岩石学报  2021, Vol. 37 Issue (3): 794-804, doi: 10.18654/1000-0569/2021.03.10   PDF    
湘中矿集区杨家山石英脉白钨矿床的白云母40Ar-39Ar和LA-ICP-MS锡石U-Pb年龄及其地质意义
张志远1, 谢桂青2, 李伟3     
1. 河北地质大学, 河北省战略性关键矿产资源重点实验室, 石家庄 050031;
2. 中国地质大学科学研究院, 北京 100083;
3. 中国地质科学院矿产资源研究所, 自然资源部成矿作用与资源评价重点实验室, 北京 100037
摘要: 杨家山中型石英脉型白钨矿床位于华南湘中低温锑金钨矿集区。白钨矿石英脉产于晚泥盆世黑云母二长花岗岩和新元古界板岩中,是全球为数不多的与花岗岩有关的石英脉白钨矿床,其成矿机制还不清楚。本文开展了与白钨矿共生的锡石的LA-ICP-MS微区原位U-Pb同位素测年,获得206Pb/238U加权平均年龄值为410.4±5.7Ma(MSWD=1.5,n=24)。同时,获得了与白钨矿共生的白云母的40Ar-39Ar同位素坪年龄为395.4±3.2Ma,等时线年龄398.2±4.4Ma,这些定年结果暗示杨家山钨矿床形成于晚泥盆世,与黑云母二长花岗岩的成岩时代在误差范围内一致。结合前人的年代成果,暗示湘中矿集区发育一期晚泥盆世的钨金成矿事件。通过以上研究,不仅能深化湘中矿集区钨金矿床的成矿规律认识,而且可以为找矿勘查取得突破提供重要的理论支撑。
关键词: 白云母40Ar-39Ar同位素年龄    锡石LA-ICP-MS U-Pb年龄    杨家山钨矿床    湘中矿集区    华南    
Muscovite 40Ar-39Ar and LA-ICP-MS cassiterite U-Pb dating of Yangjiashan quartz scheelite deposit, Xiangzhong ore district and its geological significance
ZHANG ZhiYuan1, XIE GuiQing2, LI Wei3     
1. Hebei Key Laboratory of Strategic Critical Mineral Resources, Hebei GEO University, Shijiazhuang 050031, China;
2. Institute of Earth Sciences, China University of Geosciences, Beijing 100083;
3. MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
Abstract: Yangjiashan scheelite deposit is a quartz vein type one hosted in Late Devonian biotite monzogranite that intruded in the surrounding Neoproterozoic slate in the Xiangzhong low-temperature Sb-Au-W ore district, South China. It is one of the few granite-related quartz vein scheelite deposits in the world, and its ore-forming mechanism is still unclear. This paper reported the U-Pb isotope dating of cassiterite by the method of in-situ LA-ICP-MS from Yangjigshan tungsten deposit. Cassiterite coexisting with scheelite yields a weighted mean 206Pb/238U age of 410.4±5.7Ma(MSWD=1.5, n=24). Muscovite associated with scheelite yielded a 40Ar-39Ar plateau age of 395.4±3.2Ma, with an isochron age of 398.2±4.4Ma. We infer that Yangjiashan tungsten deposit was formed in the Late Devonian, which is consistent with the zircon U-Pb ages of the biotite monzogranite within analytical uncertainties. Combined with previous ages in the literature, it is suggested that a Late Devonian W-Au metallogenic event developed in the Xiangzhong ore district. The above research results not only deepen the understanding of metallogenic theory of tungsten gold deposits in Xiangzhong ore district, but also provide important theoretical support for the breakthrough of prospecting and exploration.
Key words: Muscovite 40Ar-39Ar isotope age    Cassiterite LA-ICP-MS U-Pb age    Yangjiashan tungsten deposit    Xiangzhong ore district    South China    

