岩石学报  2014, Vol. 30 Issue (4): 979-994   PDF    
安徽沙溪斑岩型铜金矿床成岩序列及成岩成矿年代学研究
王世伟1, 周涛发1 , 袁峰1, 范裕1, 俞沧海2, 葛岭虹1, 石诚2, 池月余3    
1. 合肥工业大学资源与环境工程学院, 合肥 230009;
2. 铜陵有色金属集团控股有限公司, 铜陵 244000;
3. 安徽省地质矿产勘查局327地质队, 合肥 230001
摘要:沙溪矿床是长江中下游成矿带中典型的斑岩型铜金矿床,位于庐枞盆地北外缘、郯庐断裂内,矿床成岩成矿时代确定对该矿床成因研究及区域成矿规律的认识具有重要意义。在详细野外地质工作的基础上,采集沙溪矿床与成矿有关的主要岩浆岩样品(粗斑闪长玢岩、黑云母石英闪长玢岩、中斑石英闪长玢岩、细斑石英闪长玢岩和闪长玢岩)和与黄铜矿密切共生的辉钼矿,分别利用Cameca、LA-ICP-MS U-Pb和Re-Os同位素定年方法,获得矿床内主要岩浆岩的成岩年龄(130.60±0.97Ma、129.30±1.00Ma、127.10±1.50Ma、129.46±0.97Ma和126.7±2.1Ma)以及成矿年龄(130.0±1.0Ma),并重新厘定了沙溪岩体从早到晚岩浆的侵位序列。通过区域对比,提出长江中下游存在两阶段斑岩型铜金矿化,沙溪矿床为长江中下游成矿带第二阶段形成的斑岩型矿床,沙溪矿床的成岩成矿作用既不同于庐枞盆地,也不同于断隆区第一阶段的斑岩矿床,而是受郯庐断裂和长江断裂动力学演化联合作用的产物。
关键词成岩成矿年代     沙溪矿床     斑岩型铜金矿床     两阶段斑岩矿化    
Emplacement sequences and geochronology of the Shaxi porphyry copper-gold deposit, Anhui Province, East China
WANG ShiWei1, ZHOU TaoFa1 , YUAN Feng1, FAN Yu1, YU CangHai2, GE LingHong1, SHI Cheng2, CHI YueYu3    
1. School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China;
2. Tongling Nonferrous Metals Group Holding co., Ltd., Tongling 244000, China;
3. No.327 Geological Team, Bureau of Geology and Mineral Resources of Anhui Province, Hefei 230001, China
Abstract: Shaxi deposit is a typical porphyry Cu-Au deposit, which locates in the Tancheng-Lujiang fault and the northwest margin of the Luzong volcanic basin in the Middle and Lower Yangtze River metallogenic belt. The Shaxi porphyry deposit contains >1Mt Cu and 40t Au, which is dominated by vein-style mineralisation with chalcopyrite and gold hosted in a quartz stockwork vein that is intimately associated with potassic alteration in the core of the deposit. To determine the age of mineralization and the magmatic rocks of Shaxi deposit is very important for the study of deposit genesis and regional metallogenic regularities. On the basis of geological and petrographic studies, we collect the primary types of magmatic rocks (coarse grained diorite porphyry, biotite quartz diorite porphyry, medium quartz diorite porphyry, fine quartz diorite porphyry and diorite porphyry) and molybdenite with paragenetic chalcopyrite and get the age of the magmatic rocks (they are 130.60±0.97Ma, 129.30±1.00Ma, 127.10±1.50Ma, 129.46±0.97Ma and 126.7±2.1Ma, respectively) by zircon Cameca, LA-ICP-MS U-Pb dating and mineralization (130.0±1.0Ma) by Re-Os dating of Shaxi deposit, and reclassify the emplacement sequences of magmatic rock. Our work also indicates that Shaix deposit is the second stage porphyry mineralization, which is different from the deposit and magmatic rocks of Luzong basin and the first stage porphyry deposits, imply that Shaxi deposit is the product of the geotectonical evolution of bith the Tancheng-Lujiang fault and the Changjiang fault.
Key words: Geochronology     Shaxi deposit     Porphyry Cu-Au deposit     Two stages porphyry mineralization    

1 引言

长江中下游成矿带是我国东部重要的金属成矿带之一,长期的构造作用、岩浆作用和成矿作用形成了断隆区和断凹区的构造格局(图 1a)。成矿带内主要发育矽卡岩型、矽卡岩-斑岩复合型、玢岩型和热液脉型铜铁金多金属矿床(常印佛等,1991唐永成等,1998吴言昌等,1999邢凤鸣和徐祥,1996任启江等,1991王强等,2001储国正,2003陈江峰等,2001周涛发等, 200520082011)。断隆区主要发育与高钾钙碱性岩石有关的斑岩-矽卡岩型(铜金)矿床,成岩成矿作用主要集中于146~135Ma 之间(毛景文等,2004Zhou et al., 2007王彦斌等,2004张达等,2006张乐骏等,2008蒋少涌等,2010王世伟等,2012徐晓春等,2012郭维民等,2013),断凹区主要发育与橄榄安粗岩系(钾玄岩)有关的玢岩型铁矿床,成岩成矿年龄主要集中于135~126Ma(楼亚儿和杜杨松,2006范裕等,2008周涛发等, 200820102011)。

