ZINC ORES OF THE EDWARDS DISTRICT 1 7 



increase in volume may cause marked mechanical disturbance in 

 the mass, with resultant slickensiding. On the other hand, when 

 the calcite of a limestone is altered to the denser dolomite, the 

 decrease in volume renders the rock porous and may thus convert it 

 into a water-, gas-, oil- or ore-bearing formation. 



In the present case, it is evident from the study of thin sections 

 that the serpentine, resulting from the alteration of diopside, fills 

 the entire space originally occupied by the latter mineral. More- 

 over, it is evident, both from microscopic study and from mere 

 inspection of hand specimens, that the serpentine developed not 

 only by the alteration of grains and crystals of diopside, thus 

 forming pseudomorphs, but also filled cracks in the rocks and 

 replaced calcite, in this way often segregating in large masses. 

 Clearly, then, the formation of serpentine is a more complex 

 process than the mere alteration of diopside in place for, when 

 serpentine fills fissures in the limestone, it is evident that all the 

 constituents of the mineral must have been in solution in, and 

 deposited by, the water circulating through the fissures. In some 

 large masses, as well as in many microscopic grains, serpentine has 

 been deposited from solution, replacing calcite which goes into 

 solution as the serpentine is deposited. In so far as the limestone 

 is magnesian, it may contribute a portion of the MgO to the ser- 

 pentine, but it is probable that most of the MgO is furnished by the 

 solutions, which may get it either from the older silicates or, more 

 probably, from a magmatic source. Thus, the serpentine of the ore 

 deposits and country rock is, in part, an alteration in situ of diopside 

 (and to less extent tremolite) and, in part, a deposit from solution, 

 either in cracks or replacing calcite. Between the two methods of 

 origin there is a marked contrast. In the first case, the diopside 

 itself furnishes most of the material of the serpentine and may be 

 said to " alter " into serpentine, which is an " alteration product " 

 of diopside. The calcite, on the other hand, supplies little, if any, 

 material for the serpentine which takes its place and, thus, the latter 

 can not be called an alteration product of the former. The relation 

 between the two minerals is that of " replacement," not alteration, 

 the serpentine " replacing " the calcite. 



In alteration there is always some more or less close chemical 

 relation between the primary and secondary minerals involved in 

 the process. In replacement, on the other hand, this is not the 

 case, and one mineral may be replaced by another of totally dis- 

 similar composition. The replacement of pure calcite, calcium car- 

 bonate, by serpentine, a magnesium silicate, is of this nature. 



