Dictionaries tend to be concise- exfoliation is caused by weathering and weathering occurs near surface. Spalling is due to exfoliation (in the dictionary, I am sure it says see Exfoliation)..
>>But if the stress exists in the rock at depth isn't drilling through it or opening up a stope going to release the stress and cause spalling?<<
It would be better to say caving instead of spalling and rock burst instead of exfoliation. The stress in the surrounding rock resulting from a small opening at the depth of the TG-3 mineralization are weaker than the strength of the rock (we are talking about a very fine grained felsic volcanic). The resulting strain will not cause rock bursts. IMO every hole in TG will remain open for the rest of our lives- I hope they are cementing them after they are completed. The large openings will not remain open for long enough to have to be supported- even so, they will scale and use rock bolts to support the walls. Every underground mine in the world scales and uses rock bolts.
Lets use an example of a producing mine where I have alot of info:
Hudson Bay Mining and Smelting Co. Ruttan Mine, Leaf Rapids, Manitoba, Canada.
An underground Cu-Zn VMS with proven and probable reserves of 11.5 million tonnes averaging 1.1% Cu and 1.7% Zn as at Jan 1, 1998. The mine began producing in 1973 and is still in operation today. It produces 5,900 tonnes per day (approximately 13 million lbs) of ore which is now situated well below 150 meters depth. Their mining costs were Canadian $5.02 per tonne in 1996, using blasthole open stopes with delayed backfill. Of that cost, ground support and backfill costs were C$0.07 and C$0.41 respectively. That makes ground support and backfill only 10% of total mining costs.
As you can see, what we have been talking about in these posts is really a very small issue.
The Ruttan Mine was discovered in 1969 by drilling a single hole on a target delineated by regional electromagnetic and magnetic surveys. Subsequent drilling outlined an an orebody containing 45.5 million tonnes of sulfide ore with a grade of 1.47% copper and 1.61% zinc to 600 meters below surface. Production commenced at 9,000 tonnes per day from an open pit in 1973 and moved underground in 1979. As far as I know, Sherrit (the discoverer of the Ruttan, also a publically traded company) never published their core logs. |
