How bogus is Barrick Gold's $3.8 billion writedown of its African copper mine?

It is a source of amazement to me that we routinely see these stories and accept them at face value.

In this particular case it's about Barrick, the worlds biggest gold producer, but it's a common enough story. Company A buys company B for X billlions. A year or two later it's "oops, did we ever step in a multi-billion dollar cow-pie on that one! Faces red! So sorry!"

There follows a massive writedown of their asset value. That translates into a loss for tax purposes. That creates massive tax deductions.

There is absolutely no way to confirm the legitimacy of the writedown. Did a bunch of extremely well paid and well educated MBAs really step into a multi-billion dollar cow-pie?

Or are these writedowns just a massive tax dodge?