Figure 1 – Visual inspection of drill core from the Chilean IOCG site showing the difficulty of distinguishing barren core (<0.05% Cu) from weakly mineralized core (0.05–0.25% Cu) by visual logging alone. This ambiguity is central to the proxy logging approach. 3.5 Evaluation Models at all three sites were evaluated through prospective blind reconciliation: models were trained on data collected up to a specified cutoff date and subsequently tested against drilling data acquired thereafter. Two metrics were computed at each site’s respective cut-off grade: • Precision (false positive rate): The fraction of blocks predicted as high-grade (HG) that reconciled as HG upon drilling. • Recall (missed mineralization rate): The fraction of blocks that reconciled as HG and had been predicted as HG prior to drilling. The evaluation objective was to achieve improvement in recall—identifying a greater proportion of mineralized material—without reducing precision below the level demonstrated by the site Kriging model. Elevated false positive rates are characteristic of resource definition drilling in underground deposits, reflecting the inherently sparser data environment relative to gradecontrolled regions; false positive rates below 50% are routinely observed in grade control settings (First et al., 2023). 4. RESULTS 4.1 Chilean IOCG Copper Deposit The Chilean study site is a Manto-type IOCG deposit in the Candelaria–Punta del Cobre district, Region III, Chile. The deposit is hosted in an underground operation with a 0.5% Cu economic cut-off. This district constitutes the largest IOCG cluster in the Chilean IOCG belt, containing in excess of 13 Mt of contained copper, and is characterized by an Early Cretaceous volcanic-sedimentary arc sequence. Mineralization is hosted in altered andesites of the Punta del Cobre Formation, with orebodies comprising magnetite, chalcopyrite, and pyrite controlled by northwest-striking fault systems. Deposit geometry encompasses both stratabound replacement bodies (“Mantos”) and sub-vertical feeder structures (del Real, Thompson and Carriedo, 2018; Ichii et al., 2007). A distinctive early sodic-calcic alteration assemblage is locally overprinted by potassic alteration texturally associated with the main chalcopyrite-forming hydrothermal event (del Real et al., 2023). Approximately half of the drillhole database was unassayed.
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