张云辉,张寿庭,王世炎,,云 辉,谭和勇,张 浩
(1. 四川省地质工程勘察院,四川成都 610072;2. 中国地质大学 地球科学与资源学院,北京 100083;3. 河南省地质调查院,河南郑州 450007)
东秦岭南泥湖钼(钨)矿床和秋树湾铜(钼)矿床成岩成矿特征对比研究
张云辉1,2,张寿庭2,王世炎2,3,云 辉3,谭和勇3,张 浩2
(1. 四川省地质工程勘察院,四川成都 610072;2. 中国地质大学 地球科学与资源学院,北京 100083;3. 河南省地质调查院,河南郑州 450007)
东秦岭钼矿带内的南泥湖钼(钨)矿床和秋树湾铜(钼)矿床同为斑岩型矿床,地理位置相近,却在成矿类型和规模上差异显著。通过对两个矿床的地质特征、成矿斑岩体的地球化学特征、成矿时代及其成矿物质来源(S同位素、流体包裹体和Re含量)进行对比分析,确定南泥湖钼(钨)矿床和秋树湾铜(钼)矿床类型均为壳幔混源型,后者成岩成矿过程有更多的幔源物质参与,并且两个矿床同时形成于秦岭造山带中生代燕山期伸展减薄的机制下,但所处构造单元(华北陆块南缘和北秦岭)不同,以上这些都导致了成矿的差异性,并为日后在东秦岭钼矿带中钼铜两类矿床的找矿工作提供参考。
南泥湖钼钨矿床 秋树湾铜(钼)矿床 地球化学 构造环境 东秦岭钼矿带
Zhang Yun-hui, Zhang Shou-ting, Wang Shi-yan, Yun Hui, Tan He-yong, Zhang Hao. Comparison of petrologic and metallogenic characteristics between the Nannihu Mo(-W) deposit and Qiushuwan Cu(Mo) deposit, east Qinling[J]. Geology and Exploration, 2014, 50(4):0700-0711.
东秦岭钼矿带是我国最主要的钼金属资源产地,在全球也仅次于美国西部的Climax-Henderson斑岩钼矿带,钼金属储量达500万吨以上,带内陆续发现了金堆城、南泥湖-三道庄、上房、东沟等五个世界级超大型(>5×105t Mo)钼矿床和雷门沟、鱼池岭五个大型((5~10)×104t Mo)钼矿床以及一系列中小型(<5×104t Mo)钼矿床(李诺等,2007;李永峰等,2005)。此外,在东秦岭钼矿带还伴生有以南泥湖钼钨矿床和秋树湾铜钼矿床为代表的钨、铜、金、铅、锌、银等其它金属元素的矿化,其中南泥湖钼钨矿床中的钨达到超大规模,其储量达到中国第二位(Maoetal.,2002;郭保健等,2006;王长明等,2006)。
如此大规模的钼、钨、铜、金等多金属矿产资源的产出,吸引了大量国内外地质工作者前往探究,在其构造演化,成岩成矿规律及其地球动力学背景等问题的研究上取得了重大成果,研究结果一致表明东秦岭钼矿带主要由斑岩型矿床组成,燕山期小斑岩体是带内钼多金属矿床的成矿母岩(Steinetal.,1997;徐兆文等,2000;李永峰,2005;Maoetal.,2008a;杨宗峰等,2011a; Lietal.,2012a;Lietal.,2012b)。
南泥湖钼(钨)矿床和秋树湾铜(钼)矿床同为斑岩型矿床,位置上都处于东秦岭钼矿带的东缘(图1)。其中南泥湖钼(钨)矿床主要以产钼为主,以探明钼金属量66.46万吨,钼品位一般为0.06%~0.2%,平均为0.079%;伴生钨金属量13.54万吨,品位0.099%,为超大型矿床(李永峰等,2003);而秋树湾铜(钼)矿床主要以产铜为主,品位为0.5%~1%,全矿区平均品位达到0.718%,铜矿中钼含量一般为0.01%~0.018%,矿床规模为中型(伏雄等,2003)。
