Is citing data from a presentation not trustful? There is this investment bank specialized in the mining sector. I had access to one of their presentations where they have data regarding the production and end use of some chemical elements. They do not cite the data.

Do you think that is good or bad, trustful or not, to cite these data in my PhD thesis?
If the source has been used, it generally must be cited, otherwise you assign yourself results or conclusions obtained by other researchers. 

Providing the source is not a question for its reputability. If you think the source is not reputable enough, do not use it in your work. It is not very common but I have even seen "personal communication" as a type of the reference.