Sadly I'm not even sure that's entirely correct - Equatorial Guinea is arguably the richest state in African (GDP) and they climbed there by banishing free and fair elections (after the first) and cozying up to communist states - then of course cronyism after oil was discovered.
Gabon is probably the best example for a capitalist model (after tourist-island states). If anything, arguably the strongest correlation between African wealth and any other factor is population size.
Also Gaddafi's Libya.
Like Gabon, an authoritarian petrostate with a relatively small population.