福建平和锦溪铜(钼)矿床锆石U-Pb和辉钼矿Re-Os年龄及其地质意义

潘天望,张 达*,倪建辉,陈泉流,袁 远,狄永军

(1.中国地质大学 地质过程与矿产资源国家重点实验室,北京 100083;2.福建省地质矿产勘查开发局 第八地质大队,福建龙岩 364012)

0 引 言

上杭-云霄火山喷发带是福建省重要的多金属成矿带之一[1–2],在中生代存在大量的火山盆地以及相应的侵入岩浆系统,并形成了大量与之相关的矿床。上杭-云霄构造岩浆成矿带广泛分布有铜、金、银、钼等金属矿产及其他非金属矿化和异常点,重要的矿化集中区包括闽西南上杭紫金山矿集区、闽中永定山口及大排矿集区、闽东平和及云霄一带矿集区等。近年来前人对紫金山地区的矿床成因、构造及岩浆作用特征以及成岩成矿关系等进行了大量的研究[3–12],认为紫金山铜金矿田属于典型的高硫浅成低温热液型-中低温热液型-斑岩型成矿系统[3,4,6,13,14],该地区大规模的成矿作用主要与燕山晚期早白垩世(93～111 Ma)火山-侵入作用相关,且形成于区域拉张伸展的构造环境[3,5,11,15],如典型的罗卜岭斑岩型铜(钼)矿床辉钼矿样品 Re-Os同位素等时线年龄为(104.9±1.6) Ma[15],而与成矿关系密切的罗卜岭花岗闪长斑岩 LA-ICP-MS锆石 U-Pb谐和年龄为(103～105) Ma[16–17],成岩成矿时代基本一致。区域矿产地质调查表明,位于上杭-云霄铜、金、钼等多金属成矿带东端的平和地区也存在大量晚中生代火山盆地以及相关的侵入岩浆活动,局部可见与紫金山火山盆地相似的陆相火山机构及明矾石次生石英岩化蚀变分带[18–19]。由于与紫金山火山机构处于同一构造岩浆成矿带上,因此平和一带可能存在与“紫金山”式矿集区相类似的成岩成矿年代学记录,也可能成为寻找“紫金山”式铜、金多金属矿床新的重要找矿靶区。

平和锦溪铜(钼)矿床位于NW向上杭-云霄构造岩浆岩带东南部的平和火山盆地内,并紧邻钟腾铜(钼)矿床[20],为受构造控制的中高温热液型矿床,铜(钼)主要赋存于花岗闪长岩断裂破碎带中。上杭-云霄构造岩浆岩带上紫金山铜金矿田中四坊岩体的主要围岩为燕山期的黑云母花岗岩,在岩体内及其边缘常分布有稍晚侵入的浅成斑岩体,如罗卜岭和紫金山斑岩体,这些斑岩体与四坊岩体呈明显的侵入接触关系,是紫金山地区高硫浅成低温热液型-中低温热液型-斑岩型“三位一体”铜矿的主要成矿母岩[14]。锦溪花岗闪长岩体地质特征与同属上杭-云霄成矿带上的四坊花岗闪长岩体相似,目前钻孔资料显示锦溪花岗闪长岩体内及其边缘也具有寻找斑岩体的潜力,且岩体的主要围岩为燕山期的黑云母花岗岩,这说明锦溪铜(钼)矿床同样具有寻找斑岩型铜(钼)矿床的潜力。但由于其成矿时代及与之相关的岩体形成时代尚不清楚,一定程度上制约了矿床成因及其形成背景的认识,以及与紫金山矿田典型矿床的对比分析。为此,拟在野外考察的基础上,对平和锦溪铜(钼)矿床辉钼矿及与成矿作用相关的花岗闪长岩分别开展 Re-Os同位素及锆石 LA-ICP-MS U-Pb年代学研究,获得矿区高精度成岩成矿年龄,在此基础上,通过对比上杭-云霄成矿带晚中生代铜、钼等多金属矿床成岩成矿时空分布特征,探讨平和锦溪铜(钼)矿床成因及其构造背景。

