吳興源,劉曉陽(yáng),王 杰,任軍平,何勝飛,龔鵬輝,劉 宇
(中國(guó)地質(zhì)調(diào)查局天津地質(zhì)調(diào)查中心,天津 300170)
南非地處非洲大陸最南端,礦產(chǎn)資源豐富,成礦類(lèi)型多樣,其資源總量占非洲的50%,居全球第5位。目前,南非境內(nèi)已發(fā)現(xiàn)礦產(chǎn)達(dá)60多種,優(yōu)勢(shì)礦種多。其中,鉑族金屬、金、紅柱石和螢石儲(chǔ)量居世界第一位,分別占世界的88.7%,12.8%,17.8%,37%;錳、鉻、鋯、蛭石儲(chǔ)量居世界第二位,分別占24.1%,37.1%,25%和35.9%;釩和磷酸鹽儲(chǔ)量居世界第三位,分別占23.1%和9.4%;金剛石和鈦金屬儲(chǔ)量居世界第四位,分別占12.1%和9.8%[1];煤炭、鐵、鉛、鈾、銻與鎳礦等資源儲(chǔ)量也均位于世界前列,但較缺乏石油和天然氣。
南非擁有許多世界級(jí)大型礦床,如威特沃特斯蘭德盆地金-鈾礦床、德蘭士瓦盆地鐵-錳-石棉礦床、布什維爾德雜巖體鉑族元素礦床、帕拉博拉雜巖體銅-磷灰石-蛭石礦床等,均為著名的超大型礦床[1]。作者在研究南非的區(qū)域地質(zhì)背景與相關(guān)礦床成因規(guī)律后,發(fā)現(xiàn)不同礦床與其所處的構(gòu)造背景能大致相聯(lián)系,即不同的構(gòu)造背景對(duì)應(yīng)不同的成礦專(zhuān)屬性。為此,本文提出了南非成礦區(qū)帶的劃分方案,概略介紹了每個(gè)成礦帶內(nèi)的一些典型礦床特征,以期對(duì)下一步的找礦和資源潛力分析提供一定的借鑒。
南非是世界上少數(shù)幾個(gè)保存有完好太古宙克拉通的地區(qū)之一,地質(zhì)演化歷史復(fù)雜,其構(gòu)造單元包括卡普瓦爾太古宙克拉通和一系列元古宙—古生代造山帶,以及一個(gè)代表弧后前陸沉積系統(tǒng)的卡魯盆地。
卡普瓦爾克拉通由一系列中-低級(jí)變質(zhì)的太古宙地塊拼貼而成,其形成及演化過(guò)程包括早期陸核形成階段(3 100~3 700Ma)和晚期克拉通化階段(2 600~3 100Ma)[2-4]。該克拉通內(nèi)部以科爾斯伯格磁性線(xiàn)新?tīng)顦?gòu)造(Colesberg manetic Lineament)、塔巴津比—默奇森線(xiàn)性構(gòu)造(Thabazimbi-Murchison Lineament)和Inyoka斷裂為界進(jìn)一步劃分出4個(gè)次一級(jí)地塊:斯威士蘭(Swaziland)地塊、蘭德(Witwatersrand)地塊、彼得斯堡(Pietersburg)地塊和金伯利(Kimberly)地塊[5](圖1)。
圖1 南非大地構(gòu)造單元?jiǎng)澐謭D(據(jù)文獻(xiàn)[5]修改)Fig.1 Geotectonic division of South Africa
1.1.1 斯威士蘭地塊
斯威士蘭地塊代表了卡普瓦爾克拉通的陸核部分,出露有巴伯頓(Barberton)綠巖帶及南非最古老的巖石單元——古老片麻雜巖(Ancient Gneiss Complex)。
古老片麻雜巖(AGC)的主體由古太古代—中太古代的英云閃長(zhǎng)巖-奧長(zhǎng)花崗巖-花崗閃長(zhǎng)巖質(zhì)片麻巖(TTG巖系)構(gòu)成,夾一些角閃巖(變質(zhì)基性火山巖,原巖包括玄武質(zhì)科馬提巖、富鎂/富鐵拉斑玄武巖等)及少量的綠巖帶殘留體,并被大量后期的花崗巖侵入[6-8]。姆普魯茨(Mpuluzi)巖基與皮格斯皮克(Piggs Peak)巖基(侵位時(shí)代約3 100Ma)作為AGC的北部邊界,也被視為與巴伯頓綠巖帶的天然分界線(xiàn)[9]。AGC內(nèi)部最重要的地質(zhì)單元為恩格瓦尼(Ngwane)片麻巖[10],形成時(shí)代3 200~3 660 Ma,大部分遭受高角閃巖相變質(zhì),部分達(dá)到麻粒巖相[11-12]。此外,在產(chǎn)自Phophonyane內(nèi)露層(Inlier)或Phophonyane剪切帶的條帶狀英云閃長(zhǎng)-奧長(zhǎng)花崗質(zhì)片麻巖(恩格瓦尼片麻巖)中獲得的卡普瓦爾克拉通最古老的鋯石Pb-Pb年齡為3 640~3 660 Ma[13-15]。
巴伯頓綠巖帶(BGB)主要由綠片巖相變質(zhì)(局部達(dá)角閃巖相)的火山-沉積地層組成,內(nèi)部以Inyoka斷層為界,分為南北2部分。該區(qū)因發(fā)現(xiàn)科馬提巖而聞名,得到地質(zhì)界的持續(xù)關(guān)注[16-18]。巴伯頓綠巖帶的形成時(shí)代為3 200~3 530Ma,從下到上可分為3個(gè)群:昂弗瓦特(Onverwacht)群、無(wú)花果樹(shù)(Fig Tree)群和摩德斯(Moodies)群[19]。
1.1.2 蘭德地塊
因蘭德盆地中盛產(chǎn)礫巖型金礦而馳名世界,地塊的大部分區(qū)域被中太古代—古元古代的變質(zhì)火山-沉積地層所覆蓋。老的基底物質(zhì)只在局部出露,約翰內(nèi)斯堡(Johannesburg)穹窿和弗里德堡(Vredefort)穹窿是為數(shù)不多的太古宙花崗片麻巖露頭。
(1)約翰內(nèi)斯堡穹窿。出露面積約700km2,主體為T(mén)TG系列片麻巖與少量的太古宙綠巖殘留體。太古宙綠巖帶物質(zhì)包括一些具有科馬提巖、高鎂玄武巖及拉斑玄武巖親緣性的超鎂鐵-鎂鐵質(zhì)火山巖及相應(yīng)的侵入巖,它們被后期TTG巖石侵入并經(jīng)歷變質(zhì)、破裂、混合巖化等過(guò)程[20-22]。最老的TTG系列巖石是古太古代奧長(zhǎng)花崗質(zhì)片麻巖(約3 340Ma)[23],但主體還是較年輕的花崗巖類(lèi),包括中?;◢弾r(3 120Ma)與似斑狀花崗巖(3 110 Ma)[23]。
(2)弗里德堡穹窿。位于約翰內(nèi)斯堡東南約120km處,由于存在假玄武玻璃質(zhì)角礫巖(pseudotachylitic breccias)與弗里德堡脈巖(Vredefort Granophyre)、超高壓變質(zhì)石英礦物(柯石英、斯石英)等沖擊變質(zhì)的證據(jù),多數(shù)研究者認(rèn)為其形成過(guò)程與隕石撞擊作用(2 020Ma)有關(guān)[24-25]。通常把它分成兩個(gè)部分,包括直徑為40~45km的核部(core)及寬度為20km 的環(huán)形外帶(collar)[26]。其中,核部由太古宙花崗片麻巖(>3 100Ma)組成,外環(huán)則由一些近直立甚至倒轉(zhuǎn)的中太古代—早元古代(2 250~3 070Ma)變質(zhì)火山-沉積地層構(gòu)成[25]。另外,在其東南方位殘留太古宙綠巖帶物質(zhì)(Greenlands Greenstone Complex),與巴伯頓綠巖帶的特征極其相似,且形成時(shí)代也相近(3 300Ma)[27]。
1.1.3 彼得斯堡地塊
