the secret prospector, page-28

my guess is what you are seeing is feed from the primary crusher with 2 conveyers under the 2 different sized screens on a single deck, one conveyer takes the fines to a ball mill one takes the medium sized rock to a secondary crusher and the oversized gravel vibrates off the screen falling in a pile for a FEL to return to the primary crusher.
Robert or a poster familiar with the plant probably have a better understanding though and may explain better.

You are on the right track most mills I have seen would have a double or triple deck vibrating screen rather than a single deck screen with 2 different apperture sizes so this is quite novel.