GOLD DEPOSITS. 73 
their heat and chemical energy through a large mass of rock. Such a dissipation of energy 
in the replacement bodies would account for the lower intensity of alteration in them than 
in the wall rocks of the fissure veins. The difference in chemical composition of the wall 
rocks in the two cases would explain the mineralogical differences in the two types of 
deposits. 
This hypothesis assumes that the rock invaded by the solutions was at a lower tempera- 
ture than the solutions. The solutions from lower depths and probably from the vicinity 
of recent intrusions can hardly have failed to transfer heat to the places of ore deposition 
more rapidly than could be accomplished by conduction from the same source through 
the rocks. The possibility that secondary enrichment has modified the original deposition 
of the ores has already been referred to (pp. 65-66). 
CONCLUSIONS. 
-The gold deposits of this area are of two types — fissure veins and replacement deposits — 
with transitional members between these two extremes of the series. Concentrated solu- 
tions containing gold, silica, potash, sulphides, and oxides of iron and titanium ascended 
from great depth at high pressure and temperature. In the denser rocks they forced their 
way along crevices, forming veins, at the same time exerting so marked metasomatic action 
on the walls of these fissures that the result bears resemblance in many ways to the effect 
of contact metamorphism. Gold, pyrite, ilmenite, and other materials were deposited in 
the veins" and in the wall rocks. In the more porous rocks the solutions permeated large 
masses and replaced the original rock fragments by quartz, sericite, gold, pyrite, apatite, 
etc. The mineralogical differences in these two types of deposits are due to the differences 
in chemical composition of the rocks invaded by the solutions and to the different degrees 
of concentration of action of the solutions. The source of the ore materials was at great 
depth below the surface of that time, but of unknown distance below what is the present 
surface. It seems certain that the vein solutions are genetically connected with the gran- 
ite intrusions, representing the final product of the granite magma and bearing close rela- 
tion to the tourmaline-quartz veins and the pegmatite dikes carrying tin, which were formed 
immediately preceding the formation of the gold veins. The deposition of these materials 
was probably due partly to decrease of pressure and temperature attained by the passage 
to a higher level, but was doubtless caused mainly by a disturbance of the nice equilibrium 
of solubility and concentration by the accession of substances dissolved from the wall rock. 
These deposits differ markedly from the common western type of gold-quartz veins. 
From the amount of erosion which has taken place since their formation, as well as from 
the character of the deposits themselves, it is certain that they represent the deeply buried 
stumps of quartz veins which originally extended much higher. Whether or not the upper 
portions of these veins, now eroded away, more closely resembled the typical gold-quartz 
vein it is impossible to say. 
The numerous nuggets of large size found in the placer deposits may be an indication 
that the upper eroded portions of the veins were richer than those portions now known. 
The ore has become too lean to be profitable or has pinched out at depths not exceeding 
400 or 500 feet in so many cases that this may be considered the general occurrence. 
It may therefore be that an uneven zone not many hundred feet below the present sur- 
face represents the lower limit of the gold — namely ; the place of beginning of deposition. 
It is extremely doubtful, however, if the difference of conditions in this distance of a few hun- 
dred feet — a very slight distance compared with the original vertical extent of the depos- 
its — could have caused so great a result. It is possible instead that ore exists far below 
the present surface, but that the individual bodies of pay ore known at present give out 
at comparatively small depths, to be succeeded farther down by other and still other ore 
bodies. Thus, while the ore may persist with depth, it is by no means certain that mining 
operations will succeed in finding the lower ore bodies. If secondary enrichment has had 
any marked influence on the value of the ore, the prospects for large bodies of workable 
ore occurring at depth are not bright 
