西藏雄村礦集區(qū)含礦斑巖成因及構(gòu)造意義：來自年代學(xué)及地球化學(xué)的約束*

郎興海 郭文鉑 王旭輝 鄧煜霖 楊宗耀 謝富偉,3 李壯 張忠 姜楷

1. 成都理工大學(xué)地球科學(xué)學(xué)院，自然資源部構(gòu)造成礦成藏重點(diǎn)實(shí)驗(yàn)室，成都 6100592. 西南交通大學(xué)地球科學(xué)與環(huán)境工程學(xué)院，成都 6117563. 中國地質(zhì)科學(xué)院礦產(chǎn)資源研究所，北京 1000374. 中國地質(zhì)大學(xué)地球科學(xué)與資源學(xué)院，北京 1000835. 西藏天圓礦業(yè)資源開發(fā)有限公司，日喀則 857000

西藏岡底斯銅礦帶位于我國青藏高原拉薩地體南緣，是一條特殊的銅礦帶，因?yàn)槠洳粌H分布有與中生代新特提斯洋殼俯沖相關(guān)的斑巖型礦床，還分布有與新生代印度-歐亞大陸碰撞造山環(huán)境相關(guān)的斑巖型礦床(Houetal., 2009, 2015b; Taftietal., 2009; Langetal., 2014)。目前在該成礦帶發(fā)現(xiàn)產(chǎn)于碰撞造山環(huán)境的斑巖型礦床有甲瑪、驅(qū)龍、邦鋪、朱諾、沖江、廳宮、白容、沙讓、吉如等(侯增謙等, 2003; 鄭有業(yè)等, 2007; 張剛陽等, 2008; Houetal., 2009; 唐菊興等, 2009; Zhengetal., 2015)；產(chǎn)于俯沖構(gòu)造環(huán)境的僅有雄村斑巖型銅金礦床(Taftietal., 2009; Langetal., 2014)。由于礦床數(shù)量眾多，研究者更多的關(guān)注于岡底斯銅礦帶內(nèi)新生代時(shí)期產(chǎn)于碰撞造山環(huán)境的斑巖型礦床，對(duì)它們的空間分布特征、地球動(dòng)力學(xué)背景、含礦斑巖的源區(qū)進(jìn)行了系統(tǒng)的解剖(Houetal., 2009, 2013, 2015a, b)。但對(duì)于中生代與新特提斯洋殼俯沖相關(guān)的斑巖型礦床研究還相對(duì)薄弱、成礦理論不夠完善、且找礦勘查也一直遭遇瓶頸。盡管前人對(duì)于雄村礦集區(qū)的成礦作用的研究也有所涉及(Taftietal., 2009; Langetal., 2014; Tangetal., 2015; Yinetal., 2017)，但主要的研究工作側(cè)重于單個(gè)礦床的研究。為了全面厘定礦集區(qū)的巖漿與成礦作用的關(guān)系，本文對(duì)雄村礦集區(qū)新發(fā)現(xiàn)的3號(hào)礦體的含礦斑巖開展了鋯石U-Pb定年、巖石地球化學(xué)和Sr-Nd-Pb-Hf同位素地球化學(xué)研究，同時(shí)結(jié)合前期的研究成果，深化對(duì)礦集區(qū)含礦巖體成因、地球動(dòng)力學(xué)背景及岡底斯成礦帶巖漿作用與成礦關(guān)系的認(rèn)識(shí)，為下一步礦區(qū)找礦工作部署和區(qū)域找礦突破提供重要的理論依據(jù)。

1 區(qū)域及礦床地質(zhì)

岡底斯銅礦帶位于拉薩地體南緣，長(zhǎng)約400km，寬約50km。帶內(nèi)已發(fā)現(xiàn)數(shù)個(gè)形成于碰撞造山環(huán)境中的大型-超大型銅礦床，如甲瑪、驅(qū)龍、白容、沖江、廳宮等，這些礦床與中新世埃達(dá)克質(zhì)斑巖體關(guān)系密切(圖1a)。另外在岡底斯銅礦帶上還存在與新特提斯洋殼俯沖相關(guān)的斑巖型成礦作用——雄村銅金礦床。雄村礦集區(qū)位于西藏岡底斯銅礦帶南緣，其南側(cè)緊鄰日喀則弧前盆地(圖1a)。礦集區(qū)出露的地層為中-下侏羅統(tǒng)雄村組火山-沉積巖(圖1b)(丁楓等, 2012; Langetal., 2019)，其巖性組合主要為火山集塊巖、火山角礫巖、凝灰?guī)r，其間夾少量的砂巖、粉砂巖和灰?guī)r。礦集區(qū)內(nèi)主要的侵入巖形成時(shí)代為侏羅紀(jì)和始新世(圖1b)。侏羅紀(jì)侵入體包括早侏羅世石英閃長(zhǎng)斑巖(181～175Ma; Langetal., 2014)、早-中侏羅世石英閃長(zhǎng)斑巖(～174Ma; 郎興海等, 2014)、中侏羅世石英閃長(zhǎng)斑巖(167～161Ma; Langetal., 2014)和輝綠巖脈(165Ma; Langetal., 2018)；始新世侵入體主要包括礦區(qū)東側(cè)的黑云母花崗閃長(zhǎng)巖(47Ma; 唐菊興等, 2010)、石英閃長(zhǎng)巖和少量的煌斑巖脈(47Ma; Langetal., 2017)。礦集區(qū)構(gòu)造較為發(fā)育，主要為東西向、北東-南西向、北西-南東向斷層構(gòu)造(圖1b)，以及位于礦區(qū)南部的褶皺構(gòu)造。

圖1 岡底斯斑巖銅礦帶地質(zhì)圖(a,據(jù)Yang et al., 2009)及雄村礦集區(qū)地質(zhì)圖(b,據(jù)Tang et al., 2015)Fig.1 Geological map of Gangdese porphyry copper belt (a, after Yang et al., 2009) and Geological map of Xiongcun district (b, after Tang et al., 2015)

礦集區(qū)內(nèi)1、2、3號(hào)礦體呈北西向近等距展布，平面上為近橢圓狀(圖1b)。1號(hào)礦體Cu、Au、Ag金屬量分別為1.04×106t @ 0.48%、143.31t @ 0.66g/t、900.43t @ 4.19g/t，其蝕變類型為鉀硅酸鹽巖化、絹英巖化和青磐巖化，礦化呈浸染狀或脈狀產(chǎn)出，主要金屬礦物為黃銅礦、黃鐵礦、磁黃鐵礦以及少量的毒砂、方鉛礦、輝鉬礦和閃鋅礦等(圖2g-k)，缺乏磁鐵礦、硬石膏等表征高氧逸度的礦物。2號(hào)礦體Cu、Au、Ag金屬量分別為1.34×106t @ 0.35%、76.34t @ 0.22g/t、193.78t @ 1.3g/t，其蝕變類型包含鉀硅酸鹽巖化、鈉化-鈣化、絹英巖化和青磐巖化，主要金屬礦物為黃銅礦、黃鐵礦、磁鐵礦以及少量的輝鉬礦、方鉛礦和閃鋅礦等(圖2l-o)，非金屬礦物中可見硬石膏。3號(hào)礦體的圍巖蝕變、礦物組合特征與2號(hào)礦體一致，主成礦元素為銅，伴生金、銀，平均品位分別為0.26%、0.11g/t和1.2g/t。

