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物探与化探, 2020, 44(5): 1103-1115 doi: 10.11720/wtyht.2020.1554

地质调查·资源勘查

临沧地区富铀花岗岩体地球化学特征及其地质意义

田建民,1, 徐争启,1,2, 尹明辉1, 李涛1, 孙康3

1.成都理工大学 地球科学学院,四川 成都 610059

2.地学核技术四川省重点实验室,四川 成都 610059

3.机械工业第六设计研究院有限公司,河南 郑州 450000

Geochemical characteristics and geological significance of uranium-rich granites in Lincang area

TIAN Jian-Min,1, XU Zheng-Qi,1,2, YIN Ming-Hui1, LI Tao1, SUN Kang3

1.College of Geoscience, Chengdu University of Technology, Chengdu 610059, China

2.Sichuan Key Laboratory of Geoscience and Nuclear Technology, Chengdu 610059, China

3.Mechanical Design Sixth Design and Research Institute Co., Ltd., Zhengzhou 450000, China

通讯作者: 徐争启(1975-),男,教授,博士生导师,长期从事铀矿地质及地球化学教学和科研工作。Email:547510779@qq.com

责任编辑: 蒋实

收稿日期: 2019-11-27   修回日期: 2020-06-17   网络出版日期: 2020-10-20

基金资助: 国家自然科学基金项目“康滇地轴粗粒晶质铀矿标型特征及形成机理”.  41872079
四川省应用基础研究项目“四川冕宁—攀枝花地区新元古代铀成矿作用及成矿机理研究”.  2020YJ0361
中国核工业地质局科研项目“康滇地轴新元古代铀成矿关键控矿因素研究”.  201807

Received: 2019-11-27   Revised: 2020-06-17   Online: 2020-10-20

作者简介 About authors

田建民(1996-),男,硕士研究生,地质学专业。Email: 1067239658@qq.com

摘要

针对双江701和凤庆901铀矿点,对临沧地区富铀花岗岩体进行岩相学、地球化学和U-Pb定年研究,旨在探讨岩石成因类型、成岩构造背景与铀成矿的关系。结果表明:临沧富铀花岗岩体具有高硅(最高达90.11%,平均含量71.6%)、富碱、高钾、高钙特征,属于强过铝质、高钾钙碱性系列岩石。U-Pb锆石定年结论为214±12 Ma,表明临沧富铀花岗岩体形成于印支晚期。稀土元素呈微右倾“V”字形,轻、重稀土分馏明显(w(LREE)/w(HREE)平均为7.26),Eu负异常明显(δEu=0.28~0.49),相对富集Rb、U、Th,相对亏损Ba、Nb、Sr、Ti和Eu。综合分析认为,临沧富铀花岗岩属于分异明显的S型花岗岩,源于上地壳物质熔融,形成于碰撞后期造山环境之中,属于S澜沧江碰撞带有关的同碰撞花岗岩,其高含量的铀为铀成矿过程提供了部分铀源,铀元素经过淋滤作用被搬运到断层破碎带附近富集形成花岗岩型铀矿。

关键词： 临沧 ; 富铀花岗岩 ; 地球化学特征

Abstract

Petrographic and geochemical study and U-Pb dating of uranium-rich granite bodies in Lincang area were carried out for Shuangjiang 701 and Fengqing 901 uranium mines, with the purpose of exploring the relationship between petrogenesis type, diagenetic tectonic background and uranium mineralization. The results show that Lincang uranium-rich granite body has high Si (up to 90.11%, averaging 71.6%), rich alkali, high potassium and high Ca, thus belonging to high Al and high potassium calcium alkaline series rocks. U-Pb zircon dating yielded 214±12 Ma, and thus Lincang uranium-rich granite body was formed in Late Indochina. Rare earth elements show a slightly rightward "V" shape, with obvious fractionation of light and heavy rare earth elements (w(LREE)/w(HREE) 7.26 on average), obvious negative Eu anomaly (δEu=0.28 ~ 0.49), relative enrichment of Rb, U and Th, and relative loss of Ba, Nb, Sr, Ti and Eu. Comprehensive analysis shows that Lincang uranium-rich granite belongs to S-type granite with obvious differentiation, which must have originated from melting of upper crust material and was formed in a mountain-building and rift environment in late collision, belonging to the simultaneous collision of granite related to the Lancang River collision zone. Its high content of uranium provided part of the uranium source for the uranium mineralization process. Uranium elements were transported to the vicinity of the fault fracture zone and was enriched to form granite-type uranium deposits by leaching.

Keywords： Lincang ; uranium-rich granite ; geochemical characteristics

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本文引用格式

田建民, 徐争启, 尹明辉, 李涛, 孙康. 临沧地区富铀花岗岩体地球化学特征及其地质意义. 物探与化探[J], 2020, 44(5): 1103-1115 doi:10.11720/wtyht.2020.1554

TIAN Jian-Min, XU Zheng-Qi, YIN Ming-Hui, LI Tao, SUN Kang. Geochemical characteristics and geological significance of uranium-rich granites in Lincang area. Geophysical and Geochemical Exploration[J], 2020, 44(5): 1103-1115 doi:10.11720/wtyht.2020.1554