锡石含有较高的U含量和较低的普通Pb含量,并且锡石U-Pb同位素体系封闭温度高,不易受后期热液蚀变的影响,是较为理想的U-Pb定年矿物之一(Gulson and Jones, 1992Yuan et al., 2008)。因此锡石U-Pb定年可以用来约束钨锡稀有金属矿床的形成时代(Yuan et al., 2011Zhang et al., 2015, 2017a, b)。云母等含钾矿物的K-Ar和Ar-Ar法测定的年龄可能代表了岩浆结晶结束的年龄或后期多期次热事件的年龄(Li et al., 2014Yuan et al., 2018),也是确定热液矿床成矿年代的重要方法之一(Selby et al., 2002Xie et al., 2011)。两种不同测年方法给出的结果在一些矿床中可以得到相互的验证(Zhang et al., 2014Zhang et al., 2015)。

全球钨矿床主要类型包括产于钙质岩石中的矽卡岩白钨矿床和赋存于含铁片岩和角岩中的石英脉黑钨矿床(Lecumberri-Sanchez et al., 2017)。近年来,全球不断发现产于非钙质的地层但与花岗岩有成因联系的钨矿床中,白钨矿是唯一的含钨矿物(Wood and Samson, 2000),但目前很少有学者关注产于非钙质岩石中的石英脉型白钨矿床的成矿机制。

湘中矿集区是我国西南地区大面积低温成矿域(<200~250℃)的重要组成部分,发育大量的Au-Sb±W元素组合的矿床,是全球最大的锑金矿集区(Hu et al., 2017a, b)。这些锑金矿床在矿区范围内未发现大规模的侵入岩,可见少量的中酸性脉岩;而盆地边缘出露一定规模的花岗质岩体,锑金成矿作用与岩浆活动的关系还存在争议(Peng et al., 2003a, bPeng and Frei, 2004Zhu and Peng, 2015)。此外,湘中矿集区还发育许多石英脉型和矽卡岩型白钨矿床(图 1)。杨家山钨矿床是石英脉型白钨矿床的典型代表,矿体呈脉状产于加里东期黑云母二长花岗岩和新元古界板岩中(Hsu et al., 1959)。我国华南地区有许多与中生代花岗岩类有关的含钨矿床(Yuan et al., 2018, 2019Mao et al., 2019, 2021),但是对于与加里东期花岗岩有关的钨成矿作用关注相对较少。近年来,在华南大瑶山和苗儿山-越城岭地区发现和识别出少量与加里东期的花岗岩有关的钨矿床(华仁民等,2013Dang et al., 2020Zhu et al., 2020陈懋弘等,2020)。

图 1 湘中矿集区区域地质图(据Xie et al., 2019黄建中等,2020修改) Fig. 1 Regional geological map of the Xiangzhong ore district, South China(modified after Xie et al., 2019; Huang et al., 2020)

本文在对杨家山钨矿床详细地野外地质研究基础上,利用LA-ICP-MS U-Pb同位素定年和40Ar-39Ar阶段加热同位素测年方法分别对与白钨矿密切共生的锡石和白云母开展精确的年代研究,来厘定杨家山钨矿床的成矿时代。综合湘中矿集区加里东期岩体的年代资料和金矿床的成矿时代数据,以期对湘中矿集区加里东期的钨矿床和金矿床成矿规律有更明确的认识。

1 区域地质背景

华南地区分为扬子地块和华夏地块(Hu et al., 2017b),湘中矿集区位于扬子和华夏地块之间的弧形构造带内,包括西侧的雪峰山地区和东侧的邵阳盆地(图 1)。区域地层具有明显的双层结构:元古界基底和古生界至中生界沉积盖层(马东升等,2002)。元古界地层包括中元古界冷家溪群和新元古界板溪群低绿片岩相的变质碎屑岩,这些变质岩是在1000~800Ma期间经过区域变质作用形成的(湖南省地质矿产局,1988);元古界碎屑岩的层序为砾岩、砂岩、粉砂层、页岩、燧石并夹有少量碳酸盐岩。古生界至中生界沉积盖层包括寒武系到奥陶系的浅海相碳酸盐岩和硅质碎屑岩,志留系页岩和砂岩,泥盆系到二叠系的灰岩和晚三叠统到白垩系的陆相沉积岩序列(Tang et al., 2014)。湘中矿集区岩浆活动具有多期多阶段的特点,形成复式岩体,其中以三叠纪岩体分布最为广泛(Wang et al., 2007陈卫峰等,2007Fu et al., 2015);泥盆纪岩体为黑云母二长花岗岩、黑云母花岗岩、角闪石花岗岩和角闪石黑云母花岗闪长岩的组合,主要分布于白马山复式岩体中(图 1Chu et al., 2012杨俊等,2015Xie et al., 2019)。