南泥湖钼(钨)矿床成矿母岩为复式岩体,花岗斑岩出露在地表,下部为黑云花岗闪长岩,前人分别测得锆石SHRIMP年龄为(157.1±2.9)Ma(MSWD=1.8)(李永峰等,2003),LA-ICP-MS锆石U-Pb年龄为(149.56±0.36)Ma(MSWD=1.5)(包志伟等,2009)和(145.2±1.5)~(146.7±1.2)Ma(向君峰等,2012a);而辉钼矿Re-Os同位素的模式年龄为(141.8±2.1)Ma(李永峰等,2003),加权平均年龄为(145.03±0.69)Ma,等时线年龄得出为(146.3±1.1)Ma(向君峰等,2012a)。秋树湾铜(钼)矿床成矿母岩为细粒黑云花岗闪长岩,任启江等(1993)通过对前人K-Ar和Rb-Sr测年数据总结,认为145Ma为与矿化有关斑岩体的成岩年龄。铜钼矿床的成矿年龄通过对矿石中辉钼矿进行Re-Os同位素测年得到模式年龄为(145.57±1.80)~(147.98±2.21)Ma,平均146.42±1.77Ma,以及一个相关性很好的等时线年龄147±4Ma(郭保健等,2006)。
由此可见南泥湖钼(钨)矿床和秋树湾铜(钼)矿床的成岩成矿年龄基本一致,均是在燕山期形成,且地理位置相近,然而二者却在成矿类型和规模上有着显著的不同,为此本文在总结前人资料的基础上,试图通过系统对比两个矿床成矿母岩和矿床的地球化学特征以及成岩成矿构造环境等方面来解释这种差异性,为日后在东秦岭钼矿带这两类矿床下一步的勘探工作提供参考。
东秦岭钼矿带处于华北陆块南缘与秦岭造山带相接处,该矿带分别以商丹断裂和三宝断裂作为南北边界,东至河南省镇平县的秋树湾矿床,西起陕西省华县的金堆城矿床,呈近东西向条带状展布,依据栾川断裂可将其划分为华北陆块南缘和北秦岭两个构造单元(图1)(张国伟等,1996;张国伟等,2001;李诺等,2007)。带内中生代以来发生了强烈的陆-陆碰撞运动和频繁的中酸性岩浆上侵活动,导致了大规模钼、钨、铜多金属矿床的形成。区内褶皱和断裂发育,断裂构造以NNW向区域深大断裂为主,NNE向后期叠加浅部断裂次之,两组断裂组成了“格子状”构造格架,其交汇处控制了燕山期中酸性侵入体和矿床的分布(李永峰等,2005;Maoetal.,2008a;孙社良等,2013)。
2.1 南泥湖钼(钨)矿床
南泥湖钼(钨)矿床地处河南省栾川县,大地构造隶属华北陆块南缘(图1)。区内出露地层主要为新元古代栾川群滨-浅海相碎屑岩-碳酸盐岩,从老到新分为三川组大理岩、南泥湖组变斑黑云片岩和煤窑沟组白云石大理岩,北部出露有少量中元古界官道口群浅海相含燧石条带碳酸盐岩沉积,岩性主要为含燧石条带白云质大理岩。区域褶皱和断裂发育,断裂主要为NNW向,NNE向次之,规模不等(孙社良等,2013)(图2a)。
矿区内出露有一个燕山期岩体,即南泥湖岩体,呈小岩株状产出,为一复式岩体,地表出露岩性为花岗斑岩,下部为黑云花岗闪长岩。南泥湖岩体南北长约450m,东西宽近300m,据钻孔资料显示深部900m标高处岩体面积达到1.2km2,岩体呈上小下大的倒杯状(包志伟等,2009;刘永春等,2006)。
钼(钨)矿体主要赋存在岩体和南泥湖组的外接触带角岩中,呈透镜状和似层状产出。矿石主要类型为黑云长英角岩型,片状、放射状结构,细脉状、浸染状构造,主要矿石矿物为辉钼矿和黄铁矿,脉石矿物有石英、方解石和萤石等。矿区内发育有多期围岩蚀变,由内向外依次可分为钾化→绢英岩化→硅化→青磐岩化。
2.2 秋树湾铜(钼)矿床
秋树湾铜(钼)矿床位于河南省镇平县境内,大地构造上处于北秦岭构造带二郎坪地体南侧(图1),区内出露有新元古代秦岭岩群变质杂岩,由老至新依次包括郭庄组大理岩、雁岭沟组花岗片麻岩和石槽沟组斜长角闪岩,是一套经历多次构造和变质作用的杂岩,形成于早元古代裂谷环境(郭保健等,2006;秦臻等,2012;张宗清等,1994)。
矿区发育与区域构造线一致的NWW向和NW向构造和后期的NNE向断裂(图2b)。NWW和NW向构造早期表现为压扭性,后期在燕山期伸展减薄机制变为拉张性,为矿区主要控矿构造;NNE向构造为成矿后形成(朱华平等,1998)。
矿区主要出露的燕山期斑岩体为秋树湾小岩株,东西长为300m,南北宽近200m,呈椭圆状,面积约0.6m2(图2b)。岩性为细粒黑云花岗斑岩,斑状结构,块状构造。
