Is it perhaps, that someone has added radweld (or similar) to the coolant in an attempt to stop a leak? That can look like copper particles. If so you can drive it, but you need to find the leak.
I would be worried that if it was copper slip that it would block a water way.
As David suggests it could be radweld which is not the best in the K series but not as bad as copper slip.
James
Home to black Alfa Romeo 159 3.2 V6 Q4 ,green MGF VVC andred MG Maestro T16.