The influence of special emissions (possibly coal mining activity) on particulate matters
The first thing to consider is, if it is not the adjacent coal mine, what other nearby sources could produce such a result. Is there a coal fired power station or a factory nearby. If there are no other likely sources then the coal mine is still the most likely candidate as the source of the dust and metal contaminants.

The other thing to consider is how is the coal being mined in the open pit and how is dust being generated by the coal mine.

The most likely method of mining the coal would require the use of explosives and blasting. This can create large clouds of dust that can be high and wide. If the wind was not blowing when a blast occurred the dust may have reached your sensors.

If the coal is mined by mechanical cutting techniques dust will still be generated, but not in the same quantities as blasting produces.

The other ways dust is generated is by vehicles travelling on unsealed roads - the tyres of the vehicle churn up the dust on the roads.

Dust can also be blown from trucks loaded with coal or from conveyor belts conveying coal to a dump site. The loading of trucks will also generate dust, as will the dumping of coal.

From experience, the only way to control dust is to constantly spray with water. Do you know if the mine uses any for of dust suppression?

It is well known that coal contains all types of contaminants, including metals such as cadmium and mercury. Different deposits have different types and degrees of contaminants. 

From what you have presented, the coal mine is the most likely source of the dust and metal contamination.