The Jinchang deposit, Ailaoshan Belt: overprint of Miocene orogenic gold mineralization on Triassic hydrothermal nickel sulfide mineralization

Abstract

The Jinchang deposit, Ailaoshan Belt, is a hydrothermal gold-nickel deposit in which nickel mineralization formed during Triassic accretionary orogeny and gold mineralization during Miocene collisional orogeny. Although the nickel and gold orebodies largely overlap in an ophiolite melange at the contacts between ultramafic and metasedimentary sequences, nickel and gold concentrations have only a weak correlation in orebodies intersected in drill cores. The hydrothermal nickel sulfide ores are mainly concentrated at ultramafic-metasedimentary rock contacts. Broad alteration zones surround the contacts, with proximal quartz + clinochlore + magnesite in both rocks through quartz + fuchsite to distal muscovite + quartz assemblages in metasedimentary rocks. An apatite U–Pb age of 235.8 ± 1.8 Ma and a pyrite Re-Os age of 254 ± 21 Ma from the nickel mineralization indicate that it formed before the closure of the Ailaoshan Ocean. The As- and S-rich fluids during oceanic subduction leached Ni from the ultramafic rocks in the ophiolite melange forming the hydrothermal nickel sulfide ores. Orogenic gold mineralization comprises auriferous veins that host gold, Au- and Ag-rich sulfosalt. The veins cut the sulfides associated with nickel mineralization. The auriferous fluids reacted with nickel ore-stage pyrite forming porous or sieve-textures and patchy zoning in BSE images with native gold in pores. Geological and paleomagnetic evidence indicates that Miocene gold mineralization occurred in highly deformed Devonian metasedimentary rocks after the Oligocene–Miocene Ailaoshan sinistral shearing (~ 30 to 20 Ma). The auriferous fluids are most likely sourced from the metasomatized mantle lithosphere if Jinchang has a similar source to other orogenic gold deposits in the Ailaoshan Belt.