彼得斯堡地塊以穆奇森(Murchison)綠巖帶為界與蘭德地塊相分隔,其內(nèi)部則出露2條綠巖帶:彼得斯堡(Pietersburg)綠巖帶和吉亞尼(Giyani)綠巖帶,以及一些太古宙的花崗巖-花崗片麻巖(如Makoppa穹窿)。穆奇森綠巖帶是NEE走向的太古宙變質(zhì)火山-沉積地層,長(zhǎng)約140km,寬10~15km,與塔巴津比—默奇森線(xiàn)性構(gòu)造呈平行展布[28]。Vearncombe等將該綠巖帶劃分為4個(gè)單元:Rubbervale組、Murchison單元、Silwana角閃巖和La France組[29]。其中,Rubbervale組主要由片巖(含石英碎斑)和少量酸性火山巖、凝灰?guī)r及火山角礫巖組成,它們的噴發(fā)年齡約2 970Ma[30],其成分與Silwana角閃巖相當(dāng)。Murchison單元是由一大套超鎂鐵-鎂鐵質(zhì)火山巖、火山-沉積巖和片巖。La France組位于穆奇森綠巖帶的最南端,主要由石英片巖、藍(lán)晶石-十字石-石榴石云母片巖組成[31]。
(1)彼得斯堡綠巖帶。長(zhǎng)125km,寬25km,是一條NE向延伸的變質(zhì)帶,總體上為變質(zhì)超鎂鐵-鎂鐵質(zhì)巖與變沉積巖組合,二者為不整合接觸[32]。下段主要包括變質(zhì)拉斑玄武巖,變質(zhì)輝長(zhǎng)巖,蛇紋石化橄欖巖夾條帶狀鐵建造,燧石層及變質(zhì)碳酸鹽巖等;上段沉積巖稱(chēng)為Uitkyk組,巖石類(lèi)型為礫巖、雜砂巖、頁(yè)巖及火山角礫巖。從后期侵入的同變形花崗巖中獲得鋯石207Pb/206Pb年齡為2 850Ma,表明綠巖帶形成不晚于2 900Ma[32-33]。
(2)吉亞尼綠巖帶。呈NEE走向,其總長(zhǎng)度近70km,中央最寬處為15~20km,它從Klein-Letaba河以西開(kāi)始一分為二,分為Khavagari arm(北分支)和Lwaji arm(南分支)[34]。該綠巖帶主要為變質(zhì)超鎂鐵-鎂鐵質(zhì)火山巖形成的片巖(夾條帶狀鐵建造),包括透閃石-滑石-綠泥石-角閃石片巖,內(nèi)部殘留極少量塊狀超鎂鐵質(zhì)巖,主要是一些含橄欖石-蛇紋石-直閃石的巖石(已強(qiáng)烈風(fēng)化),原巖為純橄巖或方輝橄欖巖[35]。從超鎂鐵-鎂鐵質(zhì)巖石的夾層(變安山巖)中獲得鋯石207Pb/206Pb年齡為3 200Ma,近似代表該綠巖帶的形成年齡[32,36]。
(3)Makoppa穹窿。位于彼得斯堡地塊的西南端,一直延伸到博茨瓦納境內(nèi)。在南非出露面積約5 000km2,主要巖石類(lèi)型有:灰白色中-粗粒、局部變形的Vaalpenskraal奧長(zhǎng)花崗-英云閃長(zhǎng)質(zhì)片麻巖,紅色粗粒的斑狀花崗巖類(lèi)(Makoppa花崗閃長(zhǎng)-二長(zhǎng)花崗巖)以及灰白色中-細(xì)粒的Rooibokvlei花崗閃長(zhǎng)-二長(zhǎng)花崗巖[37]。最老的 Vaalpenskraal片麻巖中獲得2個(gè)鋯石Pb-Pb年齡分別為(3 013±11)Ma,(3 034±64)Ma;最年輕的Rooibokvlei花崗閃長(zhǎng)巖中則出現(xiàn)2個(gè)新太古代的鋯石年齡,分別為(2 777±35)Ma,(2 797±2)Ma[37]。
1.1.4 金伯利地塊
金伯利地塊大部分被新太古代Ventersdorp群(2 710~2 780Ma)及晚古生代—中生代卡魯巖系、新生代卡拉哈里(Kalahari)沙漠所覆蓋[4],其中出露一條近SN向延伸的Kraaipan-Madibe-Amalia花崗-綠巖帶。該帶長(zhǎng)約250km,由太古宙TTG片麻巖、石英二長(zhǎng)巖、變質(zhì)基性火山巖與條帶狀鐵建造等組成[4,38]。最老的結(jié)晶巖系見(jiàn)于金伯利金剛石礦區(qū)的英云閃長(zhǎng)-花崗閃長(zhǎng)質(zhì)片麻巖,其年齡約3 250 Ma[39],其余大部分花崗片麻巖則形成于2 800~3 000Ma[40]。北邊的Kraaipan綠巖帶火山巖由于遭受變質(zhì)/熱液蝕變改造,大都表現(xiàn)為塊狀-層狀的角閃巖與角閃石-綠泥石-綠簾石片巖,它內(nèi)部年齡約3 080Ma[41]或(3 191.1±7.9)Ma[42];南邊的阿瑪利亞(Amalia)綠巖帶加積火山角礫凝灰?guī)r的年齡約(2 754±5)Ma[43],而石英-綠泥石-絹云母片巖則產(chǎn)于 (2 740±13)Ma[4]。
林波波帶呈 NEE向,長(zhǎng)約650km,寬約200 km,是由太古宙的津巴布韋克拉通與卡普瓦爾克拉通二者之間發(fā)生碰撞、拼貼而形成的造山帶[44](圖1)。變質(zhì)帶北以北林波波逆沖帶為界與津巴布韋克拉通相接,南以赫特(Hout)河剪切帶為界與卡普瓦爾克拉通相鄰。林波波帶內(nèi)部又以 Magagohate-Triangle剪切帶和Palala-Tshipise剪切帶為界,進(jìn)一步劃分出3個(gè)次級(jí)地塊:北部邊緣帶、中央帶和南部邊緣帶。
(1)北部邊緣帶。帶中巖石包括紫蘇花崗閃長(zhǎng)-紫蘇花崗片麻巖、斑狀富鉀花崗巖、紫蘇花崗巖、超鎂鐵質(zhì)-鎂鐵質(zhì)片麻巖(不規(guī)則的透鏡狀或不連續(xù)的層狀)和極少量的變泥質(zhì)巖[45-46]。該帶巖石沿北林波波斷層逆沖到北側(cè)的津巴布韋克拉通之上,斷層的活動(dòng)時(shí)代為2 550~2 630Ma[47-48]。地表90%的露頭均為紫蘇花崗閃長(zhǎng)-紫蘇花崗片麻巖,部分已發(fā)生退變質(zhì)(紫蘇輝石被角閃石或黑云母交代)[45]。這些片麻巖在化學(xué)成分上屬于英云閃長(zhǎng)巖-奧長(zhǎng)花崗巖或二長(zhǎng)花崗巖,鋯石U-Pb定年結(jié)果為2 580~2 750Ma[49]。
(2)中央帶。巖性變化較大,主要以發(fā)育Beit Bridge變質(zhì)雜巖為顯著特征,還有一些正片麻巖(變質(zhì)深成巖體)。其中,Beit Bridge雜巖主要由淺色長(zhǎng)英質(zhì)片麻巖、石英巖與大理巖組成,夾變質(zhì)泥質(zhì)片麻巖、磁鐵石英巖與基性麻粒巖[46,50]。正片麻巖中含一部分TTG巖系,它們構(gòu)成了Sand River片麻巖,其原巖侵位年齡推測(cè)為3 170~3 310Ma[51-52];其他的片麻巖形成時(shí)代多為2 510~2 730Ma[51]。該帶主要記錄了2期高級(jí)變質(zhì)作用,第一期為2 500~2 700Ma,第二期為1 970~2 030Ma[52-53]。
(3)南部邊緣帶。南部邊緣帶的麻粒巖與相鄰的卡普瓦爾克拉通花崗-綠巖帶具有相似的地球化學(xué)及同位素特征,極可能源自同一來(lái)源,形成時(shí)代為2 900~3 050Ma[54]。南部邊緣帶的巖石沿赫特河剪切帶向南逆沖到卡普瓦爾克拉通之上[55],剪切帶的活動(dòng)被限定在2 640~2 760Ma[56]。該帶內(nèi)部以一條斜方閃石等變線(xiàn)為界(斜方輝石發(fā)生退變質(zhì),并被直閃石交代)分為北段和南段2個(gè)部分[57]。
