南非主要成矿区带的划分及成矿特征
2015-12-12吴兴源刘晓阳任军平何胜飞龚鹏辉
吴兴源,刘晓阳,王 杰,任军平,何胜飞,龚鹏辉,刘 宇
(中国地质调查局天津地质调查中心,天津 300170)
0 引言
南非地处非洲大陆最南端,矿产资源丰富,成矿类型多样,其资源总量占非洲的50%,居全球第5位。目前,南非境内已发现矿产达60多种,优势矿种多。其中,铂族金属、金、红柱石和萤石储量居世界第一位,分别占世界的88.7%,12.8%,17.8%,37%;锰、铬、锆、蛭石储量居世界第二位,分别占24.1%,37.1%,25%和35.9%;钒和磷酸盐储量居世界第三位,分别占23.1%和9.4%;金刚石和钛金属储量居世界第四位,分别占12.1%和9.8%[1];煤炭、铁、铅、铀、锑与镍矿等资源储量也均位于世界前列,但较缺乏石油和天然气。
南非拥有许多世界级大型矿床,如威特沃特斯兰德盆地金-铀矿床、德兰士瓦盆地铁-锰-石棉矿床、布什维尔德杂岩体铂族元素矿床、帕拉博拉杂岩体铜-磷灰石-蛭石矿床等,均为著名的超大型矿床[1]。作者在研究南非的区域地质背景与相关矿床成因规律后,发现不同矿床与其所处的构造背景能大致相联系,即不同的构造背景对应不同的成矿专属性。为此,本文提出了南非成矿区带的划分方案,概略介绍了每个成矿带内的一些典型矿床特征,以期对下一步的找矿和资源潜力分析提供一定的借鉴。
1 构造背景
南非是世界上少数几个保存有完好太古宙克拉通的地区之一,地质演化历史复杂,其构造单元包括卡普瓦尔太古宙克拉通和一系列元古宙—古生代造山带,以及一个代表弧后前陆沉积系统的卡鲁盆地。
1.1 卡普瓦尔克拉通
卡普瓦尔克拉通由一系列中-低级变质的太古宙地块拼贴而成,其形成及演化过程包括早期陆核形成阶段(3 100~3 700Ma)和晚期克拉通化阶段(2 600~3 100Ma)[2-4]。该克拉通内部以科尔斯伯格磁性线新状构造(Colesberg manetic Lineament)、塔巴津比—默奇森线性构造(Thabazimbi-Murchison Lineament)和Inyoka断裂为界进一步划分出4个次一级地块:斯威士兰(Swaziland)地块、兰德(Witwatersrand)地块、彼得斯堡(Pietersburg)地块和金伯利(Kimberly)地块[5](图1)。
图1 南非大地构造单元划分图(据文献[5]修改)Fig.1 Geotectonic division of South Africa
1.1.1 斯威士兰地块
斯威士兰地块代表了卡普瓦尔克拉通的陆核部分,出露有巴伯顿(Barberton)绿岩带及南非最古老的岩石单元——古老片麻杂岩(Ancient Gneiss Complex)。
古老片麻杂岩(AGC)的主体由古太古代—中太古代的英云闪长岩-奥长花岗岩-花岗闪长岩质片麻岩(TTG岩系)构成,夹一些角闪岩(变质基性火山岩,原岩包括玄武质科马提岩、富镁/富铁拉斑玄武岩等)及少量的绿岩带残留体,并被大量后期的花岗岩侵入[6-8]。姆普鲁茨(Mpuluzi)岩基与皮格斯皮克(Piggs Peak)岩基(侵位时代约3 100Ma)作为AGC的北部边界,也被视为与巴伯顿绿岩带的天然分界线[9]。AGC内部最重要的地质单元为恩格瓦尼(Ngwane)片麻岩[10],形成时代3 200~3 660 Ma,大部分遭受高角闪岩相变质,部分达到麻粒岩相[11-12]。此外,在产自Phophonyane内露层(Inlier)或Phophonyane剪切带的条带状英云闪长-奥长花岗质片麻岩(恩格瓦尼片麻岩)中获得的卡普瓦尔克拉通最古老的锆石Pb-Pb年龄为3 640~3 660 Ma[13-15]。
巴伯顿绿岩带(BGB)主要由绿片岩相变质(局部达角闪岩相)的火山-沉积地层组成,内部以Inyoka断层为界,分为南北2部分。该区因发现科马提岩而闻名,得到地质界的持续关注[16-18]。巴伯顿绿岩带的形成时代为3 200~3 530Ma,从下到上可分为3个群:昂弗瓦特(Onverwacht)群、无花果树(Fig Tree)群和摩德斯(Moodies)群[19]。
1.1.2 兰德地块
因兰德盆地中盛产砾岩型金矿而驰名世界,地块的大部分区域被中太古代—古元古代的变质火山-沉积地层所覆盖。老的基底物质只在局部出露,约翰内斯堡(Johannesburg)穹窿和弗里德堡(Vredefort)穹窿是为数不多的太古宙花岗片麻岩露头。
(1)约翰内斯堡穹窿。出露面积约700km2,主体为TTG系列片麻岩与少量的太古宙绿岩残留体。太古宙绿岩带物质包括一些具有科马提岩、高镁玄武岩及拉斑玄武岩亲缘性的超镁铁-镁铁质火山岩及相应的侵入岩,它们被后期TTG岩石侵入并经历变质、破裂、混合岩化等过程[20-22]。最老的TTG系列岩石是古太古代奥长花岗质片麻岩(约3 340Ma)[23],但主体还是较年轻的花岗岩类,包括中粒花岗岩(3 120Ma)与似斑状花岗岩(3 110 Ma)[23]。
(2)弗里德堡穹窿。位于约翰内斯堡东南约120km处,由于存在假玄武玻璃质角砾岩(pseudotachylitic breccias)与弗里德堡脉岩(Vredefort Granophyre)、超高压变质石英矿物(柯石英、斯石英)等冲击变质的证据,多数研究者认为其形成过程与陨石撞击作用(2 020Ma)有关[24-25]。通常把它分成两个部分,包括直径为40~45km的核部(core)及宽度为20km 的环形外带(collar)[26]。其中,核部由太古宙花岗片麻岩(>3 100Ma)组成,外环则由一些近直立甚至倒转的中太古代—早元古代(2 250~3 070Ma)变质火山-沉积地层构成[25]。另外,在其东南方位残留太古宙绿岩带物质(Greenlands Greenstone Complex),与巴伯顿绿岩带的特征极其相似,且形成时代也相近(3 300Ma)[27]。
1.1.3 彼得斯堡地块
