Automated quantitative mineralogy analysis reveals characteristics of Co occurrence in the Jinchang porphyry deposit, NE China

Abstract

A critical and strategic metal, demand for cobalt (Co) has increased in recent years given its widespread use in new energy batteries. This has led to the need to search for new Co sources. Porphyry deposits have provided large quantities of Cu, Mo, Au and other critical elements such as Re, Se and Te. However, there is a paucity of research on the occurrence, enrichment mechanisms, and metallogenic potential of Co in porphyry deposits. The Palaeo-Pacific slab subduction related Jinchang Au deposit (∼80 t @ 8 g/t Au; Heilongjiang province, NE China) has recently been recognized as a cobalt-rich porphyry Au deposit. We conducted MapsMin (automated mineralogy analysis) on two bulk samples (10-c and Z1-13) from the J0 orebody of the Jinchang deposit to accurately determine the elemental content and Co distribution, and to definitively identify the Co minerals. MapsMin analysis revealed Co contents of 0.27% and 0.07% for samples 10-c and Z1-13, respectively. In addition, we conducted whole-rock geochemistry on ten powdered samples with inductively coupled plasma-atomic emission spectroscopy/mass spectrometry. The results showed Co ranges from 0.04 wt% to 0.29 wt% (average 0.1 wt%). We compared the results of MapsMin on bulk samples with inductively coupled plasma-atomic emission spectroscopy/mass spectrometry on powders, and confirmed the accuracy of the MapsMin analysis. The relative difference in the content of the main ore-forming elements Cu, Fe, Ti, and Co in the samples is less than 30%. MapsMin identified two Co minerals (siegenite and cobaltite) and revealed that there is an isomorphic Co-rich zone in pyrite rims. The good negative correlation between Co, (Co + Cu + Ni) and Fe in pyrite indicates that Co can enter the pyrite lattice through the substitution Co2++ (Cu2++Ni2+)↔ Fe2+. Crystal size distribution data shows that siegenite in 10-c was formed under two different conditions and petrographic evidence indicates that siegenite was the first to precipitate with chalcopyrite and pyrite in the Jinchang deposit. The formation of pyrite spans the entire mineralization process. During late-formed pyrite precipitation, Co-rich hydrothermal fluids were reinjected, causing a second precipitation of siegenite. High levels of Co entered the rim of pyrite, forming a Co-rich zone. Finally, a decrease of fO2 led to the precipitation of cobaltite. Mapsmin can provide a fast, accurate and reliable mineralogy data which can be used to constrain the contents and occurrences of cobalt and other metals, such as nickel, copper, gold, rare earth elements, etc.