530 brown. SEDIMENTARY HOSTS FOR ORE DEPOSITS [Ch. 29 



There are, finally, a few accessory mineral deposits that may de- 

 serve mention. One type is that of barite replacements, as at Magnet 

 Cove, Arkansas. These may be more important than is appreciated, 

 having been overlooked to some extent in the past. A related group 

 is that of bedded fluorite replacements such as the Kentucky-Illinois 

 district, in part. 



With respect to the types of rock involved, if we include the various 

 stages of dolomite in the general term limestone, the list has a high 

 proportion of limestone replacements. The Sullivan Mine, British 

 Columbia, and Mount Isa, Australia, are the only important exceptions 

 in lead-zinc. With the pyritic sulphides this situation is distinctly 

 reversed. All the great fahlbands are in siliceous-aluminous rocks; 

 so also with the copper deposits for the most part, although Kennecott, 

 Alaska, and Cerro de Pasco area, Peru, including Morococha, etc., are 

 exceptions. 



We thus arrive at the conclusion that the subject of sedimentary 

 rocks as hosts for ore deposits is largely a study of the whys and 

 wherefores of replacement; replacement of limestones by lead and zinc 

 sulphides, and of shales, slates, or schistose rocks by pyritic and 

 cupriferous sulphides. 



Lead-Zinc Replacements 



On the subject of lead-zinc replacements in limestone, a vast amount 

 of specific literature covers mainly the empirical problems of individual 

 districts. There are also some important papers by way of summation 

 (Van Tuyl, 1916; Hewett, 1928; Hayward and Triplett, 1931; Brown, 

 1947b; Behre, 1947; Newhouse, 1942). 



Several of the references cited deal extensively with the subject of 

 dolomitization as a factor in ore localization. As Behre has pointed 

 out, the conclusions are by no means exact or satisfactory. Dolo- 

 mitized rock, on statistical evidence, would seem to be much more 

 favorable for ore deposition than pure limestone, although exceptions 

 do occur. But in many instances it still is undetermined whether 

 dolomitization occurred during the diagenesis of the sedimentary rock 

 (and hence long antedating ore emplacement) or only shortly pre- 

 ceding ore deposition and, therefore, probably as an early stage of that 

 process. In either event it seems likely that the preference of ore for 

 dolomite over pure limestone rests on a physical basis more than on 

 chemical factors. Much the same conclusion must be drawn with re- 

 spect to the importance of magnesian silication (by diopside, tremolite, 

 talc, serpentine, etc.) in metamorphosed limestone formations. Silica- 

 tion of this type usually develops preferred zones for ore and is likely 



