Yep, that's a major problem with surface stockpiles, it's almost...

Yep, that's a major problem with surface stockpiles, it's almost impossible to assess the grade, tonnage and value of the stockpile unless you have access to the original grade control data and trust that whomever mined it years ago did a competent job.

Another issue I have seen with old surface stockpiles is variable grade distribution in different size fractions and with hidden oversize and natural weathering and oxidation etc

As a number of experienced mine geologists and mine engineers have told me -
"expect the unexpected in surface stockpiles but they always seem to perform on the downside, no matter how pessimistic your predictions are!"