There is a good history of concessions and contracts and the CVG here... The main difference with a contract is that the rate of royalty charged can be more than the rate set down in the law (quoted below) and a contract can have other specific conditions imposed other than those for concessions.

http://www.garciavalentiner.com/Mina...

While there have been many changes as detailed in that document the current situation is summed up, and I quote...

In sum, presently the only way left to obtain mining rights in Venezuela is through discretionary concessions.

With regards to the latter, Resolution 115 of the Ministry of Energy & Mines dated March 1990 established that in order to grant discretionary concessions, i.e., no discretionary concessions, special advantages had to be offered by the interested party to explore and exploit in the areas reserved by the government. Those special advantages comprised higher tax rates with a minimum as follows:

3% for gold and silver

4% for diamonds

6% for others

In addition, the interested parties had to offer contributions to enhance the conditions of the town’s surrounding area of the concession and reduction of the term of the concession to 20 years.