圖2 雄村礦集區(qū)含礦斑巖和礦石的手標(biāo)本及鏡下照片(a)中侏羅世石英閃長(zhǎng)斑巖(1號(hào)礦體含礦斑巖)；(b)早侏羅世石英閃長(zhǎng)斑巖(2號(hào)礦體含礦斑巖)；(c)早侏羅世石英閃長(zhǎng)斑巖(3號(hào)礦體含礦斑巖)；(d)早-中侏羅世石英閃長(zhǎng)斑巖(不含礦斑巖)；(e)早侏羅世石英閃長(zhǎng)斑巖(3號(hào)礦體含礦斑巖)顯微照片；(f)早-中侏羅世石英閃長(zhǎng)斑巖顯微照片；(g-k) 1號(hào)礦體典型金屬礦物；(l-o) 2、3號(hào)礦體典型金屬礦物. Ccp-黃銅礦；Py-黃鐵礦；Po-磁黃鐵礦；Sp-閃鋅礦；Gn-方鉛礦；Mag-磁鐵礦；Mol-輝鉬礦；Cv-銅藍(lán)；Q-石英；Pl-斜長(zhǎng)石；Hbl-角閃石Fig.2 Hand specimen photos and microphotographs of ore-bearing porphyry and ore in the Xiongcun district(a) Middle Jurassic quartz diorite porphyry (ore-bearing porphyry of the No.1 deposit); (b) Early Jurassic quartz diorite porphyry (ore-bearing porphyry of the No.2 deposit); (c) Early Jurassic quartz diorite porphyry (ore-bearing porphyry of the No.3 deposit); (d) Early-Middle Jurassic quartz diorite porphyry (barren porphyry); (e) microphotograph of Early Jurassic quartz diorite porphyry (ore-bearing porphyry of the No.3 deposit); (f) microphotograph of Early-Middle Jurassic quartz diorite porphyry; (g-k) typical metallic mineral assemblages of No.1 deposit; (l-o) typical metallic mineral assemblages of No.2 and No.3 deposits. Ccp-chalcopyrite; Py-pyrite; Ccp-chalcopyrite; Po-pyrrhotite; Sp-sphalerite; Gn-galena; Mag-magnetite; Mol-molybdenite; Cv-covellite; Q-quartz; Pl-plagioclase; Hbl-hornblende

2 樣品特征及描述

雄村礦集區(qū)共發(fā)育有三期侏羅紀(jì)石英閃長(zhǎng)斑巖(圖1b)：早侏羅世石英閃長(zhǎng)斑巖(圖2b, c)、早-中侏羅世石英閃長(zhǎng)斑巖(圖2d)和中侏羅世石英閃長(zhǎng)斑巖(圖2a)。其中早侏羅世石英閃長(zhǎng)斑巖為2號(hào)、3號(hào)礦體的含礦斑巖，中侏羅世石英閃長(zhǎng)斑巖為1號(hào)礦體的含礦斑巖，前人對(duì)1、2號(hào)礦體中的這兩類含礦斑巖進(jìn)行了詳細(xì)描述(Langetal., 2014; Tangetal., 2015; Yinetal., 2017)，此處不再復(fù)述。早-中侏羅世石英閃長(zhǎng)斑巖為礦集區(qū)非含礦斑巖(圖1b)，以含大量粗粒石英斑晶而區(qū)分于含礦斑巖(圖2d, f, 郎興海等, 2014)，可見其侵入早侏羅世石英閃長(zhǎng)斑巖(圖1b)。3號(hào)礦體含礦早侏羅世石英閃長(zhǎng)斑巖蝕變強(qiáng)烈，蝕變?nèi)醯膸r石為灰白色，斑狀結(jié)構(gòu)，斑晶含量約10%～15%，主要由細(xì)粒斜長(zhǎng)石、角閃石和眼球狀(渾圓狀)石英組成(圖2c, e)，石英斑晶粒徑為1～3mm不等，角閃石和斜長(zhǎng)石蝕變強(qiáng)烈；基質(zhì)具有微粒-細(xì)粒結(jié)構(gòu)，主要由石英、斜長(zhǎng)石、黑云母和少量的角閃石構(gòu)成，斜長(zhǎng)石常發(fā)生絹云母蝕變；副礦物可見鋯石、磷灰石和磁鐵礦。本次研究的3號(hào)礦體含礦早侏羅世石英閃長(zhǎng)斑巖樣品均采自于鉆孔巖芯，采集樣品時(shí)盡量選擇新鮮、蝕變較弱的巖芯，以保證樣品化學(xué)分析的可靠性。

3 分析方法

3.1 鋯石U-Pb定年

鋯石的分選在廊坊市科大巖石礦物分選技術(shù)服務(wù)有限公司完成，首先對(duì)樣品進(jìn)行破碎、淘洗、電磁與重液分選，之后在雙目鏡下挑選出粒度大、晶型好、裂隙與包裹體較少的鋯石備用。鋯石的制靶及照相在北京鋯年領(lǐng)航科技有限公司完成，首先將挑選的鋯石置于環(huán)氧樹脂內(nèi)，對(duì)其進(jìn)行拋光清洗，露出鋯石表面，制成靶樣。之后對(duì)鋯石進(jìn)行陰極發(fā)光及透反射光圖像的采集。參照鋯石陰極發(fā)光及透反射光圖像，選擇鋯石顆粒表面無裂隙、內(nèi)部環(huán)帶清晰、無包裹體的位置作為U-Pb定年的測(cè)試點(diǎn)。鋯石U-Pb同位素測(cè)試在中國地質(zhì)大學(xué)(北京)成礦過程與礦產(chǎn)資源國家重點(diǎn)實(shí)驗(yàn)室進(jìn)行。U-Pb同位素測(cè)試中所用激光剝蝕系統(tǒng)為Geolas 193，ICP-MS為Thermo Fisher X-SeriesⅡ型四級(jí)桿等離子質(zhì)譜儀。激光束斑直徑為32μm，剝蝕頻率為8Hz，并利用He氣作為剝蝕物質(zhì)的載氣，Ar氣作為補(bǔ)償氣。測(cè)試中采用每隔5個(gè)測(cè)試點(diǎn)測(cè)定兩個(gè)鋯石91500(Wiedenbecketal., 1995)對(duì)樣品進(jìn)行校正，并用鋯石Plesovice(Slámaetal., 2008)，觀察儀器狀態(tài)和測(cè)試的重現(xiàn)性。詳細(xì)實(shí)驗(yàn)操作步驟可見侯可軍等(2009)。利用ICPMSDataCal(Ver7.2)軟件處理信號(hào)，協(xié)和圖解采用Isoplot 4.0處理。

3.2 Hf同位素分析

鋯石Hf同位素測(cè)試是在北京科薈測(cè)試技術(shù)有限公司Neptune plus多接收等離子質(zhì)譜及配套的ESI NWR193紫外激光剝蝕系統(tǒng)(LA-MC-ICP-MS)上進(jìn)行的，實(shí)驗(yàn)過程中采用He作為剝蝕物質(zhì)載氣，剝蝕直徑采用50μm，測(cè)定時(shí)使用鋯石國際標(biāo)樣GJ-1作為參考物質(zhì)，分析點(diǎn)與U-Pb定年分析點(diǎn)為同一位置。相關(guān)儀器運(yùn)行條件及詳細(xì)分析流程見侯可軍等(2007)。分析過程中鋯石標(biāo)準(zhǔn)GJ-1的176Hf/177Hf測(cè)試加權(quán)平均值為0.282007±0.000007，與文獻(xiàn)報(bào)道值(Moreletal., 2008)在誤差范圍內(nèi)完全一致。