安徽沙溪矿床为典型的斑岩型铜金矿床,位于庐枞盆地北部外缘郯庐断裂带南段、长江中下游成矿断裂带内,目前勘探工作表明,矿床的储量已达大型规模,资源量超过1Mt Cu、40t Au,其深部和周边还具有很大的找矿潜力。前人对于该矿床的地质特征、岩石成因及构造背景进行了大量的工作(任启江等,1991Sun et al., 2010Mao et al., 2011吕庆田等,2004Wang et al., 2006Yuan et al., 2011史大年等,2012袁峰等,2012;Lü et al., 2013),也开展了成岩成矿时代的研究(傅斌等,1997徐文艺等,1999徐兆文等,2000杨晓勇,2006Wang et al., 2006),但是由于定年样品(如黑云母是岩浆的还是蚀变的矿物等)和方法(全岩Rb-Sr等)选择等的限制,得到的年龄跨度较大(86~143Ma),成岩成矿的准确年龄和时代一直存在争议,成为了制约该矿床成矿作用深入研究的关键科学问题。本文通过详细的野外钻孔编录,重新划分了与成矿有关的沙溪岩体岩浆岩的类型,并系统采集岩浆岩和矿石样品,进行了锆石Cameca、LA-ICP-MS U-Pb定年和辉钼矿Re-Os定年,重新厘定了沙溪矿床主要岩浆岩的侵入次序,深化了对矿床形成的认识。本次研究还发现,沙溪矿床的成岩成矿年龄与早先发现的区域斑岩型矿床不同,可能揭示长江中下游成矿带存在两期斑岩型矿床成矿作用,这对于在成矿带内下一步的找矿工作具有重要的指导意义。

2 矿区地质特征

沙溪铜金矿床位于安徽省庐江县境内,构造上位于长江中下游成矿带庐枞火山岩盆地外围、郯庐断裂带内(图 1a),地处扬子板块的北缘,大别造山带东侧,是郯庐断裂带、黄破断裂带、滁河断裂的复合部位。

2.1 地层

沙溪地区的地层属于扬子地层区中的下扬子地层分区(常印佛等,1991任启江等,1991)。矿区内出露地层主要有志留系(高家边组、坟头组)和侏罗系(磨山组、罗岭组)砂岩、白垩系的杨湾组及第四系等,志留系(高家边组和坟头组)、侏罗系(磨山组、罗岭组)砂岩主要出露于矿区中部,侏罗系(罗岭组、龙门院组)和白垩系(杨湾组)地层零星分布在矿区西南和东南边部的地势低平处。矿区内的早白垩世陆相火山岩主要分布于矿区的西北部,该火山岩系不整合覆盖于侏罗系的地层之上(图 1b),志留系高家边组砂岩为含矿围岩。

2.2 构造

矿区内的构造以褶皱和断裂为主,可分为印支期和燕山期两期。以早侏罗世地层不整合界面为界,将前侏罗纪地层(以志留系为主)组成的褶皱构造划为印支期,侏罗纪以后包括白垩纪火山岩系在内地层所组成的构造划为燕山期。印支期区内主要发育以志留系高家边组、坟头组组成的一系列北北东向、南西倾伏的复式皱褶构造,主要有棋盘山向斜和铜泉山背斜(图 1b),是区域菖蒲山-盛桥复式背斜的组成部分。铜泉山背斜是区内的主要皱褶,位于铜泉山一带,轴面走向北北东向,近轴部岩层陡立,轴面往南东倾斜,北西翼地层较陡,一般倾角60°以上,部分直立或倒转;南东翼稍缓,多在50°以上,近轴部岩层陡立,显示强烈挤压特征,并多被岩体侵位。该背斜因受侏罗纪地层超覆沉积、断裂破坏及岩体侵入影响,形态已不完整。棋盘山向斜位于棋盘山-虎皮山附近,核部为志留系坟头组,两翼为高家边组,除东侧外,三面为断层所切割,轴向北东30°左右,北端上翘,向南西倾伏。区内断裂可归并为四组(图 1b),即近东西向、北东向、北北东向和北西向,其中北北东向断裂最为发育,北东向断裂次之。

图 1 研究区地质略图 (a)-长江中下游成矿带矿床分布图(据Pan and Dong, 1999; Mao et al., 2011修改);(b)-沙溪斑岩铜金矿床矿区地质图.TLF-郯城-庐江断裂;XGF-襄樊-广济断裂;YCF-阳新-常州断裂

Fig. 1 Sketch geological map of the study area (a)-map showing the distribution of deposits along the Middle-Lower Yangtze River Valley metallogenic belt(modified after Pan and Dong, 1999; Mao et al., 2011);(b)-geologic map of Shaxi porphyry copper-gold deposit. TLF: Tancheng-Lujiang fault; XGF: Xiangfan-Guangji fault; YCF: Yangxing-Changzhou fault

