Abstract In this work, the flotation of ultrafine pentlandite and its separation from lizardite in de-ionized water, and bore water of high ionic strength were studied. In de-ionized water, the flotation separation was poor due to low pentlandite recovery and high lizardite entrainment. However, bore water increased pentlandite flotation recovery while reducing lizardite entrainment. The possible mechanisms responsible for the improved flotation separation in bore water were investigated by electrokinetic studies, Scanning Electronic Microscopy (SEM) analysis and settling tests. It was found that the reduction of electrical double layer forces between particles in bore water might mitigate the coating of lizardite particles on pentlandite surfaces resulting in the improved pentlandite flotation. The reduction of electrical double layer forces might also induce the aggregation of lizardite particles and therefore enhance lizardite rejection. This study provides a new direction to address slime coating and high gangue entrainment in ultrafine mineral flotation by electrolytes.