Seriously??? Sustainable gold?
Here's a free tip from me to you: if you want people to live in a more sustainable manner, get your priorities straight. The last thing you want is to create a movement that seems to have no other goals than to take away every ounce of fun people might actually have. You can't be, or appear to be, against every single thing that people derive enjoyment from. Somehow it has to be balanced out.
One place to start would be to distinguish between planet-sustainability and people-sustainability. (To borrow the age-old adage of People, Planet, Profit.) The objection these people seem to have against the way gold is ordinarily produced seems to involve both "people" and "planet". They don't like the way gold mining ruins the environment, they don't like the way gold mining hurts the health of the miners, and they don't like how little these miners earn.
This is a problem because it makes me much less sympathetic for their cause. As a general rule, I would suggest that those three concerns widely differ in the extent to which people in the West will care, with the environment ("planet") easily beating out the interests of the miners ("people"). Personally, I'd certainly care more about the "planet" that can't speak for itself than about the workers who are making the best of a difficult situation. Not buying the gold they produce will only make them worse off; the only way to improve their lot it to support their efforts at collective bargaining and, more importantly, to help them improve the rule of law in their countries. Trying to get Western consumers to pay a "White Liberal Guilt" Premium on their gold is hardly the solution.