郎興海,唐菊興,李志軍,黃 勇,丁 楓,王成輝,張 麗,周 云

1)成都理工大學地球科學學院,四川成都 610059;

2)中國地質科學院礦產(chǎn)資源研究所,北京 100037;

3)中國地質調查局成都地質調查中心,四川成都 610081

西藏雄村斑巖型銅金礦集區(qū)I號礦體的硫、鉛同位素特征及其對成礦物質來源的指示

郎興海1),唐菊興2),李志軍1),黃 勇1),丁 楓1),王成輝2),張 麗3),周 云1)

1)成都理工大學地球科學學院,四川成都 610059;

2)中國地質科學院礦產(chǎn)資源研究所,北京 100037;

3)中國地質調查局成都地質調查中心,四川成都 610081

西藏雄村斑巖型銅金礦集區(qū)是近年來西藏岡底斯斑巖銅礦帶內發(fā)現(xiàn)的一處超大型銅金礦集區(qū),其形成于與新特提斯洋向北的洋內俯沖作用有關的島弧環(huán)境,成礦時代為中侏羅世。該礦集區(qū)位于岡底斯火山-巖漿弧的中段南緣,其南側緊鄰日喀則弧前盆地,目前探明I(原命名為雄村銅礦床)、II、III號銅金礦體規(guī)模達大型-超大型,同時還存在多個礦化異常帶。本文以雄村I號礦體為研究對象,對雄村I號礦體含礦斑巖、賦礦凝灰?guī)r和主要硫化物的硫、鉛同位素開展研究,結果表明: ①含礦斑巖、賦礦凝灰?guī)r和主要硫化物具有較為一致的硫同位素組成,δ34SCDT變化范圍為–3.5‰～2.7‰,平均–1.07‰,十分接近于零,塔式分布效應顯著,硫可能主要來自地幔; ②含礦斑巖、賦礦凝灰?guī)r和主要硫化物具有相對一致的鉛同位素組成,均以放射性成因鉛含量低為特征,206Pb/204Pb、207Pb/204Pb和208Pb/204Pb變化范圍分別為18.369～18.752、15.473～15.589和 38.389～39.1531,位于地幔與造山帶鉛演化線之間,并且相對靠近地幔鉛演化線,顯示出鉛主要來源于地幔,可能有少量地殼物質的混染。通過西藏岡底斯斑巖銅礦帶碰撞造山環(huán)境和島弧環(huán)境(以雄村 I號礦體為代表)斑巖型銅礦床的硫、鉛同位素組成特征對比,認為兩者的成礦物質來源是相似的,碰撞造山環(huán)境的地殼物質混染較強烈,而島弧環(huán)境的地殼物質混染較弱。

硫同位素; 鉛同位素; 雄村; 斑巖銅礦床; 西藏岡底斯帶

西藏岡底斯斑巖銅礦帶產(chǎn)于岡底斯火山-巖漿弧內,近年來該帶的資源評價工作取得了重要進展,已發(fā)現(xiàn)的礦床(點)主要位于東起工布江達縣、西到昂仁縣的區(qū)域,大致分布于雅魯藏布江北岸20～60 km范圍內(以25～35 km 最為集中),發(fā)現(xiàn)有雄村、驅龍、甲瑪、沙讓、沖江、廳宮、白容、朱諾、程巴、沖木達、拉抗俄、達布、吹敗子、吉如等斑巖型礦床,總體具有東西成帶、北東成群分布特征(芮宗瑤等,2003a,b; 李光明等,2005,2006; 曲曉明等,2001;王全海等,2002; 侯增謙等,2003; 鄭有業(yè)等,2007;黃志英等,2004; 張洪濤等,2004; 唐菊興等,2009b,2010a,b,2012; 胡正華,2011; 郎興海等2010c,2012a)。