矿床主要由南山斑岩-矽卡岩型钼矿和北山角砾岩型铜(钼)矿组成。其中靠近斑岩体的由内向外构成一个同心圆状水平矿化分带模型:Mo(Cu)矿化带→Cu、Mo矿化带→Cu矿化带→Pb-Zn-Ag矿化带。矿体表现为平缓的层状、似层状和透镜状,矿石结构为细网脉状、不等粒状和胶状等,构造以浸染状为主,致密块状次之。矿石矿物主要为黄铜矿、辉钼矿、闪锌矿、方铅矿、黄铁矿和磁黄铁矿等,脉石矿物包括石榴石、透辉石、石英、方解石和绿帘石等。自岩体中心向外发育多期热液蚀变:石英核→石英钾长石化→石英绢云母化→矽卡岩化→青磐岩化(秦臻,2013)。
图1 东秦岭钼矿带地质简图(据李诺等,2007修改)Fig 1 Geological sketch of the East Qinling molybdenum belt(modified from Li et al.,2007) 1-新生代冲积物;2-白垩纪磨拉石建造;3-三叠纪海相地层;4-嵩箕地块盖层;5-华熊地块盖层(1.4~0.5Ga);6-熊耳群(1.85~1.4Ga),华熊地块;7-华熊地块基底(>1.9Ga);8-早古生代地层,北秦岭;9-二郎坪群(1.0~0.4Ga),北秦岭;10-宽坪群(1.8~1.4Ga),北秦岭;11-秦岭群(>1.4Ga),北秦岭;12-南秦岭地层(1.0~0.25Ga);13-南秦岭基底(>1.0Ga);14-中生代花岗岩;15-前中生代花岗岩;16-断裂或推测断裂;17-斑岩型钼矿床;18-斑岩-矽卡岩型钼矿床;19-热液碳酸盐脉型钼矿床;20-铅锌多金属矿床;21-铜钼多金属矿床;22-热液爆破角砾岩型金矿床; 图中矿点:1-黄龙铺;2-石家湾;3-金堆城;4-木龙沟;5-夜长坪;6-银家沟;7-上房;8-马圈;9-南泥湖;10-三道庄;11-石宝沟;12-雷门沟;13-祁雨沟;14-黄水庵;15-前河;16-鱼池岭;17-东沟;18-赤土店;19-板厂;20-秋树湾1-Cenzoic sediment; 2-Cretaceous molass; 3-Triassic marine strata; 4-Songji Block overlying strata; 5-Huaxiong Block overlying strata(1.4~0.5Ga);6-Xionger Group(1.85~1.45Ga), Huaxiong Block; 7-Huaxiong basement(>1.9Ga); 8- Early-paleozoic strata, North Qinling; 9-Erlangping Group(1.0~0.4Ga), North Qinling; 10-Kuanping complex(1.8~0.4Ga), North Qinling; 11-Qinling Group(>1.4Ga), North Qinling; 12-South Qinling strata(1.0~0.25Ga); 13-South Qinling basement(>1.0Ga); 14-Mesozoic granitoid; 15-Pre-Mesozoic granitoid; 16-fault or inferred fault; 17-porphyry Mo deposit; 18-porphyry-skarn Mo deposit; 19-hydrothermal carbonate vein Mo deposit; 20-lead-zinc polymetallic deposit; 21-Cu-Mo polymetallic deposit; 22-hydrothermal explosive breccia gold deposit;Deposits:1-Huanglongan; 2-Shijiawan; 3-Jinduicheng; 4-Mulonggou; 5-Yechangping; 6-Yinjiagou; 7-Shangfang; 8-Maquan; 9-Nannihu; 10-Sandaozhuang; 11-Shibaogou; 12-Leimengou; 13-Qiyugou; 14-Huangshuian; 15-Qianhe; 16-Yuchiling; 17-Donggou; 18-Chitudian; 19-Banchang; 20-Qiushuwan
