The paper presents a probabilistic model for informed decision on mineral resource classification, using conditional simulation. It includes a case study which investigates the optimisation of drill hole spacing for mineral resource to mineral reserve conversion for an extensional vein package within the Invincible South deposit at Gold Fields' St Ives Gold Mine. The paper explores enhancement of resource classification aimed at reducing exploration costs. The study also investigates the optimal drill hole spacing for grade control. Results from the study supports a 40 m x 40 m drill spacing to inform an Indicated classification under the assumption that drill angles are optimal. For the grade control, the study showed that a 20 m × 10 m drill spacing is optimal to estimate tonnes, grades and ounces accurately on a quarterly basis. The findings of the study provide valuable insights for exploration strategy, resource definition and classification at Invincible and similar archaean gold mineralisation styles. The study further demonstrates the potential for significant exploration and resource drilling cost savings, and improved resource definition through targeted drilling programs.