Cobalt occurrence and sulfide geochemistry of the De’erni Cu-Zn-Co deposit in NW China

• Three mineralization stages are identified in the De’erni deposit; • Sulfur isotope data indicate a magmatic fluid mixed with seawater. • Co occurs as siegenite and is hosted in pyrite. The De’erni ultramafic-hosted volcanogenic massive sulfide deposit is a large Cu-Zn-Co deposit, located in the East Kunlun Orogenic Belt, and contains 0.57 Mt Cu, 0.16 Mt Zn, 0.03 Mt Co with a 0.09 % Co average ore grade. However, cobalt occurrence and the mechanism for its enrichment are unclear. This study investigated mineralogical characteristics using the TESCAN Integrated Mineral Analyzer (TIMA), Electron Probe Microanalysis (EPMA) analyses, Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses, and in situ sulfur isotopic analysis of minerals from different stages of ores in the De’erni deposit to understand Co mineralization. Three mineralization stages were recognized, and three types of pyrite were identified: Py1, Py2, and Py3 in the De’erni deposit. Cobalt occurs as siegenite, and a significant amount of Co is found in pyrite. The EPMA results show that the highest Co content is 0.40 wt% (0.02–0.40 wt%). The LA-ICP-MS trace element data show that Py1 is enriched in Co (208–5082 ppm), Ni (13.50–836 ppm, average 212 ppm), Py2 contains 11.07–322 ppm (average 94.45 ppm) Co, Py3 has Co contents of 0.68–31.90 ppm (average 5.79 ppm). The δ 34 S values of Py1, Py2, and Py3 range from 7.2 to 10.8 ‰, 4.6 to 6.7 ‰, 0.6 to 1.9 ‰, respectively, suggesting that the sulfur source originated from seawater-mixed magma fluids. Py1 occurs as euhedral-subhedral grains and displays oscillatory zoning, indicating a relatively unstable environment. Concentrations of Co, Ni, and As are highest in early-stage pyrite, whereas Se and Ag content gradually increased from Py1 to Py2 to Py3. The trace element data and mineral paragenesis indicate that the temperature and sulfur fugacity gradually decreased from stage 1 to 3. Furthermore, the first stage fluid is rich in Co and Ni, and the third stage fluid is rich in Ag and Se with Co and Ni depletion. These findings offer crucial insights into the De’erni deposit Co mineralization processes and evolution.