雄村斑巖型銅金礦集區(qū)位于西藏岡底斯斑巖銅礦帶西段,是近年來西藏岡底斯斑巖銅礦帶上發(fā)現(xiàn)的以銅為主伴生金、銀的超大型銅金礦集區(qū),屬西藏日喀則地區(qū)謝通門縣榮瑪鄉(xiāng)管轄。雄村斑巖型銅金礦集區(qū)大地構造位置屬岡底斯火山-巖漿弧的中段南緣,其南側緊鄰日喀則弧前盆地(郎興海等,2010c),研究表明其形成于與新特提斯洋向北的洋內俯沖作用有關的島弧環(huán)境,成礦時代為中侏羅世(唐菊興等,2009b; 郎興海等,2010c)。在雄村斑巖型銅金礦集區(qū),目前探明I(原命名為雄村銅礦床)、II、III號銅金礦體規(guī)模達大型-超大型,同時還存在多個礦化異常帶。三個主礦體呈北西-南東向展布,II號礦體位于I號礦體(原雄村銅礦床)北西向約3.4 km處,III號礦體位于II號礦體北西向約2.1 km處(圖1)。雄村斑巖型銅金礦集區(qū)的地質研究工作起步較晚,其中以雄村I號礦體(原命名為雄村銅礦床)研究最為詳盡(Qin et al.,2005; 徐文藝等,2005,2006a,b;丁楓等,2006; 張麗,2007; 唐菊興,2007,2009a,c,2010; 曲曉明等,2007a,b; 郎興海等,2010a,b,c,2011,2012b; 黃勇等,2011a,b; 黃勇,2012; 丁楓,2012),但其成礦物質來源尚未得到明確的認識。盡管丁楓等(2006)、徐文藝等(2006b)、曲曉明等(2007b)、黃勇等(2011a)對I號礦體的成礦物質來源有過研究,但由于測試數(shù)據(jù)較少且不夠全面和系統(tǒng),缺乏對成礦物質來源的深入探討,因此,本文將在前人研究基礎之上,通過系統(tǒng)研究雄村 I號礦體的含礦斑巖、賦礦凝灰?guī)r和主要硫化物的硫、鉛同位素組成,分析其成礦物質來源,并與區(qū)域上的斑巖型銅礦床進行對比,初步探討新特提斯洋俯沖階段島弧環(huán)境與印度-亞洲大陸碰撞造山環(huán)境形成的斑巖銅礦床成礦物質來源的異同,為深入研究雄村斑巖型銅金礦集區(qū)的成礦作用乃至整個岡底斯成礦帶構造-巖漿演化與成礦作用奠定基礎。

1 雄村I號礦區(qū)地質概況

雄村 I號礦區(qū)出露地層(圖 1)主要為中-下侏羅統(tǒng)雄村組(J1-2x)和全新統(tǒng)崩積物-沖積物(唐菊興等,2006)。雄村組分為酸性凝灰?guī)r、安山質凝灰?guī)r、石英玄武質砂巖、玄武質砂巖夾粉砂巖、粉砂巖夾泥質巖等巖性段,其中酸性凝灰?guī)r和安山質凝灰?guī)r是雄村 I號礦體的主要賦礦巖石之一,形成時間為176±5 Ma(唐菊興等,2010a)。

雄村 I號礦區(qū)發(fā)育成礦前、成礦期和成礦后巖漿巖(圖1)。成礦前巖漿巖主要為角閃石英閃長玢巖,侵入時間為173±3 Ma(唐菊興等,2010a)。成礦期巖漿巖為含眼球狀石英斑晶的角閃石英閃長玢巖,侵入時間為164.3±1.9 Ma(Tafti et al.,2006),該巖體是雄村 I號礦體的含礦斑巖體,斑晶主要為石英和角閃石,石英斑晶呈方形或渾圓狀(圖 2a,b),含量為10%～15%(有時大于 15%),粒徑為 1～1.5 cm,角閃石斑晶呈長條狀自形斑晶,含量 10%,粒徑1～2 cm,基質主要由細粒的石英、長石、角閃石及少量的黑云母組成。成礦后巖漿巖主要為始新世侵位的黑云母花崗閃長巖、斜長閃長玢巖、安山巖脈、花崗細晶巖脈和煌斑巖脈,其中黑云母花崗閃長巖侵位時間為 46.96±0.42 Ma(唐菊興等,2009c,2010a); 煌斑巖脈侵位時間為49.59±0.58 Ma(唐菊興等,2009c,2010a); 安山巖脈早于黑云母花崗閃長巖,花崗細晶巖脈晚于黑云母花崗閃長巖; 斜長閃長玢巖早于安山巖脈,其侵位的準確時間還未確定。