图2 南泥湖钼(钨)矿床(a)和秋树湾铜(钼)矿床(b)地质简图(据李永峰等,2003;秦臻等,2012)Fig.2 Geological maps of the Nannihu Mo(-W) deposit (a) and Qiushuwan Cu(-Mo) deposit(b)(after Li et al.,2003; Qin et al.,2012) 2a: 1-矿床;2-断层;3-地层界线;Q-第四系;新元古界栾川群:Pt3m-煤窑沟组;Pt3n-南泥湖组;Pt3s-三川组;Pt3b-白术沟组;Pt2g-中元古界官道口群;v-变辉长岩; 2b: 1-地层界线;2-断层;3-矿床位置;4-向斜;Q-第四系;新元古界秦岭群:Pt1y2-雁岭沟组上 段;Pt1y1-雁岭沟组下段;Pt1g-郭庄组;SK-矽卡岩;γ-花岗岩;γπ-花岗斑岩;po-石英斑岩2a: 1-deposit; 2-fault; 3-strata boundary; Q-Quaternary; New proterozoic Luanchuan Group: Pt3m-Meiyaogou Formation; Pt3n-Nannihu Formation; Pt3s-Sanchuan Formation; Pt3b-Baishugou Formation; Pt2g-Mesoproterozoic Guandaokou Group; ν-meta diabase; 2b: 1- strata boundary; 2-fault; 3-deposit location; 4-syncline; Q- Quaternary; New proterozoic Qinling Group: Pt1y2-Upper Yanlinggou Formation; Pty1 -Lower Yanlinggou; Pt1g-Guozhuang Formation; SK-skarn; γ-granite; γπ-granite porphyry; po-quartz porphyry
南泥湖钼钨和秋树湾铜钼矿床地质特征对比见表1。以下主要从成矿斑岩体、成矿时代及物质来源和构造环境三方面进行讨论。
3.1 成矿斑岩体对比
强烈而频繁的岩浆活动是很多内生金属矿床形成的先决条件,为成矿作用提供热源物质来源,而东秦岭钼矿带内钼多金属矿床与燕山期花岗斑岩密切相关(卢欣祥等,2002)。本次研究对南泥湖和秋树湾斑岩体的岩石地球化学特征进行了总结(见表2),由于卢欣祥等(1984)的主量元素分析结果中未对烧失量进行测试,误差较大,故在此仅作对比参考,不用来进行岩石地球化学投图。两个斑岩体SiO2含量较高,尤其是南泥湖岩体均大于72%,全碱含量(ALK=Na2O+K2O)在7.50%~9.6%范围内,差别不大,在TAS图解中都投点到花岗岩系列中(图3)。但是两者的K2O、Na2O含量相差悬殊,南泥湖斑岩体K2O含量在6.11%~7.99%之间,远大于秋树湾岩体的K2O含量(3.67%~5.11%),ω(K2O)/ω(Na2O)介于0.96~4.78,并且在图4A中可见南泥湖和秋树湾斑岩体分别投到钾玄岩和高钾钙碱性系列中。表2中可见两个斑岩体铝饱和指数(A/CNK)在0.95~1.27之间,而且在A/NK-A/CNK图解中发现南泥湖岩体和秋树湾岩体分别具有过铝质和偏铝质特征(图4B)。以上地球化学特征对比说明南泥湖斑岩体物质来源更浅,壳源成分更多,这也与卢欣祥等(2002)提出斑岩体与成矿关系的观点吻合:ω(SiO2)>72%,ω(K2O)≥ω(Na2O)时,则生成钼矿,并伴生一些金矿(雷门沟斑岩Mo(Au)矿床);当ω(SiO2)<72%,ω(K2O)≤ω(Na2O),则对金、铜矿化有利(如蒲塘、八宝山、银家沟等矿床);若ω(SiO2)>72%,ω(K2O)>>ω(Na2O)时,只生成高品位的单一钼矿,无金伴生,如上房、南泥湖、夜长坪等矿床(罗铭玖等,1991)。
