A question on overburden, page-7

Hi PG,
I did some accurate calcs for open cut a couple of months ago using only 1 DOF at 4.21m average width. We now know the widths are a bit wider...plus there are multiple DOF's intersected at various locations....so the following calcs should be very conservative.

1: calc true widths averaged=4.21m
2: the DOF is not vertical...my calcs suggest near to 50 deg dip...so for every 100m vertical depth, actual ore body length is 130.5m
3: slope of cutting in rock is more like 70 deg overall, taking into account step benching.
Comparing volumes for various depths in the above typical cutting gives the following for each vert depth using above slopes and strike length of 1km and base of 15m.

50m depth
total vol =2255000 cm(cub m)
ore vol =275000 cm=1540 t Co equ(calc at 0.2%)
Strip Ratio =8.2
Co value=$138.6 m @ 90,000/t

100m depth
tot vol =7515000 cm
ore vol=548000 cm=3069t Co equ
Strip Ratio=13.7
Co value =$276.12 m @ 90,000/t

How deep the open cut goes will depend on when underground becomes more viable in relation to above strip ratios.

So at 100m depth over 1km strike the Co vol =2148 t using grade of 0.14%

If we then assume the current Jorc will be over 6.5km strike, then that gives 6 yrs of open cut without the need to even think of going underground. That 6 yrs then gives plenty of time to prove up heaps more similar DOF's.

I realise the SS will involve underground as well as open cut, but it may never be required unless we wanted to significantly increase the vol of Co mined over a shorter time frame.
Imo only.