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



unlike rock masses, and especially in zones of mixed mineral content, 

 as half carbonate and half silicate, rather than in pure mineral types. 

 Rove concluded that effective shearing should be concentrated in rocks 

 of intermediate competence, neither too rigid nor too plastic. 



Garrels has in press a paper that deals with extensive tests of poros- 

 ity and permeability with respect particularly to their relation to dif- 

 fusion and the possible effectiveness of this process as a step in re- 

 placement. The tenor of this is to suggest that diffusion is relatively 

 independent of the size of openings and but moderately affected by 

 total porosity; also that it is rapid enough, especially at elevated tem- 

 peratures, to be important within reasonable lengths of time. 



Ohle also has in preparation a paper covering extensive laboratory 

 experiments on permeability of host rocks carried out under desirable 

 conditions of refinement. 



In my own researches and observations, I developed a strong im- 

 pression that grain size has a very significant influence on ore pre- 

 cipitation, as has been suggested by Bain (1936), who found that in 

 metasomatic replacement from aqueous solutions small capillary open- 

 ings were much more effective than larger ones, the suggested explana- 

 tion being that adsorption was an important factor and was propor- 

 tional to the area of surface exposed. My field observations in some 

 respects tend to support this conclusion, for many fine-grained rocks 

 appear to be especially effective precipitants for ore. Such rocks, how- 

 ever, are not good channels for the transportation of ore fluids over 

 long distances, for which larger openings, even though of microscopic 

 size, and better continuity are essential. But, at the site of deposition, 

 minute subcapillary openings may be very effective. This feature is 

 nicely illustrated in the southeastern Missouri lead district, where the 

 prevailing host rock is a recrystallized dolomite with a grain size of the 

 order of 0.1 to 0.3 millimeter and considerable porosity, quite com- 

 monly in the visible range. Intercalated in this type of rock are minor 

 areas of dark shaly material, which may be either in well-defined layers 

 or in rather irregular masses of conglomeratic appearance, enclosing 

 lumps of dolomite. It is within these conglomeratic shaly layers and 

 on the upper and lower margins of the shaly bands, where they grade 

 into dolomite, that ore is especially likely to be deposited in high con- 

 centration. Visible porosity in these rocks may be low or lacking, 

 but the grain size is infinitely smaller than that of the enclosing dolo- 

 mites, estimated to be in the range 0.04 to 0.01 millimeter. Although 

 grain size probably is important in the explanation of this relationship, 

 it is quite likely that other factors, such as the chemically combined 