2.3 岩浆岩

矿区岩浆岩非常发育,侵入岩岩石类型较多,深部钻孔揭露矿床主要发育有粗斑闪长玢岩、中斑石英闪长玢岩、细斑石英闪长玢岩、黑云母石英闪长玢岩,其次还有脉岩,如闪长玢岩和煌绿岩等,部分岩浆岩破碎形成角砾岩,局部发育角砾状闪长玢岩。火山岩主要在矿区西北侧福泉山一带和东南部出露,主要为安山玢岩、熔岩、凝灰角砾岩等。岩体在地表由数十个大小不一、形态不规则的出露体,呈北北东向,向北撒开,向南收敛的特点分布(图 1b)。

2.4 矿化蚀变

沙溪斑岩型铜金矿床分为四个矿段,自南而北依次为龙头山矿段、断龙颈矿段、铜泉山矿段及凤台山矿段,其中凤台山矿段和铜泉山矿段为主要的矿化富集地段(图 1b),矿床以浸染状和脉状矿化为特征,脉体具有多样性、多期次的特征,脉体从早到晚可分为钾硅酸盐阶段、石英硫化物阶段和石英碳酸盐阶段。矿床发育典型的斑岩型围岩蚀变,如钾硅酸盐化、青磐岩化、石英绢云母化和高岭土化,不同类型的蚀变在空间上具有明显的分带现象,钾硅酸盐阶段和石英硫化物阶段的脉体矿化对沙溪斑岩型铜矿床贡献最大(袁峰等,2012),矿体主要分布于岩体深部的钾硅酸盐化、长石分解蚀变叠加钾硅酸盐化的区域(图 2)。

图 2 沙溪斑岩铜金矿床地质剖面图 A-B:凤台山6线地质剖面图;C-D:凤台山14线地质剖面图

Fig. 2 Geologic map of section A-B and C-D in Shaxi porphyry copper-gold deposit A-B: geologic map of section exploration line 6 of Fengtaishan;C-D: geologic map of section exploration line 14 of Fengtaishan

3 岩浆岩类型

沙溪矿床的矿化主要赋存于侵入岩体-沙溪岩体中,通过详细野外观察、钻孔编录和室内研究,作者认为沙溪岩体主要有粗斑闪长玢岩、中斑石英闪长玢岩、细斑石英闪长玢岩、黑云母石英闪长玢岩和晚期脉岩(如闪长玢岩和煌绿岩等)等类型岩石组成,下面为岩浆岩的详细描述和相互关系。

3.1 粗斑闪长玢岩(图 3a,b)

主要多分布于凤台山一带(图 1b),多呈岩墙状产出,岩石呈灰红或绿灰色,斑状结构,块状构造,以斑晶粗大为特征,且大小不均匀。斑晶总量约40%~45%,由斜长石、碱性长石和角闪石等组成。斜长石:主要为中长石,自形-半自形长柱状,约占25%~35%,大小为2.00~6.00mm,具聚片双晶,环带结构,局部长石略显定向排列,少量被石英交代;碱性长石:主要为正长石,自形-半自形短柱状,约占5%~8%,大小为2.00~4.00mm,发育简单双晶;角闪石呈:自形-半自形长柱状,含量10%~15%,大小为0.19~2.24mm。基质由微细粒长石和少量石英(部分次生石英呈细脉状产出)、角闪石等组成,含量55%~60%。副矿物为磁铁矿、磷灰石等。

图 3 沙溪岩体主要岩浆岩特征及相互关系 (a)-粗斑闪长玢岩手标本,发育浸染状黄铁矿化;(b)-粗斑闪长玢岩显微照片(正交偏光),可见长石斑晶粗大;(c)-中斑石英闪长玢岩手标本,发育碳酸盐脉、碳酸盐-绿泥石细脉;(d)-中斑石英闪长玢岩显微照片(正交偏光),可见较自形角闪石沿解理缝发育硅化,斜长石发育弱绢云母化;(e)-细斑石英闪长玢岩与粗斑闪长玢岩的接触界线;(f)-细斑石英闪长玢岩与粗斑闪长玢岩的接触界线的显微照片(反射光),可见粗斑闪长玢岩中的斑晶被截断,细斑石英闪长玢岩中发育弱黄铁矿化和弱黄铜矿化,粗斑闪长玢岩主要发育黄铁矿化,颗粒较大;(g)-细斑石英闪长玢岩与粗斑闪长玢岩的接触界线的显微照片(正交射光),粗斑闪长玢岩中的长石斑晶在图(e)中为红色,但镜下可见已完全绢云母化,可能为早期斑晶发育钾长石化,被晚期长石分解蚀变叠加,并保留长石晶形,细斑石英闪长玢岩主要发育粘土化,长石都完全被绢云母和粘土矿物交代,保留长石的晶形;(h)-黑云母石英闪长玢岩手标本,岩石中发育石英-辉钼矿脉,被晚期碳酸盐细脉穿切,还发育稀疏浸染状黄铁矿化;(i)-黑云母石英闪长玢岩显微照片(正交偏光),岩浆黑云母颗粒较大,呈片状,自形-半自形,而热液黑云母颗粒细小,他形,呈不同方向堆积到一起;(j)-闪长玢岩手标本;(k)-闪长玢岩穿切石英闪长玢岩中的Qtz-Py-Cp和Py-Cp脉,明显晚于成矿,Cal脉穿切闪长玢岩和石英闪长玢岩,为最晚期的产物.Bt-黑云母;Cal-碳酸盐;Cp-黄铜矿;Hbl-角闪石;Kf-钾长石;Mo-辉钼矿;Pl-斜长石;Py-黄铁矿;Qtz-石英;Ser-绢云母;Clay minerals(Pl)-斜长石完全蚀变为粘土矿物,但仍保留斜长石的晶形;Ser(Pl)-斜长石完全蚀变为绢云母,但仍保留斜长石的晶形