0 引言

花岗岩型铀矿作为我国分布最广泛的铀矿之一,因其重要的战略地位一直被学者所关注。近几十年来国内外学者对我国华南地区印支—燕山期花岗岩型铀矿地质特征及其成矿规律做了大量的科研工作,取得了重大进展[1,2,3,4,5,6,7,8],确定了我国最老产铀花岗岩体(2 750~2 530 Ma)是连山关岩体,铀矿年龄为 1 832 Ma[4],提出了华南花岗岩型铀矿成矿来源多期次,构造运动主导的热液—岩浆—构造共同成矿模式[9]。

“三江”地区位于我国青藏高原的东缘,属于特提斯区域的东块,历来是学者研究特提斯构造演化的重要区域[10,11]。临沧花岗岩位于“三江”中南段,是滇西地区出露面积最广的复式岩基,其完整地记录了古生代以来的洋—陆碰撞事件,是区域地质演化历史重现的很好的研究对象[12,13]。前人对临沧花岗岩做了大量研究,彭头平认为临沧地区的花岗岩体岩基主要是为印支期的二长花岗岩,燕山期侏罗纪和白垩纪产出的花岗岩岩体主要呈较小的岩株状产出[14]。在花岗岩成因上,认为临沧花岗岩主体岩性是黑云母二长花岗岩,其次为黑云母花岗闪长岩;成因类型上临沧花岗岩属于沉积物源熔融形成的S型花岗岩[15,16]。廖世勇认为碱长花岗岩石是临沧花岗岩岩基的重要组成部分,并经历了高度的结晶分异作用[17]。近几年来,随着研究工作的深入和1:5万区域地质调查工作的完成,对临沧花岗岩取得大量共同认识:临沧花岗岩岩基主体是黑云母二长花岗岩,伴有部分花岗闪长岩产出,属于沉积岩熔融形成于碰撞环境之下[12,16-17,19]。但前人对云南“三江”地区富铀花岗岩体的研究集中在铀矿化因素和成矿前景方面[18,19],缺少对临沧富铀花岗岩地球化学特征和年代学特征研究。笔者在此基础之上,以临沧地区富铀花岗岩体(凤庆901铀矿化点和双江701铀矿化点)为研究对象,采用LA-MC-ICP-MS锆石U-Pb原位定年和主微量元素分析,探讨临沧富铀花岗岩铀成矿条件和构造动力学背景,分析临沧地区花岗岩型铀矿找矿潜力。

1 区域地质背景

“三江”地区在大地构造位置上属于东特提斯构造域,包含了以哀牢山—红河断裂为东界,以实皆断裂为西界的区域,东部为扬子地块,西部为西缅地块,区内存在不同时代的断裂构造,是一个复杂的造山带[11](图1)。由于长期受到印度洋板块、太平洋板块和欧亚板块的共同作用,“三江”花岗岩成为了中外地质矿床研究者的关注重点。本次研究的富铀花岗岩体是指花岗岩体平均铀含量高于世界酸性火山岩平均值3倍以上的花岗岩[20],分布在临沧江断裂带附近,主要的赋矿围岩为临沧花岗岩基中段的中粒黑云母二长花岗岩,其次是二云母花岗岩、白云母花岗岩和部分花岗闪长岩。矿化点受断裂带控制,主要发育NW向、NE向两组断裂构造,其中以NW向构造规模最大,发育最广,与铀矿化关系密切。

图1

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图1   三江地区地质简图(据参考文献[21]修改)

a—构造位置图;b—研究区地质简图

Fig.1   Geological map of Sanjiang area(revised according to reference [21])

a—structure location map;b—geological sketch of the study area


2 样品采集与处理

本次采样点主要位于凤庆901和双江701铀矿化点及其围岩花岗岩中(图1),共采集样品13件,其中双江701铀矿化点5件,凤庆901铀矿化点8件。样品采集后送往成都理工大学地球化学实验室进行处理,将选取好的全岩石样品送往核工业北京地质研究院分析测试中心测试。

常量元素分析采用X射线荧光光谱法(XRF)和化学分析法(CA),CA仅用于测试样品中的氧化亚铁含量,测定范围:>0.5%;XRF所用仪器为PW2404X荧光光谱仪,工作电压50 kV,电流50 mA,测定精度0.01%,分析误差<5%,实验过程遵循GB/T 14506.28-93。微量元素采用电感耦合等离子质谱法(ICP-MS)分析测定,提前将样品磨制200目制成溶液,在Finnigan MAT制造的HR-ICP-MS仪器上分析测定,实验过程遵循GB/T 14506.28-93,工作温度20 ℃,相对湿度30%,微量元素含量大于10×10-6时误差小于5%,含量小于10×10-6时,相对误差小于10%。锆石测年选取凤庆901铀矿化点的黑云母二长花岗岩,编号为YN057。先将样品破碎、清洗、烘干和筛选,再采用磁选和重液分类技术将锆石选出,然后在双目镜下挑选。锆石制靶后,送往南京聚谱分析检测公司进行LA-MC-ICP-MS锆石U-Pb原位定年。样品元素含量数据处理使用GeoKit[22]、Isoplot和Excel软件。

