青海尕林格矽卡巖鐵多金屬礦床Fe-Ti氧化物及其熱動力學意義

于 淼,豐成友,劉洪川,李定武,王 輝,劉建楠,趙一鳴,李大新

1)中國地質(zhì)科學院礦產(chǎn)資源研究所,國土資源部成礦作用與資源評價重點實驗室,北京 100037;2)北京大學地球與空間科學學院,北京 100871;3)青海省有色地質(zhì)礦產(chǎn)勘查局地質(zhì)礦產(chǎn)勘查院,青海西寧 810007

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青海尕林格矽卡巖鐵多金屬礦床Fe-Ti氧化物及其熱動力學意義

于 淼1,2),豐成友1)*,劉洪川3),李定武3),王 輝1),劉建楠1),趙一鳴1),李大新1)

1)中國地質(zhì)科學院礦產(chǎn)資源研究所,國土資源部成礦作用與資源評價重點實驗室,北京 100037;
2)北京大學地球與空間科學學院,北京 100871;
3)青海省有色地質(zhì)礦產(chǎn)勘查局地質(zhì)礦產(chǎn)勘查院,青海西寧 810007

摘 要:尕林格大型矽卡巖型鐵多金屬礦床地處東昆侖祁漫塔格山與柴達木盆地結合部位靠近盆地一側。本文主要針對尕林格礦床Ⅱ礦群內(nèi)發(fā)現(xiàn)的熱液性質(zhì)的Fe-Ti氧化物共生組合(HYM)和新鮮鎂鐵質(zhì)玄武安山巖中火成性質(zhì)的Fe-Ti氧化物共生組合(IGM)的化學成分及其熱動力學平衡進行了詳細研究。鏡下觀察發(fā)現(xiàn),HYM與鎂鐵尖晶石共生,鈦鐵礦呈葉片狀平行于磁鐵礦八面體的(111)面出溶,根據(jù)氧化反應的平衡公式計算得到出溶反應的平衡溫度集中在581.8～688.9℃之間,氧逸度f(O2)介于10(-14.74)～10(-17.94)之間;IGM主要與巖漿中的鎂鐵質(zhì)礦物相平衡,其出溶反應的平衡溫度范圍為690.7～740.3℃ ,氧逸度集中在10(-15.44)～10(-17.30)之間。計算結果表明,尕林格矽卡巖型鐵礦的最初成礦溫度可達700℃ ,接近于水飽和巖漿結晶時的溫度,因此判斷HYM形成于巖漿演化早期高溫高鹽度流體的最初冷卻過程。由于早期高溫高鹽度流體與圍巖地層中的鎂鐵質(zhì)火山巖發(fā)生交代反應,淋濾出大量的Fe、Ti、Al、Mg、Cu等金屬物質(zhì)。在高溫條件下,Ti 和Al進入磁鐵礦晶格,導致含鈦磁鐵礦在降溫過程中經(jīng)歷了氧化出溶反應形成鈦鐵礦-磁鐵礦共生組合。此外,由于熱液受到鎂鐵質(zhì)火山巖的緩沖作用,導致HYM和IGM各自平衡時的氧逸度很接近。

關鍵詞:尕林格;Fe-Ti氧化物;鎂鐵尖晶石;地質(zhì)溫度計;氧壓計;氧化出溶

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本文由國土資源部公益性行業(yè)科研專項(編號:201411025)、青海省地質(zhì)勘查基金項目(編號:201304)、中國地質(zhì)調(diào)查局地質(zhì)調(diào)查項目(編號:1212011085528)、中國地質(zhì)調(diào)查局高層次地質(zhì)人才培養(yǎng)計劃(編號:201309)和青年地質(zhì)英才計劃(編號:201112)聯(lián)合資助。