彼得斯堡地块以穆奇森(Murchison)绿岩带为界与兰德地块相分隔,其内部则出露2条绿岩带:彼得斯堡(Pietersburg)绿岩带和吉亚尼(Giyani)绿岩带,以及一些太古宙的花岗岩-花岗片麻岩(如Makoppa穹窿)。穆奇森绿岩带是NEE走向的太古宙变质火山-沉积地层,长约140km,宽10~15km,与塔巴津比—默奇森线性构造呈平行展布[28]。Vearncombe等将该绿岩带划分为4个单元:Rubbervale组、Murchison单元、Silwana角闪岩和La France组[29]。其中,Rubbervale组主要由片岩(含石英碎斑)和少量酸性火山岩、凝灰岩及火山角砾岩组成,它们的喷发年龄约2 970Ma[30],其成分与Silwana角闪岩相当。Murchison单元是由一大套超镁铁-镁铁质火山岩、火山-沉积岩和片岩。La France组位于穆奇森绿岩带的最南端,主要由石英片岩、蓝晶石-十字石-石榴石云母片岩组成[31]。
(1)彼得斯堡绿岩带。长125km,宽25km,是一条NE向延伸的变质带,总体上为变质超镁铁-镁铁质岩与变沉积岩组合,二者为不整合接触[32]。下段主要包括变质拉斑玄武岩,变质辉长岩,蛇纹石化橄榄岩夹条带状铁建造,燧石层及变质碳酸盐岩等;上段沉积岩称为Uitkyk组,岩石类型为砾岩、杂砂岩、页岩及火山角砾岩。从后期侵入的同变形花岗岩中获得锆石207Pb/206Pb年龄为2 850Ma,表明绿岩带形成不晚于2 900Ma[32-33]。
(2)吉亚尼绿岩带。呈NEE走向,其总长度近70km,中央最宽处为15~20km,它从Klein-Letaba河以西开始一分为二,分为Khavagari arm(北分支)和Lwaji arm(南分支)[34]。该绿岩带主要为变质超镁铁-镁铁质火山岩形成的片岩(夹条带状铁建造),包括透闪石-滑石-绿泥石-角闪石片岩,内部残留极少量块状超镁铁质岩,主要是一些含橄榄石-蛇纹石-直闪石的岩石(已强烈风化),原岩为纯橄岩或方辉橄榄岩[35]。从超镁铁-镁铁质岩石的夹层(变安山岩)中获得锆石207Pb/206Pb年龄为3 200Ma,近似代表该绿岩带的形成年龄[32,36]。
(3)Makoppa穹窿。位于彼得斯堡地块的西南端,一直延伸到博茨瓦纳境内。在南非出露面积约5 000km2,主要岩石类型有:灰白色中-粗粒、局部变形的Vaalpenskraal奥长花岗-英云闪长质片麻岩,红色粗粒的斑状花岗岩类(Makoppa花岗闪长-二长花岗岩)以及灰白色中-细粒的Rooibokvlei花岗闪长-二长花岗岩[37]。最老的 Vaalpenskraal片麻岩中获得2个锆石Pb-Pb年龄分别为(3 013±11)Ma,(3 034±64)Ma;最年轻的Rooibokvlei花岗闪长岩中则出现2个新太古代的锆石年龄,分别为(2 777±35)Ma,(2 797±2)Ma[37]。
1.1.4 金伯利地块
金伯利地块大部分被新太古代Ventersdorp群(2 710~2 780Ma)及晚古生代—中生代卡鲁岩系、新生代卡拉哈里(Kalahari)沙漠所覆盖[4],其中出露一条近SN向延伸的Kraaipan-Madibe-Amalia花岗-绿岩带。该带长约250km,由太古宙TTG片麻岩、石英二长岩、变质基性火山岩与条带状铁建造等组成[4,38]。最老的结晶岩系见于金伯利金刚石矿区的英云闪长-花岗闪长质片麻岩,其年龄约3 250 Ma[39],其余大部分花岗片麻岩则形成于2 800~3 000Ma[40]。北边的Kraaipan绿岩带火山岩由于遭受变质/热液蚀变改造,大都表现为块状-层状的角闪岩与角闪石-绿泥石-绿帘石片岩,它内部年龄约3 080Ma[41]或(3 191.1±7.9)Ma[42];南边的阿玛利亚(Amalia)绿岩带加积火山角砾凝灰岩的年龄约(2 754±5)Ma[43],而石英-绿泥石-绢云母片岩则产于 (2 740±13)Ma[4]。
1.2 古元古代林波波变质带
林波波带呈 NEE向,长约650km,宽约200 km,是由太古宙的津巴布韦克拉通与卡普瓦尔克拉通二者之间发生碰撞、拼贴而形成的造山带[44](图1)。变质带北以北林波波逆冲带为界与津巴布韦克拉通相接,南以赫特(Hout)河剪切带为界与卡普瓦尔克拉通相邻。林波波带内部又以 Magagohate-Triangle剪切带和Palala-Tshipise剪切带为界,进一步划分出3个次级地块:北部边缘带、中央带和南部边缘带。
(1)北部边缘带。带中岩石包括紫苏花岗闪长-紫苏花岗片麻岩、斑状富钾花岗岩、紫苏花岗岩、超镁铁质-镁铁质片麻岩(不规则的透镜状或不连续的层状)和极少量的变泥质岩[45-46]。该带岩石沿北林波波断层逆冲到北侧的津巴布韦克拉通之上,断层的活动时代为2 550~2 630Ma[47-48]。地表90%的露头均为紫苏花岗闪长-紫苏花岗片麻岩,部分已发生退变质(紫苏辉石被角闪石或黑云母交代)[45]。这些片麻岩在化学成分上属于英云闪长岩-奥长花岗岩或二长花岗岩,锆石U-Pb定年结果为2 580~2 750Ma[49]。
(2)中央带。岩性变化较大,主要以发育Beit Bridge变质杂岩为显著特征,还有一些正片麻岩(变质深成岩体)。其中,Beit Bridge杂岩主要由浅色长英质片麻岩、石英岩与大理岩组成,夹变质泥质片麻岩、磁铁石英岩与基性麻粒岩[46,50]。正片麻岩中含一部分TTG岩系,它们构成了Sand River片麻岩,其原岩侵位年龄推测为3 170~3 310Ma[51-52];其他的片麻岩形成时代多为2 510~2 730Ma[51]。该带主要记录了2期高级变质作用,第一期为2 500~2 700Ma,第二期为1 970~2 030Ma[52-53]。
(3)南部边缘带。南部边缘带的麻粒岩与相邻的卡普瓦尔克拉通花岗-绿岩带具有相似的地球化学及同位素特征,极可能源自同一来源,形成时代为2 900~3 050Ma[54]。南部边缘带的岩石沿赫特河剪切带向南逆冲到卡普瓦尔克拉通之上[55],剪切带的活动被限定在2 640~2 760Ma[56]。该带内部以一条斜方闪石等变线为界(斜方辉石发生退变质,并被直闪石交代)分为北段和南段2个部分[57]。
1.3 古元古代海斯带