海斯(Kheis)帶介于太古宙卡普瓦爾克拉通與中元古代那馬奎帶之間,是一條狹長(zhǎng)的褶皺-逆沖帶,總體上為變質(zhì)火山-沉積地層[4](圖1)。它的東北邊界及東南邊界分別被Blackridge逆沖斷層和Doringberg斷裂所限,而西邊以Trooilapspan剪切帶及Brackbosch斷層為界[58]。Moen將該變形-變質(zhì)帶從東到西劃分為4段:Olifantshoek超群、Brulpan群、Vaalkoppies群和 Wilgenhoutsdrif群[58]。最老的變質(zhì)沉積巖來(lái)自東部,其年齡為1 890~1 930Ma[59]。此外,Cornell也報(bào)道過(guò) Olifantshoek超群中約1 750Ma的基性巖[60]。
(1)Olifantshoek超群。底部的Mapedi組為礫巖、千枚狀頁(yè)巖與石英巖,厚<1km[61];上覆的Lucknow組厚約500m,巖性為粉砂巖、石英巖和礫巖;Hartley組(與Boegoeberg Dam組相當(dāng))覆蓋在Lucknow組之上,主要為杏仁狀-塊狀玄武巖,少量礫巖、集塊巖及凝灰?guī)r;最頂部的Volop群主要由雜色石英巖、砂巖及少量頁(yè)巖組成。
(2)Brulpan群包括 Dabep組、Boegoeberg組、Uitdraai組、Prynnsberg組和Groblershoop組等5個(gè)組。最底部的Dabep組與Olifantshoek超群為斷層接觸,發(fā)生強(qiáng)烈褶皺變形。巖性從石英-絹云母片巖過(guò)渡為石英-綠泥石-綠簾石片巖、陽(yáng)起石角閃巖等。Boegoeberg組與Prynnsberg組巖性類(lèi)似,均由淺灰-白色的中細(xì)粒石英巖組成,厚<550m。Uitdraai組由淺灰色石英巖、白色石英巖與片巖組成,推測(cè)其厚度200m。Groblershoop組由厚層的細(xì)粒云母-石英片巖組成,在奧蘭治河(Orange River)河谷廣泛出露。
(3)Vaalkoppies群包括Dagbreek組和Sultanaoord組。Dagbreek組主要由變泥質(zhì)巖,片巖及石英巖組成;Sultanaoord組主要為白色的塊狀石英巖,夾一些風(fēng)化呈紅色的千糜巖,巖層已發(fā)生強(qiáng)烈變形,最厚約800m。
(4)Wilgenhoutsdrif群包括Zonderhuis組和Leerkrans組。Zonderhuis組的最底部為一套厚300 m的淺紫色石英巖(Groot Drink Member),上覆巖層為雜色千枚巖與片巖,其內(nèi)部含灰?guī)r、蛇紋巖及石英巖的透鏡體。在Karos東南約20km處,Leerkrans組強(qiáng)烈褶皺,包含2個(gè)火山-碎屑沉積旋回。在Leerkrans附近,該組主要由片巖構(gòu)成。Moen指出,Leerkrans組火山巖在地球化學(xué)上表現(xiàn)出雙峰式巖套的特點(diǎn)[62]。
那馬奎帶位于中元古代那馬奎—納塔爾(Namaqua-Natal)造山帶的西段,該造山帶被認(rèn)為形成于格林維爾期(1 000~1 300Ma)Rodinia超大陸聚合過(guò)程中的那馬奎造山運(yùn)動(dòng)[63],也是中元古代基巴拉(Kibaran)造山帶的一部分[64](圖1)。它東北毗鄰太古宙卡普瓦爾克拉通,南以Beattie地球物理異常帶及相關(guān)的Southern Cape Conductive Belt為界[65]。該變質(zhì)帶那馬奎段的次一級(jí)構(gòu)造單元?jiǎng)澐忠?直 存 在不 同 意 見(jiàn)[66-67]。本 文 采 用 Eglington(2006)提出的分類(lèi)建議,將那馬奎段分為3個(gè)區(qū)域:Richtersveld subprovince,Bushmanland subprovince和 Gordonia subprovince[68]。
(1)Richtersveld subprovince。主要包括鈣堿性的Vioolsdrif巖基和變質(zhì)表殼巖(奧蘭治河(Orange River)群),前者的巖性為輝長(zhǎng)巖、閃長(zhǎng)巖、花崗閃長(zhǎng)巖、花崗巖等,侵位時(shí)代1 750~2 000 Ma[69];后者主要由中-低級(jí)變質(zhì)的鈣堿性火山巖組成。
(2)Bushmanland subprovine??蛇M(jìn)一步劃為Okiep Terrane,Aggeneys Terrane與Garies Terrane三部分。Okiep Terrane內(nèi)部以Skelmfontein逆沖斷裂為界,分成北段(Skeinkopf domain)和南段(Okiep domain),著名的Okiep銅礦床即產(chǎn)于此地。Skeinkopf domain由灰色正片麻巖(Gladkop Suite;~1 800Ma)[70]與一些角閃巖、變泥質(zhì)巖(含堇青石)及鈣硅酸鹽巖組成[71]。Okiep domain為1 000~1 200Ma的 正片 麻巖[70,72]侵入到老片麻巖中(Gladkop Suite),并發(fā)育一套麻粒巖相變質(zhì)的火山-沉積地層(Khurisberg subgroup),以出現(xiàn)二輝麻粒巖為特征[64]。Aggeneys Terrane主要出露一套變質(zhì)火山-沉積地層和大量正片麻巖。前者為Bushmanland群,巖性包括含鐵石英巖、頁(yè)巖、角閃巖并伴隨有大規(guī)模鉛-鋅-銅-銀成礦作用[70],其形成時(shí)代1 650~2 000Ma[69];后者包括Achab片麻巖、Hoogoor片麻巖等,在Achab片麻巖中獲得了單顆粒鋯石SHRIMP U-Pb年齡值約2 000Ma[73],但最新的測(cè)年結(jié)果卻顯示為新元古代末期(1 160~1 190 Ma)[74]。Garies Terrane由一系列正、副片麻巖組成,并被大量后期花崗巖侵入[68]。
(3)Gordonia subprovine??蛇M(jìn)一步分為 Kakamas Terrane和 Areachap Terrane。Kakamas Terrane東以Bovenrugzeer剪切帶與Areachap Terrane相隔,西以Hartbees River逆沖斷層為界。主要由強(qiáng)烈變形的角閃巖相副片麻巖組成,并被大量花崗巖侵入(包括一些紫蘇花崗巖),其內(nèi)部鈣硅酸鹽巖石的Pb-Pb年齡為約2 000Ma,而Rosynenbosch地區(qū)硫化物礦石的Pb模式年齡為約1 100 Ma[67-68]。Areachap Terrane是一條狹長(zhǎng)的角閃巖相變質(zhì)帶,巖性包括變玄武巖及正片麻巖,以東邊的Trooilapspan剪切帶及Brackbosch斷層為界與海斯帶分開(kāi)。原巖年齡為1 300~1 600Ma[75]。
開(kāi)普褶皺帶地處南非的南端,在南非境內(nèi)長(zhǎng)約1 300km,是形成于岡瓦納大陸聚合過(guò)程中的造山帶(圖1),其延伸至南美洲阿根廷、??颂m群島及南極大陸[76]。該褶皺帶分為3部分:構(gòu)造線(xiàn)方向近SN向展布的西分支、構(gòu)造線(xiàn)方向近EW向展布的南分支和介于二者之間過(guò)渡的銜接帶[77-78]。開(kāi)普造山運(yùn)動(dòng)引起開(kāi)普(Cape)超群及上覆卡魯(Karoo)超群底部Dwyka群巖石的明顯變形。西分支內(nèi)部變形較弱,主要產(chǎn)出一些平緩、開(kāi)闊的直立褶皺與單斜構(gòu)造;而南分支內(nèi)部則變形強(qiáng)烈,以發(fā)育一系列N傾的疊瓦狀逆沖斷層、雙沖構(gòu)造與倒轉(zhuǎn)褶皺為顯著特征[79-80]。
