An added bonus here is that you should be able to spot some small conical heaps of grey powder in the lay-bye parking area.
That is the ore (mainly containing nickel) which the trucks are transporting, and small quantities leak out when they stop here for a comfort break.
The cache is a camo PET preform.
NB: The Slaaihoek Pass road goes all the way through to Vygeboom Dam. Maps don't show the road past the mine, but it's there. Just turn sharp right onto the dirt road just before the security point at the mine entrance, and follow your GPS to the caches.