Why do you conjecture junior exploration companies are a foremost source of adjectives mine supply?
Answers:
the lease they bet on (frequently surrounded by lower developed countries) are so risky that the majors won't put the money or time into them.
what you hope happen next to a junior is that it finds a fine deposit and make the deal to develop it ... after which a mid-size outfit or senior will lift it over via merger.
still, the senior companies enjoy already explored the areas in close proximity their existing properties relatively ably [it is cheap compared to the pro of extending a mine's life] and so you can be pretty sure that key topical sources of supply won't come from them -- which leaves with the sole purpose the companies who are involved surrounded by alien areas and various of them are junior.
:-)