表1 南泥湖钼(钨)矿床和秋树湾铜(钼)矿床地质特征对比
注:参考文献据文中各处。
南泥湖和秋树湾斑岩体的微量元素蛛网图和稀土元素球粒陨石配分曲线体形态差别不大。两个岩体均富集大离子亲石元素(LILE)如Rb、Ba、Th和K,亏损高场强元素(HFSE)如Ti、Nb、P、Ta(表2)(图5A)。ΣREE值范围为90.50~252.26,图5B表现为右倾的趋势,轻稀土和大离子亲石元素(LILE)比重稀土元素的含量高,重稀土元素分馏较大,LaN/YbN介于11.77~40.54之间,LREE/HREE=11.37~25.61,轻重稀土分异明显。而南泥湖与秋树湾斑岩体相比具有明显的负Eu异常,Ba和Sr含量也更低。
图3 成矿母岩的TAS图解Fig.3 TAS diagram of ore-bearing parent rock 1-橄榄辉长岩; 2a-碱性辉长岩; 2b-亚碱性辉长岩; 3-辉长闪长岩; 4-闪长岩; 5-花岗闪长岩; 6-花岗岩; 7-硅英岩; 8-二长辉长岩; 9-二长闪长岩; 10-二长闪长岩; 11-石英二长岩; 12-正长岩; 13-副长石辉长岩; 14-副长石二长闪长岩; 15-副长石二长正长岩; 16-副长正长岩; 17-副长深成岩; 18-霓方钠岩/磷霞岩/粗白榴岩; NNH-南泥湖岩体; QSW-秋树湾岩体1-peridotgabbro;2a-alkaline gabbro; 2b-sub-alkaline gabbro; 3-gabbroic-diorite; 4-diorite; 5-granodiorite; 6-granite; 7-quartzolite; 8-monzogabbro; 9-monzodiorite,10-monzonite; 11-quartz-monzonite; 12-syenite; 13-foid-gabbro; 14-foid-monzodiorite; 15-foid-monzosyenite; 16-foid-syenite; 17-folidolite; 18-tawite/urtite/italite; NNH-Nannihu in- trusion; Qiushuwan-Qiushuwan intrusion
由此可见南泥湖和秋树湾斑岩体均为高分异、演化程度较高的花岗岩体。但是它们的Rb/Sr比值差异很大,分别为3.45~8.54和0.12~0.38,而中国东部上地壳平均值为0.31(Gaoetal.,1998),全球上地壳平均值为0.32(Tayloretal.,1995),可见南泥湖斑岩体的Rb/Sr比值远高于上地壳和秋树湾斑岩体,推测是因为其在岩体形成的过程中发生了强烈的钾化蚀变,导致了Rb/Sr比值的高异常,这也与其野外地质特征和室内实验测出的高K2O值相吻合,而秋树湾斑岩体钾化较弱,故而其Rb/Sr比值相对正常。
87Sr/86Sr的初始比值常常用来示踪岩浆源区,全球的87Sr/86Sr的初始比值是0.7052,而地壳的初始比值较高,地幔的初始比值较低,为0.7033(Rollinson,1993)。南泥湖斑岩体的87Sr/86Sr初始比值为0.7034~0.7080(罗铭玖等,1991),秋树湾斑岩体的87Sr/86Sr初始比值为0.70495(刘孝善等,1987),而壳源的酸性岩87Sr/86Sr >0.7060,这表明南泥湖斑岩体和秋树湾斑岩体的岩浆物质来源均为壳幔混源,而南泥湖斑岩体源区中地壳物质贡献更大。
3.2 成矿时代及物质来源
通过硫同位素组成的研究,可以判断矿床的成矿物质的来源。两个矿床的硫同位素分析结果如表3所示,其δ34S值均接近于0‰,与深源地幔硫的δ34S值(0‰左右)(Sakaietal.,1984)相比较可发现秋树湾矿床更接近深源地幔硫的δ34S值,推断两者成矿物源很可能来自下地壳或上地幔,而秋树湾矿床中幔源物质的贡献更多一些。
