Ann: Production Update and FY 2019 Guidance, page-117

"a BIF-hosted orebody at best would have a specific gravity of ~3.2"

OK using your number we get 1,140m x 2.7m x 1m =3,078m3 at a specific gravity of 3.2 and average thichness of 2.7m gives a tonnage of 9,850 t per vertical metre. At a stope grade of 4.7 g/t that is 46,293 gm per vm

or >1,488 ozs per vm (your value 827 ozs per vm) serviced by two declines gives >744 ozs per vm ( vs your value of 414 ozs per vm) for Beresford. Still a fair discrepancy between our methods of calculation. I could also use trigonometry to account for the extra ozs per vertical metre due to the ore body dipping at 60 to 80 degrees (I've just indicated this with the ">" sign).

You need to be careful using the blue and green image above as it is an isometric projection so the scale is not reliable. You are better getting any depth estimates straight from any published long sections and the stope heights from the feasibilty which are 13m not 20m as you have estimated. 13 m is in line with my comparison example where stope heights have been between 10 and 15m at the Kundana mine (see link below)

http://mininglink.com.au/site/kundana

"The prime method of mining at the Kundana gold mine is by long-hole bench stoping, following which it is backfilled. The usual length of the long-hole being between 12 to 15 metres if backfilled with waste rock. The height is normally between 10 and 15 metres. This is sufficient to gain access to the ore that averages a width from being only a few centimetres to one and a half metres thick."

Kundana has ore widths of only a few centimetres to 1.5 m thick and has been one of the best UG mines in WA for two decades using similar stope heights (although Kundana uses paste fill as opposed to pillar which are being used at Westralia and would contribute to additional mining dilution as you say)

This chart below is a bit old (2008, so costs are out of date) but gives an idea of the cost of mining dilution (overbreak) to an UG mining project.

Each 25cm of mining dilution (overbreak) would have cost a 500,000t/annum UG operation (in 2008) about $1,000,000/annum based on an ore body width of 2.7m.

The Westralia feasibilty has 3,834,000 tonnes of ore mined UG over 8 years for 479,240 tonnes/annum average. Based on the whole deposit (Beresford and Allason) having an average width of say 2.2 m (which is conservative as there are more tonnes in Beresford and 2.2m just represents an average not a tonnage weighted average width) the cost of dilution per 25cm of overbreak per annum (in 2008) would have been about $1.2M x (4.79/5) = $1.15M per 25cm of overbreak per annum. Not very much and I don't know how the $25/t operating assumption compares to todays costs. Not withstanding this the dilution due to the pillar mining method also needs to be comsidered. If worse comes to worse they could also look at the option of mining out any high grade pillars and replacing them with concrete pillars as they have done at RMS's Vivien gold mine.Esh

Strategies for Minimising and Predicting Dilution in Narrow Vein Mines – The Narrow Vein Dilution Method

https://espace.library.uq.edu.au/da...3M~ZRA__&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