自從Buddington和Lindsley(1964)第一次提出共存Fe-Ti氧化物地質(zhì)溫度計,很快就被廣泛認可并大量用于火成巖和變質(zhì)巖巖石學和地球化學分析當中。之后,更多學者致力于改進和完善Fe-Ti氧化物地質(zhì)溫度計和氧壓計的使用(Powell and Powell,1977;Spencer and Lindsley,1981;Lindsley and Spencer,1982;Stormer,1983;Stormer and Whitney,1985)。Lindsley和Spencer(1982)提出了FeO-Fe2O3-TiO2體系中鈦鐵礦-磁鐵礦固溶體地質(zhì)溫度計和氧壓計投影算法。此后,Ghiorso和Sack(1991)又進一步聯(lián)合了(Fe2+,Mg)-鋁酸鹽-亞鉻酸鹽-鈦酸鹽-鐵酸鹽尖晶石固溶體模型(Sack and Ghiorso,1991)和含F(xiàn)e2+-Fe3+-Mg-Mn-Ti斜方六面體氧化物固溶體延伸模型(Ghiorso,1990),提出了一種新的熱動力學計算公式。Anderson(1968)認為含鈦磁鐵礦出溶鈦鐵礦更多的是氧化反應所致。Dunlop和?zdemir(1997)研究發(fā)現(xiàn)高溫氧化反應是造成尖晶石(接近磁鐵礦)和斜方六面體相(接近鈦鐵礦)共生的主要因素。雖然Fe-Ti氧化物地質(zhì)溫度計已經(jīng)廣泛應用于火成巖和變質(zhì)巖當中,但在礦床領域研究中并不多見。

尕林格矽卡巖鐵多金屬礦床位于東昆侖祁漫塔格成礦帶與柴達木盆地的接合部位中部,東距青海省格爾木市230 km處。迄今為止,該礦床已達到大型規(guī)模,查明鐵礦石儲量1.8×108t,平均品位37.16%,是中國西部為數(shù)不多的大型矽卡巖礦床之一(于淼等,2013)。由于礦區(qū)全區(qū)第四系覆蓋平均厚度達到200 m,極大地增加了礦產(chǎn)勘查和科研工作的難度。此外,祁漫塔格成礦帶內(nèi)礦床普遍研究程度較低,特別是區(qū)域內(nèi)廣泛出露的鎂鐵質(zhì)火山巖與成礦間的關系問題并為得到解決。此次研究目的是通過對比尕林格礦床新鮮鎂鐵質(zhì)玄武安山巖中的火成鈦鐵礦-含鈦磁鐵礦固溶體共存組合和圍巖地層中熱液性質(zhì)的鈦鐵礦-含鈦磁鐵礦固溶體共存組合來證明尕林格矽卡巖礦床部分鐵質(zhì)的來源,以及應用磁鐵礦-鈦鐵礦地質(zhì)溫度計和氧壓計計算巖漿早期流體的溫度和氧逸度等成礦條件。

祁漫塔格作為東昆侖造山帶的分枝,東起格爾木市,沿NWW向延伸至阿爾金走滑斷裂,并轉向EW平行于阿爾金斷裂。帶內(nèi)構造復雜,發(fā)育多級次級構造;巖漿活動頻繁,從寒武紀至三疊紀均有不同程度發(fā)育。多期次、多旋回的構造巖漿運動為祁漫塔格提供了極有利的成礦條件(豐成友等,2011)。通過近些年的找礦勘查工作,實現(xiàn)了歷史性重大突破,查明了一批較有遠景的大中型斑巖-矽卡巖礦床(高曉峰等,2010;譚文娟等,2011)。其中包括虎頭崖銅鉛鋅多金屬礦、肯德可克鐵礦、野馬泉鐵鋅多金屬礦、它溫查漢鐵多金屬礦、卡而卻卡銅鉬多金屬礦、四角羊鉛鋅多金屬礦、長山鐵礦等(圖1)(毛景文等,2012;趙一鳴等,2013)。大量的成礦年齡數(shù)據(jù)顯示,印支中晚期爆發(fā)了大規(guī)模的成礦活動(豐成友等,2009;田承盛等,2013;于淼等,2015),與區(qū)域內(nèi)廣泛發(fā)育的中晚三疊世巖漿活動密切相關(豐成友等,2012)。

圖1　尕林格礦床綜合地質(zhì)圖(于淼等,2013)Fig.1 Generalized geological map of the Galinge deposit(after YU et al.,2013)

