An open cast is deployed where there is e.g.: coal, not necessarily where there is a cellular signal. Even if there is a good signal, the signal is transceived horizontally and inside the pit, coverage is zero.
Mines utilize heavy vehicles, supply chains, and machines on telemetry, monitoring them for performance, on-time supply as well as safety reasons. Technicians, drivers, and workers all require monitoring as well as communication during their shifts.
A simple, cost effective and yet extremely effective way is seen on the photos of an open cast mine (one of three installations) that were equipped with a repeater and doner antenna above and a service antenna via a feeder cable which was later trenched to withstand heavy vehicles.
The cable connected the service antenna which is positioned overlooking the whole pit which is approximately 2 square kilometers in size. Repeaters are available for all frequency bands used by network operators in South Africa as well as combinations of frequency bands.