岩浆演化对邯邢式铁矿成矿的制约:来自角闪石化学成分的证据

Constraints of the magmatic evolution on the mineralization of Han-Xing type iron deposit: evidences from amphibole chemical compositions

通过对比冀南邯邢地区矿山岩体和固镇岩体中角闪石的成分差异和形成的物理化学条件差异,讨论岩浆演化对邯邢式铁矿形成的制约。电子探针分析结果显示,强成矿的矿山岩体中角闪石主要为阳起石和镁角闪石,弱成矿岩体固镇岩体中角闪石主要为镁绿钙闪石、镁角闪石和阳起石,前者角闪石成分相对贫Fe、富Mg、Mn以及F和Cl等挥发分。角闪石矿物温压计计算结果显示:矿山岩体中的角闪石主要为岩浆侵位到浅部以后结晶,形成的温度和压力较低,氧逸度较高,未经历明显的结晶分异作用;固镇岩体中角闪石经历了深部岩浆房的结晶分异过程,而后侵位到浅部,大多形成于深部岩浆房,形成温度高、压力大。两者中角闪石成分特征与结晶时物理化学条件的明显差异表明,含有高挥发分F、Cl的岩浆有利于邯邢式铁矿的形成,角闪石在岩浆房深部的结晶分异过程导致硅酸盐体系中铁被过多和过早的消耗,不利于矽卡岩型铁矿的形成。

In this paper, we have discussed constraints of the magmatic evolution on the formation of Han-Xing type iron deposit by comparing differences of chemical compositions and formation physicochemical conditions of amphiboles in the Kuangshan and Guzhen intrusions in the Han-Xing area of the Southern Hebei. The results of electron probe microanalysis show that amphiboles in the strongly mineralized Kuangshan intrusion mainly consist of actinolite and magnesiohornblende, while amphiboles in the weakly mineralized Guzhen intrusion mainly consist of magnesiohastingsite, magnesiohornblende and actinolite. The former amphiboles are relatively poor in Fe but rich in Mg, Mn, F, Cl and other volatile components. The calculated results by using the amphibole thermobarometers suggest that amphiboles in the Kuangshan intrusion were mainly crystallized in the magma which had emplaced into the shallow part and formed under conditions of relatively low temperature and pressure, relatively high oxygen fugacity, with no obvious crystallization differentiation, while amphiboles in the Guzhen intrusion were originally formed in the magma crystallization differentiation process in the deep magma chamber and then the amphibole-bearing magma emplaced into the shallow part. The amphiboles were mostly formed in the deep magma chamber at high temperature and high pressure condition. The obvious differences of chemical compositions and the physicochemical conditions during crystallization of amphiboles in those two intrusions indicate that the magma with high contents of volatile including F and Cl is favorable to the formation of Han-Xing type iron deposit. The formation of amphibole in the magma crystallization differentiation process in the deep magma chamber could lead to the excessive and premature iron consumption in the magmatic silicate system. This is unfavorable to the formation of skarn type iron deposit by the amphibole-bearing magma emplaced into the shallow part.