海斯(Kheis)带介于太古宙卡普瓦尔克拉通与中元古代那马奎带之间,是一条狭长的褶皱-逆冲带,总体上为变质火山-沉积地层[4](图1)。它的东北边界及东南边界分别被Blackridge逆冲断层和Doringberg断裂所限,而西边以Trooilapspan剪切带及Brackbosch断层为界[58]。Moen将该变形-变质带从东到西划分为4段:Olifantshoek超群、Brulpan群、Vaalkoppies群和 Wilgenhoutsdrif群[58]。最老的变质沉积岩来自东部,其年龄为1 890~1 930Ma[59]。此外,Cornell也报道过 Olifantshoek超群中约1 750Ma的基性岩[60]。
(1)Olifantshoek超群。底部的Mapedi组为砾岩、千枚状页岩与石英岩,厚<1km[61];上覆的Lucknow组厚约500m,岩性为粉砂岩、石英岩和砾岩;Hartley组(与Boegoeberg Dam组相当)覆盖在Lucknow组之上,主要为杏仁状-块状玄武岩,少量砾岩、集块岩及凝灰岩;最顶部的Volop群主要由杂色石英岩、砂岩及少量页岩组成。
(2)Brulpan群包括 Dabep组、Boegoeberg组、Uitdraai组、Prynnsberg组和Groblershoop组等5个组。最底部的Dabep组与Olifantshoek超群为断层接触,发生强烈褶皱变形。岩性从石英-绢云母片岩过渡为石英-绿泥石-绿帘石片岩、阳起石角闪岩等。Boegoeberg组与Prynnsberg组岩性类似,均由浅灰-白色的中细粒石英岩组成,厚<550m。Uitdraai组由浅灰色石英岩、白色石英岩与片岩组成,推测其厚度200m。Groblershoop组由厚层的细粒云母-石英片岩组成,在奥兰治河(Orange River)河谷广泛出露。
(3)Vaalkoppies群包括Dagbreek组和Sultanaoord组。Dagbreek组主要由变泥质岩,片岩及石英岩组成;Sultanaoord组主要为白色的块状石英岩,夹一些风化呈红色的千糜岩,岩层已发生强烈变形,最厚约800m。
(4)Wilgenhoutsdrif群包括Zonderhuis组和Leerkrans组。Zonderhuis组的最底部为一套厚300 m的浅紫色石英岩(Groot Drink Member),上覆岩层为杂色千枚岩与片岩,其内部含灰岩、蛇纹岩及石英岩的透镜体。在Karos东南约20km处,Leerkrans组强烈褶皱,包含2个火山-碎屑沉积旋回。在Leerkrans附近,该组主要由片岩构成。Moen指出,Leerkrans组火山岩在地球化学上表现出双峰式岩套的特点[62]。
1.4 中元古代那马奎带
那马奎带位于中元古代那马奎—纳塔尔(Namaqua-Natal)造山带的西段,该造山带被认为形成于格林维尔期(1 000~1 300Ma)Rodinia超大陆聚合过程中的那马奎造山运动[63],也是中元古代基巴拉(Kibaran)造山带的一部分[64](图1)。它东北毗邻太古宙卡普瓦尔克拉通,南以Beattie地球物理异常带及相关的Southern Cape Conductive Belt为界[65]。该变质带那马奎段的次一级构造单元划分一 直 存 在不 同 意 见[66-67]。本 文 采 用 Eglington(2006)提出的分类建议,将那马奎段分为3个区域:Richtersveld subprovince,Bushmanland subprovince和 Gordonia subprovince[68]。
(1)Richtersveld subprovince。主要包括钙碱性的Vioolsdrif岩基和变质表壳岩(奥兰治河(Orange River)群),前者的岩性为辉长岩、闪长岩、花岗闪长岩、花岗岩等,侵位时代1 750~2 000 Ma[69];后者主要由中-低级变质的钙碱性火山岩组成。
(2)Bushmanland subprovine。可进一步划为Okiep Terrane,Aggeneys Terrane与Garies Terrane三部分。Okiep Terrane内部以Skelmfontein逆冲断裂为界,分成北段(Skeinkopf domain)和南段(Okiep domain),著名的Okiep铜矿床即产于此地。Skeinkopf domain由灰色正片麻岩(Gladkop Suite;~1 800Ma)[70]与一些角闪岩、变泥质岩(含堇青石)及钙硅酸盐岩组成[71]。Okiep domain为1 000~1 200Ma的 正片 麻岩[70,72]侵入到老片麻岩中(Gladkop Suite),并发育一套麻粒岩相变质的火山-沉积地层(Khurisberg subgroup),以出现二辉麻粒岩为特征[64]。Aggeneys Terrane主要出露一套变质火山-沉积地层和大量正片麻岩。前者为Bushmanland群,岩性包括含铁石英岩、页岩、角闪岩并伴随有大规模铅-锌-铜-银成矿作用[70],其形成时代1 650~2 000Ma[69];后者包括Achab片麻岩、Hoogoor片麻岩等,在Achab片麻岩中获得了单颗粒锆石SHRIMP U-Pb年龄值约2 000Ma[73],但最新的测年结果却显示为新元古代末期(1 160~1 190 Ma)[74]。Garies Terrane由一系列正、副片麻岩组成,并被大量后期花岗岩侵入[68]。
(3)Gordonia subprovine。可进一步分为 Kakamas Terrane和 Areachap Terrane。Kakamas Terrane东以Bovenrugzeer剪切带与Areachap Terrane相隔,西以Hartbees River逆冲断层为界。主要由强烈变形的角闪岩相副片麻岩组成,并被大量花岗岩侵入(包括一些紫苏花岗岩),其内部钙硅酸盐岩石的Pb-Pb年龄为约2 000Ma,而Rosynenbosch地区硫化物矿石的Pb模式年龄为约1 100 Ma[67-68]。Areachap Terrane是一条狭长的角闪岩相变质带,岩性包括变玄武岩及正片麻岩,以东边的Trooilapspan剪切带及Brackbosch断层为界与海斯带分开。原岩年龄为1 300~1 600Ma[75]。
1.5 古生代开普褶皱带
