When I spoke to Richard, he replied that he was not as CEO able to speak about tonnage estimates at this time but that very capable people could construct 3D models and estimate tonnage...I think he was reffering to one of our favorite posters here on Agora and the last calculation I saw was for 4Mt.  I am sure that RN reads everything on here (Hi Richard :-) ) and he is aware of the numbers floating around so if I read between the lines, I do not think the estimate of 4Mt is off by much at this point.