Why is gold often extracted from ores that contain very small amounts of metal but iron is extracted from ores?

This is largely down to something known as the 'cut off ore grade'. This is the highest concentration of economically and technologically exploitable metal in a rock. Low-grade ores yield less profit as more money is spent quarrying, transporting and processing the bulky ores than is made up for by the profits of the metal within. Because gold has far more value per Kg than iron we can afford to extract it in lower concentrations and still make a reasonable profit. It's also less costly to extract due to the fact it is unlikely to have reacted with other minerals.