1 地质概况

上杭-云霄构造岩浆带位于华夏板块的东南部,斜跨于不同的构造单元上,自西向东分别为武夷山隆起、粤东闽西南坳陷、浙闽粤坳陷带及东南沿海断隆带,崇安-石城、政和-大埔、长乐-南澳断裂分别在该带西侧、中部和东部通过[2](图 1a)。上杭-云霄断裂带主要活动期在早白垩世[1,3],且对燕山晚期岩浆活动及其成矿具有明显的控制作用[22],其中与北东向构造交汇处对成矿尤为有利。两组断裂构造的复合部位不仅控制着燕山期的岩浆活动,而且次级断裂及韧性剪切带往往是后期成矿的有利赋存空间,如紫金山矿集区、永定山口矿集区以及平和钟腾矿集区等都赋存于该断裂与北东向各个断裂交汇处或附近。平和锦溪铜(钼)矿床位于东南部的平和县境内,构造上位于华夏板块闽东燕山期陆内造山带西南部福安-南靖(北东向)深断裂与上杭-云霄(北西向)深断裂交汇部位,闽东火山活动亚带与上杭-云霄火山喷发亚带交汇处的东南部。成矿上处于上杭-云霄成矿带东南段,并紧邻钟腾铜(钼)矿床,表现出良好的成矿地质条件(图1b)。

平和地区出露地层以侏罗系陆相火山喷发-沉积岩系为主,主要有下侏罗统梨山组(J1l)内陆盆地碎屑岩、细粒砂岩,侏罗系中统长林组(J2c)中细粒石英砂岩、粉砂岩,上侏罗统南园组(J3n)中酸性火山喷发岩,以及第四系(Q)冲洪积层。区域构造活动十分强烈,以NE向和近EW向构造为主。其中近EW向断裂规模最大,生成最早,控制钟腾岩体和平和岩体的产出以及矿化蚀变带、物化探异常的分布1)中化地质矿山总局福建地质勘查院,福建省平和县锦溪矿区铜矿详查地质报告,2007。;NE向断裂构造形迹清晰,规模较大,控制着侵入岩、火山岩、矿化蚀变带和物化探异常的展布,是锦溪矿区最主要的导矿和容矿构造。平和地区主要出露燕山晚期的平和岩体与钟腾岩体,呈小岩株状分别展布于研究区的东、西部(图1)。两岩体均为复式岩体,前者主要由花岗闪长岩、石英闪长岩、(含)黑云母花岗岩组成,后者主要由石英闪长岩和花岗闪长岩等组成。两者均与Cu、Mo等多金属成矿关系较密切。此外,尚有后期侵入的石英斑岩、花岗斑岩等中酸性脉岩。

图1 福建上杭-云霄成矿带区域地质简图(a)(成矿带的划分据文献[2]修编;侵入岩的划分据文献[21]修编)及锦溪铜(钼)矿区域地质简图(b)Fig.1 Regional geological map of the Shanghang-Yunxiao mineralization belt(a) and tectonic location of the Jinxi copper(molybdenum) deposit(b)

2 矿床地质特征

2.1 矿区地质

锦溪铜(钼)矿床矿区除第四系洪积层外,其他地层均未见出露。第四纪洪积层主要分布于矿区西北部,堆积物由灰-灰黄色黏土、砂质黏土、粗砂及砂砾卵石等组成(图2)。

矿区主要出露燕山晚期花岗闪长岩,其次为黑云母花岗岩,两者均为平和复合岩体的组成部分,此外尚有晚期零星侵入的花岗斑岩脉。锦溪花岗闪长岩体在矿区大面积分布,且岩体的主要围岩为黑云母花岗岩,黑云母花岗岩时代不明,两者接触界线呈渐变关系,花岗斑岩脉呈透镜状侵入于矿区北部及北东部的花岗闪长岩中1)。锦溪铜(钼)矿床铜矿化与锦溪花岗闪长岩关系紧密,而钼矿化与花岗闪长岩及黑云母花岗岩关系紧密。花岗闪长岩体底下、黑云母花岗岩体内和边缘钼矿化规模较大,而同样部位均未见铜矿化,表明黑云母花岗岩可能与铜矿成矿无关。目前黑云母花岗岩及相关的辉钼矿测年工作正在进行中。

矿区断裂构造以NE向为主,其次为NW向(F5)和近SN向(F4)以及NNE向(F6)。NE向断裂带斜贯全区(F1–3),是区内主要的导矿和容矿构造。NW 向和近 SN向断层形成时间最晚,切割 NE向断裂带,剪节理发育,常见石英脉充填,地表多为铁锰质充填,显示正断层性质。