3.3 主微量元素分析

主量、微量元素分析在西南冶金地質(zhì)測(cè)試中心進(jìn)行。主量元素測(cè)試采用X射線熒光光譜法(XRF)，在荷蘭帕納科Axios X熒光儀完成，采用GBW07211和GBW07108為分析標(biāo)樣，分析結(jié)果顯示誤差優(yōu)于3%。微量元素測(cè)定采用電感耦合等離子體質(zhì)譜法(ICP-MS)，在NexIon 300x ICP-MS儀器上完成，將樣品研磨并用酸溶法制成溶液，然后在等離子質(zhì)譜儀上進(jìn)行測(cè)定，并用GBW07103和GBW07104為分析標(biāo)樣，分析結(jié)果顯示含量大于10×10-6的元素分析誤差小于5%，而含量小于10×10-6的元素分析誤差小于10%。

3.4 Sr-Nd-Pb同位素分析

全巖Sr-Nd-Pb 同位素測(cè)定在南京大學(xué)內(nèi)生金屬礦床成礦機(jī)制研究國家重點(diǎn)實(shí)驗(yàn)室完成。利用HF-HNO3混合酸來完全溶解200mg的粉末樣品，采用Bio-Rad50WX8陽離子交換樹脂將Sr、Nd、Pb分離提純。提純后的Sr溶液采用Thermo Finnigan公司的Triton TI熱電離質(zhì)譜儀(TIMS)進(jìn)行分析，提純后的Nd和Pb溶液采用Thermo Neptune Plus多接受等離子體質(zhì)譜儀(MC-ICP-MS)進(jìn)行分析，詳細(xì)的分離和測(cè)試流程見濮魏等(2005)。在實(shí)驗(yàn)過程中標(biāo)樣NBS987 Sr的87Sr/86Sr測(cè)定值為0.710239±0.000002(1σ)、JNdi-1 Nd的143Nd/144Nd測(cè)得值為0.512128±0.000004(1σ)、NIST-981 Pb的206Pb/204Pb、207Pb/204Pb、208Pb/204Pb的測(cè)定值分別為16.9318±0.0003(1σ)、15.4858±0.0003(1σ)、36.6819±0.0008(1σ)，標(biāo)樣測(cè)定值與文獻(xiàn)報(bào)道值在誤差范圍內(nèi)一致(Weisetal., 2006)。

圖3 雄村礦集區(qū)3號(hào)礦體含礦斑巖LA-ICP-MS鋯石U-Pb年齡諧和圖Fig.3 LA-ICP-MS zircon U-Pb concordia diagram of ore-bearing porphyry from the No.3 deposit in the Xiongcun district

4 分析結(jié)果

4.1 鋯石U-Pb年齡

3號(hào)礦體含礦斑巖樣品(ZK10346-65.2)鋯石多為自形-半自形晶體，呈短柱狀或長(zhǎng)柱狀，鋯石粒徑在80～150μm之間，長(zhǎng)寬比1:1～2:1，鋯石陰極發(fā)光圖像顯示具有明顯震蕩環(huán)帶(圖3)。鋯石的Th與U含量分別為26.1×10-6～156×10-6和57.9×10-6～303×10-6，Th/U比值在0.36～0.84，表明它們均屬于典型的巖漿鋯石(Hoskin and Black, 2000)。本次實(shí)驗(yàn)共測(cè)定了12個(gè)有效數(shù)據(jù)點(diǎn)，206Pb/238U年齡分布較集中，在174.0～180.9Ma之間變化(表1)，在鋯石U-Pb諧和圖中均落在諧和線上(圖3)，206Pb/238U年齡加權(quán)平均值為176.9±1.4Ma(MSWD=3.1)，該年齡值代表了含礦斑巖的成巖年齡。

表1雄村礦集區(qū)3號(hào)礦體含礦斑巖LA-ICP-MS鋯石U-Pb測(cè)試結(jié)果

Table 1 LA-ICP-MS zircon U-Pb analysis data of ore-bearing porphyry from the No.3 deposit in the Xiongcun district

測(cè)點(diǎn)號(hào)元素含量(×10-6)UThTh/U同位素比值年齡(Ma)207Pb/206Pb±1σ207Pb/235U±1σ206Pb/238U±1σ206Pb/238U±1σ110557.70.550.04980.00110.19280.00430.02810.0002178.81.3211595.90.840.05100.00210.19610.00770.02800.0003177.92.2313462.80.470.05110.00080.19560.00320.02790.0002177.31.342211560.710.05200.00070.20360.00390.02840.0004180.82.6513661.70.450.04980.00380.19210.01810.02790.0006177.63.6615157.50.380.05080.00110.19610.00430.02810.0002178.51.5757.935.60.610.05190.00170.20220.00680.02830.0003180.21.8861.426.10.420.04910.00330.18960.01490.02770.0003176.11.7916884.60.500.05000.00080.19000.00290.02770.0002176.01.01015477.50.500.05020.00090.18860.00330.02740.0001174.00.8113031100.360.05000.00110.19040.00410.02780.0001176.50.81282.341.50.500.05080.00180.19950.00830.02850.0002180.91.2

圖4 雄村礦集區(qū)含礦斑巖Zr/TiO2-Nb/Y(a, 據(jù)Winchester and Floyd, 1977)和Y-Zr(b, 據(jù)Barrett and MacLean, 1994)圖解Fig.4 Zr/TiO2 vs. Nb/Y (a, after Winchester and Floyd, 1977) and Y vs. Zr(b, after Barrett and MacLean, 1994)diagrams of ore-bearing porphyries in the Xiongcun district

4.2 Hf同位素特征

雄村礦集區(qū)1、2、3號(hào)礦體含礦斑巖的鋯石Hf同位素結(jié)果列于表2。含礦斑巖的鋯石176Lu/177Hf值較低(均值分別為0.0015、0.0013、0.0017)，表明鋯石在形成后具有極低的放射性成因Hf積累，因此所測(cè)定的176Hf/177Hf值可以代表鋯石結(jié)晶時(shí)體系的Hf同位素組成(Amelinetal., 2000)。1號(hào)礦體含礦斑巖鋯石εHf(t)值變化范圍為10.4～15.3，平均值為13.5；Hf同位素單階段模式年齡(tDM1)和二階段模式年齡(tDM2)分別介于189～385Ma和205～515Ma之間(表2)。2號(hào)礦體含礦斑巖鋯石εHf(t)值變化范圍為11.8～15.2，平均值為13.7；Hf同位素單階段模式年齡(tDM1)和二階段模式年齡(tDM2)分別介于208～340Ma和223～437Ma之間(表2)。3號(hào)礦體含礦斑巖鋯石εHf(t)值變化范圍為9.9～14.5(表2)，平均值為12.5；Hf同位素單階段模式年齡(tDM1)和二階段模式年齡(tDM2)分別介于254～436Ma和294～588Ma之間(表2)。

表2雄村礦集區(qū)含礦斑巖鋯石Lu-Hf同位素測(cè)試結(jié)果

Table 2 Lu-Hf isotopic compositions of zircons of ore-bearing porphyries in the Xiongcun district

