Actually the Burketon Pit, is NOT an active mining operation. There has been NO active operations there for many, many, many years.
A gravel pit is an active mining operation and as such they are subject to stringent rules and regulations regarding access and perimeter safety measures are required.
I wish I did own a gravel or sand pit, but sadly I do not.
Similar scenario you describe has been happening north of Kingston and last rumours I heard, the pit operators are now having the local riders sign waivers for weekend use. I have not been there recently so I can not 100% confirm this to be true, but it's a great place to ride so I will look into it further some time in the future. (post-covid) (I have no idea if there is a cost or club involved)