雄村I號礦區(qū)斷裂構造發(fā)育(圖1),主要呈北西、北西西或北北西向展布,其中 F1主斷層展布于礦區(qū)南部,F2主斷層展布于礦區(qū)中部,兩斷層均具有多期次活動的特點,為成礦后斷層,呈 265°～280°走向,傾向北,傾角 40°～75°不等,表現(xiàn)出先壓后張和先韌性后脆性的特征,沿兩斷層普遍分布有斷層角礫巖和構造蝕變巖,斷裂破碎帶內及上盤近側礦石破碎; 次級斷層大多具有陡傾特點,呈北西、北東或北北西走向,傾向北東或東,均為成礦后斷層,切穿礦體和F1、F2主斷層。

雄村 I號礦體產(chǎn)于含眼球狀石英斑晶的角閃石英閃長玢巖及其接觸帶的凝灰?guī)r中,屬斑巖型銅金礦床,已經(jīng)完成勘探工作,進入礦山開采階段。礦體(按50 m×50 m勘探網(wǎng)度控制)形態(tài)平面上呈大透鏡狀,寬度(南東向)約 300～600 m,長度(東西向)約1000～1400 m,走向北西-南東(圖1)。在南北向剖面上呈似層狀－層狀,傾向北東,傾角40°～53°。該礦體總體特征是規(guī)模大(含礦斑巖及接觸帶凝灰?guī)r全巖礦化)、富金而貧鉬(Au 0.56×10-6,Mo 19.7×10-6)、蝕變強烈,已探明銅資源量 100多萬噸、伴生金資源量100多噸、伴生銀資源量900多噸(Cu 0.41%,Au 0.56×10-6,Ag 19.7×10-6)。

由于含礦斑巖體的侵位,雄村 I號礦體的圍巖蝕變作用極為強烈,主要蝕變類型有鉀硅酸鹽化蝕變、強硅化蝕變、黃鐵絹英巖化蝕變和青磐巖化蝕變,其中重要賦礦蝕變?yōu)殁浌杷猁}化蝕變和強硅化蝕變。除上述熱液成礦期的蝕變類型外,在成礦后侵位的黑云母花崗閃長巖巖基的接觸帶發(fā)育一個強烈的角巖化帶,該期蝕變基本無礦化; 在成礦前侵位的角閃石英閃長玢巖中發(fā)育強烈的鈉化-鈣化蝕變。礦石構造主要為細脈—浸染狀或網(wǎng)脈狀,主成礦元素以 Cu 為主,伴生有用組分為 Au、Ag、Zn和Pb等,伴生有害元素(如As、Sb、Bi等)含量極少;主要的脈體類型有磁鐵礦-硫化物脈、黑云母硫化物脈、石英硫化物脈、黃鐵礦-磁黃鐵礦-黃銅礦脈、黃鐵礦脈、多金屬硫化物脈,其中主要賦礦脈體為石英硫化物脈和黑云母硫化物脈。礦石中主要金屬礦物為黃銅礦、黃鐵礦和磁黃鐵礦,次要金屬礦物為閃鋅礦、方鉛礦、輝鉬礦、輝銅礦、毒砂、輝砷銅礦和藍輝銅礦等; 主要非金屬礦物為石英、紅柱石和絹云母,次要非金屬礦物為長石、黑云母、綠泥石和綠簾石等。

雄村 I號礦體的礦化具有多期多階段性。根據(jù)對礦石結構、構造、有用組分及相關關系的研究,結合野外觀察的情況,將成礦過程分為巖漿期、矽卡巖期、熱液成礦期、表生期。熱液成礦期是主要的成礦期,分為早期石英硫化物階段(硫化物大量沉淀,形成礦體)、晚期石英多金屬硫化物階段(主要形成多金屬硫化物脈,出現(xiàn)方鉛礦、閃鋅礦以及有 Au和Ag等的局部富集)。在表生期,雄村I號礦體經(jīng)歷了發(fā)育較弱的表生(次生)作用。巖漿期和矽卡巖期的礦化作用極弱。

