There is huge portions of the mining sector that undergo no drug and alcohol testing at all. Most of the higher risk jobs ie working underground, operating heavy machinery do require testing but a lot of others, especially sub-contracting often do not.
Unless you fuck up really badly and kill someone or cause millions of dollars of damage you can usually get away with being under the influence on the job. If you can function and work you can usually get away with it. The more people you know the more you can get away with.