Systematic textural and geochemical variations in magnetite from a porphyry-skarn Cu (Au) system and implications for ore formation, perspective from Xinqiao Cu-Fe-Au deposit, eastern China

Abstract

Magnetite is an abundant mineral in both porphyry ore and related skarn ore; the physical and chemical characteristics of magnetite in different parts of mineralized zones may allow determination of the related ore-forming processes. We present the results of detailed textural and chemical studies of magnetite from Xinqiao porphyry-skarn Cu-Au deposit, eastern China, to constrain the causative magmatic-hydrothermal processes.Five types of magnetite are identified, magmatic magnetite (type1) occurs in the causative quartz monzodiorite intrusion; re-equilibrated magmatic magnetite (type2) occurs in endoskarn; hydrothermal magnetite aggregated with hydrothermal biotite (type3a) or disseminated (type3b) and veins (type3c) occurs in the porphyry alteration zone; massive magnetite (type4) and disseminated or banded magnetite (type5) occurs in the exoskarn. Magmatic magnetite with oxy-exsolution of ilmenite lamellae indicates the magma was oxidized, and type1 magnetite that underwent hydrothermal re-equilibration (replaced by titanite and hematite) to form type2 magnetite indicates that the early magmatic-hydrothermal fluid was high temperature and highly oxidized during endoskarn formation. The Ti-rich type3a-c magnetite and the presence of ilmenite-rutile lamellae in type3c magnetite from the porphyry alteration zone demonstrate Ti mobility in the porphyry ore-forming fluid, which was high T (450 °C), oxidized and volatile-rich (hydrothermal fluorapatite; F > 3.0 wt%). Trace elements Ti, V, Sc, Co, Ni are positively correlated and decrease from type3 to type5; Mg, Mn, Zn increase in the same sequence, consistent with the temperature of fluid decreasing from porphyry to skarn mineralization, and increasing fluid-rock interaction. The elevated Sn in type5 magnetite and its coexistence with hematite in the retrograde skarn zone also indicate that the fluid remained oxidized during early skarn ore formation. Crystallization of magnetite increased the supply of reduced sulfur, which assisted formation of the Cu mineralization at Xinqiao. We highlight that magnetite textures and compositions can effectively trace the ore-forming processes in porphyry-skarn deposits, and show that porphyry related skarn magnetites are distinguishable from those in skarn Fe, skarn Cu, IOA (iron oxide-apatite) and porphyry deposits in the MLYB.