Bulletin . isfactory, owing to the irregularity of the ore bodies andthe lack of well-defined mineral-bearing zones. Where possible, a system of underhand stoping (figs. 2 and 3) ispracticed, especially in the hard-ground or sheet-groundmines, provided the working face is high enough to warrant are left at frequent intervals, the distance between centersand the thickness of the pillars depending upon the character of the 22 MINING AND MILLING OF LEAD AND ZINC ORES. ore body and the height of face mined. The methods of mining usedmay be divided into hard-ground and soft-ground m
Bulletin . isfactory, owing to the irregularity of the ore bodies andthe lack of well-defined mineral-bearing zones. Where possible, a system of underhand stoping (figs. 2 and 3) ispracticed, especially in the hard-ground or sheet-groundmines, provided the working face is high enough to warrant are left at frequent intervals, the distance between centersand the thickness of the pillars depending upon the character of the 22 MINING AND MILLING OF LEAD AND ZINC ORES. ore body and the height of face mined. The methods of mining usedmay be divided into hard-ground and soft-ground mining. In hard-ground or sheet-ground mining, where the height of facewarrants underhand stoping (see PI. VI), a machine drill is usuallyset on a 7-foot column placed at the upper part of the working face, nearthe roof, in order to advance a heading about 6 to 8 feet high. Whenthe heading or breast has been advanced 15 to 25 feet the underlyingbench is drilled and blasted. This bench is usually 9 to 12 feet. Figure 3.—Plan and section of sheet-ground mine in JopKn district. thick, giving a total working face of 15 to 20 feet. If the thicknessof ore should be more, or the face higher, than 20 feet, an intermediatebench is worked between the heading and underlying bench. Insome of the hard-ground mines only the upper part of the ore-bearingdeposit can be worked at a profit and in others only the lower condition is chiefly due to the presence of an intermediate layerof practically barren rock, varying in thickness from 2 to 6 feet,which would have to be mined with the ore if the two ore-bearingstrata were mined together. Where stoping is not carried on, orwhere the deposit is not more than 8 to 12 feet thick (PI. VII), the BUREAU OF MINES BULLETIN 154 PLATE V
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectminesandmineralresou
