40 



ECONOMIC GEOLOGY 



Mesabi district the iron was originally deposited as the silicate, 

 greenalite. 



Metasomatic deposits of Bauxite. The Georgia-Alabama baux- 

 ites are found in Paleozoic limestones above pyritiferous shales. 

 Meteoric waters oxidize the pyrite to ferrous sulphate and sul- 

 phuric acid. The free acid attacks the aluminum silicates of the 

 shales producing alum and aluminum sulphates which are carried 

 upward by ascending currents. These solutions, in contact with 

 the overlying limestones, form calcium sulphate and bauxite. 



Lead and Zinc Metasomatic Deposits. 

 Lead and zinc form metasomatic 

 ores in the form of the sulphides, sul- 

 phates, carbonates and silicates. The 

 sulphides are of most importance. 

 Such deposits are common in the Car- 

 boniferous limestone but may occur 

 in the limestones of any age. In 

 Cumberland, England, the ore takes 

 the form of and replaces the calca- 

 reous fossils. The molecular replace- 

 ment has been so perfect that the ore 

 preserves not only the external form 

 but often the internal structure of the 

 fossils it has replaced. 



The source of the lead and zinc min- 

 erals is often in some igneous rocks or 

 sulphide-bearing sediments. These 

 may be beneath the surface or exposed to weathering agencies at 

 the surface. The transporting waters are meteoric and carry the 

 solutions downward where deposition takes place (Fig. 30). 



Metasomatic Copper Deposits. Chalcopyrite, CuFeS 2 , is the 

 most important copper ore of metasomatic origin. The materials 

 came from some igneous magma during the later stage of its 

 cooling. The aqueous emanation transported the minerals in 

 the form of sulphates. The solutions are either ascending or 

 descending and the chalcopyrite would be precipitated by hydro- 

 gen sulphide and alkaline sulphides. If the solution percolated 

 through the limestone, then malachite and azurite would be 

 formed. 



Metasomatic Gold Deposits. According to Thomas and Mac- 

 A lister, in the Transvaal district, siliceous gold-bearing solutions 



FIG. 30. Diagrammatic 

 section showing the contact 

 of porphyry and limestone 

 and the zone of ore deposi- 

 tion, Maginnis mine, Judith 

 Mountains, Montana. (After 

 Weed and Pirsson.) 