3 结果分析

3.1 岩相学特征

本次采集样品的岩性主要是黑云二长花岗岩和黑云母花岗闪长岩,典型样品的手标本照片和镜下照片见图2。黑云母二长花岗岩:中粗粒,浅肉红色—灰白色,色率10%~20%,具有中粗粒花岗结构,块状构造。矿物主要有钾长石、斜长石、石英和少量的黑云母,副矿物主要有磷灰石和锆石。矿物颗粒自形程度较高,粒径大小不一,最大可达5 mm,石英以它形不等粒粒状、集合体形式充填于长石间隙中,黑云母和长石呈自形—较自形散乱分布,镜下可见斜长石聚片双晶和黑云母包裹体(图2a、b、e、f)。前人根据临沧花岗岩体中暗色包裹体的成因和岩石类型将其分为4类:深源包裹体、同期包裹体、富云暗色团块和捕掳体[22]。本次发现的暗色包体主要是斜长石+石英+黑云母的矿物组合,其成因类型在这里不做讨论。黑云母花岗闪长岩:具有细—中粒花岗结构,块状构造,矿物主要由微斜长石、更长石、石英和部分黑云母,副矿物有磷灰石、褐铁矿、锆石,多数的野外岩体均遭受不同程度的风化作用,部分石英和长石有被拉长的现象,赤铁矿化和绢云母化发育。围岩蚀变主要有硅化、绿泥石化、绢云母化、赤铁矿化、钾长石化、碳酸盐化等。

图2

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图2   临沧花岗岩岩石学特征

a—似斑状黑云母二长花岗岩,含有多条石英脉;b—中细粒花岗岩体内发育不规则状暗色包体;c—似斑状黑云二长花岗岩内发育自形五边形石英晶体;d—自形石英颗粒,赤铁矿化严重;e—钾长石被石英包围,聚片双晶发育;f—钾长石卡式双晶,部分钾长石发生高岭土化,斜长石绢云母化;其中e、f为正交偏光图像;Q—石英;Kf—钾长石;Pl—斜长石;Bi—黑云母

Fig.2   Petrological characteristics of Lincang granite

a—porphyritic biotitemonzonite granite, containing multiple quartz veins; b—irregularly dark inclusions in medium-fine-grained granite; c—self-formed pentagonal quartz crystals in motley granite; d—self-shaped quartz particles, hematite is seriously mineralized; e—potassium feldspar is surrounded by quartz, and polycrystalline twin crystals develop; f—potassium feldspar card-type twin crystal, part of the potassium feldspar is kaolinized, plagioclase sericite;e and f are cross-polarized images;Q—quartz; Kf—potassium feldspar; Pl—plagioclase; Bi—biotite


3.2 年代学特征

本次对凤庆901矿化点YN057样品进行锆石LC-ICP-MS U-Pb定年,分析结果见表1,挑选的锆石阴极发光图像和测试点见图3。锆石颗粒晶形完整,环带清晰,晶体多为规则的长柱状,部分为短柱状。本次共测得24个点,其中U含量为(368.4~1 686.9)×10-6,Th含量为(160.8~817.9)×10-6,w(Th)/w(U)比值为0.23~0.64。根据锆石的阴极发光照片和w(Th)/w(U)比值确定本次样品锆石为岩浆锆石。测试结果表明,YN057样品在206Pb/238U-207Pb/235U 谐和图上均落在谐和线上或者谐和线附近(图4),总体上显示出协和性较好,数据较为集中,表明锆石在形成之后处于封闭体系之中,没有U、Th的带入带出,所得到的加权年龄为214±12 Ma,能够代表本次样品花岗岩的形成年龄,认为临沧富铀花岗岩属于印支晚期花岗岩。这与前人得出临沧花岗岩年龄为205~220 Ma[23]以及彭头平等给出临沧花岗岩229~233 Ma的侵入年龄相近[14],因此笔者认为该数据作为临沧富铀花岗岩体的形成年龄可靠。