开普褶皱带地处南非的南端,在南非境内长约1 300km,是形成于冈瓦纳大陆聚合过程中的造山带(图1),其延伸至南美洲阿根廷、福克兰群岛及南极大陆[76]。该褶皱带分为3部分:构造线方向近SN向展布的西分支、构造线方向近EW向展布的南分支和介于二者之间过渡的衔接带[77-78]。开普造山运动引起开普(Cape)超群及上覆卡鲁(Karoo)超群底部Dwyka群岩石的明显变形。西分支内部变形较弱,主要产出一些平缓、开阔的直立褶皱与单斜构造;而南分支内部则变形强烈,以发育一系列N倾的叠瓦状逆冲断层、双冲构造与倒转褶皱为显著特征[79-80]。
开普造山带内最老的地质单元为新元古代变质火山-沉积岩,其隶属于泛非期的Saldania造山带,大量同造山-后造山期的开普(Cape)花岗岩组合(510~550Ma)侵入其中。这套地层主要沉积在一系列裂谷盆地中,普遍遭受绿片岩相变质作用,并以一种内窗层(inliers)的形式被保留下来[80-82]。主要有开普敦以北地区的Malmesbury群、Oudtshoorn地区的Kango群、George地区的Kaaimans群和Port Elizabeth地区的Gamtoos群。
奥陶纪—早石炭世开普超群是本区内分布面积最广的地层,内部已发生强烈变形,地层最厚可达8 km,为产自大陆边缘(大陆架)环境的硅质碎屑沉积[77,83]。开普超群自下而上可分为3段:桌山(Table Mountain)群、Bokkeveld群和Witteberg群,前二者构成了开普超群野外露头的主体。其中,桌山群主要为石英岩,局部出现一些千枚岩,而Bokkeveld群则以页岩为主,次为石英岩。相比之下,Witteberg群分布较局限,仅出现在开普超群的北部边缘,岩性包括页岩与石英岩(在该群上部层位中还出现一层混杂岩),并被晚石炭世—中侏罗世的卡鲁超群不整合覆盖[77,80]。
1.6 南非古生代—中新生代卡鲁盆地
南非境内的卡鲁盆地是指狭义的卡鲁盆地,有研究认为它是弧后前陆盆地[84-85]。卡鲁盆地内的沉积地层称为卡鲁超群,其沉积长达125Ma,从晚石炭世一直持续到中侏罗世[76]。卡鲁超群从早到晚、从下到上可分为4段:Dwyka群、Ecca群、Beaufort群和Stormberg群(包括上覆的Drakensberg群玄武岩层)。其中,Dwyka群与Ecca群的中下部为深海相沉积环境,再过渡到Ecca群上部则为浅海相沉积环境,Beaufort群和Stormberg群则为河流相的沉积[86]。
卡鲁超群底部的Dwyka群与下伏开普超群最上部的Witteberg群为不整合接触关系,二者的沉积间断持续了30Ma,这一不整合的成因目前尚存争议[85,87]。在弧后前陆盆地体系模型中,Dwyka群被认为是前渊(foredeep)在处于欠补偿状态下的沉积产物且沉积时限为290~300Ma,也有人认为其沉积上限为305Ma左右[88]。它是厚达800m的冰海相杂岩,岩层基质以泥岩为主,内部含有从漂浮的冰筏上坠落的砾石[89]。
Ecca群主体是二叠纪的远洋与重力流沉积(浊积岩),其最厚可达3km,岩性为泥岩、粉砂岩和砂岩,局部含煤层[84,86]。Ecca群底部的 Prince Albert组凝灰岩夹层同位素年龄为(288±3.0)Ma和(289±3.8)Ma[88],与孢粉鉴定结果推测的290Ma较为吻合[90]。
Beaufort群是横跨二叠纪和三叠纪的陆相沉积,出露面积约200 000km2,最厚达7km[91]。其年龄主要通过地层中所含的合弓纲爬行动物化石组合加以限定,其中,二叠纪与三叠纪的界线以貘头兽(Tapinocephalus)及二齿兽(Dicynodon)两种化石的消失为标志[76,92]。Beaufort群内部以泥岩及粉砂岩为主,夹少量透镜状或条带状砂岩[93]。
Stormberg群代表晚三叠世—中侏罗世的陆相沉积,其底部 Molteno组与下伏Beaufort群顶部Burgersdorp组之间存在一个具穿时性的不整合。Molteno组为以冲积物为主的河流相楔形沉积[86,94-95]。与之不同的是,处于中间层位的 Elliot组主要由粒度总体向上变细的砂岩及泥岩旋回组成。Drakensberg溢流玄武岩直接覆盖在Stormberg群之上,火山岩层厚达1.37km,岩性以拉斑玄武岩为主。它们从晚三叠世开始喷发,一直持续到早白垩世。该期火山活动与冈瓦纳大陆裂解有关,也预示卡鲁盆地演化的结束[86]。
2 成矿区带划分
南非成矿区带的划分是在整理前人文献资料的基础上,以构造单元为基本轮廓,结合主要成矿元素和主要成矿类型,考虑成矿地质条件、控矿因素及成矿时代因素[96],按照“构造单元+主要矿种+主要成矿类型”的原则,将南非分为6个Ⅲ级成矿区带:①太古宙卡普瓦尔克拉通金-铀、铁-铜-铂族元素、金刚石成矿区;②古元古代林波波金-金刚石成矿带;③古元古代海斯铁-锰成矿带;④中元古代那马奎铜-铅-锌多金属成矿带;⑤古生代开普褶皱带钨-锡-铜-钼多金属成矿带;⑥古生代—中新生代卡鲁盆地铀-金刚石-砂金成矿区(图2)。
3 成矿区带特征及典型矿床简况
3.1 太古宙卡普瓦尔克拉通金-铀、铁-铜-铂族元素、金刚石成矿区
成矿区位于太古宙卡普瓦尔克拉通内,成矿作用主要与太古宙早期陆壳形成以及克拉通晚期陆内-陆缘裂谷、裂陷盆地的演化有关;而金刚石成矿则与后期克拉通内部陆下岩石圈地幔或软流圈地幔活动引发的金伯利岩浆爆发直接相关。矿床类型有绿岩带型金矿、兰德盆地砾岩型金(铀)矿、布什维尔德岩浆型铜镍硫化物-铂族元素矿床、德兰士瓦沉积变质型铁(锰)矿及原生金伯利岩型金刚石矿等。
3.1.1 绿岩带型金矿
南非绿岩带型金矿床主要分布于卡普瓦尔克拉通内的巴伯顿、穆奇森、彼德斯堡及萨瑟兰(吉亚尼)等绿岩带中,通常被认为属造山型金矿[97-98],其中以巴伯顿绿岩带金矿床的产量和规模最大。自1883年以来,巴伯顿绿岩带中已发现300多个金矿床,黄金产量累计超过345t,金主要来自希巴(Sheba)(123.7t)、新康索特(New Consort)(63.4t)、费尔维尤(Fairview)(63.4t)等矿山[99]。金矿床主要集中于巴伯顿绿岩带的西北部,特别是在Jamestone片岩带、希巴山地区和摩德斯山地区[100]。
图2 南非主要成矿区带划分示意图Fig.2 Sketch showing division of main metallogenic belts or areas in South Africa