图4 SiO2-K2O图解(a)(据Morrison,1980;Richawood,1989)和A/CNK-A/NK图解(b)Fig.4 SiO2-K2O diagram(after Morrison,1980;Richawood,1989)(a) and A/CNK-A/NK diagram(b)
图5 微量元素原始地幔蛛网图(a)和稀土球粒陨石配分图(b)(原始地幔和球粒陨石数据引自Sun and McDonough,1989)Fig.5 Primitive mantle normalized trace elements spider diagram(a) and chondrite-normalized REE patterns(b) (mantle and chondrite data after Sun and McDonough, 1989)
对前人成矿流体的研究结果总结,两个矿床的成矿流体均为高温、高盐度和富CO2的H2O-NaCl-CO2流体(秦臻等,2012;杨永飞等,2009)。陈衍景等(2009)提出陆陆碰撞构造背景下的成矿流体以高盐度、富CO2区别于岩浆弧区同类矿床的贫CO2的NaCl-H2O型流体,而南泥湖矿床和秋树湾矿床分别为陆陆碰撞和岩浆岛弧构造环境下形成矿床的典型代表(图6),却都富CO2;Maoetal.,(2008b)认为幔源包体赋存有大量CO2包裹体,而地幔流体中含有富CO2的熔体,因此秋树湾矿床成矿流体一部分可能是来自于地幔,导致了岩浆弧背景下富CO2流体的形成。秋树湾和南泥湖矿床各阶段石英、方解石的δ18O值分别为9.2‰~10.59‰和8.9‰~12.5‰(秦臻等,2012;向君峰等,2012b;杨永飞等,2009),显示出深源岩浆流体的特点,秋树湾矿床δ18O值较之南泥湖矿床偏低,也说明其成矿过程中有着更多的地幔物质参与。
前人通过精确的辉钼矿Re-Os同位素定年将南泥湖钼(钨)矿床和秋树湾铜(钼)矿床的成矿时代限定为140~150Ma之间,成矿时代与东秦岭钼矿带的主成矿期时代一致,而南泥湖钼(钨)矿床辉钼矿中Re含量测得为15.2×10-6~27.5×10-6,平均为22.0×10-6;秋树湾铜(钼)矿床辉钼矿中Re含量为151.8×10-6(郭保健等,2006;李永峰等,2003),并依据Maoetal.,(1999)和Steinetal.,(2001)提出的从地幔到壳幔源再到地壳,矿石中含Re量呈数量级下降,从幔源、I型到S型花岗岩有关的矿床,Re含量从n×100×10-6→n×10×10-6→n×10-6,从而确定了成矿物质来自壳幔混熔,由于秋树湾矿床Re含量远大于南泥湖矿床,推断是因为前者成矿过程中地幔物质参与更多导致的(郭保健等,2006;李永峰等2003)。
表2 南泥湖和秋树湾斑岩体主量元素、稀土元素和微量元素值
注:主量元素单位:wt%;稀土元素和微量元素单位:×10-6。
笔者认为仅凭辉钼矿中Re含量来推断成矿物质的来源问题过于草率。Berzinaetal(2005)等统计了俄罗斯和蒙古的5个铜钼矿床中的24个辉钼矿样品,总结出辉钼矿中Re含量受到成矿母岩的成分、成矿流体中Re的富集和结晶作用中物化条件(氧逸度、酸碱度、Cl活动性和温压)的改变影响。杨宗峰等(2011b)通过对国内744个辉钼矿Re-Os同位素测年数据进行统计,发现辉钼矿中Re含量具有混合分布的特征,在相同成矿母岩岩性的条件下主要因为辉钼矿共生矿物的不同而产生Re含量的差异,Re含量数量级的变化不能有效反映出其成矿物质来源。以上均说明辉钼矿中Re含量受到多种因素的影响,不仅仅是由成矿物质来源不同而导致其差异性。辉钼矿中Re含量与成矿温度和矿化蚀变有关(Giles and Shilling,1972;Newberry,1979;Filimonovaetal.,1984;Todorov and Staikov,1985),如辉钼矿中的Re主要赋存在ReS2中,随着温度的升高ReS2溶解度也上升,从而导致Re含量的减少(Xiongetal.,2002);酸性热液溶解条件下形成的Re含量比碱性热液溶解条件下的更高(Filimonovaetal.,1984)。