表1   临沧富铀花岗岩体锆石LA-ICP-MS U-Pb定年结果

Table 1  Zircon LA-ICP-MS U-Pb dating results of Lincang rich uranium granite

编号含量/10-6w(Th)/
w(U)		207Pb/206Pb207Pb/235U206Pb/238U208Pb/232Th238U/232Th年龄(Ma)±1σ
PbThU207Pb/206Pb207Pb/235U206Pb/238U
YN057-0149.9444.31427.10.310.0520.2650.0360.0102.606305.6±64.8238.5±6.3228.7±2.5
YN057-0334.2322.2877.40.370.0500.2620.0380.0122.279198.2±78.7236.3±7.6237.9±3.0
YN057-0447.5506.311360.450.0530.2660.0360.0131.863322.3±77.8239.9±7.0230.8±2.4
YN057-0555.3483.31532.90.320.0500.2460.0350.0122.623194.5±66.7223.7±5.6224.5±2.1
YN057-0731.1306.2802.90.380.0510.2550.0360.0122.160235.3±90.7230.6±7.7229.5±2.6
YN057-0831.4375.5807.40.470.0510.2460.0350.0111.779239±72.2223.4±8.6220.9±2.7
YN057-0919.4179.4465.20.390.0580.2900.0360.0142.145527.8±102.8258.4±9.8230±3.1
YN057-1033.4492.7768.10.640.0500.2360.0340.0121.291190.8±92.6215±7.4216.2±2.3
YN057-1135.5448.1893.90.50.0500.2370.0350.0111.652189±77.8215.8±6.1218.7±2.4
YN057-1247.3507.91266.80.40.0500.2450.0350.0112.081213±41.7222.8±6.1222.9±2.2
YN057-1315.9212.7368.40.580.0530.2700.0370.0111.435320.4±110.2242.7±9.7234.9±3.3
YN057-1416.3160.8436.90.370.0520.2480.0350.0122.273333.4±98.1224.6±8.5219.6±2.7
YN057-1557.4817.91280.40.640.0530.2660.0360.0121.317309.3±68.5239.6±6.7230.4±2.3
YN057-1640351.511080.320.0530.2620.0350.0122.652344.5±66.7236.1±6.3224.8±2.4
YN057-1731.3236.2879.20.270.0550.2800.0370.0133.006420.4±78.7250.4±8.0232.5±2.6
YN057-1824.8254681.90.370.0490.2350.0350.0122.247200.1±95.4214.5±7.4220.5±2.6
YN057-1932.9479.3789.70.610.0520.2460.0340.0121.378298.2±81.5223.3±7.0215.7±2.6
YN057-2036.94011031.80.390.0520.2470.0340.0112.158272.3±70.4223.8±6.1218.3±2.3
YN057-2134.6344.9957.20.360.0510.2500.0350.0122.358261.2±90.7226.8±7.2222.9±2.7
YN057-2224.9269.8652.50.410.0500.2460.0360.0122.014211.2±88.9223.6±7.3226.8±2.9
YN057-2321.8230.9570.90.40.0510.2460.0350.0122.056220.4±100223±8.5223.9±3.0
YN057-2433.9337.2890.80.380.0530.2600.0360.0132.199324.1±72.2235±6.3226.6±2.8
YN057-2536.4415.7954.10.440.0530.2600.0360.0111.909320.4±68.5235±6.0226.1±2.5
YN057-2656.4392.11686.90.230.0550.2650.0350.0133.575413±68.5238.7±6.1221.7±3.1

注:表中同位素比值为含量比值,下文同。

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图3

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图3   凤庆YN057花岗岩锆石CL图像

Fig.3   Fengqing YN057 granite zircon CL image


图4

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图4   YN057锆石U-Pb年龄协和图

Fig.4   YN057 zircon U-Pb age concord map


3.3 主量元素特征

双江701铀矿化点和凤庆901铀矿化点主量元素见表2。数据表明,两矿化点岩石具有高硅、高钾、低锰特征,双江701铀矿点SiO2含量为69.90%~72.34%,平均含量为70.96%,(Na2O+K2O)含量在4.36%~7.9%,平均含量6.27%;凤庆901铀矿点SiO2含量42.69%~90.11%,平均含量72.07%,变化范围较大,(Na2O+K2O)含量在 1.5%~7.68%,平均含量4.65%。YN055、YN056微量元素中U含量均显著高于其他样品,认为该样品为铀矿石。在SiO2-(Na2O+K2O)判别图解上,样品均位于亚碱性系列,双江样品投点于花岗岩和花岗岩闪长岩中,凤庆样品位于硅英岩和花岗闪长岩中(图5);在SiO2-K2O判别图解上,双江花岗岩样品大部分属于高钾钙碱性系列,凤庆花岗岩位于碱性系列偏多(图6)。

表2   临沧地区富铀花岗岩主量元素分析结果

Table 2  Analysis results of main elements of rich uranium granite in Lincang area %

指标双江样品凤庆样品中国
花岗岩[24]		世界
花岗岩[25]
YN027YN028YN029YN030YN031YN050YN051YN052YN053YN054YN055YN056YN057
SiO269.970.4171.6172.3470.5374.758379.3290.1172.4542.6961.7772.4971.9971.3
Al2O314.2412.9414.8614.3212.346.277.67.314.3913.721.198.0114.4113.8614.32
Fe2O31.774.081.591.473.414.793.722.731.992.1214.975.11.91.371.21
MgO1.140.820.730.620.630.280.560.240.160.540.340.660.710.810.71
CaO1.630.620.470.531.180.070.080.060.050.290.30.520.690.120.05
Na2O1.110.152.923.150.640.110.110.120.070.87.32.252.763.814.07
K2O5.134.214.574.754.72.422.43.141.4360.163.234.923.423.66
MnO0.040.020.020.040.0100.010.01<0.0040.030.030.10.031.551.84
TiO20.350.250.310.270.250.10.070.120.070.20.010.140.280.210.31
P2O50.110.190.110.10.120.030.040.050.020.120.30.430.110.20.12
FeO0.890.580.921.290.560.662.620.470.590.70.650.681.441.71.64
烧失量4.555.732.772.415.65.042.392.421.233.196.684.771.69
Total100.86100100.88101.2999.9794.52102.695.99100.11100.1674.6287.66101.43
A/NK2.282.971.981.812.312.483.022.252.932.020.161.461.88
A/CNK1.812.61.871.71.892.412.942.22.841.930.151.331.72
Na2O+K2O6.244.367.497.95.342.532.513.261.56.87.465.487.68