Fig. 3 Characteristics and relationship of magmatic rocks of the Shaxi intrusion in the Shaxi deposit (a)-photograph of coarse grained diorite porphyry with disseminated pyrite;(b)-micograph of coarse grained diorite porphyry(cross-polarized light);(c)-photograph of medium grained quartz diorite porphyry with carbonate vein and carbonate-chlorite vein;(d)-micograph of medium grained quartz diorite porphyry,with fine grained quartzs replacing hornblend along cleavage and sericites replaceing plagioclases(cross-polarized light);(e)-connect zone of fine grained quartz diorite porphyry and coarse grained diorite porphyry;(f)-micograph of connect zone of fine grained quartz diorite porphyry and coarse grained diorite porphyry(reflected light),which show the phenocrysts of coarse grained diorite porphyry with disseminated pyrites are cut by fine grained quartz diorite porphyry with disseminated chalcopyrites and pyrites;(g)-micograph of connect zone of fine grained quartz diorite porphyry and coarse grained diorite porphyry(cross-polarized light),which show the plagioclases of coarse grained diorite porphyry are replaced by sericites and of fine grained quartz diorite porphyry are replaced by clay minerals and sericites. Combined with red colour of plagioclases of coarse grained diorite porphyry(Fig. 3e),could imply that coarse grained diorite porphyry developed potassic alteration and were superposed by later feldspar-destructive alteration;(h)-photograph of biotite quartz diorite porphyry with disseminated pyrites and quartz-molybdenite vein cut by carbonate veinlet;(i)-micograph of biotite quartz diorite porphyry,in which there are euhedral- subhedral leaf-shaped magmatic biotites and accumulated fine grained anhedral hydrothermal biotites;(j)-photograph of diorite porphyry;(k)-diorite porphyry cut the Qtz-Py-Cp vein and Py-Cp vein of quartz diorite porphyry,both of which are cut through by Cal vein. Bt-biotite; Cal-carbonate; Cp-chalcopyrite; Hbl-hornblend; Kf-K-feldspar; Mo-molybdenite; Pl-plagioclase; Py-pyrite; Qtz-quartz; Ser-sericite; Clay minerals(Pl)-plagioclase have totally been replaced by clay minerals and still keep the sharp of plagioclase; Ser(Pl)-plagioclase have totally been replaced by sericites and still keep the sharp of plagioclase

3.2 中斑石英闪长玢岩

为矿区内主要的含矿岩体,呈南北条带状分布(图 1b)。新鲜岩石呈灰绿色,局部蚀变成浅红灰或灰白色,斑状结构,块状构造。斑晶总含量约占60%,主要为斜长石、碱性长石、角闪石、石英,斜长石主要为更长石,自形-半自形长柱状,约占25%~35%,大小为1.00~4.00mm。碱性长石主要为歪长石,自形-半自形柱状,约占5%~10%,大小为1.00~3.00mm,个别发育格子双晶。角闪石呈自形-半自形长柱状,含量10%~15%,大小为0.20~2.20mm,发育角闪石式解理,常沿解理发育硅化(图 3d)、绿泥石化等蚀变。石英呈半自形,常被溶蚀为椭圆形,含量约5%。副矿物为磷灰石、锆石、金红石等。在中斑石英闪长玢岩与地层和粗石英闪长玢岩接触的地段,长石斑晶变小(大小为0.3~2.0mm),岩性转变为细斑石英闪长玢岩(图 3e),后者出露面积极少,矿化很弱。

3.3 黑云母石英闪长玢岩

当中斑石英闪长玢岩中的黑云母含量>5%时,为黑云母石英闪长玢岩(图 3h,i),是矿区内次要含矿岩体,主要分布在矿床中心外围地段(图 1b),地表岩石棕灰色,新鲜呈深灰色、绿灰色,常蚀变为浅灰、红灰、绿灰色。中-细粒斑状结构,块状构造,斑晶总量约30%~40%,局部达到50%~60%,主要为斜长石、碱性长石、角闪石、石英、黑云母,粒度不均,一般2mm左右,斜长石主要为更长石,自形-半自形板条状,约占20%~30%,大小为0.4~4.00mm,细密环带结构,有碎斑和聚斑现象,常被绢云母、碳酸盐交代;碱性长石主要为歪长石,自形-半自形柱状,约占5%~12%,大小为1.00~2.50mm。角闪石呈自形-半自形长柱状,含量约10%,大小为0.30~2.00mm,发育角闪石式解理,部分蚀变为绿泥石和碳酸盐。石英斑晶呈半自形球粒状及不规则等轴体,含量约6%,一般2mm以下,常被熔蚀成港湾状。黑云母半自形鳞片状,含量一般不少于5%,粒径1~2mm,在岩体上部常蚀变呈白色,为白云母、碳酸盐交代,但晶形大体保留。基质部分变化较大,多数呈微粒状结构,有时呈似花岗结构或微晶板条状结构。副矿物为磷灰石、磁铁矿、锆石等。

