the water in their dolomite pit is probably ground water from the thin, permeable cover layer above or rainfall, not coming through any faults in the hardrock.
sea water would not be be a problem in the hillside pit. the ore in the drill holes is fresh. if the rock were permeable then the sea would have had 100's of millions of years (except for the ice ages when the gulf was above sea level) to oxidize and weather the hell out of the ore.
removing a tiny amount of rock in the form of an open pit would not relieve the pressure such that the sea just comes seeping in. if the sea were going to come in through those rocks, it would have done it by now!
pretty sure the dolomite pit isn't 400m deep and it is interesting that the pit stops at around sea level.
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