注:Total代表本次测试的氧化物含量和烧失量总和;Na2O+K2O为氧化物含量相加。

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图5

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图5   岩浆(火成岩)系统全碱—硅(TAS)分类图

Ir—Irvine 分界线,上方为碱性,下方为亚碱性。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

Fig.5   Alkali-silicon (TAS) classification map of magma(igneous rock) system

Ir—Irvine dividing line is alkaline at the top and sub-alkaline at the bottom. 1—peridot gabbro; 2a—alkaline gabbro; 2b—subalkaline gabbro; 3—gabbro diorite; 4—diorite; 5—granodiorite; 6—granite; 7—siliconite; 8—bellmanite; 9—dellmanite; 10—dellmanite; 11—quartzite diorite; 12—synorite; 13—parsonite gabbro; 14—para-feldspar diorite; 15—para-feldspar syenite; 16—para-syenite syenite; 17—para-long diorite; 18—neonite,phosphorite,coarse white garnet


图6

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图6   SiO2-K2O判别图

Fig.6   SiO2-K2O discrimination diagram


双江701铀矿点Al2O3含量12.34%~14.86%,平均含量13.74%,A/NK比值在 1.39~2.69,平均为1.91,A/CNKI在1.27~2.18,平均为1.54;凤庆901铀矿点Al2O3含量1.19%~14.41%, 平均含量7.86%, A/NK在0.16~3.02,平均为2.03,A/CNK在0.15~2.94,平均为1.94,均有铝弱饱和特点。在A/CNK-A/NK图解中,双江凤庆花岗岩均落于过铝质区域,具有过铝质—强过铝质特征(图7)。

图7

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图7   铝质—准铝质花岗岩A/NK-A/CNK判别图

Fig.7   A/NK-A/CNK discrimination diagram of aluminum-quasi-aluminum granite


在双江主量元素和采样点剖面图中(图8),由上盘到下盘,SiO2含量变化不大,有轻微增加趋势;Fe2O3和U2O3含量变化趋势一致,在破碎带中元素含量达到最高,距离破碎带越远含量越低;Al2O3和Na2O+K2O含量变化一致,Al2O3与UO2变化出现相反的趋势,在破碎带附近含量最低,远离破碎带样品元素含量较高;表明在铀矿化过程中伴随降铝、去碱、强烈的赤铁矿化和轻微的硅化。

图8

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图8   元素含量与采样位置示意

Fig.8   Element content and sampling position


3.4 微量元素特征

两个铀矿化点的样品微量元素分析结果见表3。在原始地幔标准化微量元素蛛网图(图9)上,双江701铀矿化点和凤庆901铀矿化点具有较一致的配分模式,整体配分模式出现U富集,亏损Ba、Sr、Ti。Sr、Ba亏损可能是由于长石结晶导致或物源成分导致[26],在岩石形成早期岩浆演化过程中,基性矿物相比酸性矿物要更早结晶析出,早期结晶的矿物中Sr、Ba以类质同象置换Ca离子进入晶格,导致晚期岩浆结晶形成的矿物Sr、Ba亏损;也有可能因为Sr、Ba属于大离子亲石元素,易溶于水,易迁出,流体作用将会导致大离子亲石元素的亏损。双江样品w(Rb)/w(Sr)变化范围为3.72~5.92,平均值为4.75;w(Th)/w(U)变化范围为0.07~0.64,平均值为0.30。凤庆样品w(Rb)/w(Sr)变化范围为1.96~53.45,平均值为22.57;w(Th)/w(U) 变化范围为0.2~4.38,平均值为1.70。在复式岩体中,Th、U都在分异后期更加富集,本次样品w(Th)/w(U)比值变化不大(除YN055、YN056),YN055、YN056铀含量急剧增加,认为该样品是铀矿石。

表3   临沧富铀花岗岩微量、稀土元素分析结果

Table 3  Analysis results of trace and rare earth elements in Lincang rich uranium granite 10-6

