You have to consider previous miners were being rational...

You have to consider previous miners were being rational economic agents when leaving so much zinc behind. Either they
A. Spend large resources incuding time to extract all the in situ resources
B. Extract the most resources in the shortest time frame, dump the tailings, move on to the next pile of fresh ore.

The important variable is theta, or time. If you extract less resources in even less time, returns can be higher

Additionally, once MMG got their hands on Los Bambas, as part of the divestment re: glencore/xtrata merger; then MMG had an economic incentive to dump a tailings operation and MOVE valuable management skills to better operations. Bambas is a very profitable mine.

Also MMG, MIGHT not have had the skills to operate a tailings operation. NCZ is operating the first hydro stripping operation in OZ, they also managing a giant remediation activity. These need different skills

And we can conclude these points could be true because that's what BHP did at Broken Hill in the end.

I might be wrong. I don't own a cystal ball, but nor does anyone else