NEW GENERALIZED-MODEL OF OBSERVED ORE VALUE DISTRIBUTIONS

A model of ore value distributions is presented that is the culminatio n of a statistical approach rather than simply a model that has been s uccessfully applied to the evaluation of ore reserves in complex geolo gical deposits. Detailed geological and physical observations based on large sampling data sets have been used to support the statistical mo delling. Fundamental to the modelling process is the concept-that the occurrence of a given grade value in space is a function not only of t he presence of the mineral but also of the fact that it is associated with a specific trapping mechanism. This can be expressed as P(Mineral ) approximate to P(Mineral\Trap site)f(Trap site) where P is the proba bility density or mass function of the occurrence of the mineral and f is a versatile mixing function. In statistical terms, as has been sho wn for alluvial/beach diamond deposits, P(r/lambda) defines the number of stones given a mean number, lambda, of stones per trap site. If la mbda follows a Poisson distribution and f(lambda) is a generalized inv erse Gaussian distribution, P(r) has a compound Poisson distribution o r, if f(lambda) is a gamma distribution, P(r) follows a negative binom ial model. These concepts are specifically applied to gold mineralizat ion, where the generalized compound lognormal model, Lambda(z), for th e gold values, z, is based on the development of the generalized compo und normal distribution model [GRAPHICS] of the logarithmically transf ormed gold values, x; f(x\sigma(2)) follows a normal distribution and the mixing function psi(sigma(2)) is described by the generalized inve rse Gaussian distribution. It is the authors' strongly held belief tha t the complex modelling described is not an exercise in statistical 'a erobics' but that it is essential for an understanding of the distribu tion of certain minerals. Work that is currently in progress is emphas izing the importance of modelling the trapping function correctly in t erms of the statistical density distribution of the mineral and also o f its spatial character, both of which have a direct bearing on the sa mpling of such deposits.