8 IKON ORES OF IRON SPRINGS DISTRICT, UTAH. 
The ores are closely related in origin to the intrusion of the ande- 
site laccoliths. The first effect of the intrusion was the contact 
metamorphism of the adjacent limestone, produced principally by 
driving out its lime and magnesia carbonates and recrystallizing 
the remaining constituents, but also by the introduction of silica, 
iron, and soda. The net result was diminution of volume. Lime- 
stone was actually fused, for residues of glass still remain. The net 
density of the contact minerals is somewhat higher than the density 
of the unaltered limestone, but the density of the rock as a whole is 
less than that of the fresh limestone, owing to the content of glass 
and the pore space. It is suggested that the fusion of the limestone 
contact phase denied access to the andesite solutions, thus explaining 
the lack of abundant introduction of minerals from the intrusives 
along the contact. Had the temperature not reached the fusion 
point of the limestone (probably about 1,000° C), other conditions 
remaining unchanged, the contact phase of the limestone might have 
been left porous, affording openings for the entrance of the solutions. 
The ore-bearing solutions followed shortly after the contact meta- 
morphism of the limestone, and after the outer part, at least, of the 
andesite laccoliths had crystallized, as shown by the occurrence of 
ore in fissures in the andesite and limestone contact phase. The ore- 
bearing solutions carried silicates similar to those previously formed 
in the limestone contact. The conspicuous feature was the intro- 
duction of soda by the ore-bearing solutions into both limestone and 
andesite along the contact. A comparison of the alteration of the 
andesite next to the ore with its alteration by weathering brings out 
clearly the effect of the ore-bearing solutions. 
A graphic method of comparison of the fresh and altered rocks 
brings out various possibilities in the interpretation of analyses. 
A consideration of the temperatures of crystallization of some of 
the silicates deposited with the ores and possible pressures resisting 
the solutions seems to indicate a probability that the solutions may 
have been vapors and that they were pneumatolytic after-effects of 
the andesite intrusions. 
It is suggested that the iron was carried as ferrous chloride, which 
became oxidized at a temperature above 500° C. by breaking up 
associated water and simultaneously developing free hydrogen. 
Precipitation came about partly by the neutralization of the acid 
when it came into contact with the limestone, and partly by the 
evaporation of the acid with the water which would accompany the 
diminution of temperature and pressure as the solutions left the andes- 
ite. The conspicuous introduction of soda into wall rocks suggests 
further that the same solution carried salt. This would be decom- 
posed by silicic acid and the soda be deposited as sodium silicate, 
largely albite, the form in which it is now found. 