元素双江样品凤庆样品
YN027YN028YN029YN030YN031YN050YN051YN052YN053YN054YN055YN056YN057
La25.4016.2028.2022.6028.7013.6011.7014.8011.5029.802.3720.7049.50
Ce56.9034.8057.1041.7062.4027.6023.2029.0017.4063.402.6637.9081.50
Pr7.674.486.875.147.733.373.133.402.726.870.474.9010.70
Nd25.8017.7028.0020.9027.3012.3011.2013.009.9626.302.1718.7038.80
Sm6.265.647.124.279.282.532.492.541.876.090.703.857.87
Eu0.750.881.060.531.600.210.210.220.200.510.130.390.70
Gd4.515.595.973.6411.401.811.891.841.624.590.893.326.60
Tb0.811.311.190.532.780.290.290.320.260.820.200.521.18
Dy4.117.926.342.7617.601.271.481.541.293.661.042.876.05
Ho0.741.601.110.523.970.290.300.310.260.690.220.511.16
Er2.023.722.921.4310.000.870.880.910.721.880.581.393.21
Tm0.320.590.460.231.360.170.150.150.120.340.110.250.54
Yb1.913.022.291.406.570.961.050.970.812.150.751.543.59
Lu0.260.360.320.220.770.150.140.130.100.290.090.220.45
Be6.408.216.796.146.052.063.602.081.595.262.495.766.52
Sc6.706.346.525.324.472.992.903.281.664.830.412.835.82
V45.8028.2032.1022.5038.8015.2021.8011.606.9820.7058.30383.0025.10
Cr15.9011.6012.4010.6010.603.303.403.612.356.669.648.479.24
Co8.5718.609.803.342.491.090.560.430.352.0516.4019.302.94
Ni5.6310.906.635.582.020.941.411.260.884.072.182.893.55
Cu21.0022.0023.7017.4017.70100.0050.30196.0062.2048.40727.00703.003.76
Zn12.9029.8029.6022.6023.40288.00764.00290.00146.002407.0059322.0033929.00140.00
Ga21.4021.0023.8019.0018.2013.0011.509.775.9419.0031.0025.7019.20
Rb289.00294.00306.00271.00268.00217.00212.00211.00124.00403.0013.90226.00313.00
Sr48.8058.7061.3072.9064.906.9110.605.092.3228.407.0917.5060.20
Y20.0052.9033.7014.60145.009.178.509.179.0120.507.9816.7032.20
Mo2.041.590.330.281.580.750.390.380.070.0052.401.150.17
Cd0.050.180.070.050.070.504.291.271.174.04251.00114.000.76
In0.050.040.040.040.030.160.110.530.050.031.660.800.03
Sb2.0811.603.540.7613.3047.505.6346.9031.300.65250.0046.400.30
Cs23.6070.8028.5019.7048.9012.708.175.073.1111.701.3413.8015.40
Ba496.00191.00379.00328.00445.00117.00178.00152.0061.30487.0011.40211.00456.00
Tl2.0810.101.911.5315.901.631.241.320.692.650.501.581.92
Pb37.50333.00126.0049.70620.009446.00551.0010751.00955.002219.0028776.0019703.0089.00
Bi1.062.340.520.571.6015.900.682.825.282.0729.6023.500.75
Th23.4018.7022.8019.309.973.989.549.077.3633.000.6318.6036.20
U45.90268.00158.0030.2064.007.653.4911.603.3810.901751.001179.008.26
Nb13.5011.4013.5011.209.814.933.896.213.2411.400.266.3412.40
Ta2.642.162.422.311.851.131.051.450.842.980.061.582.64
Zr97.50103.00118.00102.00110.0021.7015.2021.5014.8026.901.1525.3060.20
Hf3.723.514.043.443.250.790.610.790.631.030.051.022.32

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图9

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图9   微量元素原始地幔标准化蛛网图(标准化值引自Sun and McDonough[27])

Fig.9   Trace element original mantle standardized spider map(the standardized value is quoted from Sun and McDonough[27])


Nb、Ta、Zr、Hf作为高场强元素,性质稳定,常作为物源判断依据。本次测得双江w(Nb)/w(Ta)变化范围为4.85~5.58,平均值为5.22,w(Zr)/w(Hf)比值为26.21~33.85,平均值为29.65,凤庆w(Nb)/w(Ta)比值为3.70~4.70,平均值为4.15,w(Zr)/w(Hf)比值为23.42~27.57,平均值为25.49,均低于中国大陆地壳w(Nb)/w(Ta)平均值11.88和w(Zr)/w(Hf)平均值86.67,暗示了岩浆岩来源于地壳熔融物质。双江矿点w(Nb)/w(Ta)高于凤庆地区,表明双江地区花岗岩结晶较早于凤庆地区花岗岩,较低的w(Nb)/w(Ta)比值表明在岩浆作用过程中发生了Nb、Ta部分分离。

w(K)/w(Rb)反映岩浆分异作用的强弱,双江比值高于凤庆比值,认为凤庆矿点的岩浆分异作用更强。U、Pb含量在破碎带和围岩中变化较大,出现了明显破碎带铀富集的现象,(YN027+YN029)/(YN027+YN030)=5.6。笔者认为双江701铀矿点富铀花岗岩的形成与强烈的蚀变和构造运动分不开,其特殊的形成环境将在后面进行讨论。

3.5 稀土元素特征

稀土元素分析结果见表3,双江701铀矿点ΣREE为191.46×10-6~103.81×10-6,w(LREE)/w(HREE)=2.52~8.7;凤庆901铀矿点ΣREE为 12.37×10-6~211.85×10-6,w(LREE)/w(HREE)=2.19~10.21;两铀矿点都较为富集轻稀土,(La/Yb)N为2.27~11.58,轻重稀土元素分馏较弱;铕亏损明显,δEu=0.28~0.49,认为在岩体的形成过程中存在大量富集轻稀土的矿物(独居石、磷灰石等)和斜长石的分异结晶作用。(La/Sm)N和(Gd/Yb)N分别为2.19~4.06、0.98~1.78,均较低,表明两铀矿化点的轻、重稀土元素本身的分异较弱。在球粒陨石标准化稀土元素配分模式图上(图10),配分模式呈微右倾“V”字形(除YN055),较为一致的稀土元素球粒陨石配分曲线图表明了两矿点的岩石形成过程和形成环境较为相似,但双江矿点样品含量较高,认为这与野外观察发现的破碎带有关,破碎带中元素更加富集,更利于成矿。

图10

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图10   稀土元素球粒陨石标准化模式图(标准化值引自Sun and McDonough[27])

Fig.10   Rare earth element chondrite standardization pattern(the standardized value is quoted from Sun and McDonough[27])


