The good news is that we are not likely to run out of gold any time soon.
The US Geological Survey estimates there are 52,000 tonnes of mineable gold still in the ground and more is likely to be discovered.
A cube made of 171,300 tonnes would be about 20.7m (68ft) on each side. That is present extracted gold stocks!
Maybe this is the answer 21 m hight of a cube! Hah... 1 tonne of gold costs .... btc! Google: tonne = unces! Gold 1 oz = $/btc!