湘中矿集区作为我国最重要的锑矿产地,已发现锑金矿床/矿点170余处(Hu et al., 2017a),主要分布在元古界至寒武系的碎屑岩和泥盆系-石炭系的碳酸盐岩中。其中赋存在元古界碎屑岩中的锑金矿床中都发育有白钨矿化,有些甚至达到工业开采品位和规模,如渣滓溪锑钨矿床(Zhao et al., 2017)和沃溪金锑钨矿床(Zhu and Peng, 2015)。近些年随着找矿勘查的不断深入,湘中矿集区内陆续发现和探明了一批石英脉白钨矿床,如杨家山、木瓜园、沙溪、中村和牛角界钨矿床(孔令兵等,2014苏康明等,2016Li et al., 2018Xie et al., 2019),以及大溶溪和曹家坝矽卡岩型白钨矿床(图 1张龙升,2013张志远等,2016)。

2 矿床地质特征

杨家山钨矿床最早由湖南省地矿局在20世纪50年代发现并进行勘查,目前已探明WO3资源量3.86万吨,平均品位0.70%,矿区以开采钨为主,并伴有铜。土壤地球化学数据表明该矿床的含钨石英脉型矿体中还有很大的找矿潜力(潘飞等,2016)。

杨家山矿区内出露的地层为新元古界高涧群漠滨组和震旦系下统江口组(图 2a),其中漠滨组地层岩性为板岩、片岩和杂砂岩,厚度为1000m;江口组地层岩性为含砾砂质或粉砂质板岩、杂砂岩,赋矿的地层岩性为漠滨组砂质板岩(图 2)。矿区内广泛出露的侵入岩主要是黑云母二长花岗岩(图 2),代表了白马山复式岩体的最北段(图 1),锆石U-Pb年代工作显示,其主要形成于406.6±2.8Ma(Xie et al., 2019)。

图 2 杨家山钨矿床地质简图(a)和A-B勘探线剖面图(b)(据Xie et al., 2019修改) Fig. 2 Simplified geological map(a) and geological section along A-B exploration line (b) of the Yangjiashan tungsten deposit (modified after Xie et al., 2019)

杨家山钨矿床包括大一和九条槽两个矿段(图 2a),共包括18个北西向石英脉群,赋存于花岗岩或板岩中,其中一些矿脉群切割了两种不同岩性岩石的接触带(Hsu et al., 1959)。杨家山钨矿具有工业价值的矿脉为6号、8号和9号脉,其中以9号脉规模最大:走向长大于2000m,走向北西,倾向南西,倾角65°~80°,倾向延伸300m,破碎带宽1.0~1.2m,由石英脉、构造透镜体、碎裂岩、断层泥等组成,钨(WO3)品位0.08%~0.40%,平均品位0.20%。

石英白钨矿硫化物脉广泛发育于侵入岩和板岩中(图 3a, b),局部可以见到云英岩被石英白钨矿硫化物脉切穿(图 3c)。石英白钨矿硫化物脉宽度为10~80cm,其中金属矿物包括白钨矿、黄铁矿、黄铜矿、毒砂、黄铁矿、锡石和少量辉钼矿,非金属矿物包括石英、方解石、萤石、绿帘石、电气石。云英岩型矿化包括石英、白云母、白钨矿、毒砂和黄铜矿(图 3d)。白钨矿以集合体或者浸染状的形式产出,其中晶形较好的白钨矿颗粒长度可以达到1.5cm(图 3e-g),主要与石英、黄铜矿、白云母、锡石矿物共生(图 3b-g图 4a-d),还与少量的辉钼矿共生(图 4e)。黄铜矿在局部地方与磁黄铁矿、方铅矿、闪锌矿、辉铋矿和自然铋共生(图 4g-i)。根据矿脉穿插关系以及矿物共生组合关系,将整个成矿过程划分为两个阶段:(1)云英岩阶段,白钨矿与石英和白云母共生;(2)石英-白钨矿-硫化物阶段,钨主要形成于该阶段,此外该阶段还有少量的方解石、绿帘石、萤石、电气石与石英、白钨矿和黄铜矿共生(图 3h-i图 4f)。