測(cè)點(diǎn)號(hào)176Yb/177Hf±2σ176Lu/177Hf±2σ176Hf/177Hf±2σAge(Ma)εHf(t)tDM1(Ma)tDM2(Ma)1號(hào)礦體(樣品X-1,據(jù)Tangetal.,2015)1.10.03020310.00070860.00108740.00001730.28306540.0000281162.8613.42643252.10.03953320.00049240.00145150.00001930.28301700.0000278162.7011.73374383.10.03587320.00126170.00136870.00004380.28298320.0000248160.1510.43855154.10.04917590.00272890.00190690.00010200.28305640.0000282159.8213.02833525.10.03196230.00039770.00128080.00001710.28308350.0000222160.1514.02402866.10.03795010.00083700.00165240.00003410.28312030.0000219160.3115.31892057.10.02881360.00047480.00120150.00002230.28310390.0000197159.6014.72102408.10.02563990.00022360.00106070.00001030.28308420.0000194161.8914.12372829.10.03856540.00059360.00148750.00002340.28305130.0000221160.1912.828836110.10.03461540.00026120.00141620.00001100.28310670.0000205163.2514.920723311.10.03087770.00055870.00126800.00001880.28303440.0000237161.9912.331039712.10.03345750.00035800.00130640.00001580.28307730.0000224159.9813.82493011號(hào)礦體(樣品ZK5056-4,據(jù)Tangetal.,2015)1.10.04160970.00076250.00184800.00003440.28310560.0000209167.8714.92112362.10.02932700.00054330.00135350.00002500.28306410.0000190166.8813.42683273.10.03457740.00025970.00153510.00000990.28308560.0000217169.3914.22382784.10.04669390.00026270.00190580.00001130.28305770.0000184166.5313.12823465.10.03584580.00057500.00142710.00002020.28301040.0000227165.7511.53464506.10.04566360.00046560.00167220.00001240.28309770.0000232168.3914.62222527.10.03419670.00058950.00124920.00002580.28305360.0000250167.3313.12823508.10.05277690.00080670.00195750.00001730.28309140.0000267168.2114.42332682號(hào)礦體(樣品7226-233.7,據(jù)Yinetal.,2017)1.10.07004810.00035430.00187710.00000420.28307980.0000204181.014.22492872.10.03779020.00016080.00104710.00000400.28302640.0000186174.512.33204063.10.04738540.00053290.00128340.00000810.28304570.0000212184.013.22943594.10.03644310.00054210.00101000.00001020.28305230.0000206177.713.32823455.10.05273870.00056120.00143260.00000790.28303540.0000210187.612.93103817.10.05675580.00083580.00138640.00001030.28310610.0000229181.515.22082238.10.04036290.00088310.00103090.00001920.28306400.0000249180.513.826631710.10.04273600.00067370.00123140.00001240.28306880.0000217180.113.926030811.10.05700840.00017020.00163490.00000590.28306590.0000239184.613.826831512.10.06472030.00071730.00173010.00000870.28309080.0000223177.314.623226313.10.05272880.00011840.00140370.00000930.28304750.0000244178.813.129235814.10.04709100.00019320.00121100.00000420.28306160.0000256175.113.527132715.10.04313570.00030980.00107150.00000600.28301240.0000255183.912.034043316.10.06417580.00059410.00157570.00000820.28306270.0000290181.913.72723232號(hào)礦體(樣品7224-159.9,據(jù)Yinetal.,2017)1.10.03723990.00040560.00095490.00001030.28306660.0000181176.913.82623132.10.03208480.00031090.00095520.00000780.28308930.0000201176.614.62292613.10.04648160.00021560.00122390.00000710.28306720.0000187177.413.82633134.10.04907810.00056720.00124970.00001830.28307270.0000229176.514.02553015.10.04020630.00026050.00112760.00000250.28309210.0000241174.614.62262576.10.04161180.00067760.00100960.00002160.28309530.0000256176.414.82212487.10.04483760.00033330.00107350.00000850.28307460.0000265175.714.02512969.10.03800540.00015570.00112410.00000320.28308930.0000225171.414.5230265

續(xù)表2

Continued Table 2

測(cè)點(diǎn)號(hào)176Yb/177Hf±2σ176Lu/177Hf±2σ176Hf/177Hf±2σAge(Ma)εHf(t)tDM1(Ma)tDM2(Ma)11.10.04438960.00046720.00140070.00001200.28307530.0000181176.914.025229612.10.03040480.00041130.00090520.00001320.28301240.0000187175.011.833843713.10.05274870.00047590.00142860.00000690.28307640.0000228174.814.02512952號(hào)礦體(樣品7235-123.4,據(jù)Yinetal.,2017)1.10.08480730.00055470.00230640.00002760.28307710.0000240176.514.025629911.10.09235400.00164850.00259370.00001570.28306680.0000237178.713.62733243號(hào)礦體(樣品ZK10346-65.2,本文測(cè)試)2.10.0838030.0027620.0018030.0000710.2830650.000316176.914.02703233.10.0687290.0020350.0015060.0000290.2830170.000330176.912.43374294.10.0653700.0043230.0014170.0000720.2830270.000294176.912.73224065.10.1167240.0078480.0025300.0003100.2830230.000280176.912.53384246.10.0646120.0008350.0013470.0000350.2830000.000320176.911.83614688.10.0508730.0035540.0010320.0000680.2829800.000315176.911.13865119.10.0685370.0132980.0014400.0002620.2830210.000360176.912.633041910.10.0681090.0046990.0015030.0000880.2830470.000352176.913.429436211.10.0613110.0095740.0012740.0001750.2829470.000258176.99.943658812.10.1247100.0090730.0027080.0001950.2830800.000336176.914.5254294

圖5 雄村礦集區(qū)礦體含礦斑巖球粒隕石標(biāo)準(zhǔn)化稀土元素配分圖(a)和原始地幔標(biāo)準(zhǔn)化微量元素蛛網(wǎng)圖(b)(標(biāo)準(zhǔn)化值據(jù)Sun and McDonough, 1989)1號(hào)礦體據(jù)Tang et al. (2015)，2號(hào)礦體據(jù)Yin et al. (2017)Fig.5 Chondrite-normalized REE distribution patterns (a) and primitive mantle-normalized trace element spidergrams (b) of ore-bearing porphyries in the Xiongcun district (normalization values after Sun and McDonough, 1989)

4.3 主微量元素特征

雄村礦集區(qū)1、2、3號(hào)礦體含礦石英閃長(zhǎng)斑巖的主微量元素含量列于表3中。1號(hào)礦體含礦石英閃長(zhǎng)斑巖的SiO2含量介于63.07%～69.70%之間，Al2O3含量介于12.69%～17.71%之間，MgO含量介于0.58%～2.97%之間，Mg#值介于10～40之間(表3)。2號(hào)礦體含礦石英閃長(zhǎng)斑巖的SiO2含量介于55.81%～67.32%之間，Al2O3含量介于14.71%～19.74%之間，MgO含量介于0.93%～3.11%之間，Mg#值介于21～55之間(表3)。3號(hào)礦體含礦石英閃長(zhǎng)斑巖的SiO2含量介于50.87%～65.50%之間，Al2O3含量介于16.66%～21.76%之間，MgO含量介于1.32%～3.21%之間，Mg#值介于23～44之間(表3)。Nb/Y-Zr/TiO2圖解可以有效判斷遭受熱液蝕變的巖石類型，在Nb/Y-Zr/TiO2圖解上，雄村礦集區(qū)含礦斑巖樣品主要落在安山巖區(qū)域附近(圖4a)，這與野外觀察和鏡下鑒定結(jié)果一致；在Zr-Y判別圖解上，所有樣品落在鈣堿性區(qū)域及鈣堿性向低鉀(拉斑)系列過渡的區(qū)域(圖4b)，表明巖石屬于鈣堿性系列。

