Assessing the magmatic affinity and petrogenesis of granitoids at the giant Aktogai porphyry Cu deposit, Central Kazakhstan
Assessing the magmatic affinity and petrogenesis of granitoids at the giant Aktogai porphyry Cu deposit, Central Kazakhstan
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DOI：
10.2475/07.2016.02
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发表时间：
2016-09
影响因子：
2.9
通讯作者：
Mingjian Cao;Guangming Li;K. Qin;N. Evans;E. Seitmuratova
中科院分区：
文献类型：
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作者：
Mingjian Cao;Guangming Li;K. Qin;N. Evans;E. Seitmuratova
与大~巨型氧化斑岩铜矿床伴生的大部分矿化斑岩与高Sr/Y岩石亲和，而贫瘠或弱矿化花岗岩类则表现出典型的低Sr/Y特征。阿克托盖巨型斑岩铜矿赋存于Koldar岩体中，为研究这种关系提供了良好的天然实验室，同时确定了岩体的成因和成矿作用。对矿石前花岗闪长岩（Koldar岩体的主要成分）和矿化花岗闪长斑岩进行了锆石U-Pb定年、矿物化学、全岩地球化学、Sr-Nd-Pb和锆石Hf-O同位素分析。锆石U-Pb年龄表明，成矿前花岗闪长岩和矿化花岗闪长斑岩的侵位分别为345 Ma和328 Ma ~ 331 Ma。花岗闪长斑岩中磷灰石SO3含量明显高于花岗闪长岩，表明成矿斑岩形成过程中fO2含量增加（>NNO+1）。虽然所有岩石具有相似的地球化学特征（钙碱性、Nb、Ta、Ti强亏损、LREE、LILE富集），但矿石前Koldar岩体具有正弧相关岩浆特征（低Sr/Y、（La/Yb）N、高Y、YbN），花岗闪长斑岩和闪长岩（Koldar岩体的微量成分）具有高Sr/Y、（La/Yb）N、低Y、YbN特征。所有样品的Sr-Nd-Pb-Hf-O同位素组成相似[(87Sr/86Sr)i = 0.70369 ~ 0.70413, εNd (t) = + 3.6 ~ + 5.6, (206Pb/204Pb)i = 18.16 ~ 19.32，锆石εHf (t) = + 11.8 ~ + 15.9, δ18O = + 3.8 ~ + 5.9‰]，非常年轻的全岩T2DM （Nd）值（640 ~ 680 Ma）和锆石TDMC （Hf）值（320 ~ 590 Ma）]，可能来源于幼年下地壳的部分熔融。地球化学模式和部分熔融模拟表明，高Sr/Y岩石可能是由榴辉化、增厚的下地壳部分熔融形成的，而科尔达尔岩体是由正常厚下地壳部分熔融形成的。成矿前的低Sr/Y岩可能是通过准噶尔-巴尔卡什洋壳的俯冲作用较早形成的，而高Sr/Y岩则可能是通过富硫化物的增厚幼期下地壳的部分熔融作用较晚形成的。高Sr/Y岩石的高氧逸度和高熔融温度保证了所有硫化物溶解在岩浆中，并侵入先前侵位的低Sr/Y岩体，导致了显著的成矿作用。
Most mineralized porphyries associated with large to giant oxidized porphyry Cu deposits show an affinity with high Sr/Y rocks, while barren or weakly mineralized granitoids show typical low Sr/Y features. The Aktogai giant porphyry Cu deposit occurs in the Koldar pluton and provides a good natural laboratory in which to investigate this relationship, while determining the petrogenesis of the pluton and its mineralization. Zircon U-Pb dating, mineral chemistry, whole rock geochemistry and Sr-Nd-Pb and zircon Hf-O isotopic analyses were carried out on the pre-ore granodiorite (the major component of the Koldar pluton) and on the mineralized granodiorite porphyry. Zircon U-Pb ages indicate that the pre-ore granodiorite and mineralized granodiorite porphyries were emplaced at 345 and 328 to 331 Ma, respectively. Distinctly higher apatite SO3 contents in the granodiorite porphyry relative to the granodiorite suggest an increase in fO2 during the petrogenesis of the mineralized porphyries (>NNO+1). Although all rocks share similar geochemical characteristics (calc-alkaline, strong depletion in Nb, Ta and Ti, and enrichment in LREE and LILE), the pre-ore Koldar pluton has normal arc related magmatic features [low Sr/Y and (La/Yb)N, high Y and YbN], while the granodiorite porphyries and diorite (trace component of Koldar pluton) exhibit high Sr/Y and (La/Yb)N, low Y and YbN features. All samples show similar Sr-Nd-Pb-Hf-O isotopic compositions [(87Sr/86Sr)i = 0.70369 to 0.70413, εNd (t) = + 3.6 to + 5.6, (206Pb/204Pb)i = 18.16 to 19.32, zircon εHf (t) = + 11.8 to + 15.9, and δ18O = + 3.8 to + 5.9 ‰], and very young whole rock T2DM (Nd) (640 – 680 Ma) and zircon TDMC (Hf) (320 – 590 Ma) values, suggesting that they were probably derived from partial melting of juvenile lower crust. Geochemical patterns and partial melt modeling indicate that the high Sr/Y rocks were probably formed by partial melting of eclogitized, thickened lower crust, while the Koldar pluton formed by partial melting of normal thick lower crust. We propose that pre-ore low Sr/Y rocks were probably generated earlier via subduction of Junggar-Balkhash oceanic crust, and that the high Sr/Y rocks were formed later by partial melting of sulfide-enriched, thickened juvenile lower crust. High oxygen fugacity and the high melting temperature of the high Sr/Y rocks ensured that all sulfide was dissolved in the magma, which intruded the previously emplaced low Sr/Y pluton and resulted in significant mineralization.