Practitioners report improved productivity as one of the main benefits of using autonomous dump trucks in underground mining. However, manned vehicles are still needed to transport materials and personnel in the tunnels, which requires practices that may diminish autonomy benefits. Thus, both fleets shall be efficiently mixed to maximize the autonomy potential. In addition, sufficient safety shall be demonstrated for operations approval. This paper proposes a strategy to populate a GSN (Goal Structuring Notation) structure to argue for the sufficient safety of mixed traffic operations in underground mining. Our strategy considers SoS (System of Systems) concepts to describe the operations baseline and the initial argumentation line, i.e., risk reduction mitigation strategies for existing SoS components. Such a strategy is further detailed with risk reduction mitigation arguments for control systems. Mitigation strategies at both levels are derived from safety analysis supported by STPA (System-Theoretic Process Analysis), a safety analysis technique that aligns well with the SoS perspective. We also incorporate regulatory frameworks addressing machinery to align the arguments with mandatory statements of the machinery directive. Our strategy combines SoS concepts with analysis techniques and regulatory frameworks to facilitate safety case argumentation for operations approval in the European mining context.