表3雄村礦集區(qū)含礦斑巖主量元素(wt%)和微量元素(×10-6)分析數(shù)據(jù)表

Table 3 Major (wt%) and trace (×10-6) elements analyses of ore-bearing porphyries in the Xiongcun district

樣品號(hào)ZK5013-4ZK5021-3ZK5023-2ZK5023-1ZK5015-5ZK5015-4X5015-3ZK5015-2ZK5056-4X-17224-157.87224-419.67226-238.77229-264.2SiO268.3168.8968.2468.7463.0765.2368.0769.765.5667.0359.1762.7667.3266.50TiO20.370.400.310.330.400.390.350.360.380.410.680.480.380.41Al2O314.3517.7112.6913.4314.4414.7913.0813.7215.6416.2916.9417.1116.1414.71Fe2O31.122.116.121.029.367.416.824.872.591.453.781.021.713.07FeO5.251.021.076.230.920.840.721.444.074.582.873.931.351.81MnO0.30.050.060.590.050.050.080.060.220.060.080.130.070.07MgO2.350.790.612.970.580.620.740.61.360.783.111.661.060.94CaO0.680.270.790.590.40.430.260.490.890.534.224.793.822.99Na2O0.410.430.360.290.380.40.580.40.320.423.123.142.661.90K2O2.875.023.881.943.844.343.983.933.854.421.872.483.925.44P2O50.170.020.140.160.020.030.030.020.220.160.220.190.110.14LOI2.592.194.482.246.024.754.453.593.733.583.181.591.251.31Total98.7798.998.7598.5399.4899.2899.1699.1898.8399.7199.2499.2899.7999.29Mg#4033144310131616281947384027Sc5.13.234.134.425.375.955.617.285.464.2814.15.768.455.18V63.0756.549.6866.7449.2549.6952.945.6563.9755.3915177.475.6119Cr4.631.672.944.491.952.072.221.512.441.4533.71.749.733.1Co2.994.5110.781.578.0714.878.1312.34.9721.9922.914.64.7818.1Ni1.680.81.261.181.112.081.381.361.002.6610.52.264.182.37Ga17.0819.612.5818.2314.8413.341611.918.0316.1919.316.813.816.2Rb74.6512093.771.1790.62110.9120105.398.12105.839.239.277.876.3Sr79.6130.456.1849.2350.0653.141.649.0255.4859.99293426296279Y11.214.9411.69.9310.9912.69.2211.7713.1515.8214.213.210.28.65Zr93.2794.183.3290.7996.2998.2580.892.1799.37108.471.791.598.385.4Nb5.797.091.756.921.51.245.740.963.961.48.197.345.476.68Cs9.344.95.379.663.63.563.643.4413.115.194.833.013.113.19Ba204.3164194112.6209.1218.1167202.1279.2577.5170323800609La12.017.5811.9713.988.0211.666.077.4615.2117.5719.215106.98Ce19.7412.121.4325.6115.1930.4410.214.5530.329.5335.129.217.910.3Pr2.61.832.533.011.882.71.441.643.113.6943.42.121.58Nd12.227.9411.8614.048.9514.396.337.9715.6415.1516.413.88.916.73Sm2.291.532.42.611.942.651.51.752.933.13.433.021.981.54Eu0.680.470.850.860.680.80.460.620.980.961.10.90.740.6Gd2.291.32.332.391.912.551.81.922.912.993.623.092.271.82Tb0.340.160.360.320.290.350.260.30.410.510.520.440.320.24Dy1.830.921.641.651.881.641.822.182.552.92.561.91.43Ho0.360.20.40.320.330.370.350.380.440.530.570.520.380.3Er1.160.611.171.010.991.221.11.181.371.611.721.551.220.89Tm0.210.10.210.190.170.210.150.210.250.230.240.240.180.14Yb1.150.691.181.070.951.131.111.151.331.371.741.651.30.89Lu0.180.120.190.170.150.170.180.180.210.250.270.260.210.16Hf2.272.661.682.052.332.562.222.262.475.592.042.472.532.17Ta0.730.440.160.420.110.110.360.070.280.110.490.440.40.43Pb36.9211918879.9147.1135.8205109.4181.1116.511.8479.8113.4Th3.643.183.154.083.4533.212.84.363.824.824.574.736.32U0.30.550.230.510.240.20.540.210.390.351.791.344.323.53∑REE57.0635.5358.8867.2243.1070.5232.5941.1377.2780.0490.8175.6349.4333.60(La/Yb)N7.497.887.289.376.067.403.924.658.209.207.926.525.525.63δEu0.900.991.081.031.070.930.851.031.010.950.950.891.061.09數(shù)據(jù)來源1號(hào)礦體(據(jù)Tangetal.,2015)2號(hào)礦體(據(jù)Yinetal.,2017)