图 3 杨家山钨矿床不同阶段石英脉体及典型矿物组合照片 (a)板岩中的石英白钨矿脉;(b)黑云母二长花岗岩中的石英、白钨矿、黄铜矿脉;(c)石英、白钨矿、黄铜矿脉切穿石英、白云母、白钨矿化云英岩;(d)云英岩型矿石,石英、白云母、白钨矿、毒砂、黄铜矿矿物组合;(e)石英白钨矿脉中晶型较好的白钨矿颗粒;(f)石英脉中白钨矿与锡石和黄铜矿共生;(g)石英脉中白钨矿与锡石共生;(h)板岩中的石英、白钨矿、方解石、萤石矿物组合;(i)黑云母二长花岗岩中的石英绿帘石矿物组合. Sch-白钨矿;Ccp-黄铜矿;Qz-石英;Ms-白云母;Apy-毒砂;Cst-锡石;Fl-萤石;Cal-方解石;Ep-绿帘石 Fig. 3 Photographs of quartz veins of different stages and typical assemblage from the Yangjiashan tungsten deposit (a)quartz+scheelite veins hosted in slate; (b)quartz+scheelite+chalcopyrite vein hosted in biotite monzogranite; (c)quartz+scheelite+chalcopyrite vein crosscutting quartz+muscovite+scheelite greisen; (d)greisen ore with a quartz+muscovite+scheelite+arsenopyrite+chalcopyrite assemblage; (e)quartz-scheelite ore showing coarse scheelite crystals; (f)scheelite coexisting with quartz, chalcopyrite, and cassiterite; (g)scheelite coexisting with quartz and cassiterite; (h)calcite+fluorite+quartz+scheelite vein hosted in slate; (i)quartz+epidote vein in biotite monzogranite. Sch-scheelite; Ccp-chalcopyrite; Qz-quartz; Ms-muscovite; Apy-arsenopyrite; Cst-cassiterite; Fl-fluorite; Cal-calcite; Ep-epidote

图 4 杨家山钨矿床典型矿物组合显微照片 (a)白钨矿与锡石共生(正交偏光);(b)白钨矿与锡石共生(单偏光);(c)白钨矿与白云母共生(正交偏光);(d)白钨矿与白云母和黄铜矿共生(反射光);(e)白钨矿与辉钼矿和石英共生(反射光);(f)石英脉中的电气石(单偏光);(g)黄铜矿与磁黄铁矿、闪锌矿、辉铋矿和自然铋共生(BSE照片);(h)黄铜矿与方铅矿共生(反射光);(i)黄铜矿与磁黄铁矿共生(反射光). Mol-辉钼矿;Tur-电气石;Bis-辉铋矿;Bi-自然铋;Sp-闪锌矿;Po-磁黄铁矿;Gn-方铅矿 Fig. 4 Photomicrographs showing relationships of minerals from the Yangjiashan tungsten deposit (a)scheelite coexisting with cassiterite(perpendicular polarized light); (b)scheelite coexisting with cassiterite(plane polarized light); (c)scheelite coexisting with muscovite(perpendicular polarized light); (d)scheelite coexisting with muscovite and chalcopyrite(reflected light); (e)scheelite coexisting with molybdenite and quartz; (f)tourmaline in quartz veins; (g)chalcopyrite coexisting with sphalerite, pyrrhotite, bismuthinite and native Bi(BSE image); (h)chalcopyrite coexisting with galena(reflected light); (i)chalcopyrite coexisting with pyrrhotite(reflected light). Mol-molybdenite; Tur-tourmaline; Bis-bismuthinite; Bi-native Bi; Sp-sphalerite; Po-pyrrhotite; Gn-galena
3 样品采集及分析方法 3.1 样品采集及制备

