Well that is assumptive. If you had one ounce per ton over 3 feet, it could be mined satisfactorily for a profit and fairly easily. There are different ways depending on the variability of the vein system. The object is in my mind to generate something where you can mine above 0.35 OPT over a width of 4 to 5.5 feet. I used to mine in 3 to 6 foot wide stopes regularly. One I mined had a 250 foot length, and an average width of 4 feet. It seldom exceeded 5 feet, although it went to 15' in one place. What people seem to forget who have not mined, is that it is harder to mine the wider stopes. It is dead easy to shrink 3 feet wide. You have plenty of room. Every 75 to 100 feet you may have a 5-6 foot wide raise for air, mill-hole or access, and you knock out a bit more width, say 4.5 feet for your slusher winch. You drill patterns of 3 holes ore sometimes only two wide, at ten degrees from the vertical, in rows across the stope, every 2 feet to 18 inches apart for 100 feet. Load with ANFO, and tape fuse, connect with 5 second thermalite and light it.
At the end of the blasting row away from your access raise, you have blasted out a short raise cut with a 24 hole pattern and a 3-2-1 reaming hole cut. The blast breaks into this cut as a face to break to. The cut, at 4 feet wide, six feet deep into the back, and 100 feet long is 200 tons of blasted rock. After blasting that rock you will remove 66 tons of ore from the draw points (100 feet below) of what is called "swell-muck" -- which will allow one to stand again in the same 7 foot high void (above the broken ore pile) as before. After blasting one shift is allowed for the poisonous air to clear, although in some mines clearance was achieved within 3 hours. Upon returning and after drawing out the excess broken ore at the level below, the miner levels the broken ore in the stope by slushing the ore with a winch and a drawing scoop or scraper. Then after getting his equipment off the hangers, he is ready to drill again. A shift may have to be spent marking out ore, or sampling the back as well as surveying the new void created by the last blast.
Generally the miners get paid by ore removed and in order to calculate this, they have to survey the opening created from carried forward points in the stope. These are maintained by spads put in wooden plugs in the raise and stope back and surveyed down the raise by transit. Eventually this whole process will be automated by laser and triangulation. Surveys also serve to keep track of the position and volume of the mined ore, its proximity to known drill intersections which are also transit and mult-shot surveyed. If careful back sampling is done, the whole grade picture as the ore is mined can be tracked very accurately. at one mine I used to work at the ore grade picture could be calculated by in stope assay to within 1/100 of an ounce per ton of the returning smelter grades from custom milling.
This is unusual and it is not always possible to get this kind of grade control. However at Kerr Addison stopes we mined there were calculated by copious drill intersections and then compared to careful algebraic mixing of ore cars drawn from each stope to figure the back assay by mill recovery of removed ore to individual stopes. A figure of 95% agreement to predicted grade could be maintained for ore so-mined and closely drilled. This drill spacing required 25 to 50 foot centers at worst. What was much harder was to predict grade of carbonate stopes with exploration spaced drill holes at 150' and better drill spacings. the nuggety nature of the ore led the manager to say in retrospect to say that 40% of all drilled bodies at the Kerr were rejected unnecessarily as being sub ore when in fact they could probably have been mined. This would amount to about 5 million ounces in that area of the mine alone.
We had stopes at the Kerr that were pipe like and were mined from platforms that descended from raise borings. We would mine with two jacklegs in a horizontal breasting ring drill pattern with NRT joiner steel extensions. (Noranda rope thread, 6 foot segmented extensions steel) 2 miners could break 600 tons per day steadily in these 500 foot wide pipe-like ore bodies. Total costs to access, drill, blast muck and hoist in these simple structures all up, was $1.50 a ton in 1977. Cut and fill was $22.00 per ton, and square set was 35 dollars a ton. Cut and fill was much too expensive as it was unnecessarily involved in timbering. I could see ways were automation could have cut costs in half.
The whole mining shrinkage cycle is composed of these parts with max cycle time. Assume a 2-shift mine. Numbers are 8 hour contract shifts -- second numbers are men on shift in/surface mine min.
MOS TOD
1. raise round at end of stope, 2 men 1 10 d
1. drill 1200 feet 200 holes two men 2 5 e
2. load holes & hang equipment/blast 1 10 d
3. clear stope with air night/evg 1 2 n
4. slush ore and draw swell muck 1 10 d
5. mark out new ore and survey/sample 1.5 6 e
This assumes 2 miners, supervisor, scoop operator, truck operator, mechanic, electrician, geologist, surveyor/sampler, and assistant. Absolute min small mine. It would be better to get 4 miners working, as the supervisory staff does not increase. So 12 men could in theory mine 200 tons in 4 days, of which in initial mining, 66 tons per day is available until the stope is finished and it can go on free-pull. Two more men and in that 4 day period, 66 more tons is available. 33 tons per day until you can pull free and then you can get as much up the ramp as you can pull and truck.
8 shifts 4 days, 200 tons, 10 men. Development time is significant, as each stope requires two raises of total of 300 feet, and perhaps one or two drifts of 100 feet each or more. Two development miners take 33 days min to develop 400 feet of standard drift/raise for one 5,000 ton stope.
Let's say we use 4 miners, and develop a 5000 ton stope in 17 days, we mine it in 25 days, pulling 1650 tons, and free pull the rest in 23 days. That is 80 tons per day. 12 men. Not counting milling people, accounting and other engineering staff, 6.66 tons per day per man for a minimalist shrinkage mine the olde harde waye. Get it down to say 4 tons per man day, and a cost of 300 dollars per man day, and we have 75 dollars per labour ton. That is generally 60% of the cost of mining, so we have 125 dollars a ton. That is about right. All up with capex it is hard to say, but let's put it at $160 a ton for a fair number of tons. If the minimal mine costs 2 million to get underground on, and 2 million for the mill, we had better have 125,000 tons of ore. Grade needs to be about 0.45 OPT for fair profit. Roxmark can do that easily. So given that they can mine faster with more men, and have the mill built I would say their break even or cut-off grade is 0.23 OPT. That was our break even at a 35 man, 300 ton per day gold mine in Northern Ontario in 1987.
Slusher
Scraper
What this points to is that whilst our tiny mine has to be completely cycled out to see what its total production over manpower for initial development, swell mining, and free pull, it would appear that worst case it is more than 3 tons per man per day. It would behoove us to find faster mining and development techniques for narrow vein mining. Fortunately we have these at hand.
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