I think you should be fine. If your 50x50x10 foot deep pond were a cube, the volume would contain 250,000 cubic feet, or just over 187,000 gallons of water.
The salinity of fish blood is just over .8%. It takes 6.4 pounds per 100 gallons of salt to get to .8% salinity in water. That would be nearly 12,000 pounds of table or feed mixing salt.
I presume your pond is sloped with a max depth of 10 feet with an average depth of 3 feet. That would leave you a water volume of only 7500 gallons and it would only take 480 pounds of table or mixing salt to reach a .8% salinity concentration.
The mentioned concentrations and amount of salt presume you have zero calcium levels to start with.
You are probably okay, at least for now, but salt concentrations can accumulate over time. As ewest said, without knowing what your calcium and chloride levels actually are, it's impossible to say how much salt would become lethal.
