Rick's post is perfectly correct re grade and tonnage; another major factor is deposit geometry and the continuity of the ore shoots. For deposits that are worked by open pit you require an orebody with its largest dimensions in the horizontal plane so that you can extend your pit laterally, and still be in ore, as the pit gets deeper. Because you are bulk mining you have to take the richer with the poorer material to a certain extent (in some cases you may be able to selectively mine some areas of the orebody and separate others that can be tipped as waste), so you mine to average grades. For many open pit mines 1g/t is actually pretty good and economical to mine (but that case has to be made on a mine by mine basis - one size does not fit all), but it's early days yet to get too excited by numbers.
There's a great deal of work to do yet to formalise a ore resource and once a geological resource is defined that then has to be modelled to create a mining reserve. 3D pit modelling and optimisation inputs every possible variable from the geology, capital equipment costs, down to the fuel consumption and maintenance of the mine vehicles and wages & shift patterns of the workers. Every 1m cubed block of ore is identified and various simulations can tell you on what day/shift that block will be mined. These simulations will give you a production rate in tonnage and contained ounces that define a cost-per-ounce (including royalties and taxes). Factor that against the current gold price and you know how much profit you will make on that ounce on a day-by-day basis. These models are often live in working mines and update daily/hourly to input fluctuations in the gold price, power and fuel costs, etc. It depends on your financial model whether that level of profit is sufficient or not to make the project viable.
For porphyry systems contiguous tonnage is the key; everybody wants the grade to be as high as possible, but would you rather have 100 million tonnes at 1g/t or a billion tonnes at 0.5g/t if all the other factors were equal?
Work on the ADL thus far has defined both linear and cylindrical features, neither of which lend themselves very well to open pitting (due to the amount of waste that has to be stripped to access the ore at even shallow depths), but could be developed underground perhaps by selective extraction, if this was viable. If a porphyry system is intersected this will, of course, change things dramatically.
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