3.4 闪长玢岩

为矿区内的主要脉岩分布于矿区西北部,福泉山东侧(图 1b),岩石灰色-浅灰白色(图 3j),斑状结构,块状构造。斑晶为更-中长石,呈半自形柱状,粒径0.2~1.0mm。基质主要由细小的斜长石组成,其次为少量的石英,细粒结构。斜长石呈细小柱状,无定向排列,粒径0.1~0.3mm。石英呈他形不规则状集合体,具波状消光,局部分布于斜长石之间。副矿物有榍石等。

4 分析方法和测试结果
4.1 锆石Cameca U-Pb测年
4.1.1 样品采样

在地质观察、岩芯编录及岩相学工作基础上,本次工作采集了组成沙溪岩体主要的、较新鲜的岩浆岩样品:粗斑闪长玢岩(1406-340:采于1406钻孔的340m处,钻孔坐标X=3450119.23、Y=39529477.13)、中斑石英闪长玢岩(611-554:采于611钻孔554m处,钻孔坐标X=3448747.00、Y=39528764.50)、细斑石英闪长玢岩(609-628:采于609钻孔628m处,钻孔坐标X=3449772.04、Y=39529262.50)、黑云母石英闪长玢岩(610-961:采于610钻孔961m处,钻孔坐标X=3449831.56、Y=39528973.46)和闪长玢岩(FQS06:采于31°10.0210N,117°15.2040E),粗斑闪长玢岩、中斑石英闪长玢岩、细斑石英闪长玢岩和黑云母石英闪长玢岩(采样位置见图 2)。采用Cameca IMS-1280离子探针锆石U-Pb精确定年测定岩体的形成年龄。晚期脉岩闪长玢岩采用LA-ICP-MS锆石U-Pb定年测定方法。

4.1.2 测试方法

在岩石学研究基础上,将样品送至河北省廊坊区调研究所实验室进行破碎经重液分离和磁选对锆石单矿物分选,锆石靶的制定和CL图像的拍摄分别在中国科学院地质与地球物理研究所和北京锆年领航科技有限公司进行。

Cameca锆石U-Pb的测定在中国科学院地质与地球物理研究所Cameca IMS-1280二次离子质谱仪(SIMS)上进行,详细分析方法见Li et al.(2009)。锆石标样与锆石样品以13比例交替测定。U-Th-Pb同位素比值用标准锆石Plésovice(337Ma,Sláma et al., 2008(或TEMORA(417Ma,Black et al., 2004)校正获得,U含量采用标准锆石91500(81×10-6Wiedenbeck et al., 1995)校正获得,以长期监测标准样品获得的标准偏差(1SD=1.5%,Li et al., 2010)和单点测试内部精度共同传递得到样品单点误差,以标准样品Qinghu(159.5Ma,Li et al., 2009)作为未知样监测数据的精确度。普通Pb校正采用实测204Pb值。由于测得的普通Pb含量非常低,假定普通Pb主要来源于制样过程中带入的表面Pb污染,以现代地壳的平均Pb同位素组成(Stacey and Kramers, 1975)作为普通Pb组成进行校正。同位素比值及年龄误差均为1σ。数据结果处理采用ISOPLOT软件(Ludwig,2001)。

LA-ICPMS锆石U-Pb定年分析在合肥工业大学资源与环境工程学院开展,由ICP-MS和激光剥蚀系统联机完成。ICP-MS为美国Agilent公司生产的Agilent 7500a,该仪器独有的屏蔽炬(ShieldTorch)可明显提高分析灵敏度。激光剥蚀系统为美国Coherent Inc.公司生产的GeoLasPro,该系统为工作波长193nm的ComPex102 ArF准分子激光器,样品上的光斑大小为4~160μm,能量密度范围1~45J/cm2,单脉冲能量可达200mJ,最高重复频率20Hz。

表 1 沙溪岩体主要岩浆岩锆石U-Pb分析结果 Table 1 U-Pb dating for the zircons from the Shaxi intrusion

用NIST SRM 610进行载气和补偿气比例的最优化,并使208Pb达到最大的信号强度而保持较低的ThO/Th(0.1%~0.3%)和Ca2+/Ca+(0.4%~0.7%),用NIST SRM 610的238U和232Th离子信号强度的比值(238U/232Th≈1)指示样品完全气化。在分析过程中,激光剥蚀的斑束直径选为32μm,频率为6Hz,采样方式为单点剥蚀,以He作为剥蚀物质的载气,由于采用高纯度的液Ar和He气(99.999%),204Pb和202Hg的背景<100cps。ICP-MS数据采集选用一个质量峰采集一个点的跳峰方式,单点停留时间分别设定为6ms(Si,Ti,Nb,Ta和REE),15ms(204Pb,206Pb,207Pb和208Pb)和10ms(232Th和238U)。每测定5个样品点测定一次标准锆石91500(为减少偶然因素的影响,一般连续测定两次91500),每测10个样品点测一次NIST610和年龄监控样Mud Tank。每个实验室ELAN DRC-e ICP-MS上测定,具体实验流程见有关文章(Qi et al., 20102013)。获得沙溪矿床辉钼矿Re-Os的模式年龄为130.0±1.0Ma(表 2)。同批测试的实验标准物质GBW04436(JDC)的模式年龄和推荐值分别为223.0±2.0Ma和221.4±5.6Ma。