金主要赋存于绿岩带的石英脉(网脉)或韧性剪切带中,常与毒砂及黄铁矿密切共生,且以“非明金”和银金矿夹杂物的形式产出[101]。前人多认为金矿化受逆冲断层的控制[102-103],但也有人基于北部成矿区(新康索特、希巴和费尔维龙矿床)的资料提出金矿化是受晚期伸展阶段的(截切逆冲断层)断裂系控制[99,104]。研究表明,巴伯顿地区金矿床的成矿时代主要为中太古代。de Ronde等人通过对费尔维尤金矿的研究获得金矿的成矿年龄为(3 126±21)~(3 084±18)Ma[105];Dziggel等对新康索特金矿的研究揭示其成矿分为2个阶段,介于3 040~3 030Ma[106],而Dirks等在希巴金矿附近与成矿近于同期的岩脉中测得Pb-Pb年龄约为3 015 Ma[107]。对于金矿的成矿模式已有许多学者进行了大量探索[102,108]。最近,王杰等[109]将其归结为3种,即火山成因模式(Volcanogenic Model)[110]、转换滑脱构造模式(Transtensional Tectonics Model)[106]和构造交叉模式(Intersections Model)[107]。
3.1.2 兰德盆地古砾岩型金(铀)矿
兰德型金矿床位于维特沃特斯兰德盆地周缘,全称为维特沃特斯兰德古砾岩型金(铀)矿。盆地中沉积了巨厚的太古宙火山-沉积岩系,砾岩型金(铀)矿主要产自维特沃特斯兰德超群的中兰德群中[111]。兰德矿区一共包含8个金矿田,即东兰德(East Rand)、南兰德(South Rand)、中兰德(Central Rand)、西兰德(West Rand)、卡勒顿维累(Carliton-ville)、可莱克斯多普(Klerkdorp)、自由邦省(Welkom)与埃温德镇(Evander)金矿田[112]。兰德金矿自1886年发现以来,已产黄金3.5×104t,尚有储量约2×104t,开采至今金品位一直保持在7×10-6~20×10-6。
金主要呈细分散状态自然金形式产出,金的粒度为5~100μm,明金少见,常与铀矿、黄铁矿密切共生,局部也与沥青共生,金的平均品位3×10-6~25×10-6[100,111]。伴生的铀矿在1952—1975年间共产出150×104t的 U3O8,铀的平均品位达271×10-6[113]。该矿床的形成可能持续了很长时间(3 074~2 642Ma)[111],其成因模式一直存在争议,主要观点有3种:砂矿模式(Placer model)[114-116],热液模式(Hydrothermal model)[117-118]及介于二者之间的改造砂矿模式(Modified Placer model)[119-120]。
3.1.3 布什维尔德铜镍硫化物-铂族元素矿床
布什维尔德杂岩体(Bushveld Igneous Complex)是世界上最大的镁铁质层状侵入体,也是世界上单个蕴藏铂族元素(PGE)、铬铁矿和钒钛磁铁矿的最重要矿床[121]。其中,PGE储量为65 473t,含有全球75%的铂、54%的钯和82%的铑[122];镍金属占全球的16%,储量1 528×104t;铜金属占全球的5.5%,储量为688×104t;而铬、金、钒和磁铁矿的储量分别为40×108t,1 152t,1 680×104t,10×104t[123]。
杂岩体的侵位受NEE向塔马津比—默奇森线性构造的控制[124],其中含矿岩体是勒斯滕堡(Rustenberg)超镁铁-镁铁质层状岩套。该岩套分为西支、东支和北支3部分,最主要的赋矿层位是东、西分支的UG2铬铁岩矿层、梅林斯基(Merensky)矿层以及北支的普拉特(Plat)矿层[125]。布什维尔德矿床的成岩成矿时代为古元古代(2 050~2 060 Ma),该矿床的成矿构造背景为大陆边缘裂谷环境。勒斯滕堡层状岩套具有两大成矿特征:一是巨量的PGE与低品位、低密度的硫化物层共生;二是具有完整的层序和矿化部位。该矿床的成矿机制主要是地壳的含水硅酸盐成分遭受同化混染后导致硫化物熔离,从而形成贱金属硫化物,而PGE的富集则主要是硫化物熔离作用或吸附作用的结果[121]。
3.1.4 德兰士瓦沉积变质型铁(锰)矿
南非的德兰士瓦沉积变质型铁(锰)矿以塔巴金比(Thabazimbi)铁矿为典型矿床。
塔巴金比铁矿床主要产于Leeuwbosch和Cornwall两个矿区,是一个巨大的、高品位的BIF型赤铁矿矿床,资源储量近5×108t。另外,在Leeuwbosch矿区还伴生有一个密西西比河谷型(MVT)的铅-锌-银矿[126]。
铁矿的赋矿层位为德兰士瓦超群的Malmani亚群与Penge组。其中,Malmani亚群为一套碳酸盐岩夹页岩、燧石条带,其中凝灰岩的锆石SHRIMP U-Pb年龄为(2 583±5)Ma或(2 588±7)Ma[127];而Penge组则主要由条带状含铁纹层(赤铁矿、磁铁矿与针铁矿)、含铁硅酸盐层(燧石、黑硬绿泥石)、菱铁矿与页岩夹层组成[128],其时代大致为(2 480±6)Ma[129],明显受后期布什维尔德杂岩体侵入时岩浆热液-流体的强烈改造[130]。
矿石类型主要有脉状或鲕状矿石和角砾岩型矿石[129]。矿石矿物包括赤铁矿、假象赤铁矿及少量的褐铁矿和菱铁矿[128],脉石矿物主要有粗粒的石英、方解石和红碧玉。矿体呈细脉状或团块状,大者可达数十米,明显受断层控制,且层控型特征非常显著[129]。De Kock等人[131]通过对 Van der Bijl矿井中铁矿石的古地磁研究得到成矿年龄大致为1 930~2 050Ma。
Philpott和Ainslie[132]最早提出塔巴金比铁矿的形成与一种表生来源的成矿流体淋滤地层有关。后来,Netshiozwi[133]通过对矿区脉石矿物(石英、白云石及方解石)的流体包裹体研究证明,成矿流体盐度范围极广,w(NaCl)=6%~27%,均一温度为143~224℃,结合稳定同位素的结果(δ(18OSMOW)=-9.7×10-3~-2.2×10-3),作者提出成矿流体很可能由2种流体混合而成。最近,Möller等人[129]的工作表明,成矿流体的盐度(NaCl)=9.2%~39.9%,温度100~190℃,证实了前人关于成矿流体为混合成因的观点,并推测成矿温压条件为175℃,100MPa。