流体包裹体测温表明(秦臻等,2012;向君峰等,2012b;杨永飞等,2009),南泥湖矿床的成矿阶段(152~312℃)温度大于秋树湾矿床(250~380℃);并且南泥湖矿床钾化蚀变(碱性)明显和秋树湾矿床则发育石英-绢云母化蚀变(酸性)。所以,南泥湖矿床和秋树湾矿床的Re含量不同主要是受到成矿温度和矿化蚀变差异性影响而导致的,不能简单判断为成矿物质来源的不同。
表3 南泥湖钼(钨)矿床和秋树湾铜(钼)矿床硫同位素组成
3.3 构造环境
南泥湖钼(钨)矿床和秋树湾铜(钼)矿床分别位于华北陆块南缘和北秦岭构造带两个不同的构造单元,这可能也是导致两者成矿差异性的原因之一。华北陆块南缘在中生代处于陆-陆碰撞过程的由挤压向伸展转变的时期(陈衍景等,2000)或碰撞后造山的局部伸展作用下(李永峰等,2005),均表明华北陆块南缘是处于陆-陆碰撞之后,在增厚地壳的环境下发生的伸展运动,岩浆热液沿着深大断裂或背斜核部上侵;而秋树湾铜(钼)矿床地处北秦岭构造带二郎坪地块上,二郎坪地块是北秦岭早古生代活动大陆边缘沟-弧-盆系统的重要组成部分,记录了古弧后盆地的物质残存(卢欣祥等,2002),这也与Y+Nb-Rb图解中两者的投点区域所吻合(图6)。综上所述,东秦岭钼矿带在中生代燕山期活动中,扬子板块向华北板块进行陆陆(或A型)俯冲碰撞,岩石圈地幔发生拆沉减薄,软流圈发生上涌,加热熔融地壳和岩石圈,发生壳幔混熔作用,而钼元素主要来自于地壳中,铜元素则来自地幔中,从图中可见,南泥湖钼(钨)矿床的成矿环境地壳厚度大于秋树湾铜(钼)矿床,岩浆热液受壳源物质参与的更多,而秋树湾铜(钼)矿床由于所处的火山岛弧环境发生地壳增厚的程度低于南泥湖钼(钨)矿床,更多的受到幔源成分影响,故而形成了以铜为主的斑岩型矿床(图7)。
图6 Y+Nb-Rb图解(据Pearce et al.,1996)Fig.6 Y+Nb-Rb diagram(after Pearce et al., 1996)
图7 南泥湖钼(钨)矿床和秋树湾铜(钼)矿床成岩成矿模式图Fig.7 Ore-forming tectonic model of the Nannihu Mo(W) deposit and Qiushuwan Cu(Mo) deposit
(1) 南泥湖钼(钨)矿床和秋树湾铜(钼)矿床在成矿类型和规模上差异显著研究两者的野外地质标志(钾化和石英绢云母化蚀变)和室内实验研究(S同位素和流体包裹体)可为今后在东秦岭钼矿带钼和铜两类斑岩型矿床的勘探工作提供参考。
(2) 通过对两个矿床成矿母岩的地球化学特征研究分析,发现其在TAS图解中均投到花岗岩区域中,属于高钾钙碱性系列,偏铝质-弱过铝值特征,富集大离子亲石元素(Rb、Ba、Th和K),亏损高场强元素(Ti、Nb、P、Ta),轻重稀土元素分馏明显。然而与秋树湾斑岩体相比,南泥湖斑岩体SiO2和K2O含量更高,亏损Eu,Ba、Sr含量更低。而南泥湖和秋树湾斑岩体的Rb/Sr值和87Sr/86Sr值和地壳与地幔的比值对比后发现,南泥湖斑岩体更偏向于地壳的数值,秋树湾则与地幔的数值接近,反映了二者岩浆物质来源分别以壳源和幔源为主的特点。
(3) 两个矿床成岩时代虽然相近,但是成矿物质来源不同。根据矿床地质和地球化学特征推断绢英岩化蚀变有利于铜的富集,钾化蚀变有利于钼的富集,而S同位素和流体包裹体可以指示成矿流体是幔源或壳源为主,而Re元素的含量不作为指示成矿流体来源的依据。因此南泥湖钼钨矿床成矿物质主要来源于地壳,秋树湾铜(钼)矿床成矿物质多来自于地幔中,这也符合钼以壳源为主,铜以幔源为主的规律。
(4) 南泥湖和秋树湾矿床形成时的构造环境不同,导致了两者成岩成矿规模和类型的不同。
致谢 感谢曹华文同志在成文过程中的帮助;论文评审专家给予了很好的建议,在此一致深表感谢。
