Ch. 29] LEAD-ZINC REPLACEMENTS 531 



to have occurred long before ore deposition and over areas so broad as 

 to indicate little more than that ore fluids found such areas to be pre- 

 ferred sites, again mainly for physical reasons. In nearly all cases, 

 local, and generally minor, structural features are of prime importance 

 in localizing deposits within particular portions of the favorably dolo- 

 mitized or altered host. 



This leads to a second, and somewhat more satisfactory, generaliza- 

 tion on the subject of structural control, namely, that the presence of 

 a fine-grained or otherwise impervious cover above a suitable lime- 

 stone (or dolomite) host has a marked tendency to localize deposition 

 in the nearby parts of the underlying formation, and probably more 

 so the more nearly the rocks are flat-lying. Good examples are: (1) 

 The Tristate district of Missouri-Kansas-Oklahoma, where ore occurs 

 chiefly in the first 300 feet of Mississippian limestone beneath Pennsyl- 

 vania shale, which was almost certainly quite thick at the time of dep- 

 osition although now largely removed by erosion; (2) Leadville, Colo- 

 rado, where thick porphyry sheets in a series of sedimentary rocks seem 

 to serve a similar function; (3) Park City, Utah, where replacement 

 occurs chiefly in a rather thin zone of Paleozoic limestone beneath 

 thick Triassic shales; (4) the bedded fluorspar deposits of Kentucky- 

 Illinois, where ore occurs in Mississippian limestone beneath thin, shaly 

 members of the same series, and not far beneath a former thick cover 

 of Pennsylvanian shales. 



It is commonly accepted that the impervious cover has acted as a 

 guide and confining medium for rising ore fluids, channeling them for 

 long distances within a favorable host rock. The quite different in- 

 terpretation that the ore fluids actually circulated across the beds and 

 that the ore substances were held back while the transporting fluid con- 

 tinued on, somewhat as in osmosis, has been advanced (Mackay, 1946) 

 but can be accepted generally only with much more convincing proof. 

 The evidence of oil and gas pools trapped beneath similar barriers for 

 millions of years, even though under great pressure and probably by 

 nature more penetrant in some cases (especially gas) than an aqueous 

 solution, seems to be a strong argument against this proposal. 



Another lithologic situation that seems to be of considerable im- 

 portance in some places is the presence of a favorable limestone host 

 above a porous sandstone formation. An outstanding illustration is 

 the southeastern Missouri lead district. Ore occurs in the lower part 

 of the Bonne Terre dolomitic limestone above the Lamotte sandstone, 

 which rests on an irregular pre-Cambrian basement. There may be 

 some question whether the presence of the sandstone or the proximity 

 of the crystalline basement is the more important factor, but appar- 



