,且成矿专属性特征明显。俯冲碰撞期以前的白岗岩则未发生铀矿化,而碰撞造山期后(D3)产生的D、E型白岗岩对铀矿化具有直接指示意义。
(3)达马拉造山带内原生铀矿化主要形成于510~490 Ma,富U的前达马拉基底是白岗岩型铀矿主成矿期的成矿物质来源,成矿母岩浆是同化混染与分离结晶共同作用的结果,并适当混入中基性组分,在构造薄弱处富集沉淀成矿。
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图 1 纳米比亚达马拉造山带地质特征图(据Miller,1983b;Osterhus et al.,2014修改)
Figure 1. Geological map of the Damara orogeny in Namibia
图 2 达马拉造山带形成过程示意图(据Miller,2008;Anthonissen,2009修改)
Figure 2. The formation process of Damara orogenic belt
图 3 达马拉造山带铀矿成因模式图(据Corvino et al.,2013修改)
Figure 3. Genetic model of uranium deposits in the Damara orogenic belt
图 4 Husab铀矿Ⅰ号矿体剖面示意图(据荣建锋等,2016修改)
Figure 4. Geological section of Zone 1 in Huseb uranium deposit
图 5 Rössing铀矿地质图(据Berning et al.,1976修改)
Figure 5. The geological map of Rössing mine
表 1 达马拉造山带内主要铀矿产出层位特征表
Table 1 Characteristics of main uranium mineralization horizons in Damara orogenic belt
铀矿床 矿化白岗岩产出层位 白岗岩类型 矿化白岗岩类型 Rössing Khan组与Rössing组接触带及其组内 C~E D Husab Khan组与Rössing组接触带及Rössing组内部,少量分布于Chuos组内部 A~F D、E Etango Etusis组与Khan组及Khan组与Rössing组接触带 A~F D、E Hildenhof Khan组与Chuos组及Khan组与Rössing组接触带;Khan组及Rössing组内部 C~F D、E Ida Dome Khan组与Rössing组接触带及其各自组内 A~E D、E Holland’s Dome Khan组与Rössing组接触带,Khan组内部 C~E D、E Valencia Khan组与Rössing组及Karibib组与Kuiseb组接触带 A~F D、E Goanikontes Etusis组与Khan组,Khan组与Rössing组接触带,Khan组内部 B~F D、E -
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