續(xù)表3

Continued Table 3

樣品號(hào)7233-163.77232-4367239-247.67239-807247-75.67248-360.47251-378.710346-80.110348-89.110348-92.610348-158.910348-172.510349-46.710349-104.9SiO255.8162.0967.2366.2964.4563.9166.4853.4550.8755.2261.0760.5765.5055.47TiO20.490.40.430.480.420.400.401.131.291.060.690.600.490.53Al2O319.7416.5817.1216.8517.1416.1715.6420.5721.7618.9817.1618.1317.3816.66Fe2O34.351.691.682.031.931.531.274.633.756.012.812.363.0011.32FeO2.342.892.161.811.632.751.284.103.943.484.074.202.714.39MnO0.060.180.040.050.040.080.030.140.140.110.110.120.070.14MgO0.932.41.661.161.202.791.242.873.212.752.642.631.322.43CaO3.745.760.683.314.264.403.046.577.516.555.096.514.325.57Na2O4.562.973.102.523.542.964.444.183.653.371.311.950.720.83K2O3.452.523.633.092.252.123.362.213.552.194.872.764.332.59P2O50.100.110.120.190.120.120.180.140.330.280.170.170.140.09LOI2.931.831.871.662.411.881.723.883.614.103.652.873.257.07Total98.599.4299.7299.4499.3999.1199.0898.4498.2599.0098.0397.6898.4298.40Mg#2149453639554838443642433123Sc8.9712.110.76.710.813.94.2122.6620.3818.9711.6710.6311.0714.72V13815510910710811762.1225.2209.7211.6145.7125.0105.7114.0Cr5.314.2210.211.811.112.81.9112.7513.7910.9712.3211.919.4510.54Co28.2118.123.5312.27.524.5739.0926.6753.7234.7733.1316.8881.57Ni7.894.274.554.684.77.5157.310.985.307.059.147.834.624.06Ga19.516.315.420.616.715.414.622.6322.0818.6618.2218.1921.5420.87Rb54.84010453.384.878.358.671.98100.165.84116.474.97113.389.37Sr55034391.4205312321288417.2477.0471.1286.4366.8204.1257.7Y13.711.11111.917.113.111.325.021.327.814.015.212.718.4Zr10888.872.910585.678.290.112114211598.411410996.8Nb5.975.135.258.485.994.897.848.159.9910.005.997.633.774.23Cs3.151.3411.13.43.184.670.933.213.601.712.453.035.975.66Ba549331313497180220488213446270862477611289La11.89.35119.8910.215.814.923.78.6311.721.819.720.399.2Ce22.519.117.717.119.628.624.443.321.425.635.733.936.5164Pr2.882.182.392.292.63.163.075.293.283.513.693.794.2417.5Nd11.88.689.719.4911.512.412.622.215.115.513.314.416.861.2Sm2.871.851.992.123.032.592.614.943.823.712.442.843.399.24Eu0.950.840.590.481.030.740.721.801.111.761.150.881.082.80Gd3.062.262.262.493.82.862.824.573.623.772.452.702.897.43Tb0.440.330.320.370.580.390.380.850.700.750.450.470.480.98Dy2.632.12.062.233.522.372.194.614.044.482.432.602.444.29Ho0.60.450.440.50.750.50.440.960.890.970.510.540.490.75Er1.831.371.41.492.281.521.372.712.672.731.461.531.291.88Tm0.290.20.20.230.330.230.20.440.460.460.230.250.200.25Yb1.891.371.421.632.21.561.462.772.962.781.481.711.261.54Lu0.310.220.210.270.330.240.220.350.400.360.190.210.160.20Hf2.912.352.062.892.42.092.665.275.154.024.805.624.693.38Ta0.450.40.390.530.440.370.490.580.721.270.490.610.340.41Pb10.61787.0411.613.17.325.418.1211.7917.6516.9022.919.9011.82Th4.893.653.855.23.683.715.593.543.193.123.103.063.244.59U4.862.113.331.091.831.081.390.470.700.590.510.400.751.28∑REE63.8550.3051.6950.5861.7572.9667.38118.569.1478.1787.2485.4991.53371.1(La/Yb)N4.484.905.564.353.337.267.326.142.093.0210.568.2711.5946.14δEu0.971.250.850.640.930.830.811.140.901.421.420.961.031.00數(shù)據(jù)來源2號(hào)礦體(據(jù)Yinetal.,2017)3號(hào)礦體(本文測(cè)試)

注：3號(hào)礦體含礦斑巖主量元素含量為扣除燒失量后重新計(jì)算得出的含量

雄村礦集區(qū)1、2、3號(hào)礦體含礦石英閃長(zhǎng)斑巖稀土元素總量較低，介于32.59×10-6～80.04×10-6、33.60×10-6～90.81×10-6、69.14×10-6～371.1×10-6之間(表3)，(La/Yb)N分別介于3.98～9.37、3.33～7.92、2.09～46.14之間。球粒隕石標(biāo)準(zhǔn)化稀土元素配分模式圖顯示(圖5a)，輕重稀土分餾明顯，呈右傾趨勢(shì)。δEu分別介于0.85～1.08、0.64～1.25、0.90～1.42，3號(hào)礦體含礦斑巖Eu總體異常不明顯，僅2個(gè)樣品顯示出Eu的正異常。

雄村礦集區(qū)含礦石英閃長(zhǎng)斑巖微量元素分析結(jié)果見表3，原始地幔標(biāo)準(zhǔn)化蛛網(wǎng)圖顯示出其分配模式整體向右傾斜(圖5b)，相對(duì)富集大離子親石元素(LILEs：如Rb、Ba和Hf)和相對(duì)虧損高場(chǎng)強(qiáng)元素(HFSEs：如Nb和Ta)。

4.4 Sr-Nd-Pb同位素特征

雄村礦集區(qū)1、2、3號(hào)礦體含礦石英閃長(zhǎng)斑巖Sr-Nd-Pb同位素測(cè)試結(jié)果列于表4，所測(cè)得樣品具有相對(duì)較低的(87Sr/86Sr)i比值和相對(duì)較高的εNd(t)值。2號(hào)礦體含礦斑巖樣品(87Sr/86Sr)i比值變化范圍為0.70400～0.70509，3號(hào)礦體含礦斑巖樣品(87Sr/86Sr)i比值變化范圍為0.70311～0.70430；1、2、3號(hào)礦體含礦斑巖εNd(t)值分別介于4.5～5.9、5.5～5.9、5.4～5.6。Pb同位素比值變化較小(表4)，2號(hào)礦體含礦斑巖206Pb/204Pb、207Pb/204Pb、208Pb/204Pb比值分別在18.460～18.857、15.573～15.622、38.584～38.948之間變化；3號(hào)礦體含礦斑巖206Pb/204Pb、207Pb/204Pb、208Pb/204Pb比值分別在18.432～18.506、15.553～15.565、38.448～38.503之間變化。

圖6 雄村礦集區(qū)含礦斑巖Ta-Yb(a, 據(jù)Pearce et al., 1984)、Y-Zr(b，據(jù)Pearce et al., 1984)、Th/Yb-Ta/Yb(c, 據(jù)Gorton and Schandl, 2000)和La/Yb-Sc/Ni(d, 據(jù) Bailey, 1981)圖解Fig.6 Ta vs. Yb (a, after Pearce et al., 1984), Y vs. Zr (b, after Pearce et al., 1984), Th/Yb vs. Ta/Yb (c, after Gorton and Schandl, 2000) and La/Yb vs. Sc/Ni (d, after Bailey, 1981) diagrams of the ore-bearing porphyries in the Xiongcun district

圖7 雄村礦集區(qū)含礦斑巖207Pb/204Pb-206Pb/204Pb (a)和208Pb/204Pb-206Pb/204Pb (b)圖解(據(jù)Zartman and Haines, 1988)A、B、C、D分別代表地幔、造山帶、上地殼和下地殼的平均值Fig.7 207Pb/204Pb vs. 206Pb/204Pb (a) and 208Pb/204Pb vs. 206Pb/204Pb (b) diagrams of the ore-bearing porphyries in the Xiongcun district (after Zartman and Haines, 1988)Dashed lines enclose probable average values (A=mantle; B=orogene; C=upper crust; and D=lower crust)

5 討論

5.1 含礦斑巖侵位及礦化時(shí)間

本文獲得3號(hào)礦體含礦斑巖的年齡為176.9±1.4Ma，黃勇(2013)報(bào)道3號(hào)礦體的輝鉬礦Re-Os年齡為172Ma以及侵入3號(hào)礦體含礦斑巖中的非含礦斑巖年齡為171Ma，Langetal. (2014)報(bào)道2號(hào)礦體含礦斑巖的結(jié)晶年齡為181～175Ma，輝鉬礦Re-Os年齡為172Ma?？紤]到2、3號(hào)礦體的含礦斑巖均為早侏羅世石英閃長(zhǎng)斑巖且成礦時(shí)代和礦化特征一致，我們認(rèn)為2、3號(hào)礦體是同期斑巖成礦作用的產(chǎn)物。而1號(hào)礦體含礦斑巖的侵位時(shí)間為167～161Ma，礦化作用發(fā)生的時(shí)間為161.5Ma(輝鉬礦Re-Os年齡，Langetal., 2014)，明顯晚于2、3號(hào)礦體。綜合礦集區(qū)三個(gè)礦體的含礦斑巖特征和鋯石U-Pb年齡、礦化特征以及輝鉬礦Re-Os年齡，表明雄村礦集區(qū)存在兩期礦化事件，早期礦化事件發(fā)生在約172Ma，與早侏羅世(181～175Ma)石英閃長(zhǎng)斑巖相關(guān)，該期成礦作用形成了礦集區(qū)2、3號(hào)礦體。晚期成礦作用發(fā)生在161.5Ma，成礦巖體為中侏羅世(167～161Ma)石英閃長(zhǎng)斑巖，該期成礦作用形成了礦集區(qū)1號(hào)礦體。