開(kāi)普造山帶內(nèi)最老的地質(zhì)單元為新元古代變質(zhì)火山-沉積巖,其隸屬于泛非期的Saldania造山帶,大量同造山-后造山期的開(kāi)普(Cape)花崗巖組合(510~550Ma)侵入其中。這套地層主要沉積在一系列裂谷盆地中,普遍遭受綠片巖相變質(zhì)作用,并以一種內(nèi)窗層(inliers)的形式被保留下來(lái)[80-82]。主要有開(kāi)普敦以北地區(qū)的Malmesbury群、Oudtshoorn地區(qū)的Kango群、George地區(qū)的Kaaimans群和Port Elizabeth地區(qū)的Gamtoos群。
奧陶紀(jì)—早石炭世開(kāi)普超群是本區(qū)內(nèi)分布面積最廣的地層,內(nèi)部已發(fā)生強(qiáng)烈變形,地層最厚可達(dá)8 km,為產(chǎn)自大陸邊緣(大陸架)環(huán)境的硅質(zhì)碎屑沉積[77,83]。開(kāi)普超群自下而上可分為3段:桌山(Table Mountain)群、Bokkeveld群和Witteberg群,前二者構(gòu)成了開(kāi)普超群野外露頭的主體。其中,桌山群主要為石英巖,局部出現(xiàn)一些千枚巖,而B(niǎo)okkeveld群則以頁(yè)巖為主,次為石英巖。相比之下,Witteberg群分布較局限,僅出現(xiàn)在開(kāi)普超群的北部邊緣,巖性包括頁(yè)巖與石英巖(在該群上部層位中還出現(xiàn)一層混雜巖),并被晚石炭世—中侏羅世的卡魯超群不整合覆蓋[77,80]。
南非境內(nèi)的卡魯盆地是指狹義的卡魯盆地,有研究認(rèn)為它是弧后前陸盆地[84-85]??斉璧貎?nèi)的沉積地層稱(chēng)為卡魯超群,其沉積長(zhǎng)達(dá)125Ma,從晚石炭世一直持續(xù)到中侏羅世[76]??敵簭脑绲酵?、從下到上可分為4段:Dwyka群、Ecca群、Beaufort群和Stormberg群(包括上覆的Drakensberg群玄武巖層)。其中,Dwyka群與Ecca群的中下部為深海相沉積環(huán)境,再過(guò)渡到Ecca群上部則為淺海相沉積環(huán)境,Beaufort群和Stormberg群則為河流相的沉積[86]。
卡魯超群底部的Dwyka群與下伏開(kāi)普超群最上部的Witteberg群為不整合接觸關(guān)系,二者的沉積間斷持續(xù)了30Ma,這一不整合的成因目前尚存爭(zhēng)議[85,87]。在弧后前陸盆地體系模型中,Dwyka群被認(rèn)為是前淵(foredeep)在處于欠補(bǔ)償狀態(tài)下的沉積產(chǎn)物且沉積時(shí)限為290~300Ma,也有人認(rèn)為其沉積上限為305Ma左右[88]。它是厚達(dá)800m的冰海相雜巖,巖層基質(zhì)以泥巖為主,內(nèi)部含有從漂浮的冰筏上墜落的礫石[89]。
Ecca群主體是二疊紀(jì)的遠(yuǎn)洋與重力流沉積(濁積巖),其最厚可達(dá)3km,巖性為泥巖、粉砂巖和砂巖,局部含煤層[84,86]。Ecca群底部的 Prince Albert組凝灰?guī)r夾層同位素年齡為(288±3.0)Ma和(289±3.8)Ma[88],與孢粉鑒定結(jié)果推測(cè)的290Ma較為吻合[90]。
Beaufort群是橫跨二疊紀(jì)和三疊紀(jì)的陸相沉積,出露面積約200 000km2,最厚達(dá)7km[91]。其年齡主要通過(guò)地層中所含的合弓綱爬行動(dòng)物化石組合加以限定,其中,二疊紀(jì)與三疊紀(jì)的界線(xiàn)以貘頭獸(Tapinocephalus)及二齒獸(Dicynodon)兩種化石的消失為標(biāo)志[76,92]。Beaufort群內(nèi)部以泥巖及粉砂巖為主,夾少量透鏡狀或條帶狀砂巖[93]。
Stormberg群代表晚三疊世—中侏羅世的陸相沉積,其底部 Molteno組與下伏Beaufort群頂部Burgersdorp組之間存在一個(gè)具穿時(shí)性的不整合。Molteno組為以沖積物為主的河流相楔形沉積[86,94-95]。與之不同的是,處于中間層位的 Elliot組主要由粒度總體向上變細(xì)的砂巖及泥巖旋回組成。Drakensberg溢流玄武巖直接覆蓋在Stormberg群之上,火山巖層厚達(dá)1.37km,巖性以拉斑玄武巖為主。它們從晚三疊世開(kāi)始噴發(fā),一直持續(xù)到早白堊世。該期火山活動(dòng)與岡瓦納大陸裂解有關(guān),也預(yù)示卡魯盆地演化的結(jié)束[86]。
南非成礦區(qū)帶的劃分是在整理前人文獻(xiàn)資料的基礎(chǔ)上,以構(gòu)造單元為基本輪廓,結(jié)合主要成礦元素和主要成礦類(lèi)型,考慮成礦地質(zhì)條件、控礦因素及成礦時(shí)代因素[96],按照“構(gòu)造單元+主要礦種+主要成礦類(lèi)型”的原則,將南非分為6個(gè)Ⅲ級(jí)成礦區(qū)帶:①太古宙卡普瓦爾克拉通金-鈾、鐵-銅-鉑族元素、金剛石成礦區(qū);②古元古代林波波金-金剛石成礦帶;③古元古代海斯鐵-錳成礦帶;④中元古代那馬奎銅-鉛-鋅多金屬成礦帶;⑤古生代開(kāi)普褶皺帶鎢-錫-銅-鉬多金屬成礦帶;⑥古生代—中新生代卡魯盆地鈾-金剛石-砂金成礦區(qū)(圖2)。
成礦區(qū)位于太古宙卡普瓦爾克拉通內(nèi),成礦作用主要與太古宙早期陸殼形成以及克拉通晚期陸內(nèi)-陸緣裂谷、裂陷盆地的演化有關(guān);而金剛石成礦則與后期克拉通內(nèi)部陸下巖石圈地幔或軟流圈地?;顒?dòng)引發(fā)的金伯利巖漿爆發(fā)直接相關(guān)。礦床類(lèi)型有綠巖帶型金礦、蘭德盆地礫巖型金(鈾)礦、布什維爾德巖漿型銅鎳硫化物-鉑族元素礦床、德蘭士瓦沉積變質(zhì)型鐵(錳)礦及原生金伯利巖型金剛石礦等。
3.1.1 綠巖帶型金礦
南非綠巖帶型金礦床主要分布于卡普瓦爾克拉通內(nèi)的巴伯頓、穆奇森、彼德斯堡及薩瑟蘭(吉亞尼)等綠巖帶中,通常被認(rèn)為屬造山型金礦[97-98],其中以巴伯頓綠巖帶金礦床的產(chǎn)量和規(guī)模最大。自1883年以來(lái),巴伯頓綠巖帶中已發(fā)現(xiàn)300多個(gè)金礦床,黃金產(chǎn)量累計(jì)超過(guò)345t,金主要來(lái)自希巴(Sheba)(123.7t)、新康索特(New Consort)(63.4t)、費(fèi)爾維尤(Fairview)(63.4t)等礦山[99]。金礦床主要集中于巴伯頓綠巖帶的西北部,特別是在Jamestone片巖帶、希巴山地區(qū)和摩德斯山地區(qū)[100]。
圖2 南非主要成礦區(qū)帶劃分示意圖Fig.2 Sketch showing division of main metallogenic belts or areas in South Africa
