Mineralogy and geochemistry of the pyrite from the Henghui skarn iron deposit in central North China craton: Insights into cobalt mineralization

Skarn Fe (Cu) deposits associated with intermediate rocks are significant sources of Co-rich ores. However, the primary factors influencing this type of Co mineralization are currently the subject of ongoing debate. Pyrite from skarn deposits, as a major Co-bearing mineral in this system, provides an effective perspective to study this issue. The Henghui deposit serves as a representative example of Co-rich Fe skarn deposits in the Handan-Xingtai area of the Central North China craton. Four types of pyrite were identified by field geology and detailed petrographic observations: fined-grained pyrite (Py1) as inclusions in the magnetite stage, coarse-grained disseminated pyrite (Py2a) and coarse-crystallized crushing pyrites (Py2b) in the sulfide stage, and veined pyrite (Py3) in the carbonate stage. Analytical result from EPMA and LA-ICP-MS indicate that Co, Ni and As are predominantly incorporated into the pyrite lattice via isomorphous replacement of Fe and S. On the other hand, Pb, Cu, Bi, Ag, Mn, Mg and Ba in the pyrites occur mainly as invisible or visible inclusions. Py1 contains numerous galena and chalcopyrite inclusions, suggesting enrichment of Cu, Pb, Ag, and Bi in the magnetite stage. The elevated Mn, Mg, and Ba contents result from fluid-rock reactions triggering sulfide precipitation during the magnetite stage. The significant Co and Ni enrichment in Py2a is likely due to decreasing fluid temperature and salinity, magnetite crystallization, and multiple-stage mafic magmatic processes. Shear failure in the sulfide stage reduces fluid pressure, leading to subsequent boiling and the formation of Py2a with oscillatory zoning. The decoupling of Co and Ni is attributed to differences in their sulfide affinity and salinity sensitivity. Continued shear fracturing causes Py2a to fracture, locally pulverizing pyrite (Py2b), creating new porosity that enhances hydrothermal alteration and releases Co and Ni into the fluid. During the late mineralization period, the hydrothermal fluid, enriched in Co and Ni, reprecipitates under the influence of meteoric water, forming Co- and Ni-rich pyrite veins (Py3). Our study highlights the critical role of multiple-stage mafic magmatic hydrothermal processes in Co enrichment in the Handan-Xingtai area, with the disseminated pyrite in the sulfide stage showing high potential for Co exploration.