3.1.5 原生金伯利岩型金刚石矿
原生金伯利岩型金刚石矿是南非金刚石的主要来源,到目前为止已发现并开采的、规模较大的矿床主要有9个:金伯利(Kimberley)、德比尔斯(De Beers)、布尔丰坦(Bultfontein)、杜托伊斯宾(Dutoitspan)、韦塞尔顿(Wesselton)、芬奇(Finsch)、科菲方丹(Koffiefontein)、亚格斯丰坦(Jagersfontein)和库里南(Cullinan/Premier)。其中,以库里南矿床的规模和产量最大。金伯利岩被作为含矿母岩来寻找金刚石,南非金伯利岩浆喷发表现出阶段性爆发、离散分布的特点,时代包括早-中元古代、寒武纪、二叠纪、侏罗纪和白垩纪[134]。
库里南金伯利岩筒是南非最大的金刚石矿,位于比勒陀利亚(Pretoria)北东约37km处,由于1905年在此发现了世界上最大的、重达3 106ct(625g)的库里南钻石而闻名于世。矿坑地表出露面积32 hm2,在平面上呈椭圆状或肾状。金伯利岩筒侵入德瓦士兰超群及布什维尔德杂岩体中,其侵位年龄的测定结果为(1 179±36)Ma(单斜辉石Rb-Sr等时线)[135]和(1 202±72)Ma(钙钛矿 U-Pb 定年)[136],并被厚75m的辉长岩岩床切穿(1 150 Ma)[137]。
该处金伯利岩主要有3次侵入,2次是典型的凝灰质金伯利角砾岩(tuffisitic kimberlite breccias,即TKB),最后1次为侵入相浅成金伯利核杂岩。其中,金伯利角砾岩包括灰型(grey kimberlite)和褐型(brown kimberlite)2种类型;而浅成金伯利岩核杂岩也称黑金伯利岩(black kimberlite),它又可分出斑杂状金伯利岩变种(Piebald Kimberlite)[100,134,137]。此外,最晚期阶段还有一次高碳酸盐含量的金伯利岩脉侵入[138]。这些金伯利岩携带的捕虏体类型多样,主要有粗粒/变形石榴石二辉橄榄岩,方辉橄榄岩与少量榴辉岩[137]。该矿床于1903年开始开采,最初的钻石品级高达170cpht(ct/102t)[138],通常的品级大多为40~80cpht(随深度增加而逐渐升高),但这一数字在2005年下降到28 cpht[139]。除库里南钻石以外,该矿山随后又产出了像Centenary钻石(599ct)及Unkown Brown钻石(1083ct)一类的大钻石。Williams[140]指出库里南矿山的钻石普遍具有特征的油性光泽(“oily brilliance”),却很少具有八面体晶型[141]。这些金刚石中的少量是Ⅱ型金刚石(不含氮),当中又有极少一部分是浅蓝色的含硼Ⅱb型金刚石[137]。
3.2 古元古代林波波金-金刚石成矿带
此成矿带位于林波波带的中-南端,成矿作用受卡普瓦尔克拉通与津巴布韦克拉通之间碰撞事件(约2 600Ma)的影响。其中,绿岩带型金矿受构造控制明显,典型矿床包括吉亚尼(Giyani)金矿床等。此外,林波波带还包括一个原生金伯利岩型金刚石矿,即韦内沙(Venetia)金刚石矿床,其钻石产量极高。
3.2.1 吉亚尼(Giyani)绿岩带型金矿
吉亚尼(也叫Sutherland)绿岩带金矿床位于林波波带的最南端,毗邻赫特河剪切带或位于其中。该绿岩带目前已知有金矿产地55处,其中35处正在开采,其他20处为关停状态[142]。这些暂时关停的金矿中,6个金矿(Osprey,Louis Moore,Fumani,Franke,Klein Letaba和Birthday)贡献了该绿岩带金矿总产量的97%[143]。研究表明,这些矿床都具有明显的造山型金矿特征[97-98,144]。
吉亚尼绿岩带地层为吉亚尼群,主要由镁铁-超镁铁质火山岩夹变质沉积岩与长英质火山岩,其中的变沉积岩由BIFs、石英岩、云母片岩及少量白云岩组成[145]。矿石中金的载体主要有以下4种:石英脉、条带状铁建造、石英-硫化物脉和碳酸盐(方解石)脉[146]。在大多数矿床中,金常与石英脉中的硫化物密切共生,但有时也以石英脉中的自然金或硅酸盐中的包裹体形式出现[147-148]。石英脉的硫化物矿物组成稳定,通常包括雌黄铁矿、黄铜矿、黄铁矿及毒砂等[147,149],金矿化主要与晚期由流体引发的退变质作用有关[142]。关于金矿的成矿时代,Pretorius等[147]报道了Fumani矿床中切穿矿体的伟晶岩脉中原生白云母Rb-Sr年龄值为(2 632±53)Ma;而Barton和Van Reenen在Klein Letaba矿床附近韧性剪切带的伟晶岩中也获得相似的白云母Rb-Sr年龄结果(约2 660Ma),并认为这代表了流体活动年龄[150]。由此可见,本区金矿的成矿时代不晚于新太古代。赫特河剪切带是一级控矿构造[151],而区内一系列近EW走向、并毗邻矿床的韧性剪切带是次一级控矿构造[152]。
3.2.2 韦内沙(Venetia)金刚石矿床
韦内沙金伯利岩型金刚石矿发现于1980年,位于林波波带的中央带[134,153],是南非目前产量最高的金刚石矿床,一共有14个岩筒,其中规模最大的2个(K1和K2)正在开采。该矿为露天开采,2005年的矿石产量为580×104t,产出金刚石达7.18×106ct,平均品位为122cpht[134]。含矿的金伯利岩为Group-1型[154],金伯利岩浆喷发时代大致为寒武纪,其中金云母Rb-Sr等时线年龄为(530±4)Ma和(510±16)Ma[155],这与 Phillips等[156]对基质中的金云母进行Ar-Ar定年获得的结果((519±0.6)Ma)较为吻合。
K1金伯利岩筒在平面上呈拉长的靴形[157],可区分出喷发相的凝灰质金伯利角砾岩和浅成侵入相的金伯利岩2种类型[154]。K2岩管同样表现出两种明显不同的岩相特征,Seggie等[154]将其分为东、西两段,西段为浅成相金伯利岩,东段则为喷发相金伯利角砾岩。其中,西段的又进一步分出两种结构差异显著的球粒状金伯利岩与均一的金伯利岩。Stiefenhofer等[158]对来自该矿床的100个围岩捕虏体进行了分析,指出它们包括橄榄岩与辉石岩两大类型。另外,该矿床的包裹体研究表明,这些金刚石很可能结晶于古老克拉通的岩石圈地幔[159]。
