Extreme iron isotope variation of pyrite in the Muping gold deposit, Jiaodong: Implication for tracing metal origin

Pyrite has complicated iron isotope fractionation behavior in the hydrothermal condition. Here we present a.detailed in-situ iron isotope analyses of different generation pyrite from the Muping gold deposit, eastern China..δ56Fe values of pyrite investigated range from 4.25‰ to +1.78‰, unlike previous bulk analytic data ( 0.78‰.to +0.79‰). Iron isotopic homogeneity of single pyrite grain, which can be verified by in-situ analysis, was.recommended as the preferred and reliable criterion to judge the isotopic fractionation condition in the hydrothermal.gold deposit. Inter-grain variations in Fe isotopic composition suggest occurrence of both kinetic and.equilibrium fractionation for pyrite at Muping. Electron backscattered diffraction analysis shows that the.replacement likely occurred via dissolution-reprecipitation pathway. The replacement process resulted in.obvious kinetic isotope effect, in which pyrites are possessed in extreme negative Fe isotopic composition.(~ 4‰). Maximum δ56Fe of pyrite up to +1.78‰ implied that kinetic isotopic effect also play a significant role.in increasing the Fe isotopic composition of ore-forming fluids in the Muping gold deposit. Further model calculations.suggested the kinetic isotopic effect was dominated by pyrite fast precipitation rather than pyrrhotite.precipitation or pyrite precipitation via “FeS” pathway. This kinetic isotope effect ultimately will be partly or.completely erased due to fast isotope exchange rate at medium–high temperature condition. Pyrite with homogeneous.Fe isotopic composition can be used to trace iron source of hydrothermal gold deposit but be.cautious.

在热液条件下，黄铁矿具有复杂的铁同位素分馏行为。在此，我们对中国东部牟平金矿床不同世代的黄铁矿进行了详细的原位铁同位素分析。所研究黄铁矿的δ56Fe值范围为 -4.25‰到 +1.78‰，不同于先前的整体分析数据（ -0.78‰到 +0.79‰）。通过原位分析可验证，单个黄铁矿颗粒的铁同位素均一性被推荐为判断热液金矿床同位素分馏条件的首选且可靠的标准。铁同位素组成在颗粒间的变化表明，牟平的黄铁矿存在动力学和平衡分馏。电子背散射衍射分析表明，这种替代可能是通过溶解 - 再沉淀途径发生的。该替代过程导致了明显的动力学同位素效应，其中黄铁矿具有极负的铁同位素组成（约 -4‰）。黄铁矿的最大δ56Fe值高达 +1.78‰，这意味着动力学同位素效应在提高牟平金矿床成矿流体的铁同位素组成方面也起着重要作用。进一步的模型计算表明，动力学同位素效应主要由黄铁矿的快速沉淀引起，而非磁黄铁矿沉淀或通过“FeS”途径的黄铁矿沉淀。在中高温条件下，由于快速的同位素交换速率，这种动力学同位素效应最终将部分或完全被消除。具有均一铁同位素组成的黄铁矿可用于追踪热液金矿床的铁源，但需谨慎。