Bisbee daily review., 1916-07-23
AND GAZING SYSTEM IN THE COLE
 WORKINGS
Article Dealing With Econo- mical And Efficient System Of Mining Orebotlies Tn Large Mine Of District
ROBERT H. DICKSON
 IS ARTICLES AUTHOR
Mining And Engineering Jour
 nal Of New York Prints Or igi1al Article. Details Of Mitchell System Are Given

The orebodies at the Cole shaft of 1 the Calumet & Arizona Mining Co, Bisbee, AriZ.. consists of a series of raughly tabular lenses that are found
 in breaks cutting through the lime
 stone. The material in these breaks consists of leached limestone, broken limestone and limonitic clayey leach ed material, parts of which are i'm
 pregnated with sufficient oxides and
 carLonates of copper to make ore.
 These lenses of ore in the leached
 material vary in size: some are sew
 eral hundred feet long, 90 1t. wide and
 50 it. high. They vary in copper con
 tents from 10 per cent to as low as f
 per cent. In order to mine some of
 these lenses that assayed 3 to 4 per cent copper, it became necessary to devise a method that would be cheap CT than sQuare-settin9, and yet be safe for the heavy ground. After much ex- perimenting. NJ. w. Mitchell, mine foreman, developed the system known as the Mitchell top-slice and caving
 system. patterned after the top-slice
 and caving system as used in the
 Lake Superior mines and elsewhere. but so modified as to suit the local conditions.
 After the extent of the orebody is determined by crosscuts and raises, it is cut into a series of sections by rows of square sets extending across the orebody at 45-ft. centers. Each row of sets is a single set in width and as l9ng as the width of the ore body. Vertically this timbering eX- tends, when completed, from the top of the ore to the bottom ard is known as the gangway sets. The top of each section is then mined by the square-set method, one set high or more, so that there will be an even | floor under which to start slicing. The ore below is then mined out in slices ! s to 10 it. thick by running a series | of contiguous drifts about it. wide ! across the section from one gangway | to the other. As each slice is mined l out, the back is caved and another l slice is started below. While extract i ing the ore in these contiguous drifts, | the roof is temporarily supported by I pIops or bulkheads. The main haul i ageways are generally driven parallel !
  lenses-usually paralleling the longer dimension. Crosscuts are driven at | right angles to there across the ore | bodies at 45-ft centers. Raises are ! then driven of these cIosscuts, about | 40ft. apart up to the top of the ore ! body. These serve as manways and | chutes to the top of the stope and as | a starter for the gangway sets. A | low of lead sets. or gangway sets, is i run from one raise to another at the ! top of the orebody so that it will be | vertically over the crosscut Sn the | level In the meantime the top of the | orebody is being mined by the square ! set method until there is an even | floor of square sets in sufficient In | slicing, each 45-ft. section is consider ed one unit No sills are placed un- der the posts of the square sets; they | are placed directly on the ground. After the ore has been taken out of | the square sets, long sills of 4Xl0-in. l timber are laid across the stope from | one gangway to the other between | each row of posts. A 2-in. plank floor | is laid on there sills and serves to | keep the fine waste (capping) from | mixing with the ore during the slicing. | Then, if possible. the sets are filled with waste on top of this floor. This | is to serve as a cushion for any ground | to fall on in case the back does not l immediately follow the caved timber ] after slicing has started below.
 While the top of the orebOdy is be- ing mined by the square-set system, | second row of gangway sets is run | just below the first TOW, and then an- other row is started below this. After I the square sets have been properly | filled, or the timbers shot down in case there is no filling, a drift is I started from one gangway 10 another ! at the edge of the Oretody just below the carved or square-set ground. The first II 1t. of this drift is driven as an inclined raise on a i5' pitch. then driven horizontally until it reaches a f point halfway across the 45-ft section. A similar drift is driven al the same time from the opposite gangway. and these two hole together in the center
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