The constituent changes announced on Thursday are due to take effect on September 23.
Implats has been in and out of the Top 40 index over recent years, while Sibanye-Stillwater became the world's biggest platinum producer after a R4.3bn deal to buy Lonmin earlier this year.
The JSE Top 40 index is key for investors in various index funds like the Satrix 40 and Stanlib Top 40 who track the index. The funds only invest in the Top 40 companies.
PSG wealth manager Schalk Louw told Fin24 that the constituent changes were telling, although not entirely surprising.
"When you look at the inclusions, they are all mines, which is the sector that has seen the biggest increases over the past decade," said Louw.
"It's good to see Implats back." Its share price rocketed 136% so far this year thanks to booming metal prices - but it is still half of levels reached a decade ago.
Aspen was first included in the Top 40 index a decade ago.
Johann Rupert’s investment holdings giant Remgro [JSE:REM] was also reclassified out of the industrial sector and into the financial sector.
As Remgro was included into the FINI15 Index, PSG [JSE:PSG] was excluded. Remgro left the IND25 Index to make room for Telkom SA [JSE:TKG]. The JSE FINDI30 excluded Aspen and included Reinet Investments [JSE:RNI].