用于年代研究的白云母(YJS-25)和锡石(YJS-21)样品都采自杨家山钨矿床大一矿段800m中段的9号脉中。白云母样品经过破碎、筛选至40~60目,在双目镜下挑选,使白云母的纯度大于99%,用超声波洗净。锡石样品采用常规重选法粗选,然后在双目镜下挑选出粒度较大、透明度较好的锡石颗粒,挑纯至99%以上。在北京锆年领航科技有限公司进行锡石制靶,并进行了透射光、反射光和阴极发光(CL)图像的拍摄。

3.2 锡石U-Pb同位素分析

根据获得的锡石反射光和透射光图像,选择锡石颗粒的合适区域,避开包裹体和裂纹,以减少普通铅的影响。锡石U-Pb同位素年代分析在中国科学院广州地球化学研究所矿物学与成矿学重点实验室完成,所用仪器为美国Resonetics公司生产的Resolution S-155激光剥蚀系统和Agilent 7500 ICP-MS联机。分析过程中,采用He作为剥蚀物质的载气。实验采用标准锡石AY-4(158.2±0.4Ma;Yuan et al., 2011)作为测年外标,所测元素激光斑束直径为74μm,频率为6Hz,能量密度4J/cm2。具体实验分析方法详见(Li et al., 2016Zhang et al., 2017b)。锡石的年龄图和年龄采用ISOPLOT 4.15进行数据处理(Ludwig,2012)。

3.3 白云母40Ar-39Ar同位素分析

洗净后的样品被封进石英管中,然后在核反应堆进行快中子辐照。本次样品的辐照工作是在中国原子能科学研究院的“游泳池堆”中进行的。使用B4孔道,照射时间为24小时,积分中子通量为2.65×1013n·cm2s-1;同时接受辐照的还有监测中子通量的BSP-1角闪石国际标样。样品重量W=31.9mg,辐照参数J=0.002655±0.0000133。辐照后的样品放置3个月以上,当放射性剂量降至安全操作范围时,进行阶段升温测试工作。本次研究中,在对温控表和炉内温度进行校正后,所采用温度范围为710~1400℃,每个温度段释放的气体经过冷阱(干冰加酒精,-80℃)、一级锆铝泵(加热状态)、二级锆铝泵(一个为室温状态,一个为加热状态)纯化后,进入到质谱中进行40Ar到36Ar同位素的分析,质谱分析是在核工业北京地质研究院分析测试中心Thermo Fisher Helix SFT惰性气体同位素质谱仪上进行的。所有的数据都经过质量歧视校正、大气氩校正、空白校正和校正因子校正。中子辐照过程中所产生的干扰同位素校正系数通过分析辐照过的K2SO4和CaF2来获得,其值为(36Ar/37Ar)Ca=0.000278,(39Ar/37Ar)Ca=0.000852,(40Ar/39Ar)K=0.001147,仪器所得到的同位素强度采用Koppers编写的Ar-Ar数据处理软件ArArCALC Version 2.40进行Ar-Ar年龄计算(Koppers,2002),得到坪年龄、等时线年龄、反等时线年龄等相关年龄信息。详细实验流程见文献张佳等(2014)

4 测试结果 4.1 锡石U-Pb测年

与白钨矿共生的锡石颗粒(YJS-21)宽约600μm,长约1000μm,呈暗棕色,CL显示具有明显的振荡环带(图 5a)。在不同锡石颗粒上测定了25个分析点,挑选测试点具有很好的震荡环带结构,并且没有矿物和流体包裹体的干扰。其中,207Pb/235U的比值变化范围为0.4321~0.6199,206Pb/238U的比值变化范围为0.0623~0.0706(表 1),得到207Pb/235U-206Pb/238U谐和年龄为410.2±2.3Ma(图 5a),206Pb/238U加权平均年龄值为410.4±5.7Ma(MSWD=1.5,n=24;图 5b)。

图 5 杨家山钨矿床中锡石阴极发光(CL)图像和U-Pb年龄谐和图(a)及年龄加权平均值直方图(b) 白色实线圆表示LA-ICP-MS U-Pb年龄分析点位置,403.9±7.0Ma表示测点206Pb/238U年龄及误差 Fig. 5 Cathodoluminescence(CL)images and U-Pb concordia diagram(a) and age histogram(b)of cassiterite from the Yangjiashan tungsten deposit The white solid circles indicate the location of LA-ICP-MS U-Pb analysis and 403.9±7.0Ma represents that the 206Pb/238U age is 403.9Ma with an error 7.0Ma

