An exploration-adjusted mineral occurrence model.

Abstract

A mathematical model describing the probability for n mines or prospects occurring within an elementary unit (cell) of an area has been referred to as an occurrence model. Estimation of parameters of occurrence models has been plagued by the effect of area delineation on the parameters. Moreover, incompleteness of exploration creates a bias in parameter estimates. This study proposes that when the model is to describe the probability for occurrence of mines or prospects, the appropriate area is a metallogenic unit of mining district scale. Accordingly, this study examined the delineation of area by successive expansion of a polygon seeking that size of area which provides the best fitting of a truncated and effort-adjusted exponential model. However, estimation of occurrence model parameters was found to be sensitive to location of polygon on the cluster. Consequently, this approach was abandoned in favour of geologically-defined metallogenic units referred to as Intrinsic Samples. Truncated and effort adjusted occurrence models were fitted to Intrinsic Samples which included the mining districts of the Walker Lake Quadrangle of Nevada and California. The estimated model for each metallogenic unit is used to estimate the gold-silver metal endowment of the unit. This represents a departure from previous studies, e.g. Allais, in which a single parameter estimate from a control area is used to estimate the mineral endowment in all parts of a large study area. Furthermore, the study addresses the issue of economic truncation of occurrence data used in exponential model construction. Because a metallogenic unit is less than completely explored, estimated parameters based on observed occurrences provide a biased description of the number of occurrences present (i.e. endowment). The transition from sample to endowment (population) parameter is achieved by parameterizing the exponential model for a metallogenic unit on exploration effort deployed in a unit area. Thus, fitting the model to observed data and evaluating it at infinite effort yields the model for gold and silver metal endowment in a metallogenic unit.