2 樣品采集及測試

本文采集了雄村I號礦體含礦斑巖(含眼球狀石英斑晶的角閃石英閃長玢巖)、賦礦凝灰?guī)r以及主要硫化物(黃鐵礦、磁黃鐵礦、黃銅礦、閃鋅礦)進行硫、鉛同位素測定,樣品共計15件,均采自鉆孔巖芯。樣品由核工業(yè)北京地質研究院同位素室完成測試,硫同位素測試儀器采用MAT-251質譜計,檢測方法和依據(jù)為DZ/ T0184.14-1997《硫化物中硫同位素組成的測定》,測試結果見表1; 鉛同位素測試儀器采用 ISOPROBE-T熱電離質譜儀,檢測方法和依據(jù)為GB/ T17672-1999《巖石中鉛鍶釹同位素測定方法》,測試結果見表1。

3 硫、鉛同位素特征及成礦物質來源

3.1 硫同位素特征

雄村I號礦體硫化物的δ34SCDT較為均一,變化范圍為–3.5‰～2.7‰,平均–1.06‰,其中黃鐵礦的δ34SCDT為–2.92‰～2.7‰(平均–0.61‰),黃銅礦為–1.1‰～–1.7‰(平 均 -1.4‰), 閃 鋅 礦 為 –0.8‰～–3.5‰(平 均 –1.84‰), 磁 黃 鐵 礦 為 –1.1‰ ～–3.1‰(平均-2.3‰)。

雄村I號礦體含礦斑巖和賦礦凝灰?guī)r的δ34SCDT也較為均一,變化范圍為–1.7‰ ～ –0.4‰,平均–0.775‰,其中含礦斑巖的δ34SCDT為–1.7‰ ～–0.4‰(平 均 –1.05‰),凝 灰 巖 的 為 –0.6‰ ～–0.4‰(平均–1.05‰)。

雄村 I號礦體含礦斑巖、賦礦凝灰?guī)r以及主要硫化物的δ34SCDT與典型斑巖銅礦床的大體相似(圖3)。

3.2 鉛同位素特征

雄村 I號礦體主要硫化物的鉛同位素組成較為均一,放射性成因鉛含量較低,206Pb/204Pb、207P b/204P b 和208P b/204P b 變化范圍為18.034～ 8.425、15.473～15.589 和 37.918～38.593,其中黃鐵礦分別為 18.104～18.422、15.473～15.589和37.923～ 38.593,黃銅礦分別為 18.384～18.425、15.78～15.581和 38.491～38.584,閃鋅礦分別為18.034～18.408、15.479～15.581 和 37.918～38.562,磁黃鐵礦分別為 18.145～18.15、15.511～15.516 和38.065～38.074。

雄村 I號礦體含礦斑巖和賦礦凝灰?guī)r的鉛同位素組成也較為均一,放射性成因鉛含量較低,與硫化物的鉛同位素組成相近,206Pb/204Pb、207Pb/204Pb和208Pb/204Pb 變化范圍為 18.17～18.247、15.517～15.525和 38.087～38.196,其中含礦斑巖分別為 18.17～18.183 、 15.517～15.525 和 38.087～38.196,凝灰?guī)r分別為 18.183～18.247、15.527～15.549 和38.125～ 38.178。

3.3 成礦物質來源

硫、鉛同位素示蹤是一種研究成礦物質來源的有效手段(Zartman et al.,1981; Rollinson,1993; Zhu et al.,2011)。

在207Pb/204Pb-206Pb/204Pb和208Pb/204Pb-206Pb/204Pb 圖解(圖 4),含礦斑巖和硫化物投點于地幔與造山帶鉛演化線之間,并且相對靠近地幔鉛演化線,這說明Pb主要來源于地幔,可能有少量地殼物質的混染。含礦斑巖和硫化物的鉛同位素組成特征相似,說明硫化物中的Pb主要來自斑巖體,也說明成礦流體從圍巖中萃取成礦物質的作用不強,主要成礦物質還是來自斑巖本身。

雄村 I號礦體缺乏硫酸鹽,說明成礦流體中主要為還原性硫,硫化物的平均δ34S值可以代表成礦流體的δ34S值。礦石的主要硫化物是黃鐵礦、磁黃鐵礦、黃銅礦和閃鋅礦,它們硫同位素組成特征相似,變化范圍較窄(–3.5‰～2.7‰),平均值為–1.06‰,十分接近于零(圖 3),塔式分布效應顯著,與隕石硫的同位素組成特征相似,說明硫可能主要來自地幔,與 Pb來源相似。雄村 I號礦體的含礦斑巖體(含眼球狀石英斑晶的角閃石英閃長玢巖)的δ34S值變化范圍為–0.4‰至–1.7‰,平均–1.05‰,與硫化物的硫同位素組成特征相似,說明硫化物的硫主要來自斑巖體,地殼物質的混染較弱。