表 2 沙溪矿床辉钼矿Re-Os同位素年龄 Table 2 Re-Os isotopic age of molybdenite from the Shaxi deposit
5 讨论
5.1 成岩年代及岩石侵位序列
5.1.1 成岩时代

前人对沙溪岩体已进行了很多的同位素测年工作,傅斌等(1997)采用40Ar-39Ar对石英闪长玢岩中黑云母进行测定,获得岩石的形成年龄为126.8±1.0Ma。杨晓勇(2006)采用40Ar-39Ar分别对含铜石英闪长玢岩中的黑云母和斜长石进行了测定,获得40Ar-39Ar平均坪年龄为132.62±0.28Ma,等时线平均年龄为132.59±0.46Ma,代表成矿岩体的侵入时代。徐文艺等(1999)徐兆文等(2000)采用Rb-Sr法分别对岩石中的斜长石、岩石全岩进行了测试,获得的年龄值为143.3±5.17Ma和127.9±1.6。Wang et al.(2006)采用SHRIMP法对于石英闪长玢岩中的锆石进行了测试,获得年龄值为136±3Ma。

图 4 沙溪岩体主要类型岩石锆石U-Pb谐和一致曲线图

Fig. 4 U-Pb zircon concordia plots of magmatic rocks in the Shaxi intrusion

通常情况下,40Ar/39Ar年龄代表着所测矿物的冷却年龄(Mcdougall and Harrison, 1988),由于黑云母中氩同位素体系的封闭温度为300~350℃(Mcdougall and Harrison, 1988),所获得黑云母的Ar-Ar年龄为岩体冷却到300~350℃以下的年龄,同时,沙溪矿床普遍存在长石和黑云母化蚀变等,在挑取岩石定年单矿物样品时,无法避免蚀变矿物的混入,从而导致定年结果不可信,因此,上述的Ar-Ar年龄皆不能代表岩体的结晶年龄。同样,由于岩体蚀变作用的存在,上述研究者的Rb-Sr年龄也可能受到了热液蚀变的影响而无法获得真正的岩体结晶年龄。而Wang et al.(2006)的SHRIMP定年仅开展了岩体一个样品的年代测定,结果也明显不同于他人。由此可见,前人不同方法已获得的沙溪岩体同位素年龄的跨度太大,无法确定沙溪岩体形成的精确年龄,也不能厘定不同岩浆岩的侵位序列。因此,有必要重新深入系统地开展沙溪岩体成岩时代的研究。

本次研究中采集的组成沙溪岩体最主要的四种主要岩浆岩:粗斑闪长玢岩、中斑石英闪长玢岩、细斑石英闪长玢岩和黑云母石英闪长玢岩,其中锆石除少部分具有浑圆的外形外,绝大多数结晶较好,呈典型的长柱状晶形,具有典型的岩浆震荡环带,指示其主体为岩浆结晶的产物。由锆石的阴极发光图像(图 4)可以看出,所有锆石均具有清晰的内部结构。其中,中斑石英闪长玢岩中大部分锆石显示具有核幔结构,但大多数锆石具有典型的单期生长的同心环带特征。沙溪矿床岩浆岩锆石中Th/U比值基本都大于0.1,属典型的岩浆成因锆石,并且岩石的谐和年龄与坪年龄在误差范围内一致,能代表岩浆岩的形成年龄。因此,通过锆石定年,获得沙溪矿床中粗斑闪长玢岩、中斑石英闪长玢岩、细斑石英闪长玢岩、黑云母石英闪长玢岩和闪长玢岩的成岩年龄分别为130.60±0.97Ma、127.10±1.50Ma、129.46±0.97Ma、129.30±1.00Ma和126.7±2.1Ma,均为燕山期早白垩世岩浆活动的产物。

表 3 沙溪岩体岩浆岩侵入次序表 Table 3 Sequence of magmatic rock in the Shaxi deposit
5.1.2 侵位序列

前人对沙溪矿床的岩浆岩类型和岩浆的先后侵入次序进行了初步的研究(任启江等,1991;安徽省地质矿产局327地质队和南京大学地球科学系,1995),但是所描述的岩浆岩的类型种类繁多,有些先后顺序的划分,如粗斑闪长玢岩的次序排列等,没有明确的地质证据。
①安徽省地质矿产局327地质队,南京大学地球科学系.1995. 庐江地区铜铁勘查研究. "八五"国家科技攻关计划专题成果报告,1-247