2.2 矿体特征

锦溪铜(钼)矿体主要赋存于北东向断裂破碎带中,容矿围岩为锦溪花岗闪长岩。矿体形态、产状及规模主要受NE向F1、F2和F3断层控制(图3),已探明钼矿储量有望达到中型规模,铜矿储量为小型规模。锦溪矿区目前共圈定15个矿体,其中铜矿体6个,钼矿体 9个。铜矿体走向长 65～586 m,厚度2.33～15.55 m;钼矿体走向长 65～230 m,厚度1.18～8.39 m,各矿体形态在平面和剖面上均呈脉状或透镜状产出1)。钼矿体基本分布在铜矿体的上部,主要集中于 F1～F2断层构造破碎带之间。近矿围岩和矿体顶板、底板均为矿化、蚀变花岗闪长岩。矿体与顶板、底板接触界线不清,多为渐变关系。

2.3 矿石类型及特征

根据肉眼识辨及物相分析,锦溪矿区矿石的自然类型以硫化物矿为主。矿石金属矿物以黄铁矿、黄铜矿、辉钼矿为主,次为磁铁矿、磁黄铁矿,极少量辉铜矿,偶见闪锌矿、方铅矿等;辉钼矿和黄铜矿集合体常呈细脉状或星点浸染状等产于次生石英、钾长石、黑云母和磁铁矿等矿物的粒间,辉钼矿常被黄铜矿细脉切穿或被黄铜矿、黄铁矿所溶蚀,代表锦溪铜(钼)矿床至少有两阶段成矿作用(图4a和4b)。

矿石结构多样,主要为他形粒状镶嵌结构,其次为半自形-他形粒状镶嵌结构,少量自形-半自形他形粒状结构等;矿石构造以不均匀稀疏细脉浸染状构造为主,浸染状、细脉条带状构造次之,少量致密块状构造。脉石矿物以斜长石、钾长石、石英为主,其次为绿泥石、绢云母、方解石等。

2.4 蚀变类型

矿区蚀变发育,且明显受构造控制,构造节理愈发育,蚀变作用愈强烈,蚀变矿物多沿裂隙充填交代呈脉状产出,根据蚀变矿物组合与分布特征,蚀变具有分带性,大致可分为上、下两个带。上带以硅化为主,伴有绢云母化、钾长石化、磁铁矿化、黄铁矿化等,多呈脉状与辉钼矿体相伴生;下带以钾长石化为主,伴有硅化、绿泥石化、绢云母化等,往往与黄铜矿体相伴生1)中化地质矿山总局福建地质勘查院,福建省平和县锦溪矿区铜矿详查地质报告,2007。。近矿围岩蚀变主要为黄铁矿化、钾化、硅化和绿泥石化。

3 样品的采集及测试结果

3.1 样品采集及分析方法

本次研究锆石单矿物样品选自矿区 28号勘探线平硐内的花岗闪长岩中,岩石样品新鲜,蚀变污染少。锦溪花岗闪长岩呈灰色或暗灰色,以中细粒花岗结构(粒径 1～4.5 mm)、块状构造为主,未见显微文象结构和晶洞构造(图4c和4d)。组成矿物以斜长石(25.1%～42.9%,An=30～50)和石英(20%～23%)为主,并含有一定量钾长石,总量在 18.5%～35.5%之间,铁镁矿物主要为少量黑云母(7%～8%)和普通角闪石(5%～7%),角闪石多与黑云母共生,未出现碱性铁镁矿物;副矿物组合中普遍出现榍石而未见富铝矿物,以富含Fe、Ti、Ca成分的副矿物为特征。

图2 锦溪铜(钼)矿矿床地质简图1) 福建省闽东南地质大队,福建省平和县钟腾矿区深部查证报告,1990。Fig.2 Geological map of the Jinxi copper(molybdenum) deposit

图3 锦溪矿区16号勘探线剖面图示意图Fig.3 A–B section of the Jinxi mining area

图4 锦溪矿床测试岩体及矿石样品照片岩相学特征Fig.4 Photographs showing ore samples and petrographical characteristics of the Jinxi deposit

在 22号勘探线平硐内选取一件辉钼矿含量充足的样品(X22-b2)进行辉钼矿 Re-Os同位素模式年龄研究,矿石中辉钼矿呈铅灰色,半自形叶片状或不规则粒状集合体,粒径在0.5～15.0 mm之间,集合体呈脉状分布于次生石英颗粒间,基本与硅化同时或稍晚晶出,晚期含Cu黄铁矿化脉将早期含Mo硅化脉所截切,矿脉辉中钼矿被黄铜矿和黄铁矿所溶蚀(图4a和4b),这些特征表明黄铜矿和黄铁矿形成的时代要比辉钼矿稍晚。

