The Iron Isotope Fractionation And Its Significance During Alteration Process Of The Skarn Type Iron Deposit,Laiwu,Shandong

Fe isotope system is a new isotope tracer method,and its geochemical behavior in the process of mineralization is an international frontier and hot spot in recent years.Located in the Eastern North China Craton,Zhangjiawa deposit in Laiwu,is a typical skarn iron deposit,and the source and mineralization of ore-forming materials have been controversial.Based on the study of petrology and geochemical characteristics of the deposit,we focus on the iron isotope composition to investigate iron isotope fractionation during alkaline alteration and to restrict the mineralization mechanism of Zhangjiawa iron deposit.The main achievements that we obtained were as follows:1.Deffrent from the Hanxing iron deposit,a wide range of alkaline alteration has occurred in the ore forming diorite pluton in Zhangjiawa deposit,with the existence of the process of potassium and sodium.The Fe element was brought out by the alteration,which lead to the increase of the content of alkaline substance(Na+K),and the Fe₂O₃content of the altered rock shows a negative correlation with the content of Na₂O+K₂O.2.The average Fe isotope composition of the unaltered diorite is+0.03‰,the alterated diorite is+0.27‰.The results show that Fe isotope fractionation occurred during alkaline alteration process,the light Fe isotope was extracted by hydrothermal fluids and the unaltered diorites had heavier Fe isotope composition caused by the alteration.Different from our study,previous studies found that the heavy Fe isotope firstly separated out during alteration process.3.The fluids produced during the evolution of magma and enriched the light Fe isotopes,and further enriched during the alkaline alteration process.In the process of mineralization,the Fe(III)in the fluids was preferentially deposited into the rich-iron ores,which made the iron-rich ores enriched the heavy Fe isotopes.During the late fluids mineralization progress,the iron element enriched the light Fe isotope got into the skarn-diorites.4.Isotopically light Fe isotopes enriched in Majiagou limestone,and the?⁵⁷Fe values are between-0.50‰?-0.32‰.While the magnetite ores are enriched the heavy isotopes,?⁵⁷Fe_average=+0.11‰±0.017.It's indicating that the limestone was not the origin of the deposit.The Fe element in the rich-iron ores mainly came from the fluids and the Fe in the fluids mainly came from the process of alkaline alteration.