I work in the dredging industry (engineering consultancy side)...

I work in the dredging industry (engineering consultancy side) and can confirm that, in my opinion, turbidity created by dredging operations is likely to cause many headaches if that is the path GOT choose to take. Dredging of hard rock quite often creates a larger turbid plume than dredging softer material (unconsolidated sand mud etc). This is due to the creation of extremely fine 'rock flower' though the grinding/cutting of the rock by the cutter.

I see that the Geraldton Port project has been noted on WTN/GOT threads on a couple of occasions as an example of a large scale dredging project working with very hard rock. It was also a classic example of a nasty turbid plume - which basically covered all of Champion Bay for in excess of 9 months and had the greenies in despair. It could even be seen from the air off Kalbarri at one stage. My point here is that the plume issue can not be dismissed.

(before I get shot down by others more qualified than I, I should also note that now many of the seagrass areas are in better/equal condition to before the job and the dump areas have become new crayfish habitats!!)

Anyway, I digress slightly from GOT. On not much more than 'eyeballing' the project, I suspect that a mining scenario could potentially utilise a medium to large cutter suction dredge; it is certainly within the capabilities of current and cost effective technology. On the other hand, the reality is that it would probably be environmentally unacceptable for it to operate in 'open water' over a long term basis (20year + mine life) - i.e. you would probably need to put up a bund and then sit the dredge in the pond (similar to some mineral sand operations). Having reached that point, it would really come down to the economics of 'dredging everything and then separating the manganese' VS 'mining using land based plant and direct shipping the ore'.

Dredging even relatively hard material (up to say 25MPa UCS strength) should be reasonably cost effective with long term contracts or a owner operated dredge (say $15-30/m3 or $6-12/tonne).

Unfortunately, I dont know enough about traditional mining costs, or about the technology or cost of separation so I cant really comment to much future on the total cost and therefore attractiveness of either scenario in full, but hopefully this may be of some interest to people out there to at least give a ball park estimate of the marine aspects of a potential mining scenario.