圖2　尕林格磁鐵礦-鈦鐵礦-尖晶石組合顯微照片F(xiàn)ig.2 Microphotograph of ilmenite-magnetite-spinel assemblage

尕林格礦區(qū)地層主要由一套由硅質(zhì)巖、中基性火山巖和大理巖組成的海相沉積建造構成,由于后期區(qū)域變質(zhì)作用和熱液蝕變作用,巖相變得較為復雜,應歸屬于奧陶系灘間山群下巖組。礦區(qū)自西向東劃分為六個主要礦群(圖1),其中Ⅰ、Ⅲ礦群主要產(chǎn)于花崗閃長巖捕虜體內(nèi);Ⅱ礦群產(chǎn)于花崗閃長巖與白云質(zhì)大理巖接觸帶內(nèi);而Ⅳ、Ⅴ、Ⅵ礦群產(chǎn)于外接觸帶灘間山群大理巖和蝕變玄武安山巖中。磁鐵礦礦體多呈層狀、似層狀和透鏡狀,鐵礦石最高品位可達46.67%。方鉛礦礦體多產(chǎn)出于Ⅵ礦群,Pb平均品位2.32%,Zn平均品位1.48%。此外,還可見少量的Cu、Au、Co礦體產(chǎn)于Ⅱ礦群和Ⅳ礦群,平均品位分別為0.1%～0.12%、3.04～8.01 g/t和0.022～0.125 g/t。尕林格部分Cu-Fe礦化與富揮發(fā)分流體交代淋濾中基性玄武安山巖密切相關。礦區(qū)內(nèi)常見的矽卡巖礦物有鎂橄欖石、粒硅鎂石、尖晶石、金云母、硅灰石、石榴子石、透輝石、鈣鐵輝石、錳鈣鐵輝石、斧石、電氣石、透閃石、綠鈣閃石、蛇紋石、綠簾石、綠泥石等。由于圍巖性質(zhì)不同,以及與成礦巖體距離不等,形成了不同的巖石礦物組合,大致可分為三種類型:含F(xiàn)e的鎂質(zhì)矽卡巖(Mg-SK)、含F(xiàn)e(磁鐵礦和磁黃鐵礦)-Cu的鈣質(zhì)矽卡巖(Ca-SK)、含Pb-Zn的錳鈣質(zhì)矽卡巖(Mn-Ca-SK)(圖1)(于淼等,2013)。礦區(qū)內(nèi)發(fā)現(xiàn)侵入巖主要為花崗閃長巖、閃長巖和閃長玢巖。成礦巖體主要為中酸性的花崗閃長巖,年代學顯示為三疊紀晚期侵位,鋯石LA-ICP MS年齡為(229±0.8)Ma(于淼,2013)。

1 巖相學

尕林格礦床存在兩種性質(zhì)的磁鐵礦-鈦鐵礦出溶組合。一種是熱液性質(zhì)的磁鐵礦-鈦鐵礦共生組合(HYM),與鎂鐵尖晶石共生產(chǎn)于矽卡巖化圍巖中(圖2)。尕林格發(fā)現(xiàn)的尖晶石((MgFe)Al2O4)主要呈深綠色,他形粒狀,大小不等(圖2a,b)。?？梢娂饩w粒間縫隙出溶磁鐵礦,也見長葉狀和水滴狀磁鐵礦被尖晶石封閉(圖2c,d),這一現(xiàn)象反映了尖晶石與磁鐵礦幾乎同時結晶。鏡下觀察尕林格熱液性質(zhì)的磁鐵礦帶有淡藍色調(diào)而與棕色調(diào)的鈦鐵礦相區(qū)別(圖2c)。在鈦鐵礦-鈦磁鐵礦共生背散射圖像中,鈦鐵礦呈深灰色,磁鐵礦為淺灰色,可見較寬的鈦鐵礦呈長條葉片狀和格狀平行于含鈦磁鐵礦(111)面出溶(圖3a,b)。

圖3　尕林格HYM背散射圖片(a,b)和IGM背散射圖片(c,d)(礦物簡寫同圖2)Fig.3 Back scattered electron images of the HYM(a,b)and IGM(c,d)(the abbreviation as for Fig.2)

