The paper describes the application of statistical analysis in computing ore reserves in Acupan in general and in particular using statistical grade calculations to close the gap between mill heads assays and mine sample assays. The paper also aims to establish a method of evaluating assay results for better grade control in the face of erratic gold deposition in most of the active vein systems in the mine.

Field, former General Superintendent in the mid-sixties, recognized that the behaviour of gold deposition in very narrow veins does not follow a normal distribution. Using assay frequency distribution methods, it was found that lognormal distribution correlates very well with erratic mineralization. The lognormal method of computation had since been used in ore blocking of veins less than 30-crn (one foot) wide.

Results of several years operation showed that the weighted average method use for ore grade calculations is only 70% reliable. The reliability factor, however, approaches 95% if a geometric relationship is applied statistically.