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Comparison of Petrologic and Metallogenic Characteristics Between the Nannihu Mo(-W)Deposit and Qiushuwan Cu(-Mo) Deposit, East Qinling
ZHANG Yun-hui1,2, ZHANG Shou-ting2, WANG Shi-yan2,3, YUN Hui3, TAN He-yong3, ZHANG Hao2
(1.SichuanInstituteofGeologicalEngineeringInvestigation,Chengdu,Sichuan610072;2.SchooloftheEarthSciencesandResources,ChinaUniversityofGeosciences,Beijing100083;3.HenanInstituteofGeologicalSurvey,Zhengzhou,Henan450007)
The Nannihu Mo(-W) deposit and Qiushuwan Cu(-Mo) deposit are both porphyritic deposits in East Qinling molybdenum belt, but they are distinct in metallogenic type and scale. This study explores the geological characteristics of both deposits, geochemical features of ore-forming porphyries, metallogenic epoch and ore-forming material source (S isotope, fluid inclusions and Re content), and the result shows that they both are derived from a mixed source of mantle and crust, and the Qiushuwan Cu(-Mo) deposit involves more mantle materials. The two deposits were formed under the same mechanism of extension and thinning during Yanshanian epoch in Qinling, but they are located in different tectonic units. All above result in the metallogenic difference and provide a reference for the exploration of Mo and Cu deposits in East Qinling molybdenum belt in the future.
Nannihu Mo(-W) deposit, Qiushuwan Cu(-Mo) deposit, geochemistry, tectonic environment, East Qinling molybdenum belt
2013-07-28;
2013-11-29;[责任编辑]郝情情。
栾川钼铅锌多金属矿集区大型-超大型矿床形成的地球动力学背景、过程与定量评价(编号:1212011220925)和河南省栾川钼铅锌矿整装勘查综合研究及熊耳山南部地区矿产远景调查(编号:1212011220502)联合资助。
张云辉(1990年-),男,硕士研究生学历,现于四川省地质工程勘察院从事矿产勘查工作。E-mail: z374175083@163.com。
P581+P611.1
A
0495-5331(2014)04-0700-12