金主要賦存于綠巖帶的石英脈(網(wǎng)脈)或韌性剪切帶中,常與毒砂及黃鐵礦密切共生,且以“非明金”和銀金礦夾雜物的形式產(chǎn)出[101]。前人多認(rèn)為金礦化受逆沖斷層的控制[102-103],但也有人基于北部成礦區(qū)(新康索特、希巴和費(fèi)爾維龍礦床)的資料提出金礦化是受晚期伸展階段的(截切逆沖斷層)斷裂系控制[99,104]。研究表明,巴伯頓地區(qū)金礦床的成礦時(shí)代主要為中太古代。de Ronde等人通過(guò)對(duì)費(fèi)爾維尤金礦的研究獲得金礦的成礦年齡為(3 126±21)~(3 084±18)Ma[105];Dziggel等對(duì)新康索特金礦的研究揭示其成礦分為2個(gè)階段,介于3 040~3 030Ma[106],而Dirks等在希巴金礦附近與成礦近于同期的巖脈中測(cè)得Pb-Pb年齡約為3 015 Ma[107]。對(duì)于金礦的成礦模式已有許多學(xué)者進(jìn)行了大量探索[102,108]。最近,王杰等[109]將其歸結(jié)為3種,即火山成因模式(Volcanogenic Model)[110]、轉(zhuǎn)換滑脫構(gòu)造模式(Transtensional Tectonics Model)[106]和構(gòu)造交叉模式(Intersections Model)[107]。
3.1.2 蘭德盆地古礫巖型金(鈾)礦
蘭德型金礦床位于維特沃特斯蘭德盆地周緣,全稱(chēng)為維特沃特斯蘭德古礫巖型金(鈾)礦。盆地中沉積了巨厚的太古宙火山-沉積巖系,礫巖型金(鈾)礦主要產(chǎn)自維特沃特斯蘭德超群的中蘭德群中[111]。蘭德礦區(qū)一共包含8個(gè)金礦田,即東蘭德(East Rand)、南蘭德(South Rand)、中蘭德(Central Rand)、西蘭德(West Rand)、卡勒頓維累(Carliton-ville)、可萊克斯多普(Klerkdorp)、自由邦省(Welkom)與埃溫德鎮(zhèn)(Evander)金礦田[112]。蘭德金礦自1886年發(fā)現(xiàn)以來(lái),已產(chǎn)黃金3.5×104t,尚有儲(chǔ)量約2×104t,開(kāi)采至今金品位一直保持在7×10-6~20×10-6。
金主要呈細(xì)分散狀態(tài)自然金形式產(chǎn)出,金的粒度為5~100μm,明金少見(jiàn),常與鈾礦、黃鐵礦密切共生,局部也與瀝青共生,金的平均品位3×10-6~25×10-6[100,111]。伴生的鈾礦在1952—1975年間共產(chǎn)出150×104t的 U3O8,鈾的平均品位達(dá)271×10-6[113]。該礦床的形成可能持續(xù)了很長(zhǎng)時(shí)間(3 074~2 642Ma)[111],其成因模式一直存在爭(zhēng)議,主要觀點(diǎn)有3種:砂礦模式(Placer model)[114-116],熱液模式(Hydrothermal model)[117-118]及介于二者之間的改造砂礦模式(Modified Placer model)[119-120]。
3.1.3 布什維爾德銅鎳硫化物-鉑族元素礦床
布什維爾德雜巖體(Bushveld Igneous Complex)是世界上最大的鎂鐵質(zhì)層狀侵入體,也是世界上單個(gè)蘊(yùn)藏鉑族元素(PGE)、鉻鐵礦和釩鈦磁鐵礦的最重要礦床[121]。其中,PGE儲(chǔ)量為65 473t,含有全球75%的鉑、54%的鈀和82%的銠[122];鎳金屬占全球的16%,儲(chǔ)量1 528×104t;銅金屬占全球的5.5%,儲(chǔ)量為688×104t;而鉻、金、釩和磁鐵礦的儲(chǔ)量分別為40×108t,1 152t,1 680×104t,10×104t[123]。
雜巖體的侵位受NEE向塔馬津比—默奇森線(xiàn)性構(gòu)造的控制[124],其中含礦巖體是勒斯滕堡(Rustenberg)超鎂鐵-鎂鐵質(zhì)層狀巖套。該巖套分為西支、東支和北支3部分,最主要的賦礦層位是東、西分支的UG2鉻鐵巖礦層、梅林斯基(Merensky)礦層以及北支的普拉特(Plat)礦層[125]。布什維爾德礦床的成巖成礦時(shí)代為古元古代(2 050~2 060 Ma),該礦床的成礦構(gòu)造背景為大陸邊緣裂谷環(huán)境。勒斯滕堡層狀巖套具有兩大成礦特征:一是巨量的PGE與低品位、低密度的硫化物層共生;二是具有完整的層序和礦化部位。該礦床的成礦機(jī)制主要是地殼的含水硅酸鹽成分遭受同化混染后導(dǎo)致硫化物熔離,從而形成賤金屬硫化物,而PGE的富集則主要是硫化物熔離作用或吸附作用的結(jié)果[121]。
3.1.4 德蘭士瓦沉積變質(zhì)型鐵(錳)礦
南非的德蘭士瓦沉積變質(zhì)型鐵(錳)礦以塔巴金比(Thabazimbi)鐵礦為典型礦床。
塔巴金比鐵礦床主要產(chǎn)于Leeuwbosch和Cornwall兩個(gè)礦區(qū),是一個(gè)巨大的、高品位的BIF型赤鐵礦礦床,資源儲(chǔ)量近5×108t。另外,在Leeuwbosch礦區(qū)還伴生有一個(gè)密西西比河谷型(MVT)的鉛-鋅-銀礦[126]。
鐵礦的賦礦層位為德蘭士瓦超群的Malmani亞群與Penge組。其中,Malmani亞群為一套碳酸鹽巖夾頁(yè)巖、燧石條帶,其中凝灰?guī)r的鋯石SHRIMP U-Pb年齡為(2 583±5)Ma或(2 588±7)Ma[127];而Penge組則主要由條帶狀含鐵紋層(赤鐵礦、磁鐵礦與針鐵礦)、含鐵硅酸鹽層(燧石、黑硬綠泥石)、菱鐵礦與頁(yè)巖夾層組成[128],其時(shí)代大致為(2 480±6)Ma[129],明顯受后期布什維爾德雜巖體侵入時(shí)巖漿熱液-流體的強(qiáng)烈改造[130]。
礦石類(lèi)型主要有脈狀或鮞狀礦石和角礫巖型礦石[129]。礦石礦物包括赤鐵礦、假象赤鐵礦及少量的褐鐵礦和菱鐵礦[128],脈石礦物主要有粗粒的石英、方解石和紅碧玉。礦體呈細(xì)脈狀或團(tuán)塊狀,大者可達(dá)數(shù)十米,明顯受斷層控制,且層控型特征非常顯著[129]。De Kock等人[131]通過(guò)對(duì) Van der Bijl礦井中鐵礦石的古地磁研究得到成礦年齡大致為1 930~2 050Ma。
Philpott和Ainslie[132]最早提出塔巴金比鐵礦的形成與一種表生來(lái)源的成礦流體淋濾地層有關(guān)。后來(lái),Netshiozwi[133]通過(guò)對(duì)礦區(qū)脈石礦物(石英、白云石及方解石)的流體包裹體研究證明,成礦流體鹽度范圍極廣,w(NaCl)=6%~27%,均一溫度為143~224℃,結(jié)合穩(wěn)定同位素的結(jié)果(δ(18OSMOW)=-9.7×10-3~-2.2×10-3),作者提出成礦流體很可能由2種流體混合而成。最近,M?ller等人[129]的工作表明,成礦流體的鹽度(NaCl)=9.2%~39.9%,溫度100~190℃,證實(shí)了前人關(guān)于成礦流體為混合成因的觀點(diǎn),并推測(cè)成礦溫壓條件為175℃,100MPa。
3.1.5 原生金伯利巖型金剛石礦
