Determination of the Risk Index Reference Values Applied to Brazilian Phosphate Mines

摘要

A study for resource classification using the Risk Index method was proposed by Amorim & Ribeiro (1996) aiming to combine the geological uncertainty and the kriging variance in a single, impersonal and mathematically simple criterion. The two authors have developed this study for Brazilian Iron Ore mines with similar geological settings. The reference values established for the Risk Index were 0.6 to classify measured resources and 0.9 to indicated resources, respectively. Those values are coherent with the geological setting of the studied deposits.Since the geological features change from one commodity to another and the use of risk index to resources classification for phosphate mines is not yet a common practice it was decided to analyze which reference values that are more applicable to resource classification for supergene carbonatitic series rocks hosted phosphate deposits. Three Brazilian phosphate mines were analyzed for this study: Catalao Mine (Goias State), Tapira Mine and Salitre Deposit (Minas Gerais State). Based on the analysis of the geological vertical sections from the geological model, combined with geostatistical calculations and cross-checked with other resource classification methods formerly applied to those deposits, risk index reference values of 0.45 and 0.65 were established for measured resources and indicated resources, respectively.