A tentative discussion on zircon U-Pb geochronology and geochemistry of ore-bearing intermediate-acid intrusive rocks in the Bainaimiao copper ore district and the metallogenic epoch
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摘要:
为了限定白乃庙铜矿的成矿时代和成矿环境,对含矿中酸性岩类进行同位素年龄和岩石地球化学研究。获得的3个含矿花岗闪长岩样品的LA-MC-ICP-MS锆石U-Pb年龄分别为421.5±1.8Ma、438.6±3.4Ma和459.3±3.6Ma。含矿中酸性岩类属于过铝质钙碱性系列岩石,稀土元素曲线显示为轻稀土元素富集型,具有弱正Eu异常,微量元素曲线表现为明显的Ta、Nb亏损。综合含矿中酸性岩类时代、地球化学特征及区域构造背景,认为白乃庙铜矿总体形成于早古生代,具有多期成矿的特点,成矿作用与古蒙古洋早古生代俯冲消减事件有关。
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关键词:
- 白乃庙铜矿 /
- 中酸性侵入岩 /
- LA-MC-ICP-MS锆石U-Pb年龄 /
- 地球化学特征 /
- 成矿时代
Abstract:In order to determine the metallogenic epoch and ore-forming environment of the Bainaimiao copper deposit, the au-thors carried out research on isotope geochronology and geochemistry of ore-bearing intermediate-acid rocks. LA-MC-ICPMS zir-con U-Pb dating shows that the ages of three ore-bearing granodiorites samples are 421.5±1.8Ma, 438.6±3.4Ma and 459.3±3.6Ma respectively. These rocks have peraluminous calc-lkaline characteristics, and are enriched in LREE. There is a weak positive anomaly of Eu. The trace elements are obviously depleted in Ta and Nb. According to comprehensive ages and geochemical characteristics of ore-bearing granodiorites and the regional tectonic background, the authors consider that the Bainaimiao copper deposit was on the whole formed in the early Paleozoic and characterized by multi-period mineralization. In addition, the metallogenesis was related to the subduction event of ancient Mongolia Ocean in the Early Paleozoic.
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致谢: 成文过程中天津地质矿产研究所李俊建、周红英研究员提出了宝贵意见，耿建珍、张建工程师在实验测试方面给予了帮助，在此表示衷心感谢。
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图 2 白乃庙铜矿含矿中酸性侵入岩岩石特征
a—花岗闪长岩中的白乃庙群绿片岩捕虏体；b—片理化孔雀石化花岗闪长岩；c、d—正交偏光。Pl—斜长石；Mc—微斜长石；Q—石英；Ms—黑云母
Figure 2. Petrology features of intermediate-acid rocks in Bainaimiao copper deposit
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图 3 白乃庙铜矿含矿中酸性侵入岩代表性锆石CL图像及测点位置
Figure 3. Representative CL images and dating spots of zircons from intermediate-acid rocks in the Bainaimiao copper deposit
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图 4 白乃庙铜矿含矿中酸性侵入岩锆石U-Pb谐和图
Figure 4. U-Pb concordant curves of zircons from intermediateacid rocks in the Bainaimiao copper deposit
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图 5 白乃庙铜矿中酸性岩类A/CNK-A/NK判别图(a)和TAS分类图解(b)
Figure 5. A/CNK-A/NK discrimination diagram (a) and TAS diagram (b) of intermediate-acid rocks in the Bainaimiao copper deposit
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图 6 白乃庙铜矿中酸性岩类稀土元素配分模式图（a）及微量元素蛛网图（b）（球粒陨石和原始地幔标准化值均据参考文献[20]）
Figure 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized spidergrams (b) for intermediate-acid rocks in the Bainaimiao copper deposit
