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



ently ore deposits at this horizon are almost unknown where the in- 

 tervening sandstone is absent. 



This district also serves well to illustrate the great importance of 

 minor or even trivial features in localizing ore where preferred struc- 

 tures are lacking. Many of its deposits are known to be controlled 

 structurally by slight domes of initial sedimentation plus differential 

 compaction, overlying buried knobs in the pre-Cambrian basement. 

 A typical illustration is given in Fig. 1. Moreover, within ore bodies, 

 rich concentrations may be determined by minute rolls of anticlinal 

 character beneath a favorable ore-bearing band. 



A mineralized sedimentary sequence similar to that in southeastern 

 Missouri exists in several places in the United States, as in the Llano- 

 Burnet uplift of Texas, the Arbuckle Mountains of Oklahoma, and 

 near the base of the Cambrian succession at various places in the Ap- 

 palachians, although these are mostly uneconomic occurrences. 



A possible variation of this situation is presented by the Wisconsin- 

 Illinois zinc-lead field, where ore occurs in dolomitized limestone not 

 far above the widespread St. Peter sandstone. The pre-Cambrian 

 basement in this area is many hundreds of feet below the sandstone. 



In all these occurrences, there is the question, worthy of more care- 

 ful evaluation, of the extent to which the sandstone may have served 

 to collect ore fluids and distribute them laterally to favorable struc- 

 tural traps. Usually there is also the possible question whether the 

 presence of the highly pervious sandstone below is of great importance 

 or, rather, whether impermeable barriers not far above, present- in most 

 areas, has exercised more control. 



The subject of impermeable barriers deserves more careful summa- 

 tion than seems to be available anywhere, but an interesting start has 

 been made by Newhouse (1942), and most textbooks treat the matter 

 in some fashion. 



Concerning microscopic structures and details of replacement, 

 considerable valuable information is accumulating. My conclusion in 

 the Edwards-Balmat, New York, district was that microbrecciation 

 seemed to be a major factor in controlling the flow of ore fluids, and 

 that probable continuity of openings seemed much more important 

 than absolute porosity. Rove (1947) attempted to evaluate the physi- 

 cal features of favorable horizons by tests of grinding resistance, 

 crushing strength, toughness, etc., but he failed to develop any conclu- 

 sive correlations and ended by suspecting that secondary permeability 

 induced by shearing probably was the most important factor in deter- 

 mining channels of circulation. My data tended to show that the need- 

 ful microbrecciation was likely to be concentrated along contacts of 