尕林格礦床中的另一種Fe-Ti氧化物共生組合產(chǎn)自于新鮮的鎂鐵質(zhì)玄武安山巖中,為火成性質(zhì)磁鐵礦-鈦鐵礦共生組合(IGM)。尕林格玄武安山巖中的主要礦物為斜長石、鉀長石、單斜輝石、角閃石、黑云母,副礦物為白云母、榍石、磁鐵礦、鈦鐵礦等,基質(zhì)主要為鎂鐵質(zhì)礦物。斜長石斑晶含量大于70%,聚片雙晶極為常見,局部可見斜長石包裹單斜輝石、白云母、磁鐵礦和鈦鐵礦等礦物。部分斜長石具有環(huán)斑結構,為巖漿多階段結晶作用的產(chǎn)物,通常核部為中長石(An38.3～46.7),邊部為基性的拉長石(An61.5～5.7)。單斜輝石的主要成分為普通輝石,常蝕變成為角閃石和綠泥石等。角閃石的電子探針數(shù)據(jù)顯示化學成分為淺閃石,綠泥石化較強烈。在新鮮鎂鐵質(zhì)火山巖背散射圖像中(圖3c,d),含鈦磁鐵礦呈亮白色。局部可見磁鐵礦出溶鈦鐵礦,鈦鐵礦呈深灰色為,磁鐵礦為淺灰色。

圖4　FeO-Fe2O3-TiO2三元體系Fig.4 FeO-Fe2O3-TiO2ternary system

2 實驗原理

2.1 FeO-Fe2O3-TiO2三元體系

在FeO-Fe2O3-TiO2三元體系中,由TiO2·Fe2O3-2TiO2·FeO固溶體系列和含鈦磁鐵礦,以及鈦鐵礦-赤鐵礦體系,這三條固溶體線共同構成(Akimoto et al.,1957)(圖4)。含鈦磁鐵礦(簡寫TM,xFe2TiO4·(1-x)Fe3O4或Fe3-xTixO4)是具有反向尖晶石結構的立方體礦物。Ti(Ti4+)的摩爾分數(shù)根據(jù)成分系數(shù)x測量。TM0(x=0)為磁鐵礦,TM100為鈦鐵尖晶石。氧化程度由反應參數(shù)z來測定,z被定義為原始Fe2+轉化成Fe3+的百分數(shù)。Ozima和Sakamoto(1971)發(fā)現(xiàn)含鈦磁鐵礦的居里溫度和晶格常數(shù)是氧化反應參數(shù)z的函數(shù)。在FeO-Fe2O3-TiO2相平衡體系研究中,溫度和fO2決定了許多巖石中的含鈦磁鐵礦和鈦鐵礦的共存組合(Buddington and Lindsley,1964)。

2.2 地質(zhì)溫度計

磁鐵礦-鈦鐵礦共存氧化物的反應公式如下:

FeTiO3(ilm)+Fe3O4(mt)=Fe2O3(ilm)+Fe2TiO4(mt)

該反應的平衡常數(shù)為:

KD=(aFe2TiO4,mt·aFe2O3,ilm)/(aFe3O4,mt·aFeTiO3,ilm)

則有該反應的平衡關系:-ΔG0=RTln(aFe2TiO4,mt·aFe2O3,ilm)/(aFe3O4,mt·aFeTiO3,ilm)

根據(jù)熱動力學數(shù)據(jù)計算得出該反應在298°K溫度下自由能為+4100。

Ghiorso和Sack(1991)得出Fe3O4-Fe2TiO4固溶體是以近程有序結構為特征的,因此得出aFe3O4=xmt,aFe2TiO4=xusp,其中xusp為磁鐵礦固溶體中鈦鐵尖晶石的百分數(shù)。而近程有序效應在鈦鐵礦固溶體中也是存在的,從而aFeTiO3= xilm,aFe2O3= hxhem。h為亨利定律常數(shù),其在二元體系中與成分無關,但與溫度和壓力相關。

因此對于FeO-Fe2O3-TiO2體系中,二元鈦鐵礦固溶體和二元磁鐵礦固溶體化學反應平衡有:

-ΔG0=RTln [xusp(1-xilm)]/[(1-xusp)xilm]+RTln h。

平衡常數(shù):

