Well, the motives of their take are obviously disingenuous, but the illegal aspect is that there are mining syndicates mining illegally with child labour or forced labour.

Allegedly, these groups smuggle while the Venezuelan Government turns a blind eye for a cut of the gold. There are also apparently mines controlled by the ELN, funding guerilla war in Colombia.

The other illegal aspect comes from environmental laws about mercury runoff and deforestation which is happening at a high rate in the Venezuelan mining regions. This also displaces indigenous folk from their lands in many cases.

On mercury runoff and deforestation - a country has a right to its own resources and land. The mercury runoff could potentially fuck them over, and I'm sure it's not good for the workers, but fast gold mining is required for the economy which is being unnecessarily crippled by sanctions. It's mining and deforestation or nation wide instability. I trust that Maduro as an eco-socialist will not let the deforestation get out of hand.

On illegal gangs and the ELN - the ELN is designated as terror group by the Venezuelan Government, but that doesn't necessarily mean there's no underhand deals going on. The military could, and has to some extent, ramped up security and elimination of these gangs, but again, the country is crippled and Maduro would rather spend on social programs than military funding. Perhaps the government does see gold dividends from the illegal mining groups, but crucially, it sends those funds to the people rather than pocketing it for themselves.

On indigenous groups - well, from the original Bolivarian revolution and onwards, no one has treated indigenous folks better than the Bolivars have. The west is in no position to criticize their actions.

A lot of the 'problems' with Venezuela could be solved by undoing the sanctions, really.