A time-series audit of Zipf s Law as a measure of terrane endowment and maturity in mineral exploration

Abstract

Controversy has surrounded the application of Zipf’s Law as a potential exploration tool since it was first proposed by Folinsbee in 1977. The current study empirically assesses, with the wisdom of hindsight, the realism of predictions that could have been made at various times in the past about the number and size of lode gold deposits yet to be discovered in the Archean Yilgarn craton of Western Australia. The size distribution of gold deposits predicted by the Zipf curve at any point in time represents an estimate of the original “natural” endowment for this terrane based on the knowledge available at the time. Predicted gold deposits not matched by known deposits, by contrast, represent the “residual” endowment, for deposits yet to be discovered. The study concludes that a Zipf curve based on the size distribution of gold deposits known in 1973, 1989, 2003, and 2008 would have provided remarkably realistic predictions of the size and number of the gold deposits that were discovered subsequently, or of the degree to which known deposits have grown in size in subsequent years as a result of better delineation.Because lode gold orebodies are notoriously difficult to fully delineate and their resources tend to grow with time, initial Zipf predictions generally prove conservative. Predictions are particularly sensitive to the accuracy of the resource inventory of the largest rank 1 deposit on which the Zipf curve is based—in our case, the Kalgoorlie Golden Mile—which has virtually doubled in size during the period covered by the study, from 36 to 72 Moz of contained gold. Nevertheless, the degree of exploration maturity could have consistently been estimated with a high degree of accuracy as 17 percent of predicted gold endowment discovered in 1973, 33 percent in 1989, 62.5 percent in 2003, and 75 percent in 2008. In conclusion, application of Zipf’s Law would have represented an effective motivator to embark into a well-resourced gold exploration campaign in the Yilgarn craton.