锆石与辉钼矿的分选由河北省廊坊市地科勘探技术服务有限公司完成。锆石样品的挑选、制靶和阴极发光(CL)成像在北京锆石领航科技有限公司实验室完成。挑选出的辉钼矿质纯,无包体,无污染且纯度达98%以上。锆石U-Pb测年分析在天津矿产研究所实验室利用LA-ICP-MS方法完成,具体测试原理及仪器配置和实验流程详见文献[23–24]。采用Andersen[23]方法对普通Pb进行校正。采用TE-MORA作为外部锆石年龄标准,数据处理采用 ICP-MS Data Cal 4.3程序[25],加权平均年龄计算及处理采用Isoplot软件完成[26]。利用NIST612玻璃标样作为外标计算锆石样品的Pb、U、Th含量。测试结果见表1。

由于地球样品自约0.8 Ga以来放射性衰变引起的207Pb/235U比值的增幅较小,年轻锆石的207Pb丰度较低而难以测量,并且由于该类锆石中常含有低的、不等量的普通 Pb,这就使得年轻锆石在常规的206Pb/238U-207Pb/235U 谐和曲线上常常出现偏离谐和线的情况[27],造成这种常规的谐和曲线难以判断谐和年龄与存在不同放射成因 Pb丢失年龄之间的区别[28],且目前 LA-ICP-MS方法尚无法准确获得204Pb的含量,因此,为了评价207Pb/206Pb组成和普通 Pb的相对比例[29],同位素年龄的确定使用 Tera-Wasserburg谐和图解[30],图解的横坐标和纵坐标分别为238U/206Pb和207Pb/206Pb,将所有测试结果不进行普通Pb扣除而直接投图,形成一条不一致线,不一致线与谐和线的下交点即代表样品的形成年龄,图解的生成及处理亦采用Isoplot软件完成。

3.2 锆石U-Pb年代学

对锦溪花岗闪长岩体选取 2件代表性样品,进行LA-ICP-MS锆石U-Pb测年。所选锆石呈自形长柱状或短柱状,粒径一般在 80～200 μm 之间,长宽为 1∶1～3∶1之间(图 5)。CL图像显示大部分锆石内部可见较清晰的韵律环带和条带结构,表现出典型岩浆锆石特征。少量锆石具有继承锆石的核,锆石内部显示其核部震荡环带较宽,呈现亮白色-灰白色,而边部呈暗灰色,振荡环带较窄而密,是岩浆锆石早期处于高温下结晶而成的[31]。在分析过程中,避开裂隙和包裹体发育的锆石或微区,选择环带发育的锆石边部打点,两件样品锆石U、Th、Pb同位素成分数据及谐和年龄见表1。

LA-ICP-MS 锆石 U-Pb同位素测年结果显示,可能受到蜕晶化作用的影响[32],两个花岗闪长岩样品大部分测点都偏离协和线。样品JX28-b1中除了2个测点(测点 7和测点 8)显示较大的锆石206Pb/238U年龄(126±1) Ma和(138±2) Ma,其余17个测点则显示了较为一致的206Pb/238U年龄(104～114 Ma)。19个锆石测试点在 Tera-Wasserburg图解中给出了(104.5±1.0) Ma(MSWD=1.8)的下交点年龄(图 6a)。样品JX14-b2的Tera-Wasserburg图解显示,17个测试点中除了测试点 10显示出较高206Pb /238U年龄((122±2) Ma)外,剩余16个测点206Pb/238U年龄集中于107～112 Ma之间,所有测试点在Tera-Wasserburg图解中给出了(106.8±0.8) Ma(MSWD=0.89)的下交点年龄(图6b)。

上述两件样品锆石 Th/U比值变化于0.32～0.59(Th/U ＞ 0.1)之间,表明了锆石为典型的岩浆锆石[33]。由于锦溪花岗闪长岩体形成时代较新,且锆石样品中常含有低的、不等量的普通Pb,因此两个花岗闪长岩均未能获得206Pb/238U加权平均年龄,但2件样品下交点年龄代表了锦溪花岗闪长岩体的侵入年龄,表明该岩体形成于105～107 Ma之间。

图5 锦溪矿床锦溪花岗闪长岩部分锆石阴极发光图Fig.5 Cathodoluminescence(CL) images of some zircons from the Jinxi granodiorite in the Jinxi deposit

图6 锦溪花岗闪长岩中锆石LA-ICP-MS U-Pb定年结果Fig.6 LA-ICP-MS U-Pb ages of zircon from the Jinxi granodiorite

