ORE DEPOSITS 



373 



depend either upon the width of the original fissure or upon the 

 amount of the replacement of the walls, or upon both. If replace- 

 ment has not occurred, the boundary of the vein may be distinct. 



Weathering and Concentration of Ores. — As a metalliferous 

 vein is eroded, it is attacked by the agents of the weather and under- 

 ground water. The result of such action is the removal of the more 

 soluble minerals in the surface zone, (i) If the minerals removed are 

 worthless, the portion remaining will be richer. For example, in 

 gold-bearing quartz veins in which the gold is contained in pyrite, 

 the solution of the pyrite leaves the pure gold in a honeycombed, 

 rusty quartz. It was 

 such quartz veins 

 which delighted the 

 old-time prospector. 



(2) If the minerals 

 removed are valu- 

 able and are depos- 

 ited lower in the vein 

 by the percolating 

 water, a rich deposit 

 may result. In a 

 vein containing chal- 

 copyrite and pyrite, 

 for example, the iron 

 may be left in the 

 upper part of the 

 vein in the form of 

 limonite. This is 

 called the gossan (chapeau de fer and eisen hut) and may be in suffi- 

 cient quantity to be mined as iron ore (Fig. 358). 



Lower in the vein, in the oxidized or middle zone, the ores are in 

 the form of oxides, carbonates, etc., and may be enriched by the 

 addition of metallic minerals brought down from the weathered 

 zone. In some deposits the oxidized zone is the only portion of the 

 vein in which the mineral occurs in sufficient quantities to be ex- 

 tracted with profit. 



Beneath the oxidized zone, which extends to or below the level 

 of ground water, lies the unaltered vein material of the unoxidized 

 zone. Here the ores occur as they were originally deposited. These 

 three zones are not usually separated by well-defined boundaries, 



Fig. 358. — Vein showing three zones: A, surface or 

 weathered zone; B, oxidized or middle zone; C, unaltered 

 or sulphide zone. The weathered zone, A, is often largely 

 composed of iron hydroxide and is called gossan. 