圖8 雄村礦集區(qū)含礦斑巖εNd(t)-(87Sr/86Sr)i(a)、εHf(t)-t(b)和εHf(t)-εNd(t) (c)圖解

馬里亞納島弧巖漿巖據(jù)Linetal. (1990)；安第斯山弧巖漿巖據(jù)Pankhurstetal. (1999)；桑日群火山巖據(jù)Kangetal. (2014)；葉巴組火山巖據(jù)Weietal. (2017)；澤當(dāng)?shù)伢w巖漿巖據(jù) Zhangetal. (2014)；岡底斯巖基據(jù)Jietal. (2009)和Wuetal. (2010)；印度洋MORB據(jù)Chauvel and Blichert-Toft (2001)和Ingleetal. (2003)

Fig.8εNd(t) vs. (87Sr/86Sr)i(a),εHf(t) vs.t(b), andεHf(t) vs.εNd(t) (c) diagrams of the ore-bearing porphyries in the Xiongcun district

Magmatic rocks in Marianas are from Linetal. (1990); magmatic rocks in Andes arc are from Pankhurstetal. (1999); Sangri Group volcanic rocks are from Kangetal. (2014); Yeba Formation volcanic rocks are from Weietal. (2017); magmatic rocks in the Zedong terrane are from Zhangetal. (2014); Gangdese batholiths are from Jietal. (2009) and Wuetal. (2010); Indian Ocean MORB are from Chauvel and Blichert-Toft (2001) and Ingleetal. (2003)

5.2 構(gòu)造背景探討

圖9 雄村礦集區(qū)含礦斑巖構(gòu)造背景及成巖模式圖(據(jù)Tang et al., 2015修改)Fig.9 Cartoon showing tectonic setting and petrogenesis of the ore-bearing porphyries in the Xiongcun district (after Tang et al., 2015)

5.3 含礦斑巖成因

圖10 雄村礦集區(qū)含礦斑巖Th/Nb-εNd(t)(a, 據(jù)Wei et al., 2017)和Th/Yb-Ba/La(b, 據(jù)Woodhead et al., 2001)圖解Fig.10 Th/Nb vs. εNd(t) (a, after Wei et al., 2017) and Th/Yb vs. Ba/La (b, after Woodhead et al., 2001) diagrams of the ore-bearing porphyries in the Xiongcun district

在大洋島弧環(huán)境，由于缺乏大陸地殼，巖漿不可能起源于地殼或在上升的過程中被地殼物質(zhì)混染。雄村礦集區(qū)含礦斑巖顯示出均一的εNd(t)值(圖10a)，進(jìn)一步證實(shí)了巖漿在上升過程中未受到地殼物質(zhì)的混染。因此其源區(qū)主要由兩個(gè)來源：地幔楔和俯沖的洋殼。俯沖洋殼對(duì)源區(qū)的貢獻(xiàn)又包括俯沖洋殼釋放的流體、俯沖沉積物、洋殼直接部分熔融加入巖漿源區(qū)。俯沖洋殼直接部分熔融的形成的巖漿通常形成高Sr(>400×10-6)低Y(<18×10-6)、Yb(<1.9×10-6)的埃達(dá)克巖(Defant and Drummond, 1990)。雄村礦集區(qū)含礦斑巖Sr含量在30×10-6～550×10-6之間變化(Tangetal., 2015; Yinetal., 2017)。表明他們不可能直接來源于洋殼的部分熔融。含礦斑巖的Sr-Nd-Hf同位素結(jié)果顯示它們具有低的(87Sr/86Sr)i比值和相對(duì)較高的εNd(t)、εHf(t)值，在εNd(t)-(87Sr/86Sr)i圖解中(圖8a)，含礦斑巖體主要位于地幔演化序列中，在εHf(t)-t圖解中(圖8b)，它們主要落在虧損地幔附近，在εHf(t)-εNd(t)圖解中(圖8c)，它們主要位于印度洋洋中脊玄武巖(MORB)附近。綜合Sr-Nd-Hf同位素結(jié)果，表明巖漿源區(qū)主要起源于虧損地幔的部分熔融。在洋殼俯沖的構(gòu)造環(huán)境，俯沖洋殼釋放的流體或上覆沉積物熔體對(duì)地幔橄欖巖的交代是誘發(fā)其部分熔融的最為重要的機(jī)制。俯沖沉積物熔體交代源區(qū)會(huì)使得地幔中Nb、Th、Nd的含量顯著增加；反之，俯沖洋殼釋放的流體交代源區(qū)會(huì)使得地幔中Ba、Sr和Pb的含量顯著增加(Kelemenetal., 2003; Castillo and Newhall, 2004)。雄村礦集區(qū)含礦斑巖體具有顯著變化的Sr(30×10-6～550×10-6)、Ba(113×10-6～862×10-6)含量(表3, Tangetal., 2015; Yinetal., 2017)，暗示是俯沖板片釋放的流體對(duì)巖漿源區(qū)發(fā)生了交代作用。在Th/Yb-Ba/La圖解中(圖10b)，變化較大的Ba/La比值進(jìn)一步支持了地幔橄欖巖受到俯沖板片釋放流體的交代的觀點(diǎn)。此外，雄村礦集區(qū)含礦斑巖樣品缺少Eu的負(fù)異常，甚至部分樣品顯示出Eu的正異常，暗示流體參與交代地幔源區(qū)，因?yàn)樾遍L(zhǎng)石是Eu的主要攜帶礦物，在富水的條件下，斜長(zhǎng)石的分異結(jié)晶作用將受到明顯的抑制作用，其結(jié)晶晚于角閃石和石榴子石，造成殘留熔體缺少Eu的負(fù)異常，甚至出現(xiàn)正異常(Münteneretal., 2001; Groveetal., 2002)。另外含礦斑巖具有較高的Th含量(2.8×10-6～6.3×10-6)，接近全球俯沖沉積物的平均值(6.9×10-6, Plank and Langmuir, 1998)，并顯著高于原始地幔Th的含量(0.09×10-6, Sun and McDonough, 1989)，表明俯沖沉積物對(duì)源區(qū)的也具有顯著的貢獻(xiàn)。同時(shí)在207Pb/204Pb-206Pb/204Pb和208Pb/204Pb-206Pb/204Pb圖解中(圖7)，雄村礦集區(qū)含礦斑巖樣品靠近遠(yuǎn)洋沉積物區(qū)域，進(jìn)一步暗示了俯沖沉積物熔體交代地幔也是必不可少的。綜上，筆者認(rèn)為是新特提斯洋殼在早-中侏羅世甚至更早時(shí)期發(fā)生北向俯沖作用，俯沖洋殼釋放的流體和俯沖沉積物熔體同時(shí)交代了地幔橄欖巖，進(jìn)而發(fā)生部分熔融形成母巖漿(圖9)。此外，含礦斑巖顯示出變化較大的Mg#值(10～55)和Cr(1.5×10-6～34×10-6)、Ni(0.8×10-6～57×10-6)含量(表3)，暗示母巖漿在上升侵位過程經(jīng)歷了鐵鎂質(zhì)礦物的結(jié)晶分異作用，這一觀點(diǎn)也被La/Sm-La圖解所支持(圖11)。因此筆者認(rèn)為，母巖漿起源于地幔部分熔融后經(jīng)歷了結(jié)晶分異作用，最終上升侵位于近地表形成了雄村礦集區(qū)含礦石英閃長(zhǎng)斑巖(圖9)。