3.3 辉钼矿Re-Os测定

辉钼矿单矿物的挑选在室内无污染环境下进行,挑选出符合测试要求的辉钼矿,Re-Os同位素年龄测试由中国地质科学院国家地质实验测试中心完成,采用电感耦合等离子质谱仪TJA X-erie ICP-MS进行测量,样品的化学处理流程和分析方法见文献[34–35],模式年龄t按以下式计算:

式中:λ(187Re 衰变常数)=1.666×10–11a–1。

本次工作在锦溪矿区22号勘探线平硐内选取1件具有代表性且含量充足的辉钼矿样品(JX22-b2)进行样品的Re-Os同位素测试,结果表明辉钼矿的模式年龄为(105.7±1.7) Ma。辉钼矿的Re含量为2.058 μg/g,187Re含量为 1.576 μg/g,187Os含量为2.778 ng/g,而普Os含量为0.001 ng/g(表2),远低于样品的187Os含量,反映出187Os都是187Re的衰变产物,符合计算模式年龄的条件,也说明所获得的模式年龄是有效的。

4 讨 论

4.1 成岩成矿时代约束

本文获得的锦溪花岗闪长岩 LA-ICP-MS锆石U-Pb年龄在105～107 Ma之间,代表了锦溪花岗闪长岩的侵位年龄区间。闽西南紫金山盆地白垩世(94～128 Ma)火山-侵入岩分布主要受NW向上杭-云霄深断裂的控制[3],平和火山盆地与之相似,所以该地区近年来成为寻找“紫金山式”铜金矿床的重点区域[18–19]。平和锦溪铜(钼)矿床与罗卜岭斑岩型铜(钼)矿床同属NW向上杭-云霄铜、金、钼等多金属成矿带,钼矿床的成矿年龄((105.7±1.7) Ma)与罗卜岭铜(钼)矿床((104.9±1.6) Ma)以及相邻钟腾斑岩型铜(钼)矿床((109.0 ± 2.8) Ma)(辉钼矿 Re-Os)[36]的成矿年龄在误差范围内较为一致,锦溪花岗闪长岩的侵位年龄(105～107 Ma)与四坊花岗闪长岩的侵位年龄(锆石 U-Pb 年龄,104～107 Ma)[14,37]近乎一致,且与四坊岩体呈明显的侵入接触关系的罗卜岭花岗闪长斑岩的侵位年龄(103～105 Ma)亦非常接近,同时罗卜岭花岗闪长斑岩是“罗卜岭”式斑岩型铜(钼)多金属矿床的成矿母岩。因此,平和锦溪地区也可能为寻找斑岩型铜(钼)多金属矿床新的找矿靶区。

辉钼矿封闭温度较高(约 500 ℃),冷却速度较慢的且不易受后期蚀变和构造事件的影响[38]。因此,对矿石中分离出的辉钼矿进行Re-Os同位素定年是目前确定成矿时代的最好方法,辉钼矿的 Re-Os同位素定年结果也是精确厘定矿床成矿时代的最好证据[39–40]。本次工作在锦溪矿区22号勘探线平硐内获得的含辉钼矿石样品(JX22-b2)中,含辉钼矿硅化脉常被含黄铜矿细脉切穿,辉钼矿常被黄铜矿、黄铁矿所溶蚀,代表锦溪铜(钼)矿床至少存在两阶段成矿作用,铜矿的形成普遍比辉钼矿要略晚。本文首次对样品中的辉钼矿进行 Re-Os同位素定年,获得辉钼矿Re-Os同位素模式年龄为(105.7±1.7) Ma,该年龄与铜(钼)矿含矿围岩锦溪花岗闪长岩的侵位年龄(105～107 Ma)在误差范围内基本一致,因此认为该矿床钼矿成矿年龄在105～107 Ma年之间。

4.2 成矿物质来源

Re-Os同位素体系不但可以精确测定硫化物矿床的成矿时代,而且金属硫化物矿床中辉钼矿Re含量对成矿物质来源有一定的示踪作用[41–43]。Maoet al.[44–45]通过对中国各种成因类型钼矿床中辉钼矿Re含量统计与分析后指出,从幔源到壳幔混源再到壳源,辉钼矿中的Re含量从n×10～n×103μg/g →n×10 μg/g →nμg/g呈数量级逐次降低。此次测得的锦溪铜(钼)矿床中辉钼矿187Re为(1.576 ± 0.011) μg/g,相当于壳源岩浆矿床的Re含量,指示锦溪铜(钼)矿床的钼矿成矿物质来源以壳源为主。

