If the 60% sale to the Central Bank makes GRZ's gold mine uneconomical, does this law make all the gold mines in VZ uneconomical?

The short answer is yes, if the law was applied equally to every miner. However this policy allows Hugo to fine tune who gets what. While he may force all producers to sell to the CB and they all get paid in local currency for their 60% of production, that isn't the final result.

Certain miners could be favored by awarding them the rights to convert more of their bolivars to foreign currencies at the official rate. If the miner was allowed to convert 100% of his bolivars to US dollars then in effect he is paid at world prices. If he is denied access to any dollars at the official rate and is forced to go to the alternate market, the return becomes only 1/3 of the world market price and this could become even worse if the back market rate for bolivars weakens further against the official rate.

What has happened in the past is no indication of the future. Not only does the government strictly control who has access to the official rate of exchange but the difference between the official rate and the real market rate varies and could weaken even further if Vz financial situation becomes more volatile. Hugo could and probably would use the access to external currency as a weapon and a source of cash for the treasury as some of the posts here have indicated. The only way that gold sales could make money for the treasury is for them to be paying less than the real market price, and they are doing that by currency manipulation not actual price fixing.