Mineralization Mechanism of Cobalt in the Dahongshan Copper-Iron-Cobalt Polymetallic Deposit in Yunnan, China

Abstract

The Dahongshan Cu–Fe–Co deposit, a key example of the Kangdian Cu–Fe polymetallic belt, features several forms of mineralization, including volcanic exhalative-sedimentation, metamorphism, and hydrothermal superposition. This deposit is rich in Cu, Fe, Co, Au, and Mo and exhibits strong structural control. Although recent studies have demonstrated highly variable enrichment of Co in ores, knowledge about its occurrence, enrichment patterns, and mechanisms is lacking. This study examined these aspects using an electron probe micro-analyzer and laser ablation inductively coupled plasma mass spectrometry. This revealed clear Co enrichment in key ore minerals, indicated by the presence of cobaltite, pyrite, chalcopyrite, and magnetite. Cobaltite comprised 31.00%–34.26% of the Co detected. Pyrite, indicating isomorphic substitution by Co, accounted for 0.01%–5.38% of the Co detected. Lower Co concentrations were detected in inclusions in magnetite (55.37 ppm–226 ppm) and chalcopyrite (0–290 ppm). Mapping of lithofacies at the 280 m level revealed clear spatial differences. Cobalt enrichment was highest in sulfide ores, averaging 261.63 ppm, followed by oxide ores (42.7 ppm–241.9 ppm), metasomatic albitized tuff (18.7 ppm–148.1 ppm), garnet-biotite schist (80.0 ppm–143.3 ppm), carbonate minerals (21.1 ppm–70.7 ppm), and quartz (2.5 ppm–59.8 ppm). Co-reduction from the anticlinal core (exploration lines B136–B120) to the edges highlights how the Dibadu anticline affects the structure. The metallogenic model outlines a three-stage enrichment process: (1) Volcanic exhalative sedimentation occurs, and Co forms sulfides as Co–Cl (CoCl₄^2-) complexes break down. This breakdown occurs when igneous fluids mix with seawater and during oxidation events, resulting in significantly enriched cobaltite and stage pyrite (I). (2) The second stage involves regional metamorphism at 290–330 °C and 16%–32% salinity, which helps to remobilize Co via fluid extraction and recrystallization, thereby adding Co to stage pyrite (II) via isomorphism. (3) Hydrothermal superposition creates zoned mineral groups ([pyrite + chalcopyrite] → bornite → chalcocite) along fractures at lower temperatures (245.1–92.3 °C) and with varying redox states. In this stage, the presence of Co supports stage (III) pyrite formation.