4.3 成岩地质意义

华南中生代大规模构造岩浆作用被认为与特提斯构造域向环太平洋构造域的转换有关[46–48]。已有研究表明,华南在中生代至少存在 3期挤压构造运动,印支期挤压构造变形表现为近东西向褶皱和北东向断裂发生右旋走滑运动(239～230) Ma[49],造成了早-中三叠世与晚三叠世的不整合[50–51],这指示华南早中生代遭受南北向挤压作用,其动力来源为印支陆块与华南陆块南缘之间发生的碰撞以及华北陆块与华南陆块北缘之间的碰撞[52–53],而晚三叠世以后,受古太平洋板块北西向俯冲影响,中国东部大陆边缘逐渐成为活动大陆边缘[54–58];175～150 Ma是古太平洋板块斜向俯冲占主导的陆内造山时期[59–60],而特提斯构造体系向环太平洋活动边缘构造体系转换的开始时间约为(165±5) Ma[61],这一时期古太平洋板块向欧亚板块俯冲,表现为大规模陆内挤压,在中侏罗世(169～161 Ma)挤压达到高潮,中国东南部发育了大规模的S型过铝一弱过铝质花岗岩和二长花岗岩[62],晚期高分异岩株显示出铝质A型花岗岩的地球化学特征,例如闽西南地区以带内双峰式火山岩、拉斑质玄武岩和同时代A型-I型花岗岩、橄榄辉长岩为特征[63],其中以汤泉富钠埃达克质花岗闪长岩(157 Ma)[64]和高Sr初始值S型花岗岩(才溪岩体和紫金山岩体(164～150 Ma)[5,37,65]为代表,这类花岗岩形成对应于古太平洋俯冲所导致的活动大陆边缘环境,同时表明在164～155 Ma时期,古太平洋板块可能俯冲到紫金山地区[66];在150～100 Ma,中国东南部在古太平洋动力学体系的影响下,造成沿海地区拉张伸展,岩石圈减薄,地幔上隆,地幔来源玄武岩质岩浆底侵[67–72],并使下部地壳部分熔融形成花岗质岩浆,形成了中国东南部地质历史上规模最大的花岗质火山-侵入杂岩[59,67,73,74]。紫金山地区的四坊花岗闪长岩体、罗卜岭花岗闪长斑岩体、浸铜湖石英正长斑岩体即形成于这种拉张伸展构造背景之下[14–15]。

表2 锦溪矿床辉钼矿Re-Os同位素年龄测试结果Table 2 Re-Os isotopic results of the molybdenite form the Jinxi deposit

综合以上研究成果,锦溪花岗闪长岩体不但与四坊花岗闪长岩体和罗卜岭岩花岗闪长斑体成岩成矿年龄基本一致,而且这些岩体的分布主要受 NW向上杭-云霄构造岩浆岩带控制。因此,本文推测平和锦溪花岗闪长岩体可能与紫金山四坊岩体和罗卜岭岩体一样形成于拉张伸展构造背景之下。

4.4 成矿地质意义

东南沿海成矿带构造-岩浆活动以及相应的成矿作用与华夏古陆的形成演化、特提斯向环太平洋构造成矿域的转换及中新生代中国东部大陆边缘构造活动关系密切[47,48,61]。NW 向上杭-云霄成矿带作为东南沿海成矿带的重要组成部分,成矿作用时代的分布与区域构造演化、岩浆作用、沉积作用及变质作用有关(表3),不同类型矿床的分布格局是该成矿带长期构造岩浆成矿演化的结果。上杭-云霄成矿带中生代成矿作用积极响应了中侏罗世(175±5 Ma)以来华夏古陆地质作用的全过程,但仍以燕山期成矿作用占主导地位,目前还未发现印支期前(包括印支期)相应的岩浆-成矿活动。

