西天山阿吾拉勒成矿带群吉A型花岗岩成因、地质意义及成矿潜力评价

摘要

群吉钠长斑岩具有富SiO2 (65.3％ ～76.9％)、Na2O+ K2O(6.02％ ～9.66％)、FeOT/MgO(6.01～21.4),中等的A/CNK(0.90～1.04),低Al203 (9.82％ ～ 15.64％)、CaO(0.47％ ～ 1.84％)、MgO(0.13％～0.45％)的主量元素特征,同时富集Th、U、Ta、Zr、Hf等HFSE,轻稀土富集、重稀土相对平坦分布.高的Zr(468×10-6～707×10-6)、Y(20.7×10-6 ～91.4×10-6)、Nb(21.3×10-6 ～ 57.7×10-6)、Ga(11.2×10-6 ～ 19.7×10-6)以及Ce(36.7×10-6 ～ 98.2×10-6)和Zr饱和温度(＞880℃)特征表明岩体具有A型花岗岩的特征.群吉钠长斑岩的Isr值较低(0.70203 ～ 0.70549),εNd(t=303Ma)为正值(+4.1～+5.2),同时全岩Pb同位素落于地幔和下地壳之间的区域,表明形成该岩体的源岩可能为下地壳玄武质岩石.地壳的伸展引起了软流圈地幔底侵,在异常地温梯度下被底侵的玄武质下地壳发生部分熔融,形成该地区晚古生代A型花岗岩岩体.群吉地区A型花岗岩的发现,表明在晚石炭纪西天山阿吾拉勒地区为伸展的构造背景,同时岩石圈的拆沉、下地壳的加厚在晚石炭纪就已发生.群吉钠长斑岩中有局部或全部的铜矿化,该岩体是阿吾拉勒成矿带主要的含矿岩体.钠长斑岩基质中含有自形-半自形的黄铁矿及斑岩中锆石较低的Ce4+/Ce3+(19.5 ～93.0,平均为39.6),反映了岩浆低氧逸度的成岩条件,这种条件使得S从高价态变为了低价态,有利于成矿.岩浆中Zr的含量与岩浆中的水分含量成反比,H2O的加入也可以使岩浆具有高的氧逸度,因此岩浆中高的Zr含量及低的氧逸度,说明岩浆的源区为“干”体系,H2O的加入很少,而较“干”的体系对形成大型斑岩矿床不利.%The Qunji albite porphyry contains high SiO2 (65.30％ ～76.93％),Na2O + K2O (6.02％ ～ 9.66％) and FeOT/MgO (6.01 ～21.4) and low Al2 O3 (9.82％ ～ 15.64％),CaO (0.47％ ～ 1.84％),MgO (0.13 ％ ～ 0.45 ％) and is enriched in Th,U,Ta,Zr,Hf,Ga and Y.The porphyry has high Zr (468 × 10-6 ～707 叁 10-6),Y (20.7 叁 10-6 ～91.4 叁 10-6),Nb (21.3 × 10-6 ～ 57.7 × 10-6),Ga (11.2 × 10-6 ～ 19.7 × 10-6) and Ce (36.7 × 10-6 ～ 98.2 × 10-6) concentrations.Zircons from the porphyry are calculated to have saturation temperature ＞ 880℃.All of these indicate that the Qunji albite porphyry has an affinity of A-type granite.The porphyry has low Isr (0.70203 ～ 0.70549) and positive εNd (t =303Ma) value (+ 4.1 to + 5.2),and the whole rock Pb isotopic compositions are plotted in the field between the mantle and lower crust,indicating that the Qunji albite porphyry is likely sourced from the lower crust.The Qunji albite porphyry is therefore ascribed to the partial melting of the lower crust that was stimulated by the underplating basaltic magma from the asthenosphere,indicating an extension setting of this region in the late Paleozoic.The Qunji albite porphyry has potential of Cu mineralization.Zircons from the albite porphyry have low Ce4 +/Ce3 + (19.5 ～ 93.0) with a mean value of 39.6.The presence of euhedral and subeuhedral pyrite in the matrix indicates low oxygen fugacity of magma when the porphyry formed.The Cu mineralization was likely developed in a transition from oxidized S to reduced S,indicating that the parental magma of the Qunji albite porphyry is "dry",which has little potential for economic important porphyry Cu deposit.