原生金伯利巖型金剛石礦是南非金剛石的主要來(lái)源,到目前為止已發(fā)現(xiàn)并開(kāi)采的、規(guī)模較大的礦床主要有9個(gè):金伯利(Kimberley)、德比爾斯(De Beers)、布爾豐坦(Bultfontein)、杜托伊斯賓(Dutoitspan)、韋塞爾頓(Wesselton)、芬奇(Finsch)、科菲方丹(Koffiefontein)、亞格斯豐坦(Jagersfontein)和庫(kù)里南(Cullinan/Premier)。其中,以庫(kù)里南礦床的規(guī)模和產(chǎn)量最大。金伯利巖被作為含礦母巖來(lái)尋找金剛石,南非金伯利巖漿噴發(fā)表現(xiàn)出階段性爆發(fā)、離散分布的特點(diǎn),時(shí)代包括早-中元古代、寒武紀(jì)、二疊紀(jì)、侏羅紀(jì)和白堊紀(jì)[134]。
庫(kù)里南金伯利巖筒是南非最大的金剛石礦,位于比勒陀利亞(Pretoria)北東約37km處,由于1905年在此發(fā)現(xiàn)了世界上最大的、重達(dá)3 106ct(625g)的庫(kù)里南鉆石而聞名于世。礦坑地表出露面積32 hm2,在平面上呈橢圓狀或腎狀。金伯利巖筒侵入德瓦士蘭超群及布什維爾德雜巖體中,其侵位年齡的測(cè)定結(jié)果為(1 179±36)Ma(單斜輝石Rb-Sr等時(shí)線(xiàn))[135]和(1 202±72)Ma(鈣鈦礦 U-Pb 定年)[136],并被厚75m的輝長(zhǎng)巖巖床切穿(1 150 Ma)[137]。
該處金伯利巖主要有3次侵入,2次是典型的凝灰質(zhì)金伯利角礫巖(tuffisitic kimberlite breccias,即TKB),最后1次為侵入相淺成金伯利核雜巖。其中,金伯利角礫巖包括灰型(grey kimberlite)和褐型(brown kimberlite)2種類(lèi)型;而淺成金伯利巖核雜巖也稱(chēng)黑金伯利巖(black kimberlite),它又可分出斑雜狀金伯利巖變種(Piebald Kimberlite)[100,134,137]。此外,最晚期階段還有一次高碳酸鹽含量的金伯利巖脈侵入[138]。這些金伯利巖攜帶的捕虜體類(lèi)型多樣,主要有粗粒/變形石榴石二輝橄欖巖,方輝橄欖巖與少量榴輝巖[137]。該礦床于1903年開(kāi)始開(kāi)采,最初的鉆石品級(jí)高達(dá)170cpht(ct/102t)[138],通常的品級(jí)大多為40~80cpht(隨深度增加而逐漸升高),但這一數(shù)字在2005年下降到28 cpht[139]。除庫(kù)里南鉆石以外,該礦山隨后又產(chǎn)出了像Centenary鉆石(599ct)及Unkown Brown鉆石(1083ct)一類(lèi)的大鉆石。Williams[140]指出庫(kù)里南礦山的鉆石普遍具有特征的油性光澤(“oily brilliance”),卻很少具有八面體晶型[141]。這些金剛石中的少量是Ⅱ型金剛石(不含氮),當(dāng)中又有極少一部分是淺藍(lán)色的含硼Ⅱb型金剛石[137]。
此成礦帶位于林波波帶的中-南端,成礦作用受卡普瓦爾克拉通與津巴布韋克拉通之間碰撞事件(約2 600Ma)的影響。其中,綠巖帶型金礦受構(gòu)造控制明顯,典型礦床包括吉亞尼(Giyani)金礦床等。此外,林波波帶還包括一個(gè)原生金伯利巖型金剛石礦,即韋內(nèi)沙(Venetia)金剛石礦床,其鉆石產(chǎn)量極高。
3.2.1 吉亞尼(Giyani)綠巖帶型金礦
吉亞尼(也叫Sutherland)綠巖帶金礦床位于林波波帶的最南端,毗鄰赫特河剪切帶或位于其中。該綠巖帶目前已知有金礦產(chǎn)地55處,其中35處正在開(kāi)采,其他20處為關(guān)停狀態(tài)[142]。這些暫時(shí)關(guān)停的金礦中,6個(gè)金礦(Osprey,Louis Moore,F(xiàn)umani,F(xiàn)ranke,Klein Letaba和Birthday)貢獻(xiàn)了該綠巖帶金礦總產(chǎn)量的97%[143]。研究表明,這些礦床都具有明顯的造山型金礦特征[97-98,144]。
吉亞尼綠巖帶地層為吉亞尼群,主要由鎂鐵-超鎂鐵質(zhì)火山巖夾變質(zhì)沉積巖與長(zhǎng)英質(zhì)火山巖,其中的變沉積巖由BIFs、石英巖、云母片巖及少量白云巖組成[145]。礦石中金的載體主要有以下4種:石英脈、條帶狀鐵建造、石英-硫化物脈和碳酸鹽(方解石)脈[146]。在大多數(shù)礦床中,金常與石英脈中的硫化物密切共生,但有時(shí)也以石英脈中的自然金或硅酸鹽中的包裹體形式出現(xiàn)[147-148]。石英脈的硫化物礦物組成穩(wěn)定,通常包括雌黃鐵礦、黃銅礦、黃鐵礦及毒砂等[147,149],金礦化主要與晚期由流體引發(fā)的退變質(zhì)作用有關(guān)[142]。關(guān)于金礦的成礦時(shí)代,Pretorius等[147]報(bào)道了Fumani礦床中切穿礦體的偉晶巖脈中原生白云母Rb-Sr年齡值為(2 632±53)Ma;而B(niǎo)arton和Van Reenen在Klein Letaba礦床附近韌性剪切帶的偉晶巖中也獲得相似的白云母Rb-Sr年齡結(jié)果(約2 660Ma),并認(rèn)為這代表了流體活動(dòng)年齡[150]。由此可見(jiàn),本區(qū)金礦的成礦時(shí)代不晚于新太古代。赫特河剪切帶是一級(jí)控礦構(gòu)造[151],而區(qū)內(nèi)一系列近EW走向、并毗鄰礦床的韌性剪切帶是次一級(jí)控礦構(gòu)造[152]。
3.2.2 韋內(nèi)沙(Venetia)金剛石礦床
韋內(nèi)沙金伯利巖型金剛石礦發(fā)現(xiàn)于1980年,位于林波波帶的中央帶[134,153],是南非目前產(chǎn)量最高的金剛石礦床,一共有14個(gè)巖筒,其中規(guī)模最大的2個(gè)(K1和K2)正在開(kāi)采。該礦為露天開(kāi)采,2005年的礦石產(chǎn)量為580×104t,產(chǎn)出金剛石達(dá)7.18×106ct,平均品位為122cpht[134]。含礦的金伯利巖為Group-1型[154],金伯利巖漿噴發(fā)時(shí)代大致為寒武紀(jì),其中金云母Rb-Sr等時(shí)線(xiàn)年齡為(530±4)Ma和(510±16)Ma[155],這與 Phillips等[156]對(duì)基質(zhì)中的金云母進(jìn)行Ar-Ar定年獲得的結(jié)果((519±0.6)Ma)較為吻合。
K1金伯利巖筒在平面上呈拉長(zhǎng)的靴形[157],可區(qū)分出噴發(fā)相的凝灰質(zhì)金伯利角礫巖和淺成侵入相的金伯利巖2種類(lèi)型[154]。K2巖管同樣表現(xiàn)出兩種明顯不同的巖相特征,Seggie等[154]將其分為東、西兩段,西段為淺成相金伯利巖,東段則為噴發(fā)相金伯利角礫巖。其中,西段的又進(jìn)一步分出兩種結(jié)構(gòu)差異顯著的球粒狀金伯利巖與均一的金伯利巖。Stiefenhofer等[158]對(duì)來(lái)自該礦床的100個(gè)圍巖捕虜體進(jìn)行了分析,指出它們包括橄欖巖與輝石巖兩大類(lèi)型。另外,該礦床的包裹體研究表明,這些金剛石很可能結(jié)晶于古老克拉通的巖石圈地幔[159]。