綜上所述,雄村 I號礦體的成礦物質是由含礦斑巖帶來,主要來自地幔,地殼物質的混染作用不強。

4 討論

已有的研究資料表明,西藏岡底斯斑巖銅礦帶存在三個構造演化階段的斑巖成礦作用(唐菊興等,2009b; 郎興海等,2010c): ①新特提斯洋俯沖階段的斑巖成礦作用(65～200 Ma),僅發(fā)現(xiàn)有雄村斑巖型銅金礦集區(qū),形成于與新特提斯洋洋內俯沖作用有關的島弧環(huán)境; ②印度—亞洲大陸主碰撞匯聚階段的斑巖成礦作用(65～40 Ma),發(fā)現(xiàn)有吉如斑巖型銅(鉬)礦、沙讓斑巖型鉬礦和沖木達斑巖-矽卡巖型銅金礦; ③印度—亞洲大陸后碰撞伸展階段的斑巖成礦作用(25～0 Ma),是西藏岡底斯斑巖銅礦帶的主要成礦期,發(fā)現(xiàn)有眾多的斑巖型礦床,如甲瑪、沖江、吹敗子、廳宮、達布、拉抗俄、南木、驅龍、朱諾、程巴、白容等。曲曉明等(2002)、孟祥金等(2006)和 Qu等(2007)對西藏岡底斯斑巖銅礦帶印度-亞洲大陸碰撞造山環(huán)境的斑巖型銅礦床S、Pb同位素組成及成礦物質來源進行了較為系統(tǒng)的研究,將他們的研究結果與本文研究成果進行對比可以得出一些初步的認識: ①碰撞造山環(huán)境和島弧環(huán)境斑巖型銅礦床的含礦斑巖和礦石硫化物的δ34SCDT變化范圍分別為–6.3‰～1.2‰(平均–1.27‰)和–3.5～2.7‰(平均–1.07‰)(圖 3),兩者的δ34SCDT塔式分布效應顯著,硫可能主要來自地幔,但碰撞造山環(huán)境的含礦斑巖和礦石硫化物的δ34SCDT更偏負值,這可能說明有更多的地殼物質混染; ②碰撞造山環(huán)境和島弧環(huán)境斑巖型銅礦床的含礦斑巖和礦石硫化物的206Pb/204Pb、207Pb/204Pb、208Pb/204Pb 變化范圍分別為18.369～ 18.752、15.473～15.589、38.389～39.1531 和18.034～ 18.425、15.502～15.7329、37.918～38.593,前者相較于后者更富放射性成因鉛,表明有更多地殼物質的混染。在207Pb/204Pb-206Pb/204Pb 和208Pb/204Pb-206Pb/204Pb 圖解上(圖 4),碰撞造山環(huán)境的位于地殼、造山帶與地幔鉛演化線之間,更趨向于造山帶鉛演化線,而島弧環(huán)境的位于地幔與造山帶鉛演化線之間,更趨向于地幔鉛演化線,兩者構成一條線性關系很好的由地殼到造山帶至地幔鉛的演化線,這種鉛同位素組成特征可能暗示兩者具有相似的鉛源,但地殼物質的混染程度不同,碰撞造山環(huán)境的有更多的地殼物質混染,而島弧環(huán)境的地殼物質混染較弱。根據(jù)碰撞造山環(huán)境和島弧環(huán)境的斑巖型銅礦床的硫、鉛同位素組成特征對比,兩者成礦物質來源是相似的,但由于碰撞造山環(huán)境地殼很厚(約70 km),含礦斑巖巖漿上侵過程中地殼物質混染作用較為強烈,而島弧環(huán)境地殼較薄或缺失,含礦斑巖巖漿上侵過程中地殼物質混染較弱。

5 結論

1)雄村I號礦體含礦斑巖、賦礦凝灰?guī)r和主要硫化物具有較為一致的硫同位素組成,δ34SCDT變化較小,變化范圍為–3.5‰～2.7‰,平均–1.07‰,十分接近于零,塔式分布效應顯著,與隕石硫的同位組成特征相似,硫可能主要來自地幔,地殼物質混染較弱。