表 1 杨家山钨矿床LA-ICP-MS锡石原位U-Pb测年结果 Table 1 In-suit LA-ICP-MS U-Pb dating results of cassiterite from the Yangjiashan tungsten deposit
4.2 白云母40Ar-39Ar同位素测年

与白钨矿共生的白云母40Ar-39Ar阶段升温测年数据见表 2,相应的坪年龄谱和等时线年龄如图 6。在710~1400℃温度范围内,对杨家山钨矿的白云母进行了11个阶段的释热分析,其中860~1400℃构成的坪年龄为395.4±3.2Ma(图 6a),对应了93.98%的39Ar释放量,相应的39Ar/36Ar-40Ar/36Ar等时线年龄为398.2±4.4Ma(图 6b),与坪年龄在误差范围内一致。

表 2 杨家山钨矿床白云母40Ar-39Ar阶段升温测年数据结果 Table 2 40Ar-39Ar stepwise heating analytical data of muscovite from the Yangjiashan tungsten deposit

图 6 杨家山钨矿床白云母40Ar-39Ar坪年龄图谱(a)和等时线年龄图解(b) Fig. 6 40Ar-39Ar spectrum age(a) and isochron age(b)for muscovite from the Yangjiashan tungsten deposit
5 讨论 5.1 成矿时代

锡石U-Pb体系的封闭温度较高,1mm级的锡石颗粒中Pb的封闭温度为860℃(张东亮等,2011)。本次分析所用锡石颗粒明显大于1mm,此外杨家山钨矿床白钨矿中流体包裹体测温数据表明其成矿温度为200~300℃(未发表数据),低于锡石U-Pb体系的封闭温度。因此本次测试所获得的锡石U-Pb定年结果可以代表其结晶年龄。

本次获得了锡石207Pb/235U-206Pb/238U谐和年龄为410.2±2.3Ma,206Pb/238U加权平均年龄为410.4±5.7Ma,二者在误差范围内一致,并且与前人获得的锡石206Pb/238U加权平均年龄(409.8±5.9Ma;Xie et al., 2019)相吻合。结合本次用于U-Pb同位素测年的锡石样品均采集于杨家山矿区石英-白钨矿-硫化物阶段的矿脉中,为该矿床的主要矿石类型,锡石与白钨矿密切共生(图 3f图 4a-b),因而锡石207Pb/235U-206Pb/238U谐和年龄(410.2±2.3Ma)可以直接代表该矿床的形成年龄。

图 6可以看出,白云母的40Ar-39Ar坪年龄(395.4±3.2Ma)和相应的等时线年龄(398.2±4.4Ma)在误差范围内一致,表明白云母定年结果可靠。同样,由于杨家山钨矿床的形成温度(200~300℃)低于白云母的封闭温度(350±50℃;Chiaradia et al., 2013),因此本次测试所获得的白云母40Ar-39Ar定年结果可以代表其结晶年龄。白云母的40Ar-39Ar等时线年龄在误差范围内晚于锡石的206Pb/238U加权平均年龄和谐和年龄,可能是由于白云母的封闭温度低于锡石的封闭温度所致(Chiaradia et al., 2013)。本次用于40Ar-39Ar同位素测年的白云母采集于杨家山钨矿床的云英岩化阶段,白云母与白钨矿密切共生(图 4c, d)。结合锡石U-Pb和白云母的40Ar-39Ar同位素年龄,限定杨家山钨矿床的成矿时代为晚泥盆世。

5.2 成矿与成岩的关系

本次工作获得的杨家山钨矿床的成矿时代(410.2±2.3Ma)与前人获得的矿区内黑云母二长花岗岩锆石的207Pb/235U-206Pb/238U谐和年龄(406.6±2.8Ma;Xie et al., 2019)在误差范围内一致,并且与前人获得的白马山复式岩体黑云母花岗岩、角闪石花岗岩和角闪石黑云母花岗闪长岩的锆石U-Pb年龄(406.6±2.8Ma~416±4Ma;图 1表 3)在误差范围内基本一致。