3.3 古元古代海斯铁-锰成矿带
该成矿带内铁锰成矿作用显著,均为卡普瓦尔克拉通西部被动大陆边缘(海相)环境下形成的、具层控型特征的沉积变质成因矿床,明显受Eburnean期(1 900~1 800Ma)的海斯造山运动改造。在与卡普瓦尔克拉通交界处产有若干个大型-超大型矿床,锡兴(Sishen)铁矿(赤铁矿为主)、卡拉哈里(Kalahari)锰矿及波斯特马斯堡(Postmasburg)锰矿可作为其中的典型矿床。
3.3.1 锡兴(Sishen)铁矿
南非北开普省的锡兴铁矿位于金伯利以西约200km处,该矿床产于Maremane背斜的西北边缘。含矿地层主要为前寒武纪德兰士瓦超群的Gamagara 及 Asbesheuwels 亚 群[128]。Gamagara亚群的底部为层纹状矿体,向上依次为锡兴页岩(含赤铁矿砾石及砾岩型矿体)、马萨斯波特组含铁石英岩及帕林组页岩[160];而Asbesheuwels亚群则主要由条带状铁建造、碧玉铁质岩和页岩组成,常含不规则的透镜状矿体。
矿体受开阔的向斜构造控制,SN向延伸约50 km,宽约2km,向 W倾,倾角10°,并被后期的辉绿岩脉所截切[128]。矿区矿石类型主要有3种:①层纹状矿石(包括薄层与厚层两种)[161];②锡兴页岩中的砾岩型矿石;③赋存在下伏白云岩中铁矿层内的次生矿石,即撒巴津比(Thbazimbi)型矿石。另外,矿石的储量分布统计信息表明,层纹状矿石所占比重最大,约占85%;砾岩型矿石次之,约占12%;而撒巴津比型矿石只占3%。关于矿床成因,Page研究指出,在条带状铁建造沉积之后,下伏的白云岩被溶蚀,形成岩溶及地堑构造,形成了撒巴津比型矿石及层纹状矿石,后期进一步的区域抬升及剥蚀最终出现了砾岩型矿石[162]。
3.3.2 卡拉哈里锰矿
北开普省的卡拉哈里锰矿床是一个世界级锰矿,其资源总储量约为80×108t,锰的品位20%~48%[163]。该矿床位于库鲁曼西北约60km处,矿区东西长15km,南北宽35km[164]。
含矿地层是新太古代-古元古代德兰士瓦超群最上部的Hotazel组,矿体主要有3层,与互层的条带状铁建造同时沉积[165],而又以最底部的层状单元含矿性为最好[166]。该矿床可以分为南、北两个矿区,分别以出现低品位(<40%)富含碳酸盐的矿石及高品位(>44%)富含氧化物的矿石为特征[163]。其中,低品位矿石称为马马特旺(Mamatwan)型矿石[167],是由成岩作用-低级变质作用过程形成的泥质岩,在成分上包括微晶锰白云石、褐锰矿及赤铁矿,此种矿石约占总储量的97%;而高品位矿石则称为Wessels型矿石[168],它们仅局限分布于北部矿区,由粗晶黑锰矿、褐锰矿与方铁锰矿组成,约占矿石总储量的3%。目前的研究认为,这种矿石品位从低到高的转变现象很可能与断层控制的低温热液-流体交代作用有关[169-170]。锰的富集与先后经历的3次由构造引发的热液蚀变事件相关,分别是Wessel事件(1 000~1 250Ma),Mamatwan事件(550~600Ma)和Smartt事件(10~1 000Ma)[166]。
3.4 中元古代那马奎铜-铅-锌多金属成矿带
该成矿带位于南非的西北端,并向北一直延伸进入纳米比亚境内。在构造位置上属于太古宙卡普瓦尔克拉通的西南缘,成矿作用受中元古代那马奎造山运动的显著影响。该带的铜-铅-锌多金属矿化主要与古老地壳物质(古元古代,甚至太古宙)在中元古代时期再循环(改造)引发的岩浆-沉积作用有关。该地区产出一些世界级的大型-超大型矿床,著名的矿床包括Aggeneys铅-锌-银矿床、Okiep铜矿和普里斯卡(Prieska)铜-锌矿等。
3.4.1 阿赫内斯(Aggeneys)铅-锌-银矿床
阿赫内斯矿床位于那马奎变质带西段中部的阿赫内斯镇附近,一共包括4个矿区,从西到东分别是黑山(Black Moutain)、布罗肯希尔(Broken Hill)、大向斜(Big Syncline)和甘斯堡(Gamsberg),这是世界级的布罗肯希尔型铅-锌-铜-银矿床[171-172]。该矿床的资源总储量约370Mt,以布罗肯尔矿区的储量为最大(38Mt),其中铅占7.8%,锌占2.9%,铜占0.5%,伴生的银品位达113×10-6[171]。
布罗肯希尔矿区的含矿岩层为高角闪岩相的Bushmanland群,自下而上分为Namies片岩组、布罗肯希尔石英岩组、Ore片岩组、Shaft片岩组和Koeris组[173]。矿体赋存层位是Ore片岩组,主要由块状硫化物、重晶石矿,以及条带状铁建造和富铝片岩组成,矿体包括上矿层(Upper Ore Body)和下矿层(Lower Ore Body)[172,174]。矿物组成主要有闪锌矿、方铅矿、磁铁矿、磁黄铁矿、黄铜矿与黄铁矿[174],其中,下矿层富集闪锌矿、方铅矿及磁黄铁矿,而上矿层则富集磁铁矿。矿石主体均为块状,硫化物含量大于60%,粗粒结构(0.5~2mm),表现出一定的变质特征,其中的硫化物矿物还可能发生过部分熔融[174]。该矿床形成于同沉积阶段,因为Bushmanland群变质沉积岩原岩沉积时代为1 200~1 640Ma,故成矿时代近于沉积期[173,175]。
3.4.2 普里斯卡铜-锌矿
普里斯卡铜矿位于南非北开普省,是世界上最大的30个火山成因块状硫化物(VMS)型矿床之一[176]。该矿床于1968年被发现,其开采时间从1972年一直持续到1994年。Wagener和Van Schalkwyk[177]报道该矿床原始储量为47Mt,铜品位1.7%,锌品位3.8%(开采到地下900m)。Galley等[176]统计后进一步将资源储量修正为47.2 Mt,铜品位0.98%,锌品位1.98%,伴生银矿品位20×10-6。