該成礦帶內(nèi)鐵錳成礦作用顯著,均為卡普瓦爾克拉通西部被動(dòng)大陸邊緣(海相)環(huán)境下形成的、具層控型特征的沉積變質(zhì)成因礦床,明顯受Eburnean期(1 900~1 800Ma)的海斯造山運(yùn)動(dòng)改造。在與卡普瓦爾克拉通交界處產(chǎn)有若干個(gè)大型-超大型礦床,錫興(Sishen)鐵礦(赤鐵礦為主)、卡拉哈里(Kalahari)錳礦及波斯特馬斯堡(Postmasburg)錳礦可作為其中的典型礦床。
3.3.1 錫興(Sishen)鐵礦
南非北開(kāi)普省的錫興鐵礦位于金伯利以西約200km處,該礦床產(chǎn)于Maremane背斜的西北邊緣。含礦地層主要為前寒武紀(jì)德蘭士瓦超群的Gamagara 及 Asbesheuwels 亞 群[128]。Gamagara亞群的底部為層紋狀礦體,向上依次為錫興頁(yè)巖(含赤鐵礦礫石及礫巖型礦體)、馬薩斯波特組含鐵石英巖及帕林組頁(yè)巖[160];而Asbesheuwels亞群則主要由條帶狀鐵建造、碧玉鐵質(zhì)巖和頁(yè)巖組成,常含不規(guī)則的透鏡狀礦體。
礦體受開(kāi)闊的向斜構(gòu)造控制,SN向延伸約50 km,寬約2km,向 W傾,傾角10°,并被后期的輝綠巖脈所截切[128]。礦區(qū)礦石類(lèi)型主要有3種:①層紋狀礦石(包括薄層與厚層兩種)[161];②錫興頁(yè)巖中的礫巖型礦石;③賦存在下伏白云巖中鐵礦層內(nèi)的次生礦石,即撒巴津比(Thbazimbi)型礦石。另外,礦石的儲(chǔ)量分布統(tǒng)計(jì)信息表明,層紋狀礦石所占比重最大,約占85%;礫巖型礦石次之,約占12%;而撒巴津比型礦石只占3%。關(guān)于礦床成因,Page研究指出,在條帶狀鐵建造沉積之后,下伏的白云巖被溶蝕,形成巖溶及地塹構(gòu)造,形成了撒巴津比型礦石及層紋狀礦石,后期進(jìn)一步的區(qū)域抬升及剝蝕最終出現(xiàn)了礫巖型礦石[162]。
3.3.2 卡拉哈里錳礦
北開(kāi)普省的卡拉哈里錳礦床是一個(gè)世界級(jí)錳礦,其資源總儲(chǔ)量約為80×108t,錳的品位20%~48%[163]。該礦床位于庫(kù)魯曼西北約60km處,礦區(qū)東西長(zhǎng)15km,南北寬35km[164]。
含礦地層是新太古代-古元古代德蘭士瓦超群最上部的Hotazel組,礦體主要有3層,與互層的條帶狀鐵建造同時(shí)沉積[165],而又以最底部的層狀單元含礦性為最好[166]。該礦床可以分為南、北兩個(gè)礦區(qū),分別以出現(xiàn)低品位(<40%)富含碳酸鹽的礦石及高品位(>44%)富含氧化物的礦石為特征[163]。其中,低品位礦石稱(chēng)為馬馬特旺(Mamatwan)型礦石[167],是由成巖作用-低級(jí)變質(zhì)作用過(guò)程形成的泥質(zhì)巖,在成分上包括微晶錳白云石、褐錳礦及赤鐵礦,此種礦石約占總儲(chǔ)量的97%;而高品位礦石則稱(chēng)為Wessels型礦石[168],它們僅局限分布于北部礦區(qū),由粗晶黑錳礦、褐錳礦與方鐵錳礦組成,約占礦石總儲(chǔ)量的3%。目前的研究認(rèn)為,這種礦石品位從低到高的轉(zhuǎn)變現(xiàn)象很可能與斷層控制的低溫?zé)嵋海黧w交代作用有關(guān)[169-170]。錳的富集與先后經(jīng)歷的3次由構(gòu)造引發(fā)的熱液蝕變事件相關(guān),分別是Wessel事件(1 000~1 250Ma),Mamatwan事件(550~600Ma)和Smartt事件(10~1 000Ma)[166]。
該成礦帶位于南非的西北端,并向北一直延伸進(jìn)入納米比亞境內(nèi)。在構(gòu)造位置上屬于太古宙卡普瓦爾克拉通的西南緣,成礦作用受中元古代那馬奎造山運(yùn)動(dòng)的顯著影響。該帶的銅-鉛-鋅多金屬礦化主要與古老地殼物質(zhì)(古元古代,甚至太古宙)在中元古代時(shí)期再循環(huán)(改造)引發(fā)的巖漿-沉積作用有關(guān)。該地區(qū)產(chǎn)出一些世界級(jí)的大型-超大型礦床,著名的礦床包括Aggeneys鉛-鋅-銀礦床、Okiep銅礦和普里斯卡(Prieska)銅-鋅礦等。
3.4.1 阿赫內(nèi)斯(Aggeneys)鉛-鋅-銀礦床
阿赫內(nèi)斯礦床位于那馬奎變質(zhì)帶西段中部的阿赫內(nèi)斯鎮(zhèn)附近,一共包括4個(gè)礦區(qū),從西到東分別是黑山(Black Moutain)、布羅肯希爾(Broken Hill)、大向斜(Big Syncline)和甘斯堡(Gamsberg),這是世界級(jí)的布羅肯希爾型鉛-鋅-銅-銀礦床[171-172]。該礦床的資源總儲(chǔ)量約370Mt,以布羅肯爾礦區(qū)的儲(chǔ)量為最大(38Mt),其中鉛占7.8%,鋅占2.9%,銅占0.5%,伴生的銀品位達(dá)113×10-6[171]。
布羅肯希爾礦區(qū)的含礦巖層為高角閃巖相的Bushmanland群,自下而上分為Namies片巖組、布羅肯希爾石英巖組、Ore片巖組、Shaft片巖組和Koeris組[173]。礦體賦存層位是Ore片巖組,主要由塊狀硫化物、重晶石礦,以及條帶狀鐵建造和富鋁片巖組成,礦體包括上礦層(Upper Ore Body)和下礦層(Lower Ore Body)[172,174]。礦物組成主要有閃鋅礦、方鉛礦、磁鐵礦、磁黃鐵礦、黃銅礦與黃鐵礦[174],其中,下礦層富集閃鋅礦、方鉛礦及磁黃鐵礦,而上礦層則富集磁鐵礦。礦石主體均為塊狀,硫化物含量大于60%,粗粒結(jié)構(gòu)(0.5~2mm),表現(xiàn)出一定的變質(zhì)特征,其中的硫化物礦物還可能發(fā)生過(guò)部分熔融[174]。該礦床形成于同沉積階段,因?yàn)锽ushmanland群變質(zhì)沉積巖原巖沉積時(shí)代為1 200~1 640Ma,故成礦時(shí)代近于沉積期[173,175]。
3.4.2 普里斯卡銅-鋅礦
普里斯卡銅礦位于南非北開(kāi)普省,是世界上最大的30個(gè)火山成因塊狀硫化物(VMS)型礦床之一[176]。該礦床于1968年被發(fā)現(xiàn),其開(kāi)采時(shí)間從1972年一直持續(xù)到1994年。Wagener和Van Schalkwyk[177]報(bào)道該礦床原始儲(chǔ)量為47Mt,銅品位1.7%,鋅品位3.8%(開(kāi)采到地下900m)。Galley等[176]統(tǒng)計(jì)后進(jìn)一步將資源儲(chǔ)量修正為47.2 Mt,銅品位0.98%,鋅品位1.98%,伴生銀礦品位20×10-6。
礦床位于那馬奎變質(zhì)帶東部的南端(Arechap Terrane),礦區(qū)內(nèi)出露的巖層是Copperton組,為高角閃巖相-低麻粒巖相的變質(zhì)巖[178-179],主要經(jīng)歷了3期變形[180-181]。該組被劃分為Smouspan片麻巖、普里斯卡銅礦組(Prieska Copper Mines Assemblage)與Vogelstruisbult單元3個(gè)部分[182]。其中,賦礦層位是普里斯卡銅礦組,其底部為富鋁的巖石(電氣石-金云母-夕線(xiàn)石-堇青石片麻巖),中間為塊狀硫化物礦石,頂部則為黃鐵礦化石英巖、碳酸鹽-石膏層及錳磁鐵礦層等巖石組合[177,183]。
