The fractionation of iron isotope and its constraints on the sources of ore-forming materials in the Jinchanghe skarn polymetallic deposit in Sanjiang region, Southwest China

Abstract

金厂河矿床是西南三江成矿省保山地块最具代表性的远端矽卡岩型多金属矿床之一，查明其成矿金属来源对理解该类矿床成因以及区域成矿规律具有重要意义。本文通过分析不同成矿阶段代表性含铁矿物的铁同位素组成，探讨其在成矿过程中的分馏机制，从而示踪成矿金属的源区特征。金厂河矽卡岩型矿床中成矿前阶段未蚀变的石榴子石和氧化物成矿阶段的磁铁矿均相对富集铁的重同位素，其δ⁵⁶Fe值分别为0.05‰~0.16‰和0.07‰~0.18‰，而硫化物成矿阶段的黄铁矿和黄铜矿则相对富集铁的轻同位素，其δ⁵⁶Fe值分别为-0.12‰~0.17‰和-0.54‰~-0.38‰，整体显示出从高氧逸度的成矿前阶段向低氧逸度的硫化物成矿阶段演化过程中矿物δ⁵⁶Fe值逐渐降低的趋势，指示Fe³⁺富集铁的重同位素，Fe²⁺富集铁的轻同位素。同时，金厂河矿床各阶段矿物的δ⁵⁶Fe值均显著低于碳酸盐围岩，而接近全球花岗岩的δ⁵⁶Fe值，表明成矿的铁不是由围岩贡献，而是来自于隐伏的中酸性岩体。;Jinchanghe is a representative distal skarn polymetallic deposit in Baoshan block in the Sanjiang metallogenic province, finding out the sources of ore-forming metals is of great significance to understand the genesis of deposit and regional metallogeny. In this study, we analyzed the iron isotopic compositions of Fe-bearing minerals from different stages, to investigate the fractionation mechanism of iron in ore-forming process, and to trace the sources of ore-forming metals. The unaltered garnet from pre-ore stage and magnetite from oxide-ore stage in Jinchanghe skarn deposit are relatively rich in heavy isotopes of iron, with δ⁵⁶Fe values of 0.05‰~0.16‰ and 0.07‰~0.18‰, respectively. Whereas, the pyrite and chalcopyrite from sulfide-ore stage are relatively rich in light isotope, with δ⁵⁶Fe values of -0.12‰~0.17‰ and -0.54‰~-0.38‰, respectively. All results show that, from pre-ore stage of high oxygen fugacity to sulfide-ore stage of low oxygen fugacity, the δ⁵⁶Fe values of minerals gradually decrease, indicating that the heavy isotope of iron is enriched in Fe³⁺, but the light isotope of iron is enriched in Fe²⁺. Furthermore, the iron isotopes of minerals from different stages in Jinchanghe are significantly lower than that of carbonate wall rock, and close to that of global granite, implying that the ore-forming iron are not contributed by wall rock, but from the concealed granitic rocks.