Full year result, page-37

Chris1983,
Following on from my previous post where I raised the issue of delayed draw-down of ore from shrinkage stoping. I thought it might be constructive to raise a '*** model' of eventual sustainable production in order that it might be pulled apart, in whole or part, for the furtherance of discussion.

My aim being to: (a) estimate the average time to complete a stoping cycle on one stope panel. (b) estimate the sustainable level of development to stope ore. (c) test the predicted recoverable grade (ie 8g/t). (d) estimate the overall availability of the Co-O operation per qtr.

Assumptions:
SG of rock = 2.62
Along strike extent of a single stope panel ~ 50m
Average vein width ~ 1.2m

So, to calculate the tonnage of stope ore per panel:
Vertical extent of mining ~ 48m (allowing for crown pillar)
Volume of ore = 48 x 50 x 1.2 = 2,880 cub metres
Therefore stope tonnage = Vol x SG = 7,546t

To calculate the average development tonnage per stope panel:
Ore drive width = 2m
Ore drive height = 2.5m
Ore drive length = 50m (same as stope panel)
Volume = 250 cub metres
Vertical Raise = 250 cub metres
Access Drifts = 40 cub metres
Ore Chutes + Draw point = 50 cub metres
Haulage Drift = 250 cub metres
Therefore total development volume = 840 cub metres
Development tonnage per stope panel = 2,201t

Total mined tonnage per stope panel (Dev + stope) = 9,746t
Percentage of development ore to stope ore = 29% (don't forget, this is an estimate of sustainable production - not the accelerated development rate they have been working at since 2011. And given the sacking of 1,000 contractors, it appears likely that the development rate will now drop to sustaining levels).

MML are reporting mining of 100 stopes. Therefore if 100 stopes is to be the sustainable level of production it would imply that total tonnage currently being worked is 100 x the individual stope tonnage - ie 974,640t

However, maximum haulage capacity of the mine is now rated at 2,500tpd.
Hence, the minimum number of days required to fully mine and haul the total ore tonnage from 100 stopes is 974,640/2,500 = 390 days.
Therefore, the percentage of currently worked tonnage that can be mined and hauled per qtr is 23% at a maximum.
Applying this percentage at an individual stope level infers that of the 9,746t of total ore per stope only c. 2,250t can be mined and hauled per qtr. This equates to just c. 25t/day on average.

To test for the predicted recoverable grade once at full sustainable production:
Average resource grade = 9.9g/t (Indicated and Inferred)
Assume development ore grade is 50% of stope grade due to dilution from sterile rock. Then Development grade ~ 4.95g/t

Tonnage of stope ore per panel mined = 7,546t
Contained gold = 74,701 gram
Tonnage of development ore mined per panel = 2,201t
Contained gold = 10,894 gram
Therefore, total contained gold per panel advanced = 85,595 gram
Total tonnage per panel mined = 9,746t
Resulting blended grade at mill = 8.8 g/t
Average recovery ~ 90% (long term average has been c. 93%)
Recovered grade = 7.9 g/t (PHB has predicted recovered grade at 8 g/t)
Recovered gold per stope panel = 2,477 oz
Recovered gold per 100 stopes (on an annual basis) = 231,911 oz (ie applying reduction factor of 365/390)
Target production level = 200,000 oz
Difference = 86% (infers allowance for plant/mine availability)
Hence availability pa = 315 days or 79 days per quarter.

So the numbers appear to add up to the predicted levels and the overall time to mine and haul also imply a significant natural delay on full extraction from each stope on a rolling basis. If correct, then tonnage from the mine should increase on a quarterly basis as more stopes can be fully drawn down and should be eventually limited by the total haulage capacity of the mine infrastructure at c. 2,500tpd.

But it's one thing to do the math and quite another to make it work in the real world of underground narrow-vein mining! We shall see!
CPDLC