表 3 湘中矿集区加里东期成岩/成矿年龄 Table 3 Petrogenetic and metallogenic ages of Caledonian igneous rocks and deposits in the Xiangzhong ore district

前人对杨家山钨矿床流体包裹体研究表明,成矿流体温度变化较小,结合H-O同位素研究,认为成矿流体以岩浆流体为主,晚期有少量大气降水的加入(Xie et al., 2019);硫化物的δ34S值(-2.9‰~-0.7‰;Xie et al., 2019)与岩浆硫(+1.0± 6.1‰;Seal,2006)的范围一致,显示硫主要来源于岩浆体系。含钙的砂质岩石和钙质斜长石的绢云母蚀变在岩浆-热液体系中为白钨矿的形成提供了钙,形成了石英白钨矿脉(Xie et al., 2019)。

湘中地区加里东期花岗质岩浆活动形成于峰期变形(410Ma)之后挤压减弱、应力松弛的后碰撞构造环境,与之相伴发生了局部的内生热液成矿作用,因此,发育了与花岗质岩浆活动相关的钨矿化(柏道远等,2020)。综上所述,本文研究表明杨家山是与区内发育的同期的白马山岩体的黑云母二长花岗岩有成因联系的石英脉型白钨矿床。

5.3 湘中矿集区加里东期W-Au成矿作用

湘中矿集区西侧的雪峰山地区是华南最重要的金成矿区带之一,分布着一系列的金矿床(点),是湖南省最重要的黄金生产基地之一(Deng et al., 2020),其中以沃溪大型金锑钨矿床为代表(彭建堂,1999),该带目前仍有良好的找矿前景(黄建中等,2020)。多数金矿赋存于前寒武系地层中,特别是冷家溪群和板溪群中,赋矿围岩富含火山凝灰质物质和原生沉积的草莓状黄铁矿(彭建堂,1999)。由于区域内岩浆活动微弱,绝大多数金矿的矿区及其外围并无岩浆岩出露,物探资料显示大部分地段重磁平缓,并无隐伏岩体存在,岩浆岩提供成矿物质的可能性不大(彭建堂,1999)。但是,饶家荣等(1999)认为湘中地区矿床深部存在隐伏岩体,而且还有学者认为岩浆活动及地热升温促进了矿源岩石中的金活化,并在断裂带中沉淀形成金矿床(王秀璋等,1999)。因此,也不能排除岩体为成矿提供成矿流体或者能量的可能性。雪峰山地区的金矿床成矿时代主要为加里东期(表 3),与白马山复式岩体内的加里东期岩体和杨家山钨矿床成矿时代具有较好地一致性,暗示加里东期钨矿床和金矿床为同一成矿事件的产物,其是否具有成因联系还需要进一步研究。

6 结论

(1) 杨家山钨矿床LA-ICP-MS锡石的206Pb/238U加权平均年龄为410.4±5.7Ma(MSWD=1.5,n=24),白云母40Ar-39Ar等时线年龄(398.2±4.4Ma),二者在误差范围内基本一致,锡石和白云母都与白钨矿密切共生,限定杨家山钨矿床的成矿时代为晚泥盆世。

(2) 基于前人对杨家山矿区内白马山岩体的黑云母二长花岗岩的成岩年龄和同位素数据,本文认为杨家山是与区内发育的同期岩浆侵入活动有成因联系的石英脉型白钨矿床。

(3) 杨家山钨矿床与雪峰山地区发育的加里东期金矿床成矿时代基本一致,暗示它们为晚泥盆世同一成矿事件的产物,但是其是否具有成因联系还需要进一步研究。

致谢      野外工作期间得到了湖南省地质矿产勘查开发局418队的支持与帮助;LA-ICP-MS锡石U-Pb测试过程中得到了南京大学章荣清副教授的热心帮助和指导;白云母40Ar-39Ar测试得到了核工业北京地质研究院分析测试中心张佳工程师的全力支持;两位审稿专家和本刊主编提出了宝贵的修改意见,让本文质量有了很大提高;在此一并表示感谢!

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