If it is gummy (you said the carts are gummed up by it) then it is probably from the endcaps at either end of the cart. The color tends to turn a greenish blue color and is gummy or sticky. Copper carbonate is not gooey and will tend to deposit as a blue green stain on the cart and in the pool itself. In either case, it is not from the borax but is a result of low pH conditions in the water. IF the total alkalinity was very high (doubtful if the pH was so low unless you also used pH increaser, sodium carbonate, or added sodium bicarbonate to raise the TA) then the low pH would have depleated the bicarbonate in the water so the precipitation of copper carbonate is doubtful even if there is a lot of dissolved copper in the water. If there is high TA and high pH then it is possible for copper carbonate to precipitate but with an extended low pH situation I don't think that is what occurred. There very well might be copper in the water from the heat exchanger also.