2)雄村I號礦體含礦斑巖、賦礦凝灰?guī)r和主要硫化物具有相對一致的鉛同位素組成,均以放射性成因鉛含量低為特征,206Pb/204Pb、207Pb/204Pb和208Pb/204Pb 變 化 范 圍 為 18.369～18.752、15.473～ 15.589 和 38.389～39.1531,位于地幔與造山帶鉛演化線之間,并且相對靠近地幔鉛演化線,顯示出鉛主要來源于地幔,可能有少量地殼物質的混染。

3)西藏岡底斯斑巖銅礦帶碰撞造山環(huán)境和島弧環(huán)境形成的斑巖型銅礦床的硫、鉛同位素組成特征對比顯示: 兩者的成礦物質來源是相似的,碰撞造山環(huán)境的地殼物質混染較強烈,而島弧環(huán)境的地殼物質混染較弱。

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Sulfur and Lead Isotope Characteristics of No.I Ore Body in the Xiongcun Porphyry Copper-Gold Ore Concentration Area of Tibet:Implications for the Source of Metals in the Ore Deposit

LANG Xing-hai1),TANG Ju-xing2),LI Zhi-jun1),HUANG Yong1),DING Feng1),WANG Cheng-hui2),ZHANG Li3),ZHOU yun1)
1)College of Earth Science,Chengdu University of Technology,Chengdu,Sichuan610059;
2)Institute of Mineral Resources,Chinese Academy of Geological Sciences,Beijing10037;
3)Chengdu Center of China Geological Survey,Chengdu,Sichuan610081

The Xiongcun porphyry copper-gold ore concentration area,one of the large-size metallogenic areas discovered along the Gangdise porphyry copper belt,occurs in an island arc environment associated with northward intra-oceanic subduction of Neo-Tethys oceanic crust.The ore-forming age of the Xiongcun porphyry copper-gold ore concentration area is middle Jurassic.It is located in the middle of the southern margin of theGangdise orogenic belt,and its south margin is Shigatse forearc basin.The latest exploration data indicate that there are several potential mineralization-alteration areas and three large-size copper-gold ore bodies (No.I,II and III) in the Xiongcun porphyry copper-gold ore concentration area.In this paper,Xiongcun No.I ore body was chosen as the research object.According to sulfur and lead isotope composition of ore-baring porphyry,tuff and main sulfides of the ore,the authors have reached the following two conclusions: 1) Sulfur isotopic compositions vary in a narrow range of δ34S values from ?3.5‰ to +2.7‰ (?1.07‰ on average) and are close to zero,indicating a mantle sulfur signature.2) They have uniform Pb isotope compositions with low content of radiogenic Pb.The206Pb/204Pb,207Pb/204Pb,and208Pb/204Pb ratios vary in the ranges of 18.104-18.432,15.473-15.533 and 37.918-38.3072,respectively.They are located in the transition zone of mantle Pb and orogen Pb but are closer to the former,reflecting a mixing between the mantle material and minor crust material.Through a comparative study of sulfur and lead isotopes of porphyry copper deposits of the island arc environment (e.g.,Xiongcun No.I ore body) and those of the collisional orogenic environment in the Gangdise porphyry copper belt,the authors found that they have similar metal sources,but porphyry copper deposits of the island arc environment have experienced weak contamination of crust materials,whereas porphyry copper deposits of the collisional orogenic environment have experienced strong contamination of crust materials.

sulfur isotope; lead isotope; Xiongcun; porphyry copper-gold deposit; Gangdise belt in Tibet

P588.13; P597 文獻標志碼: A doi: 10.3975/cagsb.2012.04.07

本文由國家自然科學基金(編號: 41172077)、國家973項目(編號: 2011CB403103)、西藏天圓礦業(yè)資源開發(fā)有限公司項目(編號: 2006G-07)和中國地質調查局地調項目“西藏大型礦床成礦專屬性研究”(編號: 資[2012]03-002-055)聯(lián)合資助。

2012-06-03; 改回日期: 2012-06-21。責任編輯: 魏樂軍。

郎興海,男,1982年生。博士。從事礦床學、礦產(chǎn)普查與勘探的教學和研究工作。E-mail: langxinghai@126.com。