KD=xusp(1-xilm)/(1-xulv)xilm

由于RTln h和吉布斯自由能是溫度的線性函數(shù),與氧逸度無關,因此可寫成:

A/T+B=ln[xusp(1-xilm)]/[(1-xulv)xilm]。

2.3 氧壓計

反應方程:

6FeTiO3(ilm)+2Fe3O4(mt)=6Fe2TiO4(mt)+O2

可作為氧壓計標準計算。該反應的平衡關系為:

-ΔG0=RTln(a6Fe2TiO4,mt)/(a6FeTiO3,ilm·a2Fe3O4,mt)+ RT lnaO2

假設該反應的活度與上述地質(zhì)溫度計反應一致,這一平衡關系則變?yōu)?

A′/T+B′=ln[x6usp/(1-xusp)2x6ilm]+ln aO2=ln K′D。

表2　尕林格共存Fe-Ti氧化物平衡反應熱動力學實驗計算數(shù)據(jù)Table 2 Calculated equilibrium thermodynamic data of the Galinge iron-titanium oxides

圖5　共存Fe-Ti氧化物次要元素含量變化投影圖Fig.5 Projection of the varying content of minor elements in the co-existing iron-titanium oxides

由此可以看出氧活度只與磁鐵礦和鈦鐵礦的成分有關,與溫度無關。這一公式只限于二元固溶體體系。地質(zhì)溫度計和氧壓計圖解只能用于自然條件下共存的Fe-Ti氧化物,不能脫離FeO-Fe2O3-TiO2體系(Buddington and Lindsley,1964)。

3 實驗結果

共存磁鐵礦-鈦鐵礦中的次要元素的分布對于理解鈦鐵礦的出溶也很重要(圖5)。鈦鐵礦明顯比共存的磁鐵礦更加富集M n O ,而相對貧Al2O3+Cr2O3+V2O3。共存的鈦鐵礦和磁鐵礦在變得逐漸貧MgO的同時,鈦鐵礦更加富集MnO+ZnO,+3價元素則保持不變,而此時磁鐵礦則更加富集+3價元素,但MnO+ZnO的含量則幾乎保持不變。這一相反的過程顯示出在含鈦磁鐵礦出溶反應過程中次要元素明顯存在分異作用。從不同性質(zhì)的磁鐵礦-鈦鐵礦組合來看,HYM中MnO+ZnO和MgO的含量明顯比IGM的含量要高,但Al2O3+Cr2O3+V2O3的含量要低于IGM。此外,尕林格矽卡巖中熱液性質(zhì)磁鐵礦的Ti和V的含量比尕林格火成性質(zhì)磁鐵礦中的Ti和V的含量高出一個數(shù)量級(表1)。

根據(jù)共存Fe-Ti氧化物地質(zhì)溫度計計算的化學反應平衡熱動力學數(shù)據(jù)列于表2。實驗數(shù)據(jù)計算得到的HYM和IGM各自的ln KD和ln K′D與1/T夠成良好的線性關系(圖6)。將xulv和xilm投影到xulvvs.xilm溫度等高線和lnaO2等高線投影圖中(Powell and Powell,1977)(圖7),可以看出溫度和氧逸度對xilm的變化非常敏感。

圖6　尕林格磁鐵礦-鈦鐵礦共熔體化學反應平衡常數(shù)與溫度間的函數(shù)關系(a-ln KDvs.104/T;b-ln K′Dvs.104/T)Fig.6 The functional relationship between equilibrium constant and temperature for the Galinge iron-titanium oxides(a-ln KD vs.104/T;b-lnK′D vs.104/T diagram)

圖7　尕林格磁鐵礦-鈦鐵礦礦物對xuspvs.xilm溫度等高線(a)和lnaO2等高線(b)(底圖據(jù)Powell and Powell,1977)Fig.7 The geothermometer of iron-titanium oxides:a.xuspvs.xilmcontoured for temperature;b.xuspversus xilmcontoured for lnaO2(base map after Powell and Powell,1977)