4 讨论

4.1 构造环境

临沧花岗岩带属于“三江”次级构造单元,是环特提斯构造域的重要组成部分,位于扬子地块、西缅地块和松潘—甘孜地块交接部位。关于临沧花岗岩成因和构造环境仍然有较大争议。杨振德认为临沧花岗岩岩基是外来岩体或者解释为岩浆弧[28,29],孔会磊等、王舫等、李兴林认为临沧花岗岩是由于古特提斯洋俯冲闭合时同碰撞造山后伸展作用所形成的[16,23,30]。刘德利[15]认为临沧花岗岩属于S型花岗岩,形成于同碰撞阶段,而Sone M S等认为临沧花岗岩主要是S型花岗岩,但有一定的I型花岗岩[31]。判别S型花岗岩和I型花岗岩目前采样的标准是A/CNK值[32],即I型花岗岩的A/CNK值小于1.10,w(K2O)/w(Na2O)一般小于1[33,34],S型花岗岩A/CNK值大于1.10,w(K2O)/w(Na2O)一般大于1[35]。本次在矿化点采集的样品,在岩相学上没有发现堇青石、石榴子石等S型花岗岩的判别矿物,但在化学成分上除YN055铀矿石样品外,其他样品的A/CNK均大于1.1,w(K2O)/w(Na2O)均大于1,同时在Zr-TiO2判别图解和花岗岩ACF图解中,富铀花岗岩均落于S型花岗岩之中,因此笔者认为双江701和凤庆901铀矿化点中的花岗岩属于S型花岗岩(图11)。临沧花岗岩沿澜沧江断裂带西侧分布,地处我国“三江”地区特提斯造山带南段,由昌宁—孟连石炭—二叠纪火山岩带和澜沧江二叠—三叠纪火山岩带组成,与古特提斯洋闭合和板块碰撞密切相关,在早二叠世向东俯冲消减形成二叠纪俯冲低钾—中钾钙碱性中酸性火山岩[36]和具岛弧特征花岗闪长岩岩基[37]。莫宣学提出在二叠纪时期,洋盆由于板块运动开始缩小,到早三叠世消失,南北大陆开始对接碰撞,早期的临沧花岗岩(俯冲花岗岩)和二叠纪火山岩形成古特提斯洋东侧的俯冲花岗岩,完整地记录了古特提斯洋的发展,从岩体同位素年龄判断临沧花岗岩碰撞时间为印支期[38]。

图11

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图11   富铀花岗岩岩石成因类型判别图解

a—TiO2-Zr判别图;b—花岗岩ACF图解

Fig.11   Discrimination diagram of genetic types of uranium-enriched granites

a—TiO2-Zr discrimination diagram;b—granite ACF diagram


本次测试的临沧富铀花岗岩锆石U-Pb年龄显示花岗岩形成于印支晚期,该年龄与洋盆闭合、南北大陆碰撞接近。根据花岗岩主、微量元素构造环境判别图解(图12),在Al2O3-SiO2判别图解中,样品位于与构造和裂谷有关的后造山花岗岩区域, 可确定研究区富铀花岗岩岩形成于后造山环境裂谷环境之中,构造环境为大陆碰撞花岗岩类,与肖庆辉研究结果一致[39]。在Y+Nb-Rb构造环境判别图中,投影点主要位于同碰撞花岗岩区域,表明临沧富铀花岗岩主体是与澜沧江碰撞带有关的同碰撞花岗岩。本研究区富铀花岗岩主要构造作用时期为印支晚期,由南北大陆对接碰撞形成的同碰撞花岗岩是洋盆消减、大陆碰撞的产物。

图12

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图12   花岗岩主、微量元素构造环境判别图解

IAG—岛弧花岗岩类;CAG—大陆弧花岗岩类;CCG—大陆碰撞花岗岩类;POG—后造山花岗岩类;RRG与裂谷有关花岗岩类;CEUG—与大陆的构造抬升有关花岗岩类;syn-COLG—同碰撞花岗岩类;VAG—火山弧花岗岩类;WPG—板内花岗岩;ORG—洋脊花岗岩类

Fig.12   Distinguishing diagram of the main and trace element structure environment of granite

IAG—island arc granite; CAG—continental arc granite; CCG—continent collision granite; POG—post-orogenic granite; RRG—rift-related granite; CEUG—continental tectonic uplift and granite; syn-COLG—co-crash granite; VAG—volcanic arc granite; WPG—in-plate granite; ORG—ocean ridge granite


4.2 矿化特征

从岩性特征上看,双江701和凤庆901矿化点富铀花岗岩大部分为黑云母二长花岗岩,少量为黑云母花岗闪长岩,与临沧花岗岩类别一致,同一矿化点中,岩体均遭受了不同程度的硅化、绢云母化和赤铁矿化,蚀变程度越高的岩石铀含量越高。本次研究认为,黑云母二长花岗岩本身的孔隙度较高,有利于热液渗透和淋滤作用,在矿化较强的岩体中,有利于铀的吸附和还原沉淀,在蚀变岩中形成富铀岩体。

从成矿空间上看,两矿点均位于澜沧江大断裂带附近,小型断裂带的交合部位,严格受到构造控制,良好的区域地质背景为含铀热液迁移富集提供了导矿和容矿的有利场所。

从元素特征上看,双江701和凤庆901矿化点的富铀花岗岩有相似的微量元素和稀土元素配分模式,指示了两矿点形成环境一致,物源均为上地壳熔融物质。

从成矿年龄上看,临沧富铀花岗岩形成年龄与临沧花岗岩形成年龄接近,为同一时代产物。因此,印支期花岗岩为矿化点主要的侵入花岗岩,同期伴随碰撞运动,在构造和蚀变较强区域富集成矿,形成目前的铀矿化点。

