Go to the "select" menu and click on "color range". Here you can choose a specific colour and produce a selection based on that colour. You can sample different tones of gold and add them to the selection, and adjust the tolerance. What's even better is that you can preview your selection in realtime if you chose "grayscale" or "black mask" from the preview dropdown menu.
Here I am selecting just the gold colour.
Press OK once you are happy and that will create a selection. If you save the selection to a channel, then copy and paste the channel into a layer you'll have a greyscale image representing the amount of gold. If necessary you can convert this to a pure black/white image using the threshold tool.