鄂东南地区龙角山-付家山斑岩体成因及其对成矿作用的指示

摘要

龙角山铜钨矿床是鄂东南地区典型的层控式矽卡岩型矿床。本文利用LA-ICP-MS锆石U-Pb法及辉钼矿Re-Os法对该矿床进行了详细的年代学研究。结果表明，矿区内的花岗闪长斑岩形成于144±1Ma，成矿作用发生于144．7±2．9Ma，成岩成矿作用近于同时发生，均为早白垩世。这意味着成矿作用与花岗闪长斑岩的演化存在密切联系。龙角山-付家山花岗闪长斑岩为高钾钙碱性系列，具有高 Al2 O3（14．99％～16．16％）、Sr（751×10-6～1382×10-6）含量和 Sr/Y（65～99）、La/Yb（40～48）比值，以及低 Y（12×10-6～15×10-6）、Yb（0．93×10-6～1．23×10-6）含量，与典型埃达克质岩的地球化学特征基本一致。全岩（87Sr/86Sr）i值为0．70603，εNd（t）值为-5．1，锆石εHf （t）值介于-0．5～-4．8之间。元素地球化学、全岩Sr-Nd同位素及锆石 Hf同位素特征一致表明龙角山-付家山花岗闪长斑岩主要来源于富集岩石圈地幔的部分熔融作用，在成岩过程中可能伴随着镁铁质矿物的分离结晶作用。此外，与铜山口成矿斑岩体相比，龙角山-付家山花岗闪长斑岩具有较低的氧逸度，这很可能是导致两斑岩体在矿化类型上（前者以铜钼矿化为主，后者以铜钨矿化为主）存在差异的主要原因。%Longjiaoshan Cu-W deposit is a strata-bound skarn deposit in southeastern Hubei Province.In this paper,the authors carry out detail geochronological study by using LA-ICP-MS zircon U-Pb and molybdenite Re-Os analyses.The results indicate that the granodioriteporphyries in the Longjiaoshan orefield were emplaced in 144 ± 1Ma,and mineralization occurred in 144. 7 ± 2. 9Ma,suggesting that diagenesis and mineralization were broadly coeval and occurred in Early Cretaceous.This means that the mineralization was closely related to the evolution of granodiorite porphyries.The Longjiaoshan-Fujiashan granodiorite porphyries are high-K calc-alkaline,and show adakite-like geochemical characteristics,such as high Al2O3(14. 99%~16. 16%),Sr(751×10-6~1382×10-6)contents and Sr/Y(65~99),La/Yb(40~48)ratios,as well as low Y(12×10-6~15×10-6),Yb(0. 93×10-6~1. 23×10-6)contents.(87Sr/86Sr)i andεNd(t)value of the whole rock is 0. 70603 and -5. 1,respectively,and zirconεHf(t)values range from-0. 5 to -4. 8.Geochemical,Sr-Nd isotopic of the whole rock and zircon Hf isotopic characteristics consistently indicate that the Longjiaoshan-Fujiashan granodiorite porphyries were mainly derived from partial melting of enriched lithospheric mantle,and possibly followed by fractional crystallization of mafic minerals in the course of the diagenesis. Moreover, compared with the Tongshankou granodiorite porphyry,the Longjiaoshan-Fujiashan granodoirite porphyry has lower oxygen fugacity,which likely resulted in their difference of mineralization type (Cu-Mo mineralization predominated in the former, however,Cu-W mineralization predominated in the latter).