圖8　尕林格Fe-Ti氧化物溫度和氧逸度(底圖根據(jù)Frost and Lindsley,1992)Fig.8 Temperatures and oxygen fugacities of the Galinge Fe-Ti oxides(base map after Frost and Lindsley,1992)

4 討論

4.1 尕林格Fe-Ti氧化物溫度氧逸度

由于磁鐵礦中鈦鐵礦的溶解性太低而不能單一靠出溶解釋大部分鈦鐵-磁鐵礦共生組合。而磁鐵礦-鈦鐵尖晶石固相線之下的氧化反應形成鈦鐵礦-磁鐵礦共生組合已經(jīng)被實驗證實,并且發(fā)生在許多火成巖和一些變質(zhì)巖的降溫過程。Hu等(2015)總結了大部分矽卡巖礦床中的磁鐵礦都經(jīng)歷的溶解-再沉淀和氧化出溶-再結晶的過程,而其中氧化出溶-再結晶僅發(fā)生在高Ti磁鐵礦中,導致出溶形成葉片狀Fe-Ti-Al氧化物。Frost和Lindsley(1992)將FeO-CaO-MgO-SiO2-TiO2火成巖體系中相關的Fe-Ti氧化物、鈣鎂鐵輝石、橄欖石以及石英共存組合計算的熱動力學模型應用到火山巖和侵入巖的化學平衡當中,發(fā)現(xiàn)氧化物相成分與共生的鐵鎂硅酸鹽的種類密切相關。圖8中HM(hematite-magnetite)曲線限定了赤鐵礦-磁鐵礦組合緩沖區(qū)范圍(Myers and Eugster,1983;Sack et al.,1980),普通輝石、斜方輝石和含鈦磁鐵礦的基性組合(COM)的緩沖區(qū)高于橄欖石、石英和磁鐵礦組合(QFM)的緩沖區(qū),ASMW(2kb)曲線表示在Ptotal=PH2O=2kbars了鐵云母-透長石-磁鐵礦-H2O緩沖區(qū)(Helgeson,1978),而角閃石、黑云母和磁鐵礦的酸性組合緩沖區(qū)(ABM)高于COM緩沖區(qū)(Carmichael,1966)。隨著巖漿分異作用的進行,從基性巖漿到酸性巖漿fO2逐漸升高,而鐵鈦氧化物的TiO2的含量逐漸減少(岳樹勤等,1982)。通常鈦鐵礦要不在黑云母和角閃石結晶時消失,要不隨著環(huán)境條件的改變繼續(xù)保持平衡(Carmichael,1966)。

尕林格礦床內(nèi)HYM的溫度和氧逸度投影點全部落在HM(赤鐵礦-磁鐵礦共生組合)氧緩沖線和ABM氧緩沖線之間(圖8),與ABM組合非常接近,而IGM的溫度和氧逸度投影點全部落在QFM氧緩沖線和COM氧緩沖線之間。二者出溶平衡時的特征反映出尕林格熱液性質(zhì)的的磁鐵礦-鈦鐵礦出溶組合與中酸性巖漿演化流體有關,與基性巖漿的出溶無關。

尕林格熱液性質(zhì)磁鐵礦-鈦鐵礦組合的形成溫度為580～690℃,氧逸度(fO2)分布在10-14～10-18(圖8)。尕林格玄武安山巖中火成性質(zhì)的磁鐵礦-鈦鐵礦組合的形成溫度為680～750℃,氧逸度(fO2)分布在10-15～10-18。通常大部分的巖漿成因含鈦磁鐵礦-鈦鐵礦礦床都是在巖漿溫度下形成的(Buddington et al.,1955),溫度大約會在700℃以上。一般水飽和巖漿的形成溫度都大于700℃,水不飽和巖漿的的形成溫度也大概在900℃左右。因此,尕林格礦床圍巖中發(fā)現(xiàn)的共存Fe-Ti氧化物并非巖漿成因,而是形成于巖漿早期富揮發(fā)分高溫高鹽度流體的最初冷卻過程。隨著巖漿的冷卻結晶,早期富揮發(fā)分高溫高鹽度流體從巖漿中分異出來,與地層中的鎂鐵質(zhì)火山巖發(fā)生滲濾交代反應,導致鎂鐵質(zhì)火山巖發(fā)生強烈的褪色蝕變,Fe、Mg、Al、Ti和Cu等金屬元素含量明顯降低(表3)。由于流體運移過程中受到火山巖的緩沖作用,導致流體的氧逸度與火山巖中礦物平衡時的氧逸度很接近。隨著流體溫度的降低和氧逸度的改變,導致Fe、Ti、Al和Mg等元素發(fā)生氧化沉淀,形成含鈦磁鐵礦并出溶鈦鐵礦組合。

