Exploration to grade control: Quantifying information gain and spatial uncertainty in a structurally controlled gold deposit

Higher data density is widely assumed to improve grade estimation and reduce spatial uncertainty, but this remains insufficiently tested in structurally controlled, high-nugget gold systems with short-range variability. This study compares exploration- and grade control-scale datasets from the same deposit using consistent geological and block models. Ordinary kriging (OK), simple kriging (SK), and sequential Gaussian simulation (SGS) were applied to evaluate grade estimation, spatial continuity, and uncertainty. Increased sampling raised estimated mean Au grade (3.6-fold under kriging; 5.4-fold for SGS E-type estimates) and reduced variogram range from 111.3 to 44.0 m, indicating that exploration drilling smoothed short-range variability and overestimated continuity. SGS realizations became more spatially coherent with grade control, reflecting reduced structural uncertainty, but conditional variance remained high, indicating nugget-driven uncertainty. Deterministic uncertainty differed by method, with OK variance increasing and SK variance decreasing. Uncertainty therefore depends on variogram scale effects and estimation methodology, not sampling density alone.