Zircon U-Pb Geochronology and Geochemistry of Koka Granite and Its Geological Significance, Eritrea
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摘要: Koka花岗岩位于厄立特里亚Nakfa地区以西，是Koka金矿床的主要赋矿围岩.岩体具有富SiO2（67.94%~78.40%）、Na2O+K2O（5.86%~8.76%）、Al2O3（11.05%~16.51%）、FeOT（2.46%~3.80%），弱过铝质－强过铝质（A/CNK为1.09~1.55），低CaO（0.06%~1.85%）、MgO（0.15%~0.39%）的主量元素特征，同时轻稀土富集，重稀土相对亏损，强烈亏损Sr、P、Ti元素，REE分配曲线呈现燕式分布和明显的负铕异常，表明岩体具有A型花岗岩的特征.岩体锆石LA-ICP-MS U-Pb年龄显示其成岩年龄为851.2±1.9 Ma，属早新元古代，不同于区域上广泛分布的与造山后伸展作用相关的A型花岗岩（650~540 Ma），结合区域研究成果认为，其可能形成于由俯冲作用而引起的弧后拉张环境.岩体锆石具有一定的Ce正异常，Ce4+/Ce3+变化范围为3.86~146.31，平均为32.4，指示岩浆的氧逸度相对较低，结合岩浆源区为较“干”的体系特征，暗示该岩体成矿潜力较低，难以形成相关的大型、超大型矿床.Abstract: Koka granite, located in the west of the Nakfa region in Eritrea, is the main host rock of Koka gold deposit. The major elements of the granite are characteristiced by high SiO2 (67.94%-78.40%), Na2O+K2O (5.86%-8.76%), Al2O3 (11.05%-16.51%) and FeOT (2.46%-3.80%), weakly peraluminous to strongly peraluminous (A/CNK is 1.09-1.55), and low CaO (0.06%-1.85%), MgO (0.15%-0.39%). It is also enriched in LREEs and relatively depleted in HREEs, strongly depleted in Sr, P, Ti. The REE distribution curve shows characteristics such as swallow distribution and apparent negative europium anomalies. All of these indicate that the Koka granite has an affinity of A-type granite. Zircon LA-ICP-MS U-Pb dating shows the granite formed in early period of the Neoproterozoic with the diagenetic age of 851.2±1.9 Ma. This is different from the A-type granite which is widely distributed and related to the extension after collisional orogen (650-540 Ma). Combined with the regional geological research results, it is suggested that Koka granite was formed in a back-arc extensional environment which resulted from subduction. The zircon has a certain Ce positive anomaly, and variation range of the Ce4+/Ce3+ is 3.86-146.31, with an average of 32.4, indicating low degree of magmatic oxygen fugacity. Meanwhile the parental magma of Koka granite is "dry", suggesting that the metallogenic potentiality is low, and it is hard to form related large or super large deposits.
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Key words:
- Koka gold deposit /
- zircon U-Pb dating /
- geochemistry /
- Eritrea
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图 1 区域地质简图(a，b)及Koka金矿区地质图(c)
图a据Johnson et al.(2011)修改；图b据Teklay et al.(2006)修改；图c据Chalice金矿有限公司, 2010, 厄立特里亚Koka金矿地质勘查报告，珀斯
Fig. 1. Sketch of regional geology (a, b) and geology map of Koka gold deposit (c)
图 2 Koka花岗岩岩体露头(a)、手标本(b)与镜下照片(c，d)
Q.石英；Pl.斜长石；Kfs.钾长石；Py.黄铁矿
Fig. 2. Outcrop photograph (a), hand specimen photograph (b) and photomicrographs (c, d) of Koka granite
图 4 Koka花岗岩锆石REE球粒陨石配分模式(a)和Ce4+/Ce3+值分布(b)
Fig. 4. Primitive mantle normalized REE diagram (a) and scattergram of Ce4+/Ce3+value (b) of the zircon from Koka granite
图 5 Koka金矿区花岗岩TAS图解(a)，Na2O-K2O图解(b)
图a据Middlemost(1985)；图b据Maniar and Piccoli(1989)
Fig. 5. TAS diagram (a), Na2O-K2O (b) diagram of granite from Koka gold deposit
图 6 Koka花岗岩稀土元素球粒陨石标准化(a)和微量元素原始地幔标准化图(b)
Fig. 6. Primitive mantle normalized REE diagram (a) and chondirite normalized multi-element diagram (b) of zircon for Koka granite
图 7 Koka金矿区花岗岩(Na2O+K2O)/CaO(a)、FeOT/MgO与Zr+Nb+Ce+Y(b)和FeOT/(FeOT+MgO)与SiO2(c)判别图
图a、b据Whalen et al.(1987)；图c据Frost et al.(2001)
Fig. 7. Diagram of classification of granite origin from Koka gold deposit:(Na2O+K2O)/CaO (a), FeOT/MgO vs. Zr+Nb+Ce+Y (b), FeOT/(FeOT+MgO)vs. SiO2 (c)
图 8 Koka金矿区花岗岩构造判别图
底图a据Harris et al.(1986)；图b~d据Pearce et al.(1984)；ORG.洋脊花岗岩；WPG.板内花岗岩；VAG.火山弧花岗岩；Syn-COLG.同碰撞花岗岩；LCC-PCG.碰撞晚期-碰撞后花岗岩
Fig. 8. Structure discrimination diagram of granite from Koka gold deposit
图 9 Koka金矿区花岗岩构造环境判别图
图据Eby(1992)；IAB.岛弧玄武岩；OIB.洋岛玄武岩
Fig. 9. Tectonic discrimination diagram of granite from Koka gold deposit
图 10 厄立特里亚部分前/碰撞花岗岩、火山岩及后碰撞花岗岩年龄分布
年龄数据据Teklay et al.(2002a, 2002b, 2003)
Fig. 10. The distribution of pre-syn granites, volcanic and post granites, and their geochronological data in Eritrea
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