在(175±5) Ma,古太平洋板块向欧亚大陆斜向俯冲已占主导[52,59,60],引发路块间的强烈挤压,华南发生了大规模花岗质岩浆-成矿活动(170～150 Ma)[54,55,75],作为华南陆块重要组成部分之一的华夏地块也一起转入古太平洋斜向俯冲占主导的陆内造山体系,处于华夏地块西南缘上杭-云霄成矿带也有相应的岩浆-成矿活动。以永定山口钼矿床为例,其辉钼矿样品Re-Os同位素等时线年龄为(165.3±3.5) Ma[76],才溪岩体和紫金山岩体的成岩年龄在(164～150 Ma)之间[5,37,65];晚侏罗世(约150 Ma)以后,中国东南大陆在古太平洋板块斜向俯冲构造背景下,受到华南陆内多块体之间从挤压到伸张一系列构造运动以及深断裂再活化影响,发生大规模陆内火山-侵入活动,其中 135 Ma是个重要的由挤压向伸展的转换时间[57,77,78]。该时期NW向上杭-云霄深断裂再次活化,形成了武平十二排为代表岩浆-热液型钼矿床,其辉钼矿样品 Re-Os同位素等时线年龄出现(150.8±1.3) Ma[22]和(143.9± 2.1) Ma[79]两期成矿年龄,与成矿关系密切的武平十二排岩体晚期黑云母花岗岩锆石 U-Pb谐和年龄为(143.47±0.42)Ma[79],同时紫金山矿田在该时期则发生了金龙桥岩体细粒花岗岩和五龙寺岩体中细粒花岗岩(SHRIMP锆石 U-Pb年龄均为(149±2) Ma[65])的侵入活动。

早白垩世(135 Ma)以后中国东南大陆边缘进入了拉张伸展阶段,形成了一系列北西向的横张断裂和断陷盆地[80],同时出现大规模的岩浆作用与伸展空间而成矿,在 130 Ma左右,北西向上杭-云霄大断裂也从挤压向伸展机制转变[81],该断裂再次活化诱发深部中酸性岩浆向上运移,形成广泛的燕山晚期花岗岩和次火山岩。侵入的花岗岩以含幔源物质较少且为壳源型为主要特征。多阶段侵入的花岗岩不但为成矿提供了充足的热源、流体来源及绝大部分的铁质来源,而且大量含矿流体进入早期沉积的富含Fe、Cu、Mo、Pb、Zn的一套碎屑岩和碳酸盐岩之间的层间断裂,并发生了强烈的夕卡岩化交代作用,形成层控夕卡岩型铁、铅、锌、钼矿床,如永定火山盆地中大排铁多金属矿床((133±2) Ma,辉钼矿Re-Os同位素等时线年龄)以及相应的大排矿区成矿相关的似斑状花岗岩(132.7～132.5 Ma,锆石 U-Pb法谐和年龄)[82],盆地外围的大洋-莒州岩体(LAMC-ICPMS锆石U-Pb年龄为136～125 Ma)正是在挤压向伸展转变的构造背景下(约135 Ma)形成的[82–83];约125 Ma以后,中国东部转入了太平洋板块正向俯冲构造体系[67,69,84–86],其中太平洋板块俯冲方向、俯冲速率、俯冲角度等的变化可能是东南沿海地区能形成晚中生代宽广火山-侵入岩带的重要动力学原因[69,87–89]。该时期中国东南部挤压构造的表现形式以下白垩统地层翘倾和长乐-南澳大型左旋走滑韧性剪切带的形成(121～117 Ma)[58]为代表,而东南沿海结合带的上火山岩系和宁镇山脉则有从挤压到伸展构造的岩浆岩记录[89],该阶段紫金山地区发生较大规模早白垩世火山喷发与岩浆超浅层就位,形成石帽山群下段的英安玢岩及紫金山次火山岩(全岩Rb-Sr等时线年龄为(125～118 Ma)[3,90],平和长崎后地区也有Pb-Zn矿床的成矿记录(Pb-Pb同位素年龄为133 Ma)[2]。

表3 福建上杭-云霄成矿带部分矿床和岩体同位素年龄Table 3 Isotopic ages of some igneous rocks and related ore deposits in the Shanghang-Yunxiao mineralization belt,Fujian Province