4.3 铀成矿作用

长期以来,“三江”地区花岗岩一直受到中外学者的关注,许多研究者从花岗岩的主微量元素、构造背景、U-Pb锆石定年分析,认为“三江”地区岩浆活动不是连续的,呈现间歇活动特征。如前文所述,双江701铀矿化点和凤庆901铀矿化点花岗岩有相似的地球化学特征,高钾钙碱性系列,表现出了明显的S型花岗岩特征。前人在对临沧花岗岩做Hf分析得出εHf(t)<0,推断其为古老地壳的熔融产物[16],在本次样品中Nb轻微的弱负异常证明了这一观点。稀土元素U含量为3.38×10-6~1 751×10-6,大于2×10-6,Pb含量为35.75×10-6~28 776×10-6,大于20×10-6,Th含量变化较大,大部分大于9×10-6,δEu=0.28~0.49,小于1,具有上地壳物质特征[37];w(Rb)/w(Sr)为1.96~53.45,平均值15.72,为高度成熟壳源物质,因此认为铀矿化点中花岗岩是由上地壳物质组成。结合“三江”地区铀地球化学异常分布图(图13),异常区域主要分布于临沧花岗岩一带及察隅—腾冲云南花岗岩一带,在花岗岩的成分组成特点和成因类型上与华南花岗岩相似,具有华南产铀花岗岩的一般特点[24]。

图13

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图13   “三江”云南段铀地球化学异常分布(据参考文献[24]修改)

Fig.13   Uranium geochemical anomaly map of the Yunnan section of the Sanjiang (revised according to reference [24])


目前对花岗岩型铀矿的铀源分析认为主要有两点:富铀岩体提供铀源;铀矿床为后期热液活动形成。研究表明,双江701铀矿点中和凤庆901铀矿点中的黑云二长花岗岩铀含量分别为30.2×10-6~268×10-6、平均113.22×10-6和3.38×10-6~1 751×10-6、平均371.91×10-6,明显高于地壳平均值(2.80×10-6),因此认为花岗岩体具有提供丰富铀源的潜力。年代学研究表明,双江701铀矿化点富铀花岗岩的U-Pb锆石成岩年龄为214±12 Ma,与“三江”其他地区花岗岩年龄229~233 Ma相近,属于同一构造期次的产物,直接从上地壳物质携带铀源富集;w(Th)/w(U)比值可以反映花岗岩是否经过后期热液作用或表生作用改造,当w(Th)/w(U)稳定在3~5则表明花岗岩未遭受热液作用,本次双江701铀矿点花岗岩w(Th)/w(U)比值0.15~0.70,平均为0.30,凤庆901铀矿点花岗岩w(Th)/w(U)比值0.004~4.383,平均1.705,表明岩体均遭受了一定程度的流体作用影响。w(Th)/w(U)比值最小为0.000 4,w(U)>10×10-6,证实了存在花岗岩中铀元素在大气降水和流体作用下淋滤出来,被搬运到花岗岩破碎带附近,参与成矿作用。根据双江701铀矿点采样点和U含量关系(图8),对比分析矿化点破碎带、近破碎带、正常花岗岩和两侧蚀变围岩,表明铀明显在破碎带中富集,其次为近破碎带、蚀变围岩、正常花岗岩。本次研究认为破碎带中铀源并不单一,推断在铀富集过程中,有部分铀通过断层破碎带参与深部铀源富集,结合测年结果,YN057测年结果为214±12 Ma,彭头平得出澜沧江花岗岩南北段年龄分别为230±3.6 Ma和229±3.0 Ma[14],莫宣学认为岩基年龄在288~138 Ma[38],刘德利分别得出246 Ma、250 Ma、49 Ma、61 Ma[15],认为在喜马拉雅期岩浆活动过程中临沧复式岩基是多期次、多阶段侵入活动形成,它的形成经历了后期强烈改造作用。

5 结论

1) 凤庆901矿化点YN057样品的U-Pb锆石测年结果为214±12 Ma,属于印支晚期花岗岩,表明临沧富铀花岗岩形成在印支晚期。

2) 临沧富铀花岗岩体具有富硅、富钾、强过铝质特征,属高钾钙碱性系列;稀土元素配分模式呈右倾“V”字形,较富集轻稀土,轻重稀土元素分异明显,铕亏损明显;微量元素U明显富集,Rb、Th、La、Nd较富集,明显亏损Ba、Sr、Ti;w(K)/w(Rb)比值双江高于凤庆,认为凤庆矿点的岩浆分异作用更强;微量和稀土配分模式指示两矿点形成环境相似;破碎带附近的花岗岩体铀含量较高,铀含量呈现由破碎带花岗岩、近破碎带花岗岩、蚀变花岗岩、正常花岗岩逐步降低的趋势,在铀富集过程中伴随降铝、去碱、强烈的赤铁矿化和轻微的硅化。

3) 临沧富铀花岗岩体属于S型花岗岩,形成于碰撞后期造山和裂谷环境,属于碰撞后花岗岩,是由上地壳物质熔融形成;矿化点分布在断裂带交合附近,严格受到构造控制;铀源主要来源于富铀岩体,成矿过程中遭受流体作用,铀元素被搬运到破碎带附近富集成矿。

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