Ch. 28] RESEARCH NEEDED 521 



RESEARCH NEEDED 



Although the general source, methods of transportation, and modes 

 of deposition of iron are well-known we are often at a loss to outline 

 the origin of specific iron deposits with any exactitude or to explain 

 why given types of deposits were formed only during certain geologic 

 periods. The Clinton type of iron deposit is certainly of marine origin, 

 but where was the source of the iron, how was it transported to the 

 site of deposition, and as what mineral and under what chemical con- 

 ditions was it deposited? Hawley and Beavan (1934, p. 510) con- 

 cluded from a study of the Mayville ores of eastern Wisconsin that 

 ". . . the ore minerals throw no light on the baffling problem of their 

 source," although the presence of clastic grains of fragmental lava sug- 

 gests derivation from a pre-Cambrian volcanic terrane. 



It is often difficult to determine the identity of the original iron- 

 bearing minerals, because subsequent oxidizing or reducing environ- 

 ments, during diagenesis or at a much later date, may have materially 

 altered the mineral composition of the deposit. For instance, Van Hise 

 and Leith (1911, pp. 529-537) state that the original siderite and 

 greenalite of the pre-Cambrian iron formations of the Lake Superior 

 region were extensively oxidized to hematite and limonite long after 

 deposition. Perhaps this also is true for other iron deposits. 



There is often question why the pre-Cambrian iron formations are 

 characterized by the association of greenalite, siderite, and chert, 

 whereas the iron deposits of later geologic periods are composed of 

 hematite, limonite, chamosite, glauconite, or siderite and are not as- 

 sociated with chert. These are fundamental problems which must be 

 answered before we shall be able to understand fully the origin of 

 sedimentary iron deposits. Furthermore, many of the pre-Cambrian 

 sedimentary iron deposits, such as the itabirite of Brazil and Venezuela 

 and the ferruginous quartzites of South Africa and Australia, have 

 been described as alternating laminae of iron oxide and clastic quartz. 

 Recent studies by Tyler (1948, pp. 86-87) indicate that the "clastic 

 quartz" of the Brazilian deposits is recrystallized chert. The mineral 

 composition of the original iron-bearing materials is unknown. Addi- 

 tional studies are needed to clarify these problems. 



Other lines of research of perhaps a more practical nature are those 

 that lead to a more concise explanation of the origin of the higher 

 grade deposits and the physical and chemical factors that localized 

 them. The laterites present many problems of this nature. Under 

 what specific chemical and physical conditions do laterites form, and 

 where are deposits of laterite most likely to be found? Although later- 