表3　尕林格火山巖化學成分Table 3 Chemical composition of Galinge volcanic rocks

圖9　尕林格礦床兩期石榴子石BSE圖像,其中核部富Ti貧Fe,而邊部富Fe貧TiFig.9 The BSE photograph of the two generation garnets from the Galinge deposit

4.2 金屬元素遷移

在巖漿體系中,磁鐵礦中的Al和Ti的含量與溫度密切相關(Nielsen et al.,1994),含量越高指示的溫度就越高。這一規(guī)律也同樣適用于熱液性質(zhì)的斑巖和矽卡巖磁鐵礦(Nadoll et al.,2014)。通常來自于鈣質(zhì)或鎂質(zhì)矽卡巖礦床的熱液磁鐵礦中的Ti和V的含量要比火成性質(zhì)的磁鐵礦的含量低至少一個數(shù)量級(Ray and Webster,2007;Nadoll,2011),尕林格發(fā)現(xiàn)的熱液磁鐵礦-鈦鐵礦組合中磁鐵礦的Ti和V的含量就具有類似特征。有證據(jù)表明,如Ti和Al等元素在熱液中的溶解性是與溫度強烈相關的(Van Baalen,1993),通常在中低溫熱液體系下是難運移的(Verlaguet et al.,2006),而在高溫火成條件下Ti 和Al是可以進入磁鐵礦中的,因此火成磁鐵礦中Ti和Al的平均含量明顯要比熱液磁鐵礦的高。也有證據(jù)表明,大量火成巖的矽卡巖化可以導致Ti、V、Al等元素在矽卡巖礦物中富集(Meinert,1984),這一特點可以在尕林格早期結晶矽卡巖礦物石榴子石中得到證明。尕林格早期結晶石榴子石根據(jù)化學成分的不同可以劃分兩個階段(圖12),早期核部中TiO2(5.24 wt.%～5.49 wt.%)和Al2O3(6.06 wt.%～6.19 wt.%)的含量明顯高于晚期邊部TiO2(0.1 wt.%～0.3 wt.%)和Al2O3(0.67 wt.%～4.47 wt.%)(另文發(fā)表),充分說明Ti和Al在巖漿演化早期高溫高鹽度流體中可以大量運移。

5 結論

(1)尕林格矽卡巖礦床存在兩種性質(zhì)的磁鐵礦-鈦鐵礦出溶共生組合,其中矽卡巖中的熱液性質(zhì)組合與鎂鐵尖晶石共生,根據(jù)Fe-Ti氧化物熱力學計算公式換算得到的溫度范圍為581.8～688.9℃,氧逸度分布在10-14.74～10-17.94之間;鎂鐵質(zhì)安山巖中的火成性質(zhì)組合主要與巖漿中的鎂鐵質(zhì)礦物相平衡,其溫度主要集中在690.73～740.26℃,氧逸度集中在10-15.44～10-17.30之間。

(2)尕林格矽卡巖型鐵礦的最初成礦溫度可達到700℃左右,接近于水飽和巖漿結晶時的溫度,與巖漿演化早期高溫高鹽度流體密切相關。由于巖漿演化熱液受到鎂鐵質(zhì)火山巖的緩沖作用,使得HYM的氧逸度和IGM的氧逸度極為接近。

(3)熱液性質(zhì)的磁鐵礦中Ti、V和Al的含量比火成性質(zhì)的磁鐵礦低一個數(shù)量級,這與元素在熱液體系中的溶解性有關。通常Ti和Al在巖漿演化早期高溫高鹽度流體中可以大量運移。

