v116n3a10 Trends in productivity in the South African gold
Figure 3 indicates that gold ore grades have constantly declined over the past 80 years. Müller and Frimmel (2010) predicted that if only the decrease in gold grade since 1968 is plotted and extrapolated into the future the average gold grade will be as low as 0.9 g/t by 2050. This means that most current gold operations will be marginal.