早白垩世晚期(约 105 Ma),沿一系列北西向的断裂带和断陷盆地内的较大规模的陆相火山喷发和岩浆-成矿活动,标志着 105 Ma左右华夏板块已全面处于伸展构造环境[91],此时NW向上杭-云霄深断裂广泛分布的基性岩墙群说明该断裂一定程度的联通或该断裂连接了古俯冲带或(和)古拼接带等板块边界构造,而沿深断裂上升侵入的新生钙碱性花岗闪长岩-花岗闪长斑岩浆因则继承了早先形成的与俯冲或和碰撞有关的幔源物质[92],地幔组分逐渐增多,形成了诸如闽西南四坊岩体(104～107 Ma)、罗卜岭岩体(103～105 Ma)和悦洋花岗闪长斑岩体((105.0±7.2) Ma)等,以及各盆地典型火山机构中的英安玢岩(如紫金山英安玢岩锆石 U-Pb法年龄为(105.0±0.7) Ma)、隐爆角砾岩和凝灰熔岩(如包围山凝灰熔岩((104.6±1.1) Ma))等。在岩浆活动过程中,岩浆气液所携带的成矿物质沿次火山通道、NW 向构造带向浅部迁移,在接触带、隐爆角砾岩带、裂隙带等有利的位置沉淀成矿,形成了以平和包围山钼矿床((95.2±1.5) Ma)等受火山机构控制为代表的火山热液型钼矿床,以及紫金山矿田中一套典型的斑岩型-高硫浅成低温热液型成矿系统。同时平和火山盆地以锦溪花岗闪长岩(105～107 Ma)、钟腾花岗闪长斑岩(101.9 Ma)等为代表的成矿母岩就位,形成以锦溪中高温热液脉型铜(钼)矿床((105.7± 1.7) Ma)和钟腾斑岩型铜(钼)矿床((109.0±2.8) Ma)等为代表的矿床。随后92 Ma左右东南大陆岩石圈则进入伸展-地壳减薄(伸展作用加强)阶段[91,93],该时期上杭-云霄深断裂中火山再度活化,发生较大规模的陆相火山喷发和岩浆-成矿活动,形成以紫金山盆地中悦洋低硫型银多金属矿床(冰长石40Ar-39Ar年龄为((91.5±0.4)～(94.7±2.3) Ma)[94]以及浸铜湖斑岩型铜钼矿床(含矿围岩石英闪长斑岩(LA-MC-ICPMS锆石U-Pb年龄为(95.3±0.9) Ma 和(96.7±0.9) Ma)[15]等为代表的矿床,同时形成紫金山石帽山群流纹岩(SHRIMP锆石U-Pb年龄为(94.0±7.7) Ma)[65]、石英斑岩和花岗斑岩(SHRIMP锆石U-Pb年龄分别为((96±2) Ma和(93±2) Ma[65])等无矿次火山岩脉。

综合以上研究资料成果,受古太平洋构板块俯冲作用影响,上杭-云霄构造岩浆成矿带构造环境经历了170～150 Ma持续挤压阶段、150～125 Ma挤压向伸展转换阶段、125～92 Ma持续伸展阶段以及随后92 Ma左右伸展作用加强阶段过程。早白垩世晚期-晚白垩世早期(125～92 Ma)东南沿海地区在古太平洋板块俯冲作用机制影响下,岩石圈拉张伸展构造背景下下地壳部分熔融所诱发NW向上杭-云霄深断裂发生的构造-岩浆-成矿事件是导致锦溪铜(钼)矿床和锦溪花岗闪长岩体形成的主导因素。锦溪铜(钼)矿床与紫金山罗卜岭铜(钼)矿床同处于上杭-云霄构造岩浆成矿带上,与罗卜岭铜(钼)矿床形成背景相似,两者相关岩浆-成矿活动年龄非常相近。因此锦溪地区可能为寻找“罗卜岭”式铜、金多金属矿床新的重要找矿靶区,具有寻找斑岩型铜(钼)矿床的潜力。

5 结 论

锆石 LA-ICP-MS U-Pb定年结果表明,锦溪花岗闪长岩体成岩年龄为105～107 Ma;通过对矿体中辉钼矿的 Re-Os同位素年龄分析,获得其模式年龄为(105.7±1.7) Ma,与岩体成岩年龄非常相近。矿体中辉钼矿的Re含量指示钼矿成矿物质来源于地壳。研究结果指示锦溪铜(钼)矿床在早白垩世晚期至少存在2个阶段较大规模的成矿作用。根据锦溪铜(钼)矿床辉钼矿 Re-Os同位素年龄和成矿围岩锆石LA-ICP-MS U-Pb定年结果,结合中生代上杭-云霄成矿带成岩成矿的大地构造背景,认为早白垩世中-晚期(125～92 Ma)东南沿海地区在古太平洋板块俯冲作用机制影响下,岩石圈拉张伸展构造背景之下导致下地壳部分熔融所诱发上杭-云霄构造岩浆带发生的构造-岩浆-成矿事件是导致锦溪铜(钼)矿床和锦溪花岗闪长岩体形成的主导因素。

辉钼矿样品测试得到了国家地质实验测试中心屈文俊研究员和李超博士的帮助;锆石 U-Pb测年实验过程中得到了天津地质矿产研究所袁海帆老师的热情帮助;匿名审稿人的建设性意见对完善本文很有价值,在此谨致感谢！

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