Magnetite geochemistry of the Heijianshan Fe–Cu (–Au) deposit in Eastern Tianshan: Metallogenic implications for submarine volcanic-hosted Fe–Cu deposits in NW China

Abstract

The Heijianshan Fe–Cu (−Au) deposit is located in the Aqishan–Yamansu belt in Eastern Tianshan, NW China. As a typical Fe–Cu deposit in the region, Heijianshan is hosted in the Upper Carboniferous Matoutan Formation submarine volcanic/volcaniclastic rocks. Alteration styles, mineral assemblages and vein crosscutting relationships divide the hydrothermal alteration and mineralization processes into seven stages, namely the chromite (Stage I), epidote (Stage II), magnetite (Stage III), pyrite (Stage IV), Cu (–Au) (Stage V), late veins (Stage VI) and supergene (Stage VII) alteration/mineralization stages. Magnetite mineralization comprises the hematite (Stage III–A) and main magnetite (Stage III–B) mineralization sub-stages.The Heijianshan magnetite ores consist of massive (with mushketovite (MOM) or sulfides (MOS)), disseminated (DO) and magnetite clasts (with chromite (MWC) or without chromite (MNC)) ores. Magnetite in massive- and disseminated ores is featured by (1) depletion in Zr, Nb and Ta; (2) low Ti (<2wt.%) and Al (<1wt.%); and (3) Ni/Cr≥1, which all reflect a hydrothermal origin. Moreover, magnetite in massive- and disseminated ores has lower Cr (MOM: 0–13.2ppm; MOS: 0–12.9ppm; DO: 3.57–133ppm) than magnetite clasts ores (MNC: 849–2544ppm; MWC: 835–44,132ppm). However, the high Cr in the magnetite clasts ores may have been inherited from the chromite they replaced. From the magnetite clasts to disseminated/massive ores, formation temperature decreased and fO2 increased, which may represent the major controls on the formation of the various magnetite ore types. Compositions of the ore fluids and host rocks, formation of coexisting minerals and other physicochemical parameters (such as T and fO2) may have variably influenced the magnetite geochemistry in the different Heijianshan ore types, with fluid compositions probably playing the most important role.Discrimination diagrams, for instance, Cr vs. Co/Ni, Cr vs. Ti, V vs. Cr and Ni vs. Cr, may be useful for magnetite mineralization type differentiation in other submarine volcanic-hosted Fe/Fe–Cu deposits in the Aqishan–Yamansu belt. Geochemical discrimination diagrams, alteration and mineralization paragenesis indicate that the Heijianshan Fe–Cu (–Au) deposit is best classified as an IOCG-like deposit, which offers a new insight for classifying and characterizing ore genetic types for similar Fe and Fe–Cu deposits in Eastern Tianshan.