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图 7 白乃庙铜矿中酸性岩类Y-Nb(a)及(Yb+Nb)-Rb(b)图解[23]
WPG—板内花岗岩；ORG—洋脊花岗岩；VAG—火山弧花岗；syn-COLG—同碰撞花岗岩
Figure 7. Y-Nb (a) and (Yb+Nb)-Rb (b) diagrams for intermediateacid rocks in the Bainaimiao copper deposit
表 1 白乃庙铜矿中酸性岩类LA-MC-ICP-MS锆石U-Th-Pb同位素分析结果
Table 1 LA-MC-ICP-MS zircon U-Th-Pb dating results of intermediate-acid rocks in the Bainaimiao copper deposit
点号 同位素含量/10-6 232Th/238U 同位素比值 年龄/Ma Pb U 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U 测值 1σ 测值 1σ 测值 1σ 测值 1σ 测值 1σ 测值 1σ 09NM401 1 22 220 0.46 0.0553 0.0018 0.524 0.017 0.0687 0.0004 426 73 428 14 428 3 2 38 333 0.32 0.0545 0.0016 0.503 0.014 0.0669 0.0005 394 64 414 12 417 3 3 40 390 0.42 0.0549 0.0015 0.510 0.014 0.0673 0.0004 408 60 418 11 420 3 4 42 424 0.44 0.0533 0.0014 0.496 0.013 0.0674 0.0004 343 59 409 11 421 3 5 42 380 0.34 0.0552 0.0015 0.512 0.014 0.0673 0.0004 421 59 420 11 420 3 6 33 334 0.44 0.0533 0.0015 0.487 0.014 0.0663 0.0004 340 63 403 11 414 2 7 48 485 0.46 0.0552 0.0014 0.507 0.013 0.0667 0.0004 419 57 416 11 416 2 8 39 359 0.37 0.0550 0.0015 0.516 0.014 0.0680 0.0004 413 61 422 12 424 2 9 36 326 0.35 0.0552 0.0015 0.508 0.014 0.0668 0.0004 418 62 417 12 417 2 10 36 343 0.41 0.0546 0.0014 0.505 0.013 0.0671 0.0004 394 59 415 11 419 2 11 40 384 0.41 0.0550 0.0015 0.510 0.014 0.0673 0.0004 413 59 419 11 420 3 12 48 411 0.33 0.0548 0.0017 0.516 0.016 0.0683 0.0004 403 69 423 13 426 2 13 29 311 0.53 0.0553 0.0018 0.514 0.017 0.0673 0.0005 426 71 421 14 421 3 14 39 366 0.40 0.0554 0.0015 0.520 0.014 0.0681 0.0004 428 61 425 12 425 3 15 34 356 0.51 0.0541 0.0016 0.509 0.015 0.0683 0.0004 374 66 418 12 426 3 16 35 336 0.43 0.0554 0.0016 0.522 0.015 0.0684 0.0005 427 63 427 12 427 3 17 39 338 0.34 0.0553 0.0017 0.523 0.016 0.0685 0.0004 426 68 427 13 427 3 18 49 402 0.30 0.0554 0.0015 0.524 0.015 0.0686 0.0004 429 61 428 12 428 3 19 35 383 0.53 0.0553 0.0014 0.518 0.014 0.0680 0.0004 423 58 424 11 424 2 20 43 363 0.32 0.0534 0.0015 0.502 0.015 0.0682 0.0004 345 65 413 12 425 2 21 35 318 0.37 0.0546 0.0017 0.504 0.016 0.0669 0.0005 396 68 414 13 418 3 22 40 400 0.42 0.0534 0.0014 0.492 0.014 0.0669 0.0004 345 60 406 11 417 2 23 63 183 0.06 0.0554 0.0022 0.522 0.021 0.0683 0.0005 428 88 426 17 426 3 24 39 398 0.46 0.0546 0.0014 0.503 0.013 0.0668 0.0004 398 58 414 11 417 2 25 30 266 0.37 0.0543 0.0016 0.504 0.016 0.0673 0.0004 385 67 415 13 420 3 09NM401 1 36 503 0.65 0.0537 0.0010 0.494 0.010 0.0661 0.0008 359 41 408 8 413 5 2 27 365 0.50 0.0556 0.0013 0.551 0.013 0.0712 0.0007 435 51 446 11 444 4 3 21 288 0.39 0.0556 0.0017 0.551 0.018 0.0711 0.0008 437 69 446 14 443 5 4 27 354 0.64 0.0556 0.0014 0.542 0.014 0.0700 0.0004 438 57 440 12 436 3 5 154 963 0.12 0.0755 0.0005 1.775 0.016 0.1688 0.0014 1082 14 1036 10 1006 8 6 41 562 1.49 0.0567 0.0030 0.506 0.027 0.0640 0.0012 481 115 416 22 400 7 7 14 