(4)從花崗閃長質(zhì)巖漿結晶分異出的早期高溫高鹽度流體與鎂鐵質(zhì)安山巖發(fā)生滲濾交代反應,淋濾出大量的Mg、Ti、Al、Fe和Cu等金屬元素,隨著流體溫度的降低,Ti和Al等金屬元素進入到磁鐵礦晶格中。而伴隨流體氧逸度的升高,導致含鈦磁鐵礦在降溫過程中經(jīng)歷了氧化出溶反應形成鈦鐵礦-磁鐵礦共存組合。

Acknowledgements:

This study was supported by the Special Scientific Research Fund of Public Welfare Profession of Ministry of Land and Resources of the People’s Republic of China(No.201411025),the Geological Prospecting Fund of Qinghai Province(No.201304),and China Geological Survey(Nos.201411025,201309 and 201112).

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The Iron-Titanium Oxides in the Galinge Iron Polymetallic Skarn Deposit of Qinghai Province and Their Thermodynamic Significance

YU Miao1,2),FENG Cheng-you1)*,LIU Hong-chuan3),LI Ding-wu3),WANG Hui1),
LIU Jian-nan1),ZHAO Yi-ming1),LI Da-xin1)
1)MLR Key Laboratory of Metallogeny and Mineral Assessment,Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing 100037;
2)School of Earth and Space Sciences,Peking University,Beijing 100871;
3)Qinghai Institute of Nonferrous Metal and Geological Exploration,Xining,Qinghai 810007

Abstract:The Galinge skarn iron deposit is located at the junction between the Qimantag metallogenic belt in the East Kunlun Mountains and the Qaidam Basin.There are two types of Fe-Ti oxides coexisting assemblages in the Galinge skarn deposit:one is the hydrothermal magnetite-ilmenite coexisting assemblage(HYM)which was discovered in No.2 Ⅱ ore group,whereas the other one is the igneous magnetite-ilmenite coexisting assemblage(IGM)which is developed in the fresh mafic basaltic andesite.This research aimed to analyze the distinctivebook=205,ebook=80composition and significant thermodynamic features of the HYM and IGM.The HYM shows an intergrowth with ferromagnesium spinel,and the crystallographic orientation of growing ilmenite lamellae are parallel to the(111)planes of the magnetite octahedron.Based on the equilibrium of oxidation,the equilibrated temperatures range between 581.8C and 688.9℃,and the oxygen fugacities vary between 10(-14.74)and 10(-17.94).The IGM is mainly in equilibrium with silicate minerals in the mafic magma.The equilibrated temperatures of the IGM range between 690.73℃ and 740.26℃,and the range of oxygen fugacities is 10(-15.44)-10(-17.30).It is reasonably inferred that the primary metallogenic temperature might have reached 700℃,approaching the crystallographic temperature of water saturated magma.Therefore,the HYM was formed after the primary cooling procedure of the early high-temperature and high-salinity hydrothermal fluid which evolved from the magma.Because of the infiltration metasomatism between the hydrothermal fluid and the mafic igneous rocks,large amounts of metal elements such as Fe,Ti,Al,Mg and Cu were leached out.In the early high temperature setting,Ti and Al were incorporated into the magnetite,and consequently the oxyexsolution of titaniferous magnetite resulted in the intergrowth of magnetite and ilmenite.As the oxyexsolution did not rapidly re-equilibrate under new conditions,the iron-titanium oxide relationship could be preserved and hence could indicate the equilibrium temperature and the oxygen fugacity of the oxidation.

Key words:Galinge;Fe-Ti oxides;spinel;geothermometer;oxygen barometer;oxyexsolution

*通訊作者:豐成友,男,1971年生。博士,研究員,博士生導師。主要從事礦床地質(zhì)、地球化學研究。

通訊地址:100037,北京市百萬莊大街26號中國地質(zhì)科學院。E-mail:fengchy@cags.ac.cn。

作者簡介:第一于淼,男,1987年生。博士研究生。礦物學、巖石學、礦床學專業(yè)。E-mail:540052547@qq.com。

收稿日期:2015-08-12;改回日期:2015-11-04。責任編輯:閆立娟。

中圖分類號:P618.31;P574

文獻標志碼:A

doi:10.3975/cagsb.2016.02.08