Don't know much about mining but I think this may have been a collection pond for the chemical solution containing dissolved precious metals from heap leaching. Heap leaching involved dripping a dilute cyanide solution through a heap of crushed ore on a plastic lined leach pad and collecting this solution in a storage pond where the dissolved gold and silver could then be precipitated out. A solution of sulfuric acid was used if copper was the target metal. Not exactly a process folks downstream would want on a hillside that often gets swept by flash floods. Note that the pond, although no longer in use, has probably overflowed with the recent rains.