Ch. 28] IRON SULPHIDES 519 



alternate laminae of chert and siderite, with subordinate but locally- 

 important quantities of greenalite and hematite. Subsequent oxidation 

 of the siderite and greenalite has produced the ferruginous cherts and 

 jaspers, which consist of alternate beds of iron oxide and chert. The 

 siderite-chert deposits which are observed today in these iron forma- 

 tions have been protected from oxidation and are thus the remnants of 

 much more extensive deposits. The well-developed bedding of the iron 

 formations and the gradation laterally and stratigraphically into sands 

 and shales indicate deposition in standing bodies of water. Van Hise 

 and Leith (1911, p. 516) suggest that the iron and silica was either con- 

 tributed to the areas of deposition directly from a magmatic source, 

 or that the iron and silica were released by the reaction of hot sub- 

 marine lavas with sea water. Gruner (1922, pp. 459-460) is of the 

 opinion that the iron and silica were derived from the thorough weath- 

 ering of an adjacent land mass composed of basic Keewatin lavas 

 and deposited as chemical sediments in a standing body of water. 

 Woolnough (1941, pp. 465—489) suggests that deposition may have 

 taken place in isolated basins on a peneplane surface during periods of 

 low seasonal rainfall. 



The siderite iron formation and its oxidized equivalents provide an 

 enormous tonnage of low-grade siliceous material which averages about 

 28 percent iron. Although these deposits are not being used extensively 

 at present, they represent a vast reserve of iron from which high-grade 

 concentrates may be manufactured through the use of proper ore- 

 dressing techniques. 



IRON SULPHIDES 



The iron sulphides — pyrite (FeS 2 ), marcasite (FeS 2 ), melnikovite 

 (FeS 2 ), and hydrotroilite (FeS-nH 2 0) — form in a sedimentary envi- 

 ronment. These minerals usually occur as aggregates or individual 

 grains disseminated through sedimentary rocks, and they do not form 

 important deposits of iron. Iron sulphides are particularly abundant 

 in coal deposits and in carbonaceous shales and slates. 



Thin sedimentary beds of oolitic pyrite or marcasite are associated 

 with the oolitic hematite ores of Wabana, Newfoundland, and the 

 sideritic ores of Cleveland Hills, England, but they are not of economic 

 importance at present. The oolitic iron sulphide deposits of Meggen, 

 Germany, range from 12 to 20 feet in thickness and are associated with 

 Devonian limestones and slates. They have been worked for the iron 

 sulphide content. 



