Most mineral deposits provide challenges in resource estimation, including the quantification of minable minerals, as well as the quantification and location of waste-rock materials that might generate acid rock drainage (ARD) during the development, operation, closure and post-closure phases.
The identification of waste-rock materials with ARD potential is a crucial issue during project planning since it affects the economic fate of the project directly when closure costs are not well constrained. Once identified, non-acid generating (NAG) and potential acid generating (PAG) waste-rock materials can be handled separately, allowing mine companies to: (i) use NAG waste material for construction (e.g. backfills, covers), (ii) design waste rock dumps to isolate PAG material, and (iii) predict hydrochemistry of contacted waters, among others.
In this regard, this paper presents an example of a resource estimation of NAG waste-rock done in a high-sulfidation Au-Ag deposit located in the north of Chile. The evaluation has been done in three phases, using more than 90,000 rock samples taken from exploration drill holes. These samples were geochemically characterized through the following analytical battery: (i) modified ABA and NAG test analyses (N° 145), (ii) S Leco analysis (N° 13,118), (iii) total S analysis (N° 89,259), and (iv) whole-rock analysis (N° 93,020).
On phase 1, the S total, S-S(II) and S-S(VI) contents, as well as paste pH, NAG pH and NAG to pH 7 values were extrapolated using the whole rock analysis dataset (N° >90,000). This was numerically done based on mass transfer theory (Law of Mass Action and Pearce Element Ratio Methodology). Once the missing data were estimated, both the S total contents and the categorical NAG/PAG classification were extra/interpolated from the drill-holes to the block model (phase 2). Finally, using the mine planning (phase 3), both volume and extraction timing of NAG waste-rock material were estimated.
