Bulletin . be-tween the bedded layers of cherts. The seams of ore vary in thick-ness from a fraction of an inch to several inches, and here and therecontain cavities lined with crystals of lead and zinc minerals, withminor amounts of iron sulphides and other associated minerals. Theore-bearing beds are 6 to 20 feet thick, and are more uniform inrichness and less liable to terminate abruptly than the runs.Brecciation is relatively unimportant in the sheet-ground deposits. The runs and the other irregular bodies of ore may be dividedinto two classes—those in fairly hard ground, where little timb

- Image ID: 2AXDMNF
Bulletin . be-tween the bedded layers of cherts. The seams of ore vary in thick-ness from a fraction of an inch to several inches, and here and therecontain cavities lined with crystals of lead and zinc minerals, withminor amounts of iron sulphides and other associated minerals. Theore-bearing beds are 6 to 20 feet thick, and are more uniform inrichness and less liable to terminate abruptly than the runs.Brecciation is relatively unimportant in the sheet-ground deposits. The runs and the other irregular bodies of ore may be dividedinto two classes—those in fairly hard ground, where little timb
The Reading Room / Alamy Stock Photo
Image ID: 2AXDMNF
Bulletin . be-tween the bedded layers of cherts. The seams of ore vary in thick-ness from a fraction of an inch to several inches, and here and therecontain cavities lined with crystals of lead and zinc minerals, withminor amounts of iron sulphides and other associated minerals. Theore-bearing beds are 6 to 20 feet thick, and are more uniform inrichness and less liable to terminate abruptly than the runs.Brecciation is relatively unimportant in the sheet-ground deposits. The runs and the other irregular bodies of ore may be dividedinto two classes—those in fairly hard ground, where little timberingis necessary, and those known as soft-ground deposits, which requiretimbering in nearly all the drifts. These ore bodies usually have anelongated form and many are of considerable length and thickness. Afew deposits are circular or semicircular in shape and are known ascircle deposits. The so-caUed runs and the irregular ore bodiesare foimd at shallower depths than deposits of the sheet-ground type,. ORE DEPOSITS. 7 and are as a rule considerably richer in mineral content, often con-taining large pockets of nearly pure blende and galena. These min-erals are commonly associated with a dark-colored secondary chertas the main filling material of fractures and cavities. GANGUE MATERIALS. Chert and dolomite are the most important gangue minerals.Limestone, shale, soapstone, and mud are less important. In someof the deposits the chert and dolomite are intimately mixed, the zincblende being embedded in a black secondary chert that cements thegangue and the blende. Chert, or flint, is the characteristic ganguematerial of the ore deposits and can be recognized in three diJfferentvarieties: The white chert, the blue to gray chert, and the black orsecondary chert. The blue chert is known as hve flint, and itsoccurrence usually indicates the presence of blende. The black va-riety, which has already been mentioned as filhng fractures, cavities,and the interstices of the brecciated ro

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