186 0.47 0.0558 0.0022 0.552 0.023 0.0710 0.0008 445 90 446 18 442 5 8 31 403 0.59 0.0553 0.0011 0.545 0.012 0.0708 0.0006 425 45 442 9 441 4 9 32 428 0.61 0.0554 0.0011 0.543 0.012 0.0703 0.0007 430 46 440 10 438 4 10 22 281 0.61 0.0549 0.0015 0.548 0.015 0.0717 0.0008 407 59 444 12 446 5 11 27 369 0.63 0.0559 0.0013 0.535 0.012 0.0687 0.0004 448 51 435 10 428 2 12 27 372 0.63 0.0568 0.0014 0.531 0.014 0.0672 0.0008 484 54 433 11 419 5 13 38 500 0.70 0.0555 0.0008 0.550 0.011 0.0711 0.0012 433 34 445 9 443 8 14 20 260 0.52 0.0553 0.0018 0.549 0.018 0.0712 0.0008 426 72 444 15 444 5 09NM401 15 29 386 0.55 0.0542 0.0011 0.539 0.012 0.0713 0.0007 381 45 438 9 444 5 16 32 415 0.63 0.0551 0.0010 0.544 0.010 0.0709 0.0006 416 41 441 8 442 4 17 21 273 0.52 0.0566 0.0016 0.565 0.017 0.0718 0.0008 474 62 455 13 447 5 18 23 304 0.63 0.0560 0.0014 0.559 0.015 0.0716 0.0007 453 56 451 12 446 4 19 22 288 0.62 0.0546 0.0014 0.551 0.016 0.0724 0.0012 396 57 445 13 451 7 20 18 233 0.56 0.0549 0.0021 0.547 0.021 0.0715 0.0011 409 84 443 17 445 7 21 37 475 1.18 0.0547 0.0008 0.542 0.013 0.0711 0.0018 401 34 440 10 443 11 22 27 341 0.64 0.0544 0.0012 0.553 0.014 0.0730 0.0014 388 48 447 11 454 9 23 19 250 0.66 0.0533 0.0024 0.536 0.026 0.0721 0.0016 343 103 436 21 449 10 24 26 337 0.53 0.0544 0.0015 0.550 0.019 0.0727 0.0021 388 63 445 15 452 13 25 26 335 0.67 0.0550 0.0019 0.550 0.020 0.0719 0.0012 410 78 445 16 447 7 26 33 436 0.79 0.0572 0.0019 0.551 0.016 0.0692 0.0034 499 73 446 13 431 21 27 45 595 1.21 0.0604 0.0009 0.535 0.008 0.0635 0.0003 619 31 435 6 397 2 28 24 311 0.58 0.0546 0.0014 0.550 0.016 0.0724 0.0014 394 57 445 13 451 9 29 35 469 0.72 0.0590 0.0011 0.560 0.011 0.0682 0.0006 566 41 452 9 425 4 30 40 499 0.89 0.0554 0.0010 0.555 0.011 0.0719 0.0010 429 40 448 9 448 6 09NM41 1 21 275 0.58 0.0538 0.0018 0.544 0.020 0.0733 0.0010 362 75 441 16 456 6 2 26 333 0.52 0.0546 0.0011 0.565 0.014 0.0751 0.0009 395 47 455 11 467 5 3 30 361 0.73 0.0571 0.0009 0.604 0.012 0.0766 0.0009 496 34 480 9 476 5 4 29 346 0.82 0.0571 0.0014 0.585 0.016 0.0743 0.0010 494 53 468 13 462 6 5 35 421 0.73 0.0575 0.0009 0.605 0.012 0.0763 0.0009 509 35 480 9 474 5 6 12 149 0.52 0.0586 0.0022 0.617 0.026 0.0764 0.0008 550 82 488 21 474 5 7 41 529 0.60 0.0562 0.0007 0.580 0.009 0.0748 0.0006 461 29 464 7 465 4 8 13 167 0.59 0.0543 0.0021 0.563 0.023 0.0751 0.0007 386 88 453 18 467 4 9 45 562 0.85 0.0564 0.0006 0.572 0.008 0.0735 0.0007 470 23 459 6 457 4 10 38 479 0.70 0.0551 0.0008 0.563 0.008 0.0742 0.0007 415 32 454 6 461 4 11 29 369 0.73 0.0569 0.0010 0.572 0.010 0.0729 0.0007 489 37 459 8 453 4 12 33 416 0.81 0.0565 0.0008 0.566 0.008 0.0726 0.0007 473 31 455 7 452 4 13 38 493 0.63 0.0562 0.0007 0.564 0.007 0.0728 0.0006 458 27 454 6 453 4 14 40 518 0.62 0.0563 0.0007 0.567 0.007 0.0730 0.0007 464 29 456 6 454 4 15 40 528 0.70 0.0568 0.0007 0.554 0.007 0.0707 0.0007 484 28 447 6 440 4 16 46 580 0.76 0.0573 0.0006 0.569 0.006 0.0720 0.0007 504 23 457 5 448 4 17 38 467 0.78 0.0558 0.0008 0.573 0.010 0.0745 0.0007 444 31 460 8 463 4 18 22 271 0.74 0.0549 0.0013 0.557 0.014 0.0735 0.0006 410 52 450 11 457 4 19 32 370 0.97 0.0555 0.0009 0.578 0.010 0.0755 0.0006 434 35 463 8 469 4 20 37 479 0.64 0.0554 0.0007 0.565 0.009 0.0740 0.0008 427 27 455 7 460 5 21 39 488 0.82 0.0572 0.0008 0.576 0.013 0.0730 0.0013 499 31 462 10 454 8 22 42 548 0.59 0.0559 0.0006 0.568 0.008 0.0738 0.0007 446 22 457 6 459 4 23 42 564 0.58 0.0560 0.0005 0.552 0.008 0.0715 0.0008 453 21 446 7 445 5 24 24 312 0.64 0.0557 0.0012 0.566 0.014 0.0736 0.0008 441 47 455 11 458 5 25 15 194 0.54 0.0566 0.0020 0.586 0.022 0.0750 0.0009 477 78 468 17 466 6 26 58 680 0.90 0.0559 0.0006 0.582 0.010 0.0754 0.0010 450 22 466 8 469 6 
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表 2 白乃庙铜矿中酸性岩类主量元素成分
Table 2 Major element composition of intermediateacid rocks in the Bainaimiao copper deposit
% 样品号
岩性08MD101 08MD102 09NM401-1 09NM401-2 09NM401-3 花岗闪长岩 花岗闪长岩 SiO2 66.93 65.56 63.90 65.70 63.70 AI2O3 15.03 15.12 16.1 15.45 16.1 Fe2O3 2.59 2.82 3.08 2.45 3.82 FeO 1.09 1.39 2.38 2.68 1.78 CaO 2.94 3.61 3.35 1.94 3.28 MgO 1.52 1.62 1.98 1.94 1.94 K2O 3.69 3.56 2.78 4.22 3.34 Na2O 2.44 2.14 3.48 2.74 3.06 TiO2 0.38 0.42 0.49 0.44 0.52 P2O5 0.13 0.13 0.17 0.15 0.2 MnO 0.04 0.06 0.046 0.036 0.058 烧失量 2.98 3.00 1.76 1.84 1.91 total 99.77 99.43 99.52 99.59 99.71 A/CNK 1.12 1.08 1.08 1.22 1.10 δ 1.57 1.44 1.88 2.13 1.98
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表 3 白乃庙铜矿中酸性岩类微量元素成分
Table 3 Trace element composition of intermediateacid rocks in the Bainaimiao copper deposit
10-6 样品号 08MD
101-108MD
101-209NM
401-109NM
401-209NM
401-3岩性 花岗闪长岩 花岗闪长岩 La 28.82 21.77 45.20 38.80 45.00 Ce 59.93 39.29 67.60 68.00 69.60 Pr 4.72 3.95 8.00 7.45 8.16 Nd 16.58 13.83 27.40 26.40 28.20 Sm 2.87 2.53 4.90 5.00 5.10 Eu 0.91 0.81 1.72 1.62 1.90 Gd 2.34 2.03 4.36 4.28 4.50 Tb 0.32 0.27 0.56 0.59 0.58 Dy 1.78 1.52 2.53 3.02 2.72 Ho 0.36 0.30 0.50 0.60 0.52 Er 1.04 0.92 1.38 1.63 1.46 Tm 0.17 0.17 0.22 0.27 0.24 Yb 1.31 1.23 1.56 1.92 1.67 Lu 0.21 0.19 0.26 0.31 0.26 ZREE 121.36 88.80 166.19 159.89 169.91 LREE/HREE 15.13 12.39 13.62 11.67 13.22 (La/Sm)N 6.48 5.55 5.96 5.01 5.70 (La/Yb)N 15.75 12.68 20.78 14.50 19.33 δEu 1.04 1.05 1.11 1.04 1.19 Sr 635.59 547.04 614.00 473.00 578.00 Rb 80.28 56.19 97.50 114.50 102.55 Ba 1053.45 864.15 684.50 1290.00 953.00 Th 21.83 14.70 11.80 13.20 13.80 U 3.11 1.89 2.56 2.90 2.49 Ta 0.66 0.60 0.64 0.62 0.58 Nb 9.25 8.38 8.45 9.09 8.58 Ta 0.66 0.60 0.64 0.62 0.58 Zr 136.78 116.72 108.00 122.00 108.00 Hf 3.62 3.14 3.39 3.70 3.40 Y 10.30 8.18 14.70 16.60 14.40 Sr/Y 61.73 66.88 41.77 28.49 40.14
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