由上述岩石类型研究可知,沙溪岩体主要岩石类型有粗斑闪长玢岩、中斑石英闪长玢岩、细斑石英闪长玢岩、黑云母石英闪长玢岩,其次脉岩(如闪长玢岩、煌绿岩)等。根据我们的观察和编录,细斑石英闪长玢岩和中斑石英闪长玢岩为不同的岩相,在岩心中可见存在过渡的关系,细斑石英闪长玢岩没有露头,钻孔揭露也仅有少量产出,分布于中斑石英闪长玢岩与地层和粗石英闪长玢岩接触的地段(图 3e),因此,细斑石英闪长玢岩和中斑石英闪长玢岩可合并为石英闪长玢岩。少量出露的黑云母石英闪长玢岩和石英闪长玢岩没有明显的界线,当石英闪长玢岩中的黑云母含量>5%(图 3h,i;安徽省地质矿产局327地质队和南京大学地球科学系,1995),岩石则转变为黑云母石英闪长玢岩,但局部可见黑云母石英闪长玢岩中发育的脉体被石英闪长玢岩截断,表明黑云母石英闪长玢岩略早于石英闪长玢岩。石英闪长玢岩和黑云母石英闪长玢岩是主要的赋矿岩石,其中,石英闪长玢岩赋存了沙溪矿床绝大部分矿体,少量矿体赋存于黑云母石英闪长玢岩和地层中。晚期的脉岩(闪长玢岩、煌绿岩)等穿插或包裹石英闪长玢岩,明显晚于成矿岩体(图 3k)。而对于同样无矿的粗斑闪长玢岩是形成于成矿前还是成矿后,有不同的认识(任启江等,1991;安徽省地质矿产局327地质队和南京大学地球科学系,1995)。作者通过野外编录认为粗斑闪长玢岩应于成矿前形成,主要基于以下几点:①粗斑闪长玢岩的斑晶被细斑石英闪长玢岩破坏(图 3f,g);②粗斑闪长玢岩发育早期蚀变(磁铁矿化、钾长石化);③粗斑闪长玢岩被石英闪长玢岩包裹(图 2)。同时,上述年代学显示粗斑闪长玢岩略早。通过上述研究可得沙溪矿床主要岩浆岩的侵位序列如表 3,这基本与上述年代学结果一致。

图 5 沙溪矿床辉钼矿Re-Os定年用样品Qtz-Anh-Mo-Cp-Py脉手标本(a)和显微照片(b) Qtz-石英;Anh-硬石膏;Mo-辉钼矿,Py-黄铁矿;Cp-黄铜矿

Fig. 5 Sample(a) and micrograph(b)of Qtz-Anh-Mo-Cp-Py vein in ore for Re-Os dating from Shaxi deposit Qtz-quartz; Anh-anhydrite; Mo-molybdenite; Py-pyrite; Cp-chalcopyrite

5.2 成矿年代

前人对沙溪矿床的成矿时代也进行了一些研究工作。徐文艺等(1999)测定了沙溪斑岩矿床的成矿时代,采用Ar/Ar快中子活化定年法对钾化蚀变岩进行了测定。分析样S2-170采自沙溪矿区钻孔ZK802深558m处,该样品为遭受强烈钾质蚀变的石英闪长玢岩全岩样,获得全岩样40Ar/39Ar年龄谱的坪年龄为123.6±0.7Ma。由于沙溪矿床中钾硅酸盐阶段并不是主成矿阶段(袁峰等,2012),并且钾硅酸盐化样品常受到晚期长石分解蚀变的叠加,并且这一由全岩样品获得的40Ar/39Ar年龄显然不能代表成矿年龄。杨晓勇(2006)采用40Ar-39Ar分别对含铜石英闪长玢岩中的黑云母和斜长石进行了测定,获得最小平均视年龄86Ma,认为可能表示后期一次热事件,对应于斑岩体与Cu-Au矿化有关的热液活动。而斑岩矿床形成时多次侵入活动导致的重复循环的热液过程持续最长时间约2Myr(Chiaradia et al., 2013),可见这一成矿年龄明显估算过低。

由本次辉钼矿Re-Os同位素定年测试结果可知,与黄铜矿共生的辉钼矿的Re-Os模式年龄为130.0±1.0Ma,同批测试的实验标准物质GBW04436(JDC)的模式年龄(223.0±2.0Ma)与推荐年龄(221.4±5.6Ma)基本一致,表明本次由辉钼矿测定的模式年龄比较精确。因此,130.0±1.0Ma可代表沙溪矿床的成矿年龄,表明沙溪矿床形成于早白垩世,辉钼矿的Re-Os年龄与赋矿岩浆岩(沙溪岩体的中细斑石英闪长玢岩和黑云母石英闪长玢岩)的Cameca U-Pb年龄在误差范围内一致,早于闪长玢岩等晚期脉岩,说明矿床是中细斑石英闪长玢岩和黑云母石英闪长玢岩有关岩浆热液作用的产物。

图 6 沙溪岩体与区域侵入岩的SiO2-K2O(a)和Y-Sr/Y(b)对比图解 数据来自蒋少涌等(2008)任启江等(1991)王强等(20012003)、徐文艺等(1997)刘珺等(2007)杨荣勇等(1993)曹毅等(2008)赵振华和涂光炽(2003)郭维民等(2013) Fig. 6 Diagram of SiO2-K2O(a) and Y-Sr/Y(b)of magmatic rock in the Shaxi deposit and other deposits Data from Jiang et al.(2008),Ren et al.(1991),Wang et al.(2001,2003),Xu et al.(1997),Liu et al.(2007),Yang et al.(1993),Cao et al.(2008),Zhao and Tu(2003),Guo et al.(2013)