圖11 雄村礦集區(qū)含礦斑巖La/Sm-La圖解Fig.11 La/Sm vs. La diagram of the ore-bearing porphyries in the Xiongcun district

5.4 找礦意義

斑巖型礦床，無論形成于俯沖構(gòu)造環(huán)境(陸緣弧或大洋島弧)還是碰撞造山構(gòu)造環(huán)境，一個(gè)最重要的特征就是常成群、成帶分布(Cookeetal., 2005; Singeretal., 2005; Houetal., 2009; Sillitoe, 2010)，即一個(gè)斑巖型礦床的發(fā)現(xiàn)，往往可能在區(qū)域上尋找到更多的斑巖型礦床。近年來在拉薩地體南緣相繼報(bào)道了與新特提斯洋早期(晚三疊世-中侏羅世)俯沖作用相關(guān)的長(zhǎng)英質(zhì)弧巖漿巖，如湯白巖體(Guoetal., 2013; 王旭輝等, 2018)、雄村巖體(唐菊興等, 2010; Langetal., 2014)、努瑪巖體(Jietal., 2009)、南木林巖體(Zhuetal., 2011)、大竹卡巖體(Jietal., 2009)、若措巖體(郎興海等, 2017; Wangetal., 2019)、卡如巖體、塔瑪巖體、臥布巖體、宗噶巖體等(鄒銀橋等, 2017; Zouetal., 2017)。然而目前僅在雄村礦集區(qū)發(fā)現(xiàn)了與新特提斯洋早期俯沖作用相關(guān)的斑巖型礦床。那么目前未發(fā)現(xiàn)其他斑巖型礦床的原因是除雄村巖體外其他巖體都不具斑巖型礦化條件呢？還是目前的勘探程度不夠呢？

雄村礦集區(qū)含礦斑巖體具有較高的εNd(t)(>4.5)、εHf(t)(>10)值(圖8)，其巖漿起源于虧損地幔的部分熔融。近年來，一些學(xué)者在拉薩地體的南緣發(fā)現(xiàn)了含孔雀石化和石英-硫化物的晚三疊世-中侏羅世中-酸性斑巖體，如若措巖體(郎興海等, 2017; Wangetal., 2019)、湯白巖體(白云等, 2019)、卡如巖體、塔瑪巖體、臥布巖體、宗噶巖體(鄒銀橋等, 2017; Zouetal., 2017)，同時(shí)這些巖體也顯示出高εHf(t)值(>10; 鄒銀橋等, 2017; Zouetal., 2017; Wangetal., 2019)，類似于雄村礦集區(qū)的含礦斑巖。這一現(xiàn)象表明在拉薩地體南緣具有虧損Nd-Hf同位素組成(εHf(t)>10，εNd(t)>4.5)的晚三疊世-中侏羅世巖體有利于形成斑巖型銅礦床(Houetal., 2015a)。斑巖型礦床通常形成于近地表1～5km(Cookeetal., 2005; Sillitoe, 2010)，地殼的抬升和剝蝕會(huì)部分或全部破壞形成時(shí)代較老的斑巖型銅礦，使之難于保存下來。在雄村礦集區(qū)除侏羅紀(jì)含礦斑巖體外，還存在一套同時(shí)期的火山-沉積巖(圖1b)，該礦集區(qū)含礦斑巖體侵入同時(shí)期的火山-沉積巖中，其中一個(gè)重要的原因可能是該套火山沉積巖起到了一個(gè)良好的蓋層作用，有效保護(hù)了雄村斑巖銅金礦床被剝蝕。近年來在拉薩地體南緣發(fā)現(xiàn)的剝露的礦化斑巖體，其經(jīng)濟(jì)意義不大的可能原因就是它們已經(jīng)遭受了強(qiáng)烈的剝蝕作用。但是在拉薩地體的南緣除有侏羅紀(jì)侵入體報(bào)道外，還報(bào)道了一套早-中侏羅世火山巖，即桑日群火山巖(Kangetal., 2014; 黃豐等, 2015)。該套火山巖顯示出較高的εNd(t)(>4)值(圖8a)，表明其起源虧損地幔部分熔融(Kangetal., 2014)，同時(shí)也被認(rèn)為形成于大洋島弧環(huán)境(Kangetal., 2014; 黃豐等, 2015)，鄒銀橋等(2017)在桑日群比馬組火山巖中已發(fā)現(xiàn)多處銅礦化和石英-綠簾石脈體。上述信息表明桑日群火山巖及其中同時(shí)代的侵入體也具有較大的斑巖型礦化潛力，同時(shí)該套火山巖的存在有效的保護(hù)了下伏巖體免受剝蝕，因此在拉薩地體南緣尋找與新特提斯洋俯沖相關(guān)的斑巖型礦床的重點(diǎn)區(qū)域應(yīng)該是侏羅紀(jì)巖體被同期火山巖覆蓋的區(qū)域。另外，值得注意的是筆者在雄村礦集區(qū)西北部的洞嘎普-則莫多拉一帶識(shí)別出了保存較完整的侏羅紀(jì)火山機(jī)構(gòu)(洞嘎普火山機(jī)構(gòu))，圍繞火山機(jī)構(gòu)存在多處Cu-Au-Ag-Pb-Zn巖石-土壤地球化學(xué)異常(郎興海等, 2012)，顯示出明顯的火山機(jī)構(gòu)控礦特征，目前雄村礦集區(qū)發(fā)現(xiàn)的1、2、3號(hào)礦體就位于洞嘎普火山機(jī)構(gòu)旁側(cè)，因此在區(qū)域找礦過程中也應(yīng)重視侏羅紀(jì)古火山口的識(shí)別及其與成礦關(guān)系的研究。

6 結(jié)論

(1)雄村礦集區(qū)存在兩期礦化作用，早期礦化事件發(fā)生在約172Ma，與早侏羅世(181～175Ma)石英閃長(zhǎng)斑巖相關(guān)，形成了礦集區(qū)2、3號(hào)礦體；晚期成礦作用發(fā)生在161.5Ma，成礦巖體為中侏羅世(167～161Ma)石英閃長(zhǎng)斑巖，形成了礦集區(qū)1號(hào)礦體。

(2)雄村礦集區(qū)形成于新特提斯洋殼北向俯沖的大洋島弧環(huán)境而非陸緣弧環(huán)境。含礦斑巖起源于虧損地幔的部分熔融，且源區(qū)同時(shí)受到了俯沖洋殼釋放的流體和俯沖沉積物熔體的交代。

(3)拉薩地體南緣具有虧損Nd-Hf同位素組成(εHf(t)>10，εNd(t)>4.5)的侏羅紀(jì)斑巖體有利于形成斑巖型礦化，尋找與新特提斯洋俯沖相關(guān)的斑巖型礦床的重點(diǎn)區(qū)域應(yīng)該是侏羅紀(jì)巖體被同期火山巖覆蓋的區(qū)域。