在平面上,礦體呈NW走向,延伸約2km;在剖面上,礦體往深部延伸穩(wěn)定,長(zhǎng)度至少達(dá)1km,產(chǎn)狀陡傾,傾角為45°,甚至大于60°[184]。礦層厚度0.5~32m[185],平均厚約10m。礦體頂部發(fā)育厚度近100m的淋濾層(鐵帽),可見(jiàn)一些褐鐵礦、針鐵礦和藍(lán)銅礦等。礦石中黃鐵礦最為常見(jiàn),次為黃銅礦與閃鋅礦。該礦床形成時(shí)代為1 280~1 290Ma,礦石中獨(dú)特的金屬分層、獨(dú)具特色的鐵帽及礦化蝕變與硫化物礦石密切相關(guān)等現(xiàn)象都顯示該礦床為一大型VMS型礦床[184]。
開(kāi)普褶皺帶被認(rèn)為形成于岡瓦納大陸的聚合過(guò)程之中,是一個(gè)類(lèi)似于安第斯型大陸邊緣的造山帶。該帶的礦產(chǎn)資源主要集中產(chǎn)在西段內(nèi),成礦作用與古生代巖漿活動(dòng)密切相關(guān)。礦床往往賦存在寒武紀(jì)或更老的花崗巖體中(Cape Granite Suite),例如:Kuils River錫-鎢礦、Durbanville錫-金礦及Riviera鎢-鉬(銅)礦床。
3.5.1 Kuils River錫-鎢礦
Kuils River錫-鎢礦床位于泛非期Saldania造山運(yùn)動(dòng)形成的開(kāi)普褶皺帶西段,它是南非境內(nèi)發(fā)現(xiàn)的第一個(gè)錫礦床,斷斷續(xù)續(xù)均有進(jìn)行開(kāi)采,一直持續(xù)到1956年才最終停止。礦床類(lèi)型包括原生石英脈型、細(xì)晶巖脈型錫礦以及沖積-殘積型砂錫礦,至少已經(jīng)有778t的錫石礦產(chǎn)出(錫品位70%),主要來(lái)自沖積型砂錫礦床[186]。然而,后續(xù)的一些勘探活動(dòng)發(fā)現(xiàn)礦化點(diǎn)零散,且礦石品位較低(錫品位0.26%,鎢品位0.48%)[187]。
含礦巖體為中粗粒結(jié)構(gòu)、含電氣石的黑云母(似斑狀)花崗巖,有時(shí)出現(xiàn)鉀長(zhǎng)石巨晶,它是Kuils River-Stellenbosch侵入體的一部分,特征類(lèi)似S型花崗巖[188]。礦化集中分布在一條NW向延伸超過(guò)2km、寬約500m的狹窄地帶中,礦體為一系列不連續(xù)的石英脈與細(xì)晶巖脈,它們大小不一,局部厚達(dá)3 m。礦石礦物主要為錫石與鎢錳礦[189],還有極少量輝鉬礦、毒砂,而脈石礦物包括電氣石、石英及云母等[190]。往巖體深部錫石逐漸減少,取而代之的是黃銅礦與黃鐵礦[191]。已開(kāi)采的2條主礦脈的礦石品位極其可觀,錫品位9%~26.7%,鎢品位6%,石英脈型礦石品級(jí)往往較高。該地區(qū)歷史上產(chǎn)量最好的是一些沖積-殘積型砂錫礦,含Sn為0.68%~5.6%[192]。
3.5.2 Riviera鎢-鉬(銅)礦床
Riviera鎢-鉬(銅)礦床位于開(kāi)普褶皺帶西段馬姆斯伯里(Malmesbury)群露頭區(qū)域的Boland Terrane內(nèi),該礦床的資源總儲(chǔ)量為460×104t,鎢品位0.216%,鉬品位0.02%[193],為典型的夕卡巖型礦床。
含礦巖體Riviera侵入到其圍巖馬姆斯伯里群中,內(nèi)部含有許多圍巖捕虜體(以變質(zhì)碳酸鹽巖為主),礦化蝕變主要集中在巖體的頂部,尤其以絹云母化、黏土化和硅化最為明顯[186]。Riviera巖體為近等粒(中-細(xì)粒)結(jié)構(gòu)的偏鋁質(zhì)-過(guò)鋁質(zhì)巖石,巖性從石英二長(zhǎng)巖到花崗巖[188],巖相學(xué)與地球化學(xué)特征均類(lèi)似于I型花崗巖[194]。礦體在接觸帶形成的內(nèi)、外夕卡巖帶中均有分布。其中,內(nèi)夕卡巖帶中的白鎢礦(輝鉬礦)賦存于石英-方解石脈中,白鎢礦與鈣鐵榴石-透輝石-陽(yáng)起石緊密共生;外夕卡巖帶中的典型礦物組合是白鎢礦-符山石-方解石-綠簾石-鈣鐵輝石[186]。此外,還出現(xiàn)黃銅礦、黃鐵礦及閃鋅礦。這些礦脈大小不一,通常在巖體頂部非常發(fā)育。鎢與鉬的含量之間并無(wú)顯著的時(shí)空聯(lián)系或共生關(guān)系,表明二者的分布很可能與多期次巖漿活動(dòng)及相關(guān)成礦流體有關(guān)[186]。伴生的輝鉬礦脈屬于疊加成因,可能與后期侵入的A型花崗巖有關(guān)[195]。Rozendaal等通過(guò)區(qū)域構(gòu)造研究揭示含礦巖體的侵位與Saldania造山運(yùn)動(dòng)形成的穹狀構(gòu)造有關(guān)[195]。
南非的卡魯盆地是狹義的,其構(gòu)造屬性可能是弧后前陸盆地。盆地內(nèi)產(chǎn)出與沉積作用有關(guān)的礦床,如砂巖型鈾礦、砂金礦等。卡魯盆地的含鈾地層包括Beaufort群、Molteno組和Elliot組,尤以晚二疊世Beaufort群最顯著。盆地的砂巖型鈾礦床主要分布于西南部,其范圍大致從Sutherland,F(xiàn)raserburg, Beaufort West 到 Prince Albert[196]。Beaufort群內(nèi)至少可分為5個(gè)沉積旋回,礦化集中在2個(gè)砂巖單元[197-198]。此外,位于二者下部層位的Koornplaats單元可能也是一個(gè)重要的賦礦層位。這些礦體呈不規(guī)則透鏡狀,單個(gè)透鏡體厚1~5m,延伸方向與局部的古流方向近于平行[199]。礦石礦物包括鈾石(U(SiO4)1-X(OH)4X)和瀝青鈾礦(UO2),以鈾石為主,二者的鈾含量變化范圍均較大;次要礦物有輝鉬礦、黃鐵礦及毒砂等[200]。其中,鈾石被包裹在碳質(zhì)碎屑內(nèi)或呈集合體與方解石、赤鐵礦等共生,還常沿著瀝青鈾礦顆粒邊緣生長(zhǎng)[201-202]。關(guān)于成礦年齡,Duane等[203]獲得(262±15)~(185±49)Ma,而 Molteno組的礦石年齡則為(132±33)Ma,這顯示成礦在二疊紀(jì)時(shí)開(kāi)始,并一直持續(xù)到白堊紀(jì)[196]。鈾的來(lái)源復(fù)雜,風(fēng)化后的花崗巖碎屑和火山噴發(fā)物是最主要的物源[199,203-204]。該礦床的形成受同沉積構(gòu)造、沉積環(huán)境和成巖作用共同影響[205]。
按照“構(gòu)造單元+主要礦種+主要成礦類(lèi)型”的原則,在分析各成礦帶所處區(qū)域地質(zhì)背景的基礎(chǔ)上,結(jié)合礦床的成礦地質(zhì)條件、控礦因素、成礦時(shí)代和成礦規(guī)律等研究,將南非劃分為6個(gè)不同的成礦區(qū)帶,并明確了各個(gè)成礦帶內(nèi)的主要優(yōu)勢(shì)礦種與成礦類(lèi)型。初步分析了南非境內(nèi)不同時(shí)代的構(gòu)造單元成礦專(zhuān)屬性,這種認(rèn)識(shí)對(duì)后續(xù)的項(xiàng)目安排部署具有非常重要而現(xiàn)實(shí)的指導(dǎo)意義。
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