5.3 区域对比

长江中下游成矿带成岩成矿时代大致分为145~137Ma、135~127Ma和126~123Ma三个阶段(周涛发等,2008)。目前,成矿带内发现斑岩型铜(钼、金)矿床主要分布于鄂东南(铜山口矿床)、九瑞(城门山矿床、封三洞矿床)、庐枞(沙溪矿床)和铜陵(冬瓜山深部、舒家店铜矿床)等矿集区(常印佛等,1991唐永成等,1998Pan and Dong, 1999谢桂青等,2006刘延年和钱应敏,2001徐文艺等,1999王立本等,1997杜建国等,2003储国正,2003),这些斑岩型矿床的成岩成矿年龄主要集中于145~136Ma(王世伟等, 20112012吴才来等,2010赖小东等,2012陆三明,2007徐晓春等,2012Yang et al., 2011郭维民等,2013谢桂青等,2006Li et al., 2008),而沙溪矿床的成岩成矿时代则主要集中于130~127Ma,说明长江中下游成矿带存在两阶段斑岩型铜金矿化,沙溪矿床为区域第二阶段的斑岩成矿作用的产物。迄今为止,长江中下游成矿带已发现的斑岩矿床多为第一阶段的斑岩型矿床,如冬瓜山铜矿、城门山铜矿、铜山口矿床、舒家店矿床、武山矿床和封三洞矿床等,第二阶段的斑岩矿床目前仅发现于沙溪,沙溪斑岩型铜金矿床已达大型规模,且通过勘探其储量有继续扩大的可能,因此推测长江中下游成矿带存在寻找第二阶段斑岩矿床的巨大潜力。

长江中下游成矿带内成岩成矿作用在时空上表现出明显的分区性和演化趋势(周涛发等,2008)。145~137Ma的岩浆活动主要发生在断隆区(如铜陵地区等和鄂东南地区),是铜金矿化的主要时期;135~127Ma的岩浆活动主要发生在断陷区(如庐枞盆地、宁芜盆地等),是铁矿化的主要时期(周涛发等,2008)。第一阶段斑岩矿床的围岩常为志留系-三叠系地层,常发育矽卡岩化和代表矿化期间流体较还原的磁黄铁矿(如冬瓜山矿床、舒家店矿床),而沙溪矿床围岩为志留系高家边组砂岩,无矽卡岩蚀变,产出大量表明热液流体较氧化的热液硬石膏,可见,两阶段斑岩矿床的成矿流体明显不同。

沙溪矿床位于庐枞盆地的北外缘、郯庐断裂与罗河断裂之间,罗河断裂作为郯庐断裂的分支(董树文等,2009),是庐枞盆地重要的岩浆上涌和喷发通道(董树文等,2010),其成岩成矿年龄与庐枞盆地和郯庐断裂带内的成岩成矿年龄(Zhou et al., 2007; 周涛发等,2008; 范裕等,2008; 袁峰等,2008; Liu et al., 2010a)一致,但沙溪地区主要为斑岩型铜、金矿化,成矿岩体的岩石为钙碱性系列,具有埃达克岩的性质,明显不同于庐枞盆地和郯庐断裂带内侵入岩的性质(Liu et al., 2010a),也不同于断隆区(武山矿床、封三洞矿床、冬瓜山矿床和舒家店矿床、铜山口矿床)斑岩矿床岩石为高钾钙碱性的性质(图 6)。因此,我们认为沙溪斑岩型铜金矿床可能为郯庐断裂和长江断裂联合作用背景下的岩浆作用及其岩浆热液演化的产物。

6 结论

(1)沙溪岩体从早到晚的岩石侵位序列为粗斑闪长玢岩、黑云母石英闪长玢岩、中细斑石英闪长玢岩、闪长玢岩等脉岩,其成岩年龄分别为130.60±0.97Ma、129.30±1.00Ma、127.10±1.50Ma、129.46±0.97Ma和126.7±2.1Ma,为燕山期早白垩时岩浆活动的产物。

(2)沙溪矿床中与黄铜矿共生的辉钼矿的Re-Os模式年龄为130.0±1.0Ma,代表沙溪矿床的成矿年龄,成矿与沙溪岩体中细斑石英闪长玢岩和黑云母石英闪长玢岩关系密切。

(3)沙溪铜金矿床为长江中下游成矿带第二阶段成岩成矿作用的产物,成矿带具有寻找第二阶段斑岩型铜金矿床的潜力。

致谢

本文的研究工作得到了常印佛院士、唐永成教授、汤加富教授、李建设教授和吕庆田老师的指导和帮助;研究工作还得到了铜陵有色金属(集团)公司科技项目的支持;Cameca锆石U-Pb定年得到了中国科学院地质与地球物理研究所李献华老师和李秋立老师的大力帮助;辉钼矿Re-Os定年得到了中国科学院地球化学研究所漆亮老师的大力帮助;在此一并表示衷心感谢!

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