鄂东南矿集区鸡冠嘴矽卡岩型金铜矿床含矿岩体中辉石和角闪石成分变化特征及其对岩浆演化和成矿的指示意义

The composition of pyroxene and amphibole in ore-related pluton in Jiguanzui Au-Cu skarn deposit,Edong region: Implication for the magma evolution and mineralization

为了解长江中下游地区与金铜矿化有关的岩体在浅部的演化过程及其对成矿的作用,本文对鄂东南矿集区鸡冠嘴矽卡岩型金铜矿成矿岩体石英二长闪长斑岩进行了详细的岩相学观察,并利用电子探针(EMPA)分析了其中辉石和角闪石的主量成分。岩相学特征显示辉石形成时间早于角闪石。辉石成分变化较小,而角闪石成分变化较大。角闪石可根据Al的含量分为高低两组,即高铝含量的自形角闪石以及低铝含量的半自形及他形角闪石。高铝角闪石又不同程度的被低铝角闪石交代。通过矿物温压计估算辉石形成的温度和压力为1055~1071℃(平均1060℃)和224~312MPa(平均255MPa)。高铝角闪石形成的温度和压力为809~864℃(平均833℃),108~193MPa(平均137MPa),低铝角闪石形成的温度和压力为721~766℃(平均741℃),48~67MPa(平均56MPa)。岩浆具有较高的氧逸度,并且从高铝角闪石到低铝角闪石,熔体的氧逸度从△NNO+0.6升高到△NNO+1.9。岩浆经历了连续的侵位历史,从9.6km到5.2km的岩浆房再到2.1km的浅地壳处就位。岩浆在9.6km处经历了辉石的分离结晶,在5.2km处经历了角闪石及少部分斜长石的分离结晶,在2.1km处经历由于降压引起的流体出溶。由于深部的分离结晶作用,熔体中的水含量增加,氧逸度升高。较高的水含量使岩浆更容易演化出成矿热液,较高的氧逸度使岩浆演化早期没有硫化物的分离结晶,从而阻止了成矿元素在岩浆演化早期亏损。鸡冠嘴岩体较浅的就位深度也更有利于成矿流体的析出。这些因素共同作用形成了鸡冠嘴金铜矿床。

To get the information of the evolution of magma in the upper crust and its contribution to the mineralization in Middle Lower Yangtze River metallogenic belt, China, detailed petrographic observation and EMPA analysis of chemical compositions of pyroxene and amphibole have been carried out on the ore-related quartz monzodiorite porphyry in the Jiguanzui Au-Cu skarn deposit. According to the petrographic evidence, the pyroxene formed earlier than the amphibole. The amphibole can be divided into two sub-types, i.e., the high-aluminum （High-Al） amphibole and low-aluminum （Low-Al） amphibole by their aluminum contents and crystalline forms. The euhedral amphibole is mainly High-Al amphibole and subhedral and anhedral amphiboles belong to Low-Al amphibole. Some euhedral amphiboles are partially or completely replaced by the Low-Al amphiboles. The pyroxenes are formed at 224~312MPa （average 255MPa） and 1055~1071℃ （average 1060℃）. The High-Al amphiboles are euhedral and formed at 108~193MPa （average 137MPa） and 809~864℃ （average 833℃）. Whereas, the Low-Al amphiboles are subhedral and anhedral and formed at 48~67MPa （average 56MPa） and 721~766℃ （average 741℃）. The magma stalled at 9.6km depth and underwent fractional crystallization of pyroxene before it intruded up to the 5.2km depth and experienced fractional crystallization of amphibole and also a little plagioclase. The magma had a high oxygen fugacity and increased from High-Al amphiboles （△NNO＋0.6） to Low-Al amphiboles （△NNO＋1.9）, which can prevent the loss of Cu to sulphide minerals during the crystallization. Furthermore the fractional crystallization enriched the water content of the residual magma, a favorable condition for the fluid exsolution. Finally, the evolved magma intruded to the shallower depth （2.1km）, where it presumably exsolved aqueous ore-forming fluids. The formation of the large Jiguanzui deposit is likely attributed to the high oxygen fugacity, some extent of fractional crystallization, and the shallow emplacement depth.