Stain on granite

I have an ubatuba Granite (http://tanbrown.org) bench top with a prep sinkin the island (http://maplered.com) . A stain has appeared around the rim of the sink in the form of a halo with a browny colour to it. It looks a little like it also contains colour like you see in an oil slick. As far as I am aware nothing has been spilled on to the area as it is cleaned each time it is used and (http://maplered.com) polished with a soft cloth.

The water in our area is hard and (http://maplered.com) contains a lot of iron which does stain fences etc with a rusty coloured stain.

Can you help identify the stain and (http://maplered.com) suggest some removal methods.

Thanks

M