PGE Deposits are Peculiar: Is the Cause Crustal Abundances or Formation Mechanism?

Abstract

The grade–cumulative tonnage relation of Au deposits is approximated by the combination of three exponential functions, which forms a gradually decreasing curve being convex downward in a semilog plot (hereinafter referred to as “the typical pattern”). The typical pattern is also found in the ore value (OV)–cumulative tonnage relation of metallic deposits in both cases including and excluding PGE, where OV is the sum of products of metal grades and metal prices in each deposit. The enrichment ratio (ER)–cumulative tonnage relation shows the typical pattern in the case excluding PGE, where ER is the sum of the metal grades normalized by the corresponding crustal abundances in each deposit. In the case including PGE, however, the relation does not show the typical pattern, but has a hump (convex upwards), because the ERs of PGE deposits are extremely high. Though several reasons causing this exception are discussed mainly on the statistical point of view, any cause is not persuasive. In conclusion, the discussion suggests that we need to clarify why many cases show the typical pattern, yet there is an exception showing an atypical pattern with a hump.