矿床位于那马奎变质带东部的南端(Arechap Terrane),矿区内出露的岩层是Copperton组,为高角闪岩相-低麻粒岩相的变质岩[178-179],主要经历了3期变形[180-181]。该组被划分为Smouspan片麻岩、普里斯卡铜矿组(Prieska Copper Mines Assemblage)与Vogelstruisbult单元3个部分[182]。其中,赋矿层位是普里斯卡铜矿组,其底部为富铝的岩石(电气石-金云母-夕线石-堇青石片麻岩),中间为块状硫化物矿石,顶部则为黄铁矿化石英岩、碳酸盐-石膏层及锰磁铁矿层等岩石组合[177,183]。
在平面上,矿体呈NW走向,延伸约2km;在剖面上,矿体往深部延伸稳定,长度至少达1km,产状陡倾,倾角为45°,甚至大于60°[184]。矿层厚度0.5~32m[185],平均厚约10m。矿体顶部发育厚度近100m的淋滤层(铁帽),可见一些褐铁矿、针铁矿和蓝铜矿等。矿石中黄铁矿最为常见,次为黄铜矿与闪锌矿。该矿床形成时代为1 280~1 290Ma,矿石中独特的金属分层、独具特色的铁帽及矿化蚀变与硫化物矿石密切相关等现象都显示该矿床为一大型VMS型矿床[184]。
3.5 古生代开普褶皱带钨-锡-铜-钼多金属成矿带
开普褶皱带被认为形成于冈瓦纳大陆的聚合过程之中,是一个类似于安第斯型大陆边缘的造山带。该带的矿产资源主要集中产在西段内,成矿作用与古生代岩浆活动密切相关。矿床往往赋存在寒武纪或更老的花岗岩体中(Cape Granite Suite),例如:Kuils River锡-钨矿、Durbanville锡-金矿及Riviera钨-钼(铜)矿床。
3.5.1 Kuils River锡-钨矿
Kuils River锡-钨矿床位于泛非期Saldania造山运动形成的开普褶皱带西段,它是南非境内发现的第一个锡矿床,断断续续均有进行开采,一直持续到1956年才最终停止。矿床类型包括原生石英脉型、细晶岩脉型锡矿以及冲积-残积型砂锡矿,至少已经有778t的锡石矿产出(锡品位70%),主要来自冲积型砂锡矿床[186]。然而,后续的一些勘探活动发现矿化点零散,且矿石品位较低(锡品位0.26%,钨品位0.48%)[187]。
含矿岩体为中粗粒结构、含电气石的黑云母(似斑状)花岗岩,有时出现钾长石巨晶,它是Kuils River-Stellenbosch侵入体的一部分,特征类似S型花岗岩[188]。矿化集中分布在一条NW向延伸超过2km、宽约500m的狭窄地带中,矿体为一系列不连续的石英脉与细晶岩脉,它们大小不一,局部厚达3 m。矿石矿物主要为锡石与钨锰矿[189],还有极少量辉钼矿、毒砂,而脉石矿物包括电气石、石英及云母等[190]。往岩体深部锡石逐渐减少,取而代之的是黄铜矿与黄铁矿[191]。已开采的2条主矿脉的矿石品位极其可观,锡品位9%~26.7%,钨品位6%,石英脉型矿石品级往往较高。该地区历史上产量最好的是一些冲积-残积型砂锡矿,含Sn为0.68%~5.6%[192]。
3.5.2 Riviera钨-钼(铜)矿床
Riviera钨-钼(铜)矿床位于开普褶皱带西段马姆斯伯里(Malmesbury)群露头区域的Boland Terrane内,该矿床的资源总储量为460×104t,钨品位0.216%,钼品位0.02%[193],为典型的夕卡岩型矿床。
含矿岩体Riviera侵入到其围岩马姆斯伯里群中,内部含有许多围岩捕虏体(以变质碳酸盐岩为主),矿化蚀变主要集中在岩体的顶部,尤其以绢云母化、黏土化和硅化最为明显[186]。Riviera岩体为近等粒(中-细粒)结构的偏铝质-过铝质岩石,岩性从石英二长岩到花岗岩[188],岩相学与地球化学特征均类似于I型花岗岩[194]。矿体在接触带形成的内、外夕卡岩带中均有分布。其中,内夕卡岩带中的白钨矿(辉钼矿)赋存于石英-方解石脉中,白钨矿与钙铁榴石-透辉石-阳起石紧密共生;外夕卡岩带中的典型矿物组合是白钨矿-符山石-方解石-绿帘石-钙铁辉石[186]。此外,还出现黄铜矿、黄铁矿及闪锌矿。这些矿脉大小不一,通常在岩体顶部非常发育。钨与钼的含量之间并无显著的时空联系或共生关系,表明二者的分布很可能与多期次岩浆活动及相关成矿流体有关[186]。伴生的辉钼矿脉属于叠加成因,可能与后期侵入的A型花岗岩有关[195]。Rozendaal等通过区域构造研究揭示含矿岩体的侵位与Saldania造山运动形成的穹状构造有关[195]。
3.6 古生代—中新生代卡鲁盆地铀-金刚石-砂金成矿区
南非的卡鲁盆地是狭义的,其构造属性可能是弧后前陆盆地。盆地内产出与沉积作用有关的矿床,如砂岩型铀矿、砂金矿等。卡鲁盆地的含铀地层包括Beaufort群、Molteno组和Elliot组,尤以晚二叠世Beaufort群最显著。盆地的砂岩型铀矿床主要分布于西南部,其范围大致从Sutherland,Fraserburg, Beaufort West 到 Prince Albert[196]。Beaufort群内至少可分为5个沉积旋回,矿化集中在2个砂岩单元[197-198]。此外,位于二者下部层位的Koornplaats单元可能也是一个重要的赋矿层位。这些矿体呈不规则透镜状,单个透镜体厚1~5m,延伸方向与局部的古流方向近于平行[199]。矿石矿物包括铀石(U(SiO4)1-X(OH)4X)和沥青铀矿(UO2),以铀石为主,二者的铀含量变化范围均较大;次要矿物有辉钼矿、黄铁矿及毒砂等[200]。其中,铀石被包裹在碳质碎屑内或呈集合体与方解石、赤铁矿等共生,还常沿着沥青铀矿颗粒边缘生长[201-202]。关于成矿年龄,Duane等[203]获得(262±15)~(185±49)Ma,而 Molteno组的矿石年龄则为(132±33)Ma,这显示成矿在二叠纪时开始,并一直持续到白垩纪[196]。铀的来源复杂,风化后的花岗岩碎屑和火山喷发物是最主要的物源[199,203-204]。该矿床的形成受同沉积构造、沉积环境和成岩作用共同影响[205]。
4 结论
按照“构造单元+主要矿种+主要成矿类型”的原则,在分析各成矿带所处区域地质背景的基础上,结合矿床的成矿地质条件、控矿因素、成矿时代和成矿规律等研究,将南非划分为6个不同的成矿区带,并明确了各个成矿带内的主要优势矿种与成矿类型。初步分析了南非境内不同时代的构造单元成矿专属性,这种认识对后续的项目安排部署具有非常重要而现实的指导意义。
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